Equatorial Upwelling

Equatorial Upwelling (UE) is upward water’s motion in the upper layer of the equatorial ocean. It occurs when a persistent easterly wind is blowing over the equatorial zone. Maximum upward velocity in the UE occurs just at the equator. The EU is a result of a permanent divergence of a westward surface South Equatorial Current in the narrow equator vicinity forced by the southeast trade wind. Divergence of the westward current at the equator is caused by the change of the 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 entrainment of more cold water of thermocline into the upper mixed layer because associated vertical velocity in the equatorial thermocline is typically ~10-5 m per sec. 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 vicinity of the equator than in the interior equatorial ocean outside of it.

Location of Pure Equatorial Upwelling

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 thermocline 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 just in the upper layer of the eastern equatorial oceans. Therefore, such cooler sea-surface water looks like a long, thin tongue along the equator, spreading from the eastern equatorial oceans. There is also high biological activity in the vicinity of this relatively cold tongue.

The thickness of the UE is restricted by the upper boundary of equatorial undercurrent because the eastward current is accompanied by equatorial convergence and, hence, downward water motion. That is why this thickness varies from about 330–660 ft. (100–200 m) (in the western equatorial Atlantic or Pacific oceans, respectively) to 33–66 ft. (10–20 m) (in the eastern equatorial Atlantic and Pacific oceans, respectively). The UE is quite a 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, the 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 a seasonal cycle of the southeast trade wind (with some delay, which does not typically exceed a month); that is, in boreal late summer to early fall. Interannual variations of UE are mostly due to the El Niño/La Niño phenomena, especially in the Pacific Ocean. Just before the development of an El Niño event (the anomalous warming of the upper layer in the equatorial Pacific), the southeast trade wind dramatically weakens and UE is over.

In contrast, during a La Niño event (a cold episode in the equatorial Pacific Ocean), UE is strongly developed as a result of anomalous intensification of the southeast trade wind, and hence the South Equatorial Current. Interannual variability of UE in the equatorial Atlantic follows to Pacific variability with some delay, which is typically not more than a few months. However, the magnitude of interannual 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 seasonal UE variations is two to three times larger than interannual variations.

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 just in the Indian Ocean. Interannual UE variability in the Indian Ocean is controlled by Indo-Ocean Dipole, which is the inherent Indo-Ocean mode interrelated with the Pacific interannual variability (the El Niño/La Niño phenomena), as can be seen in the 2007 results from researchers Swadhin Behera, Toshio Yamagata, Alexander Polonsky, and colleagues. Low-frequency (decade-to-decade) variability of the southeast trade wind and/or northeast monsoon would generate quasi-equilibrium Upwelling Equatorial variations. A more (less) intense southeast trade wind and northeast monsoon would lead to more (less) intense Upwelling Equatorial.