Ocean Surface Heat Flux

Summary

This project aims to develop a new climate data record for total ocean surface heat flux (OSHF), a key variable in understanding Earth’s energy balance and climate change.

Unlike existing products, this record will be derived from fundamental energy conservation principles and changes in ocean heat content within the upper ocean layer (Mixed Layer). This innovative approach addresses limitations in current in situ and satellite-based flux estimates, reduces uncertainties, and avoids uncertain parameterisations.

Accurate OSHF estimates are crucial for closing the ocean energy budget and improving climate models. The project supports GCOS requirements and responds to key knowledge gaps identified by the IPCC and WCRP, particularly regarding heat uptake by the ocean – a central component in tracking progress toward Paris Agreement targets and understanding long-term climate trends.

Background

The GCOS Implementation Plan 2022 highlights the urgent need to improve estimates of latent and sensible heat fluxes (Action B9). It recommends greater use of satellite data to address this gap (Action A3) and encourages Space Agencies to enhance long-term observation capabilities. The report also notes that while in situ ocean observing systems, such as the Argo program, are essential, they are not sufficient to provide the high-resolution Ocean Surface Heat Flux (OSHF) data required for climate modelling and model validation.

According to GCOS, accurately measuring surface heat fluxes in the open ocean is crucial for improving climate change projections and supporting adaptation strategies. However, direct observation of these turbulent fluxes is challenging, expensive, and not feasible at a global scale.

Although satellite-based products can estimate these fluxes using indirect methods, they rely on parameterisations informed by limited in situ data. This leads to two major sources of uncertainty: first, in the empirical models used to estimate fluxes from easier-to-measure variables, and second, in the global measurement of key inputs to those models, such as near-surface temperature and humidity.

In contrast, the approach of this project, the estimation of net radiative fluxes at the ocean surface, derived from satellite and auxiliary data, is generally estimated with less uncertainty.

Aims and objectives

The Climate-Space: OSHF ECV focuses on the generation of Ocean Surface Heat Flux (OSHF) from Earth Observation datasets. This new satellite-based OSHF product is designed to meet the stringent GCOS 2022 requirements in terms of spatial and temporal resolution, timeliness, uncertainty, and long-term stability.

By avoiding uncertain parameterisations and relying on fundamental physical principles, the product aims to reduce uncertainties and improve consistency across climate datasets. The OSHF dataset will undergo comprehensive validation using model outputs, existing EO-based products, and in situ measurements.

Its scientific and policy relevance will be demonstrated through key applications: improving climate model evaluation via its planned inclusion in the obs4MIPs database, supporting operational climate monitoring with a focus on marine heatwaves and heat budget analysis in the Mediterranean, and exploring the assimilation of OSHF into global ocean models.

Project plans

The technical approach of this project is structured into six distinct work packages.

Work Package 1 (WP1) focuses on the analysis of requirements. In this phase, we will review a broad range of user needs and define the product specifications accordingly.

WP2 is dedicated to algorithm development. This includes collecting the necessary input data for OSHF computation, conducting a round-robin exercise to select the final input datasets, defining the algorithms for generating the satellite-based products and their associated uncertainty estimates, and designing the product validation plan.

In WP3, the prototype processing software will be developed, the Climate Data Record will be generated, and the product will be prepared for inclusion in WCRP obs4MIPs and CEOS/CGMS ECV repositories.

WP4 will carry out the validation of the new satellite product, including both large-scale and small-scale validation exercises. The application of the product in three use case studies, along with an assessment by climate service users, will be addressed in WP5. Finally, WP6 will handle the overall coordination and management of the project.