78 Wind System over the North Atlantic 



INTERRUPTIONS IN THE NORMAL CIRCULATION 



Chase (1951, p. 6) also pointed out that only about half of the daily surface- 

 pressure maps look at all Kke the monthly normal maps: 'A large pro- 

 portion of the daily maps resemble the average maps in position and 

 orientation of the Bermuda-Azores High and in the circulation about it. 

 These situations are most common in the summer (frequency estimated at 

 some 60% of the time, and least common in winter [about 30%]). The 

 frequency for fall is slightly higher than spring.' The rest of the daily 

 maps are quite irregular, showing a number of interruptions which he has 

 classified as : fronts ; stagnant lows ; linkages of the Bermuda- Azores High 

 to continental highs; and hurricanes. 



Some fronts pass to the north of the Bermudar-Azores High without 

 particularly disturbing it. Others, such as the front showTi in fig. 49, seem 

 to force it toward the south. Normally these frontal interruptions occur 

 about twice as often in the winter as in the summer, the duration of each 

 interruption being about a week. 



The stagnant lows generally persist over the North Atlantic for a 

 longer time. Such lows occur about half as often as the frontal 

 passages. 



Frequently a continental high coalesces with the Bermuda- Azores high, 

 causing interruptions of the normal circulation, and these in turn result in 

 widespread irregularities and shifts in wind. 



Hurricanes occur in summer and autumn, but primarily in September. 

 They are by far the least frequent interruption to the mean circulation. 

 Fig. 50 represents a very rare situation, that of four hurricanes occurring 

 simultaneously. 



THE WIND STRESS ON THE OCEAN 



Even though we have a good idea of average wind conditions over the 

 ocean, we can only make crude approximations to the stress produced by 

 these winds on the ocean surface (Rossby, 19366). 



The first approximate values of wind stress were computed (Ekman, 

 1905) from the mean slope of the water surface in the Baltic Sea, as 

 observed by tide gauges. According to a first-order theory, the local slope 

 of a water surface depends only upon three factors, namely, the surface 

 stress, the bottom stress, and the depth of the water (if the water is 

 assumed to be vertically homogeneous). The vertical internal turbulent- 

 eddy viscosity does not appear in this expression. In a natural body of 

 water where neither the depth nor the wind distribution is uniform, and 

 very httle is known concerning the bottom stress, this method is very 



