Imprint of Intrinsic Oceanic Variability on Interannual Variability in the Southwest Tropical Pacific

The Southwest Pacific Ocean plays an important role in the Pacific Ocean. It is on the pathways of waters redistributed from the southern subtropical gyre to the equatorial region. The westward South Equatorial Current divides into intense zonal jets when encountering the islands of Fiji, Vanuatu or New Caledonia. These zonal jets enter the Coral Sea and flow to the Australian coast, where they bifurcate toward the Solomon Sea or the Tasman Sea. The variability of these currents, resulting in a variability of the recharge of the equatorial band, is thought to participate to the modulation of ENSO (El Nino Southern Oscillation) cycle.

Yet, the processes governing this interannual variability are not completely understood. The dominant mode of interannual variability in the Tropical Pacific is ENSO, but its impacts on currents in the Southwest Pacific are not well understood. An ensemble of 50 global 1/4° ocean simulations, driven by the same realistic atmospheric forcing over 1960-2015 from slightly perturbed initial conditions, is analyzed over 1980-2015 to interpret the currents interannual variability. The "deterministic" variability driven by the atmosphere (e.g. ENSO) is estimated from the ensemble mean evolution. The intrinsic ocean variability is then quantified from the random dispersion of the simulations around the ensemble mean. We show that there are large parts of the region where interannual transport variability is firstly driven by stochastic processes, which probably arise from a rectification of the lower-frequency signal by mesoscale eddies activity. This important role played by ocean-only stochastic dynamics may thus hamper our capability to predict the interannual variability of the oceanic circulation in the Southwest Pacific.