Uncorrelated anomalies often exist in areas of vertical boundaries 

 where freq.uent advections at both levels may cause strong anomalies of 

 different origin. However, the extent of such abnormal areas is small 

 in comparison with the entire area of the ocean, and the resulting un- 

 correlated anomalies are normally produced by different causes than 

 tongue-like advection in boundary areas. 



The number of uncorrelated pairs of anomalies does not provide a 

 sufficient base for firm conclusions; however, monthly weather charts 

 for the 37 cases indicate two principal causes of reversed anomalies at 

 two levels. The first is horizontal advection at one level or different 

 horizontal advection at both levels, and the second is sixrface heating. 

 The basic annual anomaly is probably established in winter in the entire 

 mixed layer, which usually extends below UOO feet. If positive anomalies 

 exist at both levels in winter, they will be present at both levels after 

 the thermocline exists only above the 400-foot level. Later, if pro- 

 longed strong westerly winds develop, the mean southward transport of water 

 in the mixed layer seems to produce a negative anomaly at the surface 

 while a positive anomaly is maintained below the thermocline at ^00 feet. 

 If negative anomalies originally exist at both levels, prolonged strong 

 easterly winds may produce a positive anomaly at the surface, and the 

 negative anomaly remains at 400 feet. 



The second main cause of uncorrelated anomalies (surface heating) 

 occurs in the upper mixed layer in areas of prolonged weak variable winds 

 and calms. Studies of surface anomalies (3^^) established no correlation 

 between heating processes and surface anomalies; however, subsurface 

 conditions were not considered. The mixed layer thickness must be an 

 important factor; and the results would probably have been different, if 

 thermocline depths had been included in the computations. Obviously, a 

 shallow mixed layer is heated more easily than a deep mixed layer. In- 

 creased heat in the mixed layer is not increasingly conducted to the 

 iiOO-foot level owing to increased stability of the thermocline. Admixture 

 of cool water from deeper strata may cause the anomaly at 400 feet to 

 become less positive or more negative. This change of value results 

 from stirring caused by internal waves and horizontal flow. Reversed 

 anomalies of this origin seem to occur more frequently in areas south of 

 the North Atlantic Current than they do to the north. 



A typical example of a large positive surface anomaly produced by 

 heating in summer during periods of very limited mechanical mixing is 

 shown in figure 10. The three BT's were taken at 1-hour intervals with- 

 in the same 1-degree square (50°N,10°W). Note the change of the very 

 thin mixed layer of about 12 feet in the first BT to a thermocline ex- 

 tending from the surface to about 200 feet 2 hours later in the third 

 BT. The surface anomaly is 5.3°F^ and the anomaly at 400 feet is 0.5°F. 

 The 400-foot anomaly was computed with the mean obtained with equation 

 (9). 



^ 



