Summer phytoplankton blooms off the Somali coast in the South-western Arabian Sea from remote sensing observations

Abstract

Summer phytoplankton blooms appear frequently off the Somali coast in the southwestern Arabian Sea (AS), where strong reversal monsoon and summer upwelling is prevailing. Distinct high chlorophyll-a (Chl-a) concentrations in summer were displayed in the western AS, especially in the region off the Somali coast. The spatial and inter-annual variations of the summer high Chl-a were studied using satellite data including ocean color and wind vectors. Under ocean conditions including monsoon winds, Ekman transport (ET) and Ekman pumping velocity (EPV), as well as geostrophic current and aerosol precipitation, the possible mechanisms of high Chl-a was investigated. The summer high Chl-a presented strong inter-annual variations in the southwestern AS. The results of simple correlation analysis indicated that there were good correlations between the ET and Chl-a, as well as between EPV and Chl-a. These implied that the ET and EPV may cause uplift of nutrients into the upper layer of the western AS from subsurface or coastal regions, inducing high Chl-a in the southwestern AS, especially in the region off the Somali coasts in summer. The multiple/partial correlation analysis implied further that EPV-induced upwelling may be more helpful than the ET-induced upwelling in the coastal region off Somalia, leading to probably more significant influence of EPV upwelling on the phytoplankton bloom than upwelling by ET. Aerosol precipitation in the southwestern AS also played an important role in high Chl-a in the deep offshore AS (i.e., Section B in Fig. 1(a)), as second only to ET and sea surface temperature (SST), and even higher than EPV. A novel finding is that the influence of aerosol optical thickness (AOT) is evident in the offshore region and the dust precipitation is more important sources to oligotrophic water. Both the stability of the upper ocean and the aerosol precipitation may play more evident roles in the open regions of the southwestern AS off Somali.

Graphical abstract

Keywords

Somali Peninsula / the southwest Arabian Sea / chlorophyll-a / wind speeds / upwelling / nutrients

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Acknowledgments

The present research is supported by the National Natural Science Foundation of China (Grant No. 42076162), the Natural Science Foundation of Guangdong Province, China (No. 2020A1515010496) and project supported by Innovation Group Project of Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai) (No. 311020004). We thank GlobColor’s Working Group for providing merged Chl-a data (available at ACRI website) and the European Centre for Medium-Range Weather Forecasts for providing monthly SST and SSW products (available at ECMWF webstie).

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