Control of the Oceanic Heat Content of the Getz‐Dotson Trough, Antarctica, by the Amundsen Sea Low

Abstract

The changing supply of warm Circumpolar Deep Water (CDW) to the West Antarctic continental shelf is responsible for the basal melting and thinning of the West Antarctic ice shelves that has occurred in recent decades. Here we assess the variability in CDW supply, and its drivers, from a multiyear mooring deployed in, and a regional ocean model spanning, the Getz‐Dotson Trough, Amundsen Sea. Between 2010 to 2015, the CDW within the trough underwent a pronounced cooling and freshening, associated with changes in thermohaline properties on isopycnals. Variability in the rate of CDW inflow is controlled by local wind forcing of a shelf break undercurrent, which determines the hydrographic properties of inflowing CDW via tilting of density surfaces above the continental slope. Local wind is coupled to the Amundsen Sea Low (ASL) low‐pressure system, which is modulated by large‐scale climatic modes via atmospheric teleconnections. For the period analyzed, a deeper ASL was associated with westward wind anomaly at the shelf break. Changes in the sea surface slope decelerated the shelf break undercurrent, resulting in less heat accessing the continental shelf and, consequently, a cooling of the Getz‐Dotson Trough. Therefore, the present work suggests that the fate of the West Antarctic ice shelves is closely tied to the future evolution of the ASL.

Reconstructing atmospheric circulation and sea-ice extent in the West Antarctic over the past 200 years using data assimilation

“…In addition to continental changes, a stronger ASL is also associated with reduced sea-ice cover in the Bellingshausen/Amundsen Sea sector and a sea-ice cover expansion in the Ross Sea sector (e.g., Raphael and Hobbs, 2014). A deeper ASL also drives oceanic changes, enhancing warm ocean water upwelling towards ice shelves, inducing basal melting (e.g., Mankoff et al, 2012;Dotto et al, 2020).…”

Interannual Variation of Modified Circumpolar Deep Water in the Dotson‐Getz Trough, West Antarctica

“…We therefore identify an interesting new mechanism in which the interannual variability of mCDW in front of DIS previously revealed by Jenkins et al (2018) could be explained by the variability of Ekman pumping de- rived from local atmospheric circulation in DGT, seasonally modulated by the movements of the sea ice edge. This is complementary to theories that relate heat content variability to anomalous heat advection at the continental shelf break driven by changes in shelf break undercurrent amplitude and winds (Carvajal et al, 2013;Dotto et al, 2020;Kim et al, 2017;Thoma et al, 2008;Wåhlin et al, 2013). Since both shelf break winds and continental shelf winds share similar variability and drivers, further investigation is needed to detail the relative role of each in dictating the interannual variability of mCDW in front of DIS.…”

“…Many scholars have investigated the variability and mechanism of mCDW flowing into the DGT based on field observations (e.g., Dotto et al, 2020;Kim et al, 2017;Wåhlin et al, 2012Wåhlin et al, , 2013. Wåhlin et al (2012) argued that the intrusion of warm and salty water over the continental slope may be due to upslope transport in the bottom Ekman layer.…”

“…Using the same mooring observations, Kim et al (2017) demonstrated that the seasonal variability of the warm layer was influenced by Ekman upwelling. More recently, Dotto et al (2020) analyzed multi-year mooring data from the DGT to determine the variability and cause of CDW inflowing onto the continental shelf. The authors concluded that the weakening of this inflow was a response to the strengthening of easterly winds at the shelf break, which is associated with the Amundsen Sea Low (ASL), a climatological low-pressure system located in the southern Pacific Ocean (e.g., Turner et al, 2017).…”

Seasonal variability of ocean circulation near the Dotson Ice Shelf, Antarctica

“…The driving mechanism behind seasonality of the heat transport into the DIS cavity is interpreted to be caused by local OSSC, induced by changes in sea ice and wind distribution, in turn linked to changes of the polynya area and sea ice distribution 40 . Previous studies have shown that the wind field and atmospheric circulation affects not only the seawater circulation at the ice shelf front but also the variability of warm water flowing into the DGT near the center of the continental shelf 10 , 11 , 23 , 41 . Also, local wind forcing in front of PIG has been attributed a significant modulator for the short-term (less than one month) variability of the upper thermocline depth and basal melting there 42 .…”