the result of local heating or cooling. 



The dotted lines on the horizontal 



plot of surface temperature (fig. 34) show the 



0,0,0 o 



mean position of the 70 , 65 , 60 , and 55 F. 



isotherms. They have been interpolated from 



the U.S.N.H. O. Surface Temperature Atlas 



(1944) to correspond to the approximate time 



of transit on each leg, thus permitting a 



comparison of the temperature encountered on 



the cruise with mean conditions. The greatest 



and most consistent difference from normal 



occurs west of 155 W. , where all the isotherms 



are farther north than normal. East of 155 W. 



the 55 F. isotherm, which is farther south 



than usual, is the only one that is consistently 



different. 



The most significant features of 



the temperature field were several abrupt 



meridional changes of 2 -4 F. They are 



shown by the large gradients on the surface 



temperature plot (fig. 34) and on the surface 



temperature profiles (figs. 24-33). The 



changes were sharpest in the western part of 



the area, where they coincided with steep 



gradients in the dynamic topography. The 



most striking example occurred on the 160 W. 



transect, where the thermograph record (fig. 



35) shows a rise (vessel course was 180 T. ) 



from 58 F. to 60 F. in about 4 minutes 



followed by a series of more gradual rises to 



almost 63 F. over the following 2 hours and 



45 minutes. At the 8. 5-knot average speed of 



o 

 the Smith this represents a rise of almost 5 F. 



in 25 miles. According to Mackintosh (1946) 



such abrupt changes in temperature are 



characteristic of areas of convergence between 



water masses. 



Major changes in the vertical 

 temperature structure were frequently associ- 

 ated with surface temperature discontinuities. 

 At many, the south to north shallowing of the 

 isotherms (see figs. 24-33) increased sharply. 

 The increase in slope affected all of the iso- 

 therms instead of only the shallow ones, as in 

 the case of changes of slope which are attribu- 

 ted to internal waves. 



North of the discontinuities which 

 occurred at temperatures of 60 F. or less, the 

 homogeneous surface layer was almost entirely 

 missing, the bathythermograph trace showing 

 either a gradual decrease to 900 feet or a 

 series of small steplike changes. Again, the 

 phenomenon was most pronounced in the west- 

 ern part of the area, where the flow was basic- 

 ally zonal. The BT traces from the stations on 

 160 W. have been reproduced in figure 36 to 



illustrate the change in their structure from 

 south to north. Both the abrupt change in slope 

 of the isotherms and the absence of a homogene- 

 ous surface layer are characteristic of the zones 

 where mixing and sinking take place at the con- 

 vergence between two water types (Sverdrup and 

 Fleming 1941). 



Density 



The internal distribution of density 

 (sigma-t) in the ocean reflects both the field of 

 motion and the modifying processes occurring 

 at the surface. The effect of motion is seen in 

 the distributions of mass giving rise to the 

 pressure gradients associated with horizontal 

 currents; by this association an examination of 

 the sigma-t plots can yield a qualitative descrip- 

 tion of the major features of the circulation. The 

 surface modifying processes directly affect a 

 relatively thin upper layer, the "surface layer, " 

 keeping the layer nearly vertically homogeneous, 

 and causing large time and space variations in 

 its properties. 



Turning first to this surface layer 

 (fig. 37), we can see the general northward in- 

 crease in density associated with the decrease in 

 temperature. The meridional change in density 

 in the area of our sections is less in the east; 

 this is because water of lower salinity (more 

 northern origin) is present farther to the south 

 in the eastern part of the region. The greater 

 northward decrease in salinity here tends to off- 

 set the decreasing temperature in determining 

 the density of the surface water. 



The sections (figs. 38-47) show the 

 general northward thinning of the surface layer 

 connected with the net easterly flow of the North 

 Pacific Current. The surface layer becomes ex- 

 tremely shallow at the northern end of the eastern 

 sections, another indication that this region is 

 influenced by more northerly conditions than the 

 western part of the area. The local irregularities 

 and inversions to be expected in this area of 

 rapidly changing conditions and slow water motion 

 are evident on the sections. 



The relatively sharp density gradient 

 of the thermocline lies directly below the surface 

 layer. The sigma-t surfaces show a net north- 

 ward decrease in depth, again an indication of the 

 easterly North Pacific Current. At the depths of 

 the surface layer and upper thermocline the 

 southern boundary of this current is at or beyond 

 the southern limit of our sections, for there is 

 no change in the slope of the upper thermocline 

 to indicate a transition to the westerly flow of the 

 North Equatorial Current. The large irregularities 



