Drivers and Mechanisms of the 2015–16 Record-High Sea Level and Ocean Heat Content in the Southeast Pacific

Abstract While the global sea level has been rising, the regional sea level exhibits substantial variations on various temporal and spatial scales. Using satellite altimetry data, we find that the southeast Pacific (SEP) stands out from the leading mode of sea level variations in the Southern Ocean and exhibits unprecedentedly high sea level (>12 cm, over 3 times the standard deviation) during 2015–16. The thermosteric effect from sustained and deep-reaching ocean warming dominated during this period, which also led to a concurrent record high in the regional ocean heat content. Heat budget analysis reveals the primary role of zonal advection. Particularly, driven by anticyclonic wind anomalies, the reduced turbulent heat losses into the atmosphere first induced anomalous warming in the upper layer of the central South Pacific. The warm anomalies were then advected eastward and accumulated in the SEP through increased downwelling, accompanied by the subsequent removal of relatively cold water on the eastern side. In terms of the atmospheric forcing, while the poleward-propagating Rossby wave response to the development of the 2015–16 strong El Niño gradually enhanced the anticyclonic wind anomalies over the SEP, the zonal wave 3 not only had a relatively large contribution during the El Niño developing phase but also dominated the spatial–time evolution of the extratropical wave-like patterns through its phase shift. Our results provide deep insight into this extreme event in terms of both atmospheric drivers and oceanic mechanisms, with important implications for understanding the ongoing and future ocean climate changes. Significance Statement The regional sea level changes exhibit considerable spatial differences and temporal variations. Based on satellite altimetry data which provide near-global sea level observations over the past three decades, this study identifies the southeast Pacific as a distinct region for sea level variations in the Southern Ocean. We further examine the unprecedentedly high sea level (>12 cm) in the southeast Pacific during 2015–16, driven by anticyclonic wind anomalies through anomalous ocean heat and mass convergence, resulting from both remote forcing from the tropical Pacific El Niño and the extratropical Southern Hemisphere climate variability (known as zonal wave 3). Our study improves the understanding of atmospheric and oceanic processes that are responsible for regional sea level and ocean heat content changes.