Eddy-mean flow interactions along the Kuroshio Extension (KE) jet are investigated using a vorticity budget of a high-resolution ocean model simulation, averaged over a 13-yr period. The simulation explicitly resolves mesoscale eddies in the KE and is forced with air-sea fluxes representing the years 1995-2007. A mean-eddy decomposition in a jet-following coordinate system removes the variability of the jet path from the eddy components of velocity; thus, eddy kinetic energy in the jet reference frame is substantially lower than in geographic coordinates and exhibits a cross-jet asymmetry that is consistent with the baroclinic instability criterion of the long-term mean field. The vorticity budget is computed in both geographic (i.e., Eulerian) and jet reference frames; the jet frame budget reveals several patterns of eddy forcing that are largely attributed to varicose modes of variability. Eddies tend to diffuse the relative vorticity minima/maxima that flank the jet, removing momentum from the fast-moving jet core and reinforcing the quasi-permanent meridional meanders in the mean jet. A pattern associated with the vertical stretching of relative vorticity in eddies indicates a deceleration (acceleration) of the jet coincident with northward (southward) quasi-permanent meanders. Eddy relative vorticity advection outside of the eastward jet core is balanced mostly by vertical stretching of the mean flow, which through baroclinic adjustment helps to drive the flanking recirculation gyres. The jet frame vorticity budget presents a well-defined picture of eddy activity, illustrating along-jet variations in eddy-mean flow interaction that may have implications for the jet's dynamics and cross-frontal tracer fluxes.

Anomalous sea surface temperature (SST) cooling south of Java, initiated during May–July, is an important precursor to positive Indian Ocean Dipole (pIOD) events. As shown previously, the Java SST anomalies are spatially and temporally coincident with seasonal upwelling induced locally by southeasterly trade winds. However, we confirm earlier findings that interannual variability of the Java cooling is primarily driven by remote wind forcing from coastal Sumatra and the equatorial Indian Ocean (EqIO); we also find an influence from winds along the Indonesian Throughflow. The wind forcing in the EqIO and along coastal Sumatra does not initiate SST cooling locally due to a deep thermocline and thick barrier layer, but can force upwelling Kelvin waves that induce substantial surface cooling once they reach the seasonally shallower thermocline near the coast of Java. Satellite altimetry is used to obtain a Kelvin wave coefficient that approximates Kelvin wave amplitude variations along the equator. All pIOD years in the satellite record have anomalous levels of upwelling Kelvin wave activity along the equator during April–June, suggesting that upwelling waves during this season are necessary for pIOD event development. However, a change to wind‐forced downwelling Kelvin waves during July–August can abruptly terminate cool Java SST anomalies and weaken the pIOD event. Upwelling Kelvin wave activity along the equator and wind stress anomalies west of Sumatra are both robust predictors of the IOD index later in the calendar year, while values of the Kelvin wave coefficient are the most reliable predictor of pIOD events specifically.

Negative sea surface temperature (SST) anomalies associated with positive Indian Ocean Dipole (pIOD) events first appear in the seasonal upwelling zone along the southern coast of Java during May–July. The evolution of anomalous SSTs in this coastal region is analyzed by computing a temperature budget using output from a strongly eddy‐active ocean general circulation model. The seasonal cooling south of Java in May–July is driven by a reduction in incoming shortwave radiation and by vertical mixing, consistent with earlier studies in the region; however, the model budget also shows an advective contribution that drives anomalous cooling at the onset of pIOD events. To identify which process(es) are responsible for the anomalous advective cooling during pIOD events, a novel process index regression method is used to estimate the contributions of wind stress, equatorial Kelvin waves, mesoscale eddies, and Lombok Strait flow to anomalous cooling south of Java. Using this method, wind stress forcing along the west coast of Sumatra is found to make the most substantial contribution to anomalous cooling south of Java, with lesser contributions from equatorially sourced Kelvin waves and local wind stress. Mesoscale eddies redistribute heat from the Lombok Strait outflow, and have an anomalous warming effect on the eastern side of the upwelling region. The process‐specific temperature budget south of Java highlights the importance of wind stress forcing west of Sumatra relative to equatorial and local forcing, and explains most of the mixed layer temperature anomaly evolution associated with advection during pIOD events.