Interannual Variability of Isopycnal Ocean Heat Content in the Subtropical Northeast Pacific

Abstract

AbstractThe subsurface thermal anomaly originating from the mid‐latitude Pacific significantly influences climate variability in the tropical Pacific. Traditionally, the spiciness anomaly, defined on an isopycnal surface, is used to estimate this thermal anomaly. However, the thermal anomaly is confined to an isopycnal layer with a certain thickness, not spread on a specific surface. Its impact on the tropical Pacific depends on the amount of heat reaching the equator. Therefore, the thickness‐weighted isopycnal ocean heat content (IPOHC), defined as the ocean heat content in an isopycnal layer of a given potential density range, can more accurately estimate the subsurface thermal anomaly. This study focuses on the interannual variability of the IPOHC in the subtropical northeast Pacific, a region crucial for extratropical‐tropical oceanic exchange. The IPOHC in the 24.8–25.6 kg/m3 potential density layer exhibits a significant interannual variability with a 5–6‐year period. Subduction and salt fingering are identified as key processes driving this variability. Specifically, increased subduction volume and enhanced salt fingering both can induce positive IPOHC anomalies, while the opposite results in negative anomalies. Late winter variations in subduction volume and salt fingering strength, which originate from the winter mixed layer depth variability and halocline intensity variability, work together to cause the observed interannual IPOHC anomalies. We emphasize that the subduction‐induced IPOHC anomalies are determined by anomalous subduction volume rather than mixed layer temperature. Furthermore, salt fingering plays an essential role in driving substantial IPOHC anomalies in the deep, poorly‐ventilated halocline/thermocline.