J F M A M J Jasond

diminishes with latitude and not even the combination of the tilt of the Earth's axis and its annual excursion round the sun even this out. The plots in Fig. 2.2b show the annual variation in the undepleted daily flux at each of the selected northern-hemisphere latitudes.

Although the highest potential daily heat flux is everywhere quite similar, sustained heating through the year is always likely to be greatest in the tropics but never for such long diurnal periods as occur at high latitudes in summer. On a rotating but homogeneously water-covered Earth having a continuously clear atmosphere, there would be considerable latitudinal differences in the heat flux directed to the surface

1 (a) The spectrum of the solar flux at ground level compared to that of the 'solar constant' at the top of the atmosphere; the visible wavelengths (light) are shown hatched. (b) Daily integrals of undepleted solar radiation at the top of the atmosphere, shown as a function of latitude (degrees) and time of year in the northern hemisphere. The approximate match for the southern hemisphere is gained by displacing the horizontal scale by 6 months. Redrawn, with permission, from Reynolds (1997a).

(other things being equal, sufficient at the equator to raise the temperature of the top metre by ~1 °C every daylight hour, for 12 months of the year). Ignoring night-time losses and any convec-tional heat penetration, the expectation is that the now less dense surface water, heated in the tropics, would spread out to the higher latitudes, at least until it had cooled to the temperature of the high-latitude water. In compensation, water must be drawn from the higher latitudes, via a deeper return flow. In this way, we may visualise the initiation of a convectional circulation of hemispheric proportions.

This simplified conception is complicated by several interacting factors. The rotation of the Earth causes everything on it, including oceanic drift currents, to move eastwards. As surface water moves poleward, however, the rotational speed of the ground under it lessens and the inertia of the trajectory tends to pull it ahead of the solid surface, the relative motion thus drifting further east. This easterly deflection, known as Coriolis' effect, acts like a laterally applied force. The positions of the continental land masses, of course, obstruct the free development of these motions while the irregularity of their distribution gives rise to compensatory latitudinal flows among the major oceans (especially in the southern hemisphere). The variable depth of the ocean floor also interferes with the passage of deep return currents which, locally, may be forced to deflect upwards and to 'short-circuit' the potential hemispheric circulation.

Also superimposed upon the circulatory pattern are the tidal cycles exerted by the variable gravitational pull on the water exerted by the rotation of the Moon around the Earth, having frequencies of ~25 hours and ~28 days. The effect of tides on the pattern of circulation may not be large in the open ocean but may dominate inshore circulations near blocking land-forms that may trap tidal surges (the Bay of Fundy, between Nova Scotia and New Brunswick, experiences the greatest tidal extremes in the world - over 13 m - and some of the most aggressive tidal mixing).

Surface currents, especially in lakes, are prox-imally influenced by wind. Wind is the motion of air in the adjacent fluid environment, the atmosphere; its movements are subject to analogous global-scale forces. It is its much lower mass, density and viscosity that gives the impression of a different behaviour. In fact, there is close coupling between them, in the sense that strong winds generate waves, drive surface drift currents and force the transfer of some of mechanical energy to the water column. At the same time, differences in inertia and in specific heat bring differential rates of warming over land and water, leading to differences in air pressure and the superimposition of prevalent wind conditions.

The predictability of wind action on individual water bodies is generally difficult (as are most aspects of weather forecasting), save in probabilistic terms, based on the statistics of experience and pattern recognition. However, the linkage between wind effects and the motion of water in which phytoplankton is resident has been deeply explored. Broad flow patterns of surface currents in the oceans have been discerned and described by mariners over a period of centuries and since committed to oceanographers' maps (for an overview, see Fig. 2.3). Patterns of circulation in certain large lakes have been described over a rather shorter period of time (e.g. Mortimer, 1974; Csanady, 1978) and those of many smaller lakes have been added in recent years; the example in Fig. 2.4 is just one such instance.

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