Possible interannual to interdecadal variabilities of the Indonesian throughflow water pathways in the Indian Ocean

Abstract

A few possible interannual to interdecadal pathways of the Indonesian throughflow (ITF) water in the Indian Ocean are found from ocean reanalysis data provided by the European Centre for Medium‐Range Weather Forecasts under the name Ocean Re‐Analysis version S3 (ORA‐S3). The data was used in an off‐line mode to evolve a prognostic passive tracer. The tracer sources were prescribed at the ITF entrance channels and the spreading of the ITF in the Indian Ocean were tracked using interannually varying circulations for two separate episodes of 21 years spanning from 1960 to 1980 and from 1980 to 2000. A climatological run of 21 years was carried out for each time period and the differences in tracer spreading between the control and the climatological runs were computed. The empirical orthogonal function (EOF) analysis shows that the dominant mode of the interannual ITF pathway variability in the Indian Ocean is focused on the ITF entrance region into the southeastern tropical Indian Ocean. The first three EOFs collectively explain a total of 50% of the interannual pathway variability out of which EOF‐1 and 2 are significantly correlated with the Indian Ocean Dipole/Zonal Mode (IODZM). The IODZM‐related meridional circulation anomalies cause the ITF axis from the entrance region to be relatively closer to 10°S than the climatological axis which is shifted southward to 15°S∼20°S. The ITF pathways in the Indian Ocean show decadal shifts in the axis with a more direct departure from the entrance region to the western Indian Ocean prior to 1980, whereas a southwestward shift, subduction, and northwestward thermocline trajectories from the entrance region is seen after 1980. The dramatic shift in the surface meridional velocities in the ITF entrance region revealed from the ORA‐S3 data is a potential cause for the interdecadal changes in ITF pathways. The interannual pathways retrieved in this study thus complement the mean pathways of the ITF that were reported in previous studies. The variability in the pathways we found has implications for the crucial region of the southwestern thermocline ridge and the associated ocean‐atmosphere response.