Cool‐skin and warm‐layer effects on sea surface temperature

Abstract

To obtain bulk surface flux estimates approaching the ±10 W m−2 accuracy desired for the Tropical Ocean‐Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (COARE) program, bulk water temperature data from ships and buoys must be corrected for cool‐skin and diurnal warm‐layer effects. In this paper we describe two simple scaling models to estimate these corrections. The cool‐skin model is based on the standard Saunders [1967] treatment, including the effects of solar radiation absorption, modified to include both shear‐driven and convectively driven turbulence through their relative contributions to the near‐surface turbulent kinetic energy dissipation rate. Shear and convective effects are comparable at a wind speed of about 2.5 m s−1. For the R/V Moana Wave COARE data collected in the tropical western Pacific, the model gives an average cool skin of 0.30 K at night and an average local noon value of 0.18 K. The warm‐layer model is based on a single‐layer scaling version of a model by Price et al. [1986]. In this model, once solar heating of the ocean exceeds the combined cooling by turbulent scalar heat transfer and net longwave radiation, then the main body of the mixed layer is cut off from its source of turbulence. Thereafter, surface inputs of heat and momentum are confined to a depth DT that is determined by the subsequent integrals of the heat and momentum. The model assumes linear profiles of temperature‐induced and surface‐stress‐induced current in this “warm layer.” The model is shown to describe the peak afternoon warming and diurnal cycle of the warming quite accurately, on average, with a choice of a critical Richardson number of 0.65. For a clear day with a 10‐m wind speed of 1 m s−1, the peak afternoon warming is about 3.8 K with a warm‐layer depth of 0.7 m, decreasing to about 0.2 K and 19 m at a wind speed of 7 m s−1. For an average over 70 days sampled during COARE, the cool skin increases the average atmospheric heat input to the ocean by about 11 W m−2; the warm layer decreases it by about 4 W m−2 (but the effect can be 50 W m−2 at midday).