Antarctic Bottom Water Formation Mechanisms Across different Periods: Insights from Water Mass Transformation Analysis Using AWI-ESM

Antarctic Bottom Water (AABW) is a critical component of the global meridional overturning circulation, driving abyssal ventilation, heat storage, and carbon sequestration on centennial to millennial timescales.Despite its importance, the mechanisms governing AABW formation across different climate states remain poorly understood. Here we investigate AABW production during five paleoclimate periods including preindustrial (PI), mid-Holocene (MH), Last Interglacial (LIG), Last Glacial Maximum (LGM), and Marine Isotope Stage 3 (MIS3) using the AWI-ESM coupled climate model. Through water mass transformation (WMT) analysis and surface buoyancy flux decomposition using the xbudget framework, we quantify the relative contributions of thermal (longwave, shortwave, sensible, and latent heat) and haline (sea ice and precipitation-evaporation-runoff) forcing to AABW precursor water formation.Our results reveal a fundamental shift in formation mechanisms between climate states: during interglacial periods (PI/MH/LIG), heat fluxes dominate AABW precursor water production, whereas glacial conditions (LGM/MIS3) exhibit enhanced sea ice-driven transformation. Surface flux decomposition reveals that glacial conditions paradoxically reduce ocean heat loss despite colder atmospheric temperatures, as expanded sea ice insulates the bulk ocean while concentrating brine rejection. Ideal age tracer simulations demonstrate that AABW ventilation ages during LGM/MIS3 exceed preindustrial values by approximately 1,500 years, consistent with paleoceanographic proxy reconstructions. The LIG Ross Sea exhibits anomalously young ventilation ages attributed to reduced sea ice and enhanced thermal forcing.