Upper-Ocean Lateral Heat Transports in the Niño3.4 Region and Their Connection with ENSO

In the Niño3.4 region (tropical Pacific, 5°S–5°N, 170°–120°W), sea surface temperature (SST) changes are highly correlated with temperature variations in the upper 40-m layer. This study explores the upper-ocean heat budget in the Niño3.4 region using Ocean Reanalysis System 5 (ORAS5) monthly data from 1979 to 2018, with a focus on ocean heat transports at lateral boundaries in the top 40-m layer and their correlation with temperature variations. In the region, there is a well-defined structure of opposite meridional circulation in the upper and lower parts of the thermocline, characterized by divergence in the upper layer above 40 m and convergence in the lower layer. The change of mean temperature in the upper layer is determined by the sum of zonal, meridional, and vertical heat transports, which, however, tend to largely compensate for each other. In general, part of the surface heat flux from the atmosphere to the ocean and the heat transport from the subsurface ocean are transported out of the domain by meridional and zonal currents, leaving only a tiny part to warm or cool the upper ocean. The amplitude of the net surface heat flux effective for the entire 40-m layer of the ocean is weaker than the lateral heat transport. On an interannual timescale, variations of heat transports in both zonal and meridional are positively correlated with temperature anomalies, while the vertical heat transport from the subsurface ocean is negatively correlated. Composite analyses for five El Niño events and five La Niña events also revealed that there is a positive contribution of horizontal transport convergence to temperature anomalies during the evolution of El Niño (warming) and La Niña (cooling), while vertical transport acts against temperature variations.