Dancing with ENSO: A decade of volume transport in the western tropical North Pacific from a numerical state estimate

In the far western tropical Pacific, the wind-driven North Pacific and South Pacific subtropical gyres feed low-latitude western boundary currents that meet in the western Pacific warm pool, the location of deep atmospheric convection and a strong ocean-atmosphere coupling. Using the data-assimilative Western Pacific Ocean State Estimate (WPOSE) (Jan. 2009 - Dec. 2017) we calculate the annual and interannual anomalies of along-isopycnal (surface to 27.3 kg m^-3) volume transport through the far western tropical North Pacific, from 125ºE to 150ºE and equator to 16ºN. The region spans the partitioning of the equatorward Mindanao Current (MC) and the poleward New Guinea Coastal Current / Undercurrent (NGCC/NGCUC) and South Equatorial Current (SEC) into the Indonesian Throughflow (ITF) and North Equatorial Countercurrent (NECC). Results show relatively small annual variability in the MC and NEC, with interannual anomalies double that of the annual variability. For the NECC and NGCUC/SEC, annual variability and interannual variability are both large. Transport in the NEC, MC, and NECC led the mature El Niño event in 2015/16. Interannual transport responds to thermocline shifts that occur with the El Niño/Southern Oscillation phenomena (ENSO), however, these shifts match up poorly with linear-Rossby wave theories, likely caused by the nonlinear interaction of planetary waves and currents in the region.