Regional topography and winds drive a strong upwelling of cold ocean waters and an increase in phytoplankton growth in the Gulf of Tehuantepec.
21, December 2020
By Hayley Evers-King and Jochen Karl Kerkmann (EUMETSAT), Ivan Smiljanic (CGI) and Ben Loveday (Innoflair)
Tehuantepecer, or Tehuano wind, occurs between October and March, e.g. in winter, related to large-scale cold air outbreaks from the north.
A strong wind episode in the Chivela Pass (between Mexican and Guatemalan mountains) was dynamically forced by the plunging of cold air behind an intense and very elongated cold front. This front stretched from the south-western area of the Gulf of Mexico to the north-eastern coast of America (Figure 1), bringing the cold air from the far continental north all the way down to south of the Mexican coast.
Clouds streets are very prominent features of strong cold advection — clearly seen behind the frontal line in the quasi-linear cumulus cloud formations (Figure 2).
Advection of cold air, more than -18 °C further south was channeled through Chivela Pass, with accelerated flow (Figure 3).
A consequence of this strong wind circulation, was upwelling in the ocean waters in the Gulf of Tehuantepec. The winds drove an offshore movement of surface waters, which were then replaced by water which upwelled from the depths. This water was both colder and more nutrient rich than the surface water it replaced.
Sentinel-3 products comparison
The signature of this phenomena can be clearly seen in satellite-derived sea surface temperature (SST) and in chlorophyll-a concentrations derived from satellite ocean colour measurements (Figure 4), as measured by the SLSTR and OLCI sensors aboard the Sentinel-3A satellite on 3 December 2020.
A longer term perspective on this event can be gained from looking at a multi-sensor merged record of sea surface temperature. Figure 5 shows the SST in the region from November to December 2020. The enhancement of the cold upwelling feature grows clearly over the month. The upwelling in this region, as a result of the wind circulation, is observed on an annual basis, with variability in its strength influenced by the El Nino Southern Oscillation (ENSO). Ultimately upwelling dynamics play an important role in nutrient cycling and primary production in the oceans, with the potential to affect higher tropic levels, such as fish and mammal populations, on which many economic activities depend.
The phenomenon can also be seen on the GOES-16 Natural Color RGB with ASCAT winds overlaid (Figure 6).
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