Relative phase between tidal and solar cycles influences the heating of a coral atoll lagoon

We seek to understand the physical processes driving a recent warming event in Dongsha Atoll lagoon, a coral atoll in the South China Sea. The system has experienced hypoxia‐related fish kills and seagrass die‐offs, further motivating our understanding of circulation and exchange within the lagoon. Hydrographic observations were collected in summer 2019 to examine circulation and ocean–lagoon exchange. A heat budget was constructed to investigate the relative influence of tides, wind, waves, surface heating, and advective ocean–lagoon exchange on the lagoon heat content. Volume fluxes also allow for the estimate of bulk lagoon flushing time, which is critical in driving water temperatures, biogeochemical cycles, and reef ecosystem functions. Over the 22‐d study period, the lagoon warmed by 1.5°C, driven primarily by solar radiative heating. Surface heat fluxes were balanced by cooling from advective exchange through deeper channels and over the shallow reef flat surrounding the lagoon. Without these cooling flows, the lagoon would have heated by 4.5°C over the study period. Estimated flushing times for the Dongsha Atoll lagoon range from 4.6 to 6.1 d for spring and neap tide. A phase‐locked interaction between the diurnal solar cycle and the diurnal tide drives advective cooling over the reef flat into the lagoon at night during the summer, especially during spring tide. These results highlight a unique mechanism shaping seasonal patterns in circulation and heat fluxes in a diurnal‐tide‐dominated reef system, which is likely important for other biogeochemical parameters and in shallow coastal ecosystems with similar tidal forcing.