Global ocean surface heat fluxes derived from the maximum entropy production framework accounting for ocean heat storage and Bowen ratio adjustments

Abstract

Ocean evaporation, represented by latent heat flux (LE), plays a crucial role in global precipitation patterns, water cycle dynamics, and energy exchange processes. However, existing bulk methods for quantifying ocean evaporation are associated with considerable uncertainties. The maximum entropy production (MEP) theory provides a novel framework for estimating surface heat fluxes, but its application over ocean surfaces remains largely unvalidated. Given the substantial heat storage capacity of the deep ocean, which can create temporal mismatches between variations in heat fluxes and radiation, it is crucial to account for heat storage when estimating heat fluxes. This study derived global ocean heat fluxes using the MEP theory, incorporating the effects of heat storage and adjustments to the Bowen ratio (the ratio of sensible heat to latent heat). We utilized multi-source data from seven auxiliary turbulent flux datasets and 129 globally distributed buoy stations to refine and validate the MEP model. The model was first evaluated using observed data from buoy stations, and the Bowen ratio formula that most effectively enhanced the model performance was identified. By incorporating the heat storage effect and adjusting the Bowen ratio within the MEP model, the accuracy of the estimated heat fluxes was significantly improved, achieving an R-2 of 0.99 (regression slope: 0.97) and a root mean square error (RMSE) of 4.7 W m-2 compared to observations. The improved MEP method successfully addressed the underestimation of LE and the overestimation of sensible heat by the original model, providing new global estimates of LE at 93 W m-2 and sensible heat at 12 W m-2 for the annual average from 1988-2017. Compared to the 129 buoy stations, the MEP-derived global LE dataset achieved the highest accuracy, with a mean error (ME) of 1.3 W m-2, an RMSE of 15.9 W m-2, and a Kling-Gupta efficiency (KGE) of 0.89, outperforming four major long-term global heat flux datasets, including J-OFURO3, ERA5, MERRA-2, and OAFlux. Analysis of long-term trends revealed a significant increase in global ocean evaporation from 1988-2010 at a rate of 3.58 mm yr-1, followed by a decline at -2.18 mm yr-1 from 2010-2017. This dataset provides a new benchmark for the ocean surface energy budget and is expected to be a valuable resource for studies on global ocean warming, sea surface-atmosphere energy exchange, the water cycle, and climate change. The 0.25° monthly global ocean heat flux dataset based on the maximum entropy production method (GOHF-MEP) for 1988-2017 is publicly accessible at 10.6084/m9.figshare.26861767.v2 (Yang et al., 2024).