SOME CHARACTERISTICS OP SEA WATER STRUCTURE 653 



homogenizing is indicated in structures C and D. Eventually a step 

 structure such as E may result. It is important to realize that each 

 advection must tend toward congruity as it becomes deeper, and more 

 mature, because once removed from the source at the surface no further 

 advection can take place, and the homogenizing tendency proceeds un- 

 checked. Thus step structure degenerates towards a simple logarithmic 

 gradient at maturity. Obviously step structure with respect to any 

 property can occur at any depth or zone where a new source of sink is 

 provided. Excellent examples of such deep phenomena occur in the 

 margins, and confluences of the great ocean currents. 



The Meaning of Structure 



Studies now in progress indicate that each structure is a historical 

 record of the influences and processes which the water masses have 

 experienced in the sea. These are too extensive for general discussion 

 here, but one simple example may be cited for illustration. 



The origin of the minimum temperature zone in the northeast Paci- 

 fic Ocean has been made apparent by the seasonal sequence of structures 

 shown in Figure 10. In March, towards the end of winter the upper 

 zone waters are isothermal and at a minimum temperature, somewhat 

 colder than the top of the deep zone. As the season advances the sur- 

 face waters are warmed by vernal heating. This process may be re- 

 garded as the accumulation of heat from successive afternoon effects 

 and wind mixing, as illustrated in the sequence of structures in Figure 9. 

 The warm upper zone continues to increase through the summer to 

 mid-September. However, in the autumn the surface cools and the 

 violent winter storms mix the waters. In this process the upper zone 

 cools but continues to become deeper. Eventually its boundary inter- 

 cepts the remnants of the previous winter boundary, and finally the 

 characteristic late winter structure is formed. 



Conclusion 

 It is evident from these remarks that the concepts of zones and 

 logarithmic structure are at least plausible, and the consequent tech- 

 nique of the log-plot affords a new and powerful oceanographic tool. 

 The present examinations have been limited to the coastal and oceanic 

 waters of the northeast Pacific Ocean, but it appears that the principles 

 involved should be general. However, such a conclusion must await 

 study of data from other places, and examination of the ideas by other 

 oceanographers. It is hoped that such studies will enrich the present 

 experience. 



