Interannual volume transport anomalies in the far western tropical North Pacific were calculated using the Western Pacific Ocean State Estimates (WPOSE; 2009–2017; 115°E−170°E, 15°S–27°N) to assess the magnitude, phase, and vertical structure of volume transport, and the relationship of interannual transport to the El Niño-Southern Oscillation phenomena. The mean velocity and thermohaline structure for the WPOSE were verified using Argo climatology and autonomous glider observations. Transport was calculated for both an upper layer (surface-26 kg m−3) and a lower layer (26–27.3 kg m−3). Annual volume transport anomalies were small north of 8°N but varied by 50% of the mean volume transport between the equator and 8°N. Interannual anomalies exceeded annual anomalies throughout the region. Volume transports of the North Equatorial Current, the Mindanao Current, and the South Equatorial Current/New Guinea Coastal Undercurrent increased beginning in September 2014, leading to a large increase in transport in the North Equatorial Counter Current before the mature phase of El Niño in 2015/2016. The increase in transport was related to the meridional gradient in thermocline depth centered at 5°N at the southern part of the Mindanao Dome, where extreme shoaling of the thermocline took place. Lower-layer volume transports were not always in phase with those of the upper-layer and made considerable contributions to total transport variability.
Volume transport variability was greatest equatorward of 8°N, at the intersection of North and South Pacific currents
Subthermocline (lower-layer) interannual volume transport anomalies were of the same magnitude of those of the thermocline (upper-layer)
Interannual volume transport anomalies in the western tropical North Pacific increased before the mature 2015/2016 El Niño event
Plain Language Summary
The Western Pacific Ocean State Estimates (WPOSE) estimate the ocean state by combining observations and ocean model simulations through data assimilation. These estimates were made from 2009 to 2017 in the far western tropical Pacific. This region encompasses warm, tropical surface water and subtropical thermocline water (upper-layer) and polar intermediate water (lower-layer) that are sourced from the North and South Pacific and spread into the Indian Ocean through the Indonesian throughflow. The WPOSE were used to calculate volume transports to understand the modulation of major ocean currents as a system. Annual volume transport variability was small to the north of 8°N, and large to the south of 8°N and near the equator. Interannual volume transport variability was much greater than annual variability for both the upper and lower isopycnal layers and had a clear relationship with the El Niño-Southern Oscillation phenomenon. The period encompassed two El Niño events, a weak event in 2009/2010, and a strong event in 2015/2016. Regional transport increased during both events, but in the 2015/2016 El Niño, an increase in volume transport led the El Niño event by 2–3 months.
Data Availability Statement
Glider data are available at spraydata.ucsd.edu (Rudnick, 2020). The WPOSE are available upon request to B. Cornuelle and G. Gopalakrishnan and as the Northwest Pacific State Estimate/Reanalysis (NWPac) at http://www.ecco.ucsd.edu/nwpac.html. The 1/12° global HYCOM/NCODA Ocean Reanalysis was funded by the U.S. Navy and the Modeling and Simulation Coordination Office. Computer time was made available by the DoD High Performance Computing Modernization Program. The output is publicly available at http://hycom.org. The HYCOM/NCODA 1/12° global daily analysis can be obtained from the HYCOM Consortium (http://hycom.org/dataserver/). Roemmich and Gilson Argo Climatology are available at http://sio-argo.ucsd.edu/RG_Climatology.html. The Argo data are collected and made freely available by the International Argo Program and the national programs that contribute to it. (http://www.argo.ucsd.edu, http://argo.jcommops.org). The Argo Program is part of the Global Ocean Observing System (Argo, 2020). Aviso absolute dynamic topography MADT-H (2014), is available from CLS Space Oceanography Division at http://www.aviso.altimetry.fr/. The AVISO SSH maps used to compare to MADT WPOSE were obtained from the SSALTO/Developing Use of Altimetry for Climate Studies (DUACS) altimeter product on a ¼º longitude × ¼º latitude grid, produced and distributed by the Copernicus Marine and Environment Monitoring Service (CMEMS; http://marine.copernicus.eu/). The MITgcm code used in this study is checkpoint 64Y and can be obtained from http://mitgcm.org/. The along-track altimetry data can be obtained from Radar Altimetry Database System (RADS) (http://rads.tudelft.nl/rads/index.shtml). The SST data can be obtained from Remote Sensing Systems Inc (http://www.remss.com/). The NCEP/NCAR-Reanalysis-1 atmospheric forcings can be obtained from http://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.html.
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