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3) Interannual variability of temperature and heat content in the Southern Ocean

by Webmaster Legos last modified Aug 27, 2020 10:46 AM


Auger et al (2021) identified the water masses across the SURVOSTRAL line where the interannual variability is strong, and dominates the trend (science result 2). Figure 3.1 shows the temporal evolution of the summer mean NDJF temperature anomalies over 25 years, for 3 key regions :

In zone (A) in the Subantarctic Zone, strong interannual variations exist in the heat and haline content (Sokolov and Rintoul, 2003; Morrow and Kestenare, 2014), linked to movements of the Subtropical Front. These variations also respond to changes in Mode Water volume in the area, modified by interannual variations in the input of warmer, saltier water from the Tasman Outflow - a southward extension of the East Australian Current. Altimetry tracking of eddies has allowed us to identify years when more warm-core eddies propagating from the north cross the Survostral line (2001, 2002, ..), or years when more cold-core eddies spawned from the SAF are observed by the XBT profiles (1994-1996) (Morrow et al., 2004; Morrow and Kestenare, 2014; Pilo et al., 2015). The large input of heat from the north in 2000-2001 follows the strong La Nina events in the southwest Pacific (Morrow and Kestenare, 2014; Pilo et al., 2015), and La Nina conditions are more prevalent over the last decade (2007-2017).

In zone (B) close to the Antarctic continent, there is a cooling trend in the upper 200 m with large interannual variations. The stronger interannual variability is evident in temperature, SSS and sea-ice, with cooler, low salinity waters in summer linked to larger sea-ice content in spring (Auger et al., 2021; Morrow and Kestenare, 2017). Lower temperatures from 2011 onwards are impacted by local coastal circulation changes and increased ice flow, following the Mertz Glacier calving just upstream. Morrow and Kestenare (2017) proposed that the cooling and freshening of the subpolar AASW waters near 140°E from 1999 onwards, may be impacted by meridional shifts of the zonal wind position and an associated change in the sea-ice cover.

The generalized warming trend of the Upper Circumpolar Deep Waters in zone (C) shows little interannual variation, being below the seasonal mixed layer.


Figure 3.1 : The evolution (black line) and trend (red line) of the temperature anomalies within zones [A]: Subantarctic Zone, [B] close to the Antarctic continent and [C] for Circumpolar Deep Waters, respectively. Green line is the NDJF SST Reynolds interpolated onto the SURVOSTRAL line for each surface zone. From Auger et al., 2021. 

Antarctic Surface Water and Winter Water interannual variability.
The Antarctic Surface Water (AASW) region from 53-62°S has strong interannual variability over the surface mixed layer and the Winter Water layer (Chaigneau et al., 2004; Auger et al., 2021);
In the core of the Winter Water layer, defined as the layer with temperature colder than 2°C between 55°S and 61.5°S, the temperature undergoes large interannual variations that overwhelm any long-term change, with peak-to peak temperature ranging from 0.92 to 1.28°C (Auger et al., 2021). These temperature variations within the Winter Water core along the Survostral line near 140°E are positively correlated (r=0.68) with the sea surface temperature of the previous winter further upstream in the subpolar Australian-Antarctic basin (120-145°E; 57-61°S), where the Winter Waters were modified at the surface (altimetric lagrangian backtracking was used to identify when the sub-surface winter water layer was last in contact with the atmosphere).



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