Ocean water is constantly moving around Earth as a result of two
circulation systems, the surface system (the upper 400 meters of
an ocean) and the deep-water system (deeper than 400 meters). The
surface system is primarily wind driven and involves only 10% of
total ocean volume. The deep-water system is mostly density driven
and involves 90% of total ocean volume.
Four forces influence ocean water movement within these circulation
- Solar heating causes water to expand,
actually raising sea surface height in areas around the equator.
- Winds push water up in a “mound”
in the direction the wind is blowing.
- Gravity pulls water down the “mound”
of water against the pressure gradient.
- Coriolis effect: The rotation of the Earth causes a clockwise
shift in the direction of wind and ocean current flow in the Northern
Hemisphere and a counterclockwise shift in the Southern Hemisphere
known as the “Coriolis effect”. Similarly, the direction
of ocean current flow rotates in a clockwise or counterclockwise
direction with increasing depth from the ocean surface, depending
on the location in the Northern or Southern Hemisphere. Over large
distances, these surface currents form ocean “gyres”
that rotate around large areas of relatively stationary water.
Because of the Coriolis effect, winds blowing from the north along
the west coast of Northern Hemisphere continents tend to drive surface
ocean currents to the right of the wind direction, namely offshore
toward the west. Colder, nutrient-rich deeper water is then drawn
up from beneath to replace the water that was pushed offshore, resulting
in an “upwelling of cold” zone. Upwelling zones occur
on the west coasts of Southern Hemisphere continents where there
are strong northerly winds, and also occur in some equatorial zones
of where winds and ocean currents diverge. Because of the rich supply
of nutrients upwelling zones support abundant phytoplankton and
zooplankton on which larger animals feed.
Density-driven circulation is called thermohaline circulation. Cold,
dense water from the polar regions sinks and flows along the ocean
floor toward the equator where it slowly rises to the surface, circulating
in a great convection system. Circulation in deep ocean water takes
a long time, because there is less energy there than in surface
systems. Denser water masses sink below lighter ones and spread
laterally when they meet other water masses at what are called convergence
As these water masses mix, their salinity, temperature, and oxygenation
gradually change. In fact, large water masses are identified by
these physical chemical characteristics. A deep-water mass called
the Mediterranean Outflow Water is a result of the high evaporation
rate in the Mediterranean, which increases the salinity of the water
mass. As this water leaves the Mediterranean basin, it spreads out
and sinks into the Atlantic Ocean until it meets the Atlantic Deep
Water mass, which is much cooler and denser.
Models of the continuing cycle of ocean circulation depict
trends in currents and circulation that can be used to forecast
climate, sea levels, and oceanic-atmospheric interactions. One of
these types of atmospheric interactions is El Niño, a disruption
of the ocean-atmosphere system in the tropical Pacific that has
widespread consequences for weather around the planet.
Under normal weather conditions, trade winds blow west across the
tropical Pacific and pile up warm surface water so that the sea
surface is about one-half meter higher at Indonesia than at Ecuador.
The normal upwelling of cold water from deeper sea levels keeps
the temperatures off the west coast of South America cool. During
El Niño, however, the trade winds are weaker in the central
and western Pacific, and ocean waters in the eastern Pacific become
cooler. Upwelling of cold ocean water off the coast of South America
becomes less efficient, and the surface temperature of the ocean
there rises. Rainfall follows the warmer water eastward, so that
rainfall increases, sometimes catastrophically, across the southern
United States and Peru, while droughts occur in the West Pacific.
The ability to predict these trends will enable us to better prepare
for changes in our climate and weather.