SECT. 2] LARGE-SCALE INTERACTIONS 181 



arrow denotes the per cent of the time the wind blows from the specified 

 direction ; the mean wind speed in that directional category is symbolized by 

 the barbs, one for each Beaufort force number. Since a single Beaufort force 

 number may cover a range of about 5 knots, to compute the resultant stress 

 it is imperative to know or assume something of the speed distribution within 

 each Beaufort category for each direction. 



In addition to these problems, our greater uncertainty in the results of 

 momentum-exchange calculations compared to those of heat and moisture is 

 compounded by fewer and less definitive independent checks from budget 

 studies. In constructing momentum budgets, the to of (17) cannot be pinned 

 down by finding it a residual in an equation with all other terms readily com- 

 puted, since the lateral and other stress components can rarely even be esti- 

 mated. In kinetic energy budgets, the dissipation by surface stress cannot be 

 so isolated either, since the uncomputable internal dissipation may be com- 

 parably large (see, for example, Palmen and Riehl, 1957; Malkus and Riehl, 

 1960). Nor is the dissipation of kinetic energy a non-negligible and, therefore, 

 an assessable term in any of the heat budgets. 



The best hope of improved surface-stress determinations lies, first, in the 

 joint theoretical and experimental programs currently underway by many 

 turbulence study groups and individuals (reviewed, for example, by Sheppard, 

 1958; Lettau and Davidson, 1957; Priestley, 1959) and, secondly, in careful 

 testing of (17) under known and/or controllable conditions where other methods 

 of stress evaluation are simultaneously employed. Among these have been the 

 method of relating the stress to the departure between actual and geostrophic 

 wind (Charnock, Francis and Sheppard, 1956), which requires extensive and 

 accurate pressure field determination and a relatively simple meteorological 

 situation. Also, the direct method of determining momentum fluxes from air- 

 craft [see equation (22), page 113, and Bunker, 1955] is now improved to 

 include a broad spectrum of eddy sizes, but is still beset by a sampling problem 

 and expensive laborious data reduction. Meanwhile, with these reservations, 

 we shall present the best large-scale determinations currently available. 



B. Momentum Exchange Climatology 

 a. Computations of mean shearing stresses oter the oceans 



The most careful, systematic and extensive mappings of oceanic stresses 

 were carried out in 1948-50 at the Scripps Institution of Oceanography (Oceano- 

 graphic Reports No. 14 and No. 21). Their purpose was to present resultant 

 stress maps for the Pacific (5°S to 60°N latitude) and Atlantic (20°S to 60°N 

 latitude) Oceans by 5° quadrangles for each month. Formula (17) was used with 

 the cd dependence of Montgomery (1940), which changed discontinuously from 

 0.8 X 10-3 for Wa<6.7 m/sec (smooth surface) to 2.6 x 10-3 for Wa>6.7 m/sec 

 (rough surface) as shown by the dashed line in Fig. 6. Hidaka (1958) has 

 extended their method to produce charts for the Southern Hemisphere and 

 Indian Ocean. 



