Impact of North Atlantic Abrupt Cold Events on Monsoonal Phasing

The question of orbital-scale periodicity and phasing of tropical monsoon variations has developed as a long- standing problem, due to inconsistencies between modeling results and paleo-monsoon records. In contrast to modeling studies, which point to an in-phase relation of the Monsoon system with Northern Hemisphere peak summer insolation, reconstructions from marine sediment cores showed an extended lagged response of around 8000 years. On the other hand radiometrically dated East Asian Monsoon records derived from speleothems have been interpreted to be in-phase with July insolation. Here, we present a Bromine record with a centennial scale resolution from the northern Arabian Sea capturing variability of the Indian Monsoon over the past 450 000 years together with newly derived transient modeling results including global ice volume changes. Comparing our Indian Monsoon reconstruction with speleothem records of the East Asian monsoon shows consistent patterns and a common small phase lag (2 700 to 3 800 years) with respect to June (peak summer) insolation at the precession band. Our transient model results demonstrate that ice volume variability modulates amplitude but has no impact on the precessional phase relation. We argue that abrupt cooling events (Heinrich events) in the North Atlantic consistently lead the Northern Hemisphere peak summer insolation on average by 4 500 years and delay the onset of the summer monsoon during each precession cycle. This, in combination with nonlinear response to insolation of the proxy records, introduces a small phase lag during the late Pleistocene.