Abstract
AbstractThe Southern Ocean overturning circulation is driven by winds, heat fluxes, and freshwater sources. Among these sources of freshwater, Antarctic sea ice formation and melting play the dominant role. Even though ice-shelf melt is relatively small in magnitude, it is located close to regions of convection, where it may influence dense water formation. Here, we explore the impacts of ice-shelf melting on Southern Ocean water-mass transformation (WMT) using simulations from the Energy Exascale Earth System Model (E3SM) both with and without the explicit representation of melt fluxes from beneath Antarctic ice shelves. We find that ice-shelf melting enhances transformation of Upper Circumpolar Deep Water, converting it to lower density values. While the overall differences in Southern Ocean WMT between the two simulations are moderate, freshwater fluxes produced by ice-shelf melting have a further, indirect impact on the Southern Ocean overturning circulation through their interaction with sea ice formation and melting, which also cause considerable upwelling. We further find that surface freshening and cooling by ice-shelf melting cause increased Antarctic sea ice production and stronger density stratification near the Antarctic coast. In addition, ice-shelf melting causes decreasing air temperature, which may be directly related to sea ice expansion. The increased stratification reduces vertical heat transport from the deeper ocean. Although the addition of ice-shelf melting processes leads to no significant changes in Southern Ocean WMT, the simulations and analysis conducted here point to a relationship between increased Antarctic ice-shelf melting and the increased role of sea ice in Southern Ocean overturning.
Highlights
Before looking in more detail at the impacts of iceshelf melting on water-mass transformation (WMT) in E3SM, we investigate the fidelity of ocean temperature and salinity in simulation results from E3SM
We find no significant differences in net Southern Ocean WMT due to the differences in total surface fluxes between the two simulations
When we separate the WMT rate into its constituent processes, we find important differences in both WMT and water-mass formation (WMF) rate between the simulations
Summary
The Southern Ocean plays a large role in Earth’s climate system (Morrison et al 2011; Marshall and Speer 2012; Séférian et al 2012; Heuzé et al 2013; Merino et al 2018) as a significant sink for atmospheric heat. Snow in excess of 1 m water equivalent (‘‘snowcapping’’) and rain are immediately routed to the nearest coastal ocean grid cell and deposited at the surface with a small amount of horizontal smoothing This functions as a crude approximation to unresolved ice sheet processes (including surface processes, iceberg calving, and basal melting) in order to keep the ice sheet in instantaneous equilibrium with climate forcing, and conserves mass globally to avoid having to account for a potentially large water sink in the model. Published E3SM simulations (Golaz et al 2019; Petersen et al 2019) do not include ice-shelf cavities, but the horizontal and vertical grids are otherwise identical to these Both ISM and Ctrl include the three-dimensional ocean domain below the ice shelves, but in Ctrl the ice-shelf base is a depressed surface where no heat and freshwater exchange occur.
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