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Hans-Jörg Isemer and Lutz Hasse

Abstract

The Beaufort equivalent scale of the World Meteorological Organization (WMO), used for decades to transform marine Beaufort estimates to surface wind speeds over the oceans, contains systematic errors that depend nonlinearly on the wind speed. Applying a revised scientific equivalent scale instead of the WMO scale produces significant changes in statistics of surface wind speed U over the ocean and, consequently, in all air-sea fluxes that are related to U.

For the North Atlantic Ocean these biases are quantified as follows. The WMO scale underestimates climatological monthly means of U significantly: up to 1.6 m s−1in tropical latitudes throughout the year. In subpolar regions, differences are significant from spring through autumn and reach 1.3 m s−1. These regionally and seasonally different monthly biases are equivalent to an overestimate of the annual variation of U, which reaches 1.5 m s−1 in the westerlies. Local standard deviations may be overestimated up to 1.2 m s−1. The WMO scale underestimates climatological monthly estimates of latent heat flux up to 50 W m−2. (up to 25%). The bias of the mean annual North Atlantic evaporation rate is 0.3 m yr−1. The bias in annual net air-sea heat flux amounts to 27 W m−2, equivalent to an underestimate of the transequatorial oceanic heat transport by 1.15 PW (1 PW = 1015 W). Climatological monthly wind stress at the ocean surface is underestimated by more than 4.5 × 10−2 N m−2 (up to 50%) in the trade-wind region.

Most existing regional and global air-sea flux compilations (including COADS) have been derived using the WMO scale. Hence, large biases are included in these compilations, although they can be partially hidden by an artificial increase of parameterization coefficients. The wind statistics revised according to a more accurate scale allow the application of bulk coefficients in accordance with newer experimental results from the open ocean. Therefore means and statistics of wind speed and climatological estimates of air-sea fluxes over the World Ocean need revision.

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Hans-Jörg Isemer, Jürgen Willebrand, and Lutz Hasse

Abstract

An inverse technique is used to adjust uncertain coefficients and parameters in the bulk formulae of climatological air-sea energy fluxes in order to obtain an agreement of indirect estimates of meridional heat transport with direct estimates in the North Atlantic Ocean. Three oceanographic estimates of ocean heat transport at the equator, at 25°N, and 32°N are compatible with meteorological evidence provided that the uncertainties of both direct and indirect estimates are taken into account. The transport coefficient CE for estimation of the latent heat flux is the major contributor to the overall uncertainty in estimates of ocean heat transport. The constraint of 1 PW northward transport across 25°N leads to a set of parameterizations for which the parameter adjustments are only less than half as large as the estimated uncertainties. Based on this set of constrained parameterizations monthly climatological fields of the individual fluxes in the North Atlantic Ocean are computed which are consistent with direct transport estimates.

With a larger set of heat transport observations this method will provide a possibility to discriminate between various bulk formulations, and to obtain more accurate estimates of the air-sea energy flux.

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Claus W. Böning, Ralf Döscher, and Hans-Jörg Isemer

Abstract

The monthly mean wind stress climatology of Hellerman and Rosenstein (HR) is compared with the climatology of Isemer and Hasse (IH), which represents a version of the Bunker atlas (BU) for the North Atlantic based on revised parameterizations. The drag coefficients adopted by IH are 21% smaller than the values of BU and HR, and the calculation of wind speed from marine estimates of Beaufort force (Bft) is based on a revised Beaufort equivalent scale similar to the scientific scale recommended by WMO. The latter choice significantly increases wind speed below Bft 8, and effectively counteracts the reduction of the drag coefficients.

Comparing the IH stresses with HR reveals substantially enhanced magnitudes in the trade wind region throughout the year. At 15°N the mean easterly stress increases from about 0.9 (HR) to about 1.2 dyn cm−1 (IH). Annual mean differences are smaller in the region of the westerlies. In winter, the effect due to the reduced drag coefficient dominates and leads to smaller stress values in IH; during summer season the revision of the Beaufort equivalents is more effective and leads to increased stresses.

Implications of the different wind stress climatologies for forcing the large-scale ocean circulation are discussed by means of the Sverdrup transport streamfunction (ψs): Throughout the subtropical gyre a significant intensification of ψs takes place with IH. At 27°N, differences of more than 10 Sv (1 Sv ≡ 106 m3 s−1) are found near the western boundary. Differences in the seasonality of ψs are more pronounced in near-equatorial regions where IH increase the amplitude of the annual cycle by about 50%. An eddy-resolving model of the North Atlantic circulation is used to examine the effect of the different wind stresses on the seasonal cycle of the Florida Current. The transport predicted by the numerical model is in much better agreement with observations when the circulation is forced by IH than by HR, regarding both the annual mean (29.1 Sv vs 23.2 Sv) and the seasonal range (6.3 Sv vs 3.4 Sv).

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