Changes of Circumpolar Deep Water between 2006 and 2020 in the south-west Indian Ocean, East Antarctica

Abstract

The climatological distribution of circumpolar deep water (CDW) and its changes in austral summer between 2006 and 2020 are documented using historical hydrographic data and recent measurements in the south-west Indian Ocean, East Antarctica. The CDW is primarily distributed in the northeast of the Cooperation Sea, north of the southern Antarctic Circumpolar Current Front (sACCF), as shown by the CDW thickness (on average > 800 m) and mean depth (maxima >900 m). The CDW layer becomes thinner to the south of sACCF and is less than 100 m thick near the Southern Boundary (SB). The CDW distribution shows significant changes between years. Its extension reached further south in 2020 than in 2006, and there was a positive thickness anomaly with a magnitude of 50–200 m. There was also evidence of warmer (0.1–0.2 °C) and more saline (0.005) CDW in 2020 than 2006 along 70°E and between 62.8° and 63.7°S. Those warm and saline signals extended further south. Warmer and more saline water occupied the subsurface layers between 35° and 69°E at 63.6°S and 64°S, and occurred to the east of 55°E. The region between 55° and 65°E is a major entrance for the CDW poleward extension. Furthermore, we found that the changes of CDW between 2006 and 2020 were dominated by the sACCF shift due to enhanced northwest summer winds. The wind anomalies raised the sea surface height by the strengthening northward Ekman transport. Thus the ACC shifted southward (∼120 km in average), carrying more CDW poleward. A tongue-shaped southward shift of sACCF generated anomalous anticyclonic eddies, advecting CDW southward as far as 66° in 2020.