Spatial trend patterns in the Pacific Ocean sea level during the altimetry era: the contribution of thermocline depth change and internal climate variability

Abstract

This study investigates the spatial trend patterns and variability of observed sea level and upper ocean thermal structure in the Pacific Ocean during the altimetry era (1993–2012), and the role of thermocline depth changes. The observed sea level trend pattern in this region results from the superposition of two main signals: (1) a strong broad-scale V-shaped positive trend anomaly extending to mid-latitudes in the central Pacific and (2) another very strong positive trend anomaly located in the western tropical Pacific within about 120° E–160° E and 20° S–20° N latitude. In this study, we focus on the tropical Pacific (20° N–20° S) where the strongest trends in sea level are observed. By making use of in situ observational data, we study the impact of thermocline depth changes on steric sea level between the surface and 700 m and its relation with the altimetry-based observed sea level changes. This is done by calculating the time-varying thermocline depth (using the 20 °C isotherm depth as a proxy) and estimating the sea level trend patterns of the thermocline-attributed individual steric components. We show that it is essentially the vertical movement of the thermocline that governs most of the observed sea level changes and trends in the tropical Pacific. Furthermore, we also show that in the equatorial band, the changes in the upper ocean thermal structure are in direct response to the zonal wind stress. Away from the equatorial band (say, within 5°–15° latitude), the changes in the upper ocean thermal structure are consistent with the wind stress-generated Rossby waves. We also estimate the contribution of the Interdecadal Pacific Oscillation (IPO) on the vertical thermal structure of the tropical Pacific Ocean. Removing the IPO contribution to the upper layer steric sea level provides a non-negligible residual pattern, suggesting that IPO-related internal ocean variability alone cannot account for the observed trend patterns in the Pacific sea level. It is likely that the residual signal may also reflect non-linear interactions between different natural modes like El Nino Southern Oscillation (ENSO), IPO, etc.