SEASONAL-VARIATIONS BETWEEN SAMPLING AND CLASSICAL MEAN TURBULENT HEAT-FLUX ESTIMATES IN THE EASTERN NORTH-ATLANTIC

The two commonly used statistical measures of the air-sea heat flux, t he sampling and classical means, have been compared using hourly repor ts over a 7-year-period from a weather ship stationed in the NE Atlant ic. The sampling mean is the average over all flux estimates in a give n period, where individual flux estimates are determined from ship rep orts of meteorological variables using the well-known bulk formulae. T he classical mean is the flux derived by substituting period-averaged values for each of the meteorological variables into the bulk formula (where the averaging period employed is the same as that over which th e fluxes are to be determined). Monthly sampling and classical means a re calculated for the latent and sensible heat fluxes. The monthly cla ssical mean latent heat flux is found to overestimate the sampling mea n by an amount which increases from 1-2 W m(-2) in summer to 7 W m(-2) in winter, on average, over the 7-year-period. In a given winter mont h, the excess may be as great as 15 W m(-2), which represents about 10 % of the latent heat flux. For the sensible heat flux, any seasonal va riation between the two means is of the order of 1 W m(-2) and is not significant compared to the interannual variation. The discrepancy bet ween the two means for the latent heat flux is shown to arise primaril y from a negative correlation between the wind speed and sea-air humid ity difference, the effects of which are implicitly included in the sa mpling method but not in the classical. The influence of the dominant weather conditions on the sign and magnitude of this correlation are e xplored, and the large negative values that it takes in winter are fou nd to depend on the typical track of the mid-latitude depressions with respect to the position sampled. In conclusion, it is suggested that sampling means should be employed where possible in future climatologi cal studies.