Author Institution: F. T. Stone Institute of Hydrobiology, Put-in-Bay, Ohio, and Department of Meteorology, University of Wisconsin, Madiso

Bryson, R. A.

Suomi, V. E.

Verber, James L.

The Ohio Journal of Science

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CURRENTS IN LAKE MENDOTA, WISCONSINJAMES L. VERBER, R. A. BRYSON, AND V. E. SUOMIF. T. Stone Institute of Hydrobiology, Put-in-Bay, Ohio, and Department of Meteorology,University of Wisconsin, MadisonThe currents of Lake Mendota have been measured by using free floatingvanes during the year 1949 to 1950. Observed velocities ranged from less thanone meter per minute to 20 meters per minute in 60 feet of water. These velocitiesare not unusual and do not include those currents which may be even faster butfound only during stormy periods.According to the Ekman theory, application of a wind stress to a water surfacewill produce a net transport of water to the right of the wind. The distributionof velocity involved in this transport varies in the vertical as given by the Ekmanspiral (Sverdrug, 1946). The surface current has maximum velocity and is directedabout 45° to the right of the wind, while at greater depths the current turns moreto the right and its speed decreases. Near the bottom of the friction layer thecurrent is supposed to move opposite to the wind direction but with low velocity.This theory supposes a balance of forces, however, at our latitude, the time con-stant of a current system which is out of balance is longer than the interval betweenchanges in the wind stress. Thus, with our variable winds we can expect unbal-anced conditions prevailing most of the time. Only in periods of prolonged calmshould we expect the pressure gradient force, gravity force and coriolis to be inbalance.WIND STRESSES AND SOLENOIDSDuring periods of stratification the effect of this unbalanced wind stress onthe surface is to drive warm surface water downwind. The speeds attainedby these wind-driven currents are usually sufficient to move the upper fivemeters of water across the lake in about one day (table 1). This accumulationTABLE 1Time required to cross a 5 Km wide LakeCurrent Speed—meters/min1.02.02.8 (Ave. at 10 m depth)3.0 (Ave. at 5 m depth)A n4.1 (Ave. at 1 m depth)5.010.015.020.0 (Fastest observed)Time—hours8341.629.627.8on Q16.78.35.64.2of warm water downwind which is replaced by upwelling colder water upwindresults in the production of pressure-temperature solenoids. This upwellingof cold water was demonstrated by Church (1943). The action of the wind inthis respect is similar to the winding of a spring. When the wind stress ceasesthis spring unwinds and the cold water tends to return to its stable position underthe warmer water. Thus, the wind produced solenoids represent an accumlationTHE OHIO JOURNAL OF SCIENCE 53(4): 221, July, 1953.222 JAMES L. VERBER, R. A. BRYSON, AND V. E. SUOMI Vol. LIIIof potential energy which, when the wind ceases, will cause the wind-driven currentsto stop and reverse. On the 8th of July 1949 the wind was from ESE at 10-15 mph,producing a current towards the NW at 5 meters per minute. The 11th of Julywas calm, but the current at the same spot was towards the south 3.3 metersper minute.The hypolimnion is protected from frictional stress by the great stability ofthe thermocline, but not from pressure changes due to weight changes withinthe epilimnion. Consequently solenoids will be produced but the circulationshould be dominantly thermal rather than alternating between mechanical andthermal as in the epilimnion.LIM OF e«OM.MOTtC«FIGURE 1. Orientation of the cross-section is northeast-southwest, from Mendota Beach to asmall bay off Governors Island. Interval for the isotherms is 1° F. Note how thethermocline is stipped off in A.QUALITATIVE ASPECTSAs soon as the top layer of water has been driven downwind, horizontal tem-perature difference must prevail. The isotherms then move with very nearlythe speed of the current, and relative currents must deform them. By this factit is possible to estimate the location of tongues of higher velocity, regions of con-vergence and divergence, and large eddies from the charts of temperature. Certaininteresting conclusions may be reached by study of cross-sections of temperaturefigure 1.1. Changes in the structure of the lake from day to day are frequently observedwhich are greater than can be accomplished by horizontal currents alone. Thismeans that vertical currents must play an important part.2. On some of the vertical sections, temperatures appear in the upper epil-imnion which the day before were only found in the uppermost part of the thermo-cline. Figure 1A shows how this action takes place. The thermocline itselfthickens considerably in regions where upwelling might be expected in the epi-limnion and thins in regions of sinking. It is hard to say how these changes couldtake place unless the wind-induced currents in the epilimnion also produce move-ment of water in at least the upper part of the thermocline. The cross-sectionsindicate that the top layer of the thermocline may be actually stripped off, carriedNo. 4 LAKE MENDOTA 223to one side of the lake and there brought to the surface. However, this occursonly under rather extreme conditions. The biological consequences of this move-ment must be considerable.FIGURE 2. Frequency and transport of currents at 1 meter depth, July, 1949.102 velocity and direction measurements.QUANTITATIVE ASPECTSIn table 1 are listed the average velocities observed at various depths andthe number of hours required to cross large parts of the lake at these velocities.The currents in the epilimnion ranged from 2 to 6 meters per minute and thoseFIGURE 3. Frequency and transport of currents at 5 meters depth, July, 1949.102 velocity and direction measurements.224 JAMES L. VERBER, R. A. BRYSON, AND V. E. SUOMI Vol. LIII10 meters DEPTHFIGURE 4. Frequency and transport of currents at 10 meters depth, July, 1949.90 velocity and direction measurements.S METERS 10 METERSSOUTH WINDEAST WIND, 1 , 1 SCALEMETERS PER MINUTEFIGURE 5. Resultant Currents. Over 300 velocity and direction measurementsduring the summer of 1949.No. 4 LAKE MENDOTA 225of the hypolimnion from 1 to 3 meters per minute. However, the strongest currentobserved, over 33 cm/sec or 20 meters per minute was in the hypolimnion at adepth of 60 feet. This would suggest that the currents of the hypolimnion areproduced by direct solenidal circulation while the currents in the epilimnion arewind driven.The observations are summarized a little differently in figures 2, 3 and 4. Aninteresting feature of figure 2 is the bimodal distribution of the frequency withwhich each velocity was observed in the central portion of the lake. From figures3 and 4 it may be observed that these slower velocities are by far the most frequent.However, while the higher velocities are less frequent, they are very importantfrom the standpoint of water transport. Fifty percent of the transportationof water by surface currents is accomplished by the fastest 28 percent of thosecurrents. The corresponding figure at five meters is 18 percent and at ten meters,16 percent. Thus, not only do the cases of extreme disturbance associated withhigh winds strip the upper layer of the thermocline and bring it to the surface,but also accomplish a disproportionate amount of re-distribution in a horizontalsense.In figure 5 resultant velocities for the central portion of the lake are presented.These are separated according to wind direction and depth. In general, thecurrents at the surface are a little to the right of the wind direction while thoseat depth show far less relationship. In considering relation of current velocityto wind velocity the temperature difference between water and air must be takeninto account. A south wind is usually warmer than the water and a north oreast wind usually colder. The "frictional link" between air and the water isbetter if the air is cold, hence we should expect a closer agreement in directionbetween wind and water when the wind is from the north or east and higher currentvelocities for a given wind speed.SUMMARYThe results of this study have shown the following: (1) The wind produceswind-driven currents toward the downwind side of the lake, in the epilimnion.(2) The. surface currents average about 4 meters per minute, while those of thehypolimnion averaged about 3 meters per minute. (3) The currents of thehypoiimnion appear to be produced by direct solenoidal action, and in some casesfaster than surface currents. (4) High winds can strip the upper layer of thethermocline and bring it to the surface.REFERENCESChurch, P. E. 1943. The annual temperature cycle of Lake Michigan. A publication of theInstitute of Meteorology of the University of Chicago, Misc. Reports No. 4. 48 pp.Sverdrup, H. XL, M. W. Johnson and R. H. Fleming. 1946. The oceans. Prentice-Hall Inc.,New York. 1087 pp.

Currents in Lake Mendota, Wisconsin

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Currents in Lake Mendota, Wisconsin

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