The Effects of Sea Surface Temperature Gradients on Surface Turbulent Fluxes

A positive correlation between sea surface temperature (SST) and wind stress perturbation near strong SST gradients (ΔSST) has been observed in different parts of the world ocean, such as the Gulf Stream in the North Atlantic and the Kuroshio Extension east of Japan. These changes in winds and SSTs can modify near-surface stability, surface stress, and latent and sensible heat fluxes. In general, these small scale processes are poorly modeled in Numerical Weather Prediction (NWP) and climate models. Failure to account for these air-sea interactions produces inaccurate values of turbulent fluxes, and therefore a misrepresentation of the energy, moisture, and momentum budgets. Our goal is to determine the change in these surface turbulent fluxes due to overlooking the correlated variability in winds, SSTs, and related variables. To model these air-sea interactions, a flux model was forced with and without SST-induced changes to the surface wind fields. The SST modification to the wind fields is based on a baroclinic argument as implemented by the University of Washington Planetary Boundary-Layer (UWPBL) model. Other input parameters include 2-m air temperature, 2-m dew point temperature, surface pressure (all from ERA-interim), and Reynolds Daily Optimum Interpolation Sea Surface Temperature (OISST). Flux model runs are performed every 6 hours starting in December 2002 and ending in November 2003. From these model outputs, seasonal, monthly, and daily means of the difference between ΔSST and no ΔSST effects on sensible heat flux (SHF), latent heat flux (LHF), and surface stress are calculated. Since the greatest impacts occur during the winter season, six additional December-January-February (DJF) seasons were analyzed for 1987–1990 and 1999–2002. The greatest differences in surface turbulent fluxes are concentrated near strong SST fronts associated with the Gulf Stream and Kuroshio Extension. On average, 2002–2003 DJF seasonal differences in SHF, LHF, and wind stress over the Gulf Stream are 3.86 ± 0.096 W/m², 6.84 ± 0.186 W/m², and 0.032 ± 0.0008 N/m², respectively. In addition, smaller flux differences covering large expanses of the Atlantic and Pacific Oceans are non-negligible for most upper oceanic applications sensitive to multi-decadal changes. Due to these non-linear processes, average changes in surface turbulent fluxes are not zero.