8.16: Chapter Summary

Energy Sources. 

Winds are the energy source for most currents in the ocean’s upper few hundred meters. Deeper currents and most vertical movements of ocean water are caused by thermohaline circulation, which is driven by differences in the density of water masses due to temperature and salinity differences.

Wind-Driven Currents. 

Winds blowing across the sea surface transfer kinetic energy to the water through friction and cause wind-driven currents with speeds generally about 1% to 3% of the wind speed. Currents continue after winds abate until their momentum is eventually lost because of internal friction. Currents also continue because winds transport water, cause the sea surface to slope, and create horizontal pressure gradients that drive water motion. Current speed and direction are modified by friction, the Coriolis effect, horizontal pressure gradients, and land and seafloor topography.

Ekman Motion. 

Surface water set in motion by winds is deflected cum sole relative to the wind direction by the Coriolis effect. Wind energy is transferred down into the water column as each water layer transfers its energy to the layer below. If winds blow consistently for long enough over deep water, an Ekman spiral develops in which the surface current is deflected 45° cum sole, current speed decreases with depth, and current direction is progressively deflected cum sole with depth. Mean water transport (Ekman transport) in the layer within which the Ekman spiral develops is at 90° cum sole to the wind.

Wind-driven currents rarely reach deeper than 100 to 200 m. Where the seafloor is shallower than the Ekman spiral depth, friction reduces the surface water deflection below 45° and the mean transport below 90°. If a pycnocline is present within the Ekman spiral depth, downward transfer of wind energy is inhibited, and wind-driven currents do not occur below the pycnocline.

Geostrophic Currents. 

Ekman transport causes surface water to “pile up” in some areas. The resulting sloping sea surface results in a horizontal pressure gradient within the upper several hundred meters. Water flows on such gradients and is deflected by the Coriolis effect. The deflection continues until the flow is along the gradient (parallel to the pressure contours) at a speed where the Coriolis deflection is just matched by the pressure gradient force. This is geostrophic flow.

Ocean Surface Currents. 

Trade winds drive surface waters generally west in the Equatorial Currents. Westerly winds drive surface waters generally east in the subtropics. Continents deflect the flow north or south. Thus, current gyres are created between trade wind and westerly wind latitudes in each hemisphere and ocean. These gyres flow clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Ekman transport by trade winds and westerlies piles up water in the gyre center, creating horizontal pressure gradients that maintain geostrophic gyre currents even when winds cease. Western boundary currents are faster, deeper, and narrower than eastern boundary currents.

In the high latitudes of the North Atlantic and Pacific Oceans, counterclockwise subpolar gyres are complex, weak, and variable. At high southern latitudes, no continents block the Antarctic Circumpolar Current that flows westward around Antarctica.

Upwelling and Downwelling. 

Upwelling occurs where Ekman transport moves surface waters away from a divergence or coast. Downwelling occurs where Ekman transport adds to the surface layer depth at convergences or coasts. Upwelling areas are usually highly productive because upwelling can bring cold, nutrient-rich water from below into the mixed layer, where nutrients are essential for phytoplankton growth. Divergences and upwelling occur in part of the area between the North and South Pacific Equatorial Currents, around Antarctica, and in many coastal locations. Convergences occur in the centers of ocean gyres and in many coastal regions. Coastal upwelling is more prevalent inshore of eastern boundary currents than inshore of western boundary currents.

Coastal Currents. 

Local winds determine coastal current directions and they may be opposite those of the adjacent gyre currents. Coastal currents are variable and are affected by freshwater inflow, friction with the seafloor, steering by seafloor topography and coastline, and tidal currents.

Eddies. 

Ocean eddies are similar to, but smaller than, the atmospheric eddies seen on satellite images as swirls of clouds. Satellites can be used to observe some ocean eddies. Gulf Stream rings are mesoscale eddies 100 to 300 km wide that form cold-core (counterclockwise-rotating) or warm-core (clockwise-rotating) rings when a meander of the Gulf Stream breaks off. The rings, which may last several months, may reattach to the Gulf Stream as they drift slowly south. Mesoscale eddies are present throughout the oceans, generally have slower currents than Gulf Stream rings, may extend to the deep-sea floor, and are 25 to 200 km wide.

Inertial Currents. 

Once established, wind-driven currents that continue to flow and are deflected into circular paths by the Coriolis effect are called “inertial currents.”

Langmuir Circulation. 

Strong winds can set up a corkscrew-like Langmuir circulation aligned in the wind direction. Langmuir cells are a few meters deep and about 30 m wide, and they lie side by side. Foam or floating debris collects in the linear downwelling regions between cells.

Thermohaline Circulation. 

When surface water density is increased by cooling or evaporation, the water sinks and spreads horizontally at a depth where the water above has a higher density and the water below has a lower density. Throughout most of the oceans, the water column consists of a uniform-density mixed surface layer (about 100 m deep), a permanent pycnocline zone (extending from the bottom of the mixed layer to a depth of 500 to 1000 m) in which density increases progressively with depth, and a deep zone in which density increases slowly with depth. Most high-latitude regions where surface waters are cooled lack a pycnocline. Secondary, temporary pycnoclines can develop in the mixed layer in summer. Pycnoclines act as a barrier to vertical mixing.

Deep water masses are formed by cooling and ice exclusion at high southern latitudes, in the Arctic Ocean, and at high latitudes in the North Atlantic Ocean. Antarctic Bottom Water is formed primarily in the Weddell Sea and flows north as the deepest and densest water layer. North Atlantic Deep Water is formed in the Greenland and Norwegian Seas and flows south. Water masses at intermediate depths are formed by the sinking of cooled water at the Antarctic Convergence and of warm, high-salinity water produced by evaporation at subtropical convergences and in the Mediterranean Sea. Deep and intermediate-depth water masses are subject to the Coriolis effect, and thus, currents in the deep layer tend to be intensified along western boundaries and tend to flow in gyres. They are also affected by seafloor topography, which generates eddies that enhance vertical mixing.

Ocean Circulation and Climate. 

Ocean circulation transfers heat from the tropics toward the poles, moderating mid- and high-latitude climates. North Atlantic Deep Water forms near Greenland and Iceland. It sinks and spreads south in the deep Atlantic and then north into the Pacific and Indian Oceans. It mixes progressively upward to the surface layer, which is warmed and eventually transferred back to the North Atlantic by the Gulf Stream.

This Meridional Overturning Circulation (MOC) has varied in intensity and switched off and on in the past. Periods when it has not operated seem to be associated with colder climates, especially in Europe. The change to a colder climate appears to take place abruptly when the MOC is switched off.

Tracing Water Masses. 

Water masses are characterized by their salinity and temperature, which are conservative properties everywhere but at the surface and in some areas of the seafloor. Nonconservative properties such as oxygen concentration, as well as concentrations of human-made radionuclides and persistent organic compounds, are also used as water-mass tracers.