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

The Antarctic Intermediate Water (AAIW) is an essential global ocean water mass at intermediate depths. Coupled climate models in isotope‐enabled (δ18O, δD), fully coupled Community Earth System Model and Paleoclimate Model Intercomparison Project Phase 3 consistently show shallower AAIW depth at the Last Glacial Maximum (LGM) due to the northward shift of AAIW. More importantly, modeling results suggest that the northward shift of AAIW can be caused by sea ice expansion and the weakened hydrological cycle under the glacial climate. On the contrary, the AAIW under global warming tends to shift poleward compared to the pre‐industrial period driven by the retreating sea ice and strengthened hydrological cycle. However, the AAIW depth will shallow in response to the ongoing warming, likely due to the overwhelming effects of enhanced stratification and shallowing mixed layer. Plain Language Summary The Antarctic Intermediate Water (AAIW) is an important component of global ocean circulation, and processes involved in AAIW variability have been extensively studied, particularly for its role in ventilating CO2 from deep oceans to the atmosphere during the last deglaciation. However, there is scarce information about changes in AAIW vertical extent and existing studies even seem to be controversial. In this study, modeling results consistently show a shallowing AAIW depth at the Last Glacial Maximum (LGM), mainly because of the equatorward shift of the entire AAIW. More importantly, we found that the northward shift of AAIW can be caused by the northward expansion of sea ice and the weakened hydrological cycle at the LGM. Under global warming conditions, the AAIW shifts poleward due to the retreating sea ice and strengthened hydrological cycle. However, the AAIW depth also shallows, likely due to overwhelming effects of enhanced stratification and shallower 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. Key Points The Antarctic Intermediate Water (AAIW) depth shallows due to the northward shift of AAIW at the Last Glacial Maximum (LGM) The northward shift of AAIW can be forced by sea ice expansion and the weakened hydrological cycle at the LGM