Large-Scale Ocean Circulation
Water has the rare property of existing primarily as a liquid, but also as a gas and a solid within the
normal range of temperatures found on the earth's surface and in the atmosphere. Water in its liquid form
supports life on earth, and its movement is as critical to life as is its chemistry. The scales of water move-ment
in the ocean range from the microscopic dimensions (micrometers) of molecular diffusion to the
thousands of kilometers spanned by major current systems. Large-scale ocean circulation patterns have
an important effect on many different kinds of ocean life.
- Freshwater flowing into coastal waters from land carries with it sediments and nutrients which promote
high productivity in estuaries and nearshore waters, as well as pollutants that are unfavorable to the
growth and survival of many species. Some of these pollutants are also carried on air currents and are
introduced into the ocean from the atmosphere. The exchange of substances, both benign and harmful,
across the thin skin of the ocean surface, called the microlayer, is significant both to local microscopic
processes and to the global distribution of chemicals.
- Many toxic pollutants are persistent and mobile in aquatic environments. Consequently, they may be
moved far from their sources, across political boundaries as well as natural boundaries. For instance,
persistent pollutants, carried in both ocean and atmosphere currents, tend to accumulate in polar
regions, where the net flow ultimately carries them.
- Many marine animals (e.g., salmon and squid) have migration patterns that rely upon transport in major
ocean current systems, and other species rely on currents to distribute their larvae to new habitats.
- Populations of ocean species naturally fluctuate from year to year, and ocean currents often play a
significant role. The survival of plankton, for example, is affected by where the currents carry them;
and food supply varies as changing circulation patterns lead to higher or lower nutrient concentrations.
- The natural upwelling of ocean bottom waters in certain coastal locations carry to the surface nutrients
that have accumulated in the deep. This nutrient rich water supports high productivity and is often asso-ciated
with important fisheries, like the anchovetta fishery off the coast of Peru. This upwelling may
vary from year to year, which can result in significant fluctuations in productivity--and therefore in
fisheries yields--from year to year. El Nino is the best studied of the recurring variations in large-scale
circulation, and its disruptive effects on coastal weather and fisheries are well-known.
- Currents form boundaries that help define distinct habitats. Such boundaries may isolate different
genetic strains of the same species as well as different species.
- There is a global pattern of vertical ocean circulation, called the "conveyer belt," which circulates
throughout the entire expanse and depth of the world's ocean system. If there is a starting place that sets
this belt in motion, it is the Arctic formation of cold, dense, surface water that sinks to become the layer
of water moving along the ocean bottom. The primary source is off Greenland, where the surface water
becomes heavy (or denser) as it cools and as freshwater freezes out of the ocean making the liquid water
more saline (and therefore denser). The dense water feeds the circulation of cold water southward along
the bottom of the Atlantic, is augmented by sinking Antarctic water, moves around Africa into the
Indian Ocean and then around Australia into the Pacific, where it circulates in the basin and warms and
wells up toward the surface. From there, surface currents move in the opposite direction into the Atlantic
and the Arctic, where the water again cools and sinks.
- At the ocean surface, there are permanent current patterns, on the scale of thousands of kilometers,
driven by the dominant wind systems of the world. Converging from the north-east and south-east
winds blow westward around the equator; at mid-latitudes (30 - 60 N and S) they blow from the
west; and around the north and south poles winds are from the northwest and southwest respectively.
The combined influences of these dominant winds, the Coriolis force, and the continents, result in
large scale patterns of ocean circulation expressed as giant gyres: counterclockwise gyres in the subarctic
Pacific and Atlantic; clockwise gyres in the subtropical Pacific and Atlantic north of the equator;
counterclockwise current patterns in the subtropical Pacific and Atlantic south of the equator; and
clockwise currents in the Indian Ocean (south of the equator). The Antarctic Circumpolar current
runs uninterrupted toward the east.
- The currents of the gyres, which separate masses of water with very different physical and biological
characteristics, are always strongest along the western continental boundaries (or east coasts of the
continents)--for example, the Gulf Stream. Along the fronts of these strong currents, eddies and rings
encircle and isolate small sections of water (tens to hundreds of miles in diameter) from one side of
the current and may carry them, intact with living organisms, into the midst of the water mass on the
other side. Along the eastern continental boundaries (or west coasts) are weaker currents--such as the
California Current--which are associated with areas of coastal upwelling along their fronts.
- The defining currents of the gyres vary in intensity and precise location over time (on time scales of
seasons, years, decades, or greater) due to changes in the prevailing winds and sea surface temperatures.
One of the effects is the periodic loss of upwelling areas, such as occurs off the coast of Peru during
El Nino years.
- Climate change can significantly affect patterns of circulation. For instance, the conveyer belt could stop
or change due to warming Arctic waters. Similarly, global climate change can affect wind-sea surface
interactions, which may lead to changes in velocities, locations, and seasonality of major currents as well
as the location, magnitude, and timing of upwelling. The frequency and magnitude of ocean storms may
also be affected by climate change. The impacts on ocean life of changing circulation regimes could be
huge, including significant impacts on the species used by humans.
Mann, K.H. and Lazier, J.R.N. 1996. Dynamics of Marine Ecosystems: Biological-Physical Interactions in the Oceans,
2nd ed. Blackwell Science, Inc.: Cambridge, MA.
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