A Holistic Approach to Upwelling and Downwelling along the South-West Coast of India

Abstract

An attempt has been made to develop a holistic understanding of upwelling and downwelling along the south-west coast of India. The main objective was to elucidate the roles of different forcings involved in the vertical motion along this coast. The south-west coast of India was characterized by upwelling during the south-west monsoon (May to September) and by downwelling during the north-east monsoon and winter (November to February). The average vertical velocity calculated along the south-west coast from the vertical shift of the 26 C isotherm is 0.57 m/day during upwelling and 0.698 m/ day during downwelling. It was concluded that upwelling along the south-west coast of India is driven by offshore Ekman transport due to the alongshore wind, Ekman pumping, horizontal divergence of currents and by the propagation of coastally trapped waves. Whereas downwelling along the coast is driven only by convergence of currents and the propagation of coastally trapped Kelvin waves. Along the west coast of India, the downwelling-favorable Kelvin waves come from the equator and upwelling-favorable waves come from the Gulf of Mannar region. ARTICLE HISTORY

Improving landings forecasts using environmental covariates: A case study on the Indian oil sardine (Sardinella longiceps)

“…The monthly average upwelling metrics we used are those used in many other studies of upwelling and nearshore productivity in this region. In most coastal upwelling systems, local wind‐stress forces are the most important driver of upwelling; however, the southwest coast of India is unique in that remote‐forcing and local currents and gyres play an unusually important role in driving the seasonal upwelling intensity (BR, 2010; BR et al, 2008; Rao et al, 2008; Shah et al, 2019). Our nearshore‐offshore SST differential index should have captured upwelling intensity due to either wind stress or remote forcing, and we expected that it would perform better than the wind‐based metrics.…”

“…The second index was the Ekman Mass Transport (EMT; kg m −1 s −1 ) perpendicular to and within 2° longitude of the coast (Schwing et al, 1996; Shafeeque et al, 2019). The third index was the Ekman Pumping (W e ; m s −1 ) at the tip of India (Shah et al, 2019). EMT and W e are computed from surface winds and capture upwelling due to local wind stress.…”

“…It is in the Southeast Arabian Sea, one of the world's important seasonal upwelling zones (Habeebrehman et al, 2008; Madhupratap et al, 2001). The portion of the study area falling between 9°N and 13°N has especially intense upwelling in June–September due to the combined effects of wind stress (Figure 2) and remote forcing (BR, 2010; BR, Sanjeevan, Vimalkumar, & Revichandran, 2008; Shah et al, 2019). Upwelling gives rise to a strong temperature differential between the nearshore and offshore, and high primary productivity and surface chlorophyll (BR, 2010; Chauhan et al, 2011; Habeebrehman et al, 2008; Shafeeque et al, 2019).…”

The puzzle of the anchovy–sardine inverse fishery at the south‐eastern coast of the Arabian Sea and climate variability

“…In this region, coastal upwelling is driven by offshore Ekman transport due to the alongshore wind, horizontal divergence of currents and the propagation of coastally trapped waves from the Gulf of Mannar during the southwest monsoon period. The southern region around the Kerala coast experience more pronounced upwelling during this season (Shah et al, 2018, 2019). Therefore, the marine environment along the Kerala coast (southern region of the south‐west coast) is enriched with nutrients driven by upwelling (Sajna et al, 2019; Shankar et al, 2019) and it is an important fishery region for small pelagics, including sardine and anchovy.…”

“…In this region, coastal upwelling is driven by offshore Ekman transport due to the alongshore wind, horizontal divergence of currents and the propagation of coastally trapped waves from the Gulf of Mannar during the southwest monsoon period. The southern region around the Kerala coast experience more pronounced upwelling during this season (Shah et al, 2018(Shah et al, , 2019.…”