Heat flux parameterizations for the Mediterranean sea : the role of atmospheric aerosols and constraints from the water budget

The semi-enclosed nature of the Mediterranean Sea makes it an excellent place to test data sets and standard formulae used to estimate the heat flux into the world's oceans. Oceanographic observations at the Strait of Gibraltar imply a mean annual net loss of 5.2±1.3 W m−2 from from the sea surface (Macdonald et al., 1994). Investigations using bulk heat-flux formulae, however, generally give a large heat gain (≈ 20 to 30 W m−2) through the Mediterranean Sea surface. This discrepancy has been attributed by different authors to an underestimation of the latent heat flux, an underestimation of the longwave radiation, or an overestimation of the incoming shortwave radiation. Examination of the heat flux formulae themselves, and of the transport of anthropogenic and mineral aerosols, suggests that the shortwave radiation has been previously overestimated by approximately 20 W m−2. This overestimation results from an incorrect use of a commonly used bulk formula at very low cloud levels, and direct radiative attenuation due to aerosols. Allowing for the seasonal variability of aerosols over the Mediterranean basin, rather than employing a constant aerosol attenuation factor, improves the agreement of the annual Qs cycle with ground measurements at one location. Current data support a maximum increase in the net longwave radiation of 10 W m−2 from previous estimates. Recent estimates of the Mediterranean water budget do not indicate that the latent heat flux estimate is incorrect.

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