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2) Long-term trend in upper ocean temperature

by Webmaster Legos last modified Aug 27, 2020 11:43 AM


The 25-year Survostral time series has allowed us to better document the long-term temperature changes within different water-masses across our section, and how the temperature trend compares to the interannual variability (Auger et al., 2020). Three regions stand out as having strong change that is radically different from the interannual variability (Figure 2.1):
(A) warming of the deep-reaching subantarctic waters (0.29±0.09 °C/dec) in the northern part of our section, with similar values to those described by previous studies (Gille, 2008; Böning et al., 2008; Giglio and Johnson, 2017);
(B) cooling of the near-surface subpolar waters that extends down to 200 m depth (-0.07±0.04°C/dec), in a region where the long-term trend in SST also shows cooling over 25-year (Figure 5a), as described by Amour et al., (2016);
(C) significant warming of the subsurface subpolar deep waters (0.04±0.01°C/dec).

Our results highlight that this subsurface warming of subpolar circumpolar deep waters is, counter-intuitively, the most distinct change of the section with regard to interannual variability.

We observe larger warming in the upper part of the layer, which may be due to increased stratification at the base of the Winter Water layer due to surface freshening, that would reduce vertical mixing and heat removal from below (Marshall 2015, Armour et al. 2016). This robust warming of the circumpolar deep waters is associated with a large shallowing (39±11 m/dec), which has been significantly underestimated by a factor of 3 to 9 in past studies (Schmitdko et al., 2014). These Circumpolar Deep Water temperature changes are of comparable magnitude to those reported in West Antarctica (Rignot et al., 2019). This may have important consequences for warmer deep waters impinging onto the continental shelf downstream (Shen et al., 2018), with an impact for our understanding of future Antarctic ice-sheet mass loss.

These deep-reaching warming trends are associated with sea level rise, and the Southern Ocean south of Australia shows a fairly significant sea level rise during the 1990s (Lombard et al., 2006), and this increase is accelerating in the Southern Ocean in the last two decades (NASA). In an earlier study, Morrow et al. (2008) calculated the contribution of the deep-reaching temperature changes over the upper 800m from SURVOSTRAL data to the equivalent altimetric sea level variations here. A simple Sverdrup transport model showed how large-scale changes in the wind forcing, related to the Southern Annular Mode, may contribute to the deeper warming observed south of the SAF.


Figure 2.1: Top panel: Summer Reynolds SST Trends from 1993 to 2017 (NDJF). Black box indicated the region of SURVOSTRAL transects. Middle panel: Temperature trends from SURVOSTRAL XBT data. Hatched data represent zones where abs(Trends*25)/STD<1, i.e.. where the trends are smaller than the interannual variability. Bottom panel is the ratio between the trend signal and interannual variability. Auger et al. (2020)

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