U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
An eddy-resolving model of circulation on the western Gulf of Alaska shelf. 2.
Comparison of results to oceanographic observations
P. J. Stabeno
Pacific Marine Environmental Laboratory, NOAA, Seattle, Washington
A. J. Hermann
Joint Institute for the Study of the Atmosphere and Ocean, University of Washington,
Seattle
(Also at Pacific Marine Environmental Laboratory, NOAA, Seattle, Washington)
Journal of Geophysical Research, 101(C1), 1151-1161 (1996).
Copyright ©1996 by the American Geophysical Union. Further electronic distribution is not
allowed.
Data
The data fall into the following two categories: (1) observations which are
compared with the model simulations and (2) the forcing functions for the model.
The first category consists of records from moored current meters and the trajectories
of satellite-tracked drifting buoys. The second category contains the time series
for winds and freshwater runoff (buoyancy flux). In addition, data from hydrographic
surveys from 1981 were used to initialize the model density field.
Current Meters
In the spring of 1989, seven taut-wire moorings with 41 current meters were
deployed off Wide Bay (moorings 21-25, 28, and 29 in Figure
1). In the spring of 1991, three taut-wire moorings containing 12 current
meters were deployed off Cape Kekurnoi (moorings 1-3 in Figure
1). The 1989 data set is discussed by Bograd
et al. [1994] and the 1991 data set by Stabeno
et al. [1995b]. All current meters were Aanderaa model RCM 4 or RCM
6. Current speed and direction, temperature, and conductivity were sampled at
hourly intervals. The current meter data were low-pass filtered with a cosine-squared,
tapered Lanczos filter (half amplitude 35 hours, half power 42 hours) and resampled
at 6-hour intervals. (The same filter was applied to the currents generated
by the model prior to output.) Estimates of transport were calculated from the
current velocity components normal to each section, multiplied by estimates
of cross-sectional area [Bograd
et al., 1994; Stabeno
et al., 1995b].
Satellite-Tracked Drifting Buoys
From 1985 to 1994, 51 satellite-tracked drifting buoys were deployed off Cape
Kekurnoi. All buoys were drogued at ~40 m. During 1986-1988 and 1994, holey
sock drogues were used, while between 1987 and 1993, tristar drogues were used.
An average of ~10 satellite fixes were obtained daily, with a standard position
error of ~0.2 km. The buoys deployed since 1989 had tilt switches, which indicated
when the drogue was lost. In earlier years the loss of the drogue was determined
from examination of the relationship between the wind and the buoy's trajectory
[Stabeno
and Reed, 1991]. Since these buoys showed both inertial and tidal currents,
the data were first splined (using an Akima spline) and sampled at 1-hour intervals
and then a 35-hour low-pass filter was applied so that the data would be comparable
to the model simulations.
Wind Forcing
Although direct measurements of winds in the northern Gulf of Alaska are rare,
geotriptic winds calculated from sea level pressure provide fair representation
of winds in the region west of Kodiak Island [Macklin
et al., 1993]. Surface winds were computed from 12-hourly atmospheric
surface pressure supplied by Fleet Numerical Oceanographic Center. The geotriptic
winds were rotated 15° counterclockwise, speeds were reduced by 30% from the
geostrophic value, and the results interpolated to the model grid.
The geotriptic winds in Shelikof Strait proper must be modified, since ageostrophic
(downgradient) winds are common here. The terrain-induced wind variations in
Shelikof Strait have been documented with research aircraft [Lackman
and Overland, 1989] and a special observing network [Macklin
et al., 1993]; since these variations are large and persistent, they
can have a significant local impact on the upper ocean. A simple algorithm was
developed from these observations to approximate the winds in Shelikof Strait
from the large-scale sea-level pressure field [Stabeno
et al., 1995a]. For a prescribed range of geostrophic wind directions
the surface winds are assumed to be channeled and enhanced within the strait.
Buoyancy Forcing
High precipitation rates along the coast of the northeast Pacific Ocean produce
a large freshwater discharge (annual mean 23,000 m
s
), which enters the shelf waters through
many streams and rivers. Acting as a line source of buoyancy at the coast, it
provides the freshwater input for the ACC [Royer,
1982]. Freshwater discharge is maximum in the early fall, decreases through
the winter, and generally reaches a minimum in the late winter or early spring.
Time series for the 3 years discussed in this paper are shown in Figure
2. During the winter, runoff was greatest in 1987 and smallest in 1991.
The runoff from May through October was similar during each of the 3 years.
Figure 2. Time series of the monthly mean freshwater input for 1987,
1989, and 1991 (data from T. Royer, personal communication, 1994).
Return to previous section or go to next section
PMEL Outstanding Papers
PMEL Publications Search
PMEL Homepage
