First, let's take a look at how the Earth's surface temperature varies with the seasons. Figure 7.2 shows the zonal-mean air temperature near the surface for representative months in each of the four seasons. The first thing we note is that the temperature is fairly uniform in the tropics (30S-30N), but declines sharply as the poles are approached. The temperature difference between the Equator and 60N is 39K in the Winter but only 12K in the Summer. The Southern Hemisphere has a much weaker seasonal cycle, except over the Antarctic continent: The temperature difference between the Equator and 60S is 26K in the Winter and 22K in the Summer. However, over Antarctica, poleward of 60S the seasonal cycle is extreme. Noting that the Northern Hemisphere has more land than the Summer, the data imply that the oceans have a strong moderating effect on the seasonal cycle. The temperature patterns in Figure 7.2 are what we seek to explain in terms of the response of climate to the geographically and seasonally varying Solar forcing.
An even better appreciation of the effect of land masses on the seasonal cycle can be obtained by examining the map of July-January temperature differences, shown in Figure 7.2. This map shows that the strongest seasonal temperature contrast occurs in the interior of large continents, and that the ocean temperature varies by at most a few degrees over the year - and even less in the Tropics. The strong seasonal cycle of the Northern Hemisphere continents extends very little beyond the coastlines, and the seasonal cycle of the Northern oceans has similar magnitude to that of the more extensive Southern oceans.
Figure 7.1: Observed zonal mean surface air temperatures for January, April, July and October. Computed from NCEP data for 1970-2000.
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