Phenology of coastal upwelling in the Southeastern Arabian Sea: Insights from in-situ and satellite observations

Abstract: The marine ecosystem responses in the Southeastern Arabian Sea (SEAS) during the summer monsoon are mainly regulated by the periodic cycle (phenology) of coastal upwelling, which varies spatially and temporally. Hydrography associated with the initial phase of upwelling during different years was studied using in-situ and satellite observations. The research further investigated the relative contributions of wind-driven and remotely forced upwelling. Offshore Ekman transport and local temperature anomaly are significantly correlated for the coastal region from the Cape (8°N) to Mangalore (13°N). The upwelling phenology was addressed in terms of the dates of onset, withdrawal, amplitude, and duration of the process. Daily climatological patterns indicated a latitudinal gradient in upwelling, with southern latitudes (8–9°N) exhibiting earlier, longer, and more consistent upwelling. Meanwhile, northern latitudes (12–13°N) had a delay in onset, shorter duration, and increased variability. The study also examined interannual variability in upwelling onset, withdrawal, and duration, highlighting the sensitivity of upwelling systems to climatic events. Extreme climatic events such as El Niño in 2015 were associated with a delayed upwelling onset and short duration, while a strong positive Indian Ocean Dipole (IOD) in 2019 caused early cessation at north of 10°N. The Chlorophyll-a (Chl-a) concentration was also decreased notably during those periods, reflecting reduced upwelling strength. However, the La Niña/negative IOD events in 2016 prolonged the upwelling season, especially between 9 and 11°N. The derived relationship between upwelling and biological productivity is reflected in the strong coupling between upwelling indices and Chl-a. Trend analysis showed a warming trend, along with a decreasing trend in Chl-a. Despite a weakening in wind-driven coastal upwelling, the negative sea surface temperature trend observed off the southern tip of India suggests complex sub-surface processes that may counteract the warming trend. The present study defines and details the phenology of coastal upwelling in the SEAS that determines the onset-withdrawal timing, intensity, amplitude, duration, etc., and the impacts of teleconnections. Research highlights: Phenology of coastal upwelling in the SEAS is detailed through daily climatological patterns, and interannual variability is also studied. Wind-driven upwelling is higher at Tvpm, while remote forcing plays a significant role at Kollam and Kochi. El Niño (2015), positive IOD (2019), and La Niña/negative IOD (2016) modulate upwelling timing, intensity, and Chl-a concentrations. Long-term trends indicate a warming of 0.02°C per year, a decline in Chl-a of 0.01 mg/m³ per year, and a decrease in wind-driven coastal upwelling.