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1) Seasonal variability of heat content across the Southern Ocean at 140°E

by Webmaster Legos last modified Aug 27, 2020 10:53 AM


Our knowledge of heat content variability in the surface layer of the Southern Ocean has been greatly improved by SURVOSTRAL's XBT observations. In the Subantarctic Zone, between the Subtropical Front and the Subantarctic Front, SURVOSTRAL XBT data have been used to better understand variations in the summer mixed layer, and temperature variations in the underlying winter layer (Trull et al., 2001; Rintoul and Trull, 2001; Chaigneau 2003; Chaigneau et al., 2004). The impact of these surface variations on the characteristics of sub-Antarctic modal waters was also analysed using SURVOSTRAL observations (Herraiz-Borreguero et al., 2010).

More recent analyses of the monthly mean temperature structure based on the 25-years of Survostral data by Auger et al. (2020) have revealed the stability of the seasonal cycle described by Chaigneau et al. (2004) based on only 8 years of data. The monthly mean temperature sections from October to March (Figure 1.1) calculated from the 25-year time series are quite similar to those calculated by Chaigneau et al. (2004). The water masses with the strongest seasonal changes are at the surface: the Antarctic Surface Waters (AASW) south of the Polar Front show the largest monthly mean variations over the summer warming cycle with coolest waters observed at the end of winter in late Oct-Nov. In the north of the section, there is a seasonal southward and deepening expansion of Subtropical waters throughout the summer season.



Figure 1.1. Monthly mean temperature values over the upper 800 m depth between Hobart (43°S) and Dumont D’Urville (66°S) based on SURVOSTRAL XBT data over the period 1993-2017

In 2004, comparisons with the Levitus T-S climatology in this pre-Argo era highlighted that the smoothing in the climatology was much too strong in this area, the seasonal cycle was poorly represented in surface salinity and in mixed layer depth (Chaigneau et al., 2004). Since 2005, there has been a large increase in the number of T/S profiles available from the Argo network over the upper 2 km, and daily and weekly gridded data sets are now available at 0.5° resolution, such as the ISAS/CORA gridded data from Coriolis (Cabanes et al., 2013). Although the ISAS gridded product includes the SURVOSTRAL XBT data, it is smoothed over 300 km in radius, which reduces the sharp frontal structures, compared to the Survostral gridded product with much less smoothing  (0.5° resolution increasing to 0.25° resolution across the fronts from 49-54°S; Auger, 2019; Figure 1.2). Maintaining the highest resolution along the Survostral repeat transects is valuable for studying the fine-scale frontal processes. 


Figure 1.2. Monthly mean section along the SURVOSTRAL line from a) the ISAS gridded product, and b) the SURVOSTRAL gridded XBT data. Note the sharper fronts in the SURVOSTRAL high-resolution gridded data.

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