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

Abstract

To obtain bulk surface flux estimates approaching the _+10 W m -: 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 -•. 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 Dr 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 rn s -•, the peak afternoon warming is about 3.8 K with a warmlayer depth of 0.7 m, decreasing to about 0.2 K and 19 rn at a wind speed of 7 m s -•. For an average over 70 days sampled during COARE, the cool skin increases the average atmospheric heat input to the ocear/by about 11 W m-:; the warm layer decreases it by about 4 W m -: (but the effect can be 50 W m -: at midday). 1. Introduction Sea surface temperature (SST) is a key variable driving air-sea interaction. SST and air-sea fluxes were a dominant component of the study of the tropical western Pacific warm pool in the Tropical Ocean-Global Atmosphere (TOGA) Coupled Ocean-Atmosphere Response Experiment (COARE) held in 1992-1993 [Webster and Lukas, 1992]. Uncertainties in air-sea temperature difference represent a major uncertainty in assessing the heat balance of the warm pool [Lukas, Paper number 95JC03190. 0148-0227/96/95JC-03190505.00 1989]. Fairall et al. [1996a] have shown that to estimate this heat balance to an accuracy of 10 W m '2 requires specification of the SST to an accuracy of _+0.2 K. Bulk flux routines are based on the empirical relationship between the turbulent fluxes and the air-sea contrasts of wind, humidity, and temperature; the SST is the lower thermal boundary condition. Logically• the proper temperature is taken at the air-sea...

LakeSST: Lake Skin Surface Temperatures in French inland water bodies for 1999–2016 from Landsat archives

“…Skin temperatures can differ from SWT because the thermal structure of the first meters of the water column is not uniform under all conditions. At-sea measurements, of the skin surface temperature are usually lower than the temperature of the underlying water (cool skin effect), showing a dependence on wind speed (Fairall et al, 1996a;Minnett et al, 2011), and similar differences have been observed in freshwater environments (Cardenas et al, 2008). A warm near-surface layer can also appear on calm and sunny 30 days, without necessarily excluding the presence of a cool skin (Donlon et al, 1999).…”

“…Both effects are related to thermal inhomogeneities in the temperature distribution near the surface. The warm layer forms during the day, and is due to surface stratification caused by the absorption of solar radiation in the first meters of the water column (Fairall et al, 1996a). The magnitude of the warm layer effect can attain several degrees (Fairall et al, 1996a;Kawai and Wada, 2007;Gentemann and Minnett, 2008).…”

“…The warm layer forms during the day, and is due to surface stratification caused by the absorption of solar radiation in the first meters of the water column (Fairall et al, 1996a). The magnitude of the warm layer effect can attain several degrees (Fairall et al, 1996a;Kawai and Wada, 2007;Gentemann and Minnett, 2008). The 15 cool skin effect is almost always present and arises from the cooling of the first millimetres of the water column because of the joint action of longwave radiation, sensible and latent heat fluxes (Fairall et al, 1996a).…”

“…The magnitude of the warm layer effect can attain several degrees (Fairall et al, 1996a;Kawai and Wada, 2007;Gentemann and Minnett, 2008). The 15 cool skin effect is almost always present and arises from the cooling of the first millimetres of the water column because of the joint action of longwave radiation, sensible and latent heat fluxes (Fairall et al, 1996a). Its magnitude is of -0.1 ºC to -0.5 ºC (Fairall et al, 1996a;Donlon et al, 2002).…”

“…There are several models that take into account the cool skin and the warm layer effect at the sea (Price et al, 1986;Kantha and Clayson, 1994;Fairall et al, 1996a;Kawai and Kawamura, 2000;Gentemann et al, 2009). To analyse both the cool skin and the warm layer effects for the case of the reservoir of Bimont, we used the model proposed by Fairall et al (1996a), and 5 implemented in the COARE bulk flux algorithm (Fairall et al, 1996b), that depends on the surface heat (shortwave irradiance, longwave irradiance, latent and sensible heat exchange) and moment fluxes (wind shear).…”