Deep Circulation, Central North Pacific Ocean: 



1961, 1962, 1963 



Abstract. Methods are reviewed for obtaining velocity reference values such that the absolute flow in the 

 deep sea can be described in terms of these reference values and the dynamic height computations. The 

 method of Murty and Rattray (1962) for obtaining absolute flow by adjusting the transports from dynamic height 

 computations to analogous values of transport determined from the curl of the wind stress was examined and 

 used for calculating velocities for a portion of the central North Pacific Ocean between the Hawaiian and Aleutian 

 Islands from observations made during the summers of 1961, 1962, and 1963. 



A predominantly eastward flow was found at depths of 250 and 500 meters. This flow is similar to the 

 accepted pattern of flow in this area. Over much of the area velocities at 4,000 and 5,000 meters were on the 

 order of 1 cm/sec and in a direction opposite to those near the surface. The flow near the bottom tends to con- 

 form to salient features of the bathymetry. By comparison with flow inferred from the distribution of tem- 

 perature and dissolved oxygen, it is concluded that the flow at depths of 4,000 meters and more computed for 

 the summer months does not reflect the net annual circulation, but is a periodic barotropic flow. 



Errors in the assumptions in the formulation of the equations used for computing the flow, in the inadequacies 

 in tabulations of wind-driven transport, and in observational errors are examined. 



1. Introduction 



1.1 Objectives 



The method of computing relative velocities by the dynamic heights method has be- 

 come standard in oceanography. Dynamic height computations can give a realistic 

 representation of the absolute velocity field in respect to the earth if proper allowances 

 can be made for wind drift and if the relative velocities can be referred to absolute veloci- 

 ties at some level. However, the estimation of absolute velocities for reference has often 

 lacked rigor. 



The main objective of this paper is to describe as realistically as possible, the 

 circulation in a portion of the North Pacific Ocean. A method is developed which relates 

 transports calculated from oceanographic station data to geostrophic transports deter- 

 mined from the wind-stress field to find values of the velocity near the bottom. These 

 velocities near the bottom are then used as references for the dynamic height velocities. 

 The method is applied using three recent series of oceanographic observations and tabu- 

 lations of wind-driven geostrophic transports from Fofonoff (1960). 



Defant (1961) reviewed methods of defining and delineating a reference surface for 

 dynamic topography. He cited the method of determining absolute from relative values 

 by comparing direct current measurements with the relative velocities. However, he 

 pointed out that the difficulty in making a sufficient number of direct measurements, each 

 long enough to define the mean field of motion, drastically limits the use of this method. 



In practice, a reference surface is selected arbitrarily near the ocean bottom or as 

 deep as the observations allow. Surface and near-surface flow generally can be re- 

 alistically estimated because velocities at the reference depth are usually negligible in 

 comparison to velocities near the surface. However, this arbitrary selection of a reference 

 level can result in relatively large errors in computing velocities of the deeper currents, 



