30 years of sea surface temperature and salinity observations crossing the Southern Ocean near 140°E: Trends and rollercoaster variability

Abstract

Observations of high-resolution sea surface temperature, salinity and density are analysed over 30 years along a transect crossing the Southern Ocean from Tasmania to Antarctica near 140°E during austral summer. Three dynamical regions are investigated in fixed latitude bands. The Subtropical and Subantarctic Zones show a surface warming trend of 0.33 °C/decade in accord with previous studies, and surface salinity has increased by 0.05/decade. This trend towards warmer, saltier surface waters is mostly density compensated; the large trend dominates over the interannual variability. The increased presence of warm, salty subtropical waters is linked to the poleward shift in the Subtropical Front which is now located 160 km further south than in the early 1990s, whereas the Subantarctic Front and Polar Front positions remain stable. This reduces the effective summer surface area of the Subantarctic Zone by one-third over 30 years. Interannual changes in the influx of subtropical waters across 140°E are linked to the main climate modes. Austral summers having warmer, saltier conditions occur during the positive Southern Annular Mode (SAM) and La Nina phases; cooler fresher conditions occur during negative SAM and El Nino. In the Antarctic Zone south of the Polar Front, the freshening trend reported over the 1990s and 2000s has stabilized. The mean austral SSS increased slightly over the last decade, and the 30-year trend in surface salinity is now zero. The Sea-Ice Zone south of 60°S around 140°E was largely impacted by the Mertz Glacier calving in 2010, releasing a large volume of sea-ice over the following years. Interannual changes in summer surface salinity are anti-correlated with the available sea-ice the previous spring. Over the 30-year period, the region south of 60°S has freshened by 0.012 per decade, and cooled by 0.06 °C per decade. Coastal waters off Antarctica show the largest impact: here surface waters have freshened by 0.1 over three decades, even as the summer sea-ice concentration has been reduced by 2.5 %. The Subantarctic Zone and the Sea-Ice Zone exhibit a rollercoaster of large interannual changes since 2017, and continued monitoring is required to understand the long-term effects of this recent increased variability.