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| Abstract Submission No. | ABS-2022-03-0368 |
| Title of Abstract | Annihilation of the Somali current upwelling system and its impact on the biogeochemistry during summer monsoon |
| Authors | Abhisek Chatterjee*, Lakshmi R Shenoy |
| Organisation | Indian National Centre for Ocean Information Services |
| Address | INCOIS Hyderabad, Telengana, India Pincode: 500090 Mobile: 8332050495 E-mail: abhisek.c@incois.gov.in |
| Country | India |
| Presentation | Oral |
| Abstract | Somali coast is argued to be one of the strongest upwelling zones in the Indian Ocean during summer monsoon. Observations and ocean model experiments show that upwelling along the Somali coast is limited to the early phase of the summer monsoon when the alongshore Findlater Jet sets in across the Arabian Sea. At this time, the windstress curl turns negative over most part of the Arabian Sea south of approximately 15degN. As the wind strengthens during July/August, the windstress curl also gets intensified results in strong Ekman pumping (negative vertical velocity) over the interior of the basin and therefore deepening the thermocline. Subsequently, these downwelling signals then propagate west to interfere with the upwelling signal off Somalia. As a response, thermocline along the major part of the Somalia coast (~60%) deepen by about 40-60 m, particularly in the central part of the Somali coast. Moreover, strong alongshore winds and weaker stratification allows more mixing in the bottom of the mixed layer which further deepens the thermocline there. As a result, during the peak summer months, upwelling becomes limited primarily to the eddy dominated frontal flows in the northern and to some extent in the southern part of the coast. Further, thermal surface flux and entrainment driven cooling play a key role in strong sea surface temperature cooling observed along this coast. This heterogeneity in the evolution of thermocline along the coast is also reflected in the primary productivity and phytoplankton distribution of this region. Both observation and the model simulations, suggests that the intense chlorophyll concentration is restricted to the northern part (north of 9 degN) of the Somali coast in response to the strong nutrient influx induced by upwelling. The maximum chlorophyll (~5μg/L) and nutrients (Nitrate: 8-10 μmol/kg, Phosphate: 05-0.7 μmol/kg) concentration observed in the northern section is during the peak summer monsoon. In addition to the coastally upwelled nutrients in the north, the strong boundary currents along the Somali coast facilitate the advection of nutrients from the central and southern section to the northern flank of the Great Whirl. The low chlorophyll concentration to the south of 9 degN is driven by entrainment due to the deepening of the thermocline in the central section. Owing to the large concentration of nutrient flux the northern section of the Somali transect is dominated by large phytoplankton, whereas in the rest of the coast small phytoplankton dominants. We have also discussed possible limiting factors that determine the growth of these phytoplankton communities along this coast. We believe that our results not only augment the existing knowledge of the Somali current system but also raise serious implications on our existing understanding of the processes important for primary productivity of this region and also underscores the use only alongshore winds based projection of future climatic changes in the upwelling intensity along this coast. This abstract is based on the following papers: Abhisek Chatterjee, Praveen Kumar B, S. Prakash, and P. Singh (2019). Annihilation of the Somali upwelling system during summer monsoon, Scientific Reports, https://doi.org/10.1038/s41598-019-44099-1. L R Shenoy, Abhisek Chatterjee, S. Prakash, T. Mathew (2020). Biophysical interactions in driving the summer monsoon chlorophyll bloom o_ the Somalia coast. Journal of Geophysical Research, 125, https://doi.org/10.1029/2019JC015549. |