Evolution of upwelling in the eastern boundary of Arabian Sea: Based on In-situ Observations
Ch V Ramu*, Vishnu N N S, Y V B Sarma, G V M Gupta
Centre for Marine Living Resources and Ecology (CMLRE)
Centre for Marine Living Resources and Ecology, LNG Road, Puthuvype Kochi, Kerala, India Pincode: 682508 Mobile: 9299850830 E-mail: firstname.lastname@example.org
The upwelling along the west coast of India is one of the typical eastern boundary coastal upwelling systems. In recent times, ocean eastern boundary processes have received greater attention due to their ecological and economic importance in view of global warming and climate change scenarios. For the first time, utilizing the in-situ time series (ten times) data collected across seven to ten coast-offshore transects between December 2017 and January 2019, the spatial-temporal variability of the evolution of upwelling along the entire eastern boundary of the Arabian Sea has been described. This upwelling phenomenon is divided into the southern upwelling system (SUS) and the northern upwelling system (NUS). Additionally, remote sensing data of sea surface winds and sea surface height anomaly (SSHA) were also utilized. The depth of 26ºC isotherm (D26) has been used as a proxy to describe the progression of upwelling. The upwelling over the southernmost shelf (off Cape, 7ºN) initiated as early as in January as a subsurface process but remained at subsurface until the first week of May due to mesoscale processes and intrusion of the warm pool, thereafter it quickly progressed from offshore to nearshore surface by June at a rate of 3 km/day. The onset of SUS at mid-shelf off Kochi (10ºN) occurred in the third week of May, but its northward progression took about 40 days to reach off Goa (~15ºN) at a rate of 16-18 km/day. Similar to the northward progression of upwelling its withdrawal also commenced in the same pattern from September in the south to October in the central region. The evolution of NUS over the northern shelf off Okha (22ºN) followed a similar pattern as that of the central shelf, and the central shelf is a part of SUS although upwelling between these regions is not continuous due to their differential governing factors. The upwelling had a strong vertical upliftment rate in the south (0.7 - 1.4 m d-1) relative to central/north (0.25 - 0.54 m d-1) at the shelf edge. Of the entire eastern Arabian Sea shelf, the upwelling over Mumbai (19ºN) always remained at subsurface depths with the lowest residency period due to the widest shelf, strong and persistent pycnocline, dominant cross-shore winds, currents, positive alongshore Ekman transport, and low Ekman pumping velocities. Overall, the cross-shore upwelling withdrawal rates were almost double to that of its progression rate and the sustained period of upwelling south of 15ºN was relatively longer than north of it.
Key Words: Southern Upwelling System, Northern Upwelling System, Upwelling progression rates, Upliftment of Isotherms, Ekman Indices.