Understanding and Constraining Interfacial and Sea Spray Heat Fluxes in High Winds Using Direct Covariance Heat Flux Observations

Air-sea fluxes of sensible and latent heat are fundamental to the structure and energetics of the air-sea transition zone and storm systems in extreme conditions. In high winds (i.e., 10-m windspeed U₁₀ ≳ 20 m s^-1), sea spray droplets ejected from breaking wave crests provide pathways for heat transfer (i.e., droplet cooling and evaporation) that are not represented in the bulk heat flux algorithms widely used in extreme weather forecast models. Sea spray generation and heat fluxes are controlled by complex air-sea-wave interactions, but co-located and simultaneous in situ measurements of high-wind spray generation and air-sea heat fluxes, as well as the wind, wave, and turbulent upper ocean conditions that produce them, are extremely limited. Because of this, ongoing challenges remain in robustly demonstrating spray’s presence in high-wind heat flux observations and in calibrating surface heat flux algorithms that incorporate coupled interfacial and spray heat flux physics.

In this work, we examine a large and diverse collection of direct covariance sensible and latent heat flux datasets and compare these fluxes to those computed using the Coupled Ocean-Atmosphere Response Experiment (COARE) version 3.6 algorithm, a bulk turbulent flux algorithm that does not include spray physics. Observations come from a diverse set of field campaigns and sites, including the CLIVAR Mode Water Dynamic Experiment (CLIMODE), the Salinity Processes in the Upper-ocean Regional Study (SPURS) 1 and 2, the Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign, and multi-year measurements from the NSF Ocean Observatories Initiative (OOI) Pioneer, Endurance, Southern Ocean, and Irminger Sea Arrays. We find a systematic departure of observed heat fluxes from COARE calculations in high winds that is consistent with the physics of spray evaporation in the near-surface layer. We then incorporate a new parameterization for heat fluxes with spray (Barr et al. 2023) into the COARE algorithm and find that the new spray physics reproduces the observed flux behavior. Finally, we calibrate both interfacial and spray model physics in the combined algorithm using the observed heat flux datasets, providing indirect constraints on spray generation and heat fluxes for U₁₀ up to almost 30 m s^-1. These results are promising but are limited by the continued scarcity of direct covariance heat flux measurements in high winds – particularly for latent heat flux – highlighting the need for systematic, long-term, and simultaneous high-wind observations of wind, waves, surface thermodynamics, sea spray, and direct covariance sensible and latent heat fluxes.