In Situ Temperature Trends of the Antarctic Circumpolar Current South of South Africa

Abstract The Southern Ocean plays a critical role in climate change while it remains incompletely understood due to lack of observations. This work utilizes the unique collection of temperature measurements from eXpendable BathyThermographs (XBTs) deployed along repeated transects together with the available Argo profiles across the Southern Ocean south of South Africa to examine long‐term temperature changes in this region. Summertime temperature sections in the upper 800 m were constructed along the XBT transect using 2,833 XBT and 1,631 Argo profiles during the period 2004–2020. The results show different temperature changes north and south of the Polar Front (PF): (a) north of the PF: warming from 2004/2005 to 2011/2012 and cooling from 2011/2012 to 2019/2020 in the top 300 m, which are linked to the wind field changes associated with the Southern Annual Mode, and meridional shifts of the PF and the Subantarctic Front; (b) south of the PF: consistent warming in the Circumpolar Deep Water (CDW) below 200 m and cooling above in the surface layer during the whole study period. The warming trend in the CDW is small but significant compared with the interannual variability. These temperature changes south of the PF might be due to the reduced vertical mixing caused by the freshening of the surface water induced by the increased sea ice melting. These findings improve understanding of observed temperature changes in the south of South Africa and provide means to establish a connection across the Southern Ocean.