Energy Imbalance in the Sunlit Ocean Layer

Abstract The sunlit ocean layer (SOL), above 200m depth, is where marine life is most abundant and critical climate feedbacks are enabled. Satellite and in-situ temperature data make it evident that this crucial ocean layer is already reacting to global warming, as underscored recently by unprecedented peaks in marine heat waves. Yet, the SOL heat budget is not well understood, globally or regionally, as it remains difficult to estimate processes that control it from observations alone. Here we resolve this issue using an ocean reanalysis and provide the first operational definition of SOL’s energy imbalance (SOL-EI), as a heat budget anomaly relative to the 1981-2010 period. Our estimate of SOL-EI shows that it amounts to a relatively small imbalance between air-sea heat flux and ocean heat transports. Further analysis reveals a recent, tenfold increase in SOL-EI by comparing 2013-2022 with the previous decade, which could have catastrophic environmental consequences.