The Representation of Oceanic Movements and Kinematics 359 



Explanation (to Fig. 149) 

 The current roses are drawn from observations within the areas shown by the pecked Hnes. 

 Arrows indicate direction of current; north arrow current towards N. Velocity of current 

 in nautical miles per day is represented as follows : e-iz '^-^-^ ?5-4b^ 49-72 TSon^ove _ Length 

 of arrows represents frequency, 1 mm 3-7%: j j j i [ . The lower 



o 50 I °° % 



figure within the circle gives the total number of observations, the upper figure the per- 

 centage frequency of currents less than 6 miles per day. 



falls within a quadrant this will be already predominant and its middle line can be 

 regarded as the direction of the prevailing current. If the percentage of the ship dis- 

 placements falling within the quadrant is between 33% and 66% then the prevaiUng 

 current is termed ''variable''. The next grade ''rather steady is reached when at least 

 33% of all observations fall not only within one quadrant but within one octant. If 

 more than 61% of observations fall within a quadrant and between 33% and 66% 

 within an octant within the quadrant then the prevailing current is denoted "steady \ 

 if both quadrant and octant contain more than 67% of all observations the current is 

 ""'very steady"'. This characterization of stabihty is undoubtedly more illustrative 

 than the ratio of the vectorial and scalar sums of the velocities. 



An example of this type of representation is given in Fig. 1 50 which shows the chart 

 for August of the surface currents in the North Atlantic as given by Schumacher. The 

 length of the arrows indicating the prevailing direction has no significance here. The 

 velocity is given by feathering or for large values by barbs at the arrow-heads; for 

 the grade of the stability see the explanation on the chart. 



A similar evaluation of ship displacements has also been given by Schumacher 

 (1943) for the South Atlantic so that modem monthly charts are now available for the 

 whole of the Atlantic Ocean. 



(c) Current Patterns and their Interpretation 



Certain definite properties of the current field must be borne in mind in plotting 

 stream lines on the basis of the current vectors. In the j&rst place it should be noted 

 that except at singular points and lines : 



(1) the individual stream lines are not allowed to intersect; 



(2) the stream lines are curves that neither start nor finish in the current field ; 



(3) the stream lines are always continuously curved lines. 



The stream lines are drawn mostly by vectorial interpolation by the eye. Such a 

 graphical interpolation usually offers little difficulty if the current vectors cover the 

 whole chart uniformly. However, this is usually not the case and the lines must some- 

 times be drawn with a minimum of observational values. For this it is necessary to 

 have some idea of the singularities in the current field (Bjerknes and co-workers 1912, 

 1913). Because the position of these singularities fixes the general outline of the field 

 and to complete the pattern then offers little difficulty. 



The simplest singularities and their relationship to the structure of the water masses 

 in the oceans will be described in the following section. 



Lines of convergence a?id divergence. Figure 1 5 1 shows convergence and divergence 

 from only one and from both sides of the stream lines. In case (a) and {b) there is 

 an infinitely rapid convergence and divergence; cases which are rarely found in this 

 extreme form. An infinite number of stream lines leaves or enters asymptotically 



