Fluctuations 149 



graphic-station data) does not respond completely to those fluctuations in 

 the wind field which have periods of much less than a year. Since such 

 arguments as the ones given under a-c might very well be misleading, 

 Dr George Veronis and I (1956) have studied in detail the response of a 

 two-density ocean to a variable apphed wind stress. The model studied 

 does not have boundaries, but the apphed wind stress is taken as periodic 

 in space. Friction does not appear to be an important factor for periods 

 of a year or less, and inertial-gravitational wave motions are neghgible for 

 periods of more than several days. In order that the motion for a very long 

 period shall approach the motion represented in the steady-state solutions 

 by Sverdrup, Reid, and Munk (see Chapter VII), it is necessary to include 

 /?, the variation of CorioHs parameter with latitude. For very long periods 

 (thirty years or more), the theoretical currents induced by the wind are 

 confined entirely to the upper layer, as is very nearly true of wind-driven 

 currents of the real ocean, and this surface intensification of currents pro- 

 duced by a transient wind system does not occur without y?. 



The essential point to make clear is that for periods of between a week 

 and a year the density field in the ocean at mid-latitudes does not attain 

 a full equilibrium response to the variable wind-stress pattern at any 

 instant ; but it does respond. Thus, in the North Atlantic, we cannot expect 

 the depth of the main thermochne to adjust itself completely to the 

 seasonal progress of the Bermuda-Azores anticyclonic wind system ; on the 

 other hand, the main thermochne cannot be wholly insensitive to the 

 seasonal wind changes, although it would be essentially insensitive to 

 wind-stress changes with periods less than a week. The currents induced by 

 wind-stress patterns with periods between a week and a year are not 

 confined to either layer, but the vertically integrated transport is very 

 nearly in equihbrium with the applied wind stress at any instant. Thus, so 

 far as vertically integrated transport is concerned, the results obtained by 

 Veronis and Morgan (1955) using a homogeneous model and a viscous 

 Gulf Stream should be apphcable even for a density-stratified ocean. How- 

 ever, the wind-induced variations of oceanic circulation should not be 

 accurately determinable by the traditional method of geostrophic computa- 

 tion from the density field ; and the currents are so small that they cannot 

 be observed directly by current meter; and the horizontal displacements 

 involved are too small to produce noticeable changes in the distribution of 

 deep water masses. 



Apparently, in central oceanic areas (away from the equator) the chief 

 directly observable effect of the purely wind-induced seasonal variation of 

 oceanic circulation is the change in mean sea level, but even this is nearly 

 obscured by seasonal changes associated with changes of local density 

 arising from local heating, sahnity variations, and so on. The island of 



