oceanographic features of an area should not 

 be undertaken without an understanding of at 

 least the major features of climate and wea- 

 ther. The general ocean currents are more 

 or less directly produced by the prevailing 

 winds (Sverdrup 194 3, p. 92) and hence should 

 show some seasonal changes as the intensity or 

 direction of the winds change. Locally, the 

 major source of change in the thermal struc- 

 ture of the water is the mixing induced by wind 

 waves or current shear caused by local vari- 

 ance in wind stress. Over wide areas, the 

 major sources of change are from variations 

 in the rate of exchange of heat by conduction, 

 evaporation, precipitation, and radiation . 

 These changes occur when water is transported 

 by the general currents from warmer to colder 

 regions (low to high latitudes), or vice versa, 

 or when the atmospheric circulation carries 

 air to an ocean area having a different temper- 

 ature. Extensive changes in salinity in the 

 open ocean can be caused only by an exchange 

 of moisture with the atmosphere. Thus, we 

 must consider features in the atmospheric or 

 oceanic circulation which would be conducive 

 to evaporation from the sea, such as the move- 

 ment of cold air over warm water, or move- 

 ment of warm water to a region of cooler air, 

 or features conducive to precipitation, such as 

 areas of frontal activity in the air. The dis- 

 solved oxygen capacity of the sea is a function 

 of temperature, salinity, and vapor tension, 

 as well as pressure, and is therefore subject 

 to modification by the exchange of heat or 

 moisture that takes place at the sea surface. 



The mean monthly sea level 

 pressure charts (U.S.W.B. 1952) showthatthe 

 wind and weather of the North Pacific are domi- 

 nated by a subtropical pressure maximum, 

 commonly known as the Eastern North Pacific 

 High or the North Pacific anticyclone, and a 

 subpolar pressure minimum, commonly known 

 as the Aleutian Low. These features define 

 the principal wind belts. They undergo a north- 

 ward shift from winter to summer and during 

 this migration the subpolar low diminishes , 

 practically disappearing during June and July, 

 and the subtropical high increases (Namias 

 1953, Byers 1944). 



The Eastern North Pacific High is 

 the most permanent of the climatic features of 

 the North Pacific. Its center is characterized 

 by light winds and little or no storminess, but 

 its migration affects the storm paths of the en- 

 tire ocean. If the center is displaced farther 

 north than normal, storms occur farther north; 

 if it is displaced farther south, the storms 

 sweep across the oceans at more southerly 



latitudes. 



Actually, the Eastern North Pacific 

 High is a great area of subsiding air. The ef- 

 fects of subsidence are most noticeable in the 

 eastern sector as a result of air that has de- 

 scended as it swept around the cell from the 

 northwest and north. On the south side, as the 

 air joins the trades, it begins to ascend again 

 until it reaches a high point on the southwestern 

 part of the cell. On the west side of the cell, 

 fronts may be formed as the warmer air from 

 the cell encounters the cooler air to the north. 

 As a result of this circulation the areas of 

 greatest evaporation are likely to be found to the 

 east of the cell and areas of greatest precipitation 

 to the west. 



The Aleutian Low is maintained by 

 intermittent outbursts of cold polar air which is 

 steadily generated in the interior of Asia during 

 the winter months. The greatest percentage 

 (about 70 percent) of these outbreaks occur in the 

 region of Japan, the China Sea, or the Yangtze 

 Valley (Byers 1934). As the cold air moves 

 over the warm sea and encounters the warmer 

 tropical air masses, strong cyclonic vortices 

 are formed. They have well-marked warm 

 fronts of advancing tropical air with open warm 

 sectors and well-defined cold fronts. The normal 

 path of these storm centers is in a northeasterly 

 direction. In most cases the fronts have occluded 

 and died out by the time they reach Alaska. How- 

 ever, they are occasionally regenerated by in- 

 fluxes of cold air from the Yukon Valley. The 

 cyclonic disturbances tend to occur in cycles, 

 that is, large cyclones are followed by progres- 

 sively smaller ones, each carrying the cold 

 front farther toward the Equator than the pre- 

 ceding one until finally polar air flows directly 

 into the trade wind region. Precipitation occurs 

 when the warm, moist air of tropical or sub- 

 tropical origin is forced to ascend over the 

 colder, denser air (Byers 1944). 



During the summer months, the 

 Eastern North Pacific High is well developed and 

 is the predominant climatic feature of the North 

 Pacific. The mean position of the center of the 

 high lies within the area covered by cruise 25 

 from April to September and just to the east of 

 the area during the remainder of the year. Con- 

 sequently, the prevailing winds of the area vary 

 considerably from season to season as a result of 

 the migration and fluctuation of the high. During 

 the period from April to September, when it has 

 its greatest development, strong, steady northeast 

 trade winds prevail over the southern part of the 

 cruise area. The mean northern limit of the 

 northeast trades extends to 33 N. latitude on 



