Subantarctic Mode Water and Its Long-Term Change in CMIP6 Models

The Subantarctic Mode Water (SAMW) is a major water mass in the southern Indian and Pacific Oceans and plays an important role in the ocean uptake and anthropogenic heat and carbon. The characteristics, formation, and long-term evolution of the SAMW are investigated in the "historical" and "SSP245" scenario simulations of phase 6 of the Coupled Model Intercomparison Project (CMIP6). Defined by low potential vorticity, the simulated SAMW is consistently thinner, shallower, lighter, and warmer than in observations because of biases in the winter mixed layer properties and spatial distribution. The biases are especially large in the South Pacific Ocean. The winter mixed layer bias can be attributed to unrealistic heat loss and stratification in the models. Nevertheless, the SAMWis presented better in CMIP6 than in CMIP5 with regard to its volume, location, and physical characteristics. In warmer climate, the simulated SAMW in the southern Indian Ocean consistently becomes lighter in density, with a reduced volume and a southward shift in the subduction region. The reduced heat loss, instead of the increased Ekman pumping induced by the poleward-intensified westerly wind, dominates in the SAMW change. The winter mixed layer shoals in the northern outcrop region, and the SAMW subduction shifts southward where the mixed layer remains deep. The projected reduction of the SAMW volume is likely to impact the heat and freshwater redistribution in the Southern Ocean.