Seasonal and Interannual Variability in Cross-Indian ITF Plume Propagation

The Indonesian throughflow (ITF) entering the eastern tropical Indian Ocean spreads westward as a low salinity plume within the thermocline, centered at around 11°S. This feature is not static, varying in shape, latitudinal range, and westward extent. Using gridded Argo datasets (2005-2018) and SODA reanalysis (1980-2015) we investigate the variability in this plume at seasonal and interannual timescales by examining thermocline salinity, along the 24σ surface, within the Indian basin from the equator to 20°S. Salinity contours 34.6 and 34.7, in the eastern section of the basin, show the furthest propagation during austral spring to early summer and attenuated westward flow during fall and winter, with the difference between minimum and maximum seasonal propagation at the plume axis of 10° of longitude. A shifting of the seasonal cycle occurs in salinity contours further to the west. The 34.8 salinity contour reaches maximum propagation in late austral summer to early fall, and the 34.9 and 35.0 contours see maximums during the austral fall and late austral fall to winter, respectively. The two furthest west contours also display a distinctive southward shift during maximum propagation (5° latitude further south compared to seasonal minimum). The westward portion of the ITF plume (west of 90°E) is affected by the development of the Seychelles Chagos Thermocline Ridge (SCTR). In this region maximum propagation, and southward shift of salinity contours occurs during the seasonal growth and strengthening of the SCTR. In addition to the seasonal cycle of the ITF plume interannual variability is also examined. Here we focus on two end-members of plume propagation identified using SODA. Anomalously large westward propagation is identified during 1998 when the 34.8 salinity contour traversed the Indian Ocean and was present immediately north of Madagascar at 50°E. During 2011, this same salinity contour remained east of 80°E and the western Indian Ocean was anomalously salty. The mechanisms controlling ITF plume changes are investigated, including associated variability in the SCTR and the South Equatorial and Leeuwin Currents. Quantifying the changes in cross-Indian propagation of the ITF is critical to our understanding of how, and whether, this watermass reaches the Agulhas Current system of the western Indian Ocean.