An investigation of the oceanic skin temperature deviation

Abstract

Satellite and in-situ radiometric measurements of sea surface temperature (SST) together with conventional SST and meteorological parameters are used to provide a description of the ocean surface skin temperature deviation (skin temperature - bulk temperature, AT) for a transect made across the Atlantic ocean from 50°N 00°W to 23°S 35°W during September and October 1992. Methods of in-situ SST measurement are discussed and the errors associated with each technique are given. The principles of infra red radiometry are explained. The differences between the calibration strategies used to determine SST using infra-red radiometers from both in-situ and satellite platforms are reviewed and the errors associated with each technique are given. Differences between published in-situ infra red SST data indicate that there may be a bias in these data as a consequence of the calibration strategy adopted. The need for an inter calibration of in-situ infra red radiometer systems used for the validation of satellite SST is highlighted. Satellite SST algorithms are discussed and the principles of atmospheric correction are explained. The difference between the radiometric 'skin' temperature of the ocean and the conventional 'bulk' temperature at depth is defined. A review of current observations of AT is given. Several theoretical treatments of AT are reviewed. The definitions of the surface fluxes of heat and momentum are given. A description of the collection of data and an analysis of the calibration of the infra-red radiometer used to measure the skin temperature is presented.
Data have been processed to obtain AT and the surface fluxes of heat and momentum have been evaluated according to the bulk aerodynamic formulae. The relationships between AT and the measurements made are presented for the entire data set and for day and night time observations separately. Four time series of observed data are presented and the local conditions during the time of measurement are used to discuss AT. AT has a mean value of 0.39°C ±0.3°C and is shown to be a persistent feature of the Atlantic ocean. Correlation analyses reveal the skin and bulk temperature fields to be correlated at length scales > 155 km. Night time correlations are consistently higher than the day time at all length scales. For this reason it is recommended that satellite validation data are only collected during the night. High sea states are shown to affect both in-situ and satellite observations of SST biasing these data warm. The regional nature of AT is presented which is related to the dominant atmosphere-ocean conditions for each region. AT is shown to be greatest at the higher latitudes and weak in the tropical regions.
Several parameterisations of AT are used to obtain estimates of AT using the data collected. These are found to be inadequate to predict AT at small temporal scales. A regional dependence of AT is found in these parameterisations. The coefficient A, of the Saunders (1969) parameterisation has been evaluated and is shown to have a regional dependence on the local atmosphere ocean conditions. The coefficient Ci and Ci of the Hasse (1971) parameterisation have been evaluated using the data collected. These are Ci=4.74 and C2=1.22.

A comparison between the Along Track Scanning Radiometer Average SST is presented. Satellite - in-situ bulk AT has been obtained and shown to be comparable to that observed in-situ. This comparison highlights the need to make skin SST validation measurements rather than bulk SST measurements. The ATSR ASST data are shown to return a SST accurate to better than 0.3°C.

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