Assessment of Indian Ocean upwelling changes and its relationship with the Indian monsoon

Abstract

Present study aims to assess the Indian Ocean upwelling zones and its relationship with the Indian monsoon. Assessment of upwelling zones are examined by using fine resolution satellite sea surface temperature (SST), chlorophyll-a concentration and reanalysis nitrate (NO 3 ), surface wind speed, mean sea level pressure (MSLP) and precipitation products. This study uses the high-resolution (12 km) satellite era reanalysis Indian Monsoon Data Assimilation and Analysis (IMDAA) data for the evaluation of upwelling and summer monsoon relationship. Long-term and decadal trends of SST, chlorophyll-a and NO 3 are estimated by employing the non-parametric Mann–Kendall test and Sen's slope method for the summer monsoon (June–September) and winter monsoon (October–December) season. Long-term seasonal trends suggest the significant warming and weakening of upwelling over western tropical Indian Ocean, however, decadal trends reveals that signal is more prominent during the period 1999–2008. In the northern Bay of Bengal, the NO 3 trend shows significant seasonality. A zonal SST gradient-based index (upwelling index (UI)) is adopted to identify the upwelling zones, results are presented for the prominent upwelling zones of Somalia and Oman. The UI is used to understand the long-term and decadal variation of Somalia and Oman upwelling intensity with the Indian summer monsoon MSLP, surface wind and rainfall variations. Results suggest that Oman upwelling intensity is highly correlated with the monsoon heat low over the northwest India and Pakistan and Indian summer monsoon rainfall over the core region. However, MSLP over the Indian subcontinent and rainfall over the west coast of India is correlated with the Somalia upwelling intensity. Here, we show that surface wind over the central Arabian Sea (monsoon trough) region are correlated with the intensity of UI of Oman (Somalia). The decadal study provides compelling evidence that the relationship between MSLP, near surface wind, upwelling and ISMR weakens over time in Somalia coast but in the Oman coast this relationship strengthens with recent decades. • Long-term and decadal trends of SST, chlorophyll-a and NO 3 are investigated over the Indian Ocean. • Significant warming and decline of chlorophyll-a is depicted in the western and North Indian Ocean. • Recovery of upwelling zones in the western Indian Ocean is evident in the recent decade (2009-2018). • Strong correlation between the Oman upwelling intensity and Indian summer monsoon rainfall is found in the recent decades.