ot Columbia Gniversity in the Cityol PewPork Hudson Laboratories “ Dobbs Ferry, N.Y. TECHNICAL REPORT No. 90 Temperature Measurements In the Ocean Near Eleuthera by T. Arase Se ° \b Contract N6-ONR-27135 TO SyGQ) Asc? CU-112-50-ONR-271-Phys. Columbia University Hudson Laboratories Dobbs Ferry, N.Y. R. A. Frosch Director Technical Report No. 90 TEMPERATURE MEASUREMENTS IN THE OCEAN NEAR ELEUTHERA by T. Arase September 15, 1960 This report consists Copy No. foes of 21 pages. of 100 copies. This work was supported by the Office of Naval Research, under Contract N6-ONR-27135. Reproduction in whole or in part is permitted for any purpose of the United States government, ABSTRACT Temperature records have been obtained at depths of 130, 150, 170, 190, and 250 ft from five buoys in- itially separated from one ancther by six miles. Each record had a duration of about one day and records for three successive days were taken. These records have heen analyzed for autocorrelation and power spectrum. The power spectrum for a typical record is (180 £°)geg?/eph in the frequency range from 0.5 to 12 cph. No lines were observed in the spectrum, showing that the temperature re- cord cannot be synthesized by a linear combination of a few simple harmonic waves. INTRODUCTION This report descvzibes the results of the second of a series of experiments v.hich are being conducted with the tvofold aim of obtaining extended temperature measurements in the ocean and of diveloping reli- able equipment for such measuvements. Temperature measuvements are impovtant in understanding such features of near-su~face ocean behavior as internal waves or surface sound channel propagation, Early experinents have quite often led to the conclusion that the observed temperatuve fluctuations could be described as a simple harmonic wave o~ as a combination of a few such waves Usually these analyses have been pe-formed on data of inadequate duration o~ with im- propevly designed instvuments. More recent experiments, of adequate time duration, show markedly different zesults. The first, Operation STANDSTILL, utilizing BT's taken every half-hour for 25 days from an anchoved ship, finds peaks at periods of one and five days in the power grrecine 2 The second, utilizing data taken by thermistors placed on the ocean bottom near Bermuda, for many months, finds a spect7um which decveases monotonically with frequency as -3 3 . a s Sle ft as vell as evidence for the existence of a peak at the principal lunar tide and a peak at O 5 cycles per none. 8 The experiment described here is for data of one day's duration taken at two-minute intervals and hence yields information on the power spectra of the temperature fluctuations to a higher» frequency range than given in the references cited in Footnotes 1 and 2. 2 Brown, Corton. and Simpson, Power Spectrum Analysis of Internal Waves f-om Operation STANDSTILL (U. S. Navy Hyd-ographic Office, 1955), Technical Report TR-26 Hauzwitz, Storimel, and Munk, On the thermal unrest in the ocean. In Rossby Memorial Volume (Rockefelle> Institute P~ess, New York, 1959). THE EXPERIMENT The experiment was conducted from the R/V GERDA during the period August 11 to 14, 1957, approximately 15 miles off the coast of Eleuthera Island, BWI. Figures 1 (a-c) show the tracks of the five buoys which sup- ported the thermometers. Each buoy consisted of a spar buoy from which were suspended four thermometer units and one temperature-pressure unit. Figure 2 illustrates the geometry employed. Each thermometer unit consisted of an open-aperture camera which photographed a mercury-in-glass thermometer illuminated at approximately two-minute intervals. The motive power for the film drive as well as the timing of the illumination was supplied by a 24-hour clock. Standard ther- mometers calibrated to 0.1°C were enclosed in a well to protect them from hydrostatic pressure. Good thermal contact between the thermometer and the well was secured by filling the well with water. The time constant for the complete unit was of the order of 1 to 2 minutes. On the first morning, the five buoys were launched six miles apart and allowed to drift. The next morning, the thermometer units were retrieved and replaced with another set of thermometer units and similarly on the third day. Hence the final result is a set of one-day thermometer records from five buoys at four depths. Due to various difficulties with the appa- ratus, usable data was obtained from only 36 of the 60 units launched. Of these, 29 records were processed for autocorrelation and power spectrum analysis. RESULTS A. Temperature Records The temperature records are typically fairly irregular functions of time with some short sections being oscillatory, with a vague hint of a daily or semidaily period, and with root-mean-square fluctuations in the order of 0.3°C. No obvious correlations of the temperature fluctuations exist with itn til H OPERATION 6l RUN | FROM 1130 6/10/57 TO 1007 4/II/57 0947 8/il 0350 2300 BUOY I 4/47 ELEUTHERA ISLAND PALMETTO PT. Figure la 76° OPERATION 6I 1 RUN 2 1015 6/12 + HOO 6/2 FROM 0947 °/\i/57 TO 124€ 38/12/57 2400 KAS 8/12 25° 30 + —_______—__1_1 4 1 127 6/1 1 BUOY II 2332 ] 2137 1 1248 | BUOY Iv T ELEUTHERA ere ISLAND PALMETTO PT. 76° OPERATION 6l RUN 3 0835 a/13 23408 0922 6/13 ’ 2320 2035 2045 x\ 005 (IY) BUOY TI af ioist/ }'*29 00 6/I2 2247% BUOY II 2122 Nee Ww 45 8/12 na FROM ©0900 8/12/37 TO 10 8/13/57 110 8/i3 2205 BUOY Y 1248 6/I2 ELEUTHERA \. 1SLAND PALMETTO POINT 25° 30 GEOMETRY OF THERMOMETER UNITS SPAR BUOY w/ LIGHT AND RADAR REFLECTOR I30' THERMOMETER UNIT I50' THERMOMETER UNIT 170' THERMOMETER UNIT 1I90' THERMOMETER UNIT 250' THERMOMETER PRESSURE UNIT WEIGHT Figure 2 ~(R — regard to either local tides or times though one might expect to find them present. However the presence of these effects may be determined only by records of much greater time duration. We shall separate the discussions of the temperature records by classifying them as to day, buoy, and depth at which taken. Table I gives the figures illustrating the three temper- ature record categories TABLE I DAY BUOY DEPTH Fig. 3 same same different Fig. 4 same different same Fig. 5 different same same Records taken on the same day at various depths at the same buoy (see Fig. 3) generally have the same characteristics, that is, if the temperature increases at one depth, it increases at all depths. The magni- tude of the temperature change is different for the different depths, being greatest for the unit at the depth where BT's indicated a large change in temperature with depth. Records taken on the same day at different buoys, and at the same depth are illustrated in Fig. 4. We note that there are no obvious corre- lations between these records. From Fig. 1(a), we see that the tracks of the buoys range from northerly to northwesterly with slightly differing speeds. Table II gives the spacing of the buoys at various times after launching. TABLE II Spacing of buoys in miles at various times during the first day's vun TIME BUOY At launching 2000 2400 0400 - 6 5-3/4 5-1/2 6 2 12 11 10-1/4 8-3/4 : 6 7 7-1/4 7-1/2 5 The mean temperature observed at each unit during four-hour periods is given in Table III. BS a ae i iy ae lat a aatatias ) pKa euHOe Tis ides tere yl baie: Vite a es ee rE BA ae CRSs2 UU eT I eM f i ee rs ” Kt MEG) Rid Re RE EERE Sl wait as i NEE = . i i y m i 0 > ne | ti Ee i = e Ants a5 Ta ae 7 i Mikron usteras? poieyh. LPH ih PR Re) Cen My ik. Beh ; hee eS ea a ee LOW HIGH LOW HIGH LOW TIDE TIDE TIDE TIDE TIDE 1357 2006 0224 0830 1435 SUNSET BE? 28° TEMPERATURE (C°) 26° Figure 3. Temperature records for the first day, Buoy 5, at various depths. HIGH LOW HIGH LOW HIGH TIDE TIDE TIDE TIDE TIDE 0830 1435 2046 0257 0906 28° cooo BUOY | cx00 BUOY 4 aire a anh So a aa atch TEMPERATURE (C°) 26° Figure 4, Temperature records for the second day, Buoys 1 and 4, at 170 ft. HIGH LOW HIGH LOW HIGH TIDE TIDE TIDE TIDE TIDE 0906 151 2115 0329 0940 29° SUNSET THIRD RUN SUNRISE = 8-12 8-13 iS} W ° Wi 28 =) — Ps °o Ms BY a = uJ F 26° THIRD RUN SECOND RUN 1435 2046 0257 0906 LOW HIGH LOW HIGH TIDE TIDE TIDE TIDE 8 - Il SECOND RUN Figure 5. Temperature records for second and third runs, Buoy 1, at 170 ft. mwa * Asatieee y - ‘ es id in 88 ere 3 Lines nae Thar we i a ise % 3 - vy \ Pe ae ; ' Py i 3 cis er pee rere nial rey : A ; Oe : res nw re) ae ee) r ‘4 . = : i : nee t i om 7 it a sf yee ‘aie yo pals dunie A badiaa aan Reser eine rae Deane + 4 i ahh ih | ~ \ ’ : ; ee a ‘ 7 - i LB i ES 2 Bh TABLE III Four-hour average temperatures in °C observed at 150-ft depth at various times during the first day's run TIME BUOY 2000 2400 0400 1 27.7 PAS U 27.75 2 28.25 28.0 28.2 4 27.75 27.75 27.95 ) 27.6 27.65 28.25 Comparing the entries in this table, we see that the greatest average temperature difference for a given separation is that between Buoys 1 and 2 at 2000 hr, of magnitude 0.1°C per mile. This difference is small in comparison to the difference in average temperature at dif- ferent depths at the same buoy on this day, which averaged 0.1°C for 5 ft. Thus, there is a factor of about 1000 in the ratio of horizontal to vertical average temperature differences. Part of this ratio can be accounted for if the depths of these units differed by several feet. This may have been due to differential stretchage, tangling, or slippage of the suspension points on the Manila rope to which all the units were connected. However, every effort had been made prior to the launching to make the buoys identical in construction. Records obtained on successive days at the same buoy and depth are illustrated in Fig. 5. Note that they appear to share the same general char- acter. These records will be further analyzed and illustrated in the next section. $Kei gf he cae, WOKATHLT LS CLR! | eel ke. 1 7 a han sees bce AP ase eS, Ve Tad. eee i} Poanseks th Cys siea ss | Memes Olea 4 “4 Avy os { esail ft ; =. 7 wieey 5 . 4 ae s : ee Vinee TEA ES 1, q ry i u 4 xi BS) a ee ea grnogke bi oY ves ai " a x eo 4 > hae ehh res = & Ho) Tas Vath tiie . | ‘ = = ; Ss — Fi) : ns Basan F ‘ ao) op ih - iy B. Correlation and Spectrum Analysis Autocorrelation and power spectrum computations were carried out on all records according to the methods of Tukey, ‘> who gives formulae for the digital computation of these quantities for data obtained at equal time intervals. The formulae are summarized below. Let the N+l numbers A OSS aL be the time series taken at time intervals At , with zero mean. The quantity corresponding to the autocorrelation function is the mean lagged product C == X X E-On Ane Siegen The raw spectral density estimate is formed by m-1] agli SLs Ve = ne(c, + 2 2 oe cos a + Ch cos | _ which is the estimate of the two-sided spectral density at the frequency ie = (r/2mAt) Peaw Ok Mee 2ee 2 em This process of obtaining spectral density estimates is analogous to ob- taining the power spectrum of electrical signals by determining the strength of the signal transmitted by various bandpass filters. Just as these filters respond to components of the signal whose frequencies lie outside the main passband and at a side lobe, so the raw spectral density estimate gives an erroneous reading for signals at the side lobes of the effective passband for this computation. Several procedures are available for reducing this error by reducing the amplitude of the side lobe response. We have used the pro- cedure known as hanning, which reduces the maximum side lobe response to 3 R. B. Blackman and J. W. Tukey, The Measurement of Power Spectra (Dover Publications, New York, 1958). babyy is Reon mri oe c wl = a = = s: pe 8 es =. pati = = : = <2 + aa Me Sona ae ngy’ if lig 6 Pi | pastes Weal Sey eu ne eae 4 Lee Ae RE RO Ree HSs re one owl Py as RT a ye ous kot DT SO De mpeg eee y er, 8 nang seat te : i ore ee. eee | dy Vue Sear : e = ¥) Ad Dine : : = $e a ) SA ee ote etme Lercamus teh nef ae i ie orl Fae” a aE ie bi ie } % x st : i: a iy a, aol eal i Atenas ERLENGL Let Meta bia ) Exevoeg ; a Hats irene Fis eS etn Reet ap | a Pep ba , aT Se: att hy ; wee ees 2 Se seh Me iia a hoger eeaienn ss cat ie iE Si dd Too (Vin Goria 24 fe woe & CE & i. ee i hal + vy ev} Pts Boe, aien Sl is Sal 2 pe / i a and: $i Oe Centon esti tte oily Ve send TAN ORES RE UALR RED ee Si ite | eas WEE Ta parte Shad Vidoaiy p etthae Fite Hs epptheubsd det ee hae ay oh haw hina a Tae ied Wak ET Le ps fen Reet Re 3 ar u “a eae Gdns me D> WEEE Eee. a SIE ses Pe aye NSD ESE a 3) ry ny ¥ ae u iia Liamicsiocgeecil ex tiids So sot Fi 2 ries aoe Gad! dee algae At a AaON way aE aR aes | cori: Z 2 percent of the main passband. The result of applying the hanning pro- cedure yields the refined spectral density estimates. Further details are given in the reference cited in Footnote 4. In this method of calculating the power spectrum, error may be introduced by aliasing, which is serious because its presence cannot be detected in the final result. Aliasing (also known as spectrum folding) arises from the fact that the data is taken at discrete time intervals At . The highest frequency at which spectral density estimates can be made is the Nyquist frequency fy = (at)! If appreciable power is present at frequencies greater than f, , they will appear at the fre- N quencies For example, Fig. 6 shows how the aliased spectrum of the form faa will appear. Hence, in the design of the experiment, one must insure that the upper frequency cutoff of the measuring instruments is less than fy Alternately, the data may be lowpass filtered by digital computation before the spectrum analysis is performed. An example of aliasing is shown in the experimental data taken by BI's in Fig. 9, discussed later. Some typical results of the autocorrelation computation are given in Fig. 7. For zero delay, the value of the autocorrelation is the vari- ance of the record. For the three records shown, this varies by a factor of four, which is typical of the range of values obtained for all the units. The autocorrelation decreases with delay time at differing rates, reaching zero most quickly fo: the second day while remaining large for the third day. It is tempting to interpret the autocorrelation in terms of the re- sults for known functions. For example, if the time series were composed of a set of random numbers, then the autocorrelation should be a delta function. Again, if the time series were a cosine function,then the auto- 4 T. Arase, A Guide to the Usage of Power Spectrum Calculations According to the Methods of Tukey (Columbia University, Hudson Laboratories, 1960), Technical Memorandum-to-File No. 47. siilke EFFECT OF SPECTRUM FOLDING ORIGINAL SPECTRUM, f 2 SPECTRUM AFTER FOLDING ABOUT f=/7 FREQ Figure 6 10 Sie oO © WW a 05 ) (3 (e) 1.0 25 DELAY TIME (HR) Figure 7. The temperature records of mean lagged products for three successive days, Buoy 2, at a depth of 150 ft. ola wot correlation should also be a cosine function. Hence, one might conjecture that the oscillatory portion represented by the autocorrelation for the second day is indeed due to a periodic component in the time series. Simi- larly, the time in which the autocorrelation decreases to some small number is a measure of the correlation time for the phenomena beyond which the time series is uncorrelated. However, a study of the time series does not appear to support these conjectures. In no case can one point out the differences in the time series which would account for the differences in their autocorrelations. It is not a function which one can compute mentally. Suffice it to say that it is useful as an intermediate step in the computa- tion of the power spectrum, except for certain exceptional cases. The power spectra corresponding to the autocorrelations previously discussed are given in Fig. 8. The very interesting result is obtained that the power spectra all have the same form within the error associated with the results. There are slight differences but these reflect large differences in the autocorrelation. For example, at 0 frequency the auto- correlation which decreased slowest has the highest value, exceeding by an order of magnitude the results of the other two. Again, the autocorrelation with one oscillatory cycle shows peaks in its spectrum. These peaks, however, are within the errors to be associated with the computation (shown as the error bar) and hence must be ignored. The overwhelming similarity is in the form of the power spectra, for, excluding the data obtained at frequencies less than 1 cycle per hour as reflecting errors due to the finite sample time, and cxcluding the data at the highest frequency as perhaps being in- Sake by the response time of the thermometers, the power spectra follow the f at the same buoy and at the same time; it applies also to data obtained at law as indicated. This is true for data obtained at various depths the same depth at different buoys; and the data shown is data obtained at the same depth, at the same buoy, and on successive days. Power spectra representing the extremes of those calculated were plotted with those of the bottomed thermistors at Bermda ‘=? (see Fig. 9). The difference in slope between the thermistor data and the thermometer data 13 in - a Vik - hen 7 i a a Pe ee deta ee meer ‘ald ' Me ‘ aa Wide h bie et aie, EAP eats +t 4 Soe hy a “$13 (30 Cia a) > ee Se eee ee ine Fare Be We a, aid: mes a cor te 4 eet oo ; : =e ea fee Py SA Le 3 iy iets So ee te & i . be r iF , fe a ae re ee ee Te: , 2 i, ho | Mate ga Age i 7 Sees + P) s! ‘ } . Ree Te a eSrS che ~ ee ee ’ a Pe ee ae = : d Shan FS Be eG ae Ae ee ei Sin ee aan aes eet y me vs e cote. Gad Bt, (EL PEe ee ee | te : nem 1 b th : = = Kl Sh Abie ae Ses i - 1 foie : °¢2/CPH FOR 50 METERS -2 10 a m o Ls “~ oO Uv ac 10-5 SECOND DAY ERROR IN SPEC - TRUM ESTIMATES (Ome 10-5 0 4 8 12 CYCLES / HOUR Figure 8. Spectral density estimates for the temperature records of three success- ive days, Buoy 2, at 150 ft. Onl) Gn a uJ | WwW Ol = {e) ° wo -3 10 5 we ze lo-4 6 ~“ Nn > Og? 0 | 1.0 2.0 CYCLES PER HOUR Figure 9. Comparison of power spectra of temperature fluctuations obtained by ther- mistors at Bermuda2 (dotted lines) and by thermometer units at Eleuthera (solid lines). The corresponding estimates of error are given by the error bars. Eyes AND PRESENT DATA — nll ill iad a 7 ey. Scshaeel { ‘ be 3 Me ; = - oe mes ls 5 , 7 s ‘ aintinrent “ i ana yee ) a rb | a Bee . = Ea - ; i i vs. << ¢) i Lfeel pe fv ae ‘ tha re ue ¢ + a hk i : it Sy fy ; \ Wa Pus 2 fn it ea ene i is evident even for this frequency range. What is more important is that the orders of magnitude of the power spectra from these two experiments are similar, The data of 50 m is puzzling because it is so much lower in level than that of 500 m. This difference was explicitly pointed out in Reference 2. The power spectra of the temperature at 150-ft depth as obtained by BT’s taken during the course of the three days, every half-hour, is also plotted in this figure. Here again the order of magnitude and the rate of falloff are similar to the data of 500 m. The rise at 1 cycle per hour is due to aliasing, which shows that the BT responds to temper- ature changes at a greater rate than it cought to for observations taken at this slow rate. A second important result shown in Fig. 8 is the absence of lines of any sort in the power spectra. Essentially this means that this data is not susceptible to interpretation as being composed of a si:ple har- monic temperature wave or even as a combination of a few simple harmonic waves. This is quite different from the results obtained by early experi- menters who reported the frequency and wavelength of the simple harmonic waves with which they reconstructed the experimental temperature record. Perhaps the temperature records are analogous to sea surface records, which are describable only by their power spectra and by statistical measures. CONCLUSIONS For the data obtained in this set of one-day observations, spread over 24 miles horizontally and 120 ft vertically, the power spectra of the temperature “hojimtinns have been found to obey the fe law in the fre- quency range from 0.5 to 12 cycles per hour. No lines have been observed in the spectra. These results differ from those of early experimenters. However, they cannot be considered to be at varia:ce with the data reported in the references cited in Footnotes 1 and 2 bocause the frequency range covcred differs so widely. SiGe met ley icin Tis toatl ah thas Sikh, Staey Ste 4 tl uae shud a5 pia i - ) i 4 ee aS oy wee on) | a ' i ; Date Ly i 1 ivan ) Tn Additional experiments are planned to extend the recording time so that such a comparison can be made, An experiment has been completed in the Bermuda area, but the longest record obtained of the temperature was less than two days. A power spectra analysis of this data is in progvess. ACKNOWLEDGMENTS Captain Kou Walter and the crew of the R/V GERDA weve of great help in the conduct of this experiment, Tom Farrell designed the ther- mometer units. Tom Kelly was responsible for the maintenance, launching, and vecovery of the thermometer units. “is DISTRIBUTION Office of Naval Research Navy Department Washington 25, D. ©. (Code 466) copies 1-2 (Code 411) copy 3 Commanding Officer and Director U, S. Navy Underwater Sound Laboratory Fort Trumbull New London, Connecticut copies 4-5 Director, U. S. Naval Ordnance Laboratory White Oak Silver Spring 19, Maryland copies 6-7 Commander U. S. Naval Ordnance Test Station China Lake, California Attn: Technical Director copy 8 Commander (Code 753) U. S. Naval Ordnance Test Station China Lake, California Attn: Technical Library copy 9 Commander U. S. 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