Analysis of marine heatwaves and biogeochemistry in the Northern Arabian Sea

Abstract

Marine heatwaves (MHWs) are abnormal Sea Surface Temperature (SST) conditions in which temperature exceeds the 90th percentile threshold limit. It has significant impacts on marine biodiversity causing large-scale coral bleaching, fish deaths or migration, and local weather changes. Analyzing MHWs and their causative mechanism can help to understand the notable influence on biodiversity. In this paper, we have investigated the MHW events in the Arabian Sea (AS) for the period 1982 to 2021. Our study reveals that an average of two MHW events occur every year with an increasing linear trend of 1.22 MHW events per decade in the northern AS region. In addition, we found a category-II MHW event that lasted about 134 days in the year 2020. This northern AS MHW event has a maximum, mean, and cumulative intensity of about 2.76 °C, 1.94 °C, and 259.35 °C-days (above the climatological mean), respectively, from 9thJune 2020 to 20thOctober 2020. According to our analysis, the western AS showed relatively higher intensity than the eastern part, especially near the coasts. The entire AS showed an increasing trend of MHW. We found that the decrease in wind velocity, increase of net downward heat flux, and lowering of surface ocean current is the main cause of the 2020 northern AS MHW event. Many of the biogeochemistry parameters, namely, chlorophyll (-16.2255%), net primary production (-25.5948%), dissolved oxygen (-0.7323%), nitrate (-23.9284%), and phosphate (-49.2542%) show the lowest values during the 2020 MHW event period except for silicate (13.9649%) for their corresponding climatological mean. Decreasing trends are also observed for chlorophyll (-0.0020 mg/m3-year), net primary production (-0.1747 mg/m3-year), oxygen (-0.1079 mmol/m3-year), nitrate (-0.0080 mmol/m3-year) and phosphate (-0.0076 mmol/m3-year) with increasing MHW days except for silicate that shows an increasing trend of 0.0018 mmol/m3-year with increasing MHW days with time.Plain Language SummaryForty years of SST data is used to find and analyze the most intense MHW events in the Arabian Sea (AS). We calculated its characteristics such as maximum, mean, cumulative intensity, and duration to understand and further analyze these MHW events. We have analyzed the time series of the number of MHW events per year from 1982 to 2021 alongside the trend to understand and predict the future MHW events in the northern AS region. We generated spatial plots of different MHW parameters over the entire AS and tried to understand their behavior. We compared changes in six marine biogeochemistry parameters spatially and temporally with the yearly MHW days and observed their relationship.