Makassar Strait Throughflow Seasonal and Interannual Variability: An Overview

The Makassar Strait throughflow of similar to 12-13 Sv, representing similar to 77% of the total Indonesian Throughflow, displays fluctuations over a broad range of time scales, from intraseasonal to seasonal (monsoonal) and interannual scales. We now have 13.3 years of Makassar throughflow observations: November 1996 to early July 1998; January 2004 to August 2011; and August 2013 to August 2017. Strong southward transport is evident during boreal summer, modulated by an ENSO interannual signal, with weaker southward flow and a deeper subsurface velocity maximum during El Nino; stronger southward flow with a shallower velocity maximum during La Nina. Accordingly, the southward heat flux, a product of the along-channel current and temperature profiles, is significantly larger in summer and slightly larger during La Nina. The southward flow relaxed in 2014 and more so in 2015/2016, similar though not as extreme as during the strong El Nino event of 1997. In 2017, the throughflow increased to similar to 20 Sv. Since 2016, the deep layer, 300- to 760-m southward transport increases, almost doubling to similar to 7.5 Sv. From mid-2016 into early 2017, the transports above 300 m and below 300 m are about equal, whereas previously, the ratio was about 2.7:1. Near zero or northward flow occurs in the upper 100 m during boreal winter, albeit with interannual variability. Particularly strong winter reversals were observed in 2014/2015 and 2016/2017, the latter being the strongest winter reversal revealed in the entire Makassar time series. Plain Language Summary Pacific water flows into the Indian Ocean within the passages and basins of the Indonesian Seas: the Indonesian Throughflow (ITF), driven by the pressure gradient between the Pacific and Indian Oceans. The ITF affects heat and freshwater inventories of the Pacific and Indian Oceans. The primary inflow path of Pacific water into the Indonesian seas is the Makassar Strait, channeling 12.5 x 10(6) m(3)/s, about 77% of the total ITF. During the 21 years, November 1996 to August 2017, we have recorded 13.3 years of Makassar throughflow, exposing a broad range of spatial and temporal scale patterns. The Makassar Strait annual cycle transport ranges from about 7 to 16 x 10(6) m(3)/s. Strong southward transport occurs during boreal summer, with weaker (stronger) southward flow and a deeper (shallower) subsurface velocity maximum during El Nino (La Nina). The throughflow relaxed in 2015 into 2016 during an El Nino. In 2017, the southward transport increased to 20 x 10(6) m(3)/s. The 300- to 760-m transport increased in 2016 into 2017, equaling the 0- to 300-m transport, from the more typical 30% of the 0- to 300-m transport. Near zero flow occurs in the upper similar to 100 m during boreal winter with particularly strong winter reversals in 2014/2015 and 2016/2017.