Lagrangian observations of the near-surface circulation in the North Pacific, 1990-1995

• Main Author: Bograd, Steven J.
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/9453

An analysis and interpretation of the position time series, and accompanying derived velocities, obtained from a large set of satellite-tracked surface drifters deployed and tracked throughout the North Pacific Ocean over the period 1990-1995 is presented. As part of Canada's contribution to the World Ocean Circulation Experiment's Surface Velocity Program (WOCE-SVP), 102 drifters were released in the North Pacific in 12 deployments between August 1990 and November 1994. Each of the drifters was equipped with a drogue centered 15 m (62 drifters) or 120 m (40 drifters) below the surface, i.e., within the mixed layer or in the underlying pycnocline. The principal objective of this work was to describe the observed circulation and its variability at both drogue depths. This was accomplished through several independent analyses, each focusing on a particular suite of statistical methods or subset of drifters. As a first step, the effects of reduced sampling schedules (duty cycles) on derived velocity statistics were investigated by degrading continuous data segments from a subset of drifters to match three different duty cycles. It was found that strong high-frequency (primarily inertial) motions prevalent in the drifter records resulted in significantly biased statistics derived from the degraded series, and that reproduction of the original prime and rotary spectral statistics required an interpolation which took into account the oscillatory component of the drifter motions. The trajectories of all drifters were used to characterize the upper-ocean mean circulation and eddy variability in the North Pacific Ocean over the period 1990-1995. A l l branches of the Alaskan Gyre were well-sampled at both drogue depths, revealing a weak Subarctic Current and strong, variable flow in the Alaska Current and Alaskan Stream. At 15 m depth, the bifurcation of the Subarctic Current was observed near 48°N, 130°W, while at 120 m, northward flow in the Alaska Current occurred much further offshore. A minimum in eddy kinetic energy was observed in the northern subtropical gyre (the "eddy desert"). Eddy kinetic energies were nearly twice as high in the mixed layer compared to below, and 2-3 times larger in winter than in summer throughout most of the near-surface Alaskan Gyre. Taylor's theory of single particle dispersion was applied to the drifter ensembles to estimate Lagrangian decorrelation scales and eddy diffusivities. Both the initial dispersion and random walk regimes predicted by Taylor's theory were identified in the dispersion time series computed for several regions of both ensembles. The consistency of the results with previous studies suggests that the simplifying assumptions of Taylor (1921) are reasonably valid throughout the upper ocean, which bodes well for the effective parameterization of near-surface diffusivities in general circulation models. Subsets of drifters were used to examine eddy activity in the vicinity of the Emperor Seamount Chain (ESC) and the Kuril-Kamchatka Trench (KKT) in the western North Pacific. In both regions, drifters were trapped within topographically-controlled mesoscale eddies. The trajectories of two deep-drogued drifters revealed a pair of counter-rotating mesoscale eddies attached to the leeside of Ojin/Jingu Seamount. One of the drifters made five loops within the cyclonic eddy over a period of 62 days, providing one of the first observations to demonstrate an extended attachment of a topographically-generated eddy to a seamount. The observations of attached leeside eddies (or lack thereof) at the ESC match the predictions of numerical and analytical models very well. Further west, drifter trajectories revealed the presence of large anticyclonic eddies positioned over the deepest part of the KKT. These observations support the implication from historical data that long-lived anticyclonic eddies are common in this region. However, the origin and longevity of these eddies remain uncertain.