Evolution to decay of upwelling and associated biogeochemistry over the southeastern Arabian Sea shelf

Abstract

Observations along 10 shelf transects in 2012 near 10°N in the southeastern Arabian Sea revealed the usual warm oligotrophic conditions during the winter monsoon and upwelling of oxygen-deficient, nutrient-rich cool water during the summer monsoon (SM). By changing an oligotrophic to a nutrient-replete condition, the upwelling is the major process that regulates the biogeochemistry of this shelf. Its onset is perceptible at 100 m depth between January and March. The upwelling reaches the surface layer in May and intensifies during June-July but withdraws completely and abruptly by October. Despite the nutrient injection, the primary production during SM, integrated for euphotic zone, is comparable to that during the preceding spring intermonsoon (SIM). Again, as usual, the high oxygen demand coupled with low concentration in the upwelled subsurface waters causes severe oxygen depletion below the shallow pycnocline. The oxygen concentrations/saturations of 2012 on the midshelf are similar from those of mid-1958 to early 1960, except for marginally higher values during the peak upwelling period due to relatively weak upwelling in 2012. This implies little anthropogenic influence on coastal hypoxia unlike many other coastal regions. In 2012, the inner shelf system shifted from net autotrophy in SIM to net heterotrophy in SM but on an annual basis it was net autotrophic (gross primary production to community respiration ratio, GPP/R:1.11 ± 0.84) as organic production exceeded consumption.

Positive Indian Ocean Dipole events prevent anoxia off the west coast of India

“…The oxygen concentration on the shelf is expected to be lower than that offshore because of a higher oxygen consumption on the shelf, particularly associated with the sediment respiration process, which in our model is a very simple representation. Despite lower oxygen content on the shelf, there is a very good phase agreement between the oxygen seasonal variations from CaTS in situ measurements and the corresponding offshore variations derived from the WOA and model, suggesting a strong offshore-shelf connection through dynamical upwelling process, as suggested by Banse (1959), Carruthers et al (1959), and Gupta et al (2016).…”

“…Neutral or negative IOD years are, however, not necessarily anoxic, indicating that a neutral or negative IOD is a necessary but not a sufficient condition for severe anoxia. A recent study by Gupta et al (2016) revealed that the oxygen deficiency in 1959 along the WCI was more severe than in 2012, a conclusion consistent with the occurrence of a negative IOD in 1959 and a positive one in 2012. Similarly, in situ measurements also revealed that subsurface oxygen concentrations were significantly lower at the turn of the 20th century than in the 70s ): our simulation exhibits a similar behavior (see Fig.…”

“…With the advent of the summer monsoon (June-August), the westerly monsoon winds drive a very strong offshore Ekman transport in the western Bay of Bengal and near the STI , resulting in an upwelling signal, which shoals the OCD and TCD up to 60 m at the STI ( Fig. 6b and f; Smitha et al, 2008;Gupta et al, 2016). Further north, the winds are almost perpendicular to the coast of western India ) and hence do not induce upwelling there.…”

“…While the spatiotemporal density of observations in the eastern Pacific margin is adequate to accurately describe the monthly oxygen evolution along the west coast of South America over the past 3 decades (Helly and Levine, 2004;Arntz et al, 2006), there is a dearth of long-term data from fixed sites in the Indian Ocean as compared to the Pacific and the Atlantic (Gupta et al, 2016), which in turn prevents building a reliable time series that could depict the year-toyear variations. On the other hand, three-dimensional coupled physical-biogeochemical models that include the oxygen cycle have contributed to improve the description and understanding of dynamics of hypoxic events in various coastal regions (Peña et al, 2010), such as the Gulf of Mexico (e.g., Hetland and DiMarco, 2008), Black Sea (e.g., Gregoire and Friedrich, 2004), and Baltic Sea (e.g., Eilola et al, 2009).…”

“…A similar connection has also been reported by Carruthers et al (1959) off Mumbai at 19 • N in early November 1958 and later by Naqvi et al (2009) off Mangalore at 13 • N by late summer for several years. A recent study by Gupta et al (2016) used 10 shelf transects near 10 • N during 2012 to conclude that the upwelling of oxygen-deficient waters along the shelf break during the monsoon is the major process regulating the biogeochemistry on the shelf. Although these studies indicate a clear connection between the shelf oxygen content and upwelling at the shelf break, the actual processes in these exchanges are not well understood, owing to the lack of continuous and high-frequency observations.…”