Verh. Internat. Verein. Limnol. 28 299-302 Srungan, February 2002

Coastal exchange characteristics during unstratified season in

southern Lake Michigan

C. R. Murthy,Y.R. Rao,M. J. McCormick,G. S. Miller and J. H. Saylor

Introduction

In the Great Lakes, as well as in the coastal oceans,the gradients of many biogeochemically importantmaterials (BIMs) are considerably higher in the off-shore direcrion than in the alongshore direction(BRINKet al. 1992). In the presence of these largegradients, cross-isobath circulation is a primarymechanism for the exchange of material berweennearshore and offshore waters. In the coastal regionsof the Great Lakes it has been observed that the

mean alongshore transport is much larger than thecross-shore transport. However, both the alongshoreand cross-shore currem components exhibit strongepisodic behavior due to wind forcing. In order £0understand the cross-shore transport of BIMs, andto quantify the physical processes that are responsi-ble for the nearshore-offshore mass exchange, a mul-tidisciplinary research program, EEGLE (EpisodicEvents Great Lakes Experiment) was recently initi-ated by NOAA (National Oceanic and AtmosphericAdministration) and NSF (National Science Foun-dation) in Lake Michigan.

Circulation in the lakes is driven by wind, bur theeffects of earth's rotation, basin £Opography, and ver-tical densiry structure are also important. During theunstratified season, the higher wind speeds and theabsence of the thermocline allow the effecrs of wind

action to penetrate deeper into the water column(BOYCEet al. 1989). In shallow water, the entirewater mass moves in the direction of the wind, whilereturn flow occurs in the deeper pans of the lake.This forms rwo coumer-rotating closed gyres (SAY-LORet al. 1980), a cyclonic gyre to the right of thewind and an amicyclonic gyre £0 the left. Theserotary motions or voniciry waves have been sug-gested as a main mechanism for nearshore-offshoretranspon in the Great Lakes. SCHWABet al. (2000)observed the presence of this rwo-gyre circulationpattern in their numerical experimems during awind evem in March 1998. MCCORMICKet al.

(2000) reported time series of currems at a few sta-tions in southern Lake Michigan during this event.

. -... -- -.. . ------ - - .". _.u___

In the presem srudy, the focus is on the analysis oftime series data at selected stations during the winterof 1999, with particular reference to a nonherlywind event in March 1999.

Experimental data

The observation strategy for obtaining the cross-shore and alongshore currents, physical environ-mem, and temperature consisted of three compo-nems: (a) moored instruments (b) Lagrangianmeasuremems, and (c) ship-board surveys. In themoored instrumentation time series of currents,winds and temperature data were obtained for thefield Yl:arsof 1997-2000. A maximum of 17 moor-ings of ADCPs and VACMs were deployed from the20- to 60-m depth contours by GLERL (Great LakesEnvironmemal Research Labora£Ory).As part of theprogram, Canada Cemre for Inland Waters (CClW)deployed additional instrumentation consisting ofseven SACMs and tWoADCPs in the shallowwaters,at a depth of 12 m, along with tWocoastal meteoro-logicalstations. The complete details of the observa-tional strategy are reported on the EEGLE website(http:\\www.glerl.noaa.gov\eegle\). A subset of cur-rent meter data was chosen for further analysis.Among the sevenSACMs deployed in shallowwater,tWo of them returned good qualiry data for thewhole winter period from I January to 10 April1999. Four ADCPs at 20, 30 and 40 m have alsoreturned high-qualiry data during this period(Fig. I).

Results and discussion

Wind-driven transport is a dominant feature ofthe circulation in the lakes. The spatial variabil-ity of the wind field can have a considerableinfluence on the circulation pattern in the lake.However, for the time series analysis in thisstudy the winds observed at tWo meteorologicalstations - Michigan City, Indiana and St.

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300 Verh. Internat. Verein.Limno/. 28 (2002)

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/"..I8rJ~~. (

(:J

(1f /&J)

(d42.2

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41.8

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~MI

I-87.2

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Fig. 1. Map of southern Lake Michigan showing instrumented moorings.

Joseph, Michigan (MI and St. Joseph, respec-tively, in Fig. 1) - are considered as a represen-tative forcing. The wind Stress was obtainedfrom the quadratic law. The wind stress andcurrent velocities were rotated ro conform ro

the local shore and depth contours for calculat-ing alongshore and cross-shore components.The alongshore and cross-shore components ofwind stress and currents were subjected to alow-pass filter with a cutoff period of 18 to 24h. Furthermore, the currents at tWo ADCP sta-

tions were averaged over the depth.

A time series of low-pass filtered currentsshow the flow reversals coupled with prevailingwinds (Fig. 2). The alongshore currents exhib-ited oscillations corresponding to 2-5 days.The mean currents are towards the north at

these locations. During a northerly (southward)storm from March 1 to March 10, 1999, the

offshore flow increased considerably at near-shore stations; however, at station A5, located at

a 40-m depth, no significant offshore flow isobserved. The mean low-pass filtered currentsshow increased southwesterly (offshore) flowduring the storm event compared to averagewinter conditions (Fig. 3a).

~ Winds

o ADCP~ SACM (11 m)

/>5 (40mb L:.c.1

A4 (21 mJo 0"-

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A 1 (20 m)Q.S4

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In order to estimate the horizontal exchangesin low frequency (>0.0416 cph) and high fre-quency «0.0416 cph), the time series of low-pass filtered flow values u(t) and v(t) are sub-tracted from the observed hourly values roobtain u'(t) and v'(t). The root-mean-squarevalues ("ii'2 and "V'2) are used as a measure ofthe magnitude of velocity fluctuations. Thekinetic energy of low-frequency motion(LFKE) and high-frequency motion (HFKE)are then simply given as {LFKE, HFKE} ={O.5(ic+ iY), 0.5(U'2+ V'2)}. The increase in thekinetic energy due to storm-forced winds atthese frequency bands is calculated as the differ-ence betWeen the kinetic energy during stormevents and the kinetic energy during the wholewinter season (~LFKE and ~HFKE). The rmsvalues at station A4 over the depth (Fig. 3b)show that the fluctuations are non-isotropicand the magnitude of the cross-shore compo-nent is marginally higher than the alongshorecomponent. During the March 1999 stormevent, both alongshore and cross-shore fluctua-tions increased slightly in the upper layers. Fig-ure 3c shows ~LFKE and ~HFKE for three

ADCP stations (AI, A4 and A5). This figure

...--. .--.

C. R. Murthy et aI., Analysis of time series data 301

4

rWirdstre

:A( em2) - A/a1gshore

- Ooss-shae

o -A'HY": . ii .:;J;'rf~\",""IJ\.rJ~V~~1\f\~J~r/\-V'\J~_--JIYv~,\,./ ',l \' :. \.i ';J ~I . \.

-4

40 1.J~ 85-11 m (crrVs)

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40

F-11 m(crrVs)

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40 A5-Mean(em's)

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o 10 20 30 40 50 60 70

Julian Cays,19!:E(..Em1to Ap'. 10)

80 90 100

Fig. 2. Time series of alongshore and cross-shore low-pass filtered windstress (St. Josc:ph) and currents. Theshaded area shows the period of the March 1999 storm event.

Fig. 3. (a) Means of low-pass filtered currents as a function of depth at station A4, (b) rms values of fluctua-tions at station A4, and (c) dLFKE and dHFKE at three ADCP stations.

0-.-- 0 0(a) (b) (e) . I:.. I:. 0. _0 I:. 0

0 . . 0 .. I:. 0t . - 0 I:. C>

-4 0 . ....- . " C>r . ..' .. u '" "

0 . -10.. '" 0. . 0 I:. c-

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t]0 .' . . .a '" A0 '"0 . . 0D0 .. . . -20 00 <__I . . 00 . . -12 D0 .. . . OJ0 . . D0 . . -30 D

-16.j D . . -16- l> AI(AlFI<E)D . AIu b""-<Jon.1 b.Ap-.IQ

0 ""'!Fa8(Jon 1tD""'.1I). cra;s."'8(Jon 1tD""'.IO)0 lFI<E)D

..CI'Q'hn(.....1 b.Ap-.1Q - ""'!Fa8(MII'.1tDMIr.IQ . M(AI-fKE)D .......sIu8(MIr.1 tDIQ - cra;s.Ihr1(MIr.1tDMIr.q 0 .o6(AlFI<E)

-:<I) -20 -40 .-6 -4 -2 0 2 4 6 0 5 10 -10 0 10 :<I)

anls em's (anls/2

302 Verh. Internat. Verein. Limno/. 28 (2002)

clearly shows that the storm episode duringMarch 1999 significantly increased the low-fre-quency currents, and comparatively small varia-tion is observed in high-frequency fluctuationsin the shallow waters. In the deeper waters,although low-frequency currems increased mar-ginally, fluctuations have not shown significamvariation. Although the rime series of filteredcurrents at nearshore stations (55 and 56)showed several occasions of offshore flow dur-

ing the winter season, at intermediate depths(20-30 m), northerly storm evenrs seem to be amajor mechanism for offshore flow. The futureanalysis of these data will be directed toward thequanrification of low-frequency motions suchas vorticity waves that are responsible for cross-shore transport in sourhern Lake Michigan.

References

BOYCE,EM., DONElAN, M. A., HAMBLIN,P. E, MURTHY,C. R.

& SIMONS,T. J., 1989: Thermal structure and circulation in

the Great Lakes. -Atmosphm-Ouan 27(4): 607-642.

BRINK, K. H., BANE,J. M., CHURCH, T. M., FAIRALL,C. W.,

GEERNAERT,G. L., HAMMOND, D. E., HENRICHS, S. M.,

MARTENS,C. 5., NITTROUER,C. A., ROGERS,D. P., ROMAN,

M. R., ROUGHGARDEN,J. D., SMITH, R. L., WRIGHT, L. D.

& YODER,J. A., 1992: CoastalOcran Procmrs: a Scirna Pro-

sprctUS. - Woods Hole Oceanographic Institution, WoodsHole, MA, USA.

MCCORMICK,M. J., MILLER,G. 5., MURTHY,C. R. & SAYlOR,J.H., 2000: Obsavatiom on thr Coastal Flow in Southrrn lAkr

Michigan. - Ocean Sciences Meeting, ASLO-AGU, Jan24-28, San Antonio, TX., USA.

SAYlOR,J. H., HUANG, J. C. K. & REID, R. 0., 1980: Vortex

modes in Lake Michigan. -- j. Phys. Oaanogr. 10(11):1814-1823.

SCHWAB,D. J., BRETSKY,D. & Lou, ]., 2000: The 1998

coastal turbidity plume in Lake Michigan. -- Estuar. Coast.Shr/fSci. 50: 49-58.

Authors' addresses:

C. R. MURTHY,Y. R. RAO,National Water Research

Institute, CCIW, Burlington, L7R4A6, Canada.M. J. MCCORMICK, G. S. MILLER AND SAYLOR, J. H.,

Great Lakes Environmental Research Laboratory,Ann Arbor, MI 48105, USA.