Remotely induced warming of Antarctic Bottom Water in the eastern Weddell gyre

Abstract

Four repeat hydrographic sections across the eastern Weddell gyre at 30°E reveal a warming (by ~0.1°C) and lightening (by ~0.02–0.03 kg m−3) of the Antarctic Bottom Water (AABW) entering the gyre from the Indian sector of the Southern Ocean between the mid‐1990s and late 2000s. Historical hydrographic and altimetric measurements in the region suggest that the most likely explanation for the change is increased entrainment of warmer mid‐depth Circumpolar Deep Water by cascading shelf water plumes close to Cape Darnley, where the Indian‐sourced AABW entering the Weddell gyre from the east is ventilated. This change in entrainment is associated with a concurrent southward shift of the Antarctic Circumpolar Current's (ACC) southern boundary in the region. This mechanism of AABW warming may affect wherever the ACC flows close to Antarctica.

Distributions, trends and inter-annual variability of nutrients along a repeat section through the Weddell Sea (1996–2011)

“…3c; at 3500m, 15ºW) suggests that the processes determining the silicate concentration in Prydz Bay and surroundings have not significantly changed, i.e., apparently no enhanced upwelling in the gyre circulation of Prydz Bay (Smith et al, 1984) like in the Weddell Gyre. Couldrey et al (2013) did find changes (i.e., warming) in this water mass during the time period of our observations, and attributed this to enhanced entrainment of CDW. This would potentially cause changes (likely an increase) in silicate concentration in the core, but apparently these are not sufficiently large (or have been attenuated underway) to be observed in the Weddell Gyre.…”

The contribution of the Weddell Gyre to the lower limb of the Global Overturning Circulation

“…Some of this AABW (WSBW) must then upwell diapycnally within the gyre to be exported to the midlatitudes as WSDW. The occurrence of a significant influx of Indian‐sourced AABW to the Weddell Gyre has been reported in several transient tracer‐based investigations [ Archambeau et al ., ; Meredith et al ., ; Hoppema et al ., ] and in a numerical modeling study [ Schodlok et al ., ], and its formation traced to the Prydz Bay/Cape Darnley polynya region [see Couldrey et al ., ; Ohshima et al ., ].…”

“…[] suggest that AABW may be exported from the gyre across a wider zonal swath than previously thought, including the major topographic barrier of the South Scotia Ridge. A most unexpected finding in this context relates to the observation of a prominent flow of AABW from the Indian Ocean sector entering the southern Weddell Gyre across its eastern rim [ Meredith et al ., ; Hoppema et al ., ; Couldrey et al ., ], which has led to the (as yet untested) proposition that the role of the gyre in AABW formation has been historically overstated [ Jacobs , ].…”

Freshwater fluxes in the Weddell Gyre: results from δ 18 O

“…This water mass flows from the ACC into the Weddell Gyre at its eastern edge before recirculating as Warm Deep Water (WDW) and is the source of all water masses south of the SB [42]. A substantial input into the gyre occurs at the Antarctic Slope Front (ASF) at 30 • E, which separates the colder, fresher continental shelf waters from the warmer, saltier waters to the north, and which flows into the gyre transporting relatively recently ventilated varieties of AABW from further east [43][44][45]. Export from the gyre is concentrated at the Weddell Front (WF), which is roughly located at the eastern end of the SSR, and marks the northern limb of the gyre, separating colder WDW to the south from warmer CDW to the north [46,47].…”