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Boundary currents are
ocean current An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, s ...
s with dynamics determined by the presence of a
coast The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean, or as a line that forms the boundary between the land and the coastline. The Earth has around of coastline. Coasts are important zones in n ...
line, and fall into two distinct categories: western boundary currents and eastern boundary currents.


Eastern boundary currents

Eastern boundary currents are relatively shallow, broad and slow-flowing. They are found on the eastern side of
oceanic basin In hydrology, an oceanic basin (or ocean basin) is anywhere on Earth that is covered by seawater. Geologically, ocean basins are large  geologic basins that are below sea level. Most commonly the ocean is divided into basins fol ...
s (adjacent to the western coasts of continents). Subtropical eastern boundary currents flow equatorward, transporting cold water from higher latitudes to lower latitudes; examples include the Benguela Current, the
Canary Current The Canary Current is a wind-driven surface current that is part of the North Atlantic Gyre. This eastern boundary current branches south from the North Atlantic Current and flows southwest about as far as Senegal where it turns west and later jo ...
, the Humboldt (Peru) Current, and the
California Current The California Current is a cold water Pacific Ocean current that moves southward along the western coast of North America, beginning off southern British Columbia and ending off southern Baja California Sur. It is considered an Eastern bound ...
.
Coastal upwelling Upwelling is an physical oceanography, oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted ...
often brings nutrient-rich water into eastern boundary current regions, making them productive areas of the ocean.


Western boundary currents

Western boundary currents may themselves be divided into sub-tropical or low-latitude western boundary currents. Sub-tropical western boundary currents are warm, deep, narrow, and fast-flowing currents that form on the west side of ocean basins due to ''western intensification''. They carry warm water from the tropics poleward. Examples include the
Gulf Stream The Gulf Stream, together with its northern extension the North Atlantic Current, North Atlantic Drift, is a warm and swift Atlantic Ocean, Atlantic ocean current that originates in the Gulf of Mexico and flows through the Straits of Florida a ...
, the
Agulhas Current The Agulhas Current () is the western boundary current of the southwest Indian Ocean. It flows south along the east coast of Africa from 27°S to 40°S. It is narrow, swift and strong. It is suggested that it is the largest western boundary curren ...
, and the
Kuroshio Current The , also known as the Black or or the is a north-flowing, warm ocean current on the west side of the North Pacific Ocean basin. It was named for the deep blue appearance of its waters. Similar to the Gulf Stream in the North Atlantic, the Ku ...
. Low-latitude western boundary currents are similar to sub-tropical western boundary currents but carry cool water from the subtropics equatorward. Examples include the
Mindanao Current The Mindanao Current (MC) is a southward current in the western Pacific Ocean that transports mass and freshwater between ocean basins. It is a low-latitude western boundary current that follows the eastern coast of the Philippine island group a ...
and the
North Brazil Current The North Brazil Current (NBC) is a warm water ocean current that is part of the southwestern North Atlantic Gyre. It begins when the westward moving Atlantic South Equatorial Current splits in half and flows northwestward, following the coastline ...
.


Western intensification

Western intensification applies to the western arm of an
oceanic current An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, ...
, particularly a large gyre in such a basin. The
trade winds The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisph ...
blow westward in the tropics. The
westerlies The westerlies, anti-trades, or prevailing westerlies, are prevailing winds from the west toward the east in the middle latitudes between 30 and 60 degrees latitude. They originate from the high-pressure areas in the horse latitudes and trend to ...
blow eastward at mid-latitudes. This applies a
stress Stress may refer to: Science and medicine * Stress (biology), an organism's response to a stressor such as an environmental condition * Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...
to the ocean surface with a
curl cURL (pronounced like "curl", UK: , US: ) is a computer software project providing a library (libcurl) and command-line tool (curl) for transferring data using various network protocols. The name stands for "Client URL". History cURL was fi ...
in north and south hemispheres, causing
Sverdrup transport The Sverdrup balance, or Sverdrup relation, is a theoretical relationship between the wind stress exerted on the surface of the open ocean and the vertically integrated meridional (north-south) transport of ocean water. History Aside from the ...
equatorward (toward the tropics). Because of conservation of mass and of
potential vorticity In fluid mechanics, potential vorticity (PV) is a quantity which is proportional to the dot product of vorticity and stratification. This quantity, following a parcel of air or water, can only be changed by diabatic or frictional processes. It i ...
, that transport is balanced by a narrow, intense poleward current, which flows along the western coast, allowing the
vorticity In continuum mechanics, vorticity is a pseudovector field that describes the local spinning motion of a continuum near some point (the tendency of something to rotate), as would be seen by an observer located at that point and traveling along wit ...
introduced by coastal friction to balance the vorticity input of the wind. The reverse effect applies to the polar gyres – the sign of the
wind stress In physical oceanography and fluid dynamics, the wind stress is the shear stress exerted by the wind on the surface of large bodies of water – such as oceans, seas, estuaries and lakes. Stress is the quantity that describes the magnitude of a for ...
curl and the direction of the resulting currents are reversed. The principal west-side currents (such as the
Gulf Stream The Gulf Stream, together with its northern extension the North Atlantic Current, North Atlantic Drift, is a warm and swift Atlantic Ocean, Atlantic ocean current that originates in the Gulf of Mexico and flows through the Straits of Florida a ...
of the North
Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...
) are stronger than those opposite (such as the
California Current The California Current is a cold water Pacific Ocean current that moves southward along the western coast of North America, beginning off southern British Columbia and ending off southern Baja California Sur. It is considered an Eastern bound ...
of the North
Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...
). The mechanics were made clear by the American oceanographer
Henry Stommel Henry Melson Stommel (September 27, 1920 – January 17, 1992) was a major contributor to the field of physical oceanography. Beginning in the 1940s, he advanced theories about global ocean circulation patterns and the behavior of the Gulf Stream ...
. In 1948, Stommel published his key paper in ''Transactions, American Geophysical Union'': "The Westward Intensification of Wind-Driven Ocean Currents", in which he used a simple, homogeneous, rectangular ocean model to examine the streamlines and surface height contours for an ocean at a non-rotating frame, an ocean characterized by a constant Coriolis parameter and finally, a real-case ocean basin with a latitudinally-varying Coriolis parameter. In this simple modeling the principal factors that were accounted for influencing the oceanic circulation were: *surface wind stress *bottom friction *a variable surface height leading to horizontal pressure gradients *the
Coriolis effect In physics, the Coriolis force is an inertial or fictitious force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the ...
. In this, Stommel assumed an ocean of constant density and depth D+h seeing ocean currents; he also introduced a linearized, frictional term to account for the dissipative effects that prevent the real ocean from accelerating. He starts, thus, from the steady-state momentum and continuity equations: f(D+h)v-F\cos \left(\frac\right)-Ru-g(D+h)\frac=0\qquad (1) \quad -f(D+h)u-Rv-g(D+h)\frac=0 \qquad \qquad (2) \qquad \qquad \frac+\frac=0 \qquad \qquad \qquad (3) Here f is the strength of the Coriolis force, R is the bottom-friction coefficient, g\,\, is gravity, and -F\cos \left(\frac\right) is the wind forcing. The wind is blowing towards the west at y=0 and towards the east at y=b. Acting on (1) with \frac and on (2) with \frac, subtracting, and then using (3), gives : v(D+h)\left(\frac\right)+\frac\sin \left(\frac\right)+R\left(\frac-\frac\right)=0 \quad (4) If we introduce a
Stream function The stream function is defined for incompressible flow, incompressible (divergence-free) fluid flow, flows in two dimensions – as well as in three dimensions with axisymmetry. The flow velocity components can be expressed as the derivatives of t ...
\psi and linearize by assuming that D>>h, equation (4) reduces to \nabla^2 \psi +\alpha\left(\frac\right)=\gamma \sin \left(\frac\right)\qquad (5) Here \alpha=\left(\frac\right)\left (\frac\right) and \gamma=\frac The solutions of (5) with boundary condition that \psi be constant on the coastlines, and for different values of \alpha, emphasize the role of the variation of the Coriolis parameter with latitude in inciting the strengthening of western boundary currents. Such currents are observed to be much faster, deeper, narrower and warmer than their eastern counterparts. For a non-rotating state (zero Coriolis parameter) and where that is a constant, ocean circulation has no preference toward intensification/acceleration near the western boundary. The streamlines exhibit a symmetric behavior in all directions, with the height contours demonstrating a nearly parallel relation to the streamlines, in a homogeneously rotating ocean. Finally, on a rotating sphere - the case where the
Coriolis force In physics, the Coriolis force is an inertial or fictitious force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the ...
is latitudinally variant, a distinct tendency for ''asymmetrical'' streamlines is found, with an intense clustering along the western coasts. Mathematically elegant figures within models of the distribution of streamlines and height contours in such an ocean if currents uniformly rotate can be found in the paper.


Sverdrup balance and physics of western intensification

The physics of western intensification can be understood through a mechanism that helps maintain the vortex balance along an ocean gyre. Harald Sverdrup was the first one, preceding Henry Stommel, to attempt to explain the mid-ocean vorticity balance by looking at the relationship between surface wind forcings and the mass transport within the upper ocean layer. He assumed a geostrophic interior flow, while neglecting any frictional or viscosity effects and presuming that the circulation vanishes at some depth in the ocean. This prohibited the application of his theory to the western boundary currents, since some form of dissipative effect (bottom Ekman layer) would be later shown to be necessary to predict a closed circulation for an entire ocean basin and to counteract the wind-driven flow. Sverdrup introduced a potential vorticity argument to connect the net, interior flow of the oceans to the surface wind stress and the incited planetary vorticity perturbations. For instance, Ekman convergence in the sub-tropics (related to the existence of the trade winds in the tropics and the westerlies in the mid-latitudes) was suggested to lead to a downward vertical velocity and therefore, a squashing of the water columns, which subsequently forces the ocean gyre to spin more slowly (via angular momentum conservation). This is accomplished via a decrease in planetary vorticity (since relative vorticity variations are not significant in large ocean circulations), a phenomenon attainable through an equatorially directed, interior flow that characterizes the subtropical gyre. The opposite is applicable when Ekman divergence is induced, leading to Ekman absorption (suction) and a subsequent, water column stretching and poleward return flow, a characteristic of sub-polar gyres. This return flow, as shown by Stommel, occurs in a meridional current, concentrated near the western boundary of an ocean basin. To balance the vorticity source induced by the wind stress forcing, Stommel introduced a linear frictional term in the Sverdrup equation, functioning as the vorticity sink. This bottom ocean, frictional drag on the horizontal flow allowed Stommel to theoretically predict a closed, basin-wide circulation, while demonstrating the west-ward intensification of wind-driven gyres and its attribution to the Coriolis variation with latitude (beta effect).
Walter Munk Walter Heinrich Munk (October 19, 1917 – February 8, 2019) was an American physical oceanographer. He was one of the first scientists to bring statistical methods to the analysis of oceanographic data. His work won awards including the Nation ...
(1950) further implemented Stommel's theory of western intensification by using a more realistic frictional term, while emphasizing "the lateral dissipation of eddy energy". In this way, not only did he reproduce Stommel's results, recreating thus the circulation of a western boundary current of an ocean gyre resembling the Gulf stream, but he also showed that sub-polar gyres should develop northward of the subtropical ones, spinning in the opposite direction.


Climate change

Observations indicate that the ocean warming over the subtropical western boundary currents is two-to-three times stronger than the global mean surface ocean warming. A study finds that the enhanced warming may be attributed to an intensification and poleward shift of the western boundary currents as a side-effect of the widening Hadley circulation under global warming. These warming hotspots cause severe environmental and economic problems, such as the rapid sea level rise along the East Coast of the United States, collapse of the fishery over the Gulf of Maine and Uruguay.


See also

* * *


References


AMS glossary
*Professor Raphael Kudela, UCSC, lectures OCEA1 Fall 2007 *Munk, W.H., ''On the wind-driven ocean circulation'', J. Meteorol., Vol. 7, 1950 * * *Stommel, H., "The Westward Intensification of Wind-Driven Ocean Currents", ''Transactions American Geophysical Union'', vol. 29, 1948 * *Thurman, Harold V., Trujillo, Alan P., ''Introductory Oceanography'', tenth edition.


Footnotes


External links

*
biophysics.sbg.ac.at
(JPG)
learner.orgaos.princeton.edu
(PDF) {{Physical oceanography Physical oceanography Ocean currents