Makassar Strait Throughflow Seasonal and Interannual Variability: An Overview

Abstract

The Makassar Strait throughflow of ~12–13 Sv, representing ~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 Niño; stronger southward flow with a shallower velocity maximum during La Niña. 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 Niña. The southward flow relaxed in 2014 and more so in 2015/2016, similar though not as extreme as during the strong El Niño event of 1997. In 2017, the throughflow increased to ~20 Sv. Since 2016, the deep layer, 300‐ to 760‐m southward transport increases, almost doubling 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.

Unprecedented Response of Indonesian Throughflow to Anomalous Indo‐Pacific Climatic Forcing in 2016

“…To help simplify the analysis, we divide the water column in Makassar Strait across which the ITF transport was observed into two layers, surface to 300 m (upper layer) and 300–760 m (lower layer). The water column partition used here is consistent with that applied by Gordon et al (), in which the depth demarcating the two layers corresponds with the core depth of salinity minimum attributed to the North Pacific Intermediate Water observed in Makassar Strait (Ilahude & Gordon, ). We will hereafter, respectively, refer the layer between the surface and 300 m and 300 m and 760 m as the upper layer and the lower layer.…”

“…Our observations indicate that about 72 % of the variance of ITF transport anomalies exhibit a two‐layer vertical structure, the upper and lower layers, with a 10°C isotherm demarcating the upper from the lower (Gordon et al, ). The transport anomalies in both layers generally demonstrate an opposite sign, implying a decrease in ITF transport in the upper layer coincides with an increase in the lower layer and vice versa.…”

“…The remaining 20 % of the variance of ITF transport anomalies exhibit a vertical structure with a uniform sign across the upper 760 m. Gordon et al () reported a historical high of total ITF transport in Makassar Strait, amounting of 20 Sv, in the first half 2017, coinciding with increased negative transport anomalies across depth, particularly in the lower layer. Underlying physical processes explaining the high total transport in 2017 remains to be investigated (Gordon et al, ).…”

“…In general, the ITF seasonal cycle is in agreement with that derived from shorter time series in Makassar Strait (Gordon et al, ; Susanto et al, ), inferred from the satellite‐based proxy transport (Sprintall & Revelard, ) and estimated by model‐based studies (Masumoto & Yamagata, ; Potemra & Lukas, ; Shinoda et al, ). Readers are referred to Gordon et al () for details of the seasonal cycle of ITF transport elucidated from the long‐term measurements in Makassar Strait.…”

The Madden‐Julian Oscillation's Impact on the Makassar Strait Surface Layer Transport

“…In contrast, the opposite southeasterly winds prevailing over the course of the southeast monsoon through the months of June–September (henceforth referred to JJAS) remove the buoyant SCS water from the southern Makassar Strait (Figure , right panel), allowing the ITF to reach its maximum transport (Gordon et al, , , ). Thus, the upper layer ITF generally attains its minimum (maximum) transport through DJFM (JJAS).…”