Atmosphere-driven compound extremes: record marine heatwave and phytoplankton bloom in the Ross Sea during summer 2022–2023

Abstract

We document an extreme marine heatwave (MHW) in the eastern Ross Sea during the 2022–2023 austral summer (DJF), the most intense thermal event in the satellite record. Using satellite-retrieved sea surface temperature (SST), ocean colour, sea ice, and atmospheric-oceanic reanalysis data, we show that the eastern Ross Sea experienced a Category IV (extreme) MHW during this season, with the regional-mean cumulative intensity reaching 127.22 °C·days in January 2023 and summer-mean SST anomalies locally peaking at approximately +2.5 °C above the climatological mean. Summer-mean chlorophyll-a (Chl-a) concentration reached 3.67 mg m−3, the highest value observed in the eastern Ross Sea since satellite ocean-colour records began in 1997. The ecological response followed a threshold-dependent sequence: early sea-ice retreat initially removed light limitation, followed later by a strong Chl-a increase as sustained warming and upper-ocean restratification favoured bloom development. In contrast, persistent ice cover in Terra Nova Bay prevented a comparable thermal or biological response. A meridional wind dipole drove divergent ice transport and the early expansion of the eastern Ross Sea polynya, while low-cloud cover decreased by up to 15.43%, increasing downward surface shortwave radiation by 64.09 W m−2. Subsurface warming remained confined to the upper continental-shelf waters south of approximately 73° S, suggesting that local atmosphere-ice-radiation interactions contributed more strongly to the event than heat transport associated with ocean circulation. These results identify a distinctly polar pathway through which atmospheric forcing can generate compound thermal and biological extremes, highlighting the importance of local sea-ice coupling in shaping future Antarctic coastal productivity.