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

Abstract

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 ra•liation 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 • constant aerosol attenuation f•ctor, improves the agreement of the annual Q• 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. 10 W m -2. The flow through the Strait can be separated into a long-term two-way baroclinic exchange, and mainly barotropic subinertial and tidal flows [Lacombe and Richez, 1982]. Recent analysis of current meter data from the year-long Gibraltar Experiment gives a net advective flux into the Mediterranean equivalent to 5.2 -4-1.3 W m -2 [Macdonald et al., 1994], a slight decrease from the estimate of 7-4-3 W m -2 used in earlier studies [Bunker et al., 1982; Belhou•, 1979]. The new estimate appears to be an improvement, as it includes Paper number 93JC03069. 0148-0227]94/93JC-03069505.00 the flux associated with the correlations between inflow and outflow velocity and temperature fluctuations (u-•-•T•), so that the net advective heat transport is defined as surface __ (• T + u'T')W(z) dz J bottom where W(z) is the width of the Strait at the mooring location. The heat transport due to fluctuations was found to exceed the transport by the time-averaged components. The extent to which annual and interannual variations affect this estimate cannot be addressed with the data, however. To balance the advective heat flux, there should be an equal average heat loss from the Mediterranean Sea surface (as the contributions from the Black Sea and the ocean bottom are negligible). Detailed investigations of the Mediterranean heat budget using standard formulae and data sets, however, generally result in a large (approximately 20-30 W m -2) heat gain through the sea surface. This discrepancy has been attributed to various causes. For example, Bunke...

Seasonal heat content changes in the western Mediterranean Sea as a means for evaluating surface heat flux formulations

“…This assumption is not supported by our results. The study of Gilman and Garrett [1994] had already pointed in this direction with the introduction the irradiation-reducing effect of aerosols, being able to reduce the discrepancy between long-term average heat fluxes and the heat transport through Gibraltar.…”

“…In two subsequent studies two different paths have been followed to reduce the discrepancies of the long-term average. Gilman and Garrett [1994] used basically the same set of parametrizations as Garrett et al [1993] but found an error in the parametrization of the insolation and introduced the effect of aerosols into insolation. Castellari et al [1998] and Angelucci et al [1998] followed a second route in comparing various sets of parametrizations for the four heat flux components to examine for which set the overall agreement is best.…”

“…Bignami et al [1995] developed a new formulation for the longwave radiation correcting this underestimation. This new formulation was applied by Gilman and Garrett [1994] and gave good results for the Mediterranean heat budget. These results highlight the need to better understand the surface heat fluxes and their possible dependence on other local atmospheric parameters like humidity or cloud cover and type.…”

Summer breakout of trapped bottom dense water from the northern Adriatic

“…Ivan in intervals 1984-1986and 1988-1992 [49] Monthly values of surface heat (Q; taken as the sum of insolation Q s , longwave radiation Q l , latent Q e and sensible heat Q c fluxes) and water flux (W; difference between precipitation P and evaporation E) at Trieste, Rovinj and Mali Lošinj were computed from monthly values and then averaged, following the procedure described by Supić and Orlić [1999]. In that paper, Q s was computed after the formula proposed by Reed [1977] and adapted for use in the Mediterranean area by Gilman and Garrett [1994]. For computation of Q l , Q e and Q, three different sets of formulae previously utilized in the Mediterranean were applied: the sets proposed by Gill [1982], by Bunker et al [1982], and by Gilman and Garrett [1994].…”

“…In that paper, Q s was computed after the formula proposed by Reed [1977] and adapted for use in the Mediterranean area by Gilman and Garrett [1994]. For computation of Q l , Q e and Q, three different sets of formulae previously utilized in the Mediterranean were applied: the sets proposed by Gill [1982], by Bunker et al [1982], and by Gilman and Garrett [1994]. A simpler method, described in more detail by Lyons et al [2007], with Q s computed after Reed [1977] and Gilman and Garrett [1994], and Q l , Q e and Q, computed after Gill [1982], was used to compute daily means of SE at Trieste and Pula.…”