Observations and Modeling of Wind-driven Currents in the Northeast Pacific

Abstract

Velocity measurements from the interior of the Alaskan gyre are presented from a current meter mooring deployed in 4000 m of water at 49°33′N, 138°38′W, in the vicinity of Ocean Weather Station P. The mooring held five current meters, which spanned the depth of the water column. The data reveal surface-intensified motions with flow fluctuations in the upper layers of the water column characterized by long-period, O(100 d) time scales of variability. At abyssal depths, the flow displays shorter, O(20 d) time scales of variability. The data are compared with observations from one of the NEPAC moorings in the northeast Pacific (42°N, 152°W). Similar characteristics in kinetic energy levels, in the vertical structure of the flow, and in the vertical variation of eddy time scales are found at this location. The current measurements are considered in terms of the linear theory of directly wind-driven variability of Müller and Frankignoul. A comparison with simulated currents from a quasigeostrophic numerical model demonstrates that stochastic atmospheric forcing of the ocean can account for the observed variability. The numerical experiments and a simple extension of the linear theory suggest that the presence of bottom topography is important for the partition of energy between vertical modes and for the vertical variation of the time scales of the flow.