Variability and Remote Controls of the Warm‐Water Halo and Taylor Cap at Maud Rise

Abstract

The region of Maud Rise, a seamount in the Weddell Sea, is known for the occurrence of irregular polynya openings during the winter months. Hydrographic observations have shown the presence of a warmer water mass below the mixed layer along the seamount's flanks, commonly termed the warm‐water Halo, surrounding a colder region above the rise, the Taylor Cap. Here we use two observational data sets, an eddy‐permitting reanalysis product and regional high‐resolution simulations, to investigate the interannual variability of the Halo and Taylor Cap for the period 2007–2022. Observations include novel hydrographic profiles obtained in the Maud Rise area in January 2022, during the first SO‐CHIC cruise. It is demonstrated that the temperature of deep waters around Maud Rise exhibits strong interannual variability within the Halo and Taylor Cap, occasionally to such an extent that the two features become indistinguishable. A warming of deep waters by as much as 0.8°C is observed in the Taylor Cap during the years preceding the opening of a polynya in 2016 and 2017, starting in 2011. By analyzing regional simulations, we show that most of the observed variability in the Halo is forced remotely by advection of deep waters from the Weddell Gyre into the region surrounding Maud Rise. Our highest‐resolution simulation indicates that mesoscale eddies subsequently transfer the properties of the Halo's deep waters onto the Taylor Cap. The eddies responsible for such transfer originate in an abrupt retroflection along the inner flank of the Halo.

Ekman-driven salt transport as a key mechanism for open-ocean polynya formation at Maud Rise

“…Because positive salinity anomalies were associated with all simulated Maud Rise polynyas, local destratification via the remote advection of such anomalies was put forward as a possible mechanism for polynya initiation. Analysis of sparse observations available around Maud Rise confirms the presence of a large interannual variability in local circulation and stratification, in particular along the northern flank of Maud Rise ( 13 ).…”

“…9. In the years 2013–2015, the Taylor cap accumulated salt near the surface due to enhanced vertical mixing and enhanced eddy advection of salt anomalies from the flanks to the seamount, enabled by a stronger jet due to the spin-up of the Weddell Gyre ( 12 , 13 ). During polynya formation in 2017 over the Rise’s northern flank, there was a net buoyancy gain due to sea ice melt that compensated for the vertical mixing of salinity.…”

“…Furthermore, the gyre’s acceleration played a key role in the buildup of heat and salt over the Taylor cap. This process was driven by the heightened eddy energy in the area, enhancing the transfer of heat and salinity anomalies onto the seamount ( 13 ). Hence, it is inferred that large-scale dynamics are intricately linked to the localized mechanisms that instigated the polynya’s formation, demonstrating a coordinated interplay between these scales in the polynya’s development.…”

“…The Maud Rise is a subsurface seamount, the shallow bathymetry of which traps a column of cooler and denser waters, forming a Taylor cap surrounded by warm deep water (WDW) ( 10 ), which is derived from the CDW flowing via the Weddell Gyre into the Maud Rise region ( 11 ). This inflow, upon interacting with the topography, generates a halo of warm and saline WDW around the seamount ( 12 , 13 ) associated with a particularly weak stratification. This reduced stratification makes local conditions favorable for the formation of open-ocean polynyas ( 14 ), referred to as Maud Rise polynyas.…”

“…Maud Rise polynyas are preceded, up to 4 months in advance, by an anomalous sea ice thinning early in winter that is concurrent with enhanced cyclonic winds, reduced sea surface heights, and a stronger Weddell Gyre circulation ( 29 ). High-resolution models indicate that remote density anomalies can be advected onto the Maud Rise, affecting the stratification over the region ( 13 , 30 ). Because positive salinity anomalies were associated with all simulated Maud Rise polynyas, local destratification via the remote advection of such anomalies was put forward as a possible mechanism for polynya initiation.…”