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Friday, March 14, 2008

The Surface Exchange Process

Although solar radiation is partially absorbed by the atmosphere it is actually the transfer of heat and evaporation of moisture (latent energy) from the underlying surfaces that are the major sources of energy which drive atmospheric circulation. Over the oceans there is a lot of latent energy and the important factor is wind speed which regulates the evaporation rate. Over land it is both wind speed and surface characteristics, such as soil moisture, which play a part.

The way solar radiation is absorbed by the Earth's surface depends primarily on whether the surface is land or sea. Over land, the radiation is absorbed at the surface and it moves slowly through the earth beneath it. There is very little change in temperature, at a depth of a few metres, throughout the year. The surface, therefore, warms rapidly with increased radiation. Over winter, the land surface temperature cools somewhat because of the minimal amount of solar radiation.

In contrast, solar radiation over the oceans is absorbed and penetrates the surface layers of the oceans. The water has quite a high thermal capacity and because of the wind, any extra heat is distributed across the upper layer. As a result of this the sea surface temperature responds a lot slower to the different solar radiation at different times of the year.

Land and water that are next to each other heat and cool at different rates because of the different processes involved with absorbing solar radiation and the release of longwave radiation. In the summer, land becomes relatively hotter and then cools in the winter causing temperature and pressure gradients across boundaries areas.

Air will blow inland from the cooler ocean as the land heats in summer and will blow toward the sea as land cools during winter. The distribution of land and seas has a significant impact on global climate and its seasonal cycle because of the different responses the surface makes to varying solar radiation.

The annual average sea level pressure and wind distribution over the oceans is seen in the image below, although they do not reflect seasonal variability. However, there are clear indications of dominant processes in the Asia-Pacific region. In the Pacific there is mean high pressure in the subtropics of both the Northern and Southern Hemispheres, and in the Indian Ocean there is average high pressure and anticyclonic winds in the Southern Hemisphere, but this is not reflected in the Northern Hemisphere.

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