Southern Ocean Subsurface heating, Ice Melt MEchanisms and Responses (SO-SIMMER)

SO-SIMMER will examine and quantify the drivers and mechanisms of the observed and projected accumulation of subsurface heat below the sea-ice regions of the Southern Ocean. Using two numerical models, in combination with observational and reanalysis datasets, it will establish how this warming has and will impact ice shelf melting, sea-ice extent and air-sea fluxes. The deep Southern Ocean transports heat south from the relatively warm subtropics to the Antarctic continental margins. Here, a unique combination of ocean, cryosphere and atmospheric processes brings this deep warm water upwards to the mixed layer and surface. This subsurface heat influences air-sea heat fluxes, sea-ice concentrations and extent, as well as being the major mechanism driving ice shelf melt. The deep subsurface waters have been observed to be warming in recent decades and heat is accumulating directly below the mixed layer in the upwelling regions. Alongside atmospheric candidates, the movement of this accumulated deep heat to the surface layers was likely a major driver of the extremely anomalous low sea-ice around Antarctica that occurred in the Austral winter of 2023. While the impact of the released heat is the subject of intense scrutiny, the mechanisms driving the subsurface warming have not yet been investigated, making SO-SIMMER extremely timely. There is significant evidence to suggest that increasing surface stratification, changed ocean poleward circulation, and increased ocean eddy mixing may be occurring in the Southern Ocean. These are all known to influence heat delivery and storage below the regions of sea-ice formation. However, the relative contribution of each of these factors is presently unknown. It is also unclear how changes in these processes will influence delivery of heat to ice shelves, drive changes in sea-ice and air-sea fluxes, and how this will change in the future. This study will undertake a range of well-posed experiments using two realistic state-of-the-art numerical models; one optimised to examine the drivers of heat accumulation, and the other their impacts on the cryosphere. Both will be constrained by observations of ocean properties and surface fluxes. They will be used to answer the following questions: How does surface stratification impact subsurface heat accumulation in the vicinity of sea-ice? 2. How does heat delivery to this subsurface region change in response to changes in the residual overturning circulation, or a warming of northern source waters? 3. How does an increase in eddy driven mixing in the Antarctic Circumpolar Current (ACC) impact poleward heat delivery? 4. How do the ocean properties of the continental shelf change in response to this subsurface heat, and what are the subsequent impacts on sea-ice, the ice shelves, and the atmosphere? 5. How do we expect the above processes to change under future climate forcing and what feedbacks do the induced cryosphere changes impose? This study will identify those processes most significant in driving subpolar subsurface warming. It will deliver major improvements to our understanding of recent Southern Ocean change, and our ability to assess the potential for future ice shelf melt, sea-ice and air-sea heat flux change. This will provide information necessary to assess coupled climate model projections, which typically represent historical Southern Ocean water masses and sea-ice trends very poorly. It will also serve to constrain future projections of ocean heat change, sea-ice and ice shelf melt, as well as guide focused improvements in ocean parameterisations.