Abstract
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 Niño-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 Niña/El Niño. During the NEM, positive/negative chl-a anomaly coincided with high/low river discharge and strong/weak wind during La Niña/El Niño. Meanwhile, during NOM, positive chl-a anomaly could be attributed to anomalous high wind speed and precipitation during El Niño.
Highlights
The influence of the seasonally reversing Asian monsoons on the distribution of phytoplankton in the upper euphotic column has been reported for several regions within southeast Asia, including the South China Sea (e.g., Shen et al 2018; Tang et al 2003; Yuan-Jian et al 2012), the eastern and western coasts of Sabah (e.g., Abdul-Hadi et al 2013), the Strait of Malacca (e.g., Siswanto and Tanaka 2014), and the south-eastern Arabian Sea (e.g., Shafeeque et al 2019)
We improved the accuracy of the standard National Aeronautics and Space Administration (NASA) chl-a product of MODerate-resolution Imaging Spectroradiometer-Aqua (MODIS) for the upper Gulf of Thailand (uGoT) using a new empirical algorithm for the uGoT that was developed using in situ optical and chl-a datasets
It is worthy to note that in most of the other regions, chl-a variation during El Niño-Southern Oscillation (ENSO) or Indian Ocean Dipole (IOD) events is associated with strong upwelling and upward Ekman pumping, which is in contrast to the uGoT, where we demonstrated that variability in chl-a is the result of ENSO driven variations in river discharge, precipitation, and wind speed (Fig. 8d–o)
Summary
The influence of the seasonally reversing Asian monsoons on the distribution of phytoplankton in the upper euphotic column has been reported for several regions within southeast Asia, including the South China Sea (e.g., Shen et al 2018; Tang et al 2003; Yuan-Jian et al 2012), the eastern and western coasts of Sabah (e.g., Abdul-Hadi et al 2013), the Strait of Malacca (e.g., Siswanto and Tanaka 2014), and the south-eastern Arabian Sea (e.g., Shafeeque et al 2019) Several of these studies have shown that the strength of monsoonal winds can enhance phytoplankton biomass by stimulating oceanic physical processes (e.g., coastal upwelling and vertical mixing), that facilitate the transport of nutrient-rich waters from the deep and enhance primary productivity within the entire euphotic layer (Pinet 2014). It is believed that the variations in monsoonal rainfall, river runoff and monsooninduced changes in circulation, can potentially influence seasonal phytoplankton variability in the uGoT
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