Influence of orographically enhanced SW monsoon flux on coastal processes along the SE Arabian Sea

Abstract

[1] The Arabian Sea has an excess evaporation over precipitation regime. The southeast Arabian Sea is, however, anomalous because it has ∼2800-4800 mm rainfall during the southwest monsoon (SWM). Despite a high rainfall, the fluvial influence on supply of total suspended matter (TSM) and marine productivity is deemed insignificant and remains unevaluated. We evaluated the poorly resolved influence of fluvial influx on shelf processes. We documented low salinity, stratification, high productivity, and turbidity over the entire inner shelf (chlorophyll-a ∼2.4 mg m −3 ; PO 4 3− >2.5 mM; NO 3 − 6.8-2.1 mM; TSM 88-182 mg l −1; salinity nearshore region 26; offshore region 33.8-34.6 practical salinity units). The deeper regions (>40 m), however, had greatly reduced TSM and nutrient levels (NO 3 − and PO 4 −3 <1.0 mM), TSM (<24 mg l −1 ), and patches of high chlorophyll-a. Upon cessation of the SWM, nutrient levels and TSM reduced considerably. We identify two processes that contribute to the marine productivity and turbidity during the SWM. Over the deeper regions, there is a poor influence of fluvial supply and upwelling regulates productivity. Over the shallow inner shelf, the sequestering of fluvial influx due to the prevalence of strong winds, upwelling, and equatorward flow is dominant. The later processes induced high marine productivity and eutrophication in the nearshore region and may have implications for siltation of channels. Reduced turbidity, nutrient and chlorophyll-a levels, and higher salinity of the coastal waters during rest of the year imply a substantial role of fluvial fluxes on coastal processes.

Getting the right wind-forcing for an ecosystem model: A case study from the eastern Arabian Sea

“…These regions also recurrently witness spring bloom of Trichodesmium spp. This, along with coastal current dynamics, upwelling and high rainfall ensure high coastal productivity with elevated surface chlorophyll values up to 32-39 mg m −3 off Mangalore (Madhupratap et al 2001;Jayram et al 2010;Raghavan et al 2010;Chauhan et al 2011;Sudheesh et al 2016).…”

“…The horizontal expansion of OMZ occurs as upwelling influences the coastal productivity during ISM (Devol et al 2006;Sudheesh et al 2016). Monsoonal reversal of winds and in turn coastal currents, along with high amount of river discharges generate high degree of variability in the productivity induced from either mixing or upwelling, in the EAS (Sarupria and Bhargava 1993;Madhupratap et al 2001;Srinivas and DineshKumar 2006;Habeebrehman et al 2008;Jayaram et al 2010;Chauhan et al 2011;Joshi and Rao 2012;Gupta et al 2016;Sudheesh et al 2016). This underlines that the AS is an ideal laboratory to study the impact of forcing in simulating the ecosystem processes responsible for the variability in coastal productivity.…”

Timing, cause and consequences of mid-Holocene climate transition in the Arabian Sea

“…The exposure and inundation of the continental shelf during glacial-interglacial transitions, affects the surface circulation as well as the material flux pathways and its spatial extent. At present, a majority of the terrestrial runoff is restricted to the inner shelf (Chauhan et al, 2011(Chauhan et al, , 2012 and during the times of low sea-stand, the same material will reach deeper regions, thus affecting the flux and subsequent digenetic processes. The entire continental shelf was exposed during the last glacial maximum, as a result of lowering of sea level by~120 m (Siddall et al, 2003;Grant et al, 2012).…”

“…The entire continental shelf was exposed during the last glacial maximum, as a result of lowering of sea level by~120 m (Siddall et al, 2003;Grant et al, 2012). The subsequent inundation of the shelf, affected the spatial extent of terrigenous input as well as the coastal current intensity and direction in the southeastern Arabian Sea (Ramaswamy and Nair, 1989;Chauhan et al, 2000Chauhan et al, , 2011. A comparison of the core data with the regional as well as global sea-level curve, reveals a relative stabilization of the sea level only at~6.4 ka (Hashimi et al, 1995;Siddall et al, 2003;Grant et al, 2012), suggesting that the sea level changes also contributed towards the mid-Holocene transition noted in the Malabar and other cores (Staubwasser et al, 2003a,b;Anand et al, 2008;Naik et al, 2014) collected from the southeastern Arabian Sea.…”

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

“…Precipitation during the summer monsoon and the day of monsoon arrival is thought to act as direct or indirect cues for spawning (Antony Raja, 1969, 1974; Jayaprakash, 2002; Murty & Edelman, 1966; Pitchaikani & Lipton, 2012; Srinath, 1998; Xu & Boyce, 2009). At the same time, intense monsoon rain over land causes high nutrient flux from rivers into the shallow nearshore regions which causes eutrophication and anoxia (Chauhan et al, 2011). The strong seasonal upwelling along the southwestern coast of India is also thought to be a key driver of variability in oil sardine abundance and catches.…”