2288 VOLUME 27J O U R N A L O F P H Y S I C A L O C E A N O G R A P H Y
q 1997 American Meteorological Society
NOTES AND CORRESPONDENCE
Temporal Variability of the Large-Scale Geostrophic Surface Velocity in the
Northeast Pacific*
P. VAN MEURS
Pacific Marine Environmental Laboratory, Seattle, Washington
P. P. NIILER
Scripps Institution of Oceanography, La Jolla, California
31 December 1995 and 11 March 1996
ABSTRACT
Data from Argos-tracked mixed layer drifters in fall and winter 1987 (49 drifters) and 1989 (16 drifters) are
used to investigate the differences in the large-scale surface velocity and eddy activity in the northeast Pacific.
The velocities were corrected for wind-induced slippage and corrected for wind-driven (Ekman) flow by matching
an Ekman model to the observed currents. The model, which explains 15%–30% of the variance, indicates that
the currents are at 608 to the right of the wind. The magnitude of the currents is 30% of the magnitude of the
wind stress. In 1987–88, the geostrophic motion in the region from 46.58 to 48.58N, 1428 to 1338W was
characterized by an eastward flow of 0.9 (60.4) cm s21 and a northward flow of 0.7 (60.4) cm s21. In 1989–
90, for the same region, the geostrophic eastward component was 3.8 (60.5) cm s21, more than four times as
large as in 1987–88, and the northward component was 0.3 (60.5) cm s21. In this region ageostrophic contributions
to the velocities are small.
In 1987–88 the drifter tracks reveal evidence of the presence of several persistent, warm core mesoscale
eddies. In 1989–90 there is no evidence of any significant eddy activity. The mean speed of the drifters in 1987–
88 was 7.0 (60.3) cm s21 and in 1989–90 was 6.5 (60.4) cm s21. So, although the average speed is the same,
drifters in 1987–88 take a longer time to travel eastward because of the significant north–south excursions due
to the mesoscale eddies. Data from two drifter experiments have shown that the variability of mesoscale eddies
can result in large interannual differences in estimates of mean velocity.
1. Introduction
The northeast Pacific is characterized by low mean
and eddy kinetic energy with most of the energy related
to mesoscale eddy features. Paduan and Niiler (1993)
found that in the eastern Pacific the eddy energy exceeds
the mean kinetic energy by a factor of 4–14. Sverdrup
et al. (1942) identified two eastward flowing currents
in the interior region of the eastern Pacific: the subarctic
current located at about 458N is part of the Alaskan
subarctic gyre and the North Pacific Current located at
about 378N is part of the North Pacific subtropical gyre.
Data collected at Ocean Station Papa at 508N, 1458W
show significant year to year variability (Tabata 1965).
*NOAA Pacific Marine Environmental Laboratory Contribution
Number 1749.
Corresponding author address: Dr. Pim van Meurs, NOAA/PMEL/
R/E/PM, BIN C15700/Bldg. 3, 7600 Sand Point Way NE, Seattle,
WA 98115-0070.
E-mail: pim@pmel.noaa.gov
Reid and Arthur (1975) and Wyrtki (1975) show dy-
namic topography charts of this region indicating the
presence of the two-gyre system. Historical ship drift
data indicate a slow eastward flow in the area separating
the Alaskan subartic gyre from the North Pacific sub-
tropical gyre (Wakata and Sugimori 1990) and (Meehl
1982). These estimates of surface currents are strongly
wind driven, as they are based on ship drift observations.
Kirwan et al. (1978), McNally (1981), and McNally and
White (1985) used drifters to measure the surface and
near-surface currents in the eastern midlatitude Pacific.
Figure 1 of Kirwan et al. (1978) and McNally (1981)
shows an eastward flow and a bifurcation of the flow
with drifters released north of the Subarctic Front get-
ting caught in the northward flowing Alaskan Current
and those released south of the Subarctic Front follow-
ing the southward flowing eastern boundary current of
the subtropical gyre. Our data (Fig. 1) reveal similar
bifurcation of the flow in 1987, while in 1989 there is
less evidence of this bifurcation (Fig. 2). Unlike our
findings in 1987 (Fig. 1), McNally (1981) shows little
eddy activity, similar to the HEAVY 89 experiment (Fig.

OCTOBER 1997 2289N O T E S A N D C O R R E S P O N D E N C E
FIG. 1. Filtered drifter tracks for the STORMS 87 experiment for days 250–500, 1987. Regions
1–3 start at the bottom left corner, regions 4–6 start at the top left corner.
FIG. 2. Filtered drifter tracks for the HEAVY 89 experiment for days 250–500, 1989. Regions
1–3 start at the bottom left corner, regions 4–6 start at the top left corner.