Tropical Ocean to Southern Ocean coupling: Indonesian Throughflow and Weddell Sea Bottom Water

Abstract:

Tropical Pacific water cascades through the complex geography of the Maritime Continent into the Indian Ocean, Indonesian Throughflow (ITF), forming a key component of the Interocean Exchange ‘conveyor belt’. In a far colder part of the ocean, dense water sinks along the rim of Antarctica as Antarctic Bottom Water (AABW) that spreads along the sea floor well into the Northern Hemisphere, forming the Southern Ocean Meridional Overturning Cell. A major component of AABW is derived from the Weddell Sea, Weddell Sea Bottom Water (WSBW). Are the hot ITF and cold WSBW related? Decadal long timeseries into the 21^st century of the Makassar Strait Throughflow (MST; ~80% of the ITF) and of WSBW export reveal that in addition to seasonal variability there is pronounced interannual variability. The MST relaxed during the strong 2015 El Niño event, rebounding in 2016 into 2017, a significant part of the changes was associated with the transport below 300 m, which almost doubling, leading to nearly equal transport above and below 300 m, which usually have a ratio of 3:1. WSBW also revealed significant change in 2015 into 2016, marked by an anomalous salinity decrease, induced by injection of low salinity winter open ocean surface water, rather than to changes in the shelf water characteristics. The change is likely a consequence of wind stress curl over the Weddell Gyre. A major factor in the coupling of the ITF and WSBW may have been the synchronous occurrence of a strong positive Southern Annular Mode (SAM) and positive nino3.4: in 2015-2016, which affected the tropical and Southern Ocean wind fields.