spring and summer 1958, evidence was ob- 

 tained that the implied vertical movement 

 extended almost to the surface in spring, but 

 was suppressed during summer by the dilute, 

 less dense water flowing out of the Gulf of 

 Alaska (INPFC, 1960). We assumed that during 

 winter, the period of minimum coastal runoff, 

 upwelling might reach the surface and reveal an 

 area in which nutrients at depth were restored 

 to the surface layers; thus providing a partial 

 explanation as to why the water in the Subarctic 

 region is so fertile. 



Another feature of the region requiring 

 investigation during winter was the ever- 

 present temperature-minimum stratum. It is 

 generally accepted that the temperature -mini- 

 mum in the eastern North Pacific is the 

 result of winter overturn, and the temperatures 

 reflect the local winter surface temperatures 

 (Dodimead, 1961). Westward of the Gulf of 

 Alaska, however, the temperatures in the 

 minimum stratum are colder than could be 

 expected from winter overturn, and appear 

 to be associated with a westward intrusion 

 from the Oyashio region. The intrusion of 

 this cold water (found between lat. 45° and 

 50° N.) from long. 165° W. in 1956 to 150° W. 

 in 1959 has been shown by Favorite and Hebard 

 (1961). It was necessary to ascertain if the 

 convergence of the westward flowing Alaskan 

 Stream and the eastward flowing Oyashio or 

 Kuroshio Extension during winter could result 

 in a downward transport of cold surface 

 water which after the advent of spring and 

 summer heating would also be evident as a 

 temperature-minimum stratum and contradict 

 the previous interpretation. 



Finally, because the cruise track along 

 long. 155° W. crossed the area of divergence 

 off the Washington coast, the cruise provided 

 an excellent opportunity for an effective drift 

 bottle experiment. Assuming a drift of about 

 5 miles per day indicated by our previous 

 recoveries, the bottles released along long. 

 155° W. should pass through the divergence 

 zone in summer and come ashore on the 

 North American continent during the fall, 

 and the bottles released along long. 175° W. 

 should pass the zone during winter and come 

 ashore the following spring. 



OCEANOGRAPHIC CONDITIONS 



Density 



The vertical profiles of density expressed 

 as sigma-t' along the three cruise legs are 

 shown in figure 4. A significant feature in all 

 profiles is the crowding of the isolines be- 

 tween 100 and 150 meters, which indicates 

 the marked stability of the water column at 

 these depths. Northward of lat. 45° N., winter 

 overturn has resulted in an almost isothermal 

 layer above the halocline. Since these tempera- 

 tures approximate those in the deeper layers 

 relatively isothermal conditions exist to a 

 depth of about 800 meters. Thus, the stability 

 of the water column in this area is almost 

 entirely a function of the salinity distribution. 



Sigma-t distribution is discussed first be- 

 cause it is the basis for calculations of the 

 geostrophic currents which are presented in 

 the following section. Since these currents 

 reflect flow relative to a selected reference 

 level at which no motion is supposed to occur, 

 it is difficult to evaluate the significance or 

 validity of these currents without knowledge of 

 the density structure. The downward slope of 

 the isolines from lat. 50° N. along long. 

 175° W. and lat, 52° N. along long. 155° W. at 

 all depths indicates that a flow existed at and 

 below 1,000 meters, and geostrophic currents 

 will not reflect true current speeds. Along the 

 east-west leg near lat, 47° N,, however, the 

 isolines are relatively level and in the absence 

 of slope currents the geostrophic approxima- 

 tion may be reasonably accurate, 



Geostrophic Currents 



The geostrophic approximation neglects fric- 

 tion and acceleration; assumes a balance 

 between Coriolis and pressure forces; and 

 presupposes a known level of no net motion. 

 Using a reference level of 1,000-decibars, 

 Reid (1961) has presented the geostrophic 

 circulation for the entire Pacific Ocean and 

 found that it compares favorably with currents 

 estimated from the set and drift of vessels and 

 tabulated in various atlases. On an ocean-wide 



'Sigma-t--density as defined by: (specific gravity 1) 

 1,000, expressed as milligrams per cm^. 



