Coupling between SW monsoon-related surface and deep ocean processes as discerned from continuous particle flux measurements and correlated satellite data

Abstract

Particle flux data obtained by time series sediment traps deployed at water depths of approximately 3000 m in the western, central, and eastern Arabian Sea since 1986 were compared with wind speeds derived from measurements made by microwave radiometer flying on polar orbiting satellites and sea surface temperatures (SSTs) provided by the Physical Oceanography Distributed Active Archive Center at Jet Propulsion Laboratory. This comparison has allowed us to trace the link between the oceanographic and biological processes related to the development of the SW monsoon with the pattern and interannual variability of particle fluxes to the interior of the Arabian Sea. We could recognize the well-known upwelling systems along the coasts of Somalia and Oman as well as open ocean upwelling at the beginning of the SW monsoon. Both open ocean upwelling and coastal upwelling off Oman cause a cooling of surface waters at our western and central Arabian Sea stations. When SSTs fall below their long-term average, an increase in fluxes which are dominated by coccolithophorid-derived carbonates occurs. The timing of this increase is determined by rate of surface water cooling. Further intensification of upwelling as the SW monsoon progresses causes additional increases in biogenic opal fluxes denoting diatom blooms in the overlying waters. The total fluxes during this period are the highest measured in the open Arabian Sea. At the central Arabian Sea location the fluxes are only randomly affected by these blooms. The particle flux in the eastern Arabian Sea is as high as in the central Arabian Sea but is influenced by a weaker upwelling system along the Indian Coast. The observed interannual variability in the pattern of particle fluxes during the SW monsoons is most pronounced in the western Arabian Sea. This is controlled by the intensity of the upwelling systems on the one hand and the transport of cold, nutrient-poor, south equatorial water into the Oman region on the other. The latter effect, which is strongest during the SW monsoon with highest recorded wind speeds, reduces the influence of upwelling and the related particle fluxes at the western Arabian Sea station, where highest fluxes occur during SW monsoons with moderate wind speeds. Thus coastal and open ocean upwelling are most effective in transferring biogenic matter to the deep sea during the SW monsoons of intermediate strength