In order for the global annual mean temperature of Earth to remain constant, the energy received from the Sun (see 1, Fig. 1.01) must be balanced by a net loss of heat caused by the other processes (see 2–9, Fig. 1.01). If there is no balance, then the sea surface, and consequently Earth, will warm or cool. Figure 1.02 shows global SST averaged over 1 typical year.

Compare this to the plot in Figure 1.03, depicting the amount of solar radiation reaching the ocean’s surface as a function of latitude at 180°W (a meridian in the central Pacific Ocean) for a 4-year period. How does the average SST distribution for 1 year approximately correspond to the distribution of solar radiation received from the Sun? It’s interesting to note that, on average, both SST and solar radiation are uniform in the east-west direction, along lines of constant latitude. On the other hand, strong variations in SST and solar radiation exist in the north-south direction, along lines of constant longitude, with warmer SSTs around the equator where the incoming solar radiation is greatest, and decreasing SSTs toward the poles where the amount of solar radiation is the least.

On smaller spatial and temporal scales, this east-west distribution of properties does vary significantly. These variations are important to scientists because they represent the processes which, over the long term, help to balance out the incoming solar radiation, maintain constant SSTs, and moderate Earth's climate so it is habitable.

Figure 1.04 shows the monthly average SST for May 1990. Studying this image, we observe that there are, indeed, significant variations from the east-west distribution of properties we defined earlier, particularly along the eastern and western boundaries of the ocean basins. These variations represent the processes, in addition to the incoming solar radiation, that affect the distribution of SST throughout the oceans. They include the movement of water by ocean currents and the movement of water by winds.