Impact of river discharge on the coastal water pH and pCO₂ levels during the Indian Ocean Dipole (IOD) years in the western Bay of Bengal

Abstract

Monthly variations in inorganic carbon components during January 2010–December 2011 in the coastal Bay of Bengal (BoB) were studied in relation to the variations in Godavari River (GR) discharge using ship board measurements, buoy data and satellite remote sensing data. Higher GR discharge was observed during the 2010 monsoon (1133 m³ s⁻¹) resulting in low salinity and strong stratification in the coastal region. During 2011 monsoon season, low GR discharge (403 m³ s⁻¹) resulted in weaker stratification in the coastal waters. Contrasting atmospheric and oceanic conditions prevailed in the north Indian Ocean during 2010–2011 in association with the negative Indian Ocean Dipole (nIOD) event in 2010 and positive IOD (pIOD) event in 2011. Consequently, anomalous onshore easterly surface winds and warmer Sea Surface Temperature (SST) anomalies occurred in the BoB during nIOD event and an anomalous strong alongshore northeastward surface winds and colder SST anomalies occurred during pIOD event in the coastal western BoB. The observed weaker stratification due to low GR discharge and the anomalous alongshore surface winds favoured coastal upwelling during pIOD event in the 2011 monsoon, and consequently lowered the SST and the pH pCO₂) levels by 0.01 units (higher by 60 μatm) than those observed during the nIOD event. This study thus reveals that reduced GR discharge together with pIOD event contributed to enhanced acidification and pCO₂ levels in the coastal western BoB. It could be envisioned that the enhanced acidity in the coastal waters due to variations in river discharge and phases of IOD may significantly modify the coastal ecosystem that requires careful evaluation