Upwelling equatorial

upwelling equatorial (UE) is upward water's motion in the upper layer of the Equatorial Ocean and occurs when a persistent easterly wind is blowing over the equatorial zone. Maximum upward velocity in the UE occurs just at the equator.

UE is a result of the permanent divergence of a westward surface south equatorial current in the narrow equator vicinity, forced by the southeast trade wind. Divergence of westward current at the equator is caused by the change of sign of the Coriolis force between the Northern and Southern hemispheres. As a consequence of divergence, the upper thermocline becomes shallower at the equator. Strong permanent equatorial divergence also causes an intense entrain-ment of more cold water from the thermocline into the upper mixed layer. This leads to cooling of the upper mixed layer. As a result, the sea surface temperature is about 1.8 degrees F (1 degree C) lower in the equator vicinity than in the interior Equatorial Ocean outside of it.

Pure UE occurs in the narrow vicinity of the equator, just within the divergent zone. Because of the slope of equatorial thermocline in a zonal direction (the ther-mocline is deeper in the western equatorial Atlantic and Pacific oceans than in the eastern) and the generation of coastal upwelling in the eastern Equatorial Oceans, UE manifestation, as relatively cold surface water, is more pronounced in the upper layer of the eastern Equatorial Oceans. Therefore, the cooler sea surface water looks like a long and thin equatorial tongue spreading from the eastern Equatorial Oceans. There is also quite high biological activity in this relatively cold tongue.

The thickness of the UE is restricted by the upper boundary of the equatorial undercurrent because the eastward current is accompanied by equatorial convergence and, hence, downward water's motion. That is why this thickness varies from about 328 to 656 ft. or 100 or 200 m. (in the western Equatorial Atlantic or Pacific Ocean, respectively) to 33 to 66 ft. or 10 to 20 m. (in the eastern Equatorial Atlantic and the Pacific Ocean).

UE is a quite persistent phenomenon in the Atlantic and Pacific oceans because the westward surface south equatorial current occurs there in the equator's vicinity almost throughout the entire year. However, UE intensity varies from season to season and from year to year. Seasonally, it is at a maximum in the Equatorial Atlantic and Pacific, when the south equatorial current intensifies, following the seasonal cycle of the southeast trade wind (with some delay that does not typically exceed a month); that is, in boreal late summer to early fall. Interannular variations of UE are mostly to the result of the El Niño/La Niña phenomena, especially in the Pacific Ocean. Just before El Niño developing (i.e., an anomalous warming of the upper layer in the Equatorial Pacific), the southeast trade wind dramatically weakens, and UE is over. In contrast, during La Niña (a cold episode in

Cold-water upwelling in the Gulf of Tehuantepec: This image of the Isthmus of Tehuantepec in Mexico shows sea surface temperatures observed by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua satellite.

the Equatorial Pacific Ocean), UE is strongly developed as a result of an anomalous intensification of the southeast trade wind and, hence, the south equatorial current. Interannular variability of the UE in the Equatorial Atlantic follows Pacific variability with some delay, which is typically not more than a few months. However, the magnitude of interannular UE variations in the Atlantic Ocean is not as large as in the Pacific Ocean. A seasonal cycle prevails in the Equatorial Atlantic, where the magnitude of the seasonal UE variations is two to three times bigger than the interannular ones.

In the Indian Ocean, UE (as a persistent phenomenon) occurs only in boreal winter, when the northeast monsoon has been developing. The UE is most pronounced in the western part of this basin. Seasonal UE variability is at maximum in the Indian Ocean. Interannular UE variability in the Indian Ocean is controlled by the Indo-Ocean Dipole, which is an inherent Indo-Ocean mode interrelated with the Pacific interannular variability (i.e., El Niño/La Niña phenomena), as can be seen in the recent results of Alexander Polonsky and coauthors and of Swadhin Behera and Toshio Yamagata.

Low-frequency (decade-to-decade) variability of the southeast trade wind or the northeast monsoon generates quasi-equilibrium UE variations. A more (or less) intense southeast trade wind and northeast monsoon leads to more (or less) intense UE.

sEE ALsO: Atlantic Ocean; Equatorial Undercurrent; Indian Ocean; Mixed Layer; Pacific Ocean; Thermocline; Trade Winds; Upwelling, Coastal.

bibliography. Swadhin Behera and Toshio Yamagata, "Influence of the Indian Ocean Dipole on the Southern Oscillation," Journal of the Meterological Society of Japan (v.81/1, 2003); Eric Kraus, ed., Modelling and Prediction of the Upper Layers of the Ocean, Proceedings of a NATO Advanced Study Institute (Pergamon Press, 1977); Alexander Polonsky, Gary Meyers, and Anton Torbinsky, "Interannual Variability of Heat Content of Upper Equatorial Layer in the Indian Ocean and Indo-Ocean Dipole," Physical Oceanography (v.21/1, 2007).

Alexander Boris Polonsky Marine Hydrophysical Institute, Sevastopol

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