Multi-Decadal Decline in the Formation of Antarctic Intermediate Water: A High-Resolution Model Hindcast

The spatial and temporal variability of Antarctic Intermediate Water (AAIW) formation rates during 1948-2007 is investigated in a set of high-resolution experiments performed with the NEMO-LIM ocean sea-ice model. Two sets of experiments are analyzed: one having a global 1/4° horizontal resolution (ORCA025) and another having a resolution of 1/12° in the Southern Ocean, achieved using a two-way nesting technique. The hindcast experiments are forced with the CORE-II atmospheric reanalysis from 1948-2007, with companion climatological experiments used to correct for model spurious trends. The spatial distribution of AAIW formation rates is explored via available AAIW volume and via kinematic subduction rates. Maximum AAIW formation rates are found in the regions where the mixed layer depth (MLD) is deepest in winter, i.e. in the southeast Pacific and along the Antarctic Circumpolar Current fronts, whereas kinematic subduction rates are highest in regions dominated by lateral induction. A circumpolar multi-decadal decline of AAIW formation rates and kinematic subduction rates is found, with largest decline in the southeast Pacific. Here, a strong negative MLD trend is accompanied by freshening of the mixed layer, possibly linked to a multi-decadal increase in dominance of the freshwater versus the thermal buoyancy forcing as a response to large scale warming.