The Response of Large Diurnal Warm Layers to Short-Term Variability in Solar and Wind Forcing: Observations and Physical Modeling

Abstract In conditions of low winds and high insolation, near-surface stratification develops in the ocean that is typically referred to as a diurnal warm layer (DWL). These layers can have a substantial effect on sea surface temperature and air–sea fluxes yet are rarely accounted for in modern global models due to their small vertical scale. Here, we present collocated measurements of vertical temperature and turbulence structures in large DWLs made from a Lagrangian float featuring a robotic lead screw temperature/salinity ( T / S ) profiler and pulse-to-pulse coherent ADCP as well as a comprehensive suite of meteorological observations above the ocean surface, yielding novel observations of the response of large DWLs to variability in wind and solar forcing at subhourly time scales. Comparison between the observations and a hierarchy of upper-ocean models reveals the importance of an accurate solar heating parameterization and suggests a modification to the critical bulk Richardson number used by default in the K -profile parameterization. Comparison to a simple scaling for DWL evolution highlights its potential as a means of incorporating DWL effects into global-scale modeling, and a new extension to the scaling is developed to remedy its inaccuracy in cases of wind decrease. None of the models tested are able to reproduce the observed response to sudden insolation loss on one of the stations. Significance Statement This study presents measurements of warm layers of water that can develop on the ocean surface on a calm, sunny day. These layers are widespread in the ocean and change the relationship between the ocean and the atmosphere, but they are hard to include in large models because they are so shallow. By comparing first-of-their-kind observations of these warm layers made by our drifting buoy with several types of physical models, we improve our understanding of them and chart a realistic path toward their inclusion in global models.