NOAA Technical Report ERL 445-PMEL 40 

Satellite Observations of Mesoscale 

Features in Lower Cook Inlet and 

Shelikof Strait, Gulf of Alaska 

James D. Schumacher 

Willard E. Barber 

Benjamin Holt 

Antony K Uu 

May 1991 

U.S. DEPARTMENT OF COMMERCE 

National Oceanic and Atmospheric Administration 

Environmental Research Laboratories 

---

NOAA Technical Report ERL 445--PMEL 40 

Satellite Observations of Mesoscale 

Features in Lower Cook Inlet and 

Shelikof Strait, Gulf of Alaska 

James D. Schumacher 

Pacific Marine Environmental Laboratory 

Willard E. Barber 

University of Alaska at Fairbanks 

Benjamin Holt 

Jet Propulsion Laboratory 

Antony K. Liu 

NASA/Goddard Space Flight Center 

Pacific Marine Environmental Laboratory 

Seattle, Washington 

May 1991 

U.S. Department of Commerce 

Robert A. Mosbacher, Secretary 

National Oceanic and Atmospheric Administration 

John A. Knauss. Under Secretary for Oceans and Atmosphere I Administrator 

Environmental Research Laboratories 

Boulder, Colorado 

Joseph O. Fletcher, Director 
NOTICE 

Mention of a commercial company or product does not constitute an endorsement by 

NOAA/ERL. Use of information from this publication concerning proprietary products 

or the tests of such products for publicity or advertising purposes is not authorized. 

Contribution No. 1230 from NOAAlPacific Marine Environmental Laboratory 

For sale by the National Technical Information Service, 5285 Port Royal Road
 

Springfield, VA 22161
 

ii 
CONTENTS
 

Page 

1. INTRODUCI10N....................................... 1
 

2. REGIONAL SETIING 4
 

3. METHODS AND OBSERVATIONS 4
 

3.1. Satellite Observations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4
 

3.2. In Situ Measurements 7
 

4. RESULTS 12
 

4.1. Internal Wave Features. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 12
 

4.2. Meanders 13
 

4.3. Eddies 14
 

5. DISCUSSION 15
 

6. ACKNOWLEDGMENTS 15
 

7. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 16
 

iii 

Satellite Observations of Mesoscale Features
 

in Lower Cook Inlet and Shelikof Strait, Gulf of Alaska
 

James D. Schumacher, Willard E. Barber, Benjamin Holt, 

and Antony K. Liu 

ABSTRACT. The Seasat satellite launched in summer 1978 carried a synthetic aperture radar (SAR). 

Although Seasat failed after 105 days in orbit, it provided observations that demonstrate the potential to 

examine and monitor upper oceanic processes. Seasat made five passes over lower Cook Inlet and Shelikof 

Strait, Alaska, during summer 1978. SAR images from the passes show oceanographic features, including 

a meander in a front, a pair of mesoscale eddies, and internal waves. These features are compared with 

contemporary and representative images from a satellite-borne Advanced Very High Resolution Radiometer 

(AVHRR) and Coastal Zone Color Scanner (CZCS), with water property data, and with current observations 

from moored instruments. The results indicate that SAR data can be used to monitor mesoscale 

oceanographic features. 

1. INTRODUCTION 

It is generally accepted that physical oceanic processes influence and may even be the 

primary determinant of the recruitment of fish to oceanic fisheries. In particular, the impacts of 

these processes on survival during the early life stages of fish as they are being transported from 

spawning grounds to nursery areas are believed to be the most important factors in determining 

recruitment (Rothchild, 1986). Mortality is affected by two broad categories of physical 

mechanisms: turbulence of the water column, which influences food availability, and fluctuations 

in current patterns transporting eggs and larvae to nursery areas. Currently, there is a long-term 

interdisciplinary program, Fisheries Oceanography Coordinated Investigations (FOCI), examining 

the influence of the environment in Shelikof Strait, Gulf of Alaska (Fig. 1), on survival and 

recruitment of walleye pollock (Theragra chalcogramma) to the fisheries (Reed et aI., 1988). 

Results to date include identification of eddies (Incze et aI., 1990; Vastano et aI., 1991) and 

complex current patterns that affect transport (Schumacher et aI., 1989) and dispersion (Reed et 

aI., 1989) of larvae as they drift to nursery grounds along the Alaskan Peninsula (Hinckley et aI., 

1991). 

Advanced Very High Resolution Radiometer (AVHRR) satellite images from the study area 

have been used to construct surface current distributions that are consistent with in situ current 

and water property measurements (Vastano et aI., 1991). A marked feature of contemporary 

satellite and larval distribution data (Fig. 2) is that the greatest concentrations of larvae are 

situated in eddies. These results suggest that eddies may play a crucial role in retaining larvae 

on the shelf, thereby inhibiting transport into the Gulf of Alaska. A problem with AVHRR 

techniques in this region, however, is that cloud cover usually inhibits acquisition of clear 

imagery over a sufficiently large area to produce maps of surface temperature (Royer and 

Muench, 1977) and inferred currents. The advantage of imagery from synthetic aperture radar 

(SAR) over that from AVHRR is that it is unaffected by cloud cover. Additionally, the SAR is 

capable of producing imagery with a resolution as fine as 25 m, compared with the 1-km 

resolution of AVHRR. Over the ocean, SAR detects backscattered energy that is affected by the 

small-scale roughness arising from short surface waves. Variability in the short waves is 

produced by interactions with longer waves, currents, and atmospheric conditions to produce a 

fine-scale two-dimensional map of the ocean surface. 
600 N 

(I 

50 \ 

.... ) 

59° 

58° 

Gulf of Alaska 

I 57° / 

/ 

/ 

50 

r 

/ 

" 

156° 154° 152° 1500 W 

Fig. 1. The study region in the Gulf of Alaska. The locations of Seasat SAR imagery 

(shown in Figs. 3a, 4a, and 5) and the revolution numbers are indicated. Depth contours 

(dashed lines) are in fathoms (50 fathoms =92 m). 

The Seasat satellite, launched in summer 1978, carried an SAR. Although Seasat failed after 

105 days in orbit, it provided extensive information that demonstrates the potential to examine 

and monitor upper oceanic processes. Phenomena revealed by Seasat SAR imagery include 

eddies, fronts, bathymetric features, surface and internal waves, windrows, and ships and their 

wakes (Fu and Holt, 1982). Data gathered by Seasat have also provided information to use in 

investigating the mechanisms responsible for these phenomena (Beal et aI., 1981; Vesecky and 

Stewart, 1982; Fu and Holt, 1983). Thus, SAR may be a valuable tool to extend our capabilities 

in investigating oceanic processes and how they may affect recruitment to fisheries. 

Much of the analysis of the utility of SAR for oceanographic studies has been done using 

imagery from Seasat (e.g., Fu and Holt, 1982; Beal et aI., 1981; Vesecky and Stewart, 1982) and 

from the shuttle imaging radar (SIR) series in 1981 and 1984 (Cimino et aI., 1988). SAR 

oceanographic studies have mostly emphasized the accuracy of surface wave directional and 

height spectrums (e.g., Hasselmann et aI., 1985) and the observations of near-surface internal 

waves (e.g., Fu and Holt, 1984; Apel and Gonzalez, 1983; Liu, 1988; Gasparovich et aI., 1988). 

Several studies have examined the SAR detection of currents and mesoscale circulation features 

such as the Gulf Stream and its warm-core eddies (Lichy et aI., 1981; Hayes, 1981), cold eddies 

(Cheney, 1981), smaller-scale fronts and eddies (Fu and Holt, 1983), and wave-current interaction 

(e.g., Liu et aI., 1989; Barnett et aI., 1989; Johannessen et aI., 1990). Mesoscale features are 

2 
Pollock Larvae, #/m2 

N>300 _ 

200< N~300 _ 

100 < N 200 !",,',:,:,'/,I 

1580 1560 154°W 

Fig. 2. Surface velocity field (cm S-1) inferred from AVHRR images of sea surface 

temperature, and contours of larval abundance. The contours are in units of m-2 so that 

they are normalized by depth. The surface current distribution was made from tracking 

features from images on 28 and 29 April 1986. 

imaged by SAR through several possible mechanisms that are not well understood. These 

mechanisms include the modulation of the short surface waves by current shear, the alteration 

of the stability of the surface wind across a relatively sharp sea surface temperature gradient, 

shifts in the Doppler frequency due to variability in the surface currents, and current-induced 

wave refraction. The SAR studies of oceanographic features have shown considerable promise 

for understanding the mesoscale dynamics of the ocean circulation, especially if used in 

combination with larger-scale satellite measurements such as from altimeters, scatterometers, and 

visible imaging satellite sensors (EOS SAR Panel, 1988). 

In this report, we examine SAR imagery from several Seasat passes over the lower Cook 

Inlet/Shelikof Strait region of the Gulf of Alaska to assess the capability of SAR to detect 

mesoscale features of interest, including eddies, frontal current meanders, and internal waves in 

the northwest coastal region of Alaska. We compare the SAR imagery with imagery from 

AVHRR and from a Coastal Zone Color Scanner (CZCS), and with results of a radar scattering 

model, which incorporate in situ field measurements. Our objectives are (1) to determine the 

potential use of SAR in detecting features associated with physical processes in the study area, 

and (2) to integrate SAR information into the FOCI project. The satellite SAR may be a tool to 

aid in the management of fisheries and the development of recruitment models. 

3 
2. REGIONAL SETTING 

All the satellite images we present are of lower Cook Inlet and Shelikof Strait. The 

dominant circulation feature there is the Alaska Coastal Current (ACC), which is driven by 

coastal convergence and freshwater flux along the Kenai Peninsula (Schumacher and Reed, 1980; 

Schumacher et aI., 1989). The ACC enters lower Cook Inlet/Shelikof Strait through Kennedy 

Entrance, follows the 100-m isobath, and then flows southwestward through the Shelikof Strait 

along the sea valley. Baroclinic instability (Mysak et aI., 1981) within the strait results in 

formation of meanders throughout and eddies at the southern exit. The eddies are translated 

toward the southwest and have been observed off Wide Bay (Incze et aI., 1990). Within Cook 

Inlet, features of the mean circulation are (1) a band of southward flow along the western shore 

(with salt content diluted by river runoff), (2) a slow northward drift on the eastern side of the 

inlet, and (3) an intense westerly flow across the lower inlet along the 100-m and deeper isobaths 

(Muench et aI., 1978). The third feature is the local manifestation of the ACC. The 

instantaneous current field is dominated by wind-driven currents and tidal currents that vary from 

prominent (principal lunar component, M2, amplitude of 80 cm S-I) in the eastern lower inlet to 

weaker (M2 amplitude of 40 cm S-I) in the central and western inlet (Muench and Schumacher, 

1980; Isaji and Spaulding, 1987). 

3. METHODS AND OBSERVATIONS 

3.1. Satellite Observations 

Seasat operated during the summer and early fall of 1978 (Fu and Holt, 1982). Because onboard tape recorder capacity was limited, SAR data could be acquired only when the satellite was 

in view of a ground receiving station. About 40 hours of SAR imagery was obtained over North 

America and Europe. Within Alaska, imagery was obtained at the Fairbanks station. Areal 

coverage was of the northeast Pacific south to about 50o

N. 

For this study, there were five separate Seasat passes that covered Cook Inlet and Shelikof 

Strait. We examined upper Shelikof Strait from SAR imagery revolution (Rev) 811 (22 August 

1978), which shows a frontal current meander (Fig. 3a), and Cook Inlet imagery from Rev 289 

(17 July 1978), which shows internal waves (Fig. 4a). Lower Shelikof Strait was also covered 

by imagery from Rev 289, which shows a pair of mesoscale eddies (Fig. 5). We also examined 

imagery from Rev 323 (20 July 1978), Rev 1284 (24 September 1978), and Rev 1499 (7 October 

1978), which covered lower Cook Inlet, but these images contained no detectable ocean features 

of interest. One explanation for the lack of detectable features on these three passes is high 

winds. Several studies have shown that winds greater than about 10 m S-1 tend to obscure 

detectable ocean features on SAR (Fu and Holt, 1984; Lichy et aI., 1981). Below a threshold 

(approximately 2 m S-I), the short surface wave field is not adequately developed to produce 

images that contain mesoscale features of the ocean. Wind observations collected on five islands 

surrounding the coastal waters southwest of Kodiak Island indicate that wind speeds between 2 

and 10 m s-1 occur from 40% to 80% of the time for any given month (Macklin, 1987; Macklin, 

PMEL, personal communication). During the months of May through July when larval and early 

juvenile pollack abound in the study area, the percentage of winds in the usable band increases 

to greater than 65% and up to 85%, depending on location. Another explanation for the lack of 

detectable features is that no mesoscale features were present. The SAR imagery has been 

digitally processed into 100 km x 100 km scenes and has been ground-corrected, which removes 

skewness due to imaging geometry. 

Remotely sensed observations of the Shelikof Strait region were also gathered by National 

Oceanic and Atmospheric Administration (NOAA) satellites in 1988 using the AVHRR 

4 
c 

a b 

Fig. 3. (a) Seasat SAR image of upper 

Shelikof Strait from Rev 811, obtained on 22 

August 1978 at 1656 GMT (courtesy Jet 

Propulsion Laboratory). (b) Enlargement of 

an infrared AVHRR image of Shelikof 

Strait, obtained on 25 August 1978, from 

NOAA 5, orbit 9372. (c) Sketch map 

showing boundaries of identifiable frontal 

features seen in the Seasat SAR (dashed 

line) and the AVHRR (solid line) imagery. 

mstruments and were obtained as High-Resolution Picture Transmission (HRPT) scenes from 

NOAA's National Environmental Satellite, Data, and Information Service (NESDIS). The present 

study uses full 1.1-km-resolution, channel 4 (11-,um-band) images processed from these scenes 

with a single-channel atmospheric correction and mapped to Mercator projections (Fig. 6a). 

Analysis of these images for advective sea surface flow estimates (Vastano and Reid, 1985) 

required measurement of sea surface temperature pattern displacements with sequential images 

using an interactive visualization algorithm (Vastano and Borders, 1984). 

5 
Fig. 4. (a) Seasat SAR image of lower Cook Inlet from Rev 289, obtained on 17 July 

1978 at 0548 GMT (courtesy Jet Propulsion Laboratory). 

Figure 7 shows derived pigment images obtained from the CZCS on the NIMBUS 7 satellite, 

which passed over the lower Shelikof Strait on 24 May 1986. The CZCS, like the AVHRR, 

cannot see the ocean surface through clouds. Bright clouds (and bright land) create anomalous 

signals (known as ringing) due to saturation of the sensor as it passes over them. Ringing occurs 

only on the right side of the cloud (or land), since the sensor scans left to right. The CZCS 

pigment image in Fig. 7a uses a color scale, in which land, clouds, and extremely high values 

are white. Pigment concentrations varying from low to high are represented by purple, dark blue, 

blue, green, yellow, orange, and red. The image was mapped to an earth grid. An eddy can 

barely be seen off Wide Bay, just to the south of the small white patch in the area of apparently 

very high pigment (this signal may be contaminated by land-ringing). Off the continental shelf, 

the pigment concentrations are low, as shown by the color blue. For some areas with high 

pigment, the clarity of mesoscale features may be enhanced by setting the water-leaving radiance 

band at 520 ± 10 nm, as shown in Fig. 7b for the same scene as in Fig. 7a. The anticyclonic 

eddy/swirl is more clearly seen in Fig. 7b. Although the color scale is different from that in 

Fig. 7a, it follows the same trend, with purples and blues representing low radiances and salmon 

colors representing higher radiances. North of the eddy/swirl, the ringing area is most noticeable 

next to land, but it is unclear how far to the east its effect persists. The location of the 

eddy/swirl in the CZCS image is consistent with locations of similar features in AVHRR, SAR, 

and in situ measurements from different times. General location and size are confirmed by 

biological and physical measurements (Incze et al., 1990) collected within about a week of the 

CZCS image. 

6 
b 

LOWER COOK INLET 

• NOAA MOORINGS 

SOUNDINGS IN FATHOMS 

50' 

40' 

20' 

50 

KENNED~ ENTRANCE 

~::.;:s::~~""""""'----":::"">'-----<SL-"-'--L __L- L- L-__C:::::.8::.BL- .L- .l..- 59' 

154 D 40' 20' 1530 40' 20' 152 0 40' 20' 151 0 

WEST LONGITUDE 

Fig. 4. (b) Bathymetry map (10 fathom contour interval) of lower Cook Inlet, indicating 

the locations of internal wave packets seen in the SAR image in Fig. 4a and the locations 

of moored current meters (asterisks) from summer 1978 (see also Fig. 8). 

3.2. In Situ Measurements 

Moored current meters were deployed using taut-wire moorings and acoustic releases. 

Aanderaa RCM-4 recording current meters were used on all moorings. These meters use 

Savonius rotors to measure current speed, and vane and compass assemblies for determining 

current direction. The current meters sampled at 15- or 20-min intervals. Raw data were lowpass filtered to remove high-frequency noise; this filter passed more than 99% of the amplitude 

at periods greater than 5 h, 50% at 2.86 h, and less than 0.5% at 2 h. This series was then 

further filtered to remove most of the tidal energy. The second filter passed more than 99% of 

the amplitude at periods of more than 55 h, 50% at 35 h, and less than 0.5% at 25 h. The 

second, low-pass-filtered series was then resampled at 6-h intervals and was used for examining 

nontidal circulation. 

7 
Fig. 5. Seasat SAR image of lower Shelikof Strait from Rev 289, obtained on 17 July 

1978 at 0549 GMT (courtesy Jet Propulsion Laboratory). The sketch map of the 

mesoscale eddy in this image is shown in Fig. 6d. 

Temperature and salinity data were obtained aboard ship using a Plessey Model 9040 

conductivity/temperature/depth (CfD) profiling system during 1978 and a Sea Bird Electronics 

system during 1988. These data were recorded during the downcast, and descent rate was held 

to about 1 m S-1 or less to minimize error in the recorded data due to thermal lag in the sensors 

in regions where large vertical temperature gradients were present. Water samples were obtained 

and analyzed for conductivity and temperature at roughly half of the CfD stations to provide 

field correction offsets. The water samples were obtained using a rosette sampler equipped with 

calibrated reversing thermometers. Conductivity was measured aboard ship using a portable 

salinometer calibrated with standard seawater. These calibration data confirmed an overall 

accuracy for the CTD temperature and salinity values of ±0.02°C and ±0.02 practical salinity 

units (psu), respectively, for the Plessey data and ±0.01°C and ±0.01 psu for the Sea Bird 

Electronics data. 

During summer 1978, 11 current moorings were deployed in lower Cook Inlet. A complete 

description of the experiment is given in Muench et al. (1981), and Reed and Schumacher (1987) 

interpret the data in light of more recent advances in understanding of regional oceanography. 

Mooring locations and mean currents are shown in Fig. 8, and current statistics are given in 

Table 1. The measurements confirm results from previous studies. The current vectors at 

moorings C-1B, C-2B, and C-3B, and to a lesser degree at mooring C-13A represent the 

concentrated outflow from lower Cook Inlet, while those at moorings C-7B, C-11B, and C-12A 

represent the weak mean inflow. The ACC is manifested as the strong flow at moorings C-5B, 

C-8B, and C-lOB. Water properties vary across Cook Inlet; the most dilute water is on the 

western side (Fig. 9). 

8 
a b 

c d 

Fig. 6. (a) An interpretation of an AVHRR image of Shelikof Strait obtained on 29 April 

1988, from NOAA 9 (orbit 17402, at 0047 GMT). The shaded temperature scale ranges 

from <3.5° to >5.5°C and shows cloud cover. (b) Sea surface temperature (OC) and (c) 

geopotential topography (dyn m), both derived from a 38-station (crosses) CfD survey 

that took place on 27-29 April 1988. (d) Sketch map of detectable current features seen 

in the eddy in the SAR image in Fig. 5. 

9 
Fig. 7. A CZCS four-band composite pigment image (top) and the same image 

constructed using the radiance band at 520 nm (bottom). The data were collected by 

Nimbus 7 on 24 May 1986. Land, clouds, and extremely high radiance values are white; 

low radiances are represented by purples and blues, and higher radiances by salmon. 

10 
C-13A t..2A 

C-3BI 

/ C-6B 

I 

C-2B 

C-1B"..

I" C-5B 

C-8B 

59°N 

\C-9B 

C-10B 

I 

9 25 50 

cmls 

1540 1530 1520 

Fig. 8. Vectors showing mean currents (cm S-1) at the 11 current moorings in the study 

area for the period 6 June-11 August 1978. 

Table 1. Mooring information and mean current data for summer 1978 

Mooring 

name 

C-1B 

C-2B 

C-3B 

C-5B 

C-6B 

C-7B 

C-8B 

C-9B 

C-10B 

C-12A 

C-13A 

Observation 

period 

(JD) 

149-235 

148-291 

148-291 

148-264 

148-292 

148-291 

141-277 

149-288 

149-283 

148-229 

149-290 

Instrument 

depth 

(m) 

18 

18 

25 

27 

26 

17 

63 

63 

25 

20 

27 

Mean current speed 

± RMS error* 

(cm s-1) 

9.9 ± 1.5 

14.4 ± 1.8 

17.8±1.5 

18.7 ± 5.2 

1.4 ± 0.9 

3.8 ± 0.8 

13.1 ± 2.3 

5.0 ± 1.7 

10.5 ± 2.0 

3.9 ± 0.6 

7.7 ± 1.1 

Mean current 

direction 

(OT) 

202 

217 

236 

251 

215 

309 

283 

296 

217 

039 

199 

The longest simultaneous observation period for all moorings was JD157-JD223. 

* RMS error is the square root of the record variance divided by the ratio of record length to 

integral time scale. 

11 
31.38 

31.28 

+ 

+ 

9.95 

30.19 9.88 

.02 + 10.91 

30.09 

+ 

+ 12.09 

12.23 

--r--r-----r----.----r---+ 5830° 

151°W 

Fig. 9. Salinity (upper number, psu) and temperature (lower number, °C) at the surface 

of lower Cook Inlet on 17 August 1978. 

4. RESULTS 

4.1. Internal Wave Features 

The Seasat SAR imagery of internal waves, collected over lower Cook Inlet during Rev 289 

on 17 July 1978, is shown in Fig. 4 with interpretive line drawings of the internal waves. A 

series of internal wave packets is visible in the middle of the SAR image. Each wave is 

characterized by a bright band followed immediately by a dark band. These features are probably 

generated by the tide interacting with features of the bottom. Although no "sea-truth" is available 

for the present case, the identification as internal waves is quite certain (e.g., Liu, 1983) and has 

been verified in field tests in the New York Bight (Liu, 1988). There are several characteristics 

of the sea surface pattern induced by the internal waves. The important ones are summarized 

as follows: 

(1) The internal waves occurred in packets, separated by distances that are approximately 

20km. 

(2) On the basis of the assumption that the wave packets were generated by the 

semidiurnal tidal current, their estimated speed was 0.50 m S-1. 

(3) The wave packets in this region contained about seven waves with a packet width of 

about 4 km. 

(4) Within a given wave packet, the wavelengths appeared to decrease monotonically, 

front to rear, and the surface pattern is always more observable at the front of the 

wave packet than at the rear. 

(5) The wave packet propagation direction was refracted by the current and evolved as it 

propagated through lower Cook Inlet. 

12 
These features suggest that the intemal waves in each packet were probably rank-ordered 

solitary waves with the largest, fastest, and longest waves in front. We can examine our 

assumption that the intemal waves were generated by the semidiumal tidal current. Water 

property data were collected at several stations in central lower Cook Inlet within a day of Rev 

289. The average mixed-layer depth was 30 m in a water column 100 m deep, and the difference 

in density between the two layers was approximately 1.0 x 10-3• Using these observations in an 

equation for intemal waves, 

v. = (g. h~'"r. 

where g' is the reduced gravity g(~p/p), ~p is the density difference between lower and upper 

layers and p is the mean density of the two layers, hI and h2 are the depths of the upper and 

lower layers, and H is the overall water column depth, gives a group velocity vs. of 0.45 cm S-I. 

The close agreement between wave speeds from the model and those estimatea from the SAR 

image supports our contention that the wave packets were generated by the vigorous semidiumal 

tidal currents extant in lower Cook Inlet. 

The essential element of the surface effects of the SAR backscattering is the interaction 

between the intemal-wave-induced surface current field and the wind-driven ocean surface waves 

(Liu, 1983; Caponi et aI., 1988). Basically, the wave-current interaction is based on a nearequilibrium spectral transport model to estimate the roughness modulation by a variable surface 

current. The effect of surface current is to alter the spectrum from its equilibrium value while 

the natural processes of wave energy input from the wind, wave breaking, and other nonconservative processes act to restore the ambient equilibrium spectrum. To observe intemal 

waves in the coastal region, the strain rate (horizontal shear stress) must be of order 10-3 S-1 (Liu, 

1988). 

4.2. Meanders 

One Seasat SAR image (Rev 811, Fig. 3a) shows a cold-water filament meandering in the 

upper Shelikof Strait on 22 August 1978. For comparison, an AVHRR image from Rev 9372 

(25 August 1978) at the same location is shown in Fig. 3b. In the AVHRR image, the cold-water 

mass flowing into upper Shelikof Strait had surface temperature differences between the meander 

and adjacent water of 1°-2°C. Such differences are consistent with observations of horizontal 

surface temperature gradients in lower Cook Inlet (e.g., Fig. 9). The surface signature in the 

SAR image is very likely caused by the effects of an air-sea temperature difference on sea 

surface roughness. Basically, the cold water stabilizes the atmosphere boundary layer. Variations 

in the stable atmospheric density stratification, as a result of air-sea temperature differences, can 

dampen the turbulent momentum exchange rate in the boundary layer and later the velocity 

profile near the ocean surface. This altered wind velocity profile may significantly change the 

energy transfer rate from the wind to the ocean surface waves. The dampened surface waves and 

roughness modulation at cold-water regions show as a dark band in the SAR imagery. 

The air-sea interaction model for atmospheric stability effects on surface wave modulation 

was reported by Chen et al. (1986); the model was based on the model by Large and Pond 

(1981). Chen et al. show friction velocity as a nonlinear function of wind speed and air-sea 

temperature difference. The friction velocity u., defined as the square root of turbulent shear 

stress divided by air density, is a measure of the turbulent momentum flux. Since the friction 

velocity is directly related to the wind stress, it is closely related to the radar cross section, as 

13
 
.8 

c: 

0 

.~

(,) 

Q) 

rn 

I 

rn 

rn 

.6 

0

... 

(,)

... 

tU 

"0 .4 

* ++ 

"Best fit" 

"0 

Q) 

.!:::! 

+ Power law curve 

&0 = 6.5 (u )2.14 

* (ij 

... 

E .2 RMS error 

0 

z ",,(00 -15,1=0.076 

0 .10 .20 .30 .40 

Friction velocity [u* (m 51)] 

Fig. 10. Radar cross section data values (for stable atmospheric conditions) and model 

function from a power law fit, versus friction velocity (slope values ignored). Error is 

the root mean square of residual. [Data are a subset from Keller et al. (1985, Fig. 7b).] 

shown in Fig. 10, under stable conditions (Weissman, 1987). The best fit to data from Keller 

et al. (1985) represents a power law curve as a transfer function model. As an example, in the 

upper Shelikof Strait the air-sea temperature difference is approximately 1°_2°C. From Fig. 10, 

with an assumed wind speed of 6 m s-t, the friction velocity is 19 cm S-l for the neutral 

condition (corresponding to the surrounding environment with no air-sea temperature difference) 

and is 17 cm S-l for the stable condition. Also from Fig. 10, the normalized radar cross section 

is 0.2 for the neutral condition and 0.15 for the stable condition. A change (reduction) of radar 

cross section by 25% due to the cold-water flow into the upper Shelikof Strait is a reasonable 

estimate. For weaker wind speed (4 m S-l), the change of radar cross section is larger (by a 

factor of 2), and the SAR intensity contrast is also larger. 

4.3. Eddies 

Figure 5 shows a Seasat SAR image of eddies collected over the lower Shelikof Strait during 

Rev 289 (17 July 1978). The dark region behind Kodiak Island is caused by low radar 

backscattering from a smooth ocean surface. The relatively smooth surface may be due either 

to the surface slick or to wind shadowing by the island. Small vortices are barely observable 

behind the island. A pair of eddies, however, can be identified in the lower Shelikof Strait near 

the Alaska Peninsula. Dipole or pairs of eddies have also been identified in Landsat Thematic 

Mapper data of the Alaska Coastal Current about 700 km upstream from the pair in Shelikof 

Strait (Ahlnas et aI., 1987). In the present study area, eddies have been shown in AVHRR, 

CZCS, and in situ measurements from different times (e.g., Fig. 6). 

The generation mechanisms for eddies in this region are not well understood. Eddies may 

be caused by the baroclinic instability in Shelikof Strait or by complex patterns in surface winds 

(Schumacher et aI., 1989). It is important to note that the eddy in the 1986 AVHRR (cold ring) 

and CZCS images is anticyclonic, whereas the eddy in the 1988 AVHRR (warm ring) and in situ 

14 
measurements (Fig. 6) is cyclonic. During May 1990, three satellite-tracked buoys, drogued at 

40 m, were deployed in a region with a high abundance of pollack larvae. All three buoys 

circulated in an anti-cyclonic eddy for approximately 10 days before dispersing. The cause of 

different rotational directions and their implication for formation mechanisms and larval retention 

are not currently known. 

s. DISCUSSION 

We have compelling evidence that a wide range of scales of eddies are detectable in SAR 

imagery in Shelikof Strait. SAR imagery from Seasat demonstrated that mesoscale eddies can 

be detected by their influences on the short gravity waves responsible for microwave radar 

backscatter. The mechanisms that alter the short gravity wave field in that area are not well 

understood. In at least some situations, it is likely that current gradients near the boundary of 

an eddy modify the spectrum of short waves, similar to the internal wave packets, producing the 

microwave backscatter. In other instances, or perhaps concurrently, sea-state change can also be 

induced by sea surface temperature variations that alter the atmospheric boundary layer stability. 

This gives rise to local variation in wind stress that is similar to the process apparently induced 

by the meander observed in upper Shelikof Strait. 

It is important to understand the generation processes and persistence of these eddies and 

variability of their characteristics. Through efforts to gain better understanding of these features 

we may also gain better understanding of how the regional environment affects survival of larval 

pollock and their eventual recruitment into the commercial fishery. Future research in FOCI will 

emphasize field studies that match space and time scales of circulation with biological processes. 

SAR would be a good tool to monitor the evolution of mesoscale eddies. During the next 5 

years, three spacecraft will carry SAR in near-polar orbits. These satellites are the first European 

Remote Sensing Satellite (ERS 1), the Japanese first Earth Resources Satellite (J-ERS 1), and 

RADARSAT to be flown by the Canada Centre for Remote Sensing (CCRS). In collaboration 

with these satellite programs, NASA has instructed the Jet Propulsion Laboratory (JPL) and the 

University of Alaska at Fairbanks to design, implement, and operate an Alaska SAR Facility 

(ASP) ground station at Fairbanks to receive and process the SAR data from ERS 1. A program 

is also to be developed that utilizes the data effectively in a wide variety of scientific disciplines. 

Because of the special capabilities of SAR systems, it is anticipated that applications of SAR for 

fisheries oceanography will emerge following the launch of ERS 1 in April 1991. 

6. ACKNOWLEDGMENTS 

The authors thank Tom George (University of Alaska) for supplying the 1978 AVHRR 

image, Kristina Ahlnas (University of Alaska) for providing sea surface temperature estimates 

for the same period, and John Crawford (JPL) for image co-registration. A.K. Liu extends thanks 

to Jane Elrod (NASA) for providing the CZCS images. We thank D. Vastano (Texas A&M 

University) and T. Vance (PMEL) for the surface current vectors and AVHRR images from 1988. 

Some of the graphic work was provided by Karen Conlan (PMEL), and we thank Christy Sweet 

(ERL) for thorough technical editing. Portions of this work were performed at the Jet Propulsion 

Laboratory, California Institute of Technology, under contract to the National Aeronautics and 

Space Administration. The work performed by A.K. Liu was supported by the NASA Oceanic 

Processes Program and that of W.E. Barber by the University of Alaska, Fairbanks. This work 

was partially funded by NOAA's Fisheries Oceanography Coordinated Investigations and is FOCI 

contribution No. 0112 and PMEL contribution No. 1230. 

15 
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