Open-Ocean Convection in the Weddell Sea in a High-Resolution Earth System Model

Open ocean polynyas in the Southern Ocean are ice-free areas within the winter ice pack that are associated with deep convection, ventilation of intermediate and bottom waters and can significantly contribute to the formation of Antarctic Bottom Water and therefore, important for the Atlantic Meridional Overturning Circulation. In the early 1970’s, satellite observations unveiled the presence of a large open ocean polynya within the Antarctic ice pack in the Weddell Sea called the Weddell Sea Polynya (WSP) which were observed for three consecutive winters of 1974-1976. Although assumed to be a normal phenomenon at first, the WSP has not occurred in the same scale since with the exception of smaller Maud Rise polynyas (MRPs). Here we investigate the formation of WSPs in a high-resolution preindustrial simulation with the Energy Exascale Earth System Model version 0 (E3SMv0-HR). The simulated WSPs grow realistically out of MRPs, similarly to the observed WSP events of 1974-1976. The formation of MRPs requires the model to simulate the detailed flow around Maud Rise seamount, while a realistic formation process of WSPs requires the ability of a model to produce MRPs. Deep convection over Maud Rise being triggered by the advection of near-surface waters with anomalously high salinity. The onset of WSP also requires a prolonged build-up of a heat reservoir at depth. The prerequisite for such is a strong stratification and a thick cold and fresh surface layer, which usually prevails when the core of the southern hemisphere westerlies is at an anomalously northern position. If in a subsequent period the westerlies shift southward, the negative wind stress curl over the Weddell gyre strengthens, and leads to a spin up of foremost the eastern portion of the double-cell structure of the gyre. This appears to be a necessary condition for WSP formation in E3SMv0-HR. The ultimate trigger is the formation of pronounced MRPs, which bring warm and salty Weddell Deep Water to the surface. If the associated surface salinity anomalies are high enough, their westward propagation with the background flow of the Weddell gyre eventually triggers the formation of a WSP along with a significant contribution to deep water masses in the Weddell Sea.