noaa tm glerl-73. currents and water temperatures observed in green … · 2004-09-16 · green...

NOAA Technical Memorandum ERL GLERL-73

CURRENTS AND WATER TEMPERATURES OBSERVED IN

GREEN BAY, LAKE MICHIGAN

PART I: WINTER 1988-1989

PART II: SUMMER 1989

Erik S. GottliebJames H. SaylorGerald S. Miller

Great Lakes Environmental Research LaboratoryAnn Arbor, MichiganNovember 1990

UNITED STATESDEPARTMENT OF COMMERCE

Robert A. MosbacherSecretary

NATIONAL OCEANIC ANDATMOSPHERIC ADMINISTRATION

John A. KnaussUnder Secretary for Oceans

and Atmosphere/Administrator

Environmental ResearchLaboratories

Joseph 0. FletcherDirector

NOTICE

Mention of a commercial company or product does not constitutean endorsement by NOAA Environmental Research Laboratories.Use for publicity or advertising purposes of information fromthis publication concerning proprietary products or the testsof such products is not authorized.

For \ale by the Nawnat Technical Inkwmalion Service. 5ZR Port Royal RoadSpr mgfield. \‘A XI61

ii

CONTENTS

PAGE

ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..-.......... 1

PART I

I. 1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..*................................*..... 5

1.2. DESCRIPTION OF DATA SET AND COMPUTATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.3. DISCUSSION OF CURRENTS, TEMPERATURES, AND ICE COVER . . . . . . . . . . . . . . . . . -7

1.4. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Appendix I-A: Satellite-Derived Ice Analysis Maps ............................................................ 13

Appendix I-B: Low-Pass-Filtered, Bidaily-Averaged Currents ............................................ 21

Appendix I-C: Low-Pass-Filtered VACM Temperatures ..................................................... 25

Appendix I-D: Samples of Raw Current and VACM Temperature Data ............................. 29

FIGURES

Figure I-l--Bathymetric map of Green Bay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Figure I-2.--Map showing locations of winter moorings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure I-3.--Plots of low-pass-filtered meteorological data from the GreenBay Harbor Entrance Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 1-4.--S tick plots of monthly-averaged currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

. . .111

PAGE

Figure I-T--Plot of power spectrum from mooring 5 ........................................................... 11

Figure I-A-l.--Satellite-derived ice analysis maps from Dec. 19 and 26, andJan. 6 and 11 ................................... .............................................................. 14

Figure I-A-2.--As in Fig. I-A-l, from Jan. l&23, and 27, and Feb. 1 ................................. 15

Figure I-A-3.--As in Fig. I-A-1, from Feb. 6, 10, 13, and 17............... ................................ 16

Figure I-A-4.--As in Fig. I-A- 1, from Feb. 22, and Mar. 1, 13, and 17 ............................... 17

Figure I-A-5.--As in Fig. I-A- 1, from Mar. 22,27, and 3 1, and Apr. 3 ............................... 18

Figure I-A-6.--As in Fig. I-A-l, from Apr. 10, 17,21, and 26 ............................................. 19

Figure I-B- l..:- S tick plots of low-pass-filtered, bidaily-averaged currentsfrom moorings 1,2,3, and 6 ........................................................................ 22

Figure I-B-2.--As in Fig. I-B- 1, from moorings 4,5,6, and 7 ............................................. 23

Figure I-B-3.--As in Fig. I-B-l, from moorings 7 and 8 ...................................................... 24

Figure I-C-l.--Plots of low-pass-filtered temperatures from moorings 1,2, 3, and 6..........2 6

Figure I-C-2.--As in Fig. I-C- 1, from moorings 4,5,6, and 7 ............................................. 27

Figure I-C-3.--As in Fig. I-C- 1, from moorings 7 and 8 ...................................................... 28

Figure I-D- 1 .--Plots of samples of raw current and temperature data from moorings5 and 6 during Feb. 13-20 ............................................................................. 30

Figure I-D-2.--As in Fig. I-D- 1, during Feb. 20-27 .............................................................. 31

Figure I-D-3.--As in Fig. I-D- 1, during Feb. 27 to Mar. 6 ................................................... 32

Figure I-D-4.--As in Fig. I-D- 1, during Mar. 6- 13 ............................................................... 33

Figure I-D-5.--As in Fig. I-D- 1, during Mar. 13-20 ............................................................. 34

Figure I-D-6.--As in Fig. I-D- 1, during Mar. 20-27 ............................................................. 35

iv

PAGE

TABLES

Table I-l.--Location, water depth, deployment and recovery times, and instrumentdepths for each mooring, winter 1988-1989 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

PART II

II. 1. INTRODUCTION ....................................................................................................... 39

11.2. DESCRIPTION OF DATA SET AND COMPUTATIONS ....................................... 40

11.3. DISCUSSION OF CURRENTS AND TEMPERATURES . . . . . . . . . . . . . . . . . . . . . . . . . ..*............ 41

11.4. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Appendix II-A: Low-Pass-Filtered, Bidaily-Averaged Currents ........................................ 47

Appendix II-B: Low-Pass-Filtered VACM Temperatures .................................................. 55

Appendix II-C: Low-Pass Filtered Thermistor Chain Temperatures .................................. 63

Appendix II-D: Meteorological Data .................................................................................. 71

Appendix II-E: Samples of Raw Current and VACM Data ................................................ 75

Appendix II-F: Loran-C Tracked Drifter Paths ................................................................... 83

FIGURES

Figure II- 1 .--Map showing locations of the summer 1989 moorings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

V

PAGE

Figure 11-2.--S tick plots of monthly-averaged currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Figure II-3.--Plot of power spectrum from mooring 18 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Figure II-A- 1 .--Stick plots of low-pass-filtered, bidaily-averaged currents frommoorings 10, 11,12, and 13 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Figure II-A-2.--As in Fig. II-A-l, from moorings 15 and 16 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Figure II-A-3.--As in Fig. II-A-l, from moorings 10, 14, 17,20, and 21 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50




Figure II-B-l.--Plots of low-pass-filtered temperatures from moorings 10, 11, 12,and 13 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Figure II-B-2.--As in Fig. II-B- 1, from moorings 15 and 16 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Figure II-B-3.--As in Fig. II-B-l, from moorings 10, 14, 17,20, and 21 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58


Figure II-B-5.--As in Fig. II-B-l, from moorings 22 and 23 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60


Figure II-C-l--Contour plots of low-pass-filtered thermistor data from mooring 19 . . . . . . ...64

Figure II-C-2.--As in Fig. II-C-l, from mooring 21 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Figure II-C-3.--As in Fig. II-C-l, from mooring 24. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

Figure II-D- 1 :--Plots of low-pass-filtered meteorological data from the Green BayHarbor Entrance Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..- 72

Figure II-D-2.--Plots of low-pass-filtered meteorological data from Sheboygan,Wisconsin and NDBC Buoy 45002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

vi

PAGE

Figure II-E- 1 .--Plots of samples of raw current and temperature data from moorings18 and 20 during July 3-10 ........................................................................... 76

Figure II-E-2.--As in Fig. I-D-l, during July lo-17 .............................................................. 77

Figure II-E-3.--As in Fig. I-D-l, during July 17-24 .............................................................. 78

Figure II-E-4.--As in Fig. I-D- 1, during July 24-3 1 .............................................................. 79

Figure II-E-5.--As in Fig. I-D-l, during July 3 1 to Aug. 7 .................................................... 80 ’

Figure II-E-6.--As in Fig. I-D-l, during Aug. 7-14 ............................................................... 81

Figure II-F- 1 .--Key for loran-C-tracked drifter paths. ........................................................... 84

Figure II-F-2.--Drifter paths during July 12-14 ..................................................................... 85






TABLES

Table II-1 .--Location, water depth, deployment and recovery times, and instrumentdepths for each mooring, summer 1989 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

vii

CURRENTS AND WATER TEMPERATURES OBSERVED IN GREEN BAY

PART I: WINTER 1988- 1989

PART II: SUM- 1989

Erik S. Gottlieb, James H. Saylor, and Gerald S. Miller’

ABSTRACT. To help monitor the transport of water within Green Bay and theexchange of waters between the bay and Lake Michigan, current meter mooringswere deployed in the bay and in the passages separating the bay and Lake Michi-gan, from September 1988 to April 1989 (Part I: Winter) and again from May toSeptember 1989 (Part II: Summer). The winter deployment involved 8 currentmeter moorings, whereas summer included 21 moorings, 3 thermistor chains, and7 loran-C-tracked drifters (July only). Each mooring held two or three currentmeters, usually placed at 12 and 20 m depth and 5 m above the bottom.

To aid in understanding the winter data, maps of ice concentration andthickness are included in Part I. Although currents under the ice are surprisinglyenergetic at the lunar semi-diurnal tide and Lake Michigan surface seiche peri-ods, monthly-averaged currents reveal a very weak and poorly defined meancirculation pattern in the bay. Despite partial ice cover, bidaily-averaged currentsare strong, burstlike, and mostly outward through Death’s Door Passage, andweaker, steadily inward, and slightly warmer through Rock Island Passage.

Monthly-averaged summer currents (Part II) show a somewhat anticycloniccirculation pattern in the southern half of the bay, and a persistent inflow below20 m depth through all four major passages. Above 20 m however, outflow isnotable only through Death’s Door Passage. Comparison of bidaily-averagedcurrents and observed wind patterns indicates that north to northeast winds createa single cyclonic circulation cell in the bay, and south to southwest winds create atwo-celled pattern that has an anticyclonic cell in the south half of the bay and acyclonic cell in the north. Low-pass-filtered currents and temperatures duringJuly and August reveal a strong, persistent, well-defined, 8-day-long oscillationassociated with seiching of the thermocline in Green Bay. Thermistor chain dataindicate an amplitude of about 6 to 10 m for the internal seiche.

‘GLERL Contribution No. 7 13

PART I:

WINTER 1988-1989

Scaieinkm

! I I I 10 10 20

Contours in meters

- 45.5O

Chambers Island

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Figure I-l. Bathymetric map of Green Bay, showing locations of the major urban areas.The inset box shows an enlarged map of the mouth region and a bathymetric cross sectionalong the indicated transect. (Redrawn frorh Miller and Saylor, 1985.)

I. 1. INTRODUCTION

Green Bay is a long, narrow, shallow gulf connected to northwestern Lake Michigan (FigureI-l). According to the definitions of Mortimer (1978), the bay is 193 km long and 22 km wide,averages 15.8 m deep, is 4250 km2 in surface area, and has a volume of 67 km3. About one-thirdof the total Lake Michigan watershed drains into Green Bay. Agricultural runoff, municipalwaste water discharges, and industrial pollution during this century have adversely affected thewater quality. Problems are most serious near the city of Green Bay, Wisconsin (Figure I-l),where inadequately treated waste water (primarily from paper mills and municipalities) is dis-charged into the Fox River and subsequently enters the very shallow southernmost waters of thebay.

The northern half of Green Bay exchanges waters with Lake Michigan through the four mainpassages located across the “mouth” region (see inset, Figure I-l). The passages range between 2to 7 km wide, are about 14 to 30 m deep at their shallowest points, and are 52 km2 in cross-sectional area along a transect defined by Mortimer (1978). Exchanges of water through theSturgeon Bay Canal, transecting the Door Peninsula (Figure I-l), are relatively minor (seeSaylor, 1964). The exchange of water between the northern and southern halves of Green Bay isconfined to the passages on either side of Chambers Island. The total cross-sectional area acrossthose two passages is roughly comparable with that across the mouth passages.

Circulation in the bay is governed not only by bathymetry and the seasonal effects of icecover and stratification, but also by the Coriolis, wind, and barometric pressure forces. Usingconductivity as a tracer, Modlin and Beeton (1970) noted a tongue of diluted Fox River water(i.e., a water mass) extending 40 km from the river mouth. Currents are also known to beheavily influenced by tidal and seiche activity, thus facilitating mixing of waters within the bay.However complex, knowledge of the circulation patterns and the responses of water masses tothe forces acting on them is needed to determine the distribution and impact of pollutants on theaquatic environment.

To aid understanding of the transport of water and pollutants in Green Bay and the exchangesof water between the bay and Lake Michigan, the present study was undertaken as part of theGreen Bay/Fox River Mass Balance Study (GBMBS). In this technical memorandum Part I,which includes ice analysis maps (Appendix I-A) and current meter data from September 1988 toMay 1989 (Appendices I-B and I-C), illustrates how currents and temperatures in the bay areinfluenced by the formation, coverage, and breakup of the winter ice. Part II, which includesdata continuing through to the following October, shows how development of the thermoclineaffects circulation as well as how this surface responds to atmospheric forcing and contributes tothe resulting movement of water through all the aforementioned passages. This information willbe useful to GBMBS modelers, biologists, and physical scientists, and anyone interested in theseasonal cycle of the current patterns and thermal structure in Green Bay.

5

1.2. DESCRIPTION OF DATA SET AND COMPUTATIONS

The winter data were collected using 16 EG&G brand vector-averaging current meters(VACMs) with integral temperature sensors that were suspended in the water column beneathsubsurface floats on 8 taut-line moorings. Table I-l lists the mooring locations, water depths atthe mooring sites, deployment and recovery times, and the deployed depths of the VACMs.Figure I-2 shows a map of the mooring locations. All VACMs yielded full data returns exceptfor a complete failure of the temperature sensor on mooring 4 at depth 23.0 m and two gaps (50and 20 hours in length, respectively) in the data from mooring 8 at depth 39.0 m. The VACMdata (current velocity and water temperature) and meteorological data (wind velocity and airtemperature at 22 m height on the Green Bay Harbor Entrance Light) were recorded at a 15-minute interval, and herein will be referred to as the raw data.

After averaging all raw data at an hourly interval and filling short gaps with linearly interpo-lated values, a Cosine-Lanczos filter with a 60-point taper (40-hour half-power point), describedby Mooers and Smith (1968), was applied to remove short-period oscillations. After filtering,velocity data were averaged at a bidaily interval and temperature data at a 3- or 4-hour interval toreduce the amount of data for display. These data are graphically presented in Appendices I-B(currents) and I-C (water temperatures), and Figure I-3 (meteorological data). Samples of theraw data from moorings 5 and 6 are presented in Appendix I-D to show examples of the oscilla-tions and fluctuations removed by filtering.

Maps of ice concentration and thickness (Appendix I-A) were redrawn from the Great LakesIce Analysis maps produced by the Navy/NOAA Joint Ice Center. The analysis maps are pre-pared on Monday, Wednesday, and Friday of each week during the ice season. Data sourcesinclude imagery from NOAA’s GOES and polar-orbiting satellites, American and Canadian shipand shore reports, satellite-derived water surface temperature data, and Canadian aerial icereconnaissance data. Compared with the ice climatology during 1960-1979 (Assel et a1.,1983)the 1988-1989 Green Bay ice season (Appendix I-A) was normal except for January, when theice coverage was slightly less than normal.

-w----p ----------a--- -----------------------

Water InstrumentMooring Position LORAN-C Depth Deploy Recover Depth(s)Number ("lat./"lon.) (x/v) ( m ) (date/time)'(date/time)' ( m )- - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - -

1 44.83187.70 32339.21148148.62 16.3 18/0950 05/1355 10.02 44.94187.54 32263.79148113.29 25.0 18/1120 05/1515 12.0, 20.03 44.93187.50 32259.68148128.72 25.5 18/1205 0511545 12.0, 20.54 45.23187.43 32155.64/47962.91 28.0 20/1050 04/1410 12.0, 23.05 45.22187.40 32150.67147976.71 32.5 20/1140 04/1245 12.5, 18.56 45.14187.29 32143.84148043.72 17.0 21/0950 04/1525 12.07 45.30186.97 32015.68/48015.82 36.0 21/1220 03/1545 12.0, 20.0, 31.08 45.43186.80 31933.52147969.38 44.0 21/1435 03/1310 12.0, 20.0, 39.0

--------------------------------1_----_I-------l TimesareEST

Table I-l. Location, water depth, deployment and recovery times, and instrument depths foreach mooring, for winter 1988-l 989 (September-May).

6

1.3. DISCUSSION OF CURRENTS, TEMPERATURES, AND ICE COVER

To aid in describing general circulation patterns, monthly-averaged currents are schemati-cally diagrammed according to mooring location in Figure I-4. The circulation pattern is weaklyanticyclonic in the southern half of the bay during fall, and virtually disappears after January asice concentration in the bay approached 100% (Figure I-A-2). Despite the proximity of moor-ings 4 and 5, monthly-averaged currents at mooring 4 are mostly opposed to the currents atmooring 5 from September to December, and imply an anticyclonic shear on the west side ofChambers Island. Monthly-averaged currents through the Death’s Door, Rock Island, and EastChambers Island Passages (moorings 7,8, and 6) are strong (due to constriction of the flow)even under the ice, and all moorings show very small currents for April, at which time the icecover was breaking up (Figure I-A-6).

The opposed monthly-averaged currents at moorings 4 and 5 can be partially explained byexamining the bidaily-averaged currents (Figure I-B-2).

The currents at moorings 4 and 5 are congruent during some periods, especially duringnorthward flow events (e.g., October lo-14), but reveal the anticyclonic pattern during otherperiods (e.g., northward to northeastward currents at 4 and southwestward currents at 5 fromOctober 28 through November 4). Also, the northward flow events at 4 are typically strongerand of longer duration than at 5. The currents are notably complex during the last half of De-cember (i.e., during the formation of the ice cover, see Figure I-A-l), having a weak but constant

GREEN BAY

Winter Moorings,

Figure I-2. Map showinglocations of the winter 198%I989 moorings.

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Figure I-3.Plots of low

-pass-filtered m

eteorological data from 22 m

height on the Green

Bay Harbor

Entrance Light.The top panel show

s 3-hour-averaged air and surface w

ater temperatures, and the

bottom panel is a stick plot of bidaily-averaged

wind velocity

(the sticks are plotted in the sam

e sense asthe current velocity sticks, i.e., they point tow

ard the direction the wind is heading; north is up).

8

Green Bay

Monthly-Averaged Currents

Sep. 1988 - Apr. 1989

--YGY??. &* 4 12.0. . . .. . . . . - . . . . . . .. . , , . . . . . . . . . .@&c+c&I~~;,l:i 2. o

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Figure I-4. Stick plots of monthly-averaged currents for September 1988-April1989, computedusing the raw data from each current meter. The sticks point in the direction the current isheading; north is up. The placement of the individual plots schematically represents the place-ment of the moorings in the bay.

9

southward flow at mooring 5, very weak and variable currents at mooring 4 (12 m), and stronger,somewhat northward, but highly variable flow deeper in the water column at mooring 4 (23 m).

Also correlated with the ice cover formation was an abrupt change in the flow throughDeath’s Door Passage (mooring 7, Figure I-B-3) from mostly inward during December to mostlyoutward during January; a corresponding change in the flow through Rock Island Passage (moor-ing 8) was not observed. Complete coverage of the ice was delayed due to the atmosphericwarming trend during the entire month of January (Figure I-3). Consequently, the ice sheetunderwent several freeze-thaw cycles (Figure I-A-2) before complete solidification. In Januaryin East Chambers Island Passage (mooring 6, Figure I-B-l) strong southward flow events oc-curred during strong wind impulses directed toward the northeast; they also may have beenrelated to motions of the freezing and thawing ice sheet. The abrupt decrease in current magni-tude on February 6 undoubtedly was due to complete solidification of the ice cover.

Currents between the end of the warming trend and the onset of complete solidification areparticularly interesting; several days of strong, steady, freezing north winds were associated witha remarkable 25°C drop in air temperature during February l-4 (Figure I-3). With a 90-100% iceconcentration over the south half of the bay (Figs. I-A-2 and -3), currents under the ice weresmall (referring to Appendix I-B) but steadily southward at moorings 4, 5, and 2, and large andnorthward at moorings 3 and 6, implying a cyclonic circulation with an intensified flow along theeastern shore. The pattern was short-lived however; all currents abruptly decreased on February6 and remained very small until breakup of the ice sheet between April 10 and 17 (Figure I-A-6).

Low-pass-filtered water temperatures in the bay (Appendix I-C) show large decreases duringthe week of December 12-19, corresponding to the diminished air temperature (Figure I-3) andthe initiation of the winter ice cover. Interestingly, from mid-October to mid-December tem-peratures in the East Chambers Island Passage (mooring 6) were warmer than both southernGreen Bay (mooring 1, Figure I-C-l) and Death’s Door Passage (mooring 7, Figure I-C-2). ByJanuary, water temperatures in the bay and passages had fallen to the freezing point, except inRock Island Passage (mooring 8, Figure I-C-3) where temperatures remained 1 to 2°C warmeruntil March. Steady inflow of warmer, denser water from Lake Michigan into the bay continuedthroughout the winter months through this passage. The small but steady warming trend ob-served from December to mid-April at most moorings in the bay could have been due to solarradiational heating through the ice, geothermal heating through the bay floor, liberation of solarheat stored in the bottom sediments, adjustment of the water column to the temperature of maxi-mum density, waste water effluent, or any combination of these factors [see Parrott and Fleming(1970) for a more complete description].

Even though low-pass filtered mean currents (Appendix B) were barely observable under thesolid ice sheet, actual currents (i.e., the raw data, Appendix D) remained significant, with speedsof up to 35 cm s-l on one occasion (Figure I-D-5). Neglecting mooring 6, monthly vector aver-ages of the current velocities in the bay during February and March range between 0.3-3.4 cm s-l.Appendix D shows that the major under-ice currents are oscillatory in nature (or rotary as atmooring 6), with considerable variation in the amplitude of the oscillations. The oscillationsusually occur about twice per day, being driven by the semi-diurnal tide (period 12.4 h), but

10

sometimes occur at a frequency of slightly more than twice per day indicating excitement of thefirst longitudinal mode of Lake Michigan surface seiches [period 9.3 hours, as computed by Raoet al. (1976)]. A power spectrum of winter currents at mooring 5 (Figure I-5) reveals the domi-nance of the oscillatory components; the lunar tide has the largest energy. The very large ampli-tude of the semi-diurnal lunar tide and of the longest-period Lake Michigan seiche in Green Bayis caused by resonance of these waves with the lowest-mode free oscillation of the bay itself(Mortimer, 1965). The lowest bay mode has a period somewhere between that of the tide and ofthe Lake Michigan seiche, but is close enough in frequency with each for large amplification tooccur. Interference between these waves of similar frequency produces large-amplitude varia-tions over intervals of just a few days, as Appendix I-D clearly shows.

1.4. REFERENCES

ASSEL, R.A, F.H. QUINN, G.A. LESHKEVICH, and S.A. BOLSENGA. Great Lakes IceAtlas, Great Lakes Environmental Research Laboratory, Ann Arbor, 115 pp. (1983).

MILLER, G.S., and J.H. SAYLOR. Currents and temperatures in Green Bay, Lake Michigan.Journal of Great Lakes Research 11(2):97-109 (1985).

“a

-F = inertial oscillalion

Ml - UC. Michigan 18’ surface modeLb2 = Lk. Michigan 2nd surface modeGk? = Green Bay 2nd surface mode

-0 1 2 3 4 5 6

Frequency (cpd)

Figure I-5. Plot of powerspectrum computed usinghourly averages of the rawdata from mooring 5 (18.5m) during January throughMay 1989. The frequenciesindicated for the surface-mode oscillations are thosecomputed by Rao et al.(1976).

11

Modlin, R., and A.M. BEETON. Dispersal of Fox River water in Green Bay, Lake Michigan.Proceedings, 13th Conference on Great Lakes Research, Buffalo, NY, April 1970. InternationalAssociation for Great Lakes Research, Ann Arbor, 468-476 (1970).

Mooers, C.N.K., and R.L. Smith. Continental shelf waves off Oregon. Journal of GeophysicalResearch 73(2):549-557 (1968).

Mortimer, C.H. Spectra of long surface waves and tides in Lake Michigan and at Green Bay,Wisconsin. Proceedings, 8th Conference on Great Lakes Research. Publication No. 13, GreatLakes Research Division, University of Michigan, 304-325 (1965).

Mortimer, C.H. Water movement, mixing, and transport in Green Bay, Lake Michigan. Pro-ceedings, Green Bay Research Workshop, Green Bay, WI, September 1978. WIS-SG-78-234,University of Wisconsin Sea Grant Program, Green Bay, lo-56 (1978).

Parrott, W.H., and W.M. Fleming. The temperature structure of a mid-latitude, dimictic lakeduring freezing, ice cover, and thawing. Research Report, USA-CRREL-RR291, U.S. ArmyCorps of Engineers, Hanover, NH, 23 pp. (1970).

RAO, D.B., C.H. Mortimer, and D.J. SCHWAB. Surface normal modes of Lake Michigan:Calculations compared with spectra of observed water level fluctuations. Journal of PhysicalOceanography 6(4):575-588 (1976).

SAYLOR, J.H. Survey of Lake Michigan harbor currents. Proceedings, 7th Conference onGreat Lakes Research, Toronto, Canada, April 1964. Great Lakes Research Division Publ. No.11, Ann Arbor, 366-367 (1964).

12

Appendix I-A: Satellite-Derived Ice Analysis Maps

Maps of satellite-derived ice concentration and thickness in northwestern Lake Michigan for thewinter season of 1988-1989, for the following dates:

Figure I-A- 1 .--Dec. 19 and 26, and Jan. 6 and 11Figure I-A-2.--Jan. 18,23, and 27, and Feb. 1Figure I-A-3.--Feb. 6, 10, 13, and 17Figure I-A-4.--Feb. 22, and Mar. 1, 13, and 17Figure I-A-5.--Mar. 22,27, and 3 1, and Apr. 3Figure I-A-6.--Apr. 10, 17,21, and 26

See Section I.2 for a description of the maps.

13

m

wI

I

.co-

clIce thickness

icefme indicated in cm

.co-

clkethickness

ice free indicated in cm

•III o-1/10 19 Dec. 88

•II o-1/10 6 Jan. 89

a

vf

I

I

.cam

0ke thickness

ice free indiited in cm

ml o-1/10 11 Jan. 89

cl

ice thicknessicefree indicatedincm

•llo-1/10 26 Dec. 88

lzl l-3110I30-50

Figure I-A-1

14

.on-

l-lIcethickness

icefree indicated in cmI 4

ull o-1/10 18 Jan. 89

.Cm

cl

Ice thicknessice free indicated in cm

un O-1110 27 Jan. 89

ml l-311 0

c2

LI3

d

I

I

cl

Ice thicknessicefree indicated in cm

23 Jan. 89

cl

ke thicknessicefree indited in cm

•n o-1/10 1 Feb. 89

Figure I-A-2

15

m

Ice thicknessindiited in cm

6 Feb. 89

lee thicknessicefree indicated in cm

ull o-1/10 13 Feb. 89

l-lIce thickness

ice free indicated in cm

ml o-1/10 IO Feb. 89

izl l-311 0

0

Ice thicknessice free indiikd in cm

ml o-1/10 17 Feb. 89

EH l-3/1 0

Figure I-A-3

16

.co-

clkethkkness

icefree indicated in cm

cm o-1/10 22 Feb. 89

.co-

clke thickness


m l o-1/10 13 Mar. 89

.co-

clIce ulickness


ml o-1/10 1 Mar. 8 9

Ia l-3/1 0

.c o -

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Ice thicknessice free indiited in cm

cm o-1/10 17 Mar. 89

OS

Ib

Figure I-A-4

17

.co-

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ic%ffee indicated in cm

cm o-1/10 22 Mar. 89

.co-

clIce thickness


m l o-1/10 31 Mar. 89

.

v

#,11:.,

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rl

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ull o-1/10 27 Mar. 89

.Co-

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ml o-1/10 3 Apr. 89

I.m1:‘0,1’.1.,,.I4r

Figure I-A-5

18

.co-

cl

Ice thicknessicefree indicated in cm

•llo-1/10 lOApr.89

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ice free indited in cm

llll o-1/10 21 Apr. 89

.

.

mFigure

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clIce thickness


llno-1/10 26 Apr. 89

.co-

clIce thickness

icefree indiited in cm

m lo-1/10 17Apr.89

Appendix I-B: Low-Pass-Filtered, Bidaily-Averaged Currents

Stick plots of 40-hour low-pass-filtered, bidaily-averaged currents from the following mooringsand depths (indicated in parentheses in meters):

Figure I-B-1.--1 (lo), 2 (12 and 20), 3 (12 and 20), and 6 (12)Figure I-B-2.--4 (12 and 23), 5 (12.5 and l&5), 6 (12), and 7(20)Figure I-B-3.--7 (12,20, and 3 1) and 8 (12,20 and 39)

The sticks point toward the direction the current is heading. North is up, except in Figure I-B-3(from the mouth passages) where the currents have been rotated (by the indicated amount, indegrees clockwise from north) so that currents directed into Green Bay along the channel axisare positive.

21

Currents: Mooring (Depth in meters)T1

$ - ,~~~~~~~~~~~~~~~,~,~,~~~~~~~~~-~~~~~-l~~.,~.-~~~,,,,-~~~J~l~,~~ - 0, 2- WIq-1

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-I

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xx

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26 3 10 17 24 31 7 14 21 28 5 12 19 26 2 9 16 23 30 6 13 20 27 6 13 20 27 3 10 17 24

SEP OCT NOV DEC JAN FEB MAR APR

Green Bay 1988-1989Figure 11-B-l

l+#&+ .,~,~~,rr*,l~~~~~_,~,~~,~~~~,*,~~,J.’,~,~.~~,l,..~..,*l~~~~,~~,,,-, . L...h. . -.... . . . . . . . . ..c.*.-\m .*,@.*.......-.* . . . . . . . . . . . . ._ -*,,*pr , ,?dw-.a&, _ _.,,

&\\I/&Ld JM 1&\\I/&Ld ,IM 1/pr 4”m.,4”m.,YY~~~~il~~,4~C..~l~,~~,~~ ,,.‘~~.. L+.L*‘q&~#(,y...~ -... .- -. .‘-*- ..a. --.*-“.~~~~il~~,4~C..~l~,~~,~~ ,,.‘~w- k.+.L+~‘~ ‘,,y..‘C . . . . .- -. ...* L ..*. -..d..“. ,,*. . ..“. ,.. ._.I. .. ..“. ,.. ._.I. . . . ._..., .. . ._...,, * ,” ..* . . . . . Id,” _.* _.... Id--+sp+,,hw.-.-+‘p+,,h7,F.-. A&a?: _A&a?: _vv /pr

_ ~~,~~~,,~,~,~~,~~,l~~~~~.~~~,~,~,~l~~.~,l~~~~~~~~l~~,~.“,~..~~~~l,~.~~~. I(,,,2 .,.-..*... . ” .#. . . . . ..-..... m . . . . . . . . ._...... . . . . . . . . . . . . .“... C,., 7~%,“C.,,-,W I -

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11

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26 3 10 17 24 31 7 14 21 28 5 12 19 26 2 9 16 23 30 6 13 20 27 6 13 20 27 3 10 17 24


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24

Appendix I-C: Low-Pass-Filtered VACM Temperatures

Plots of low-pass-filtered, 4-hour-averaged water temperatures from the following moorings anddepths (indicated in parentheses in meters):

Figure I-C-1.--1 (lo), 2 (12 and 20), 3 (12 and 20.5), and 6 (12)Figure I-C-2.--4 (12), 5 (12.5 and l&S), 6 (12), and 7(20)Figure I-C-3.--7 (12,20, and 3 1) and 8 (12,20 and 39)

25

26 3 10 17 24 31 7 14 21 26 5 12 19 26 2 9 16 23 30 6 13 20 27 6 13 20 27 3 10 17 24SEP OCT NOV DEC JAN FEE MAR APR

Green Bay 1988-1989Figure I-C-1

Temperatures: Mooring (Depth in meters) a3

- 4(12.0)

- 5(12.5)

26 3 10 17 24 31 7 14 21 28 5 12 19 26 2 9 16 23 30 6 13 20 27 6 13 20 27 3 10 17 24SEP OCT NOV DEC JAN FEB MAR APR

Green Bay 1988-1989Figure I-C-Z

Temperatures: Mooring (Depth in meters)

- 7(12.0)

- fJ(l2.0)

26 3 10 17 24 31 7 14 21 28 5 12 19 26 2 9 16 23 30 6 13 20 27 6 13 20 27 3 10 17 24


Green Bay 1988-1989Figure I-C-3

Appendix I-D: Samples of Raw Current and VACM Temperature Data

Plots of samples of the raw, unfiltered (15-minute) current and temperature data from 12 mdepth, moorings 5 and 6 (West and East Chambers Island Passages, respectively) during Febru-ary 20 to March 27. Figures I-D-l to I-D-6 each show one week of data. Across- and through-passage currents are positive toward 12 1 and 3 l”T respectively for mooring 5, and 90 and O”T formooring 6.

29

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Raw Data: Moorings 5 and 6 (12 m depth) (061-5

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Chambers Island PassagesFigure I-D-3

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Raw Data: Moorings 5 and 6 (12 m depth) CDd

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II. 1. INTRODUCTION

A brief description of Green Bay, factors affecting the circulation of the bay’s waters, and thepollution problem is presented in Section I. 1. As noted, the aim of the present study is to aidunderstanding of the transport of water and pollutants in Green Bay and the exchanges of waterbetween the bay and Lake Michigan. Since currents and the related thermal structure generallyare more dynamic during summer, almost three times as many instruments were used to monitorthe currents and temperatures during summer (Part II) than during winter (Part I). A vigoroustherrnocline oscillation (internal seiche) persisted in Green Bay during July and August, and thelarge number of recording instruments incorporated in the present study allow for a unique anddetailed analysis of the resulting water movements.

Miller and Saylor (1985) used current meter data from moorings deployed from May throughSeptember 1977 at locations corresponding to the present moorings 10, l&20,22,24, and 25(Figure II-l), to determine that southwest winds set up a cyclonic circulation in the bay and thatnortheast winds set up a reverse (anticyclonic) circulation. A cyclonic circulation pattern in thebay is defined by generally southwestward currents along the western (Wisconsin-Michigan)shoreline and northeastward along the eastern (Door Peninsula) shoreline. During stratification,which usually lasts from May to October, the thermocline separates a warm, well-mixed, FoxRiver-influenced (Modlin and Beeton, 1970) water mass in the upper layer from a cold, LakeMichigan-originating water mass in the lower layer. Influenced by prevailing south-southwestwinds and the Coriolis force, the upper-layer waters flow northeastward along the eastern shore,and thus in the lower layer there should exist a “compensatory (generally) southward current

GREEN BAY

Summer Moorings,

Figure II-I. Map showing locations of thesummer 1989 moorings.

39

along the western shoreline carrying water which has been diluted by water from Lake Michiganentering at the mouth” (Mortimer, 1978). It is shown in Section II.3 that in addition to thiscompensatory current, currents driven by an internal seiche strongly enhance mixing and move-ment of the upper- and lower-layer waters.

Compared with the Miller and Saylor (1985) study, the present study allows for a muchgreater spatial resolution (both horizontal and vertical) of the general circulation pattern in GreenBay. For example, it is shown in Section II.3 that the circulation pattern set up by southwestwinds consists of two counter-rotating flow cells; an anticyclonic cell occupies the southern halfof the bay and a cyclonic cell occupies the northern half. Also, a persistent, small-scale flowfeature in the constriction west of Chambers Island causes a highly nonuniform flow through thepassage. Overall the results are both interesting and complex, and will be highly useful forverifying the results of numerical simulations of the thermal (density) structure and related flowfield in Green Bay.

11.2. DESCRIPTION OF DATA SET AND COMPUTATIONS

The summer data were collected using 21 moorings, 37 VACMs, 3 thermistor chains, 7 loran-C-tracked drifters, and 2 acoustic doppler current profilers. VACM moorings are described inSection 1.2. Table II-1 lists the mooring locations, water depths, deployment and recovery times,and instrument depths, and Figure II-1 shows a map of the mooring locations. One thermistorchain was located in Rock Island Passage (mooring 24T), and one chain and one profiler eachwere located in East (moorings 21T and 21P) and West (moorings 19T and 19P) ChambersIsland Passages. Because of the complexity of the profiler data, they will be presented separatelyin a subsequent report.

During the mooring period several instrument failures and mishaps occurred, causing loss ofdata. Shortly after deployment, VACMs failed on moorings 17 at 15.1 m depth and 21 at 12.0 mdepth. The VACM data from mooring 10 at 10.0 m depth and thermistor chain data from moor-ing 21T were truncated on September 14 and October 2 respectively, because of the increasinglyfrequent occurrences of sporadically erroneous values. A 37-hour gap in the VACM data frommooring 25 at 12.0 m depth occurred during July 30 to August 1. Mooring 13 was accidentallyretrieved by a fish trawler on June 20. After repairs were made, it was redeployed on July 12.Mooring 25, with instruments at 12.0,20.0, and 31.9 m depth, experienced both partial failureand unplanned retrieval. On May 8 the 20.0 m instrument failed. On June 6, the mooring wasreported adrift near St. Martin Island. The upper two instruments were retrieved that day, stillattached to the subsurface flotation. The entire mooring, with a new bottom instrument, wasredeployed on June 26. Data from before June 6 do not indicate when the mooring was setadrift, and the bottom instrument was not recovered.

All VACM, thermistor chain, and Green Bay Harbor Entrance Light data were recorded,averaged, low-pass filtered, and post-filter averaged as described in Section 1.2, and are pre-sented in Appendices II-A (currents), II-B (VACM temperatures), II-C (thermistor chain tem-peratures), II-D (meteorological data), and II-E (samples of raw VACM data). Because of

40

- - - -- - - - - - - - LI-------l--------------- - - - - - s - I I - - - - - - c -- - - -

- - -

Water InstrumentMooring Position LORAN-C Depth Deploy Recover Depth(s)Number (Olat.Plon.) OW ( m ) (date/time)'(date/tim@' ( m )----e-m -B---m-s-- - - - P - - - P - - - - - -----

10 44.83187.75 32352.55148137.1s 15.9 19/1220 15/1400 10.011 44.80/87.69 32345.94/48166.99 14.0 19/1320 1511135 10.012 44.94187.54 32266.28148112.56 25.0 17/1350 13/1050 14.0, 20.013 44.93/87.50 32259.93/48134.09 26.5 17/1430 1311719 12.0, 21.514 45.06/87.55 32231.96/48038.45 21.6 20/1035 13/1553 10.0, 16.615 45.05/87.44 32209.32/48069.57 32.3 05/1145 15/1630 12.0, 20.0, 27.316 45.04187.39 32197.55148082.74 31.4 04/1110 15/1725 12.0, 20.0, 26.417 45.24187.46 32160.53/47952.00 20.1 21/1230 13/1440 10.0, 15.118 45.22187.43 32157.33147966.16 29.9 2111315 1211225 12.0, 17.0, 24.919 45.21187.41 32154.30147976.62 33.2 04/1330 12/1045 12.0, 19.0, 28.219P 45.21187.40 32154.01147979.48 34.1 22/1210 120015 31.119T 45.21187.41 32154.94/47979.26 33.5 21/1515 1211145 5.0-29.020 45.14187.29 32143.58148043.89 17.4 20/1415 12/1615 12.021 45.17J87.29 32135.99148024.18 23.8 20/1240 12/1500 12.0, 18.821P 45.17/87.29 32136.16/48025.11 23.5 22/1400 12/1430 20.521T 45.17187.29 32135.67148025.33 23.8 20 /1330 12/1600 5.0-23.022 45.29186.97 32017.45148017.58 30.5 03/1650 14/1355 12.0, 20.0, 25.523 45.30/86.96 32013.58/48017.21 34.8 1811915 14/1324 12.0, 20.0, 29.824 45.43186.80 31933.2Ol47968.81 44.2 03/1350 22/1630 12.0, 20.0, 39.22 4 T 45.43186.80 31935.09/47971.19 44.8 18/1610 22/1830 5.0-41.025 45.40/86.75 31901.56/47937.05 36.9 18/1415 22/1450 12.0, 20.0, 31.9

------------------.---- psI-e---v- - - - - - - - - -~~--~-~-c-------

l limesareEST

Table II-I. Location, water depth, deployment and recovery times, and instrument depths for eachmooring, for summer 1989 (May-October). In the mooring number column, the T and P indicatethermistor chain and profiler, respectively.

equipment malfunction at the Harbor Light during June 22 to July 18, meteorological data fromSheboygan, Wisconsin, and NOAA Data Buoy 45002 are also included in Appendix II-D. Theraw data from Sheboygan and Buoy 45002 were recorded at an hourly interval, and subsequentlylow-pass filtered and post-filter averaged as described in Section 1.2. Each gap of missing valuesin the raw meteorological data (all gaps longer than 23 hours are listed in Appendix II-D) wasfilled with values linearly interpolated across the gap. Gaps of missing VACM data points weretreated likewise, and therefore no data are lost to filtering at the beginnings and ends of the gaps(e.g., Figure 11-A-6).

11.3. DISCUSSION OF CURRENTS AND TEMPERATURES

Because transient forcing can influence the currents in Green Bay more than steady, low-levelforcing for periods of up to several days or more can, monthly-averaged currents (Figure 11-2)should provide a good representation of the steady component of bay circulation (i.e., the generalcirculation pattern). During May the currents are very weak and show no recognizable circula-tion pattern. May is normally the month when atmospheric stability over the Great Lakes is at a

41

Green Bay

Monthly-Averaged Currents

May - October 1989

- = 10 cm/s (eastward)

CD = Mooring

Depth levels are indicatedIn meters below the surface

,,,;;qFi; ,f 31.g

-:.*r.~ 12.0@ ii&K 2. o

. . . . . .. . . . .. . . .+?s. .,*’ i::...

39.2

@ $?f&:!/‘O-O

l W/.?? 12.0. . . . .0

18 ::::::f$<

#?.P.//. 17.0

. . . . . . .,:~.‘.‘.‘.w 28.2

~yw.'.'* 12.0. .,I23 :-,~A&

9,

20.0. .,:$&y;: 29.8\

0!:;\?$\y\ ‘2-O

22 .:.::: 1.:.T:TJ$y:P 20*o. . . . . .; : *++i?i 25.5

0 20 t .:.:.:.:.:.:. . . , . d . f 3.1 . , 12 * 0

02, ::::::::::::.#'-4-t v 3 18.8

0, 14 I. . :. /..::::1 t.t tt 10.0e. j '2.4 116.6 ,c:?.r 12.0

0 12 *.v. ..fi '4

rc.*i ..:: 14.0i ;P 20.0

Figure II-2. Stick plots of monthly-averaged currents for May-October 1989, computedusing the raw data from each current meter. The sticks point in the direction the current isheading; north is up. The placement of the individual plots schematically represents theplacement of the moorings in the bay.

42

maximum (warm air over cold water), and thus momentum imparted from the wind to the watersurface is small (Saylor and Miller, 1979). By May 20 the air (Figure II-D-l) was indeed 7-12°Cwarmer than the water (Figure II-B-l) in Green Bay, and thus wind-driven surface water move-ment should have been minimal.

The circulation pattern (Figure 11-2) in the southern half of the bay below 10 m depth wasweak and cyclonic during June, but became stronger and anticyclonic during July and August. Itwas strongest and still anticyclonic during September. Winds (Appendix II-D) during Septemberwere unmistakably south-southwest dominated. However, monthly-averaged currents frommoorings 17, 18, and 20 indicate a counter-rotating (i.e., cyclonic) flow around Chambers Islandduring July, August, and especially September, as reported by Miller and Saylor (1985); theyresolved the cyclonic flow around the island but missed the anticyclonic flow farther southbecause of limited data coverage in that region. Thus, southwest winds set up a two-celledcirculation pattern in the water below 10 m depth in Green Bay, i.e., an anticyclonic cell in thesouthern half of the bay and a cyclonic flow around and to the north of Chambers Island.

The bidaily-averaged currents (Appendix II-A) reveal the actual response of the currents tothe winds. An example illustrating the two-celled pattern occurred during September 13-23,when warm, south-southwest winds increased in strength and became relatively strong andsteady for several days (Figure II-D-l). Figures II-A-l and II-A-2 show steady southwestwardcurrents along the eastern shore (moorings 11, 13, and 16), and Figure II-A-3 shows strong andsteady northeastward currents along the western shore (mooring 14, 10.0 m), consistent with ananticyclonic cell; moorings 18 and 19 (Figure 11-A-4) also show northeastward currents at 12 m.Near-bottom currents at moorings 14 (16.6 m) and 15 (27.3 m) were strong and southwestwardto compensate for the wind-driven, northeastward-moving surface waters; deep, compensatory,south to southwestward currents also are evident on both sides of Chambers Island (moorings 18,19 and 21). The southwestward flow at mooring 17 and northward flow at mooring 20 (Figure11-A-3) is consistent with a cyclonic cell around Chambers Island. Flow through the connectingpassages (moorings 22-25, Figs. II-A-5 and 11-A-6) was mostly out of Green Bay at the 12.0 mlevel and into the bay at the deeper levels.

To illustrate the reverse response (i.e., northeast winds), Figure II-D- 1 shows a good exampleduring June 12-17 when cold, strong winds veered from east to north across the bay. Althoughcurrents in the south half of the bay (Figs. II-A-l and 11-A-2) were mostly reverse in direction tothose during the September 13-23 event, currents at moorings 17,20, and 21 (Figure 11-A-3)were in the same direction. These observations are consistent with a single-celled cycloniccirculation under northeast winds, which disagrees with the anticyclonic circulation observed byMiller and Saylor (1985). The discrepancy probably arises from the fact that Miller and Saylor’sChambers Island Passage mooring was located near the present mooring 18. Indeed, currentsfrom all levels and especially the bottom on moorings 18 and 19 (Figure 11-A-4) are northeast-ward and are probably compensatory flows. Surface circulation however is undoubtedly cy-clonic, as seen by the strong southwestward currents at moorings 17 and 14. Thus, northeastwinds set up a cyclonic circulation pattern in the lower bay, and the flow (especially at thedeeper levels) through the eastern half of the constriction west of Chambers Island serves tocompensate for surface water movement driven by both northeast and southwest winds.

43

The drifter data (Appendix II-F) also indicate a cyclonic circulation in response to north tonortheast winds. During July lo- 14, winds (Figure 11-D-2) were generally north and relativelysteady. Drifter paths for July 12-14 (Figure II-F- 1) show a southwestward flow along the west-em shoreline and paths for July 14- 16 reveal a cyclone in the center of the bay. Winds werenortheast to north during July 18-23, and the drifter paths for July 22-23 were again cyclonic.On July 18 a wind direction reversal (from south to north-northeast) caused a corresponding flowreversal (from anticyclonic to cyclonic) along the western shoreline (Figure B-F-5), indicatingthat the general circulation pattern in the bay responds rapidly to changes in the wind forcing.

Currents in Green Bay are two-layered: a wind-driven net transport of warm, Fox River-influenced water in the surface layer, and compensatory return flows of cold, Lake Michigan-originating water in the lower layer. Low-pass-filtered VACM temperatures (Appendix II-B)and thermistor chain temperatures (Appendix II-C) should therefore reveal the origins of thewaters constituting the observed flow events and the depth at which the upper and lower layersare separated. For the June 12-17 flow event (northeast winds) Figure II-B-1 shows a rapidwarming of the deep waters first at mooring 12 and then at mooring 13, consistent with a cy-clonic transport of warm, surface-layer water. At mooring 16 (20 m), temperature (Figure II-B-2) was oscillatory but steadily warming, consistent with the oscillatory but generally northwardcurrents (Figure 11-A-2). The thermistor chain temperatures in the passage east of ChambersIsland (Figure 11-C-2) correlate well with the air temperature in Figure II-D-l (i.e., coolingduring the first half of the event followed by a major warming), indicating that the waters in thispassage were influenced more by vertical mixing than by wind-induced horizontal transport. Inthe western passage, however, the warming thermistor temperatures (i.e., the deepened 12”isotherm in Figure II-C- 1 b) on June 11 and 16 were undoubtedly correlated with the two north-eastward flow events observed at all levels, and especially at the bottom (Figure B-A-4).

Examination of the bottom-level currents on almost all moorings in the bay and even in theLake Michigan connecting passages (Appendix II-A) during mid-July to mid-August reveals apulselike oscillation with an approximately 8-day period. The oscillation is most notable atmooring 18 where alternating northeast and southwest flow events (Figure B-A-4) are seen topersist well into September; the 17.0 m level (Figure B-B-4) shows well-defined warm-tempera-ture peaks correlated with the strongest northeastward flows. Thermistor chain temperatures formooring 21 (Figure II-C-2a) show eight distinct peaks in the 18°C isotherm occurring approxi-mately on July 15,22, and 30, on August 8,18, and 26, and on September 3 and 13. The averagetime between each of these dates (i.e., the period of the oscillation) is 8.4 days, and the height ofthe peaks (i.e., the height of the oscillation) ranges between 6 and 10 m. A power spectrumcomputed for the currents recorded at mooring 18 from June through September (Figure 11-3)shows a high-energy peak at a period of 8 days (0.25 cpd). This whole-bay, long-period, internaloscillation was almost certainly caused by excitement of the lowest-mode, longitudinal seiche ofthe thermocline surface in Green Bay.

Assuming that the thicknesses of the upper and lower layers are equal, that the densitydifference between the layers divided by the lower layer density is 0.00 1, and that the accelera-tion due to gravity g is 10 m s2, the equation for the period T (in seconds) of a free, first-mode,standing internal wave (i.e., an internal seiche) is a basin of length L and depth h (both in meters)

44

is T = 20L(h)- 1/2 . Using 170,000 m and 23 m for L and h, respectively (as estimated from FigureI-l), yields a value of 8.2 days, which agrees well with the observed period. An even largervalue of L could be used, as evidenced by the oscillation-induced cooling events at moorings 10and 11 (Figure II-B-l), and by occasional reports of cold, clear, Lake Michigan water appearingin the extreme southern end of the bay. A value of h is not as easily justified, but is reasonable touse a value between the mean depth of the bay [ 15.8 m, from Mortimer (1978)] and the depth atthe center of the basin (about 30 m, see Figure I-l).

What is the forcing mechanism that initially sets the observed thermocline observation intomotion? Examination of the meteorological data (Figure 11-D-2) shows mostly steady, north-northeast winds during July 7-13, after which the first and strongest northeastward flow eventoccurred at moorings 18 and 19 (Figure 11-A-4). The winds pushed surface water into the south-ern half of the bay, causing the strong, compensatory northeastward flow of lower-layer waterand the initiation of the internal seiche. Further inspection reveals that the winds and air tem-peratures (in Figures II-D- 1 and 11-D-2) after July 13 oscillated at about an 8- to lo-day periodfor at least three distinct cycles of cold, generally north-northeast winds followed by warm,south-southwest winds. Early stages of the seiche and atmospheric oscillations thus are corre-lated well, and resonant wind forcing occurs during at least the first three seiche cycles. Forexample, at moorings 18 and 19 the southwestward flow events occurred in order of increasingstrength during the south-southwest wind events centered on July 26, August 3, and August 11.

El- WIrl

=I1 = Green Bay 1” ini’F - inertial oscillationk = lunar sem’diurnal tide

G&lWI

= Green Bay 1” surface mode

Lb2= Lk. Michigan lti surface mode

G&2- UC. Michigan end surface mode= Green Bay 2nd surface mode

0 I 2 3 4 5 6

Frequency (cpd)

Figure II-3. Plot of powerspectrum computed usinghourly averages of the rawdata from mooring I8 (I 7.0m) during June throughSeptember 1989. Thefrequencies indicated for thesurface-mode oscillationsare those computed by Raoet al. (1976).

45

Thus, the free internal seiche initiated by wind forcing subsequently continued through a longinterval of propagation, and was less influenced by wind reinforcement after the July 26 throughAugust 11 resonant excitement interval.

Samples of raw current meter data from the Chambers Island cross section are shown inAppendix II-E. The data display and the energetic tidal oscillations that characterize Green Baycurrents and also reveal interesting features of the longer period motions. Figure II-E-2 showsinitiation of the large-amplitude internal seiche motions that cause the thermocline surface tooscillate with the nearly 8-day-long period. The thermocline oscillates out of phase, from oneside of the Island to the other, because of the long-period wave’s tendency toward geostrophy.Alternating, oppositely directed flows in the upper and lower layers of the Bay cause the densityinterface to shift back and forth in an attempt to establish geostrophic equilibrium. Therefore, thethermocline oscillation propagates as a wave traveling clockwise around the bay’s perimeter.The out-of-phase thermocline displacements between moorings 18 and 20 can be traced forseveral cycles continuing throughout Figure II-E.

11.4. REFERENCES

MILLER, G.S., and J.H. SAYLOR Currents and temperatures in Green Bay, Lake Michigan.Journal of Great Lakes Research 11(2):97-109 (1985).

Modlin, R., and A.M. BEETON. Dispersal of Fox River water in Green Bay, Lake Michigan.Proceedings, 13th Conference on Great Lakes Research, Buffalo, NY, April 1970. InternationalAssociation for Great Lakes Research, Ann Arbor, 468-476 (1970).

Mortimer, C.H. Water movement, mixing, and transport in Green Bay, Lake Michigan. Pro-ceedings, Green Bay Research Workshop, Green Bay, WI, September 1978. WIS-SG-78-234,University of Wisconsin Sea Grant Program, Green Bay, lo-56 (1978).

RAO, D.B., C.H. Mortimer, and D.J. SCHWAB. Surface normal modes of Lake Michigan:Calculations compared with spectra of observed water level fluctuations. Journal of PhysicalOceanography 6(4):575-588 (1976).

SAYLOR, J.H., and G.S. MILLER. Lake Huron winter circulation. Journal of GeophysicalResearch 8413237-3252 (1979).

46

Appendix II-A: Low-Pass-Filtered, Bidaily-Averaged Currents

Stick plots of low-pass-filtered, bidaily-averaged currents from the following moorings anddepths (indicated in parentheses in meters):

Figure II-A-l.--10 (lo), 11 (lo), 12 (14 and 20), and 13 (12 and 21.5)Figure II-A-2.-- 15 (12,20, and 27.3) and 16 (12,20, and 26.4)Figure II-A-3.--10 (lo), 14 (10 and 16.6), 17 (lo), 20 (12), and 21 (18.8)Figure I&A-4.--18 (12,20, and 24.9) and 19 (12, 19,28.2)Figure II-A-S.--22 (12,20, and 25.5) and 23 (12,20, and 29.8)Figure II-A-6.--24 (12,20, and 39.2) and 25 (12,20, and 3 1.9)

The sticks point toward the direction the current is heading. North is up, except in Figs. II-A-Sand II-A-6 (from the mouth passages) where the currents have been rotated (by the indicatedamount, in degrees clockwise from north) so that currents directed into Green Bay along thechannel axis are positive.

47

..~ __.-.--_ _ -_ .._ -_ _ -. -.. - . . . . -- -- .- ._ . -. .-- - . .=-_ ----..- ...-_l.-____ _ -, - _._____? i_I_xl__I--- ..---x_-----I- _ ---,- ------I--.---- -----_-__-- ----- I-.--- --------- ----- - ~~

Currents:

~~~~~I~~‘*.~-~~~~,,, v ~-#q,,$&+&m ,h ,r,I I /I ’ I

“.q,\ +,,Ic\‘- -v y/,r‘yun . . . . --.-‘l/F”\\\\I//1*,,~,,,~ ‘(~~~~,,~,r”~~~~.~~.“~~~l~~~~,,~,.~,Jl4,~,,,4,dlvrrzrr-

, -*v,,“v ,,,.‘-“,l-,,.*,\ -+yu\y.“~~~*I+~-Z\\V ,,,, *AP- v‘ 4y f y/p //‘-P’Yl/A+ 5 #w k “~~‘/‘/“*““..~,~~,,,,,~\ 1--,~,,,IY-&-+- +

8 tit?AY 22 29 5 12 19 26 3 10 17 24 31 7 14 21 28 4JUNE 11 18 25 2JULY 9 16

AUGUST SEPTEMBER OCTOBER

Green Bay 1989Figure II-A-1

Currents: Mooring (Depth in meters)AI 1.0-Pi

- t

Ah\. .m* "1 .-%eYy, y ,,'.. (r. .A'., 4

F

,,~,~-~~l~~~,~~~~~~-~\~~~~l~~~/~~ll~,~~~~.~~~.

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/~-~,*,~/(l(.l’l’ll’~~\~~\~~~,,~,,~~~~~,~.....~~~~,~~._,y,~~,~,~~~, r’!d ,y .\I\. &,,” ,\,, L//

8 :AY 22 29 5 12 19 26 3 10 17 24 31 7 14 21 28 4 11 18 25 2 9 16JUNE JULY AUGUST SEPTEMBER OCTOBER

t

Green Bay 1989l7-i 0.*-b-n T-r-A-9

Currents: Mooring (Depth in meters)

”Al Ill d/v /Lq,,,J,~~,,,A l+./l//,Y

4y\\ lp\-y \-j

11 \\

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.p’-x.*\l4, ,vr ///,p-wv 71

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1 I I 1 1 1 1 1 1 I 1 1 I 1 I I 1 1 I I I I I I I -

8 lz4Y 22 29 5 12 19 26 3 10 17 24 31 7 14 21 28 4 11 18 25 2 9 16

JUNE JULY AUGUST SEPTEMBER OCTOBER

Green Bay 1989

8 IiAY 22 29 5 12 19 26 3 10 17 24 31 7 14 21 28 4 11 18 25 2 9 16


Green Bay 1989Figure II-A-3Figure II-A-3

4

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53

Appendix II-B: Low-Pass-Filtered VACM Temperatures

Plots of low-pass-filtered, 4-hour-averaged water temperatures from the following moorings anddepths (indicated in meters):

Figure II-B-l .-- 10 (lo), 11 (lo), 12 (14 and 20), and 13 (12 and 21.5)Figure I&B-2.--15 (12,20, and 27.3) and 16 (12,20, and 26.4)Figure II-B-3.--10 (lo), 14 (10 and 16.6), 17 (lo), 20 (12), and 21 (18.8)Figure 11-B-4.--18 (12,20, and 24.9) and 19 (12, 19,28.2)Figure 11-B-5.--22 (12,20, and 25.5) and 23 (12,20, and 29.8)Figure II-B-6.--24 (12,20, and 39.2) and 25 (12,20, and 3 1.9)

55

Temperatures: Moorjn 1 (Depth in meters)

8 ti*u 22 29 5 12 19 26 3 10 17 24 31 7 14 21 28 4 11 18JUNE

25 2 9 16JULY AUGUST SEPTEMBER OCTOBER

Green Bay 1989Figure II-B-1

Temperatures: Mooring (Depth in meters)I I-15(12.0)

I I I I I I I I I I 1 I 1 I I I I I I I I I I _I :

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9 16JULY AUGUST SEPTEMBER OCTOBER


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8 GAY 22 29 5 12 19 26 3 10 17 24 31 7 14 21 28 4 11 18 25 2 9 16JUNE JULY AUGUST SEPTEMBER OCTOBER


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8 rJ;AY 22 29 5 12 19 26 3 10 17 24 31 7 14 21 28 4 11 18JUNE

25 2 9 16JULY AUGUST SEPTEMBER OCTOBER


Appendix II-C: Low-Pass Filtered Thermistor Chain Temperatures

Contour plots (in depth-time space) of low-pass-filtered, 3-hour-averaged, thermistor-recordedwater temperatures from moorings 19 (Figure II-C-l), 21 (11-C-2), and 24 (II-C-3). Each figureincludes two parts (a and b): part a shows the 4.9,6, 10, and 18°C isotherms, and part b shows allisotherms at a 1°C interval. The thermistors, indicated on the right-side axis, were positioned atthe depths indicated on the left-side axis.

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Appendix II-D: Meteorological Data

Plots of low-pass-filtered meteorological data from the Green Bay Harbor Entrance Light (FigureII-D-l), and from Sheboygan, WI, and National Data Buoy Center (NDBC) Buoy 45002 (II-D-2). Sheboygan is approximately 90 km south-southeast of Green Bay, WI, and NDBC Buoy45002 is about 50 km to the east of Death’s Door Passage. For a description of Figure II-D-l,refer to the caption of Figure I-3. Figure II-D-2 is similar to II-D-l and includes barometricpressure. Data were collected at 19.2 m height at Sheboygan, and at 5 m height above the lakesurface from Buoy 45002.

Note: Treatment of gaps in the data is described in Section 11.2. All gaps longer than 23 hoursare listed here:

1) June 05-06,24 hour gap in data from Sheboygan and Buoy 450022) July 0% 11,72 hour gap in data from S heboygan and Buoy 450023) July 14-17,69 hour gap in data from Buoy 450024) July 28-3 1,65 hour gap in data from Sheboygan and Buoy 450025) Aug. 10-l 1,57 hour gap in data from Sheboygan and Buoy 450026) Oct. 03-05,38 hour gap in data from Green Bay Harbor Entrance Light7) Oct. 26-27,29 hour gap in data from Sheboygan and Buoy 45002

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81

iAY22 29 5 10 17 24 31 7 14 21 28 4 11 18 2512 19 26 3

JUNE

Green Bay

2

Wind Velocity, and Air and Water Temperatures

- Water

JULY AUGUST SEPTEMBER

Harbor Entrance Light, 1989Figure II-D-l

Wind Velocity, and Air Temperature and Pressure

1 - Sheboygan

I I I I I I I I I I I I I I I I I I I I I I I I I -

1 8 15 22 29 5 12 19 26 3 10 17 24 31 7 14 21MAY 28 4 11 18 25JUNE 2 9 16JULY

23AUGUST SEPTEMBER OCTOBER

Sheboygan, WI and NDBC Buoy 45002, 1989Figure II-D-2

Appendix II-E: Samples of Raw Current and VACM Temperature Data

Plots of samples of the raw, unfiltered (15-minute) current and temperature data from 12 mdepth, moorings 18 and 20 (West and East Chambers Island Passages, respectively) during July 3to August 14. Figures II-E-l to II-E-6 each show one week of data. Across- and through-passage currents are positive toward 12 1 and 3 l”T respectively for mooring 18, and 90 and O”Tfor mooring 20.

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Appendix II-F: Loran-C-Tracked Drifter Paths

The paths of loran-C-tracked drifters during July 12 to 23 from six locations in Green Bay,shown in Figure II-F- 1, are presented in Figs. II-F-2 to 11-F-7. The drifters typically weredeployed on the morning of the start date and retrieved on the afternoon or evening of the stopdate. Data points along each path (not including the ARGOS paths) are at U-minute intervals.

83

Loran-C-Tracked Drifter Paths - July 1989Enlargments are presented in chronological order on the following pages

Notes for Enlargemepts

1) Drifter paths with sparse data pdints weretracked by ARGOS only.

2) All drifters were drogued at 3 m depth duringall dates except for those denoted with the+, 0,and &whichweredmguedat6mduring the 12-14th and 14th-16th.

1

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84

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85

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start

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86

Figure II-F-4

87

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88

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89

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Figure II-F-7

90

noaa tm glerl-73. currents and water temperatures observed in green … · 2004-09-16 · green...

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