Böning, Claus W.

English

OceanRep

In: International COADS Wind Workshop. , 31.05.-02.06.1994, Kiel, Germany . Proceedings of the International COADS Wind Workshop. ; pp. 171-178 . Effects of Different Wind Stress Climatologies on the North AtlanticCirculation: Model resultsClaus W. BöningIfM Kiel, GermanyCan we use ocean circulation models to test wind stress fields? Even in the case of aperfect model, several issues need to be considered if we want to learn something fromcomparing simulated volume transports with observations. Obviously, we have to identifycirculation regimes where the oceanic transports are (predominantly) driven by the wind stress,so that changes in the winds have significant effects on the resulting circulation. In addition,we have to focus on those areas in the ocean where the transport is well-known fromobservations. In this report, I shall discuss these issues for the case of the North Atlantic. Themodel being used is that developed by Bryan and Holland (1989) as a “Community ModelingEffort” (CME) under the World Ocean Circulation Experiment (WOCE). The model spans theAtlantic Ocean between 15°S and 65°N. In recent years, a suite of model experiments has beenconducted by the CME groups at NCAR and at IfM Kiel, differing in a number of modelfactors, including horizontal resolution, frictional parameterization, wind and thermohalineforcing. For model descriptions, list of experiments, and detailed accounts of the meancirculation in different parts of the basin, the reader is referred to Bryan et al. (1994) andBöning et al. (1994). Model results discussed in this report are all from a model version with agrid resolution of 1/3 deg. in latitude, 2/5 deg. in longitude, and 30 levels in the vertical.Large-scale ocean circulation is forced by the momentum, heat and freshwater fluxesat the surface. The thermohaline forcing in the Atlantic Ocean is associated with deep waterrenewal in subarctic regions, driving an overturning motion with a northward flow of warmwater in the upper 1000 or 1200 m, and southward flow of cold, North Atlantic Deep Waterbetween 1000 m and 4000–4500 m. The deep flow has a strong impact on the vorticity balanceof the horizontal circulation in the subpolar North Atlantic. Both the CME-results (Bryan et al.1994) and diagnostic modeling by Greatbatch et al. (1991) suggest that the structure andstrength of the subpolar gyre is governed by the thermohaline-driven flow and its interactionwith bottom topography, and to a much lesser degree by the wind stress. (This does not hold,however, for the seasonal variability which, in good approximation, can be understood in termsof a linear response to the variation in the wind stress curl. Lack of data would presently notallow to use this behavior for a test of different wind stress fields in the subpolar NorthAtlantic.) A model evaluation in terms of a wind-driven transport has to be restricted to theupper-layer circulation in the tropics and subtropics, south of the Gulf Stream recirculationregime.The second requirement noted above, knowledge of volume transports fromobservations in the ocean, readily lead to further restrictions. As recently discussed in thereview of Schmitz and McCartney (1993), there is very little quantitative information on171oceanic transports; the singular exception for the North Atlantic is the transport through theStraits of Florida which had been studied over many years with different methods. The meannorthward transport is 30 - 31 Sv, with little interannual variability, and a well defined seasonalcycle. Both the mean and the seasonal variation of the Florida Straits transport had been studiedwith the CME model, using two different wind stress climatologies (i.e., HR and IH,respectively; see below) (Böning et al. 1991a). As it turns out, however, focusing on the FloridaStraits transport alone is of limited value for a test of the wind driven circulation in the subpolargyre. The seasonal variation of the Florida Current is largely due to the variation of themeridional wind stress along the coast (Böning et al. 1991a). The mean transport, on the otherhand, only partly represents the northward return flow of the wind-driven, southward flow overthe interior of the basins; about 40% is considered to be part of the meridional overturning(Schmitz and Richardson 1991). Some fraction of the wind-driven gyre flow does not enter theCaribbean Sea to feed the Florida Current, but flows northward in a western boundary currentto the east of the Bahamas (in what is sometimes called the Antilles Current, though it does notrepresent a continuous flow along the Antillean Archipelago). In the context of modelvalidation, this leads to at least two problems. First, even after several years of current metermeasurements, there is considerable uncertainty about the mean transport due to a strongvariability on monthly time scales, associated with a meandering of the current axis (Lee et al.1990); recent estimates seem to converge at 5–10 Sv (at 26.5°N). Second, in model simulationsthe fraction of the flow entering the Caribbean through the narrow island passages must besensitive to details of the topography, friction, etc. The situation for the 1/3-degree CME modelis illustrated in Fig. 1, showing the mean flow in the western subtropical North Atlantic at 232m and 2125 m, and Fig. 2, showing a zonal section of the mean meridional velocity at 26°N.According to the situation described above a quantitative model-data comparisonconcerning the wind-driven transport in the subpolar gyre cannot focus on the Florida Current;it has to be based on the total northward transport in the western boundary currents (WBQ tothe west and east of the Bahamas. Present estimates for this are 35–40 Sv (and one can expectthe uncertainties to decrease over the next several years due to ongoing measurement programseast of the Bahamas). In addition we need information about the fraction of the total WBCtransport associated with the thermohaline overturning. (Model results indicate that at thislatitude, because of the confinement of the deep flow to the western boundary, the totalnorthward transport may in good approximation be considered as a linear superposition of athermohaline and a wind-driven part.) From zonal, transatlantic hydrographic sections along25°N the overturning is estimated to be about 15 Sv, leaving 20–25 Sv for the net contributionof the wind-driven gyre to the northward, upper-layer flow at the western boundary. Thetransport budget at this latitude is schematically illustrated in Fig. 3.CME experiments have been carried out with four different, monthly mean wind stressclimatologies: the stresses based on historical marine observations given by:(i) Hellerman and Rosenstein (1983); in the following denoted HR;(ii) Isemer and Hasse (1987), denoted IH;(iii) the model stresses given by Trenberth et al. (1990) based on their analysis of the1000 mb winds of the ECMWF for the period 1980 - 1986, denoted ECMWF;and(iv) monthly mean stresses from a 10 yr. control integration of NCAR’s ClimateCommunity Model 2; denoted CCM2.172A more detailed discussion of the mean circulation in these experiments may be foundin Bryan et al. (1994). The wind stress climatologies in (i) and (ii) are compared in Böning etal. (1991b). The interesting question in the present context is to which extent theseclimatologies lead to differences in the transport of the subpolar gyre in the North Atlantic. Asoutlined above, this may be boiled down to a single number: the southward transport in theupper layer (top 1000 m) at 25°N, between the coast of Africa and the eastern edge of the WBCeast of the Bahamas. The results are24 Sv for HR31 Sv for IH25 Sv for ECMWF19 Sv for CCM2.These numbers represent averages over several years of integration. (For a discussionof the interannual variability see Bryan et al., 1994.) Comparison with the observedwind-driven transport indicates IH to be too strong, HR and ECMWF about right, and CCM2somewhat too weak at this latitude.In summary, it is important to stress the singularity of the situation at 25°N: it representsthe only latitude in the North Atlantic where we both have fairly good information on the totalvolume transport and can, to a good approximation separate the relative contributions from thethermohaline and wind-driven flows. It is important to stress also, that for a model-datacomparison a circulation model with sufficiently fine grid spacing to resolve the WBC systemis required; and that a comparison can not be based on the well-measured Florida Current alone,but has to take into account the significant, but less-known transport to the east of the Bahamas.173ReferencesBöning, C.W., R. Doscher, and R.G. Budich, 1991: Seasonal transport variation in the westernsubtropical North Atlantic: Experiments with an eddy-resolving model. J. Phys.Oceanogr., 21, 1271-1289.Böning, C.W., R. Doscher, and H.-J. Isemer, 1991b: Monthly mean wind stress and Sverdruptransports in the North Atlantic: A comparison of the Hellerman-Rosenstein andIsemer-Hasse climatologies. J. Phys. Oceanogr., 21, 221-235.Böning, C.W., F.O. Bryan, W.R. Holland, and R. Doscher, 1994: Thermohaline circulation andpoleward heat transport in a high-resolution model of the North Atlantic. Submitted toJ. Phys. Oceanogr.Bryan, F.O. and W.R. Holland, 1989: A high-resolution simulation of the wind andthermohaline-driven circulation in the North Atlantic Ocean. In: parameterization ofsmall-scale processes. Proceedings “aha huliko'a”, Hawaiian Winter Workshop,University of Hawaii, 99-115.Bryan, F.O., OW. Böning, and W.R. Holland, 1994: On the mid-latitude circulation in ahigh-resolution model of the North Atlantic. Submitted to J. Phys. Oceanogr.Greatbatch, R.J., A.F. Fanning, A.D. Goulding, and S. Levitus, 1991: A diagnosis ofinterpentadal circulation changes in the North Atlantic. J. Geophys. Res., 96, 22009-22023.Hellerman, S. and M. Rosenstein, 1983: Normal monthly wind stress over the world ocean witherror estimates. J. Phys. Oceanogr., 13, 1093-1104.Isemer, H.-J. and L.Hasse, 1987: The BUNKER Climate Atlas of the North Atlantic Ocean.Vol. 2: Air-Sea Interactions. Springer Verlag, 256 pp.Lee, T.N., W. Johns, F. Schott, and R. Zantopp, 1990: Western boundary current structure andvariability east of Abaco, Bahamas, at 26.5°N. J. Phys. Oceanogr., 20, 446-466.Schmitz, W.J., Jr. and W.S. Richardson, 1991: On the sources of the Florida Current. Deep-SeaRes., 3 8, 3 79-409.Schmitz, W.J., Jr. and M.S. McCartney, 1993: On the North Atlantic circulation. Rev.Geophys., 31, 29-49.Trenberth, K.E., W.G. Large, and J.G. Olson, 1990: The mean annual cycle in global windstress. J. Phys. Oceanogr., 20, 1742-1760.174Figure 1a: CME model estimate of mean flow in the western subtropical North Atlantic,at 2125 m depth.Figure 1b: CME model estimate of mean flow in the western subtropical North Atlantic,at 2125 m depth.175Figure 2: Zonal cross-section of mean meridional velocity near 26°N.176Figure 3: Ocean transport budget at 26°N. Values is Sv.177

Effects of different wind stress climatologies on the North Atlantic circulation: Model results

http://oceanrep.geomar.de/7964/1/Kiel.Boning.pdf

Effects of different wind stress climatologies on the North Atlantic circulation: Model results

Abstract

Similar works

Full text

Available Versions

OceanRep