Observed variability of monsoon blooms in the north-central Arabian Sea and its implication on oxygen concentration: A bio-argo study

Abstract

The central Arabian Sea (CAS) is productive during both the summer and winter monsoons owing to different physical processes. We analysed four years (2013–2016) record of chlorophyll and dissolved oxygen (DO) concentration from a Bio-Argo float deployed in this region. Though the surface blooms were observed during both the monsoons and sub-surface chlorophyll was also persistently observed, the intensity and duration of the bloom have been decreasing over the past few years. Also, the winter blooms were more prominent compared to the summer bloom in the study region. Our analysis shows that the observed inter-annual variability in the summer bloom can be attributed to the variability in wind speed, oceanic stratification and advection of nutrient rich water from the western Arabian Sea. During both the monsoons, stratification played an important role in reducing the productivity in recent years. We also found that during the winter monsoon, the upwelling Rossby wave propagating from the west coast of India influenced productivity as north as 15°N. The chlorophyll data from Bio-Argo float shows that the total surface chlorophyll concentration and backscattering (a proxy for particulate flux) in the OMZ region have been decreasing during the study period. Consequently, the DO concentration in the deeper waters has also been increasing. We believe that the decrease in surface productivity, lateral advection from the west and their manifestation on the export flux has reduced the oxygen demand in the deeper layer.