A Model-Data Assessment of Glacial to Holocene Changes in Walker and Hadley Circulations: Implications for Indian Ocean Hydroclimate and Wind System

Abstract:

We report on several proxy-based reconstructions from the tropical eastern Indian Ocean indicating a warmer and deeper thermocline during the Last Glacial Maximum (LGM) compared to the Holocene. This setting resembles a decline in the monsoon-related upwelling in the glacial eastern Indian Ocean suggestive of a stronger Indian Walker Circulation during the LGM compared to today. Analysis of climate model results, provided by the Paleoclimate Modelling Inter-comparison Project (Phase II and III), confirms the deepening of the equatorial thermocline in the eastern Indian Ocean in model simulations with a stronger Walker Circulation during the LGM. The inferred strengthening of the glacial Walker Circulation confirms theoretical considerations with respect to the tropical circulation response to global temperature change. During the last glacial period, the hydroclimate of both the monsoonal and tropical Indian Ocean was also characterized by dry conditions of millennial duration associated with North Atlantic cold spells, like the Younger Dryas and Heinrich Events. Climate model simulations suggest that the dry conditions were associated with a southward displacement of the Intertropical Convergence Zone and a weakening of the mean annual southern Hadley Cell over the Indian Ocean triggered by reduced northward heat transport in the Atlantic Ocean.

Our combined proxy data and model results suggest significant influence of both the Walker and the Hadley Circulation on tropical Indian Ocean hydroclimate and wind systems during the last glacial period. Although boundary conditions during the last glacial period were different than today, our results help to constrain the response of tropical circulation to temperature change with significant implications for the projected response of the Indian Ocean to global warming.