The map of temperatures 30 m below the surface (Figure 3.3) also shows temperatures of 24°C or more may not be reached in all tropical areas. For instance, the western tropical Pacific and Atlantic Oceans appear warmer than their eastern parts. This is another consequence of large-scale ocean dynamics and their coupling with atmospheric processes.
The general wind pattern presented in Figure 3.1 shows that the prevailing winds in the tropics are the so-called Trade Winds, blowing predominantly from the northeast in the northern hemisphere and from the southeast in the southern hemisphere. They converge from both hemispheres near the equator in the so-called Intertropical Convergence Zone, characterized by its lack of wind (the so-called Doldrums) and its sudden squalls and thunderstorms.
As mentioned earlier, friction of the winds on the ocean surface is able to set water in motion. However, the Coriolis effect makes that wind blowing in a certain direction does not displace water in the same direction of the wind, but with an angle of 45° to the right (resp. left) in the northern (resp. southern) hemisphere. The same angle applies between the uppermost water layer and its underlying neighbor and so on, thereby leading to a spiraling current structure, the so-called Ekman spiral. When integrated over depth, the mean transport of water (the so-called Ekman transport) is directed 90° to the right (resp. left) of the wind direction in the northern (resp. southern) hemisphere.
Figure 3.4: Schematic diagram of coastal upwelling induced by the Ekman transport in the northern hemisphere, generating a current 90 degrees to the right. (reproduced from Wikipedia.org http://en.wikipedia.org/wiki/File:Upwelling-labels-en.svg).
Trade winds over the tropical oceans also give rise to this Ekman transport. Along the so-called oceanic eastern boundaries (the eastern edges of oceanic basins), the divergence caused by Ekman transport (water flowing away from the coast) is replaced by deep water in a process called coastal upwelling (Figure 3.4). Since deeper water is slightly colder than surface waters (see the explanation above), areas of coastal upwelling can be characterized in temperature maps by lower (sub-)surface temperatures. Such areas can be found along almost all oceanic eastern boundaries: coasts of Marocco and Mauritania in the north Atlantic, coasts of Namibia and Angola in the south Atlantic, coasts of California and Mexico in the north Pacific, coasts of Peru and Chili in the south Pacific, and to a lesser extent coasts of Somalia and Oman in the Indian Ocean. In the Indian Ocean this eastern boundary pattern is less marked and more subject to seasonal variations correlated to the monsoon and its reversal of winds.
In addition, a similar divergence of water occurs along the equator because of the direction of the prevailing winds (Figure 3.1) whereby the Ekman transport on either side of the equator is directed away from the equator. This divergence gives rise to upwelling along the equator, as can be seen Figure 3.3, especially in the eastern Pacific Ocean. The temperature signature of this equatorial upwelling is more marked in eastern parts of the oceanic basins because it combines with the coastal upwelling along eastern boundaries. It is also more marked in the Pacific than in the Atlantic because of differences in ocean dynamics between oceanic basins that are beyond the scope of this discussion. In the Indian ocean this equatorial upwelling is also more subjected to seasonal variations (related to the monsoon) and does not translate into a significant change in water temperature (again for reasons that are too complex to be explained in this document).
Figure 3.5: Recorded tracks of tropical cyclones for the period 1945–2006, based on data from the Joint Typhoon Warning Center and the U.S. National Oceanographic and Atmospheric Administration. Reproduced from Wikipedia.org http://en.wikipedia.org/wiki/File:Tropical_cyclones_1945_2006_wikicolor.png
The ocean sea surface temperatures relates directly to the genesis of tropical cyclones. An overview of recorded tropical cyclones for the period 1945-2006 (Figure 3.5) shows that the southeastern Pacific and Atlantic Oceans are mostly void of recorded storm tracks. This is caused by the fact that the cyclogenesis of tropical cyclones requires a sea surface temperature of at least 26°C, which generally do not occur in the southeastern Pacific and Atlantic because of eastern boundary upwelling (see also Figure 3.3).
