Some Oceanographic Features of the Northeast Pacific Ocean during August 1955

Abstract

Physical oceanographic data from the international NorPac survey of August 1955, in the area north of Lat. 45°N and east of Long. 161°W, were examined. The temperature, salinity, and density distribution and structure from the surface to 2000 m are discussed.The temperature structure showed an isothermal layer to about 30 m depth, a marked thermocline to about 100 m depth, a temperature inversion in most of the area, and below this a gradual temperature decrease into the abyss. At all depths the water was coldest in a "cold core" centered about 100 miles south of Kodiak and the Shumagin Islands. From there the temperature increased at each level in all directions.The salinity structure showed an isohaline layer to about 100 m depth, a marked halocline to about 200 m depth, and below this the salinity increased slightly into the abyss. The salinity structure did not coincide with the temperature structure.The density structure showed an isopycnal layer to about 30 m depth, a pycnocline associated with the thermocline, a second isopycnal layer, a second pycnocline associated with the halocline, and below this the density increased slightly into the abyss.Variations in these structures throughout the region are discussed in some detail.There is no horizontal isosteric level in the 2000 m of depth. It is concluded that there is no level of "no net motion" in this range, but a reference level of 2000 decibars (db) for dynamic calculations is more acceptable than the usual 1000-db level. A new procedure is introduced to extend the reference level into the bottom in near coastal areas.The geostrophic currents were calculated. There was a major latitudinal drift from the west into the central part of the area. It veered northward and continued around the Gulf of Alaska, forming the Alaska Gyral, and left the area to the westward, as an intensified current (Alaskan Stream) close along the Alaskan Peninsula. This intensification is probably due to conservation of absolute vorticity through changing latitude.The circulation pattern extended to at least 2000 m depth, and probably to the bottom. It transported about 17 million m3 of water per second. There were a number of eddies in the system, some of which were observed on earlier surveys.The major flow pattern was not wind-generated within the region. The influence of local winds was limited to the upper 200 m of depth. In some areas it aided the flow, and in others retarded it.There is evidence to show that two chains of seamounts influenced the current pattern according to the Sverdrup concept. Since the major portion of these is below 2000 m depth it is concluded that currents exist in the abyss, in essentially the same direction as at the upper levels.