| Abstrakt: |
The 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. Plain Language Summary: Our study aimed to understand better the warming and cooling phenomena in the subsurface of the subtropical northeast Pacific, as the thermal variations there can impact the climate in the tropics. We focused on a specific layer in the ocean called the isopycnal layer, where the ocean heat content is stored and travels within. We found the ocean heat content in the 24.8–25.6 kg/m3 isopycnal layer of the subtropical northeast Pacific undergoes a significant variability with a 5–6‐year cycle. The variability signals show a broad distribution with the maximum amplitude centered around 135°W, 25°N. We identified two main factors driving this variability: subduction and salt fingering. Between February and May, if more mixed layer waters are pulled down into the subsurface irreversibly and the mixing of upper warm/salty waters and lower cool/fresh waters by salt fingering is enhanced, positive ocean heat content anomalies will appear and the subsurface layers warm up. Conversely, the opposite processes lead to negative anomalies and subsurface cooling. The observed surface warming or cooling results from variations in both the amount of subducted water and the strength of mixing. This study explains how the subsurface of the subtropical northeast Pacific warms up or cools down periodically. Key Points: Thickness‐weighted isopycnal ocean heat content (IPOHC) in the 24.8–25.6 isopycnal layer of the subtropical NE Pacific shows a 5–6‐year period interannual variabilityVariations in subduction volume and salt fingering strength during late winter work together to induce the interannual IPOHC anomaliesSalt fingering plays an important role in inducing substantial IPOHC anomalies in the deep, poorly‐ventilated halocline/thermocline [ABSTRACT FROM AUTHOR] |
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