In addition to daily variations, the amount of energy the surface receives from the Sun varies considerably on seasonal time scales. This variation is due to the combination of the tilt of Earth on its axis of rotation and the orbit of Earth about the Sun as depicted in Figure 1.11.
The incoming solar radiation from the Sun is less intense when a hemisphere is tilted away from the Sun during its winter than when its tilted towards the Sun during its summer. Study Figures 1.12 and 1.13, which illustrate the amount of incoming radiation from the Sun during the Northern Hemisphere summer and winter solstices, respectively, to convince yourself of this. As a result, in winter the surface receives less radiation from the Sun than it loses to longwave radiation, conduction, evaporation, or advection, and its temperature decreases. Conversely, during each hemispheres summer, the surface receives more heat from the Sun than it loses; therefore, its temperature increases. Its interesting to note that the Southern Hemisphere is slightly closer to the Sun in January, during its summer, than the Northern Hemisphere is in June, during its summer. This results in the Southern Hemisphere receiving slightly more energy from the Sun than the Northern Hemisphere, but this difference is less than 5%. Also, the Southern Hemisphere consists of a larger percentage of ocean (consequently a smaller percentage of land) than the Northern Hemisphere. Because water has a much higher heat capacity (it takes more energy to warm and cool water than it does land), the net effect is that the Southern Hemisphere has a smaller seasonal variation of SSTs than does the Northern Hemisphere.
The seasonal changes in SST are readily evident in satellite data. The top panel of Figure 1.14 shows the average SST for June 1996 and the bottom shows the average SST for December 1996. The greatest range in SSTs occur in the subtropics, with the SSTs near the Equator and at the poles relatively constant throughout the year.
6.1 Indian Ocean Monsoon
Another example of seasonal variability is the annual change in SSTs associated with the Indian Ocean Monsoon. The word monsoon is derived from an Arabic word meaning winds that change with the seasons. Indeed, during the hot summer months, India is overtaken by a humid flow from the southwest that brings extensive cloudiness and impressive amounts of precipitation. This southerly monsoon (i.e., winds from the south) is caused by the heating of the Asian land mass in summer. The land heats the air above it, causing the air to rise, and consequently drawing air inland from the oceans to replace the rising air. By October, the Sun is well into its southerly winter decline and the Asian land mass cools rapidly during the longer nights. The air over the Asian continent now cools and sinks, and the warm, moist southerly monsoon is replaced by a dryer and cooler flow from the north, descending from the slopes of the Himalayas. The surface waters of the Indian Ocean have typical temperature characteristics, a substantially east-west distribution of SST with a temperature maximum near the Equator. However, the seasonal shifts in the winds associated with the Asian Monsoon have a dramatic influence on this east-west SST distribution. The most notable signature of the Summer Monsoon in the SST of the Arabian Sea is strong upwelling that occurs along the Somali and Arabian coasts. The southwest winds associated with the summer monsoon drive an eastward Ekman Transport away from the Somali and Arabian coasts. This eastward movement of water leads to upwelling of cold, nutrient rich waters along the Somali and Arabian coasts. The SST signature of this upwelling is most notable in satellite SST images from August and September, near the end of the summer monsoon (see Figure 1.15).