Remote origins of interannual variability in the Indonesian Throughflow region from data and a global Parallel Ocean Program simulation

Abstract

The mean and interannual variability of the thermal structure of the World Ocean Circulation Experiment (WOCE) repeat IX1‐expendable bathythermograph (XBT) transect between Java and Western Australia were compared statistically for the years 1987–1997 with concurrent, co‐located output from a global eddy‐permitting configuration of the Parallel Ocean Program (POP) model forced with realistic surface fluxes. Dominant variability at long timescales for both model and data in the southern IX1 region was associated with Pacific El Niño–Southern Oscillation (ENSO) events; at the northern end it was due to remote equatorial Indian Ocean forcing and Indian Ocean Dipole Mode events. In the Indo‐Pacific domain the model reproduced the structure and magnitude of observed low‐frequency variability. Event analyses following the warm ENSO phase showed low‐frequency off‐equatorial Rossby waves interacting with the North Pacific western maritime boundary to reflect onto the equator and excite a coastally trapped response that propagated through the Indonesian seas and along the northwest coast of Australia. In turn, the signal progressively propagated away from this coast as free baroclinic Rossby waves to 90°E. Cross‐spectral analyses confirmed that on interannual timescales, both off‐equatorial and equatorial signals remotely forced in the Pacific were largely responsible for the strong observed and modeled variability at the southern end of IX1.