Introduction to Ocean Currents

0.    Objectives
1.    Introduction
2.    Open Ocean Currents
2.1      Introduction
2.2      Wind-Driven Currents
2.3      Description of Surface Currents
2.4      Undercurrents
2.5      Geostrophic Currents
2.6      Upwelling
2.7      Density-Driven Currents (Meridional Overturning Circulation)
3.    Coastal Currents
3.1      Introduction
3.2      Tidal Currents
3.3      Wind-Driven Currents
3.4      Density-Driven Currents
3.5      Geostrophic Effects
3.6      Coastal/Open-Ocean Transition
4.    Measurement Techniques
5.    Products for Describing and Predicting Currents
6.    Forecast Considerations
7.    Summary
8.    References Cited


0. Objectives

After completing this module, the learner should be able to do the following things:

  1. Identify the locations of the major and minor ocean currents and describe their origin
    1. List the factors that cause ocean currents
    2. Describe how each factor influences ocean currents
  2. Characterize open-ocean currents in terms of temperature, volume (transport), and speed.
  3. Describe the origin of strong horizontal and vertical temperature, salinity, and density gradients in both open ocean and coastal ocean environments.
  4. Describe the effects of friction, bathymetry, and Coriolis force on ocean currents in both open ocean and coastal ocean environments.
  5. Explain the role of ocean currents in the global distribution of heat (i.e., the earth's heat budget).
    1. Define global meridional overturning circulation (MOC)
    2. Describe the origin of North Atlantic Deep Water and Antarctic Bottom Water
  6. Describe current prediction methods and forecast considerations

1. Introduction

1.1.1 Why do we care about ocean currents?

90-day animation of sea surface salinity in the Caribbean Sea and Gulf of Mexico from the NRL Intra-Americas Sea Ocean Nowcast/Forecast System (IASNFS)
Click to open animation in a new window.

The world’s oceans are in constant motion. Currents move vast amounts of water at all depths. These currents directly impact many human activities on a daily basis. Activities as diverse as shipping, fishing, oil and gas activities, and military operations must account for currents. More indirectly, but no less important, ocean currents redistribute heat, which affects atmospheric processes. In this way, ocean currents impact nearly all maritime operations.

1.1.2 Hazards in Shallow Water

USS Worden sinking after she went aground in Constantine Harbor, during the occupation of Amchitka, Aleutian Islands, Alaska, on 12 January 1943.

In general, currents most strongly impact activities in shallow water, rather than deep water. In shallow water, currents are highly variable across both space and time. Prediction is difficult, yet hazards abound. Precise planning and timing of operations can be problematic.

This photograph shows the USS Worden off Amchitka Island, in the Aleutians, on 12 January 1943. While covering preliminary landings of U.S. troops, strong currents swept the Worden onto a rock pinnacle, where the destroyer was left aground and without power. The Worden was then twisted around by the current and left to the mercy of the waves. It gradually capsized to starboard, broke in two, and sank.

1.2 Deep Water vs Shallow Water

1.2.1 Drawing the Line

Bathymetry of the Gulf of Mexico with the 200-m Isobath

This module will separate the discussion of currents in deep water from currents in shallow water. We use a depth of 200 meters to divide the two domains. As we see here, 200 meters roughly corresponds to the edge of the continental shelf.

We will refer to ocean currents in deep water as open-ocean currents. This includes all currents, from the surface to the bottom, so long as the water depth exceeds 200 meters. We refer to those currents in shallow water as coastal currents. This includes currents out on the continental shelf as well as those in bays, estuaries and river mouths.

Note: The U.S. Navy refers to shallow water as less than 100 fathoms (183 m).

1.2.2 Driving Forces and Modifying Effects

Conceptual cross section showing dynamic forcing processes

At the interface between deep and shallow water, we see a change in the relative importance of driving forces and modifying effects. In shallow water, tides become important, and may be our greatest consideration. As water shallows, friction and bathymetry more strongly affect currents. Proximity to the coastline brings with it fresh water from rivers. The cumulative impact of these effects results in a shallow water current regime that bears little resemblance to currents in deep water.

2. Open-Ocean Currents

2.1 Introduction

2.1.1 Forcing Mechanisms

In the open ocean, far from the influence of land, currents are primarily forced by two mechanisms: wind and density gradients. Wind is the primary driver of surface currents, while density gradients drive deep currents.

2.1.2 How big are ocean currents?

Global ocean currents with several average current transports and river discharges labeled.
Click to open animation in a new window.

When discussing currents in the open ocean, we measure the magnitude of currents in units of sverdrups (Sv). Each sverdrup is one million cubic meters per second. For comparison, the volume of the Louisiana Superdome is approximately two million cubic meters. So a flow of one sverdrup would fill the Superdome in 2 seconds. The magnitude of the major ocean currents range from a few sverdrup to over 100 sverdrup. By comparison, the largest river in the world, the Amazon, has a flow of approximately 0.2 sverdrup. The Mississippi River has a flow of approximately 0.014 sverdrup.

2.2 Wind-Driven Currents

2.2.1 Similarity of Winds and Currents

Global ocean currents

Global Mean Surface Wind for May 1979-1995 from the NCEP/NCAR Reanalysis

When we compare maps of wind and ocean currents at a global scale we see that they share most of the same features. Note in particular the gyres that form in both wind and water in each of the world’s ocean basins. The similarity arises from the fact that wind is the fundamental driver of surface ocean currents.

2.2.2 Solar Heating

Conceptual graphic showing insolation on the surface of the earth

To understand the relationship between wind and surface currents, we start with the fundamental notion that all wind arises from uneven solar heating of the earth’s surface.

The amount of solar energy reaching the surface of the earth has a maximum at the equator (over 400 Watts per square meter) and a minimum at the poles (less than 200 Watts per square meter). Two effects cause this:

(1) At the equator, sunlight strikes the earth at a nearly perpendicular angle. As latitude increases, that angle of incidence decreases, so that the same amount of solar energy is spread over a larger area.

(2) The atmosphere absorbs a certain fraction of the sunlight striking the earth. Notice how solar radiation strikes the surface of the earth at an oblique angle at high latitude. As a result, sunlight travels a longer path through the atmosphere at high latitudes than it does at low latitudes. Thus, less solar energy reaches the surface at high latitudes.

Global Mean 1000 hPa Temperature for May, 1979-1995, from the NCEP/NCAR Reanalysis

The end result of this uneven solar heating is a warm equator and cold poles.

2.2.3 Atmospheric Convection

Side view of globe with single atmospheric convective cell shown in cross section
Click to open animation in a new window.

Because air at the equator is warmer than air at the poles, it is less dense. For a moment, imagine an idealized, non-spinning globe. Because air at the equator is less dense, it rises while cold, dense air at the poles sinks. This process sets up a large circulation cell, with air rising at the equator, flowing high in the atmosphere to the poles, then sinking and flowing at low levels back to the equator. This flow represents the root cause of all wind in this idealized system.

Side view of globe with 3 atmospheric convective cells shown in cross section
Click to open animation in a new window.

The real world is a bit more complicated. For reasons having mostly to do with the depth of the atmosphere and the heat differential between the poles and the equator, three circulation cells form between the equator and each pole.

Air rises at the equator and diverges, moving both north and south. At about 30° latitude air sinks. Air also rises at about 60° latitude, moves north and south, and sinks at both 30° latitude and the poles.

This circulation still represents a highly idealized view of the world. However, it’s a model that helps explain the major features of global circulation.

2.2.4 Prevailing Winds

Side view of globe with 3 atmospheric convective cells shown in cross section and winds that result on a rotating earth
Click to open animation in a new window.

Now let’s introduce the rotation of the earth. Due to the Coriolis effect, flow on a sphere curves: to the right in the northern hemisphere and to the left in the southern hemisphere.

When we combine our simple convective cells with the Coriolis effect, familiar patterns emerge. We find the trade winds at tropical latitudes, and westerlies at mid latitudes. These are the winds that drive the major ocean surface currents.

2.2.5 Ekman Spiral

Surface and depth averaged transport currents due to Ekman spiral for deep water.
Click to open animation in a new window.

When wind blows across water, friction between the wind and water transfers a small amount of the wind’s energy to the water, starting the water in motion. With time and persistent winds, a surface current develops.

However, the current that develops does not move parallel to the wind direction. Rather, it moves at a direction that is 20° to 60° from the wind direction, to the right in the northern hemisphere and to the left in the southern hemisphere. The Arctic explorer and oceanographer Fridtjof Nansen first described this phenomenon. He observed that icebergs consistently moved at an angle to the right of the wind direction. He gave the problem to his student, Vang Walfrid Ekman to solve. The solution is now known as the Ekman spiral.

As we show with this conceptual image, according to the Ekman spiral under ideal circumstances, surface water moves at a 45° angle to the wind direction. The water immediately below the surface doesn’t feel the wind. Instead, it feels the water immediately above. So it moves at slight angle to the right of the surface water, but not as fast due to frictional losses. This continues down the water column, each layer moving a little to the right and a little slower than the layer immediately above it. When all the incremental motions are added up, the bulk motion of the affected water column is about 90° from the wind direction.

2.2.6 Idealized Surface Currents

Side view of globe with winds ocean currents that result on a rotating earth with no continents
Click to open animation in a new window.

When we combine the trade winds and westerlies over the ocean with the Ekman spiral, we develop westward-flowing equatorial currents and eastward-flowing currents at mid latitudes.

Side view of globe with winds and  ocean currents that result on a rotating earth with continents
Click to open animation in a new window.

Now, if we add bathymetry, the currents run into continents. The equatorial currents are diverted and flow poleward along coasts until they feed into eastward-flowing mid latitude currents. When these eastward-flowing currents encounter a continent, they are diverted toward the equator. There they feed into the equatorial currents and close the loop. We refer to these loops around ocean basins as subtropical gyres.

2.2.7 Question

Why is it warmer at the equator than at the poles?
a. Ocean currents transport heat to the equator
b. Solar insolation is greater at the equator
c. The equator is closer to the sun

Feedback: The correct answer is b) Solar insolation is greater at the equator.
Mean annual solar insolation is more than twice as large at the equator than it is at the poles. This results in a warm equator and cool poles.
The difference in insolation results from 2 effects
1) The high angle of incidence of solar radiation at the equator compared to that at the poles and 2) The longer path through the atmosphere that sunlight must penetrate at the poles.
While the equator is closer to the sun than the poles, this has a negligible effect on heating. Ocean currents actually act to transport heat from low to high latitudes.

2.3 Description of Surface Currents

2.3.1 Subtropical Gyres

Global ocean currents with subtropical gyres highlighted
Click to open animation in a new window.

The world’s open-ocean currents are dominated by five subtropical gyres. These include gyres in the north and south Pacific, north and south Atlantic, and Indian Oceans. Each gyre comprises a westward-flowing equatorial current, poleward-flowing western boundary current, eastward-flowing midlatitude current, and an eastern boundary current that returns to the equatorial current. Note that in the southern hemisphere, the West Wind Drift, also known as the Antarctic Circumpolar Current, provides the eastward-flowing segment of the gyres.

In this section we will examine the common characteristics of each leg of a typical gyre. Then we will examine some of the smaller, but important surface currents that contribute to ocean circulation.

2.3.2 Equatorial Currents

Global ocean currents with equatorial currents highlighted
Click to open animation in a new window.

Equatorial currents are found in every ocean basin. They form the low-latitude leg of the subtropical gyres and are driven to the west by trade winds.

Global ocean currents with equatorial countercurrents highlighted
Click to open animation in a new window.

This simple view of equatorial currents is complicated by the presence of eastward-flowing equatorial countercurrents that split the westward-flowing equatorial currents into northern and southern branches.

When equatorial currents encounter land on the west side of an ocean basin, warm water piles up, leading to a sea surface slope back down to the east. Along the equator, between the northern and southern trades, winds become very light. Because there is no wind, the sea surface slope drives some water back to the east, resulting in an equatorial countercurrent.

2.3.3 Western Boundary Currents

Global ocean currents with western boundary currents highlighted
Click to open animation in a new window.

Western boundary currents probably constitute the most familiar ocean currents and for good reason. Because they originate at low latitudes, they bring warmth to higher latitudes. For example, heat from the Gulf Stream keeps northern Europe habitable.

Western boundary currents are among the strongest currents. For example, the Gulf Stream flows at approximately 26 sverdrup, second only to the West Wind Drift. The high speed and large volume of western boundary currents results from a process know as western intensification. In response to the spin of the earth and friction along the coast, the center of the subtropical gyres is displaced westward. In essence, water on the west side of a gyre must pass through a narrower, better-defined channel. Meanwhile, return flow on the east side of the gyre is less constrained and much more diffuse.

Beside the Gulf Stream, other notable western boundary currents include the Kuroshio Current off Japan, the Agulhas Current off southeast Africa, and the Brazil Current.

2.3.4 Eddies

NLOM Analysis of Surface Currents 21 Jan 2006 showing Gulf Stream

Global maps of ocean currents predictably fail to depict the details of the currents’ paths. The currents frequently bend and meander. This is particularly true of western boundary currents once they depart from the edge of the continental shelf. For example, this image shows the bends and meanders of the Gulf Stream northeast of Cape Hatteras along the U.S. East Coast.

NLOM Analysis of Surface Currents 8-Jan to 8 Mar 2006 showing Formation of Eddies along the Gulf Stream
Click to open animation in a new window.

Frequently, loops and meanders in the current close off to form isolated eddies that continue to spin. On the warm side of the current, eddies will have a cold core, while on the cold side of the current, eddies will have a warm core. These eddies can persist for a period of months to years and will continue to migrate along with the flow in which they are embedded.

Note that the warm core eddy spins clockwise while the cold core eddy spins counterclockwise. We discuss the reasons for this later in the module.

2.3.5 Eastern Boundary Currents

Global ocean currents with eastern boundary currents highlighted
Click to open animation in a new window.

Eastern boundary currents are slower and more diffuse than western boundary currents. Because they originate at high latitudes, they bring cool water to lower latitudes. As a consequence, the marine boundary layer above these currents tends to be cool, moist, and stable and hence, prone to fog.

Examples of eastern boundary currents include the California, Peru, and Benguela currents.

[Text Note]
To learn more about the relationship between ocean currents and fog formation, see the COMET modules West Coast Fog (http://www.meted.ucar.edu/fogstrat/ic31/ic313) and Dynamically Forced Fog (http://www.meted.ucar.edu/mesoprim/dynfog).

2.3.6 Antarctic Circumpolar Current (West Wind Drift)

Global ocean currents with West Wind Drift highlighted
Click to open animation in a new window.

In the southern hemisphere, no significant land mass lies between Antarctica and 40° S latitude, with the exception of the tip of South America. Consequently, persistent westerly winds from 30°S to 60°S drive an ocean current eastward all the way around Antarctica.

Track of Argo Drifter 24442 6-jan-1997 to 6-jan-2003 showing Circum-Antarctic Current

As this graphic shows, a drifting buoy will circle Antarctica over a period of several years.

2.3.7 Subpolar Gyres

Global ocean currents with subpolar gyres and East Wind Drift highlighted
Click to open animation in a new window.

At latitudes above about 60°, the persistent winds change to an easterly direction. This results in two types of currents worth noting: (1) an east wind drift along the coast of Antarctica and (2) subpolar gyres. While these currents are relatively unimportant in terms of global circulation, they can drive icebergs into shipping lanes, and thus bear some scrutiny.

The east wind drift, also known as the Antarctic Coastal Current, is a relatively weak current with an average speed on the order of 0.1–0.2 knots. It occurs mainly south of 66°S.

Subpolar gyres occur only in the high latitude marginal seas large enough to support them: the Greenland and Weddell seas and the Gulf of Alaska. The subpolar gyres rotate in a direction opposite to the subtropical gyres: counterclockwise in the northern hemisphere and clockwise in the southern Hemisphere.

2.3.8 Monsoon-Driven Currents in the Northern Indian Ocean

Average daily Indian Ocean surface current vectors from drifting buoy observations showing reversal of monsoon current
Click to open animation in a new window.

The northern Indian Ocean is unique among the world oceans. Driven by monsoon winds, equatorial currents actually reverse direction on a seasonal basis. Monsoon winds can be thought of as a seasonal sea breeze. During the warm months, the Asian land mass is warmer than the sea, so the flow is directed onshore, from the southwest. Conversely, during the cool season Asia is cooler than the sea, so flow is directed offshore, from the northeast.

During the summer monsoon, southwesterly flow drives a strong western boundary current. The Somali Current flows northward along the east coast of Africa into the Arabian Sea, then southward along the west coast of India. This current then merges with the eastward-flowing Summer Monsoon Current.

During the winter monsoon, the Somali Current dies and a westward-flowing northern equatorial current replaces the Summer Monsoon Current.

2.3.9 Question

Which has a larger flow, the world's largest river (Amazon) or the world's largest ocean current (West Wind Drift)?
a. Amazon River
b. West Wind Drift

The answer is b) West Wind Drift.
In fact, the West wind Drift (100 Sv) is 500 times bigger than the Amazon River (0.2 Sv). In general, the transport of any of the named open-ocean currents is several times larger than that of the Amazon river.

2.3.10 Question

Which of the following currents can be classified as an eastern boundary current?
a. Brazil
b. California
c. Canary
d. Gulf Stream
e. Kuroshio
f. Peru

The correct answers are b, c, and f.
Eastern boundary currents occur on the eastern side of oceans, so they appear along the west coasts of continents. Thus, the California, Canary, and Peru currents are all eastern boundary currents. The Gulf Stream, Kuroshio, and Brazil currents are all western boundary currents.

2.3.11 Question

This map shows where the continents stood 225 million years ago. Based on what we have discussed in the previous sections, describe the open-ocean currents.

Global map showing Pangea (225 million years ago)

Feedback:
This map shows currents as they may have existed with Pangea. We could expect to see both subtropical and polar gyres in the Panthalassa Ocean. We also could expect to see north and south subtropical gyres in the Tethys Sea.

Global map showing Pangea (225 million years ago), with hypothetical ocean currents

2.4 Undercurrents

North-south cross section of velocity across the equator at 20W in the Atlantic Ocean showing Equatorial Undercurrent.

If you dive under many open ocean surface currents, you will find a current going the opposite direction. We refer to these as undercurrents. For example, this graphic shows a north-south oriented cross section in the equatorial Atlantic Ocean from the surface down to 300 meters. North is to the right, as if viewed from Africa. Dashed contours show westward-flowing speeds, while solid contours show eastward-flowing speeds. At the surface, we see the westward-flowing equatorial current. Beneath it, we see an eastward-flowing undercurrent. This undercurrent traverses the entire ocean basin, though it is strongest in the west.

In addition to equatorial undercurrents, undercurrents have also been found beneath eastern and western boundary currents. In general, undercurrents lay at a depth of 50–200 meters, and move at speeds that may approach one meter per second, about 2 knots. The origin and significance of these undercurrents is still under debate, but it appears that they play a significant role in ocean circulation.

2.5 Geostrophic Currents

2.5.1 Geostrophic Currents

Conceptual animation showing origin of geostrophic current
Click to open animation in a new window.

A persistent slope in sea surface leads to what we call geostrophic currents. Naturally, water tends to flow downslope from an area of higher elevation to one of lower elevation. However, this flow gets deflected due to the Coriolis force: to the right in the northern hemisphere and to the left in the southern hemisphere. As the current accelerates downslope, it is progressively deflected until it flows parallel to surface elevation contours. At this point the Coriolis force balances the pressure gradient force. When this occurs, we have achieved geostrophic balance.

2.5.2 Geostrophic Currents Example

Gulf of Mexico Sea Surface Heights. IASNFS Analysis 00 UTC 1 Apr 2004

This plot shows sea surface heights in the Gulf of Mexico. Warm colors are higher than cool colors. Based on the previous discussion, describe the currents near points A, B, and C.

Feedback:
Gulf of Mexico Sea Surface Heights. IASNFS Analysis 00 UTC 1 Apr 2004

In this case, the pressure gradient force is oriented downhill from the warmer colors to the cooler colors. Because we are in the Northern Hemisphere, as water accelerates downslope, it is deflected to the right. Consequently, we see a clockwise rotating eddy at point A. Points B and C lie on the Loop Current, which flows out of the Caribbean Sea, makes a large loop in the Gulf of Mexico, then rounds south Florida and initiates the Gulf Stream.

2.5.3 Density Differences

Temperature-salinity graph showing lines of constant density (isopycnals)

Sea surface slopes arise in 2 ways: density differences and Ekman transport. Density is determined by temperature, salinity, and pressure. This graph shows density contours as a function of temperature and salinity.

Please complete the following sentence, then click Done:
Density increases as temperature [increases / decreases] and salinity [increases / decreases].

What is the density of seawater with a temperature of 4°C and a salinity of 34 psu?
The correct answer is 1.027 g/cm3.

What is the density of seawater with a temperature of 14°C and a salinity of 36 psu?
Again, the correct answer is 1.027 g/cm3. From the last 2 questions we can see that increasing salinity can offset increasing temperature in the determination of seawater density. Thus, warmer, saltier seawater can have the same density as cooler, fresher seawater.

Conceptual image showing horizontal density gradient, sea surface slope, and geostrophic current

The sea surface elevation of a column of seawater is a function of its density. A column of sea water with a lower average density will have a higher sea surface elevation than a column with a higher average density. Thus water tends to flow from low density to high density.

At a given latitude, the greater the horizontal density gradient, the greater the sea surface slope, and thus, the faster the geostrophic current.

2.5.4 Example

Gulf of Mexico Sea Surface Temperature and Currents, IASNFS Analysis 00 UTC 1 Apr 2004
Gulf of Mexico Sea Surface Salinity, IASNFS Analysis 00 UTC 1 Apr 2004
Gulf of Mexico Sea Surface Height, IASNFS Analysis 00 UTC 1 Apr 2004

This loop of images shows the sea surface salinity, temperature, and height in the Gulf of Mexico. You can clearly see that the Loop Current follows the edges of a warmer and slightly fresher body of water.

You can also see the cooler, fresher, runoff along the northern Gulf of Mexico. Note that in this case, the runoff does not produce a large height anomaly because the low salinity is balanced by the cool temperature.

Also, remember that surface images do not tell the whole story. It is the average density throughout the entire water column that determines the surface elevation. For example, looking at the surface temperature and salinity alone, it is difficult to find the source of the elevated dome of water that produces the pronounced eddy west of the Loop Current.

2.6 Upwelling

2.6.1 Equatorial upwelling

Conceptual image showing equatorial upwelling

Upwelling brings cooler, deeper water to the surface. Because this water is cooler, it is also typically denser than the surrounding water. Thus, upwelling usually results in a depression in sea surface height, leading to geostrophic currents.

Upwelling most commonly occurs in 2 settings: along the equator and along the coast. As this image shows, upwelling along the equator occurs in response to persistent easterly winds. North of the equator, Ekman transport moves water north. South of the equator, Ekman transport moves water south.

Sea Surface Height (m). TOPEX/Poseidon 10-day Mean 14 July 2002.

The resulting divergence of surface water brings deeper water to the surface. This upwelled water is cooler, and thus denser. Denser seawater leads to lower sea level along the equator, clearly seen in this plot.

2.6.2 Coastal Upwelling

Conceptual image showing coastal upwelling

Along the coast, persistent winds can move surface waters offshore, resulting in upwelling. Most often, this occurs when winds blow parallel to the coast with Ekman transport directed offshore. This figure illustrates one such case for a location in the Southern Hemisphere (note that the surface water moves to the left of the wind direction).

Many factors contribute to the strength and frequency of upwelling, most importantly a narrow continental shelf and favorable winds. Coastal upwelling most commonly occurs along the west coasts of continents, particularly North America, South America, and southwest Africa. Mountainous coastlines and cold, eastern boundary currents lead to persistent low-level coastal jets, which promote upwelling.

In Depth: Coastal Upwelling

Schematic showing the relationship between surface wind, surface water movement, and upwelling

When winds blow with persistence over an ocean surface, the Coriolis force acts to move surface water at an angle to the wind direction: to the right in the Northern Hemisphere and to the left in the southern hemisphere. When winds push water offshore, cold water rises from beneath to replace it. This upwelling substantially reduces sea surface temperatures.

QuikSCAT satellite-derived winds along California coast, 23 May 2003, showing acceleration of wind downwind of Cape Mendocino and Point Arena

This image shows satellite-derived winds along the California coast. Note the high wind speeds just offshore from Cape Mendocino to San Francisco Bay. These strong coast-parallel winds are common in the summer months and can persist for several days. Ekman transport drives water offshore under these conditions, resulting in upwelling.

Shaded relief map of central California showing topography/bathymetry

The presence of deep water close to shore enhances the cooling effect of upwelling. This results from deeper, colder water rising to the surface during upwelling. Thus, coastal areas with a narrow continental shelf or where a submarine canyon comes close to shore experience more cooling during upwelling than areas with a broad, shallow continental shelf. This map shows the bathymetry of the central California coast near Monterey. Note the deep canyon that approaches the coast and the relatively narrow continental shelf throughout the region.

Sea surface temperature (SST) off the west coast of North America, July 2003

The end result of strong coast-parallel winds and a narrow continental shelf is very low sea surface temperatures close to shore, as is shown in this plot for western North America in July, 2003. Lower temperatures mean higher density, which leads to a drop in sea surface height.

2.7 Density-Driven Currents

2.7.1 Origin of Deep Ocean Water

Conceptual image showing thermohaline circulation in the Atlantic Ocean

Currents below the level of the Ekman spiral are driven primarily by differences in density. It stands to reason that the densest water will move downhill along the bottom of the ocean. This process leads to a vertically stratified ocean, with denser sea water underlying less dense sea water. But this situation raises several questions: where does the deep water in the oceans come from? Why is it moving? Why doesn’t it just pool up and stagnate?

Temperature and salinity determine the density of sea water. Bottom water in the ocean basins is both cold and salty. Most of the deep water in the world’s ocean basins originates in the North Atlantic. There the warm, salty water of the Gulf Stream cools at high latitudes, resulting in very cold, very salty, sea water. This water then sinks, initiating what is called Meridional Overturning Circulation (MOC). This conceptual animation shows Meridional Overturning Circulation for the Atlantic Ocean, from the Arctic to the Antarctic.

2.7.2 Meridional Overturning Circulation (MOC)

Conceptual image showing global thermohaline circulation
Click to open animation in a new window.

On a global scale Meridional Overturning Circulation originates in the North Atlantic, where cold, salty water sinks and forms North Atlantic Deep Water. This water flows southward all the way to the Southern Ocean, where it turns eastward. As this deep ocean current travels east, some of it branches northward into the Indian Ocean, while the rest continues on into the Pacific Basin before turning north. Along the way, this deep ocean water mixes with other deep water, gradually becoming slightly warmer and less salty. In the North Pacific and the northern Indian Ocean, upwelling draws the water back to the surface. Surface currents then drive the water back to the west at low latitudes. As the water warms, evaporation increases its salinity. After rounding the southern tip of Africa, the water crosses the Atlantic and heads north as the Gulf Stream. The Gulf Stream returns to the North Atlantic, where the warm, salty water cools, sinks, and starts the cycle over.

2.7.3 Antarctic Bottom Water

Animation showing brine exclusion during sea ice formation
Click to open animation in a new window.

The densest water in the ocean basins actually forms off the coast of Antarctica in the Weddell and Ross Seas. Here, the highly saline water forms through a process know as brine exclusion, shown in this animation. As sea ice forms, fresh water preferentially forms the ice, leaving a remainder of cold, salty brine. This brine sinks and forms Antarctic Bottom Water.

North-south cross section of salinity in the Pacific Ocean showing Antarctic bottom water

This graphic shows a north-south cross section of salinity in the Pacific Ocean. South is to the left. Click on the Antarctic Bottom Water.

Feedback: In this cross section, we clearly see a tongue of high-salinity seawater, shown in brown, coming off of Antarctica and settling along the seafloor north of the equator.

2.7.4 Red Sea and Mediterranean Intermediate Water

East-west cross section of salinity in the eastern Atlantic Ocean showing Mediterranean intermediate water

Evaporation in the Mediterranean and Red Seas leads to very salty water. However, this seawater is also quite warm. As a result, when this water leaves the sea where it formed, it sinks to an intermediate position in the ocean.

This cross section shows the salinity of seawater near the mouth of the Mediterranean Sea. We see a salty tongue of water exiting the Straits of Gibraltar at a depth of about 1000 meters. This water forms an intermediate layer within the Atlantic Ocean.

3. Coastal Currents

3.1 Introduction

3.1.1 Shallow Water Effects

Conceptual cross section showing dynamic forcing processes

Currents in shallow water differ markedly from those in deep water. In this module, we covered the basic concepts of currents in the deep water section. So what makes shallow water different?

  1. The water is shallow enough that frictional and bathymetric effects exert a strong influence.
  2. Tidal influences are stronger and may be the dominant driving force.
  3. The proximity to freshwater runoff can create strong horizontal and vertical density differences.
  4. Currents are frequently constrained by the coastal boundary or shallow-water bathymetry.
  5. Coastal weather patterns differ substantially from those over the open ocean.

For the purposes of this module, we refer to all currents that form in shallow water as coastal currents.

3.1.2 Time Scales of Current Variability

Modeled Daily Mean Salinity (shaded) and Velocity (vectors), Shelikof Strait, AK, May to October, 1998
Click to open animation in a new window.

As a result of shallow water and coastal influences, coastal currents are highly variable across both time and space.

Time scales range from hours for semidiurnal tidal currents to days for wind-driven currents forced by weather events to weeks for density-driven currents forced by freshwater runoff events.

Note that we describe wind and freshwater runoff as events. For example, note the pulses of fresher water moving down the coast in this animation. Climatological averages do not usually capture the magnitude of these individual pulses. The timing of peak spring runoff can differ by weeks from year to year and peak river flows regularly vary by a factor of two or more.

The influence of wind events is not only sensitive to the timing and wind speed, but also to the wind direction, all of which vary across time and space. Due to this high variability, climatology underestimates the importance of weather events.

3.1.3 Spatial Scales of Current Variability

Shaded relief image of Southeast U.S.

In general, the spatial variability of currents is much greater across the continental shelf than it is along the continental shelf. The notable exceptions to this are near inlets and submarine canyons. Regardless, coastal currents exhibit much higher spatial variability than do open-ocean currents.

The high spatial and temporal variability in shallow water impacts how we measure and model coastal currents. Observation networks require a higher density at shorter time intervals. Numerical models require higher resolution with shorter time steps to accurately simulate coastal currents.

3.1.4 Mechanisms that Drive Shallow Water Currents

Montage of mechanisms that drive currents

As we have alluded to in the previous discussion, several mechanisms drive shallow water currents. The principal mechanisms include tides, wind, and horizontal density gradients. We will discuss each of these mechanisms in detail later in the module.

We introduced tides in a previous COMET module, Introduction to Ocean Tides (http://www.meted.ucar.edu/oceans/tides_intro). If you have not completed that module, doing so would help you understand several concepts that we discuss later.

In addition to tides, wind, and density gradients, we will also discuss the concept of non-local forcing. By this we mean mechanisms that operate outside of our area of interest, yet significantly impact coastal currents within it. Non-local forcing can be difficult to account for in model simulations of coastal currents.

3.1.5 Influences that Modify Coastal Currents

Montage of properties that modify currents: Bathymetry, coriolis, friction

Once a current exists, several influences act to modify that current. In this module we will examine three: bathymetry, Coriolis effect, and bottom friction.

3.2 Tidal Currents

3.2.1 Origin of Tidal Currents

Tidal currents and bulges due to lunar net tractive forces
Click to open animation in a new window.

As we discussed in the module Introduction to Ocean Tides, the gravitational attraction of the sun and moon result in horizontal tractive forces. These tractive forces slide water across the surface of the earth resulting in tidal bulges. This horizontal motion of water leads to tidal currents.

3.2.2 Relation of Tidal Currents to Tidal Elevation

Animation of a volume of water where the tidal current entering the volune varies from the tidal current exiting the volume.  This shows the change in tidal elevation (or water level) during this variation.
Click to open animation in a new window.

This animation shows the relationship between tidal currents and water elevations. The box is partially filled with water. If more water flows into the box than out of it, then the water level rises. Conversely, if more water flows out than in, then the water level falls.

3.2.3 Tidal Heights and Current Speeds

Time series of tidal elevation and current speed. Shows that the current speed correlates well with the tidal elevation cycle.

This graph depicts a time series of water elevation and current speed for a month-long period. Note the correlation between tidal range and current speed. Current speeds peak during spring tides and reach a minimum during neap tides.

3.2.4 Tidal Currents on Continental Shelves

Surface Current Vectors. PCTides Forecast 00 UTC 5 Aug to 00 UTC 7 Aug 2005
Click to open animation in a new window.

This animation shows tidal currents for an area near the Kamchatka Peninsula, determined using the Navy’s PCTides software. Note that the current vectors change direction continuously through the tidal cycle. Thus, we have a rotary motion for the tidal currents, superimposed with preferred directions of maximum speed associated with flood and ebb. This generally holds true on continental shelves, where land boundaries are far enough away that they exert little influence on tidal currents.

3.2.5 Tidal Currents in Bays and Inlets

Time series graph showing current and elevation for 4 days at the entrance to Galveston Bay. Current direction shows a rectilinear pattern.

In enclosed basins, tidal currents flow in from open water during the "flood" stage and out during the ebb stage. Tidal currents in these basins have a "rectilinear" pattern; they reverse rapidly after short periods of "slack water" and then flow in the opposite direction.

This plot shows a time series of current speed and direction for the entrance to Galveston Bay in Texas. At this location, the current speed briefly goes to zero as the current reverses direction. Where tides are semi-diurnal, the currents reverse approximately every 6 hours; where tides are diurnal, currents reverse about every 12 hours.

3.2.6 Standing Wave

Graph showing relationship between water height and current velocity for a standing wave

The time of flood, ebb, and slack water varies depending on the location. In small, enclosed basins, slack water tends to occur near or at high tide and again near low tide. We refer to this pattern as a standing wave.

3.2.7 Progressive Wave

CBOFS Nowcast/Forecast of Water height at Solomons Is. and current velocity at Cove Pt. in Chesapeake, 15 May 2006, showing a progressive wave pattern where highest current velocities occur at high and low tide.

However, in long open channels and open areas, slack water occurs closer to mid-tide. For example, these graphs show tidal current and water elevation for a location about midway up Chesapeake Bay. Note that the strongest flood occurs near high tide, the strongest ebb occurs near low tide, and slack water occurs roughly halfway between high and low tide. We refer to this pattern as a progressive wave.

3.2.8 Rivers and Disappearing Flood Current

Water Height and Along-Channel Current Velocity at Newbold, NJ, on the St John River, 12 May to 15 May 2006, showing minimal flood current.

As one travels up estuaries and inlets, inflow from rivers starts to affect tides. Under some circumstances, water elevations continue to rise and fall, even though the current in the channel never reverses. These graphs show currents and water elevations at Newbold, PA, on the Delaware River. Here, we see a very weak flood current and comparatively strong ebb current as the water level rises and falls. This pattern reflects the influence of the river current.

3.3 Wind-Driven Currents

3.3.1 Introduction

Just as the wind drives open-ocean currents, it drives coastal currents, too. However, once we are out of deep water and up on the continental shelf, shallow water and proximity to the coast strongly influence the resulting currents. In this section, we examine several of the most important influences on wind-driven currents in shallow water.

3.3.2 Vertical Shear

Time Series Graph of Wind-Driven Currents at Surface, Middle Depth, and Bottom Over several days showing decrease of current speed with depth

Relatively shallow water on the continental shelf leads to increased friction between currents and the sea floor. As a result, wind-driven current speed is frequently greatest at the surface and diminishes with depth. However this is not always the case. In reality, the vertical current shear is highly variable and depends on wind speed, duration of strong winds, water depth, and vertical density stratification.

This time series shows an example of current speeds at the surface, mid-depth, and bottom. The tidal component of the current was filtered out, leaving only the wind-driven current. Early in the record, current speed at the surface, mid-depth, and bottom are nearly equal. However, after a strong wind event at about 310 days, the current speed decreases with depth. At about 314 days, mid-depth currents lag those at both the top and bottom!

While current shear results from the complex interaction of several processes, we can make some general observations. For example, the stronger the density gradient, the more the wind drives the surface and not below, resulting in a large vertical current shear. Consequently, we tend to see stronger vertical current shear in the summer months when surface waters warm and the vertical temperature gradient strengthens.

In a related fashion, when a wind springs up, currents develop first at the surface, leading to strong current shear. With time, in response to wind-driven mixing and momentum transfer, deeper currents develop, which decreases the vertical current shear.

3.3.3 Shallow Water and the Ekman Spiral

Conceptual diagram of an Ekman spiral in shallow water

In the previous section on deep-water currents we saw how the Coriolis effect leads to an Ekman spiral. In deep water, surface water moves approximately 45° to the wind direction and the mean water motion through the depth of the spiral moves at right angles to the wind direction.

On the continental shelf and in shallower waters in bays and estuaries, the water is not deep enough for a full Ekman spiral. Thus, in shallow water, surface water moves at an angle to the wind that is substantially less than 45°. And overall, the mean water motion is typically much less than 90° to the wind direction.

In Depth: Factors controlling the shallow-water Ekman spiral

Ekman Depth for Different Latitudes and Wind Speeds
10-m Wind (m/s) Latitude
15° 45°
5 75 m 45 m
10 150 m 90 m
20 300 m 180 m

In shallow water the relative effect of Ekman transport depends on the depth of the water relative to the so-called Ekman depth. As this table shows, the Ekman depth depends primarily upon latitude and wind speed. Latitude controls the strength of the Coriolis force. Lower latitudes result in a weaker Coriolis force and, consequently, greater Ekman depths. Higher wind speeds also lead to a greater Ekman depth. Note that for moderate wind speeds and subtropical latitudes, the Ekman depth roughly corresponds to the 200-meter isobath, the depth at the outer edge of the continental shelf.

Interactive animation showing variation in ekam spiral from deep to shallow water.
Click to open animation in a new window.

This interactive animation shows how the Ekman spiral varies for different values of water depths. At the far left, the water depth exceeds the Ekman depth. Note that when the water depth equals about one half of the Ekman depth, the surface current is still about 45° to the wind direction, but that the mean water motion through the Ekman layer is closer to the wind direction than the deep water case: about 70° as opposed to 90°. As the water gets shallower relative to the Ekman depth, the current direction approaches the wind direction and the current speed decreases due to bottom friction.

3.3.4. Effects of Coriolis and Wind Direction on Coastal Sea Level

Conceptual animation showing currents from onshore and offshore wind
Click to open animation in a new window.

When the wind drives water toward shore, the local sea level rises, which we call set up. Conversely, when wind drives water away from shore, the local sea level falls, which we call set down.

The rise and fall of water in response to the wind or changes in atmospheric pressure is referred to as a meteorological tide. When meteorological tides raise or lower the water level immediately adjacent to bays or other inlets, water flows in or out of the bay in response. In areas with a relatively small tidal range, this meteorologically induced current can dominate the tidal current.

Conceptual animation showing currents from winds blowing up and down the coast
Click to open animation in a new window.

Note that the wind does not need to be blowing onshore or offshore to drive water onshore or offshore. Due to the Coriolis effect and the Ekman transport, winds blowing parallel to the coast drive water onshore or offshore.

3.3.5 Inertial Currents

Conceptual animation of buoy drift showing intertial currents
Click to open animation in a new window.

A sustained wind lasting several days will result in a steady wind-driven current. If the wind suddenly dies, the current will continue as an inertial current. However, the inertial current doesn't just continue as a straight line. As this animation shows, in response to the Coriolis effect, the current also develops a circular motion. The result looks like a series of loops. With time, the loops shrink and the current diminishes in response to friction.

Because the Coriolis effect equals zero at the equator and reaches a maximum at the poles, its effect on inertial currents is least at low latitudes and greatest at high latitudes.

3.4 Density-Driven Currents

3.4.1 Introduction

A volume with two liquids of differing densities are separated by an object and then released to show the flow of the less dense liquid over top of the more dense liquid
Click to open animation in a new window.

Density differences are the third major driving force for shallow water currents. Any time water masses with different densities are juxtaposed horizontally, there is a tendency for the lower-density water to flow over the higher-density water, while the higher-density water flows under the lower-density water.

3.4.2 Mechanisms Causing Horizontal Density Differences

Cross Section of Mean Salinity and Mean Surface Currents across Cook Inlet, Alaska

Several processes lead to horizontal differences in salinity or temperature in coastal regions, and thus to density differences. These include:
- Freshwater runoff from land, which decreases salinity
- Evaporation in shallow bays, which increases salinity
- Upwelling, which decreases temperature
- Seasonal heating and cooling
- Wind-driven vertical mixing, which cools surface water, but tends to warm deeper water
- Horizontal advection of either temperature or salinity

This graphic shows a cross section of salinity taken across Cook Inlet, near Anchorage, Alaska. You can clearly see fresher water near the surface and more saline water near the bottom of the channel.

3.4.3 Significance of Density-Driven Currents

Cross Section of Mean Velocity and Mean Surface Current Vectors across Cook Inlet, Alaska

This image shows a cross section of mean geostrophic velocity across Cook Inlet computed based on the mean salinity profile we examined previously. The geostrophic currents are forced by persistent density differences caused by freshwater runoff. We can see that the time-averaged, density-driven component of the current approaches 3 knots in the fresher water. These geostrophic currents significantly alter the magnitude and timing of the total current in an area with one of the largest tidal ranges in the world.

Under most circumstances, the density-driven component of currents in shallow water is a small part of the total current at any given moment. For example, density-driven currents are typically not felt by swimmers. However, the density-driven component may dominate the long-term, time-averaged current. Near major rivers, like the Mississippi, or along coasts that receive high rainfall, like southeast Alaska, density-driven currents can reach a speed of several knots.

3.4.4 Time Scales

Graph showing time scales for different current driving mechanisms

Density-driven currents operate across many time scales, depending on the forcing mechanism. At short time scales, ebb tides can dump fresh water from rivers into estuaries or out onto the continental shelf. Intense rainfall can result in floods that last for a day or days. Seasonal processes like spring runoff and evaporation operate on time scales of weeks or more. As this graphic shows, tidal and wind-driven currents operate over a similar range of time scales.

3.5 Geostrophic Effects

3.5.1 Geostrophic Effects on Density-Driven Coastal Currents

Flow of less dense liquid away from coast, being turned along the continental shelf by the coriolis force.
Click to open animation in a new window.

Regardless of the process, whenever a current is forced for a sustained period of time, the Coriolis effect will tend to rotate the current: to the right in the northern hemisphere and to the left in the southern hemisphere. This is true for coastal currents, just as it was for open-ocean currents. Thus, currents forced by seasonal runoff events or persistent winds will develop a significant geostrophic component, while currents forced by tides and sea breezes will not. The effect increases with latitude, as the Coriolis effect increases.

This animation shows a current that forms as a result of a large runoff event. Initially, the freshwater flows out over the top of the denser saltwater. With time, the freshwater turns down the coast and a coast-parallel, density-driven current results. This process occurs near the mouth of most large rivers, particularly at higher latitudes.

3.5.2 Geostrophic Effects on Wind-Driven Coastal Currents

Conceptual image showing development of a coastal geostrophic current in response to a coast parallel wind

Another common example of geostrophic forcing of coastal currents is a persistent wind blowing parallel to the coast, particularly when it results in upwelling. As water is pushed offshore, a landward-dipping slope develops. The result is a coast-parallel, wind-driven current with a significant geostrophic component.

3.6 Coastal/Open-Ocean Transition

3.6.1 Location of the Interface between Shallow and Deep Ocean

Conceptual image showing transition from deep water to shallow water near the Ekman depth

The most challenging place to predict ocean currents lies at the interface between deep and shallow water. Because currents in the transition zone respond to a variety of local and remote influences, they are less predictable than those closer or farther from shore.

The interface between shallow and deep water is a relatively broad zone that typically occurs near the edge of the continental shelf. If the continental shelf is narrow, deep-water effects can extend to the coast. On the other hand, if the shelf is broad, the interface is confined to the outer shelf. This leaves the inner shelf mostly free of deep water influences, so it responds primarily to local forces.

So what happens in this transition zone? On the east coasts of continents, strong western boundary currents may impinge, dominating the local current influences. Where these currents detach from the coast and move offshore, eddies may spin off and migrate back onto the continental shelf.

Another deep-ocean influence on the continental shelf occurs when cold, dense, deep-ocean water upwells onto the continental shelf. This process leads to density-driven currents and geostrophic adjustment.

3.6.2 Internal Waves and Currents at the Continental Shelf Edge

Animated simulation of internal waves meeting a steep continental shelf
Click to open animation in a new window.

Much like ocean swell at the sea surface, waves also form deeper in the ocean. We refer to these waves as internal waves. While ocean swell forms at the interface between ocean and atmosphere, internal waves form at depths with strong vertical density gradients, such as the base of the mixed layer.

Many different mechanisms generate internal waves.We are most interested in the internal waves that occur when open-ocean tides impinge upon steep bathymetry, such as the continental shelf edge and seamounts. This animation shows how these internal waves can result in strong currents near the edge of the continental shelf. Colors depict seawater density, while vectors indicate the current. When tidal waves encounter the continental shelf, they appear to break. Internal waves then propagate shoreward and seaward. In the case of a narrow continental shelf or a submarine canyon, this process may result in strong currents close to the coast.

3.6.3 Satellite Images of Internal Waves

Internal waves approaching Palawan, Phillipines. MODIS true-color image 8 April 2003

This MODIS true-color image shows internal waves approaching Palawan, Philippines. With the appropriate sun angle, you can see zones of rougher and smoother water caused by convergence and divergence at the surface. As this image shows, the wavelength of internal waves measures in miles.

4. Measurement Techniques

4.1 Introduction

We measure ocean currents in several ways.

4.2 Moored Current Meters

Photo of navigation buoy

Moored current meters provide a detailed record of currents at a given location. Most real-time current meters support navigation. Consequently, they are located at the entrances to harbors or along channels and record surface currents. Frequently, these current meters are fixed to navigation buoys, like this one.

Photo of acoustic doppler current profiler (ADCP)

Other current meters are moored to the sea floor. Some of these, known as acoustic doppler current profilers (ADCPs), can measure the current at many levels from the seafloor to the surface. This photo shows an ADCP on the seafloor in Monterey Bay National Marine Sanctuary. It measures currents flowing over the top of Davidson Seamount.

Location map showing acoustic doppler current profilers (ADCP) in the western Gulf of Mexico

Other ADCPs are located offshore. For example, this graphic shows the locations of current meters located on oil platforms in the Gulf of Mexico.

Whether they are deployed on buoys or oil platforms or fixed to the seafloor, many current meters do not provide data in real-time. These meters keep a record that must be retrieved and downloaded at a later date. This is particularly true for current meters used in engineering studies and scientific research.

4.3 Drifting Buoys

Photo of argo drifting buoy

Another way to measure current is to set a buoy free and have it send its position back to a receiver. There are several thousand buoys like the one pictured here presently drifting around the world oceans doing exactly that.

Trajectories of Drifting Buoys in the North Atlantic, 13 Nov - 11 Dec 2006

This plot shows buoy trajectories for a one-month period. You can see from the density of data that drifting buoys do not provide high-resolution data in real time. However, we can derive accurate climatologies when we compile data from several years.

Most drifting buoys float on the surface, but some sink to a depth of several hundred meters for several days, then resurface and radio back their position. These buoys provide data on deep ocean currents. As the buoys rise and fall, they also measure temperature and salinity.

4.4 Satellite

OSCAR Satellite-Derived Surface Currents 1 Dec 2006

Satellites can very accurately measure sea surface height (SSH) and surface winds. From these measurements we can estimate the geostrophic and wind-driven components of ocean currents. NOAA currently provides near real-time analyses derived in this fashion through the OSCAR (Ocean Surface Current Analyses — Real time) program. This graphic shows currents in the Indian Ocean for December, 2006. Real-time analyses like this provide mariners with timely information about variable currents, like monsoon-driven currents in the Indian Ocean.

4.5 Radar

Surface Current Vectors Measured by High-Frequency Radar 0100 UTC 11 Dec 2006

High frequency radar can provide estimates of surface currents up to 300 km from shore. For example, this graphic shows radar-measured surface currents near Cape Hatteras. Note the high velocity of the Gulf Stream as it moves offshore. At the present time, radar coverage in the U.S. is confined to a few regions along the East, West, and Gulf Coasts.

5. Products for Describing and Predicting Ocean Currents

As we have seen throughout this module, current data can be depicted in several ways. This section takes a brief look at products that depict ocean currents.

5.1 Current Maps

Maps of currents use many different combinations of arrows and color shading to depict currents. These can include the following:

  • Just vectors, where the length of the vector indicates current speed
    Surface Current Vectors Measured by High-Frequency Radar 0100 UTC  11 Dec 2006
  • Vectors or uniform length arrows over shaded velocity
    OSCAR Satellite-Derived Surface Currents 1 Dec 2006
  • Streamlines over shaded velocity
    NLOM Analysis of Surface Currents 21 Jan 2006 showing Gulf Stream
  • Trajectories, where the length of the trajectory indicates current speed
    Trajectories of Drifting Buoys in the North Atlantic, 13 Nov - 11 Dec 2006

Current maps typically show surface currents, but maps may depict currents at any depth in the ocean.

5.2 Time Series Graphs

Time series graph showing current and elevation for 4 days at the entrance to Galveston Bay. Current direction shows a rectilinear pattern.

One unfortunate aspect of map plots is that they show current information at discreet times. We can combine maps into animated loops, but at best, they may show currents on an hourly basis. In coastal areas with strong tidal currents, you may need to know more precisely when the current switches from flood to ebb. In this case, a time series plot will give you the answer you seek. However, you may need to examine several individual plots as the tide will change at different times in different places.

Using this diagram, find the current speed and direction at 0600 on 25 November.

[Answers: 1 kt at 290 degrees]

5.3 Stick Diagrams

Example of stick diagram. Part of a diagram showing water elevation, stick diagram, and current speed.

This diagram shows three time series graphs for a single location. The top graph shows water elevation, the bottom graph shows the current speed, and the middle graph is a stick diagram of currents. Stick diagrams comprise a series of vectors for a single location. Each stick is merely a current vector without an arrow on the end. Thus each stick shows the current speed and direction for a particular time. For example, a stick pointing straight up indicates a northward-flowing current, while a stick pointing down and to the right indicates a current flowing to the southeast . The length of the stick indicates the current speed.

Using this diagram, find the current speed and direction at 0000 on 18 August.

[Answers: 2 kt at 20 degrees]

5.4 Tidal Current Tables

Tide current table for San Diego Bay entrance, CA, January 1-14, 2006

Most of us are familiar with tide tables. They typically show the times of high and low tide for set locations. Similar tables are published for tidal currents. This table shows tidal currents for the entrance to San Diego Bay for the first two weeks of January, 2006. The table shows the time of slack water and the time and velocity of maximum current. Flood and Ebb current directions are shown in the header. It is assumed that the current oscillates between these two directions. Currents with positive velocities in the table flow in the flood direction; negative velocities flow in the ebb direction.

Remember that these tables only depict currents forced by astronomical tides. They will not show any effect from wind-driven or density-driven currents.

Tidal current tables are published by NOAA and the British Admiralty, just the same as water levels.

Using the tidal current table, find the current speed and direction at 1400 on 7 Jan 2006.

[Answers: 0.7 kt at 355 degrees (from header)]

6. Forecast Considerations

6.1 Forecasting Guidance for Open-Ocean Currents

When we forecast open-ocean currents, we can look at data from historical observations, near-realtime observations, climatology, and numerical ocean models.

The density of observations from current meters and drifting buoys in the open ocean is generally quite sparse. These types of observations may help us evaluate model skill or the usefulness of a particular climatology, but they only occasionally provide forecast guidance. Satellites can provide near real-time observations, which may be all we need for a short-term forecast in the open ocean.

Climatology is a long-term average of observed ocean currents. In the open ocean, climatology typically provides the best long-term forecast. In general, climatologies have several weaknesses:

  • They are only as good as the observations; areas with few observations may not have accurate averages.
  • Climatologies average out short-term events like storms and smooth out seasonal events, like spring runoff.

Ocean models predict the state of the ocean using a set of equations. There are many different models available, each with their own strengths and weaknesses. The main thing to recognize with ocean models is that their accuracy degrades with time. And the more variable the currents are, the more quickly model accuracy will diminish. At some point in the forecast period, climatology often becomes more accurate than the model prediction.

[Text Note]
To learn more about ocean models, see the COMET module Introduction to Ocean Models (http://www.meted.ucar.edu/oceans/ocean_models/).

6.2 Forecasting Guidance for Coastal Currents: Customer Considerations

Photo of SCUBA divers

Photo of landing craft on beach

Photo of helicopter rescue

The first step in forecasting shallow water currents is to understand how the forecast will be used. For Naval operations, we tend to forecast either "instantaneous currents" (for example, the current speed a diver might feel) or long-term drift currents (for example, where a disabled boat may drift over several days). The long-term drift application needs to be further refined to define "long-term." Are the currents to be vector-averaged over a few tidal cycles? Or longer? If so, you may also be averaging over wind events, and maybe even density-driven events.

While preparing the forecast, it is helpful to periodically re-assess the detailed application; in other words, how much information does the forecast need to contain to satisfy the application? For example, perhaps a customer wants to know the current speed and direction over the next three days at half hour increments for a particular mission. This information is very difficult to accurately predict (even with the use of models). However, it is often feasible to determine the times of slack water, maximum flood, and maximum ebb in tidally dominated areas accurately enough for the customer's mission. It is better to provide less information that is accurate, than more information that is wrong, relative to the mission.

6.3 Forecasting Guidance for Coastal Currents: Basic Approach

Montage of mechanisms that drive currents

In coastal areas, there are more observations than in the open ocean, but the currents are much more variable, which makes it difficult to extrapolate observations. Thus, unless we are very close to the observing site, observations provide limited information.

The basic forecasting approach for coastal currents is to estimate the relative contribution of the different forcing mechanisms to the total current, whether that be instantaneous or long-term drift. For instantaneous currents, tides often, but certainly not always, dominate or contribute significantly to the total current, especially in estuaries and bays. Winds usually dominate or contribute significantly on the open shelf. There are certainly cases where density-driven currents dominate the instantaneous current, but more often, density plays a small role compared to tides and winds.

For long-term drift applications, winds and density are usually more important than tidal currents.

Remember that non-local effects may include the impact of the coastal ocean on an estuary or bay, or the impact of the deep ocean on the continental shelf.

6.4 In Closing: Some Cautions

Forecasting shallow water currents is very difficult. Suppose one collected a time series of current speed and direction on the continental shelf for a month. Then you collected synoptic data for the same period on forcing mechanisms, such as sea level, winds, and temperature/salinity profiles. When you went to investigate cause and effect, much of the current signal would remain unexplained. You may do better explaining instantaneous currents in tidally dominated areas in estuaries and bays, but not much better explaining long-term drift.

Thus, when forecasting ocean currents, you will need to apply your knowledge of currents and their forcing mechanisms as they relate to the operations you are supporting. One should always take care to appreciate and acknowledge the limitations of numerical models, real-time observations, and climatologies.

[Text Note]
Forecasting currents near shore will be the topic of a forthcoming COMET module.


7. Summary

Ocean currents can be classified based on water depth.

  • Open-ocean currents occur in water deeper than 200 m.
  • Coastal currents occur in water shallower than 200 m.

In deep water (open-ocean currents):

  • Wind drives surface currents.
  • Density differences drive deep currents.

In shallow water (coastal currents):

  • Tides become important.
  • Friction and bathymetry more strongly affect currents.
  • Fresh water runoff changes water density
Open-Ocean Currents

Surface currents in deep water are driven by large-scale wind circulation patterns.

  • This wind circulation is driven by global temperature gradients: hot at the equator and cold at the poles.
  • In response to these temperature gradients, three convective cells form between the equator and each pole.
  • Due to the Coriolis effect, this convective circulation creates trade winds at tropical latitudes and westerlies at mid-latitudes.
  • When we combine the trade winds and westerlies over the ocean with the Ekman spiral, we develop westward-flowing equatorial currents and eastward-flowing currents at mid latitudes.
  • Where continents block equatorial currents, subtropical gyres develop.

While subtropical gyres dominate surface currents in the deep ocean, other currents also play an important role in global circulation. These include:

  • Subpolar gyres that form in the Greenland and Weddell Seas
  • Monsoon currents in the northern Indian Ocean, where the equatorial currents reverse direction during the monsoon
  • Undercurrents that underlie many surface currents, but flow in the opposite direction

In response to Ekman transport:

  • Surface water moves 45° from the wind direction.
  • The depth-averaged motion of the water column is 90° from the wind direction.
  • Upwelling occurs along the equator and in coastal areas.

A persistent slope in the sea surface leads to geostrophic currents.
Sea surface slopes arise in 2 ways: density differences and Ekman transport.

  1. A column of sea water with a lower average density will have a higher sea surface elevation than a column with a higher average density. Water tends to flow from low density to high density.
  2. Ekman transport can result in a sea surface slope, resulting in geostrophic currents.

Currents below the level of the Ekman spiral are driven primarily by differences in density.

  • Density differences arise from cooling at high latitudes and evaporation at low latitudes.
  • Most of the deep water in the world's ocean basins originates in the North Atlantic.
  • The densest water in ocean basins forms off the coast of Antarctica.
  • Downwelling at high latitudes and upwelling in other regions leads to meridional overturning circulation.

Shallow Water Currents

Currents in shallow water differ markedly from those in deep water for several reasons:

  1. The water is shallow enough that friction and bathymetry modify surface currents.
  2. Tidal influences are stronger and may be the dominant driving force.
  3. The proximity to freshwater runoff can create strong horizontal and vertical density differences.
  4. Currents are frequently constrained by the coastal boundary or shallow-water bathymetry.
  5. Coastal weather patterns differ substantially from those over the open ocean.

Shallow water currents are highly variable across both time and space.

Tidal Currents

The gravitational attraction of the sun and moon result in horizontal tractive forces. These tractive forces slide water across the surface of the earth resulting in tidal currents.

Greater tidal ranges typically result in higher tidal current velocities.

Far from land, tidal currents tend to describe an ellipse with every tidal cycle.
In confined bays and estuaries, tidal currents tend to reverse direction rapidly after a short period of "slack water."

In long open channels,the strongest currents coincide with high and low tide. We refer to this pattern as a progressive wave.
In small, enclosed basins, slack water coincides with high and low tide. We refer to this pattern as a standing wave.

In some rivers near the coast, the current in the channel never reverses, even though water elevations rise and fall.

Wind-Driven Currents

Wind-driven current speed is greatest at the surface and diminishes rapidly with depth.

Shallow water is not deep enough for a full Ekman spiral. Consequently, as water gets shallower, the current direction approaches the wind direction and the current speed decreases. 

When wind drives water toward shore, local sea level rises (set up). Conversely, when wind drives water away from shore, sea level falls (set down). In areas with a relatively small tidal range, this wind-driven current can dominate the tidal current.

When a sustained wind suddenly dies, the wind-forced current has momentum, and continues as an inertial current. This inertial current looks like a series of diminishing loops.

Density-Driven Currents

When water masses with different densities are juxtaposed horizontally, there is a tendency for the low-density water to flow over the high-density water and/or high-density water to flow under low-density water.

Several near shore processes lead to horizontal density differences:

  • Freshwater runoff from land
    • Seasonal runoff (weeks)
    • Local, heavy rainfall (hours - days)
  • Evaporation in shallow bay
  • Upwelling
  • Seasonal heating and cooling
  • Wind-driven vertical mixing
  • Horizontal advection of temperature or salinity

Currents forced by seasonal runoff events or persistent winds will develop a significant geostrophic component.

Measurement Techniques

We can measure ocean currents in many ways:
  • Moored current meters provide a detailed record of currents at a given location. Current meters may be attached to buoys, piers, drill rigs, or the seafloor.
  • The trajectories of drifting buoys also provide a measure of currents.
  • Satellite-derived wind speed and sea surface height allow us to estimate currents in the deep ocean.
  • High frequency radar can provide estimates of surface currents up to 300 km from shore.

Data Products

In the deep ocean spatial differences tend to be larger than temporal differences. As a result, current data tends to be depicted in map view. These plots can take many forms:
     1. Vectors or arrows, with or without shaded velocity
     2. Streamlines, with or without shaded velocity
     3. Trajectories
Animations are used to add the element of time.

Shallow water currents vary more with respect to time than deep water currents. Therefore, current data tends to be depicted in a time series plot. These plots can take two common forms:
     1. Graph of current speed and direction
     2. Stick diagrams show a time series of vectors for a single location.
However, while a time series provides high temporal resolution, several are required to cover a given area as speed and direction may vary greatly over short distances.

Tide current tables show times of slack water and time and speed of maximum flood and ebb currents at any time in the future. However, they are only available for limited locations and only depict the currents forced by astronomical tides.

Forecast Considerations

Whether we forecast open-ocean currents or coastal currents, we can look at data from observations, climatology, and numerical ocean models.

The first step in forecasting coastal currents is to understand how the forecast will be used. Most operations require a forecast or either instantaneous currents or long-term drift currents.

  • For instantaneous currents, tides typically dominate or contribute significantly to the total current, especially in estuaries and bays.
  • For long-term drift applications, winds and density usually are relatively more important than tidal currents.
    • Winds usually dominate or contribute significantly on the open shelf.
    • Density most often, but not always, plays a small role compared to tides and winds.

When forecasting ocean currents you will need to apply your knowledge of currents and their forcing mechanisms as they relate to the operations you are supporting.


8. References Cited

American Practical Navigator: An Epitome of Navigation, 2002
Published by the Department of Defense, National Imagery and Mapping Agency.
NIMA Pub 9. 2002 Bicentennial Edition. Originally by Nathaniel Bowditch.
(http://permanent.access.gpo.gov/lps1058/bookstore.gpo.gov/cdrom/cdrom109.html
Or available free online at http://www.irbs.com/bowditch/)

D’Asaro, E. A., C. C. Eriksen, M. D. Levine, P. Niiler, C. A. Paulson, and P. Van Meurs, 1995: Upper-ocean inertial currents forced by a strong storm. Part I: Data and comparisons with linear theory. J. Phys. Oceanogr., 25, 2909-2936.

Gordon, R.B. and Pilbeam, C.C., 1975: Circulation in central Long Island Sound. J. Geophys. Res., 80, 414-422.

Hazeleger, W., P. de Vries, and Y. Friocourt, 2003: Sources of the equatorial undercurrent in the Atlantic in a high-resolution ocean model. J. Phys. Oceanogr., 33, 677-693.

Lumpkin, R. and M. Pazos, 2004: Lifetime Statistics of Most Recent Drifter Deployments (2000-2003), Global Drifter Program / Drifter Data Assembly Center, NOAA/AOML, Miami, Florida DBCP-20,Chennai, India, October 18-22, 2004
(http://www.aoml.noaa.gov/phod/dac/DBCP20_mayra.pdf)

Okkonen, S. R., 2005. Observations of hydrography and currents in central Cook Inlet, Alaska during diurnal and semidiurnal tidal cycles. Outer Continental Shelf Study MMS 2004-058 (http://www.mms.gov/alaska/reports/2004Reports/2004_058.pdf).

Venayagamoorthy, S. K., 2007: Energetics and dynamics of internal waves on a shelf break using numerical simulations, PhD Dissertation, Stanford University.

Venayagamoorthy, S. K., and O. B. Fringer, 2007: On the formation and propagation of nonlinear internal boluses across a shelf break. J. Fluid Mech., 577, 137-159.

WOCE Hydrographic Atlas
(http://www-pord.ucsd.edu/whp_atlas/)

Related Websites

NOAA / Atlantic Oceanographic and Meteorological Laboratory (AOML)
(http://www.aoml.noaa.gov/phod/dac/dacdata.html)

NOAA / National Data Buoy Center / High Frequency Radar
(http://hfradar.ndbc.noaa.gov/)

NOAA / National Data Buoy Center /
Western Gulf of Mexico MMS ADCP Recent Marine Data
(http://www.ndbc.noaa.gov/maps/ADCP_WestGulf.shtml)

NOAA / NCEP / NCAR Reanalysis Project (CDAS)
(http://www.cpc.ncep.noaa.gov/products/wesley/reanalysis.html)

NOAA / Ocean Surface Current Analyses — Real time (OSCAR)
(http://www.oscar.noaa.gov/)

NOAA / Pacific Marine Environmental Laboratory /
Fisheries-Oceanography Coordinated Investigations /
SPEM ( Sigma-coordinate Primitive Equation Model ) Modeling of Shelikof Strait
(http://nctr-people.pmel.noaa.gov/spillane/shelikof.html)

NOAA / Tides and Currents
(http://tidesandcurrents.noaa.gov)

NRL / Intra-Americas Sea Ocean Nowcast/Forecast System
(http://www7320.nrlssc.navy.mil/IASNFS_WWW/)

NRL / Global Navy Layered Ocean Model (NLOM)
(http://www7320.nrlssc.navy.mil/global_nlom/)

NRL / Ocean Dynamics and Prediction Branch
(http://www7320.nrlssc.navy.mil/projects.php)

NRL / PCTides
(http://www7320.nrlssc.navy.mil/pctides/)

Sea Water Equation of State Calculator
(http://fermi.jhuapl.edu/denscalc.html)