Physical Oceanography: An Overview
2.7 The General Circulation of the Ocean
32 Principles of Ocean Physics
cy differences as well. It is these interchanges that bring about the turbu- lence in the planetary boundary layer; many of the properties of the layer depend on the sign of the air-sea temperature difference, To - T,, which, when negative (colder air over warmer water), leads to strong interchange, considerable turbulence, and roughened seas.
By definition, the currents induced by the wind stress make up the wind- driven circulation of the ocean. While the direct effects of the stress are felt close to the ocean surface (mainly within the so-called oceanic Ekman layer extending over the upper 10 to 100 m), it is the deeper components of circu- lation brought about by the surface motions that bring into play a signifi- cant volume of the waters of the sea. Furthermore, it is not only the stress itself, but also its curl, divergence, and gradient that drive the circulation;
the division of the wind effects into various portions is based partly on mathe- matical convenience and partly on physical response; this will be discussed in more detail in Chapters 3 and 6.
while their lengths indicate persistency. [From Dietrich, G., General Oceanography (1963); originally due to G. Schott (1943).]
Fig. 2.17 The general surface circulation of the ocean in schematic form. The dominant features are the large, anticyclonic sub- tropical gyres in each ocean basin, the equatorial current systems, and the Antarctic Circumpolar Current.
901 'L' ' ' ' ' ' " 1 ' ' " " ' ' 1 ' " 1 ' ' ' 1 " ' ' 1 ' ' ' c
-_I , , , , , , , , I I .
0 30 GO 90 120 150 180 150 120 90 GO 30 0
East West
Longitude, @ (deg)
Fig. 2.18 The general surface temperature of the ocean, averaged from nearly 1.6 million observations. The warmest major ocean area is near the Far Eastern Island Continent. [From Levitus, S., Climatological Atlas of the Wodd Ocean (1982).]
North Atlantic Current as it crosses that ocean in an easterly direction at latitudes of 40" to 50"N. Nearing Europe, it divides into a stronger Canary Current directed toward the south, and the Norwegian Current headed to- ward the north. The southern leg merges with the North Equatorial Current off Africa to return across the Atlantic near 20" to 30"N and flow on through the Caribbean to complete the circuit. This large, clockwise gyre carries a total flow of the order of lo8 m3 s - ' in its most intense regions, with the Gulf Stream portion transporting perhaps 60 x lo6 m3 s ~
' .
(The name giv- en to the unit representing a volume transport of lo6 m 3 s-I is the sverdrup, abbreviated Sv, after the Norwegian oceanographer.) The average surface flow speed is 1 to 2 m s - I , although in its most intense portions the Flori- da Current may reach speeds of 2.5 t o 3.0 m s - ' . This gyre constitutes a very large oceanic high pressure system whose surface elevation extends per- haps a meter or more above the equipotential surface and whose western interior constitutes the Sargasso Sea, a. region also known as the Atlantic Subtropical Convergence.Every ocean except the Arctic Ocean has such a basinwide subtropical gyre, although there are important modifications brought about by geography and winds in the Indian Ocean and in the Antarctic Circumpolar current. It is apparent from the latitudes of flow cited above that the eastward ocean cur- rents occur where the winds are westerlies, t o use the meteorological con- vention (i.e., from the west and directed toward the east), while the regions of flow toward the west are in the easterly trade wind zones. In the Northern
36 Principles of Ocean Physics
Hemisphere, the gyres are clockwise; in the Southern, counterclockwise. In either event, they are termed anticyclonic, reflecting their high pressure character. It is the combined forces of surface wind stress and Coriolis ac- celeration coupled with continental boundaries that establish the anticyclonic flow patterns.
The Antarctic Circumpolar Current lies between 40" and 60"s in the zone of Southern Hemisphere westerlies and in a region of ocean uninterrupted by land masses for the entire circuit around the globe. This current, whose surface speeds are of order 0.5 to 1.5 m s - ' and whose volume transport is of order 200 x lo6 m3 s - ' , is the oceanic analog of the jet streams in the atmosphere.
The Indian Ocean, which in some ways is only half an ocean, has a cur- rent system whose northern part is dominated by the variable monsoon winds.
During the Northern Hemisphere summer, the southwest wet monsoon drives surface currents north and east between Africa and India. In the winter, when the dry, northeast monsoon occurs, both wind and current reverse direction to flow south and west in the northern part of the basin. The South Indian Ocean, which is probably the least-well understood major basin, also pos- sesses a subtropical gyre.
In the equatorial zone, separating the oppositely rotating major gyres is a somewhat complicated system of currents flowing mainly in the zonal direc- tion. The westward-directed North and South Equatorial Currents are the equatorward arms of the gyres, but these are separated by an Equatorial Countercurrent and a near-surface Equatorial Undercurrent, both flowing east.
Branching from and merging with these components of the general circu- lation are numbers of secondary flows known by a variety of geographically oriented names. The major secondary systems in the north are, for example, the Norway, Greenland, and Labrador Currents in the Atlantic, and the Alas- ka and Oyashio Currents in the Pacific. These, plus other similar currents, comprise the secondary oceanic surface and near-surface circulation features.
From their latitudinal positions, it is clear that the transoceanic arms of the gyres are closely linked to the surface wind stress (in fact, to the curl of the stress, as will be shown in Chapter 6), with Coriolis effects playing a role. We will now attempt to describe how the wind stress couples into the water and how it responds over very large areas in a way that makes up the surface circulation patterns shown in Fig. 2.16. We will see that the sub- surface circulation is also strongly conditioned by the surface flows, but that cooling and sinking of water in the polar regions (i.e., the thermohaline cir- culation) also contribute to the subsurface flow in important ways.