Detection of Overflow Events in the Shag Rocks Passage, Scotia Ridge

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similarto thatobtained by visual inspec tion ofFig. 2. Since the average PWV over therespective periods was 1.2 and 0.35cm, weconclude that the fluctuation powervaries considerably more rapidly thanthe averagePWV, approximately as the square.

A matter of considerable interest in the theory of atmospheric structure is the slope of the power spectrum. Al- though the shape is found to change considerably with weather conditions, one canfit the slopes of spectrasuch as those in Fig. 2 withrelationships of the formS(F) = KF-X. However, itis clear by visual inspection of Fig. 2 that the curves canbe more satisfactorily represented by two straight lines rather than one.Suchbeing the case, the behavior of the PWVspectrum appears to change at about F = 10, correspondingtoaperiod of about 2.5 hours.

Aconcern in power spectra measure- ments isthe signal-to-noiseratio over the frequency range of interest. Low-fre- quencynoiseinamicrowave radiometric instrument is generated by effects such aschanges in the temperatures of components,gainsofamplifiers, and lackof stabilityin powersupplies.To determine the noiseperformanceof the dual-channel system, the microwave antenna (which normallyis permanently attached to generate a beam in the zenith) is disconnected and replaced by an en- closedmicrowavetermination of known temperature. Under these conditions, the radiometer is operated over a long periodoftime, and dataarerecorded in theusual way. The spectrum of the background noise of the instrument is then obtained by the same technique, and lengthofinterval, as that used in gener- ating the PWV spectra. The result (Fig.

3) shows that the most objectionable noise components occur at the lower frequencies,near two tothreecyclesper day. Acomparisonofcurve aofFig. 2, themoistcondition,with thenoise spec- trumofFig. 3 shows that the data spec- trumexceeds the noise spectrumbytwo orders of magnitude over most of the frequency range. IncurvebofFig. 2,the dry condition,thesignal-to-noise ratiois sometimes less than 10dB, especiallyat thehighfrequencies.

Although not yet applied, spectra of watervapor such as those discussed here areknown to be of interest in areas other than meteorological research on short-

termweatherprediction. They may also have impact on the choice ofthe opti- mumtime constant in thedesignof microwave radiometers, in the spacing of very-long-baseline radio interferome- SCIENCE,VOL.213,4 SEPTEMBER 1981

10-4'

ILo-

0

10-s

,,,,,1 I ,,,I I,,

10o 10o

Cycles per day

Fig. 3. A 34-hour measurement oi trum of the background noise of channel microwave radiometric sl ordinate is lowerby afactor of 103t 2.The units of the ordinate are sq meters percycleperday.

ters, and in the design of X instruments that rely on radi such as those of a global p systemformonitoringthemoti crustofthe earth. The latter ap arise because the fundamental

Inhistoric (1) and recent (2) descrip- tions of the position of the Polar Front (Convergence)inthe DrakePassageand the ScotiaSea, itsmostmeridionalcom-

ponentseemstobesituated between the Falkland Islands and Shag Rocks (Fig.

1). As insimilar casesin thePacific (3), the associated Antarctic Circumpolar Currentseemstobeguided there by the local bottom topography. The topo- graphic chart fromthe Scotia Seaarea

showsadiscontinuity atthe site where,

ontheaverage,theCircumpolarCurrent leaves theScotiaSea withastrongequa-

torwardcomponent.

Severalinvestigators have emphasized the significance of this fracture zone-

henceforth called Shag Rocks Passage (SRP)-forwaterexchange between the Pacific andAtlantic(4-6). Analyzingpo-

tential temperature and dissolved ^oxy-

gen data from higher southern latitudes

phase fluctuation in such systems is set bythevariation in thewatervaporinthe troposphere.

D. C.HOGG, F. 0. GUIRAUD W. B. SWEEZY Wave Propagation Laboratory,

Environmental Research

Laboratories, National Oceanic and AtmosphericAdministration,

102 '~ Boulder, Colorado 80303

Reences

aNd

Notes

fthe dual- 1. F. 0. Guiraud, J. Howard, D. C. Hogg,IEEE

Fsthem;due

Trans. Geosci. Electron. GE17 (No. 4), 129 ystem; the (1979).

thaninFig. 2. J. B. Snider, F. 0. Guiraud, D. C. Hogg, J.

uarecenti- Appl.Meteorol. 19(No. 5), 577(1980).

3. T. A. Orhaug, Thermal Noise Radiation from the Atmosphere (Transaction 300, Chalmers University of Technology, Gothenburg, Swe- den, 1965).

4. Wethank C. G. Little,E. R. Westwater, J.C.

Kaimal, and R. A. Kropfli of the WavePropaga- netrology tion Laboratory,oratories, National Oceanic and AtmosphericEnvironmental ResearchLb-

io phase, Administration, for their interest and sugges- ositioning tions, and E. B. Burtonof theNWS, Denver, OSitioning Colo.,^for hishospitality.Thiswork wassup- ion of the ported in part by the U.S. Army Research iplications Office,proposal 15957-GS.

I limit on 23March 1981; revised 26May1981

between 20°W and 170°W, Gordon (6) deducedthecirculationpatterns ofAnt- arctic bottomwater.He showed that the circumpolar bottomfloworiginatesfrom theRossSea,passesthroughthe Drake Passage, and mixes with Weddell Sea waterintheeastern Scotia Sea. Hecal- culated strong(> 20cmsec-') currents transporting bottom water through the SRPinto theArgentineBasin. Hisargu- ments have since been supported by bottom photographic observations indi- cating murkywaterin thedeep layersof the SRP associated with abundant evi- dence forstrong contour currents(7).

Duringthe German AntarcticExpedi- tions of 1979-1980 and 1980-1981 with researchvessels Polarsirkel(8)and Me-

teor (9), I had the rare opportunity to obtainan almost yearlongdirect obser- vation of bottom current speed, direction, and temperature from the SRP.

Detection of Overflow Events in the Shag Rocks Passage, Scotia Ridge

Abstract.During an almost yearlong period of observations made withacurrent meter inthefracturezonebetween the Falkland Islands(IslasMalvinas)andSouth Georgia, severaloverflowevents wererecordedat adepth of 3000meterscarrying cold bottomwaterfromthe Scotia Sea intotheArgentine Basin. Theoutflow bursts ofScotia Sea bottom water, a mixingproduct of Weddell Sea and easternPacific bottomwater, wereassociated with typical speeds ofmore than 28centimetersper second toward the northwest and characteristic temperatures below 0.6°C. The maximum 24-hour average speed of 65 centimeters per second, together with a temperatureof 0.29°C, was encounteredon 14November 1980at a waterdepthof 2973meters, 35 meters above theseafloor.

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on May 10, 2016http://science.sciencemag.org/Downloaded from

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Two Aanderaa current meters were mooredat53°S, 48°W,one at35 and the other at 85 m above the sea floor at a water depth of 3008 m (Fig. lI). The hourly records of both instruments ex- tended from 23 December 1979 to 19 November 1980. The results reported here arelimitedtothedailyaveragedata of the lower instrument, consisting of transtidal and transinertial fluctuations only.

The progressive vectordiagram (Fig.

lb) shows a predominantcurrent direction toward thenorthwest, which iscon- sistent with the local bottom topography of themooring site. The 3000-m isobath (Fig. la)was adapted from Gordon (6).

At themooring position it is parallel to the meanbottom current direction. The current series (Fig. 2) shows pulsations withaperiodicity of approximately2 to3 weeks. During the 11-month period, a vector-averaged speed of 17.1 cm sec-1 toward 2910 true course was recorded,

a

361

0ro

80

700 60° 500 W 400

ich would correspondto a waterdis- cementof 4900 km(A-B inFig. lb) if bathymetric restrictions were pres-

. A more detailed inspection of the rrent speed and direction record re- alsthat, withincreasingcurrents, sta- izing westerlydirections werealways served. In quite a number of cases rrent onsetscoincided with reductions temperature. The correlation coeffi- :ntsoftemperatureandscalar speed(r -.59) or temperature and the west- Lrd component (r= .67) were well ove the 95 percent significance level

uncorrelatedprocesses.

FromGordon's (6) workweknow that water leavingthe SRP in the direc- tnof the Argentine Basin has a tem- rature of approximately 0.6°C. For rity we have reproduced his 0.5°C ttomisotherm in the Scotia Sea(Fig.

). The frequency distribution of all rrent speeds shows a flat minimum lund 25 cm

sec-1,

separating two

Fig. 1. (a) Position of the near-bottom, long-term 520 current meter mooring in the Shag Rocks Passage (SRP), Scotia Sea; (b) the gia progressive vector diagram S of the 2973-m instrument, a) 35 m above the seafloor.

The 3000-m isobath and Sea

60°

0.5°C isotherm have been adapted from Gordon (6),

b

who has predicted strong bottom currents (double arrow) west of the SRP.

The numbersin(b) denote periods of oveflow as seen inFig. 2c.

A

c

yes

0.

0 No 1 iII I

° Ioa ' ' &I ii1-1 * 23 0ec.

1979

21 Feb. 21 Apr.

NE~~~~~~~IL

20 June 1980

19Aug.

1.5

0

1.0 aa

I.-

1JOct.

Fig.2. (a)Direction and(b)current speedand temperatureof the bottom water in the SRP.

Simultaneous high speed-low temperature occurrences are marked by dark peaks and^are interpretedasoverfloweventsin the formofadigitaltime series(c).Priortoplotting,thehourly datawereaveraged daily.

1114

speedranges.Considering only the high- speed range (. 28 cm sec'), we find that24 percentof all recorded speeds fall intothiscategory. Ifas afurther restric- tion on this class we impose the addi- tional requirement that T be ^- 0.60C, wefind that 15 percentof all daily values meet this condition. The direction of each ofthesehigh-speed vectors lies in the range2700to3150true course.

On theaverage, the overflowthrough the SRPischaracterized by west-north- westerly currentswith speeds of 38.2 ± 8.8 cm

sec-1

and associated tempera- turesof0.470 ± 0.11°C. In Figs. lb and 2c the important overflow events are marked by numbers; peaks in the speed and temperature serits are represented by the darkareasinFig. 2b. Thedigitsin Fig. 2crepresent thefrequency ofover- flow days. They seem to be correlated with the seasons, since we find pro- nounced occurrences inearly fall (area 1) and winter(areas 2and3). Area5 corre- spondstothespring, immediatelybefore therecoveryofourmooring. During this eventtheextremevaluesof 65cmsec-' and0.29°Cwereobtained (on14Novem- ber 1980). Exactlyatthattime, theupper instrument lost itsrotorasaresult of the extreme speed.

On the basis of these findings, I con- clude (i) that the overflow through the SRPhasanintermittent character which well meets Gordon's (6) prediction ofa

"strong" current, and (ii) that the ex- tremecurrentscausing visible signalson the sea floor, as seen in Hollister and Elder's (7) photographs, are rare, and long-termcurrentobservationsareneed- ed for their detection. Further studies combining topographicdetails and more detailed current meterdata and stratifi- cation data will benecessary before we will be able toestimate the significance of the SRPto the circulation of bottom waterin the Scotia Sea.

WALTER ZENK Institut fur Meereskunde, Universitat Kiel, D2300Kiel 1, Germany

References and Notes

1. W.Meinardus,Dtsch.Sadpolar Exped.III Me- teorol.1(No. 1),544(1923).

2. A.L.Gordon, D.T.Georgi, H. W.Taylor,J.

Phys. Oceanogr.1, 309(1977).

3. D. J.Baker, Jr., Oceanus18,8(1975).

4.G.Wuzst,Wiss.Ergeb.Dtsch. Atl.Exped. "Me- teor," 1925-1927 6,(No. 1),1(1933).

5. A. J.Clowes,Nature(London) 131, 189(1933).

6. A.L.Gordon, Deep-SeaRes.13,1125(1966).

7. C. D. Hollister and R. B. Elder, ibid. 16, 99 (1969).

8.T. Gammelsr0dand N. Slotsvikhelpedlaunch themooring.

9. B. ZeitzschelandW. Zenk,Ber. Inst. Meeres- kundeKiel No. 80(1981).

10. Ithank T.Gammelsr0dand N.Slotsvik for their assistancein the launchprocedure. Supported bythe DeutscheForschungsgemeinschaft (Son- derforschungsbereich 95, Contribution No.

366).

31 March 1981

SCIENCE,VOL. 213

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(4512), 1113-1114. [doi: 10.1126/science.213.4512.1113]

213 Science

WALTER ZENK (September 4, 1981) Ridge

Detection of Overflow Events in the Shag Rocks Passage, Scotia

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