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

Abstract

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; PO₄³⁻ 2.5 μM; NO₃⁻ 6.8–2.1 μM; 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 (NO₃⁻ and PO₄⁻³ 1.0 μM), 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.