AEROGRAPHER'S MATE 3 & 2 



When commencing the SST, the approach is 

 similar to the meteorological analysis of air 

 masses. In this case, the determination is the 

 orientation of the water masses. Since the water 

 masses In an area are dependent upon current 

 structure, consideration must also be given 

 to this element. 



Each area of the oceans has its local char- 

 acteristics such as bottom topography, current 

 flow, prevailing winds, etc. Since it would be 

 confusing to attempt to describe an analysis 

 of all the oceans at once, this discussion will 

 be confined to the Western North Atlantic area. 

 However, the same general rutes apply in other 

 areas as well, with modification dependent on 

 the local characteristics of the area. 



CONSIDERATIONS RELATING TO SST ANAL- 

 YSIS.— The western North Atlantic is dominated 

 by a large, clockwise rotating water mass, the 

 Sargasso Sea, with the Gulf Stream flowing 

 along the perimeter as a restriction to the out- 

 flow of water from the area. 



Characteristics of the Gulf Stream.— The 

 Gulf Stream flows from a region of excessive 

 evaporation and surface heating into one of 

 excessive fresh water (precipitation and thawing 

 ice) and surface cooling. Consequently, the Gulf 

 Stream waters show decreasing salinity with 

 depth; and upon moving northward, they show 

 increasing instability as a result of convective 

 mixing. These factors tend to produce more 

 pronounced velocity structures in warm water 

 currents since by their nature they exhibit 

 deeper mixing. Cold currents, such as the 

 Labrador waters, tend to become more stable 

 as they move south since, in addition to the 

 characteristic salinity increasing with depth, 

 surface heating adds to their stability. These 

 characteristics prevent deep mixing, however, 

 so that the cold currents are more poorly 

 defined with depth. 



One characteristic feature which exerts con- 

 siderable influence on the water structure is 

 the "meanders" in the Gulf Stream. Although 

 the definition of meander is "to wander aim- 

 lessly," the meanders of the Gulf Stream, pri- 

 marily due to bottom topography, are quite 

 persistent. 



As the meanders continue to occur, momen- 

 tum forces some of the water up over colder 

 more dense water to the north causing many 

 tongue like protrusions. This configuration is 



evident in the SST analysis illustrated in fig- 

 ure 11-23. This overriding effect also occurs 

 to the south; however, it is not so pronounced. 



Meanders in the Gulf Stream are also af- 

 fected by fluctuations in the wind field. For 

 example, if the Bermuda High is located south 

 of its normal position, there is a resulting 

 increase in the trade winds. This in turn causes 

 an increase in the Gulf Stream velocity. 



The rate of the meander motion may be on 

 the order of 10 to 15 nautical miles per day 

 but is often much slower. 



Seasonal Temperature Variations of the West- 

 ern North Atlantic. — During the summer season 

 the sea surface temperatures of the Gulf Stream 

 usually run 75° F or higher, and the Sargasso 

 Sea surface temperatures are in excess of 

 80°F. 



During the winter season the Gulf Stream 

 surface temperatures are normally in the 70° 

 to 80° F range and those in the Sargasso Sea 

 generally 70°F or greater. 



From the preceding discussion it is appar- 

 ent that the temperatures of the surface water 

 of the Sargasso Sea and the Gulf Stream nor- 

 mally do not vary to as great an extent as do 

 other areas adjacent to the Gulf Stream. For 

 example, water to the north of the Gulf Stream, 

 originating in the Labrador Sea or farther north 

 in the Arctic and moving southward will create 

 comparatively large temperature differences over 

 short distances. Therefore, the analysis will 

 show strong horizontal temperature gradients 

 along the northern edge of the Gulf Stream. 

 The contrast between the cold Labrador and the 

 warm Gulf Stream water creates a narrow 

 transition zone called the north wall, which is 

 similar to a front between air masses. 



Although the temperature gradients along the 

 north wall are strong during all seasons, they 

 are somewhat stronger during the winter. The 

 north wall (boundary between Gulf Stream and 

 Labrador water) is usually outlined by the 68° F 

 isotherm in September in the Cape Hatteras 

 area, with these temperatures decreasing to 

 the east. The north wall of the Gulf Stream 

 is usually not identifiable east of 60° west 

 longitude. 



The seasonal sea surface temperatures of 

 the counter current range from 75° to 77° F 

 in the summer (occasionally higher in late 

 summer) to 60° to 69"F during the winter sea- 

 son. 



220 



