LATE QUATERNARY FORCING OF INDIAN-OCEAN SUMMER-MONSOON WINDS - A COMPARISON OF FOURIER MODEL AND GENERAL-CIRCULATION MODEL RESULTS

Past fluctuations in the strength of summer monsoon winds in the Arabian Sea can be inferred from the grain size of lithogenic material blown in from the peripheral deserts. We partition a 400 kiloyear (kyr) time series of inferred wind strength into its dominant frequency components, and develop a Fourier model to evaluate the relative influence of potential monsoon forcing mechanisms operating over these dominant frequencies. Our results indicate that the primary external and internal forcing mechanisms determining the timing and strength of Arabian Sea monsoon winds over the late Quaternary are (1) solar insolation and (2) latent heat collected from the southern subtropical Indian Ocean and released over the Asian Plateau. Our results further suggest that the extent of glacial conditions plays only a minor role in determining the timing and strength of the Arabian Sea paleomonsoon. In contrast, sensitivity experiments using the National Center for Atmospheric Research community climate model (NCAR CCM) indicate that glacial boundary conditions play a dominant role in forcing the strength of Arabian Sea paleomonsoon winds. Such inconsistencies are illustrated by comparing CCM wind strengths to those inferred from geological data. CCM monsoon winds in the Arabian Sea are uniformly strong during interglacial oxygen isotope stages 1 and 5 and weak during glacial stage 2. The geological record, in contrast, indicates strong and weak monsoons during both glacial and interglacial stages. Examining CCM monsoon strength in the context of latent heat flux versus regional sea surface temperatures (SST) suggests that these discrepancies may be attributed to incorrect ocean-atmosphere heat transfer in the southern hemisphere, possibly associated with the effects of prescribed model SST fields.