J

F'M'A'M'J"J1 A 1 S 1 O 1 N 1 D 1 Month

Fig. 2.4. Monthly mean zonally-averaged pressure difference (in hPa) between 40°S and 60°S. Based on daily numerical analyses by the Australian Bureau of Meteorology over the 5-year period 1972-77 (after Streten, 1980).

Fig. 2.4. Monthly mean zonally-averaged pressure difference (in hPa) between 40°S and 60°S. Based on daily numerical analyses by the Australian Bureau of Meteorology over the 5-year period 1972-77 (after Streten, 1980).

maximum and minimum (Fig. 2.5). It is the change in the solar insolation regime and nature of the underlying surface between the two latitude bands that produces a semi-annual cycle in the temperature difference. At oceanic middle latitudes between 40°S and 50°S, the large thermal inertia of the oceans causes the atmospheric temperature cycle to lag the annual cycle in solar insolation, so that air temperatures do not start dropping until March. The higher latitude region near 65°S, being closer to Antarctica and its surrounding sea ice, displays a more "continental" climate, and air temperatures respond more quickly when solar radiation decreases after the solstitial maximum in December. The 50°-60°S temperature gradient therefore has a maximum in autumn (March).

The second maximum in the spring occurs largely by default, because the lowest temperatures of the year at 65°S are delayed until the very end of winter. To some extent this occurs because there is no clearly defined minimum of insolation during the polar night nearer the Pole, with the sun remaining below or just above the horizon for a long period. Another factor is the large amount of latent heat involved in the formation of sea ice, which acts to buffer the seasonal decline in air temperature (note that autumn is also the season of maximum ice

0 0

Post a comment