Abstract
Some of the possible interannual to interdecadal variabilities of the Indonesian Throughflow (ITF) source water pathways in the Pacific Ocean are identified from an ocean reanalysis product provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) under the name Ocean Reanalysis, version S3 (ORA-S3). The data were used in an offline mode to evolve adjoint pathways of a passive tracer, which is injected from the key channels of the Indonesian straits where the ITF enters into the Indian Ocean. The adjoint pathways are simulated using interannually varying circulations for 41 yr starting from December 2000 to January 1960 with reversed currents and other physical parameters (control run). A climatological run for the 41 yr is produced with the reversed currents and other physical parameters as a monthly climatology. The adjoint pathway variability is found by subtracting the climatological run from the control run. The empirical orthogonal function (EOF) analysis carried out over the monthly differences between the tracer concentrations of the control run and the climatological run shows that the ITF is largely supplied from the northwestern tropical Pacific during a normal year, whereas the supply from the south equatorial Pacific is dominant during El Niño–Southern Oscillation (ENSO) years at a lag of 6 months. The interannual variability of the ITF source water pathways in the Pacific is largely determined by the ENSO variability and they are confined to the tropical Pacific, whereas the corresponding interdecadal variability is controlled by the meridional overturning circulations in the tropical and subtropical Pacific. The adjoint pathways hint that the ITF volume transport may have interdecadal variability; they are closely related to the variability of the subtropical cells (STCs) in the Pacific Ocean and can be quantified using the tropical convergence changes. The ITF is just an active member of the recharge–discharge of tropical warm waters at all time scales, and its role in the coupled climate variability of the Indo-Pacific needs to be assessed in that context.
