Modeling seasonal ocean circulation of Prince FmrWikc ...Modeling seasonal ocean circulation of Prince William Sound, Alaska using freshwater of a line source /^ V.Patrick,™ J.Allen,™

Modeling seasonal ocean circulation of Prince

William Sound, Alaska using freshwater of a

line source

/̂ V.Patrick,™ J.Allen,™ S.Vaughan,™ C.

Mooers,̂ M.Jin^

International Arctic Research Center- Frontier, University of AlaskaFmrWikc, Fairbanks, Af 99775-7220, C/&4EMail: [email protected]

™ Prince William Sound Science Center, P.O. Box 705, Cordova, AK99574,

Rosenstiel School of Marine and Atmospheric Science, University ofMiami, Miami, FL 33149, USA

*• * Institute of Marine Science, University of Alaska Fairbanks,Fmr6aM&j,A# 99775- 7220, [/&4

Abstract

A three-dimensional, primitive equation ocean circulation model (Wang andIkeda[7]), was applied to Prince William Sound, Alaska (3D-PWS model) underforcing of freshwater runoff of a line source, heat flux, Gulf of Alaska (GO A)water inflow/outflow (throughflow), and daily (synoptic), spatially varyingwinds. The 3-D structures and seasonal cycles of the circulation patterns,temperature, salinity, and density were examined in real bottom topography. The"river/lake" scenarios (i.e., the weak versus strong flushing throughflow) werecaptured on a monthly basis. The freshwater runoff of the line sourcesignificantly contributes to the basin-scale cyclonic circulation which can't beseen in previous simulations without freshwater runoff (Mooers and Wang [3]).Wind forcing due to the orographic effect substantially contributes to thecirculation patterns in the Sound. Multiple circulation regimes (cyclonic,anticyclonic, and their combination) characterize the complexity of the systemwhich depends on the intensity of the GOA water throughflow, freshwaterdischarge of the line source, and the synoptic wind. A winter circulation ischaracterized by a high flushing regime due to high throughflow and northeastwinds, while the spring pattern is dominated by a basin-scale anticyclonic gyre.The summer (July to September) circulation is controlled by a basin-scalecyclonic gyre due to the maximum freshwater influence along the coastline. Theautumn circulation is driven by a combination of the throughflow and thenortheast wind-driven flow. The simulated cyclonic gyre in summer and late fallis supported by observations.

Transactions on the Built Environment vol 40 © 1999 WIT Press, www.witpress.com, ISSN 1743-3509

58 Coastal Engineering and Marina Developments

1 IntroductionPrince William Sound (PWS or "the Sound") constitutes of multiple basins,fjords, channels, islands, inlets, and estuaries along the alpine coast of southernAlaska. The complexity of the seasonal circulation patterns is due to thecomplexities and uncertainties of seasonal inflow/outflow and various directionsand magnitudes of winds due to the orographic effect in the mountainous region(Fig. 1). Mooers and Wang [3] implemented a 3-D circulation model in PWS

using idealized wind forcing and fixed throughflow (0.3 Sv, 1 Sv =10^ m̂ /s),without heat flux and freshwater discharge. Therefore, the major circulationpattern is dominated by the throughflow from Hinchinbrook Entrance toMontegue Strait. For lack of freshwater runoff, the basin-scale circulation wasnot obtained. Nevertheless, the model performance was encouraging underidealized forcing which was confirmed by a study of the tracer transportexperiments (Deleersnijder et al. [1]).The SEA (Sound Ecosystem Assessment) Program conducted majorobservational efforts. This numerical model is to 1) simulate the PWS seasonalcirculation patterns under different atmospheric forcing and coastalinflow/outflow conditions using a sophisticated 3-D numerical model; 2)examine the seasonal variations of the river/lake scenarios (hypotheses) whichare essential for understanding the ecosystem of the Sound; 3) investigate theforcing factors, such as orographic wind fields, freshwater runoff of a linesource, and importance of ACC throughflow; and 4) understand the seasonalvariations of temperature and salinity patterns.

2 The Mesoscale Ocean Circulation ModelA modified version of the POM using a predictor- corrector scheme for the timeintegration and semi-implicit scheme without mode-splitting (Wang and Ikeda[7]) was implemented to the PWS. It has the following features: (1) Arakawa Cgrid; (2) sigma coordinates in the vertical with realistic bottom topography; (3)free surface; (4) level 2.5 turbulence closure model for the vertical viscosity anddiffusivity (Mellor and Yamada [2]); (5)Smagorinsky's parameterization forhorizontal viscosity and diffusivity; (6) semi-implicit scheme for the shallowwater equations (Wang et al. [5]); and (7) a predictor-corrector scheme for thetime integration to avoid inertial instability (Wang and Ikeda [6,7,8]).The model domain includes the entire PWS with two open boundaries(Hinchinbrook Entrance and Montague Strait, Fig 1). The model grid space is1.2 km, which is eddy resolving because the internal Rossby radius ofdeformation is about 5 km in winter (50 km in summer) (Niebauer et al. [4]).There are 15 vertical sigma levels, with a relatively highresolution in the upper 50m to resolve the upper mixed layer. The integrationtime step is 100 seconds which is about ten times the CFL constraint because thesemi-implicit scheme has been used for the shallow water equations (Wang [5]).The initial temperature and salinity fields are based on a typical spring profile(Fig.2, dashed, of Mooers and Wang[3]) and specified to be horizontallyuniform. The model was spun-up for one year from these initial conditions underseasonal forcing to reach a dynamic and thermodynamic steady state.


Coastal Engineering and Marina Developments 59

3 Seasonal Forcing FactorsAccording to observations at Hinchinbrook Entrance (Niebauer et al. [4]), thecoastal inflow varies seasonally (Fig. 2a). The outflow through Montague Straitis of the same order of magnitude, although the water volume in the Sound mayincrease or decrease in response to transient forcing. Hence, an inflow wasspecified over the seasonal cycle through Hinchinbrook Entrance while aradiation boundary condition for the normal velocity was applied to MontagueStrait. The open boundary condition at Montague Strait for the temperature andsalinity is free advective.The monthly heat flux (Fig. 2b) originating from the GOADS was specifiedspatially homogenous with the restoring surface temperature boundary conditionto the seasonal climatology from 1975-1996. The monthly freshwater runoff ofthe line source (Fig. 2c) was calculated from the hydrological Digital ElevationModel (DEM) along the coast. The restoring boundary condition is the observedseasonal salinity climatologies from 1975-1996.The wind forcing has high spatial and seasonal variations over PWS due to theorographic effect and synoptic time scales from days to weeks. Three-hourlyrecords of the wind speed and direction, humidity, air temperature, andshortwave solar radiation were taken at nine meteorological stations over PWS(shown in Fig. 1). The wind speed and direction are spatially variable amongstations (Fig. 3), at Mid Sound (Fig. 3a), the wind direction varies from time totime due to a relatively flat region in the central Sound. However, at PotatoPoint (Fig. 3b) channeled by high mountains on both sides, the wind direction iseither southwestward or northeastward, and the wind speed is stronger than at theother stations. Thus, annually averaged wind is southwest. At Valdez (Fig. 3c),the wind direction is also channeled by the mountains, as at Whittier (Fig. 3d)where the wind is normally northeasterly or southwesterly, with the northeast

winds dominating. The annual mean northeast wind is about 3m s .As the orographic effect over PWS is overwhelming, theoretically, a mesoscalemeteorological boundary model is needed to calculate the surface wind fields.For the first cut, however, an empirical, wind-fetch model (nine wind fetchesresponding to 16 possible prevailing wind directions) was used to optimallyinterpolate the wind records from the nine stations.Figure 4 demonstrates the daily wind fields on days 15 (January), 105 (April),195 (July) and 285 (October). These show spatially variable features that cannotbe captured by any of the wind records at a single station.

4 Seasonal Circulation, SST, and SSS PatternsThe surface circulation patterns in January, April, July, and October are shown inFig. 5, along with the sea surface temperature (SST, Fig. 6) and sea surfacesalinity (SSS, Fig. 7) fields. Due to the strong northeast wind in winter, thesurface currents are driven out of the Sound (Fig .5a) at very high flushing rate.There are two regimes in SST and particularly SSS with a division between thewestern and eastern Sound. The SST (Fig. 6a) and SSS (Fig. 7a) signaturessuggest a significant oceanic influence on the central and eastern Sound rather



than on the west. The SST in the west (about 3°C) is about 2°C lower than in thecentral and eastern Sound (about 5°C), and the SSS difference is about 2 psu (28vs. 30 psu) which dominates the density field.When the minimum throughflow occurs in April, anticyclonic circulation occursin the central Sound (Fig. 5b). This feature cannot be seen in the previoussimulations (Mooers and Wang [3]). The SST (Fig. 6b) rises to 7-8°C due tosolar warming. Freshwater of the line source is set up along the northwesterncoast and estuaries (Fig. 7b) and generates the along-shore current. The SSSdecreases due to the setup of the freshwater discharge. As the freshwater runoffcontinues in July and southwest wind dominates, the cyclonic gyre in the centralSound starts to form, while an anticyclonic eddy is developing in the northernSound (Fig. 5c). Despite of the freshening, the northeastward current on the westcoast is generated by the southwest wind, whose direction is opposite to that ofthe density-driven along-shore current. There is freshwater tongue from ValdezArm and strong freshening along the northern and western coasts (Fig. 7c). Astrong freshening occurs during August through September, so the central Soundcyclonic circulation continues to develop from August to September (Fig. 8b).The circulation pattern in October (Fig. 5d) is dominated by a basin-scalecyclonic gyre. The northeast wind-driven current is consistent with the along-shore density-driven current, leading to stronger current along the western coast.Oceanic influence through Hinchinbrook Entrance on SST (Fig. 6d) and SSS(Fig. 7d) is a striking feature, because the ACC transports the warmer and moresaline water into PWS and leads to strong SST and SSS gradients in thesoutheast-to-northwest direction.The seasonal "river/lake" scenario can be captured by the streamfunction(volume transport). During the lower throughflow months, the circulationpattern dominated by the anticyclonic gyre (Fig. 8a) tends to be a lake scenariobecause the PWS water recirculates anticyclonically. However, in the summerseason, the cyclonic circulation dominates (Fig. 8b), enhancing the flushing ofthe water out of the Sound through Montegue Strait: the river scenario. In thewinter season, the strong northeast winds along with the strong throughflow,maximizes the river scenario (Fig. 8c). A mixed scenario between the lake andriver states can be seen in February (Fig. 8d) when the cyclonic and anticycloniceddies are comparable. There are 2-3 pairs of cyclonic-anticyclonic eddies alongthe deep basin of the Sound; the deep water allows easier development of eddiesthan the shallow region of sloping topography (Wang and Ikeda [9]). Theseseasonal circulation patterns depict the complexity caused by external forcing.Time series of the surface velocity vectors (Fig. 9) reflect the combination ofwind forcing, density, and throughflow forcing. Obviously, the winds playsignificant role in the surface current. Because no moorings were deployedinside PWS during the SEA program, no direct comparison in time seriesbetween model outputs and observations were conducted.With respect to the mesoscale eddy development and decay, the time series oftotal kinetic energy (TKE), zonal kinetic energy (ZKE), and eddy kinetic energy(EKE) of the entire PWS domain indicate the seasonal variations (Fig. 10, upperpanel), where TKE = ZKE + EKE. ZKE (that is smaller than EKE) represents



the strengths of the throughflow [which well matches the magnitude of thethroughflow (Fig. 2a)] and the density-driven flow (which peaks in August-September). For instance, ZKE is low from April through August when thethroughflow is weak and freshwater runoff starts to build up. In September, thethroughflow increases and the freshwater runoff reaches its maximum, leading toa basin-scale cyclonic circulation in the Sound. As a result, ZKE rises sharply inSeptember. In summary, ZKE represents the strength of the basin-scale gyre orthroughflow circulation, either cyclonic or anticyclonic. Another significant ZKEpeak can be seen during February and March when there are more active basin-scale gyres and eddies (Fig. 8a and 8d).Mesoscale eddies are another phenomenon in the Sound which can be seen in thetime series of EKE and the growth rate (Fig. 10, lower panel). The higher EKEis accompanied by the higher ZKE, indicating that the higher basin-scale gyresare the sources (possibly due to baroclinic instability together with thetopographic effect) of the mesoscale eddies (Wang and Ikeda [9]). EKE is muchhigher than ZKE because the active mesoscale eddies or transient currentdominates. The eddy duration time scales are from a few days to months, basedon the growth rate time series (Fig. 10, low panel). The negative values of thegrowth rate indicate the decaying phase of eddies.

5 Conclusions and DiscussionsThe 3D-PWS model has produced reasonable seasonal circulation patterns underforcing of the throughflow, freshwater runoff of a line source, and daily(synoptic) winds. The above investigations can be summarized as follows:1) Seasonal circulation patterns are generated by the equally important externalforcing factors: throughflow, freshwater runoff of the line source, and synopticwind fields. Without one of these forcing factors, the simulated seasonalcirculation would not be reliable. The circulation patterns vary from month tomonth due to the highly variable forcing fields. Generally speaking, theanticyclonic circulation patterns more likely occur during the time of lowfreshwater runoff, weak throughflow, and weak northeast wind, such as inMarch, while cyclonic circulation patterns dominate during the period of highthroughflow and high freshwater runoff, such as from July to October.2) There are more than lake and river scenarios over a seasonal cycle. The lakescenario is detected in March to June when the anticyclonic circulation gyresprevail. The river scenario most likely occurs in July to October when thecyclonic circulation patterns are strong. The most energetic river scenario mayoccur in the winter months (November to January) due to both strongthroughflow and persistent northeast wind controlled by the persistent AleutianLow (Fig. 6, Wang et al. [10]). However, in February, comparable cyclonic andanticyclonic gyre and eddy pairs mark the transit from river to lake scenario.3) Without freshwater runoff of a line source, the seasonal circulation patterns inthe upper layer would be impossible to be reproduced and the seasonal SSS andSST patterns would not be realistic. The freshening due to runoff generatescurrent along the northwest coast that has not been shown in the previoussimulations.



4) Although heat flux is not as important as freshwater runoff in terms ofdensity-driven current, it is important to determine the seasonal cycle of theupper layer temperature structure, as well as 3D structures. Therefore, accurateheat flux is necessary to the thermodynamics that affects biological components.5) Highly spatially variable directions and magnitudes of the wind fields providea topic for Mure research.

Acknowledgements: Financial support from the SEA Program through PrinceWilliam Sound Science Center, Alaska is appreciated. JW acknowledges supportfrom IARC-FRSGC for providing the computer power and from Prof. M. Ikedaof Hokkaido University, Japan, for his helpful discussions.

References1. Deleernijder D., J. Wang and C.N.K. Mooers, A two compartment model for

understanding the simulated three-dimensional circulation in Prince WilliamSound, Alaska. Continen. Shelf Res,lS, pp. 279-287, 1998.

2. Mellor, G.L. and T. Yamada, Development of a turbulence closure modelfor geophysical fluid problem, Rev. Geophys. Space Phys., 20, pp. 851-875,1982

3. Mooers, C.N.K. and J. Wang, On the implementation of a three-dimensionalcirculation model for Prince William Sound, Alaska. Continen. Shelf Res.,18, pp. 253-277, 1998.

4. Niebauer, H.J., T.C. Royer and TJ. Weingartner, Circulation of PrinceWilliam Sound, Alaska. /. Geophys. Res., 99, pp. 14,113-14,126. 1994.

5. Wang, J., L.A. Mysak and R.G. Ingram, A 3-D numerical simulation ofHudson Bay summer circulation: topographic gyres, separations and coastaljets. /. Phys. Oceanogr., 24, pp. 2496-2514, 1994.

6. Wang, J. and M. Ikeda, Stability analysis of finite difference schemes forinertial oscillations in ocean general models. Computer Modeling of Seasand Coastal Regions., Vol. 2, C.A. Brebbia. Et al, Computational MechanicsPublications, Southampton, pp. 19-27. 1995.

7. Wang, J. and M. Ikeda, A 3-D ocean general circulation model formesoscale eddies-I: Meander simulation and linear growth rate. ActaOceanologica Sinica, 15, pp. 31-58, 1996.

8. Wang, J. and M. Ikeda, Inertial stability and phase error of time integrationschemes in ocean general circulation models. Mon. Wea. Rev., 125, pp.2316-2327, 1997a.

9. Wang, J. and M. Ikeda, Diagnosing ocean unstable baroclinic waves andmeanders using quasi-geostrophic equations and Q-Vector method. /. Phys.Oceanogr., 27, pp. 1158-1172, 1997b.

10. Wang J., R.T. Cheng and P.C. Smith, Seasonal sea-level variations in SanFrancisco Bay in response to atmospheric forcing, 1980. Estuarine, Coastaland Shelf Science, 45, pp. 39-52, 1997c.



78DISTANCE (KM)

Fig.l. Bottom topography of PWSand stations. Six CM in 'o', tenTIDE in '*', and ninemeteorological stations 'A'.

POTATO POUft

C 30 68 90 120 150 240 27B 300 330 3b0

Fig.3. The 1996 time series ofdaily wind velocity vectors at MidSound (a), Potato Point (b), Valdez(c), and Whittier (d). The thicksolid line with the dot (direction)denotes the annual mean windvelocity.

Fig.2. The time series of inflowfrom the Hinchinbrook Entrance(a), heat flux (b), and freshwaterrunoff (c).

Fig.4. The snapshots of the windfields derived from the empiricalwind-fetch model on days 15 (a),105 (b), 195 (c), and 285 (d).


Fig.5. The surface circulation patterns on days 15(a), 105(b), 195(c) and 285 (d).

H9W 148W

p - 51.ON



DISTANCE (KM)

Fig.6. Sea surface temperature patterns on day 15(a), 105(b), 195(c) and 285 (d).

DISTANCE (KM)LONGITUDE

-— 61.0N

STANCE (KM

Fig.7 Sea surface salinity patterns on day 15(a), 105(b), 195(c) and 285 (d).



r - 61.0Mp— 61.0N

DISTANCE (KM)

' a-£ 61.0N

DISTANCE (KM)

Fig. 8. The simulated streamf unction (transport) in March (a), August (b),December (c) and February (d).

120 150 180 210 240 270 300 330 360210 240 270 300 330 360 SOLD) TKE

SHORT DASHED—ZKELONG D

JULIAN DATTS. 1996240 270 300 330 360

JULIAN DAYS. 1996Fig.9. The time series of thesimulated surface velocity vectorsat the five stations.

Fig. 10. The time series ofsimulated TKE, ZKE, and EKE(upper panel), and the mesoscaleeddy growth rate (lower panel)


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Modeling seasonal ocean circulation of Prince FmrWikc ...Modeling seasonal ocean circulation of Prince William Sound, Alaska using freshwater of a line source /^ V.Patrick,™ J.Allen,™