mixing. The resulting conditions include an in- 

 crease in surface temperatures, the develop- 

 ment of a thin layer of light water offshore, a 

 modification of the surface circulation depend- 

 ing on the nature of the variation in the wind 

 stress, and the biological changes associated 

 with the altered environment. At the same time, 

 in a region such as the Peruvian offing, where 

 a cold coastal current turns away from the 

 coast at low latitudes (see below), the well- 

 defined zone of transition to warmer waters 

 may be at higher latitudes than usual. Testing 

 of this hypothesis requires a better set of 

 oceanographic and meteorological observa- 

 tions than is yet available from Peru. Analog- 

 ous situations are undoubtedly present off other 

 upwellmg coasts, such as California, where 

 data may be more abundant. 



A paper on this subject, entitled "El Nino", 

 was published in California Cooperative 

 Oceanic Fisheries Investigations, Reports, 

 vol. 7, p. 43-45, 1960. 



PHYSICAL FEATURES AND PROCESSES 

 IN THE OCEAN 



This section of the paper deals with the 

 physical oceanography of selected tuna fishing 

 areas of the eastern tropical Pacific, with 

 some reference to similar areas in other 

 parts of the ocean. The work was done to 

 improve our understanding of the distribution 

 of tropical tunas in those areas; it is further 

 discussed under "Tuna ecology." 



Comparative Study of Eastern Boundary 

 Currents (W. S. Woosler and J. L. Ueid) 



The Peru Current and California Current 

 belong to a class of ocean currents known 

 as eastern boundary currents. Other members 

 of the class include the Canary and Benguela 

 Currents off the west coast of Africa. A com- 

 parative study of these phenomena has been 

 made to understand the causes of their simi- 

 larities and differences. 



The eastern boundary currents constitute 

 the eastern limbs of the anticyclonic sub- 

 tropical gyres. Flow is predominantly equator- 

 ward and is broad (about 1,000 km.), slow 

 (less than half a knot) and shallow (usually 

 above 500 m,). The difference in steric level 

 across such currents is about one-third of 

 a meter, and the average transport is of the 

 order of 15 x lO^m.^/sec. 



Surface waters are of relatively low tem- 

 perature, since flow is from high to low lati- 

 tudes and since additional cold water is in- 

 troduced seasonally along the coast by 

 upwelling. Below the surface a general coast- 

 ward rise of the thermocline is indicative of 

 the distribution of mass associated with equa- 



torward geostrophic flow. In all but the Cali- 

 fornia Current, salinity decreases with depth 

 to minimum values at depths of several hun- 

 dred to a thousand meters, so the effect of 

 coastal upwelling is to decrease surface 

 salinity. The opposite is true in the California 

 Current. 



Isopleths of dissolved oxygen ascend toward 

 the coast where the subsurface oxygenminimum 

 is shoalest and has the lowest oxygen content. 

 At times coastal upwelling is so intense that 

 nearshore surface waters are significantly 

 under saturated. Coastal surface values of 

 dissolved inorganic phosphorus may be as 

 high as l-2fig.-at./l . in contrast to the usual 

 mid- and low-latitude oceanic surface concen- 

 trations of 0.2 |J.g.-at./l. or less. To these 

 unusually high surface nutrient concentrations 

 is attributed the extraordinary productive ca- 

 pacity of these regions. 



The physical process of coastal upwelling 

 which operates along the boundaries of these 

 currents is of great biological importance. The 

 model of the process involves the action of 

 wind stress parallel to the coast transporting 

 surface water away from the coast, with a 

 compensatory replacement with deeper lying 

 waters. This implies that time and space vari- 

 ations in the intensity of upwelling depend on 

 variations in the magnitude of the wind stress 

 component parallel to a given section of coast. 



Values of average wind stress and the 

 average orientation of coast lines in 5-degree 

 squares along the eastern sides of oceans 

 were used to compute seasonal values of Ekman 

 transport of surface water away from the 

 coast. These were then examined as possible 

 crude indices of the seasonal and geographical 

 variation of coastal upwelling. 



Values of the index were plotted as functions 

 of latitude and season (figure 6), The following 

 tests of the index were made: 



1. Estimates of the speed of vertical motion. 

 For an offshore transport of 10 kg. /cm. /sec. 

 and an upwelling zone 50 km. wide, the 

 compensating vertical motion is about 50 m./ 

 month, a result comparable with previous 

 estimates. 



2. Annual range of surface temperature. 

 At midlatitudes a summer maximum of up- 

 welling will tend to increase this range, rela- 

 tive to comparable latitudes farther offshore. 

 At low latitudes a pronounced seasonal change 

 in the intensity of upwelling will tend to increase 

 an otherwise small annual range of surface 

 temperature. The observed annual range and 

 the index are quite consistent in this respect. 



3. Reported variations of upwelling. Ex- 

 amination of reports on upwelling along eastern 



17 



