Session: Glacial Climates (LGM, Last deglaciation, Ice sheet uncertainties, Glacial-interglacial cycles)
Author: Matthias Prange / firstname.lastname@example.org / MARUM - Center for Marine Environmental Sciences, University of Bremen, Germany
Co-author: Mahyar Mohtadi, MARUM - Center for Marine Environmental Sciences, University of Bremen, Germany;
Enno Schefuß, MARUM - Center for Marine Environmental Sciences, University of Bremen, Germany;
Tim Jennerjahn, Leibniz Centre for Tropical Marine Research, Bremen, Germany;
Climate models predict a slowdown of the Walker circulation for the twenty-first century. However, historical records and observations of the Walker circulation over the twentieth century disagree on the sign of change, and thus necessitate longer climate records to test both the simulated history and fate of the Walker circulation. Here we present a suite of records of sea surface and thermocline temperatures, and of the isotopic composition of rainfall, from the eastern tropical Indian Ocean for the Last Glacial Maximum (LGM) and the late Holocene. Our results indicate an increase in both thermocline depth and rainfall suggesting a stronger-than-today Indian Ocean Walker cell during the LGM. Analysis of PMIP2 and PMIP3/CMIP5 climate model results confirms the thermocline deepening in the eastern Indian Ocean in model simulations with a stronger Walker circulation during the LGM. However, our analysis of model output also reveals a considerable scatter of model results, with some models even simulating a shoaling of the thermocline along with a weakening of the Indian Ocean Walker cell under LGM conditions. In the two models with maximum Walker circulation strengthening (CCSM3 and FGOALS-g1.0) the deepening of the thermocline is sufficiently strong to induce even warmer-than-today eastern equatorial Indian Ocean subsurface temperatures in the LGM in accordance with our new thermocline records. We conclude that during the LGM, convection and rainfall over the eastern equatorial Indian Ocean was stronger than today as a result of an intensified Indian Ocean Walker circulation, while further to the east, anomalous subsidence resulted in drier conditions over the Maritime continent, as indicated by various previous proxy studies. An intensified Walker circulation during the globally cooler last glacial period underscores the sensitivity of tropical circulation to temperature change and implies a further weakening of the Walker circulation in response to greenhouse warming.