The Weddell Sea supports a range of processes that are critical to the Antarctic and global-scale ocean circulation and climate. Warm Deep Water (WDW), found offshore at mid-depth, makes incursions onto the continental shelf, and mixes with shelf water masses to produce Antarctic Bottom Water (AABW) which ventilates over one-third of the global-sub-surface ocean. Previous studies using ocean/sea ice simulations have indicated that in a warming climate, intrusions of WDW onto the southern Weddell Sea continental shelf may rapidly melt the floating Filchner-Ronne Ice Shelf (FRIS), and thereby drive rapid future retreat of Antarctic glaciers. However, the conditions required for the intrusions and rapid melt to take place were previously poorly understood.

The uncertainties associated with ongoing and future of changes in these processes derive largely from observational limitations and constraints on model resolution in the Weddell region. For example, previous modeling approaches have been unable to fully resolve eddies, which are known to play a major role in mixing and tracer transport throughout the ocean (see Figure). In contrast, decades of observational/modeling studies have established a critical role for tides in mixing and exchanging water masses across the Antarctic continental shelf and slope. However, the relative roles of these processes in controlling the transports and transformations of water masses in the Weddell Sea had not previously been quantified.

Intellectual Merit: The central focus of this project was the development of a new high-resolution regional model of the southern Weddell Sea, including the ocean beneath the FRIS. We first used this model configuration to perform a large suite of simulations of the southern Weddell Sea circulation with various idealized perturbations to the atmospheric climate that drives this circulation. We showed that the speed to the winds that blow northward off the FRIS play a key role in controlling the access of WDW to the cavity beneath the FRIS, and thus in setting the rate at which the glacier melts. Furthermore, we showed that the FRIS cavity circulation is "bi-stable", meaning that for identical atmospheric conditions and ambient ocean conditions, the water beneath the ice shelf can either be very cold or much warmer, resulting in relatively low or relatively high melt rates. Very large perturbations to the offshore-blowing winds are required to "shift" the state of the FRIS cavity from warm to cold, or vice versa.

We then ran our simulations at much higher resolution to investigate the relative roles of eddies, tides, and seasonal/interannual variability in driving water mass transports and transformations in the southern Weddell Sea. Contrary to expectations based on previous studies, we found that tides play a relatively minor role, producing only a modest decrase in the formation of AABW by slightly increasing the melt rate of the FRIS. Seasonal/interannual variability also proved to have little net effect in the formation of AABW. However, eddies play a major role, as they export most of the dense waters from the southern Weddell Sea continental shelf, and are responsible for 100% of the transfer of heat onto the continental shelf.

Broader Impacts: These findings yield new and surprising insights into the mechanisms at play in a region that exerts major influence on the global ocean circulation. Our identification of the role of offshore-blowing winds in mediating the ocean state and melt rate beneath the FRIS warrants increased focus on this aspect of the atmospheric circulation among the climate science community. Meanwhile, the muted role of tides in forming dense waters suggests that their effects could be largely parameterized in climate models as an enhancement of the input of meltwater into the ocean. Finally, the key role of eddies in exporting dense waters across the southern Weddell Sea continental shelf break indicates that this process warrants particular focus in future observational deployments and climate model parameterization development.

Last Modified: 12/30/2020
Modified by: Andrew L Stewart