Seasonal and interannual variations of MODIS Aqua chlorophyll-a (2003–2017) in the Upper Gulf of Thailand influenced by Asian monsoons

Seasonal and interannual variations of chlorophyll-a (chl-a) in the upper Gulf of Thailand (uGoT) were obtained using new regionally tuned algorithms applied to Moderate Resolution Imaging Spectroradiometer-Aqua. This long time-series (2003–2017) data were analyzed in the context of variations in environmental conditions associated with the Southeast Asian Monsoon. Chl-a distribution patterns were distinct for the non-monsoon (NOM), southwest-monsoon (SWM), and northeast-monsoon (NEM) seasons. During the SWM/NEM, high/low chl-a concentrations were associated with high/low precipitation and river discharge. During the NOM chl-a concentrations were generally low, because of low precipitation. In general, chl-a variability was tightly coupled to discharge from the Chao Phraya and Tha Chin rivers. Chl-a concentrations were generally higher in the north, but chl-a accumulation in the east/west of the uGoT could be linked to piling of freshwater to the east/west during the SWM/NEM caused by changes in wind direction and the reversal of currents. Interannual changes in chl-a were attributed to El Nino-Southern Oscillation (ENSO) rather than Indian Ocean Dipole (IOD) driven changes in precipitation, river discharge, and wind patterns. During the SWM, positive/negative chl-a anomalies coincided with high/low precipitation and river discharge during La Nina/El Nino. During the NEM, positive/negative chl-a anomaly coincided with high/low river discharge and strong/weak wind during La Nina/El Nino. Meanwhile, during NOM, positive chl-a anomaly could be attributed to anomalous high wind speed and precipitation during El Nino.