SOS-iClimate: Southern Ocean Salinity– Isotopic fingerprints and impacts on global CLIMATE

So far, the influence of changing ice masses on the ocean has largely been inferred from surface fluxes that have large uncertainties. A more direct quantification of this influence can be obtained from ocean tracers, such as seawater stable isotopes and noble gases. However, their use has been limited spatially, which is owing to a scattered data distribution and quality, even though Southern Ocean wide isotope data have been already collected in the 1970s.

This surge of data provides a unique opportunity to constrain climate model simulations and to assess long-term changes when comparing them to sparser historical data. Yet, a coordinated effort to compile the available data does not exist. Therefore, in SOS-iClimate, we will build a novel database of quality controlled seawater stable isotopes.

In summary, while most water masses in the Southern Ocean have experienced a freshening, the exact causes of these observed changes remain unknown. This knowledge gap is owing to discrep-ancies between observations and model simulations and large uncertainties in the observational constraints from surface fluxes. An additional complication arises from the unknown spreading of glacial meltwater along the continental shelf, into the open ocean, and into AABW. Addressing these issues is urgent given the abrupt recent decline in Antarctic sea ice and the accelerated melting of ice shelves.

A novel combined use of noble gas and stable water isotope measurements will allow the first comprehensive assessment of the causes of the freshening of the Southern Ocean. To link the changes in salinity to climate-relevant processes, we will investigate their influence on changes in the vertical exchange of water masses, heat and carbon in the Southern Ocean.

One of the essential questions related to the freshening of the Southern Ocean and the changing ice masses is if they will influence the planetary-scale ocean overturning circulation and might impact the climate through this process. To first order, the overturning circulation in the Southern Ocean is fueled by wind-driven divergence and convergence. While the dynamics of the overturning circulation have been fairly well understood, our understanding of the role surface fluxes of heat and freshwater that are required to convert the upwelled deep water to either new lighter or denser water masses, i.e., to become ventilated and form new water masses is still limited. However, it is this ventilation processes that is critical to exchange heat and carbon between the deep ocean and the surface.

We want to use biogeochemical tracers for this project. For example, inorganic nutrients can be combined with oxygen measurements to obtain estimates of the recirculated fraction of inorganic nutrients. In this way, we want to evaluate the relationship between the spatial and temporal variations of the various freshwater fluxes at the surface and the oceanic overturning circulation.

The observed freshening, and thus lightening, of the Southern Ocean surface waters has led to a circumpolar increase of the surface density stratification over recent decades. This increasing density stratification has critical consequences for the mixing between deep and surface waters that have possibly declined over recent decades. In my own work, I could show that this increased stratification is probably responsible for most of the observed temperature changes, since it led to a trapping of heat at depth. These changes most likely also affected the vertical mixing of carbon-rich deep waters into the surface layer and the downward mixing of anthropogenic CO₂. Yet, despite the increasing stratification over recent decades, the Southern Ocean continued to take up CO₂ from the atmosphere at an expected rate, but the changes in these CO₂ fluxes also carry large uncertainties and model simulations do not reproduce them. Thus, the exact relation between stratification changes and CO₂ exchange still remains unknown. Given that the density stratification has been changing, which has been proposed to strongly alter the CO₂ fluxes on long time scales, an investigation of this relation requires urgent attention. Recent advances in remotely observing the high latitudes from space and under water now provide new avenues to explore this critical relation.

While climate projections show a strong increase in the density stratification of the Southern Ocean, the stratification of the Southern Ocean should weaken in a warmer climate, according to paleoclimatic records. This discrepancy between model simulation and paleo-climatic records provides a major conundrum in our field that has not yet been sufficiently addressed. Part of the issue might arise from the inability of models to accurately represent the high-latitude water mass structure. In addition, these models do not account for increasing glacial meltwater and have strong biases in their representation of Antarctic sea ice, which is the main driver of the upper ocean stratification changes. the spatial spreading of the meltwater in these models is not evaluated against observations, due to a lack of observational evidence to trace meltwater fluxes. But whether it spreads at the surface and stratifies the upper ocean or enters and influences AABW could result in major differences in the response of these mod-els. All these factors lead to a poor understanding of the pathways, spatial extent, and influence of melting Antarctic ice on past and future Southern Ocean stratification and circulation changes and the related climatic impacts. This leads us to the question: Does the changing Antarctic cryosphere amplify or mitigate global warming?