Shallowing Glacial Antarctic Intermediate Water by Changes in Sea Ice and Hydrological Cycle

The Antarctic Intermediate Water (AAIW) is an important component of global ocean circulation, and processes involved in AAIW variability have been extensively studied. However, there is scarce information about changes in AAIW vertical extent and existing studies even seem to be controversial. In this study, coupled climate models in iCESM and PMIP3 consistently show shallower AAIW depth at the Last Glacial Maximum (LGM) due to the equatorward shift of the entire AAIW. More importantly, modeling results suggest that the equatorward shift of AAIW can be caused by sea ice expansion and the weakened hydrological cycle under the glacial climate. Under global warming condition, the AAIW shifts poleward compared to the pre-industrial period driven by the retreating sea ice and strengthened hydrological cycle. However, the AAIW depth also shallows in response to the ongoing warming, likely due to overwhelming effects of enhanced stratification and shallowing mixed layer. The results in this study are model-based, and further studies, including more robust paleo observations, are needed to better understand the mechanisms that control the AAIW depth and extent.