Estimation of High-Frequency Air-Sea Heat Flux Feedbacks in the Middle Latitudes

Resolute understanding of the exchange of energy between the atmosphere and ocean across time scales remains elusive due to the number of dynamical processes involved, the broad range of potentially interacting time scales, and a historical lack of high resolution data. Atmosphere-ocean interaction complicates the intrinsic variability of both the atmosphere and the ocean, and it is speculated that the effects of energy exchanges between the two systems can span disparate time scales due to the significant difference of the heat capacities of air and water. Atmospheric processes regulate radiative heat transfer to the oceanic mixed layer, generate mechanical energy input to the oceanic mixed layer, and affect turbulent heat fluxes out of the oceanic mixed layer. In contrast, ocean surface conditions can affect atmospheric vertical mixing, alter atmospheric boundary layer height, and affect secondary flows in the atmospheric boundary layer driven by horizontal pressure gradients.

In this study a one dimensional stochastic forcing model is used to estimate latent and sensible heat flux feedback factors from sea surface temperature anomalies at three-hourly, quarter degree resolution. A new dataset (SeaFlux version 1) compiled from satellite observations of diurnally-varying latent and sensible heat fluxes, accurate to within 14 W m^-2 and 6 W m^-2, respectively, is utilized. Western boundary currents and regions of strong horizontal motion in the ocean are neglected, as well as tropical latitudes, due to the assumptions of the model. Previous studies have used 30-day running means to calculate latent and sensible heat flux feedback factors. The latent and sensible heat flux feedback factors are estimated on sub-monthly and monthly time scales by utilizing discrete wavelet transforms to filter time series in a manner that retains the time localization (i.e. lag information), in addition to providing frequency decomposition and ensuring energy conservation, of the input signal. The estimation of sub-monthly feedback factors in this study not only quantify the magnitude and direction of oceanic mixed layer heat flux responses to forcing on short time scales, but also illuminate dominant atmosphere-ocean processes on different time scales.