Do Wind-based Proxies from the Arabian Sea Represent Long-term Variations in Indian Monsoon Rainfall?

Modern observations suggest a positive correlation between Indian summer monsoon rainfall and coastal upwelling along the western boundary of the Arabian Sea. It was assumed that this relation is valid in climates of the past and has been the basis of several monsoon reconstructions. Such reconstructions indicate that monsoon increases when local summer insolation decreases, contrary to many speleothem-based monsoon reconstructions. So far, a comprehensive understanding of this difference is missing. Using a set of idealized precession experiments in a high-resolution model, we have shown that :(a) the correlation between Indian monsoon rainfall and upwelling changes drastically depending on the precessional forcing. (b) the spatial extent of the region where upwelling occurs increases when summer occurs at aphelion (weaker insolation) and decreases when summer occurs at perihelion (stronger insolation). This explains the opposite trend recorded by upwelling proxies. We find that it is due to the impact of precessional forcing on the latitude, width, and strength of the low-level jet. Previously, all the changes in upwelling were attributed to the changes in the strength of the low-level jet alone. Our results underscore that proxies from the Arabian Sea do not necessarily represent the strength of the Indian summer monsoon and are influenced by the changes in the width and latitude of the jet. Hence, the implications of these proxies for the Indian monsoon must be interpreted with care.