Variability in upwelling fields in the northwestern Indian Ocean 1. Model experiments for the past 18,000 years

Authors

  • Mark E. Luther,

  • James J. O'Brien,

  • Warren L. Prell


Abstract

A nonlinear reduced gravity ocean model is used to assess the effects of changes in the monsoon winds during glacial and interglacial conditions on the seasonal circulation in the northwestern Indian Ocean. Winds from the National Center for Atmospheric Research Community Climate Model simulations for 18 kyr B.P. (the most recent glacial maximum and a period of weaker monsoon winds), 9 kyr B.P. (near the beginning of the present interglacial and a period of stronger monsoon winds), and present-day (0 kyr B.P.) conditions are used to drive the model to a steady seasonal cycle. Strength of upwelling fields are inferred in each case by integrating upward displacements of the model pycnocline over the primary upwelling season of mid-April to mid-August. In both the 0 kyr B.P. and the 9 kyr B.P. cases, a broad band of upwelling and decreased model upper layer thickness extends along the coast of the Arabian Peninsula out 350–500 km offshore during the southwest monsoon. This upwelling is driven by the strongly positive wind stress curl beneath the cyclonic side of the atmospheric Findlater Jet, located to the north and west of the jet axis. Farther offshore, in the anticyclonic region to the south and east of the jet axis, the negative wind stress curl drives downwelling and increased upper layer thickness. In the 9 kyr B.P. case, the atmospheric jet is much stronger and narrower than in the 0 kyr B.P. case, with stronger values of both positive and negative curl to the northwest and southeast, respectively, of the jet axis. This drives much stronger upwelling to the northwest of the jet axis and downwelling to the southeast. In the 18 kyr B.P. case, the Findlater Jet is very weak, and upwelling patterns are uniformly weak across the basin. The spatial distribution and temporal variation of the upwelling fauna in the sediment record is consistent with the model upwelling fields in all three cases. The ocean model thus provides the link between the climate model and the sedimentation data, verifying the hypothesis that variations in upwelling driven by variations in the strength of the monsoon jet are responsible for the observed variations in the sediment record.

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