The feedback between entrainment flux and sea surface temperature

The modulation of the ocean and resultant atmospheric variability by diurnal warming and nocturnal cooling is increasingly an area of interest. Positive feedbacks between diurnal warming and the resultant reduced nocturnal cooling and sea surface temperature have been hypothesized as playing a role in oscillations from the intraseasonal such as the Madden-Julian Oscillation to ENSO. The purpose of this research is to quantitatively determine the feedback associated with the entrainment cooling of the ocean mixed layer as it relates to sea surface temperature through the surface energy budget. A feedback structure exists in the observational data; using a one-dimensional turbulence model of the ocean mixed layer can evaluate sea surface temperature changes in the absence of such feedbacks. Thus estimation of the feedback dynamics is accomplished by comparing the frequency spectrum of the model with that of the observed system. The dsifference between the two spectra is then attributed to the feedback using the appropriate closed loop feedback structure. Rather than estimate the feedback as a gain or “feedback factor,” as is done throughout the relevant literature, this new technique seeks to estimate the feedback as a dynamic system by comparing the frequency spectra of the model and observed systems. The “reverse controller design” technique allows the estimation of feedbacks more representative of the complexity in the observed system. The results of this research using data from the TOGA COARE Intensive Observations Period in the tropical western Pacific will be shown.