Variability in Makassar Strait heat flux and its effect on the eastern tropical Indian Ocean
The heat flux anomaly (HFa) within Makassar Strait, Indonesia, is investigated using observed velocity time series and El Niño-Southern Oscillation (ENSO)- scaled temperature profiles. Direct measurements of velocity from 40 m to 740 m depth were recorded during 2004–2011 and 2013–2017 during the International Nusantara STratification ANd Transport (INSTANT) and Monitoring Indonesian Throughflow (MITF) programs. The velocity profile is thermocline-intensified, with a velocity maximum near 100 m depth. In situ temperature measurements during 2004–2006 and ship-based CTD profiles collected during these two monitoring campaigns were combined with all available World Ocean Database CTD, ocean station, mechanical bathythermograph, and expendable bathythermograph data collected within Makassar Strait since 1950 to create representative temperature profiles for positive, negative, and neutral phases of ENSO. The Makassar velocity profile displays a stronger (weaker), shallower (deeper) velocity maximum, and a deeper (shallower) thermocline during La Niña (El Niño). Southward Makassar HFa increases rapidly from 2006 to 2008, with a peak of 0.13 PW in 2008 and 2009. Afterward, Makassar HFa slowly decreases to a minimum of −0.25 PW (less southward) during 2015, after which southward heat flux begins to climb again. Variability in depth-integrated volume transport from the surface to 740 m depth explains 57% of HFa variance. However, the total volume transport does not reflect the relative contributions of the warm upper and cool lower layers. Changes in the depth-dependent velocity profile explain 72% of HFa variance, whereas temperature profile variability explains only 28%. The impact of Makassar HFa variability on the Indian Ocean is assessed through comparison with the heat content anomaly (HCa) in an eastern tropical Indian Ocean box (ETIO; 101.5°E–105.5°E, and 9.5°S–15.5°S) using gridded Argo data. The ETIO HCa follows a similar pattern (R = 0.83) when lagged 30 months behind the Makassar HFa. Although well correlated, a notable discrepancy between the two time series is present in the ETIO in 2012/2013, possibly owing to a shift of the ITF from the dominant South Equatorial Current pathway to a southward Leeuwin track.
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