20»N-' 



100° w 



Figure 5. — Volume transport streamlines for the deep (potential temperature less than 4°C) general circulation of the North 

 Atlantic according to Worthington. Numbers indicate transport in Sverdrups The locations of Array 1 (along 28°N. and 

 60°W) and Array 2 (along 55°W) are indicated by straight lines. 



The distributions of momentum flux (off-diagonal com- 

 ponent of the horizontal Reynolds stress tensor) in figure 6C 

 indicate that the dynamics of the mean flow may be significantly 

 influenced by the structure of the deep-eddy field. The mo- 

 mentum transport by the eddy field is neglected relative to the 

 momentum transport by the mean flow in inertial models of 

 the Gulf Stream or of the subtropical gyre. Taking one term as 

 an indicator of the potential relevance of this assumption, the 

 latitudinal derivatives of u v and u~v are of the same order for 

 the 4,000 m data along 55° W. In frictional models, the mo- 

 mentum transport by the mean flow is neglected relative to the 

 momentum transport by the fluctuations, and a constant posi- 

 tive eddy viscosity is used. Again looking at one term as an 

 example, the ratio (eddy viscosity) of -lTv to the latitudinal 

 derivative of the zonal mean flow is both positive and negative, 

 and varies by an order of magnitude for the data along 55°W. 

 This ratio is negative in the region of eastward mean flow to 



the offshore side of the axis of the Gulf Stream and in the 

 westward return flow. The corresponding t erm (the negative 

 product of u and the latitudinal derivative of li v) in the energy 

 budget for the mean flow, based on 4,000 m data at 55°W, is 

 the largest (and from low-frequency fluctuations to the mean 

 flow) in the regions of eastward mean flow offshore of the 

 axis of the Gulf Stream and in the westward return flow to the 

 south. 



Recent gyre-scale numerical models capable of resolving 

 eddies yield a deep-mean circulation driven by the eddies. 

 This class of models and the data described above are quali- 

 tatively similar in some respects; detailed quantitative com- 

 parisons await more thorough analysis, a more substantial 

 data base, and more realistic models. Nevertheless, it seems 

 possible that the directly wind-driven segment of the North 

 Atlantic subtropical gyre is confined to about 30 Sv moving 

 through the Straits of Florida, with a sizeable fraction of the 

 downstream increase (120 Sv according to Worthington) in 

 transport of the Gulf Stream system induced by the spatial 

 inhomogeneity in the eddy field, especially at depths below the 

 main thermocline. 



Joint U.S.-U.S.S.R. Activiries 



Work with the Soviet Union has consisted of planning 

 meetings, theoretical and experimental discussions, and joint 

 cruises to intercompare instruments. Three specific results 

 of this activity are: 



1. POLYMODE Joint Summer Theoretical Institute. A 

 3-week meeting of theoreticians and computer modelers 

 was held in Yalta, U.S.S.R., in August 1976. The meet- 

 ing brought together scientists actively involved in the 

 POLYMODE experimental program with those newly 



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