Variability in upwelling fields in the northwestern Indian Ocean 2. Data-model comparison at 9000 years B.P.


  • Warren L. Prell,

  • Rebecca E. Marvil,

  • Mark E. Luther


Theoretical investigations, in the form of general circulation model (GCM) simulations and empirical studies of pollen, lake levels, and marine sediments suggest that the Indian summer monsoon was intensified 9000 years ago, at a time of increased summer radiation (about 8%) in the northern hemisphere. The southwesterly summer monsoon winds and their pattern of wind stress curl cause coastal upwelling along the coast of Arabia. Stronger summer winds, therefore, should intensify coastal upwelling. Incorporation of the broad-scale GCM winds into a high-resolution ocean model of the Arabian Sea produces distinct patterns of stronger upwelling at 9000 years B.P. To verify these patterns in the geologic record, we mapped upwelling assemblages of planktonic foraminifera and estimated sea surface temperature (SST) to define the magnitude and spatial distribution of monsoonal upwelling at 9000 years B.P. We found that the upwelling assemblage increased in abundance and the August SSTs were several degrees Celsius cooler off Arabia than present. However, in the central and eastern Arabian Sea the tropical assemblage increased and the SSTs were warmer than present values. The SST gradient across the Arabian Sea was greater (6°C) at 9000 years B.P. than it is today (4°C). These observations are consistent with the model results that show increased divergence, upwelling, and a thinner upper layer or shallower thermocline in the westernmost Arabian Sea and increased convergence, downwelling, and a thicker upper layer in the central and eastern Arabian Sea. In this experiment, the use of low resolution winds from a general circulation model (NCAR-CCM) in a high-resolution ocean model seems to be appropriate for simulating the long-term averages observed in the marine sediments.