Title: Indonesian Throughflow Heat Transport, and Spreading within the Eastern Tropical Indian Ocean
Description: The Indonesian Throughflow (ITF) is the only low latitude connector between the Pacific and Indian Oceans affecting upper ocean stratification and regional climate. Here we focus on the Indian Ocean side of this connection, first identifying changes within the primary throughflow pathway within the Indonesian Seas, then following the throughflow as it moves within the eastern tropical Indian Ocean. Moored velocity measurements and an ENSO varying temperature profile developed from all available observations within the Makassar Strait are used to determine the southward heat flux anomaly (HFa) within this primary pathway of the ITF. Variability in the velocity profile is more important than that of the temperature profile for determining changes in the total heat flux with the former accounting for 72% of the variance in HFa and the latter 28%. As the upper layer (0-300 m) is the site of the largest volume transports and also the largest transport variability, upper layer HFa is far more dominant than the lower (320-740 m) in influencing the total depth integrated HFa.

Upper ocean heat content anomaly (0-300 m; HCa) in the eastern tropical Indian Ocean calculated from gridded Argo datasets is well correlated with Makassar HFa at interannual timescales (r = 0.8). The lag between the two is 2.5 years, indicating that this is consistent with an advective signal. From the Indo-Australian basin ITF waters flow either into the South Equatorial Current (SEC) to the west or the Leeuwin Current (LC) to the south. Gridded Argo data is used to track upper ocean heat content changes from the immediate outflow area into these two currents. The heat content anomaly timeseries in the region closest to the Indonesian Seas is well correlated with that at the easternmost section of the SEC with r = 0.8 at a 5 month lag. A notable exception occurs during 2011 when a positive heat content anomaly in the ITF outflow region is not later reflected in the SEC region, but rather expressed as an HCa increase the LC region. When compared to a previous HCa increase in the ITF outflow region during 2009, GODAS reanalysis shows that the velocity within the SEC was stronger eastward and the LC stronger southward during 2011.

The Ningaloo Niño of 2011 was characterized by a low pressure anomaly off the west Australian Coast, which induced anomalous cyclonic circulation seen in NCEP/NCAR reanalysis winds at 1000 HPa. The positive zonal wind anomalies over the SEC and the reduction of southerly winds over the LC influenced these changes in current velocity. During the Ningaloo Niño of 2000 a similar pattern in atmospheric and oceanic circulation was identified. These results confirm the importance of the Ningaloo Niño in influencing the pathways of the ITF out of the Indo-Australian basin. Additionally, over the Argo time period, volume transport via the LC and SEC pathways appears anti-correlated, with increases in SEC outflow coupled with decreases in LC outflow. As the SEC is the major pathway for the ITF within the Indian Ocean, we examine the propagation of these low salinity waters within the SEC thermocline. Using gridded Argo data, we examine the salinity along the 24σ surface as a proxy for ITF propagation, and the depth of the 20°C isotherm (d20) to determine how changes in the thermocline depth may affect the flow.

The d20 was correlated with the salinity (r=-0.5) in the region of the Seychelles Chagos Thermocline Ridge (SCTR), indicating that this region of upwelling, and the geostrophic currents that form around it, play a role in the westward propagation of the ITF. When examining the seasonal cycle, the effect of the SCTR is apparent as low salinity contours within the western portion of the basin show the furthest westward propagation during austral winter, when the SCTR is strong and most longitudinally expansive. On interannual timescales two years, 2010/11 and 2016/17, show anomalously high salinity in the SEC thermocline indicative of a reduction of ITF westward propagation. During late 2010 and 2016 anomalously strong upwelling regions are present at about 80°E and 10°S, out of the normal season for strong upwelling at this location. GODAS reanalysis velocity at 105 m shows cyclonic circulation developed around these upwelling centers, disrupting the normal zonal pathway of the SEC and reducing the amount of ITF able to propagate into the central Indian.

As seen in the 34.8 salinity contour, both 2011 and 2017 show a reduction of 20 degrees of longitude of ITF westward propagation when compared to climatology. These upwelling regions were caused by both regional winds conducive to Ekman upwelling at that location, in addition to the absence of the annual westward propagating downwelling Rossby wave. This wave was absent during both late 2010 and 2016 due to positive zonal wind anomalies in the south east tropical Indian Ocean caused by a simultaneous occurrence of La Niña and a negative IOD.