Variability of latent and sensible heat fluxes estimated using bulk formulae

Abstract

The spatial and temporal variability of monthly average latent and sensible heat flux over the oceans is explored. Monthly flux anomalies are estimated using bulk formulae applied to COADS marine data over 1946–1986. Emphasis is on behaviour during fall and winter over the well sampled North Atlantic and North Pacific oceans, but available data from the Indian Ocean, from the tropics and from the Southern Hemisphere are also included. Random observation errors and random weather sampling errors are reduced by averaging several observations together. Biases in the observations and in the bulk formulae are not automatically reduced by averaging, but because the mean of the fluxes is subtracted to provide the anomalies, the non‐time‐varying biases are diminished. Largest latent flux anomalies occur from the tropics to middle latitudes, and largest sensible flux anomalies appear in middle‐to‐high latitudes. In mid‐latitudes, monthly latent and sensible flux anomalies are strongly correlated, so that they tend to be reinforcing. The bulk parametrizations indicate that the latent and sensible flux anomalies typically outweigh those of the radiative fluxes, except in the tropics and in the summer extratropics where net solar flux variations become important. An analysis of variance, which identifies the dominant contributions by the fundamental marine variables, yields results that emphasize the importance of the mean values, as well as the anomalies of these variables, in creating latent and sensible flux anomalies. Although they contain small‐scale “noise”, there is a marked signal in the flux anomalies that is spatially organized and quite strongly related to the monthly atmospheric circulation. The first four rotated empirical orthogonal functions (REOFs) of the sum of the latent and sensible flux anomalies account for about half of the total variance in the North Atlantic and North Pacific basins during winter months. The REOFs have magnitudes that represent anomalies that typically exceed 50 W m⁻² over substantial portions of the ocean basins. Links to the atmospheric circulation also indicate a short‐period climate signal. Correlations of the amplitudes of the REOFs of the fluxes with the sea‐level pressure field exhibit patterns that strongly resemble frequently occurring modes of monthly circulation anomalies. In the extratropics during winter, the atmospheric circulation affects the Bowen ratio (sensible flux/latent flux). When the wind is more equatorward or more continental than normal, the Bowen ratio increases.