The structure of the southwest monsoon winds over the Arabian Sea during the late Quaternary: Observations, simulations, and marine geologic evidence

Abstract

Each summer strong southwest winds produced by the Asian monsoon blow across the NW Arabian Sea, creating a broad region of upwelling characterized by cool sea surface temperatures and enriched phytoplankton biomass. The integrated history of the summer plankton blooms is preserved in the form of distinct plankton fossil assemblages and sediment facies, forming a continuous geologic record of wind‐driven upwelling and thus changes in the SW monsoon winds. Sediments have been recovered from two regions in the NW Arabian Sea, one located along the Oman continental margin, and another located approximately 400 km offshore. Over the late Quaternary, these two sites record time series of upwelling that are substantially different. To understand the source of the differences, we examined the relationship between wind stress and upwelling at these sites using historical observations and model‐simulated wind fields. In the coastal region, upwelling is directly related to the Ekman transport offshore, and is thus linearly proportional to the alongshore wind stress. Farther offshore, open‐ocean upwelling is driven by the positive curl in the wind stress. In historical records, we found little correlation between stronger winds and increased open‐ocean upwelling. In the region offshore where the sediment cores were recovered, the year with the strongest winds was accompanied by weaker wind stress curl and diminished upwelling (indicated by warmer sea surface temperatures and decreased phytoplankton pigment content) because the region of maximum wind stress curl moved NW that year. In atmospheric general circulation model experiments using a range of summer insolation forcing, we found large changes in the wind stress curl occur over some areas of the NW Arabian Sea; however the offshore core locations lies outside of the region where changes in the SW monsoon drive large changes in open‐ocean upwelling. In contrast to the coastal sites where upwelling is directly related to the wind stress, the offshore sites appear to be located in a region where the wind stress curl does not change much over a large range of summer insolation. There is some evidence from both the interannual and model fields that this location is affected more by movements in the position of the low‐level jet than by changes in the over‐all strength of the SW monsoon. Although more data are required to define these patterns, these results indicate that the time series of open‐ocean upwelling observed in the geologic record from offshore sites may differ substantially from the coastal time series.