Open-ocean polynyas and deep convection in the Southern Ocean

Abstract

An open-ocean polynya is a large ice-free area surrounded by sea ice. The Maud Rise Polynya in the Southern Ocean occasionally occurs during the austral winter and spring seasons in the vicinity of Maud Rise near the Greenwich Meridian. In the mid-1970s the Maud Rise Polynya served as a precursor to the more persistent, larger Weddell Polynya associated with intensive open-ocean deep convection. However, the Maud Rise Polynya generally does not lead to a Weddell Polynya, as was the situation in the September to November of 2017 occurrence of a strong Maud Rise Polynya. Using diverse, long-term observation and reanalysis data, we found that a combination of weakly stratified ocean near Maud Rise and a wind induced spin-up of the cyclonic Weddell Gyre played a crucial role in generating the 2017 Maud Rise Polynya. More specifically, the enhanced flow over the southwestern flank of Maud Rise intensified eddy activity, weakening and raising the pycnocline. However, in 2018 the formation of a Weddell Polynya was hindered by relatively low surface salinity associated with the positive Southern Annular Mode, in contrast to the 1970s’ condition of a prolonged, negative Southern Annular Mode that induced a saltier surface layer and weaker pycnocline.

Interactions between Increasing CO2 and Antarctic Melt Rates

“…Ordinarily, in some places of rapid sea ice production over the continental shelf around Antarctica, this saline water is dense enough to sink to the depth of the shelf, and to spill over the shelf edge and spread through the deep ocean abyss as Antarctic Bottom Water (AABW) (M. van Aken 2007;Nicholls et al 2009). Another mechanism for AABW formation is prolonged deep convection in the open ocean, which is the primary mechanism by which AABW forms in most CMIP5 climate models but occurs rarely in reality (Heuzé et al 2015;Cheon and Gordon 2019). Deep water convection does not occur in this configuration of HadGEM3-GC3.1 (Menary et al 2018).…”

On the crucial role of atmospheric rivers in the two major Weddell Polynya events in 1973 and 2017 in Antarctica

“…Various processes have been proposed to explain the polynya formation and maintenance, all involving the complex atmosphere–ocean sea ice–seabed interaction system. These include (i) changes in atmospheric circulation associated with La Niña and negative Southern Annular Mode (SAM) ( 34 ), (ii) upper-ocean preconditioning ( 36 ) concurrent with severe storms ( 37 , 38 ), and (iii) intensification of westerly winds causing a spin-up of the Weddell Gyre ( 36 ).…”

Interannual Variability of the Outflow of Weddell Sea Bottom Water

“…The Weddell Gyre spin‐up may have affected the sea ice cover. A major anomaly during 2015–2017 reported in the literature is the occurrence of larger, more persistent polynya in the vicinity of Maud Rise (the Maud Rise Polynya, Cheon & Gordon, ; Campbell et al, ; Jena et al, ; Francis et al, ). The Maud Rise Polynya may be associated with stronger wind mixing, air‐sea fluxes, and increased ocean eddy activity induced by the currents over the Maud Rise, which enhances local upwelling.…”