Two-signed feedback of cross-isthmus moisture transport on glacial overturning controlled by the Atlantic warm pool

Abstract. This paper studies the control of the Atlantic Warm Pool (AWP) on atmospheric moisture transport across the Central American isthmus as a potential feedback on rapid glacial climate fluctuations. Defined as a region of the Atlantic with surface temperatures above 28.5 °C, the modern AWP expands from the tropical Northwest Atlantic up to the Gulf of Mexico during boreal summer. Due to enhanced deep convection over these warm waters, changes in AWP area cause inverse changes in the strength of the Caribbean low level jet. This low level jet drives atmospheric moisture transport from the Atlantic across the Central American isthmus towards the Pacific. Changes in cross-isthmus moisture transport, potentially related to the AWP, may therefore have affected North Atlantic salinity and the partly density driven Atlantic Meridional Overturning Circulation (AMOC) during the glacial. Based on available proxy evidence we hypothesize that the AWP evolved independent of extratropical North Atlantic temperatures during most of the last glacial, except during periods of AMOC collapse when intense extratropical North Atlantic cooling may have limited eastward AWP expansion. We investigate the implications of this hypothesis for cross-isthmus moisture transport by simulating the coupled ocean-atmosphere response to AMOC collapse and the atmospheric sensitivity to additional variations in AWP area. Our simulations suggest that a decrease in AWP area may increase cross-isthmus moisture transport, whereas extratropical North Atlantic cooling beside a persistent AWP may decrease cross-isthmus moisture transport. Interpretation of these effects throughout an idealized Bond Cycle suggests a positive feedback of reduced cross-isthmus moisture transport in response to Greenland cooling prior to AMOC collapse. During AMOC collapse, when AWP expansion is proposed to have been inhibited, this positive feedback turns negative as enhanced cross-isthmus moisture transport may help AMOC recovery. Supported by reconstructed sea surface salinity changes, we propose that the AWP may have played a key role in the glacial climate by acting as a gatekeeper to regulate moisture transport across the Central American isthmus.