boundary conditions along the equator. Another "source", which 



is probably very important, is the flow of water through the Straits 



of Florida, 



Along the northern boundary a "source" has to be considered 

 northeast of the Grand Banks due to the inflow of the Labrador Cur- 

 rent. An important "sink" between Great Britain and Iceland is 

 given by the transport of the North Atlantic Current which accounts 

 for a net balance of the water in the region covered by the model. 

 The total net intake of water between the equator and the northern 

 boundary, that is, the sum of the "sources" and "sinks", must be 

 zero. 



For a numerical integration of the differential equation (11) 

 the surface of the ocean is thought to be covered by a series of 

 lattice points as given by the intersections of a set of rectangular 

 lines in the north-south and the east-west direction. The lattice 

 of this net consists of "interior" points and "boundary-points". 

 Interior points are called such points which are surrounded by ad- 

 jacent points belonging to the lattice of the covered ocean area. 

 An interior point, therefore, is always surrounded by four adjacent 

 points, whereas a "boundary point" has less than four adjacent points. 

 The unit mesh width of the lattice was chosen as d = 222 km; how- 

 ever, in the western North Atlantic it may be necessary to use a 

 smaller unit (55*5 km) in order to get some of the details of the 

 Gulf Stream flow pattern. This section will be considered by a 

 special evaluation of the basic differential equation. 



The differential equation (11) for the horizontal mass trans- 

 port can be written in the form 



49 



