The seasonal march in upper-ocean temperature, salinity, steric height and sea surface height in the equatorial Pacific

Florent Gasparin, Scripps Institution of Oceanography, San Diego, United States and Dean H Roemmich, Scripps Institution of Oceanography, San Diego, CA, United States
The close relationship of Argo steric height (SH, 0-2000dbar) and altimetric sea surface height (SSH) is revisited in the equatorial Pacific with 5-day time-series, focusing on the climatological seasonal march (CSM) because of its large contribution to the total variance, its relationship to interannual ENSO variability, and its unique equatorial wave phenomena. Using either 11-year (2004-2014, SH and SSH) or 22-year (1993-2014, SSH) time-series for averaging, the seasonal Rossby waves stands out clearly. Eastward intraseasonal Kelvin wave propagation along the equator is strong enough in individual years to leave residuals in the 11-year or 22-year averages, particularly in October/November. SSH minus SH residuals (±1.4cm) are closely related to the annual variations of the mass related-component. The occurrence of similar intraseasonal patterns during the 1993-2003 and 2004-2014 periods suggests that the late-year intraseasonal Kelvin wave is a recurrent feature of the CSM. The Argo profile data reveal thermocline depth and near-surface salinity variations, identified as the main contributors to sea level fluctuations at intraseasonal to interannual timescales. The thermocline deepens by about 15m during the climatological October-November (equivalent to a warm anomaly of 1.5°C) and by more than 60m during El Niño October-November (anomaly of 4-5°C). Salinity variations in the western Pacific account for between a third and a half of sea level variations during ENSO episodes. In addition, these subsurface variations are shown to play a significant role in horizontal pressure gradient variations, in quasi-equilibrium with the zonal wind stress and currents from intraseasonal to interannual timescales. The comparison with the unusual evolution of the 2014-2015 El Niño is also investigated. This work provides a benchmark for assessing the accuracy of models in representing equatorial Pacific, and toward redesigning the Tropical Pacific Observing System.