University of Alaska

Coastal Marine Institute

Observations of hydrography and currents in central

Cook Inlet, Alaska during diurnal and semidiurnal

tidal cycles

Stephen R. Okkonen, Principal Investigator

Institute of Marine Science, University of Alaska Fairbanks

Final Report

October 2005

a. *. * *

OCS Study MMS 2004-058
OCS STUDY MMS 2004-058

Final Report

Observations of hydrography and currents in central Cook

Inlet, Alaska during diurnal and semidiurnal tidal cycles

by

Stephen R. Okkonen

Institute of Marine Science

University of Alaska Fairbanks

SEPTEMBER 2005
Contact information

e-mail: carter@sfos.ual.edu

phone: 907.474.1811

fax: 907.474.1188

postal: Coastal Marine Institute

School of Fisheries and Ocean Sciences

University of Alaska Fairbanks

Fairbanks, AK 99775-7220

2
Table of Contents

Table of Figures ii

Abstract 1

Background and Relevance to Framework Issues 1

Methods 2

Results 3

Mean propertyfields 3

Semidiurnal variability 4

Diurnal variability 5

Discussion 6

References 8
Table of Figures

Figure 1. Bathymetric map and place names of Cook Inlet, Alaska. The location of the

hydrographic section is shown as a red line 9

Figure 2. MODIS true color image of Cook Inlet acquired 2 September 2002 10

Figure 3. Top Mean surface currents vectors across section. Northward flow is up, southward

flow is down. Bottom Mean salinity across section. Contour interval is 1.0 psu. ... 11

Figure 4. Top Mean surface currents vectors. Northward flow is up, southward flow is down.

Bottom Mean temperature across section. Contour interval is 1.0° C 12

Figure 5. Top Mean surface currents vectors. Northward flow is up, southward flow is down.

Bottom Mean baroclinic geostrophic velocity across section. Positive speeds are

northward. Contour interval 0.5 kt 13

Figure 6. Top Mean surface currents vectors. Northward flow is up, southward flow is down.

Bottom Mean transmissivity across section. Contour interval is 10% 14

Figure 7. Top Semidiumal M2 tidal ellipses. Northward flow is up, southward flow is down.

Middle Phase of sernidiurnal tidal currents. Phase is relative to 00 hours, 9 August

2003 local time. Bottom Bottom depth profile along section 15

Figure 8. a Amplitude of semidiurnal salinity signal. Contour interval is 0.2 psu. b Phase of

semidiurnal salinity signal relative to 00 hours, 9 August 2003 local time. Contour

interval is 1.0 hours 16

Figure 9. a Amplitude of semidiurnal temperature signal. Contour interval is 0.1°C. b Phase of

semidiurnal salinity signal relative to 00 hours, 9 August 2003 local time. Contour

interval is 1.0 hours 17

Figure 10. a Amplitude of semidiurnal transmissivity signal. Contour interval 5%. b Phase of

semidiurnal salinity signal relative to 00 hours, 9 August2003 local time. Contour

interval is 1.0 hours 18

Figure 11. Top Diurnal K! tidal ellipses. Northward flow is up, southward flow is down.

Bottom Phase of semidiurnal tidal currents. Phase is relative to 00 hours, 9 August

2003 local time. Bottom Bottom depth profile along section 19

Figure 12. a Amplitude of diurnal salinity signal. Contour interval is 0.2 psu. b Phase of diurnal

salinity signal relative to 00 hours, 9 August 2003 local time. Contour interval is 1

hour 20

Figure 13. a Amplitude of diurnal temperature signal. Contour interval is 0.1° C. b Phase of

diurnal salinity signal relative to 00 hours, 9 August 2003 local time. Contour interval

is 1 hour 21

Figure 14. a Amplitude of diurnal transmissivity signal. Contour interval 5%. b Phase of

diurnal salinity signal relative to 00 hours, 9 August 2003 local time. Contour interval

is 1 hour 22

Figure 15. Top Time-longitude plot of cross-channel surface current divergence &i/x.

Contour interval is 1 x i04 s’. Convergence au/ax<0 is shaded. Divergence

&i/ax>o is not shaded. Surface current vectors are shown. Thick dotted lines denote

slack water northward and southward current components are zero. Occurrences of

High and Low tides indicated on right side of plot. Bottom Bottom depth profile

along section 23

11
Figure 16. Top Time-longitude plot of along-channel surface current shear av/ax. Contour

interval is 1 x iO s1. Negative shears av/ax<O are shaded. Surface current vectors

are shown. Thick dotted lines denote slack water northward and southward current

components are zero. Occurrences of High and Low tides indicated on right side

of plot. Bottom Bottom depth profile along section 24

Figure 17. Top Time-longitude plot of surface layer salinity 0-1 m. Contour interval is 0.5

psu. Surface current vectors are shown. Thick dotted lines denote slack water

northward and southward current components are zero. Occurrences of High and

Low tides indicated on right side of plot. White oblique lines indicate vessel position

and time. Bottom Bottom depth profile along section 25

Figure 18. Map of bathymetric gradients in Cook Inlet, Alaska 26

Figure 19. Synthetic aperture radar image 8 February 2000 of central Cook Inlet, Alaska 27

111
Observations of hydrography and currents in central Cook Inlet, Alaska during diurnal

and semidiurnal tidal cycles

Stephen R. Okkonen

Institute of Marine Science

University of Alaska Fairbanks

Abstract

Surface-to-bottom measurements of temperature, salinity, and transmissivity, as well as

measurements of surface currents vessel drift speeds were acquired along an east-west section

in central Cook Inlet, Alaska during a 26-hour period on 9-10 August 2003. These measurements

are used to describe the evolution of frontal features tide rips and physical properties along this

section during semidiurnal and diurnal tidal cycles. The observation that the amplitude of surface

currents is a function of water depth is used to show that strong frontal features occur in

association with steep bathymetry. The positions and strengths of these fronts vary with the

semidiurnal tide. The presence of freshwater gradients alters the phase and duration of tidal

currents across the section. Where mean density-driven flow is northward along the eastern shore

and near Kalgin Island, the onset of northward tidal flow flood tide occurs earlier and has

longer duration than the onset and duration of northward tidal flow where mean density-driven

flow is southward in the shipping channel. Conversely, where mean density-driven flow is

southward in the shipping channel, the onset of southward tidal flow ebb tide occurs earlier

and has longer duration than the onset and duration of southward tidal flow along the eastern

shore and near Kalgin Island.

Background and Relevance to Framework Issues

Cook Inlet, a large -350 km long tidal estuary in southcentral Alaska Figure 1, is oriented

roughly south-southwest to north-northeast. Northern inlet waters are relatively fresh <20 psu

and turbid owing to sediment-laden discharges from the Matanuska, Susitna and Knik rivers. The

southern inlet opens to Shelikof Strait and the shelf region of the northern Gulf of Alaska. The

relatively clear oceanic waters occupying the southern inlet intrude northward along the eastern

side of the central inlet Figure 2. The north-south salinity gradient is strongest in late

summer/early fall when river discharges and glacial outflows are high. Semidiurnal tidal currents

dominate circulation variability in Cook Inlet and establish strong frontal convergence zones

known locally as the West Rip, Middle Rip, and East Rip. These strong rips serve as migratory

pathways for salmon returning to their natal streams and forage sites for sea birds. During the

1987 TA’ Glacier Bay oil spill, these convergent rips collected spilled oil.

This study investigates the semidiurnal and diurnal variability of the hydrographic structure and

currents in central Cook Inlet during conditions associated with the neap-to-spring progression of

the fortnightly tidal cycle and with large freshwater inputs. However, the operative physical

forces and their periods of variability are known, so that the results can be extended to infer

conditions at other locations in Cook Inlet and at other times during the fortnightly tidal cycle and

seasonal freshwater cycle. The results from this project 1 improve the understanding of local

hydrography, surface currents, and dynamical processes, 2 provide observations for the

development and validation of numerical circulation models, and 3 provide information that

may be potentially useful for spill response planning and training.
Methods

A Sea-Bird Electronics, Inc. SBE l9plus SEACAT Profiler CTD conductivity-temperature

depth and a WET Labs C-STAR transmissometer were used to acquire 126 surface-to-bottom

measurements of temperature, salinity, and light transmissivity during nine crossings of an eastwest transect near Kalgin Island and Kenai/Soldotna in central Cook Inlet, Alaska see Figure 1.

GPS measurements of vessel drift speed and direction were also recorded at each CTD cast

location. These measurements were acquired during a 26.5 hour period beginning 12:35 hours

local time on 9 August 2003. The sampling period was chosen to occur during a period of calm

seas and light winds so as to reflect oceanographic conditions without the complicating effects of

wind and waves influencing surface currents or mixing of the water column. Seas were calm less

than 2 ft. and mean winds were light 6.3 kts, 3.2 m s during this period and, as a consequence,

wind and wave effects were considered to be negligible and vessel drift velocities were assumed

to represent surface current velocities. High tides at the Kenai River entrance 5 km north of the

eastern terminus of the transect during the sampling period were approximately 18.9 ft 5.76 m

at 15:45 9 August2003 and 21.3 ft 6.49 m at 03:13 10 August 2003 local time. Low tides at

the Kenai River entrance were approximately 5.0 ft 1.52 m at 2 1:30 9 August 2003 and -1.7 ft

-0.52 m at 10:23 10 August 2003.

Station spacing between successive CTD casts was nominally 1.82 km 1 n mi. near the coasts

and 2.73 km 1.5 n mi. along the middle portion of the transect. Cast locations were acquired

from a vessel-mounted GPS unit. Vessel drift velocities speeds and headings were calculated

automatically by the GPS unit from successive GPS fixes. The CTD was lowered at

approximately 30 m mm1 and individual measurements of temperature, salinity, transmissivity,

and pressure were acquired at a rate of four per second. Temperature, salinity, and transmissivity

data from each downcast were averaged in 1 m 1 decibar bins centered on integer depths.

Sampling along each crossing of the transect took roughly 3 hours. As a consequence, a temporal

offset is associated with each CTD station location. In order to depict instantaneous conditions

across the transect, a least squares procedure was used to compute interpolated values of

temperature, salinity, and transmissivity at 15-minute intervals for each integer depth at each

CTD station location. The underlying assumption for the interpolation was that conditions at each

location x and depth z varied according to the sinusoidal function

Pt = A1.cos2it.tJ12.42 + A2.sin21t.t112.42 + A3.cos2it.t123.93 + A4.sin2it.t/23.93 +

A5.t + A6 Equation 1

in which P is the property of interest temperature, salinity, transmissivity, or velocity, A1 ....

are least squares coefficients, and t is the time in hours. The function includes the M2 semidiurnal

12.42 hr tidal constituent, the K1 diurnal 23.93 hr tidal constituent, a linear trend, and offset.

The amplitude and phase of the M2 semidiurnal variation were computed according to

AmplitudeM2 = A1. A1 + A2. A22 Equation 2

M2 = tan’ A2/A1, Equation 3

respectively. Similarly, the amplitude and phase of the Ki diurnal variation were computed

according to

2
AmplitudeK! = A3. A3 + A.4. A4"2 Equation 4

Phase 11 = tan1 A4/A3. . Equation 5

Daily-mean property fields were computed by averaging the ninety-six 15-minute property fields

computed according to equation 1 for the 24-hour period beginning 1400 hrs, 9 August 2003.

The terms flood tide and ebb tide are used in reference to tidal currents that flow up-inlet and

down-inlet, respectively, as opposed to tidal heights.

Results

Mean property fields

The daily-mean salinity field illustrates representative structures and locations of the principal

salinity fronts in central Cook Inlet Figure 3. Two freshwater <25 psu plumes intersect the

transect. The plume along the eastern side of the inlet <23 psu derives from the Kenai and

Kasilof Rivers east plume. The surface plume in the shipping channel <25 psu is the extension

of freshwater originating in the northern inlet that has passed through the constriction between the

West Foreland and East Foreland Forelands plume. The shipping channel refers to the relatively

deep channel that intersects the section roughly between 151 .73°W and 151 .55°W. The strong

salinity gradients at the flanks of the Forelands plume define two fronts, the surface expressions

of which are locally known as the West Rip and Middle Rip. The Middle Rip front is the most

vertically extensive of these fronts. The base of the Middle Rip front intersects the bottom at a

depth of approximately 50 m. The most saline waters >27 psu are found at the bottom of the

shipping channel and in the shallows adjacent to Kalgin Island. The higher salinity values

occurring in the shipping channel reflect the northward intrusion of denser water with a more

oceanic character. Mean surface currents, shown in stick plot format above the mean salinity

field, indicate that mean flow associated with the core of the Forelands plume is southward and

northward elsewhere. The changes between northward and southward flows occur where the

depth-averaged zonal salinity gradient changes sign near 151.72°W and 151.59°W.

The structure of the mean temperature field Figure 4 is qualitatively similar to the structure of

the mean salinity field in that the locations of the thermal fronts roughly correspond to the

locations of the salinity fronts, although the temperatures and salinities are negatively correlated.

Temperatures and salinities are negatively correlated during the summer months, whereas they

are positively correlated during the winter months. While the warmest temperatures >15°C

occur along the shallow eastern side of the inlet, temperatures associated with the Forelands

plume are also slightly warmer >14°C than adjacent waters. The coolest temperatures <13.4°C

are found at the bottom of the shipping channel.

The mean salinity and temperature fields were used to compute a mean density field, from which

were computed the baroclinic geostrophic currents normal to the section line Figure 5. The

contour plot indicates that density-driven currents are generally northward across the section

except within the shipping channel where mean currents are southward. The stick plot of mean

surface currents, independently derived from vessel drift velocities, shows directions similar in

orientation to the baroclinic currents, but it shows somewhat different magnitudes. The mean

salinity field see Figure 3 and geostrophy provide the context for understanding the mean

current field. Because the temperature gradients are relatively weak see Figure 4, density

3
gradients are more greatly influenced by the relatively strong salinity gradients see Figure 3.

According to the geostrophic approximation, northward baroclinic geostrophic currents will be

associated with an eastward decrease in density fresher and warmer, whereas southward

geostrophic currents will be associated with an eastward increase in density saltier and cooler.

This description of mean surface currents differs somewhat from those provided by Burbank

1977 and Whitney 1994, in which they both show southward flow everywhere at this latitude

except very near the eastern side of the inlet.

The structure of mean transmissivity field Figure 6 differs somewhat from the general structure

of the mean salinity and temperature fields. Transmissivity is functionally related to the

suspended sediment concentration, although water samples were not taken to quantify this

relationship. Low high transmissivity is associated with a high low suspended sediment

concentration. Overall, the suspended sediment concentration increases with depth. The clearest

waters occur above the eastern flank of the shipping channel in association with higher salinity,

more oceanic waters and on the seaward flank of the East plume compare with Figure 3. The

highest suspended sediment concentrations generally occur where the mean current speeds are

high, that is, above the western flank of the shipping channel and midway between the shipping

channel and the eastern side of the inlet.

Semidiurnal variability

Mean semidiurnal surface current tidal ellipses and phases along the transect are shown in Figure

7. The largest amplitude semidiurnal currents >4 kts occur in the deep water of the shipping

channel and are more or less aligned with the NNE-SSW axis of the channel. As the bottom

shoals toward the eastern side of the inlet, the semidiumal currents generally weaken except for a

slight intensification at the two easternmost stations. Adding the semidiurnal tidal currents to their

corresponding mean currents see Figure 3 suggests that the strongest net currents during the

flood tide will be observed near 151 .63°W, whereas the strongest currents during the ebb tide will

be observed between 151 .67°W and 151 .7°W. The meridional current components at the three

stations nearest Kalgin Island are about 2 knots. Interestingly, the zonal current components at

151.8°W 3.2 kts and 151.73°W 2.9 kts exceed the corresponding meridional current

components 2 kts and 2.6 kts, respectively at these locations. The phase plot indicates that

maximum semidiurnal surface currents in the shipping channel lag the surface currents near the

eastern side of the inlet by approximately 1.5-2 hours. The tidal ellipses also provide clues as to

the locations and phasing of the tide rips. The orientations of the ellipses suggest convergence

during the flood tide at locations between 151.3°W and 151.4°W, between 151.5°W and

151.6°W, and near 151.7°W. These convergences correspond to the locations of the East Rip,

Middle Rip, and West Rip, respectively. The orientations of the ellipses also suggest

convergences occur during ebb tide between 151 .4°W and 151 .5°W East Rip and between

15 1.7°W and 151.8°W West Rip. The absence of a prominent surface convergence in the

middle of the transect suggests that the Middle Rip dissipates during the ebb tide.

There are three principal loci at which large amplitude >2 psu semidiurnal salinity signals are

evident Figure 8a. The most vertically extensive locus occurs on the western flank of the

shipping channel at depths between 35-55 m and is associated with the base of the Middle Rip

front. The local amplitude maximum near the eastern side of the inlet is associated with the East

plume, whereas the local maximum above the western shoulder of the shipping channel near

Kalgin Island is associated with the West Rip front. Note, however, that the width of these large

amplitude signals is about 1-2 n mi. 1.82-3.64 km. Broadly speaking, the amplitude of the

4
semidiurnal salinity signal within the shipping channel exceeds the amplitude in shallower

waters. The minimum amplitude for the semidiurnal salinity signal <0.8 psu occurs in the

surface layer within the shipping channel. The relatively weak semidiurnal salinity signal at this

location reflects the persistence of the Forelands plume. The companion phase plot Figure 8b

shows that semidiurnal period changes in salinity near the eastern side of the inlet lead changes in

the shipping channel by 2-3 hours. Comparison of the salinity phase plot with the surface current

phase plot indicates that, across most of the transect, the maximum surface salinities occur

approximately 2-3 hours after the maximum surface currents. It can be inferred from this

relationship that the highest salinities occur near the end of the flood tide. Conversely, minimum

salinities occur near the end of the ebb tide.

The general pattern of semidiurnal temperature signals Figure 9a is similar to the pattern of the

semidiurnal salinity signals compare Figure 8a. The largest amplitude semidiurnal temperature

signal >0.8°C occurs along the western flank of the shipping channel. Secondary amplitude

maxima >0.5°C occur in shallow water near Kalgin Island and at the seaward flank of the river

plume along the eastern side of the inlet compare with Figure 3. The amplitude minimum

<0.2°C occurs in the upper 5 m of the shipping channel. A secondary minimum occurs in

shallow water adjacent to the eastern side of the inlet. The companion phases plot Figure 9b

shows that semidiurnal temperature signal in the shipping channel lags the signal near the eastern

side of the inlet by 3-4 hours. As expected, a comparison of the temperature phase plot with the

salinity phase plot compare Figure 9b shows that the temperature maximum occurs roughly six

hours half a semidiurnal cycle after the salinity maximum. This out-of-phase relationship

implies that warm temperatures are associated with fresher waters and cool temperatures are

associated with more saline waters.

The general pattern of semidiurnal transmissivity signals Figure lOa is somewhat similar to the

mean transmissivity conditions compare Figure 6. The least variation occurs where the mean

transmissivity is lowest: along the western flank of the shipping channel and between the

shipping channel and the eastern side of the inlet. The largest amplitude semidiurnal

transmissivity signal occurs in the shallow water next to Kalgin Island. Moderate amplitude

>25% semidiurnal transmissivity signals also occur near the bottom and surface in the middle of

the shipping channel and on the seaward flank of the Kenai and Kasilof River plume. The

companion phase plot Figure lOb shows that large phase gradients occur along the western

flank of the shipping channel and midway between the shipping channel and eastern side of the

inlet, whereas small phase gradients occur within the middle portion of the shipping channel.

Comparison of the transmissivity amplitude with the mean transmissivity plot compare Figure 6

indicates that high suspended sediment concentrations and the least variability in concentration

occur in association with large phase gradients and that low suspended sediment concentrations

are associated with small phase gradients. Moreover, comparison of the transmissivity phase plot

with the salinity phase plot compare Figure 8b indicates that within much of the shipping

channel, the phase of the two properties differ by less than an hour. Stated another way, the flood

tide carries relatively clear, saline water northward and the ebb tide carries relatively turbid, fresh

water southward.

Diurnal variability

Mean diurnal surface current tidal ellipses and phases along the transect are shown in Figure 11.

In general, the diurnal amplitudes are less than half the semidiurnal amplitudes. As was the case

for the semidiurnal surface currents, the largest amplitude diurnal currents -1.5 kts occur within

the shipping channel. Zonal east-west components of the diurnal surface currents are largest on

5
the west side of the transect and midway between the shipping channel and eastern side of the

inlet. The phase plot indicates that maximum diurnal surface currents in the shipping current lag

the diurnal surface currents near the eastern side of the inlet by about 2 hours. The phases of

maximum diurnal currents at the three westernmost locations and at 151 .37°W differ substantially

from the phases at the other locations.

The largest amplitude diurnal salinity signal >.lpsu occurs along the bottom of the shipping

channel Figure 12a. The smallest amplitude diurnal signals <0.4 psu occur next to Kalgin

Island, in the surface 0-5 m layer within the shipping channel, and roughly midway between the

shipping channel and eastern side of the inlet. The phase plot Figure 12b shows that across most

of the transect, except near the western and eastern side of the inlets, the phase differential is

typically less than 4 hours 1/6 diurnal cycle.

The plot of the amplitude of the diurnal temperature signal Figure 1 3a indicates that the largest

amplitude temperature signals >0.4°C occur in the upper meter or two of the Forelands plume

and East plume. The phase plot Figure 1 3b indicates that these shallow temperature maxima

occur in the late afternoon 1600-1800 hrs local time, suggesting that these surface signals result

from solar heating. There is also a - 10-m thick layer at the bottom of the shipping channel that

exhibits a >0.3°C diurnal variation. Across much of the remainder of the transect, the amplitude

of the diurnal temperature signal is <0.2°C.

The largest amplitude diurnal variation in transmissivity >20% occurs through much of the

water column in the middle of the shipping channel Figure 14a. The diurnal signal is smallest

near the bottom the shipping channel and midway between the shipping channel and the eastern

side of the inlet. The phase plot Figure 14b shows that phase is relatively constant at mid-depths

in the center of the shipping channel. However, across most of the remainder of the transect, there

are prominent horizontal and vertical phase gradients.

Discussion

There are a number of qualifications to the observational data sets that should be kept in mind

when reviewing and interpreting these data. In order to estimate instantaneous conditions, it was

assumed that the time variations of the field variables at any location on the section plane could

be represented by linear combinations of diurnal Ki period and semidiurnal M2 period

sinusoidal functions, a trend, and constant offset. The tidal amplitudes and corresponding phase

estimates were derived from relatively few measurements nine occupations of the transect

acquired during a 26-hour period. This limitation is most severe at the endpoints of the transect

because the times between repeat measurements were greatest. The temperature and salinity

observations reflect mid-summer conditions, that is, conditions that are approaching the warmest

and freshest values, as well as strongest gradients, of their seasonal cycles. Additionally, the

observations were acquired during the beginning of the spring tide portion of the fortnightly tidal

cycle. Spring tides are characterized by stronger currents and larger tidal ranges than are neap

tides. Wind and wave effects are not represented because observations were acquired during a

period of light winds and calm seas.

The preceding descriptions of hydrography and surface currents provide important clues as to the

dominant physical forces governing circulation and the evolution of fronts in Cook Inlet. Strong

convergences and shears in the surface currents characterize Cook Inlet tide rips. The temporal

and spatial occurrences of convergence zones are depicted in a plot the zonal derivative of the

6
east-west surface velocity component au/ax for each time t. Convergences occur when and

where u/x<O, assuming that av/ay is negligible Figure 15. The surface current vector overlay

shows that the strong West Rip convergence zone results from the large east-west u velocity

component at 151.73°W relative to the u components at 151.77°W and 151.7°W see also Figure

7. The West Rip oscillates around 151 .73°W, with strongest convergences occurring within a

few hours either side of slack water. The Middle Rip convergence resides between -151 .53°W

and 151 .6°W with the strongest convergences occurring during much of the flood and continuing

through the beginning of the ebb. The East Rip convergence generally occurs during the ebb

between 151 .5°W and 151 .43°W. Slightly stronger convergences are evident near the eastern

shore during the flood. Overall, the instantaneous widths of the convergence zones are -2-4 n mi

with the strong West Rip convergence exhibiting an instantaneous width of about 2 n mi.

The underlying constraint on the location of the surface current shear zones can be deduced from

the plots of the tidal ellipses Figure 7 and Figure 11 which show that the speeds of tidal currents

vary with the water depth. That is, the faster along-channel currents generally occur in deeper

water and, conversely, the slower currents occur in shallower water. The depth dependence of the

tidal current speeds indicates that frictional effects imparted to the water column by the channel

floor are important in determining the characteristics of the tidal flow field. The relevant

dynamics are those governing turbulent flow in an open channel. A very simple representation of

this relationship steady state and no rotation states that the current velocity, V. is proportional to

the square root of the channel depth, H,

V oc H"2 . equation 5

The cross-channel derivative of equation 5

H’’2 H/x equation 6

indicates that prominent velocity shears should occur where the cross-channel bottom slope is

large. The temporal evolution of the along-channel velocity shear avIJx field Figure 16 shows

this to generally be the case. The largest magnitude shears occur over the steep western flank of

the shipping channel during ebb tide. Somewhat weaker magnitude shears occur near the eastern

flank of the shipping channel between 151 .63°W and 15 1.57°W. Even weaker magnitude shears

occur near 151 .43°W. The locations of these three shear zones corresporfd to the respective

locations of the West, Middle, and East rips as inferred from the velocity convergence plot. From

a temporal perspective, the magnitudes of the velocity shears tend be larger during the latter half

of the flood and ebb.

It is readily apparent from comparisons of the time evolution of the surface salinity field Figure

17 with the corresponding convergence and shear plots see Figures 15 and 16 that salinity

fronts are associated with the West Rip and Middle Rip convergence and shear zones. The East

Rip, however, does not exhibit significant zonal salinity gradients at any time during the sampling

period. The strong salinity front at the flank of the east plume exhibits its greatest offshore extent

near the end of the ebb and its closest approach to the eastern shore near the end of the flood. The

zonal migration of each these salinity fronts appear to be roughly 1-2 n mi.

The plot of the mean density-driven currents and mean surface currents see Figure 5 and the M2

tidal currents see Figure 7 provide a context for understanding the influence of freshwater inputs

to Cook Inlet on the variability of the vector surface currents illustrated in Figures 15-17. The

mean density-driven southward flow between 151 .7°W and 151 .6°W will augment tidal ebb

7
currents and oppose tidal flood currents. The mean density-driven northward flows east of

151 .6°W and west of 151 .7°W will augment tidal flood currents and oppose tidal ebb currents.

The result is that where mean density-driven flow is northward along the eastern shore and near

Kalgin Island, the onset of northward tidal flow flood tide occurs earlier and has longer

duration than the onset and duration of northward tidal flow where mean density-driven flow is

southward in the shipping channel. Conversely, where mean density-driven flow is southward

in the shipping channel, the onset of southward tidal flow ebb tide occurs earlier and has

longer duration than the onset and duration of southward tidal flow along the eastern shore and

near Kalgin Island.

The results presented above can be generalized to qualitatively describe conditions at other times

during the fortnightly and seasonal cycles. It can be inferred from the asymmetry of the two

semidiumal tidal cycles during the sampling period currents were weaker on 9 August than on

10 August that, over the fortnightly tidal cycle, tide rips current shears and convergences and

property gradients temperature, salinity, suspended sediments will be stronger during the spring

tide and weaker during the neap tide. Because cross-channel freshwater density gradients alter

the phases of tidal currents, it can be inferred from the seasonal cycle of freshwater inputs to

Cook Inlet high inputs in summer and low inputs in winter that density-driven currents will be

weaker and the phases of the tidal currents will be more uniform across the inlet during winter

than in summer.

The results can also be generalized to infer rip locations throughout Cook Inlet. Equation 6, along

with Figures 15 and 16, indicate that strong tide rips are associated with the steep bathymetry.

Figure 18 shows bathymetric gradients computed from digital bathymetry of the Cook Inlet

region see Figure 1. The steep bathymetry lying immediately east of Kalgin Island and

extending northward through the Forelands and southwestward to the latitude of Ninilchik is

associated with the West Rip. The bathymetric gradients associated with the Middle Rip and East

Rip are also evident, but less prominent. Strong bathymetric gradients exist to the west of Kalgin

Island. A radar image of ice distribution in central Cook Inlet on 8 February 2000 Figure 19

supports the generalization that rips/fronts ice edges are well correlated with bathymetric

gradients.

In summary, the results from this study suggest that Cook Inlet numerical circulation models

require accurate, high-resolution bathymetry to correctly locate fronts rips. Additionally, these

models need to be fully three-dimensional, incorporating buoyancy temperature and salinity

effects at seasonal to semidiurnal time scales to improve their forecasting skill.

References

Burbank, D. C. 1977. Circulation studies in Kachemak Bay and Lower Cook Inlet. Trasky, L. L.

L. B. Flagg D. C. Burbank eds. vol. III. Alaska Department of Fish and Game.

Anchorage, AK. pp. 207.

Whitney, J. 1994. Past experience with spill response in Cook Inlet. Anchorage, AK: ISCOI,

NOAA, Hazardous Materials Response and Assessment Division.

8
400 300 200 100 0

Depth m

Figure 1. Bathymetric map aM place names of Cook inlet, Alaska. The location of the

hydrographic section is shown as a red line.

9

1 I 53* 151*
Figure 2. MODIS true color image of Cook inlet acquired 2 September 2002.

10
l.0*

0.5

0.0

_Q5

-1.0

I

r

0

- I I I

20 21 22 23 24

Soli&ty psi

Figure 3. Top Mean surface currents vectors across section, Northward flow is up, southward

flow is down. Bottom Mean salinity across section. Contour interval is 1.0 psu.

19 25 26 27 28 29

0,

S.

0

C

151.8°W 151.7°W l51.6W l51.5°’ l51.4° 151.3°W
-1.0

0

10

20

£

x

Ui

0

50

Temperature t

Figure 4. Top Mean surface currents vectors, Northward flow is up, southward flow is down.

Bottom Mean temperature across section. Contour interval is 1.0° C.

II 12 13 14 lb 16 17

60

70

15t.8°W l51,7° 15I.6°W 151.5 t51.4°W 151.3°W

12
Li

P1 cc AJOJU Jfl041103

pwMquou am spaads aAnrSOd UOIJDS SSOJDE AII3OIaA oiqdoJ2soa DRfiIooilq UDjAJ uo4wq

uMop Si MOjJ PJi3M1flOS dti s! MOjJ pnMquo SJOIOOA siuanna orpns rn?gj do1 g SJfl!J

s;owt pads

£ I 0 V * I * I I

W.nct $.nci $.nc& M.rtci $.ncI $.tiQ I I * I

-09

-02

.0p

01

.0

0 -

00 o

En

01

p

[7"Y 77 I

L.x
1.0

0.5

0.0

0.5

1 .0

E

I

0-

C

40

50

60

s..L.xJ/.N.rJ.JA..JJ.J.JL.l

I U I

o 10 20

I I I

30 40 50

TronsminJvity %

Figure 6. Top Mean surface currents vectors. Northward flow is up,

Bottom Mean transmissivity across section. Contour interval is 10%.

southward flow is down.

a

10

30

/0

151.8°W 15L7w 151.6°W 151.5°W 151 YW 151.YW

I I

60 70 80

14
LO/J’t

151.8°W 151.7DW 151:6°W 151.50W 151.4°W 15L3°W

151:8°W 151:7°W 151:6°W 151.5°W 151,4°W 151.3°W

Figure 7. Top Semidiurnal M2 tidal ellipses. Northward flow is up, southward flow is down.

Middle Phase of semidiurnal tidal currents. Phase is relative to 00 hours, 9 August 2003 local

time. Bottom Bottom depth profile along section.

4-

2-

0-

-2-

-4-

6 In

0

V

In

0

4-

2-

0-

-2

E

Q.

V

151.8’W 151.7°W 151.6W 151.5°W 151,4°W 151.3°W

15
E‘a

r

E‘a

I

40

50

60

70

t51.7* 151.VW lsi.rW 151.4°W 151.3’*

FigureS. a Amplitude of semidiurnal salinity signal. Contour interval is 0.2 psu. b Phase of

semidiurnal salinity signal relative to 00 hours, 9 August 2003 local time. Contour interval is 1.0

hours.

151.8W 151.7t 151.6* 151.5°* 151.4W l5l.3j

/ a30

IllslId -

4

I

a

b A Phase hours

16
E

I

Iii

0

0

10-

20-

50

60

70-

isifl I5I.7W 1b1.&W t51.5W l5I.4 151fl

Figure 9. a Amplitude of sernidiurnal temperature signal. Contour interval is 0.1°C. b Phase of

sernidiurnal salinfty signal relative to 00 hours, 9 August 2003 local time. Contour interval is 1.0

hours.

151.8°*’ 15l.7 151.6t 1513 l5lA 151.3

b Phos. hours

17
0

I0

20

40

50

60

70

Figure 10. a Amplitude of semidiurnal transmissivity signal. Contour interval 5%. b Phase of

seniidiurnal salinity signal relative to 00 hours, 9 August 2003 local time. Contour interval is 1.0

hours.

151.8°* I51.7 151.6t 1513°* l51.4’ t51.3

151.7W 151.6W I5I5W 151.4* 151.3*

18
4-

2-

0-

-2-

-4-

120

1 18-

10-

0

E 20

3O

O2 / / -

151i 181 7 15l.V* 1B1.5e* 151A’* 181.3*

,,-----

N

//

/

,

Iblr* 151.r* 151.6’* 1815’* 1814’* 181.r*

Figure 11. Top Diurnal Ki tidal ellipses. Northward flow is up, southward flow is down.

Bottom Phase of semidiurnal tidal currents. Phase is relative to 00 hours, 9 August 2003 local

time. Bottom Bottom depth profile along section.

181J* isi.rw 181.8* 181.4’* 151.3*

19
151.8.* I51.7W I5I.6°* l51:4*

*/:-=-**-

*1

T

Lç

O

-- II!

‘I I!

/

!/

I !j

/2

-

rI--

_ThA

]J/ A

‘4

A

I

.4

Phase hours

70

Figure 12. a Amplitude of diurnal salinity signal. Contour interval is 0.2 psu. b Phase of diurnal

sahnity signal relative to 00 hours, 9 August 2003 local time. Contour interval is 1 hour.

151.8’’ t5I.1 151St I513 ISlAt I51.3

0’

10*

20

30

4o.

50-

60- b

20
0

I0

20

E 30

I

I

40

50

60

70

151 .3W

Figure 13. a Amplitude of diurnal temperature signal. Contour interval is 0.1° C. b Phase of

diurnal salinity signal relative to 00 hours, 9 August 2003 local time. Contour interva! is 1 hour.

151.8 l51.7 l5t.6 151.5 151.4’W 151.yW

151.8* 15l.7W 151.VW I51.5°W 151.4°*

2!
0

10

20

I

40

50

60

70

151 .3W

Figure 14. a Amplitude of diurnal transrnissivity signal. Contour interval 5%. b Phase of

diurnal salinity signal relative to 00 hours, 9 August 2003 local time. Contour interval is I hour.

151.8’ 151.7 151$t i5i.5°* l5l.4’ 15l.3

15I.8W 151.7W t5I.5* 15l.5W 151.4°*

22
Ov/Ox

S

E

I

E

30-

. 40-

t50.

60-

70

151.8°W I51.7°W 151.6°W 151.5°W 151.4W 151.3%’

Figure 15. Top Time-longitude plot of cross-channel surface current divergence au/ax.

Contour interva] is I x I o-4 s’. Convergence auJaxO is shaded. Divergence au/ax>O is not

shaded. Surface current vectors are shown. Thick dotted lines denote slack water northward and

southward current components are zero. Occurrences of High and Low tides indicated on

right side of plot. Bottom Bottom depth profile along section.

I

0 -

-I I

-* 4 kts

‘0- 1

20- a

a

a

4.

a

23
14:00

10:00

06:00

p0200

I

2200

18:00

1400

E

Figure 16. Top Time-longitude plot of along-channel surface current shear v/ax. Contour

interval is 1 x 1 O s. Negative shears v/x<O are shaded. Surface current vectors are shown.

Thick dotted lines denote slack water northward and southward current components are zero.

-.* 4 Ms

151.8W 151.7’* 151.6’W 151.5* 151.4* 151.3*

24
1400

10:00

08:00

p02:00

2200

18:00

14:00

E

Figure 16. Top Time-longitude plot of along-channel surface current shear v/ax. Contour

interval is 1 x 1 O s. Negative shears av/x<O are shaded. Surface current vectors are shown.

Thick dotted lines denote slack water northward and southward current components are zero.

-.* 4 Ms

151.8W 151.7’* 151.6’W 151.b’* 151.4’W 151.3’*

24
Occurrences of High and Low tides indicated on right side of plot. Bottom Bottom depth

profile along section.

151.8W lbl.r* tbl.VW tbI.5W 151.4°W 151.3°W

19 20 21 22 23 24 25 25 27 28 29

Sorinlty plo

Figure 17. Top Time-longitude plot of surface layer salinity 0-I m. Contour interval is 0.5

psu. Surface current vectors are shown. Thick dotted lines denote stack water northward and

southward current components are zero. Occurrences of High and Low tides indicated on

I

-a

0*

-4 4 kls

10-

20-

50*

60-

70 -

4d

A

25
61M

601’l

59M

Figure 18. Map of bathymetric gradients in Cook Inlet, Alaska.

right side of plot. White oblique lines indicate vessel position and time. Bottom Bottom depth

profile along section.

154* 153* 152* 151* 150* 149*

26
Figure 19. Synthetic aperture radar image 8 February 2000 of central Cook Inlet, Alaska.

27