Diurnal warming at the ocean surface

Daytime warming of the ocean surface occurs when incoming heat fluxes exceed outgoing heat fluxes. The surface expression of this warming is strongly dependent on vertical stratification and the magnitude of wind driven turbulence in the mixed layer and will directly affect air-sea heat and gas fluxes and daytime satellite measurements of Sea Surface Temperature (SST). Numerical Weather Prediction, climate modeling, and mesoscale oceanography require remotely sensed SSTs with accuracies of 0.1--0.3 K. Warming of up to 5.2 K is measured in this study; values of up to 2--3 K are not unusual and may persist for hours. To meet stated SST accuracy requirements, it is necessary to obtain a better understanding of diurnal warming variability and improve our ability to model it at the ocean surface.Measurements of skin SST collected in all major oceans were utilized to complete the first characterization of diurnal warming at the ocean surface using a large database of in situ data collected by the Marine Atmospheric Emitted Radiance Interferometer (M-AERI). The sensitivity of diurnal warming to magnitude and variability of wind speed, time of day, and insolation was determined. The largest measured diurnal warming measured was 5.2 K.A number of empirical and physics-based models of diurnal warming have been developed and are currently being employed in research to examine how diurnal variability impacts air-sea heat and gas fluxes. Sensitivity studies were utilized to explore dependencies of these models on input forcing parameters, such as wind speed and insolation. The maximum diurnal warming returned from models varied from 2.4 to 14.5 K.Comparison of the existing models simulated diurnal warming against 72 days of in situ skin SST measurements of diurnal warming revealed a number of shortcomings which motivated the development of a new model. A bulk upper-ocean model from the literature was refined to better represent upper ocean processes. The parameterization of the absorption of solar radiation was improved and dissipation of heat and momentum was introduced into the model. Non-dimensional vertical profiles of temperature within the diurnal thermocline were also incorporated into the model. These wind-dependent profiles scale with the model-calculated warm layer thickness and heat content to significantly improve the models dependence on wind speed when compared to the in situ measurements. The new model, Profiles Of Surface Heating (POSH), was best able to predict the behavior of in situ measurements of diurnal warming and provides information on the vertical structure within the diurnal thermocline