Linking Seasonal-to-Interannual Variability of Intermediate Currents in the Southwest Tropical Pacific to Wind Forcing and ENSO

Seasonal-to-interannual variability of intermediate circulations in the southwest tropical Pacific is key to interhemisphere and interbasin water mass exchange and is shown to be interconnected with wind forcing and the El Nino-Southern Oscillation (ENSO) based on numerical model results validated by mooring observations. The Lower Equatorial Intermediate Current and the intermediate part of the New Guinea Coastal Undercurrent show clear seasonal variability and a significant interannual response to ENSO at a 7-12-month lag, both of which are induced by Rossby waves. Baroclinic modes 1 and 2 of the direct wind-forced Rossby waves contribute mainly to the seasonal variability. In contrast, the Rossby waves from baroclinic modes 2 and 4 and the reflection of Kelvin waves are the main contributors to the variability on ENSO time scales. The presence of the southwest tropical Pacific coastline enhances the intermediate seasonal variance and the interannual relationship between intermediate currents and ENSO.