A Strong Sub-Thermocline Intrusion of the North Equatorial Subsurface Current Into the Makassar Strait in 2016-2017

The poorly resolved pathway of the sub-thermocline waters (>300 m) within the Makassar Strait, the primary inflow path of the Indonesian Throughflow, is investigated using in situ mooring measurements and Argo profiles at the entrance to the Indonesian Seas. We focus on the strong sub-thermocline intrusion in the summer of 2016, when significant changes of sub-thermocline transport thrice as large as the interannual standard deviation occurred. Analysis suggests that the intrusion was drawn from the North Equatorial Subsurface Current (NESC) flowing westward below the North Equatorial Countercurrent, which was composed of a mixture of intermediate-depth waters from both the North and South Pacific. The anomalously strong NESC to the Makassar sub-thermocline in the summer 2016 is suggested to be in response to the 2015/2016 extreme El Nino event, forced by the trade wind anomalies over the western-central Pacific Ocean through westward and downward propagating baroclinic Rossby waves. Plain Language Summary The transfer of waters from the tropical Pacific to the Indian Ocean via the Indonesian Throughflow (ITF), the majority of which flows through the Makassar Strait, plays an important role in global ocean circulation and climate variations. Here, we identify the sources of the sub-thermocline Makassar Strait waters by comparing their water mass properties in the Makassar Strait with those in the gateway region using ship-based and Argo Conductivity-Temperature-Depth data. Traditional understanding of the ITF source water in the Makassar Strait is that it is drawn from the western boundary currents of the tropical Pacific Ocean. Our finding identifies a new zonal pathway for the equatorial waters entering the ITF in the sub-thermocline layer (>300 m) in the summer of 2016, following the strong 2015/16 El Nino event. These results are anticipated to be the beginning of more comprehensive investigations of the Pacific-Indian Ocean sub-thermocline connection.

Automated summary

The study investigates the poorly resolved pathway of sub-thermocline waters in the Makassar Strait, the main inflow path of the Indonesian Throughflow, using in situ mooring measurements and Argo profiles. The anomalously strong sub-thermocline intrusion in the summer of 2016 is suggested to be linked to the extreme El Nino event of 2015/2016.