SECT. 2] LARGE-SCALE INTERACTIONS 217 



Large latitudinal temperature differences cause their axes to slope equatorward 

 with height, superimposing westerlies aloft over most of the tropical easterlies 

 (Figs. 22, 23, 26b and c). Pressure heads are maximal ; the average trade-flow 

 is vigorous, importing cool, dry air rapidly into the tropics, so that exchange of 

 all properties is greatest in the winter months. Interaction between the dis- 

 turbances of the mid-latitude westerlies and the trades is also common from 

 time to time, and the tropics are frequently invaded by "polar troughs" and 

 shearlines, left over remnants of the polar front which cause strong resurgences 

 of the trades in their rear (Riehl, 1945, 1954). Except in such disturbances, 

 the moist layer is generally shallow and over most of the tropics proper 

 winter is the dry season. i 



In summer, the subtropical high cells are farthest poleward, weakest, and 

 their axes nearly vertical. The trades are slower but deeper and largely de- 

 coupled from middle latitudes. The thicker moist layer is deformed by dis- 

 turbances of purely tropical origin, such as the easterly wave and equatorial 

 vortex (Riehl, 1945 ; Palmer, 1952). Since both zonal and meridional circulations 

 are more sluggish, reduced property differences and wind speed combine to 

 produce, over the sea, the minimum seasonal values of Qg, Qe and shearing 

 stress. On the other hand, the oceanic trades have a summer rainfall maximum. 



Aside from the periodic fluctuations in exchange imposed by the seasonal 

 and diurnal cycles, the two causes of the greatest alterations in air-sea inter- 

 action are aperiodic : namely, those associated with variations in overall 

 circulation strength, of one to several weeks in duration, and those associated 

 with the passage of synoptic-scale disturbances which, operating on the 1-3 day 

 scale, can create the largest disruptions of all. Unfortunately, periodic or 

 otherwise, all of these factors are mutually interdependent, as we shall see, and 

 their effects upon exchange superpose in a fashion scarcely likely to be linear. 

 In the context developed by investigating these, still longer period (climatic) 

 fluctuations will be brought up briefly in the last section. On the short side of 

 the spectrum, meso-scale (individual cloud group, 10-100 km size and duration 

 of some hours) variations in air-sea interaction have, as we saw, just been 

 opened up for study (Malkus, 1957; Bunker, 1959). 



a. Effects of changes in overall circulation strength 



Within each season, there are variations in the strength and character of the 

 trades associated with large-scale changes in the position and develojiment of 

 the subtropical high pressure cells over major parts, or all, of a hemisphere. 

 Periods of several weeks occur when the ridges show higher than normal 

 central pressures and are stretched out from east to west. The pressure dif- 

 ference from subtropical ridge to equatorial trough is then large. The trades 



1 Exceptions are found in the polewartl fringes. An oceanic example is the Hawaiian 

 region where (Usturbances of the westerly jet stream produce a winter rainfall maximum. 

 Continental examples are subtropical west coasts, such as California and Chile, where 

 upwelling enhances summer dryness. 



