Longterm Trend Of The Southeast Indian Subantarctic Mode Water In The CMIP6 Coupled Models

Subantarctic mode water(SAMW)is formed between the subantarctic front and the subtropical front.The SAMW that formed in the South Indian Ocean is called Southeast Indian Subantarctic Mode Water(SEISAMW).When it formed,seawater at the base of mixed layer subducted into the ocean interior in late winter,transmitting the air-sea interaction signals into the ocean interior and providing subsurface oceanic reservoirs of heat,carbon,and other properties that significantly affected the global climate.This study assesses the capability of 12 models in the Phase 6 of the Coupled Models Intercomparison Project(CMIP6)in simulating the SEISAMW by comparing to Argo observations.The mixed layer continues deepening from austral fall to late winter,providing the precondition of mode water formation.The results show that the simulated mixed layer associated with SEISAMW formation is biased deep compared to observations.This deep bias is governed by the combine effects of both stratification and buoyancy flux: the buoyancy loss bias occurs in CMIP6 models tending to break the stratification at the base of mixed layer and inducing the mixed layer to convect deeply.In addidtion,the CMIP6 models generally have produced artificially smaller mixed layer potential density in comparison with those of the Argo observation.Consequently,the simulated SEISAMW in the CMIP6 models are generally with smaller potential density.Meanwhile,the subduction regions of the SEISAMWs show significant differences among the analyzed CMIP6 models,which is attributed to lateral induction in the mixed layer.Nevertheless,all of the analyzed CMIP6 coupled models are able to reproduce the SEISAMW and its seasonal cycle,albeit with discrepancies of formation region and properties among the models.The SEISAMW subduction rate show significant interannual variability,which is primarily induced by lateral induction in these models,in agreement with observations.Based on the outputs of eight earth system models involved in the CMIP6,this study investigated the projected long-term trends in subduction rate,volume and properties of the SEISAMW under medium and high greenhouse gas emission scenarios(i.e.,SSP245,SSP585).The results show that in the historical,SSP245 and SSP585 outputs,the SEISAMWs show consistent decreasing trends in subduction rate and volume,increasing trend in temperature,and decreasing trends in salinity and potential density.The long-term trends of the SEISAMWs are largest under SSP585 scenario,followed by the SSP245 scenario and historical simulation.The projected trends of SEISAMW can be explained by the following mechanism: the temperature and freshwater flux in the Southeastern Indian Ocean upper layer tend to increase under enhanced radioactive forcing,resulting in shoaling in mixed layer and flattening of the mixed layer gradient.As a result,the trends of SEISAMWs in subduction rate,volume and water properties show larger values in accordance with stronger radioactive forcing.