190 MALKUS [chap. 4 



production is given by the single parameter of wind steadiness. Figs. 22 and 23 

 showed maps of the prevaihng surface winds over the oceans, their strength 

 and steadiness. These should be used in conjunction with mean stress maps 

 such as those of the Scripps report. 



c. The global distribution of momentum exchange and its time variations 



The mean annual stress distribution is well illustrated by Figs. 38 and 41, 

 which are not, however, exactly comparable due to differences in data sources 

 and computation method. Extreme seasonal variations in both oceans are 

 revealed by Figs. 39, 40, 42 and 43 (in conjunction with Figs. 22 and 23). The 

 Northern Hemisphere trade- wind stress weakens considerably from winter to 

 summer, particularly in the Pacific, while the compensating westerly drag in 

 temperate latitudes entirely shifts hemispheres with the season. The effects of 

 the Asiatic monsoon are pronounced over the Japan Sea in winter, with 

 large northerly stresses occasioned by the outflow of cold continental air. 

 The unsteady flow in that region of Fig. 22 suggests that this occurs in 

 intermittent outbursts, probably in the polar outbreaks following frontal 

 cyclones. 



Altogether, in the belt of westerlies, the resultant stress arises from a succes- 

 sion of short-lived synoptic-scale atmospheric eddies, and the mean stress 

 picture for a month says relatively little about the stress that might actually 

 be working on the ocean in a given period or even over a given season. The 

 relative constancy of the large-scale ocean currents must, therefore, be attri- 

 buted to two factors : namely, the high inertia of the ocean's response to 

 fluctuations in stress, which is beyond the scope of the present chapter ; and 

 to the reliability of the easterly trade winds, which is thus of major import to 

 ocean dynamics. The operation and steadiness of the trade system is analyzed 

 next. 



C. Momentum Production and Flow Steadiness in the Trades 



In Section 5 of this chapter, the role of the oceanic heat source in maintaining 

 the lower trade winds was introduced in terms of the downstream pressure 

 drop. But we did not explicitly explore what the pressure head was used for, 

 nor did we relate it, other than spatially, to the remarkable steadiness of the 

 flow, which also vanishes above the inversion. In the foregoing, we have shown 

 that shearing stress is the momentum link between air and sea. Stress is also 

 the internal momentum linkage within the atmosphere and its height derivative 

 is a major frictional drag term in equations of air motion. We shall now in- 

 corporate this turbulent drag with the heating in a dynamic model of the 

 trades and their stability. This paves the way to examining the mechanisms of 

 momentum production and its input to the ocean at the sea-air boundary, thus 

 completing a full cycle in the coupling of the systems. 



