Ocean currents, upwelling and downwelling

The global ocean conveyor belt shown in Fig 11.19 simplifies the shallow and deep ocean circulations, and the vertical transports between them. We gained a better understanding of the atmospheric circulation by separating between near-surface and upper tropospheric winds (Chapter 12). Similarly, a more detailed insight in the ocean circulation arises by displaying shallow and deep currents separately. By dividing upper and lower levels by some arbitrary boundary, one can compare the mass transport, i.e. the flux of water in the vertical plane normal to the current, expressed in kg/s. One study (1) chose the 3.5° C isotherm (or, more correctly, potential temperature line) as boundary, and the resulting mean currents are shown in Fig 1 and Fig 2. The depth of this boundary varies with latitude, from about 2.2 km in the western equatorial Pacific to zero at about 65° N and 55° S. An isentropic surface was chosen as boundary because a current rarely crosses such surface. That is because water, as air, conserves its potential temperature and flows along isentropes in the absence of heating or boundary forcing (Section 7.2).

One prominent current in Fig 1 is the Antarctic circumpolar current. South of Australia and New Zealand, this current is convergent, producing downwelling. The Antarctic Convergence zone is a well-defined line where northwards cold surface water from Antarctica meets warmer waters from the oceans of lower latitude. In calm weather, a band of mist hangs over the boundary, where air moistened to the north is chilled by the cold water to the south.

Fig 1. Upper flows of the oceans, i.e. in the layer above the 3.5°C isentrope (1). The fat lines show currents whose mass transport is over 20 x 10⁹ kg/s, whilst the thin lines show those between 10 - 20 x 10⁹ kg/s. The letter U indicates the position of upwelling, and D is where there is downwelling.

Fig 2. As for Fig 1 but for flows of water colder than 3.5°C, i.e. water at deeper levels.

The flows through Indonesia are relatively small, which is reasonable in view of the shallowness of the seabed there. At all depths, the dominant flow is around the South Pole, with loops into the Indian Ocean and the southern Pacific. There are fairly separate circulations in the Atlantic and northern Pacific.

A substantial surface flow northwards in the Gulf Stream (which becomes the North Atlantic Drift) is matched by a southward deep flow, so there must be is considerable downwelling in the North Atlantic (as in Fig 11.19). Conversely in the Indian Ocean, there is upwelling from a northward spur of the circumpolar deep flow. Also, there is upwelling (of cold water) offshore from the subtropical west coast of the Americas and off the Namibian desert.

(1) Macdonald, A. and C. Wunsch 1996. An estimate of global ocean circulation and heat flows. Nature, 382, 436-9.