Large scale ocean circulation

The previous section briey described some of the physical phenomena which aect plankton systems at the small (individual organism) and medium (mixed layer) scale. The models used in this thesis are applied at the medium scale, but are often applied (e.g. Fashamet al., 1993 Sarmientoet al., 1993) at a larger scale (e.g. ocean basin). To put such related work in context, this short section outlines some of the physical processes in operation at such scales.

The earth's atmosphere receives energy from direct, reected and re{emitted irradiance. This energy manifests itself in the movement of air in response to its thermal state. Since the earth does not receive irradiance evenly over its surface (the poles receive considerably less irradiance per unit surface area than the equatorial region), the heating of the atmosphere is similarly uneven. Since warm air rises, one might expect that this skewed distribution of solar heating would be resolved simply by the move- ment of warmer, lighter equatorial air over cooler, denser polar air. However, the rotation of the earth

60 N 60 S 30 S 30 N 0 North pole South pole Westerlies Northeast trades Southeast trades

Intertropical convergence zone

Westerlies

Figure 1.1: Trade and westerly wind systems in the northern and southern hemispheres of the earth. The jagged appearance of the westerlies represents their meanderings which lead to the formation of Rossby waves (see text). After Mann & Lazier (1991).

complicates the picture, leading to the formation of two major wind systems : the trade winds and the westerlies (see gure 1.1).

Ekman Transport Trade winds Winds Westerlies 40 N 30 N Latitude 20 N Sverdrup transport Pycnocline 100 m Ekman pumping

Figure 1.2: Diagrammatic representation of the processes of Ekman and Sverdrup transport (in the northern hemisphere). Ekman transport of surface water (<100 m) north and south towards 30 is caused by the action of the trade and westerly winds (out of and into the page respectively) on the surface of the ocean. This water movement causes water to be \pumped" downwards where Ekman transport converges. This in turn leads to the process of Sverdrup transport of deeper water (up to 1000 m) towards the equator (see text for more details). After Mann & Lazier (1991).

The trade winds arise in the manner already suggested, namely warmer, equatorial air rises up and moves polewards, and is replaced by cooler, denser air from the subtropics. Because of the Coriolis eect7, the ows of air from the subtropics to the equator are deected to the west, generating the northeast and southeast trade winds (these are named after the direction from which they come). The rising equatorial air cools as it moves towards the poles and descends in the subtropics (30

N/S). This circular pattern of air ow, where equatorial air is replaced by subtropical air, which it then replaces itself, creates convection cells which are known as Hadley cells. Since these cells essentially retain the thermal energy supplied to the tropics within the tropics and subtropics, this creates a thermal (and thus pressure) gradient between latitude 30 and the poles. This gradient gives rise to strong westerly 7The Coriolis eect is a consequence of the earth's rotation on its axis, and aects the movement of bodies of air and

water over the earth's surface. In the context of the oceans, it is responsible for why, relative to the surface of the earth, a moving body of water veers to the right in the northern hemisphere, and to the left in the southern hemisphere.

winds which are perpendicular the pressure gradient, and ow continuously around the earth. These winds vary in strength with altitude (the jet{stream marks their maximum) and also meander north and south. This meandering produces waves (around 10000 km long) known as Rossby waves. These have periods of around a month and are responsible for shifting weather patterns in the mid{latitudes.

0 30 W 60 W 90 W 30 E Brazil Benguela Polar Peru N. Equatorial Guinea N. Atlantic Norwegian S. Equatorial Weddell Canary Azores Antilles Circumpolar Antarctic Polar 90 W 60 W 30 W 0 30 E 30 N 60 N 60 S 30 S 0 60 S 30 S 0 30 N 60 N Stream Gulf Labrador

Figure 1.3: The major surface currents of the northern and southern Atlantic ocean. Western boundary currents (the Gulf Stream and the Brazil current) are emphasised. Note the circular pattern of ows in the northern and southern regions. After Mann & Lazier (1991). The trade winds and the westerlies play an important role in the generation of surface and deeper layer ows within the ocean. Moving in opposite directions at lower and higher latitudes respectively, they generate stress on the ocean's surface which, in combinationwith the Coriolis force, leads to the transport of water in the upper layer of the ocean. The interaction between the wind stress and the Coriolis force acts to produce water movement to the right of the wind. Consequently, surface water on the poleward side of the subtropics (>30

) moves towards the equator, while water on the equatorial side of the sub- tropics (<30

these movements are opposite to one another, the moving water has to go somewhere, leading to a region of downwelling water at the subtropics (this downwelling is known as Ekman pumping). The ocean layer beneath the surface wind{driven layer compensates for the downward Ekman pumping with a horizontal ow towards the equator (see gure 1.2). Although the surface ows are in opposite directions and head towards the subtropics, this deeper ow, known as Sverdrup transport, is equatorial. The net eect of this wind{driven circulation is an equatorial pattern of water circulation. The Californian, Canary, Peruvian, and Benguelan currents are examples of this circulation pattern. However, these currents only complete half the full circulation required. The patterns are completed by ow parallel to the equator and by relatively stronger ow at the westward boundaries of the ocean basins (the Gulf stream and the Brazil current are these western ows in the northern and southern Atlantic ocean respectively). These currents close the circular patterns of surface ow in the main ocean basins (see gure 1.38). These patterns are known asgyres, and play signicant roles in dissipating inertia and thermal energy through the oceans. In the case of the Gulf Stream, some of this energy is dissipated by occasional meanderings which break o into separate rings (the Ring Group, 1981, followed the evolution of ring \Bob" across the course of its seven month life).

Again, the processes outlined here are comprehensively discussed by Mann & Lazier (1991) and Henderson{ Sellers & Robinson (1986).