VOLUME 84 NUMBER 42

21 OCTOBER 2003

PAGES 441–456

Eos,Vol. 84, No. 42, 21 October 2003

EOS,TRANSACTIONS, AMERICAN GEOPHYSICAL UNION

The volatile constituents—H22O,CO22,and S,andCl—play an important role in the generation,evolution, and eruption of magma. Knowledgeof the abundance and flux of these volatiles isimportant for understanding the hazard impli-cations of volcanic activity, explosive eruptivebehavior of volcanoes, recycling of volatiles insubduction zones, formation of magmatic-hydrothermal ore deposits,additions of volcanicgases to Earth’s atmosphere, and potential climatic impacts of large volcanic eruptions.

Over the past 25 years, new developments intechnology have led to major advances in ourability to measure the fluxes of volatiles releasedfrom erupting volcanoes.At the same time,new micro-analytical techniques have made itpossible to measure the pre-eruptive concen-trations of magmatic volatiles through the useof tiny melt inclusions trapped in phenocrystsin volcanic rocks. Comparison of the dataacquired from these different perspectives hashighlighted some major gaps in our under-standing of both the abundance of the differ-ent volatiles,and the ways in which they aretransported through magmatic systems andreleased to the atmosphere by degassing.

Remote Sensing Approaches

Sulfur dioxide (SO22) is the easiest of themain magmatic volatiles to measure in volcanicplumes, because its concentration is relativelyhigh compared to ambient atmospheric values.However, CO22 and H22S can also be analyzedusing aircraft-based gas analyzers and sensors[e.g., McGee et al., 2001], and H22O, HCl, HF, CO22,and several other species can be measuredusing Fourier-Transform Infrared (FTIR) spec-troscopy [e.g., Burton et al., 2000]. Remotesensing of SO22 is achieved using absorptionbands of SO22 gas in the ultraviolet (UV) andinfrared (IR) regions of the electromagneticspectrum. Numerous remote measurements ofSO22 fluxes from active volcanoes have been

made, including both ground-based and air-borne use of the UV correlation spectrometer(COSPEC) since the early 1970s,and the satellite-based Total Ozone Mapping Spectrometer(TOMS; Figure 1) since 1978 [Krueger et al.,2000; see: http://skye.gsfc.nasa.gov].

Building on the measurements of volcanicSO22 from space pioneered by the UV TOMSinstruments, recent years have seen launchesof IR sensors that have augmented the SO22

mapping capabilities available to volcanologists[Realmuto et al., 1997].The Moderate Resolu-tion Imaging Spectroradiometer (MODIS) car-ried by NASA’s Earth Observing System (EOS)Terra and Aqua satellites, and the AdvancedSpaceborne Thermal Emission and ReflectionRadiometer (ASTER) on EOS/Terra offer thepotential to quantify the significant non-erup-tive or passive SO22 plumes emitted by manyquiescent volcanoes.Measurements of largerSO22 clouds in the upper troposphere and strat-osphere, traditionally the domain of TOMS,canalso now be accomplished several times dailyusing data from the High Resolution InfraredSounder (HIRS) on NOAA’s polar-orbitingsatellites [Prata et al., 2003]. Moreover, furtherdevelopments are anticipated in 2004 with thelaunch of the Ozone Monitoring Instrument(OMI) on EOS/Aura.OMI will be the successorto TOMS and is expected to be able to routinelymeasure volcanic SO22, including passive plumesin the atmospheric boundary layer.

An advanced multispectral IR sensor (SEVIRI;http://www.esa.int/msg/pag4.html) on theEuropean Space Agency’s new Meteosat SecondGeneration (MSG) satellite, expected to beoperational very soon,will provide geostationarycoverage of volcanic clouds over Europe andAfrica. Geostationary platforms stare at thesame places and allow for very frequent cov-erage, which will result in sensing of short-lived eruptions, and the IR bands are ideal forSO22 and ash sensing. Despite having greatervolcanic activity, North and South Americahave no comparable IR coverage, and from2004 to ~2012,will lose important geostationaryash detection capability on the GOES satellite.After 2012,a new sensor called ABI (AdvancedBaseline Imager), which is currently being

developed, will fly over the Americas and pro-vide robust coverage of volcanic clouds.

Measurement of volcanic clouds by multi-spectral techniques has demonstrated the utility of examining clouds from different per-spectives.There is really no one method orspectral range that is universally appropriatefor evaluating SO22 in volcanic plumes. Eachsensor is limited by its spatial, temporal, orspectral resolution, and thus, observes only aportion of a plume, or is best suited for evalu-ating a plume for a short period of time orunder a restricted range of meteorologicaland volcanological conditions. Interferenceby co-emitted ash, meteorological clouds,background reflectivity, and the presence orgeneration of aerosols, ice, and other gases inthe plumes all affect retrievals to varyingdegrees, and are unique to each setting andevent.

Knowledge of meteorological conditions(relative humidity, temperature, wind vectors)also plays a vital role in evaluating gas fluxes,particularly for effusive emissions.The scientif-ic utility of remotely sensed data is oftendependent on the ability to measure or modelsurrounding environmental conditions.A mul-ti-spectral approach is vital because volcanicclouds evolve, sometimes rapidly, both in phys-ical and chemical character during the firstfew days of emission into the atmosphere.With the addition of new tools and techniqueshas come the discovery of many new complex-ities and processes affecting the fate of volcanicemissions.

Recent Developments in Ground-based and Airborne Methods

Ground-based and airborne techniques for monitoring volcanic emissions are alsoadvancing. One important development is theapplication of miniature UV spectrometers toSO22 retrieval [Galle et al., 2002].These low-cost,lightweight devices will probably supplant theCOSPEC as the instrument of choice for proxi-mal surveillance of degassing volcanoes.

Another development is an airborne systemdevised by the U.S. Geological Survey thatdirectly measures concentrations of CO22, SO22,and H22S by extraction sampling of plumes.The system includes an infrared gas analyzer,chemical sensors,a closed-path FTIR instrument,a COSPEC, a high-precision barometer, a tem-perature probe, and a GPS receiver. Measure-ments sample a range of altitudes along traversesorthogonal to plume (wind) direction oralong orbits circling a volcano (Figure 2A).

PAGES 441, 446–447

Integrating Petrologic and RemoteSensing Perspectives on MagmaticVolatiles and Volcanic Degassing

BY PAUL J.WALLACE, SIMON A. CARN,WILLIAM I.ROSE, GREGG J. S. BLUTH,AND TERRY GERLACH

Data for all gases are recorded at 1-s intervalsand tagged with latitude, longitude, altitude,temperature, and pressure. Gas concentrationsin plume cross-sections are contoured withmapping software (Figures 2B,2C) and usedwith in-flight wind data to calculate emissionrates.

The system is modular and can be deployedon fixed-wing aircraft and helicopters, ormounted on vehicles for ground-level meas-urements, making it suitable for a wide rangeof volcanic environments. It is applicable tovolcanoes of the western United States andAlaska, where locations are remote, ground

access is difficult, and wet climates, snowpack,and glaciers foster hydrothermal and groundwaters that can scrub acid magmatic volatiles(SO2, HCl, HF), masking degassing at depthduring critical early stages of unrest.Campaignsto date have constrained CO2 emission ratesfrom as low as 200–300 tons/day at MammothMountain and Mount Baker, to as high as8000–9000 tons/day at Popocatépetl andKilauea [McGee et al., 2001; Gerlach et al.,2002, and references therein].

Except in rare cases, the gas emission moni-toring techniques covered in this article arenot likely to measure effectively the diffusedegassing flux of volcanoes.Although tendingto be smaller in magnitude than vent degassing,diffuse degassing is an important componentof the total gas emission at many volcanoes,and in some instances—Mammoth Mountainbeing a notable example—it can be the domi-nant mode of degassing. Eddy covariancemethods and soil gas efflux surveys offer alter-native monitoring techniques for diffusedegassing.

Petrologic Studies of Magmatic Volatiles

With the techniques summarized above,volcanologists now have at their disposal atoolkit of instruments capable of quantifyingemissions of SO2 and other volatiles at a rangeof temporal and spatial scales.Used in concert,these techniques have the potential to producea vastly improved and expanded volcanicdegassing data set. Complementary to thesedevelopments in remote sensing,new samplingand micro-analytical techniques have made itincreasingly possible to get information onmagmatic volatiles directly from samples of vol-canic material. Because the solubility of mostvolatile components is strongly pressure-dependent,magmas exsolve gas during ascentto the surface.

To understand eruptive behavior, it is impor-tant to know the total volatile content of magmaduring its pre-eruptive storage within the Earth’scrust.An important source of information ondissolved volatiles in magmas is through theanalysis of glass inclusions trapped inside ofcrystals.When melts crystallize, the crystalsgrow imperfectly,causing small blebs of melt tobe trapped inside of the crystals. If the magmaerupts and cools rapidly, then these trappedmelt inclusions quench to glass. Because thecrystalline host for the inclusions is relativelyrigid, they act as tiny pressure vessels and pre-vent the trapped melt from degassing, eventhough the bulk magma decompresses to sur-face pressure during eruption. For this reason,trapped melt (glass) inclusions commonlyretain their original dissolved volatiles. Meltinclusions are now routinely analyzed for manyvolatile species (H22O,CO22,S,Cl,F) using infraredand Raman spectroscopy,electron microprobe,and secondary ion mass spectrometry (SIMS).

Comparison of Remote Sensing and Petro-logic Perspectives–The Excess Sulfur Problem

It is possible to use data for S in melt inclu-sions to make an estimate of how much S would

Eos,Vol. 84, No. 42, 21 October 2003

Fig.1.Photograph of Popocatépetl volcano forcefully venting gases from its summit lava dome on19 December 2000 (top panel),along with satellite SO22 data (SO22 column amount in milli atm cmor Dobson Units) from the Earth Probe TOMS instrument collected approximately 2.5 hours afterthe photograph was taken (lower panel).The volcanic cloud observed by TOMS contained~26,000 tons of SO22 (± 30%).Some of the highest SO22 emission rates and lava dome growth ratesrecorded during Popo’s current eruption were observed in December 2000. (photo courtesy of theCentro Nacional de Prevención de Desastres [CENAPRED].)

Eos,Vol. 84, No. 42, 21 October 2003

be released in a volcanic eruption based onknowledge of the total volume of eruptedmagma. However, such estimates assume thatthe only S released during an eruption is S

that was originally dissolved in the silicate liq-uid (melt) portion of the magma.Comparisonof such “petrologic”estimates based on meltinclusion data with SO22 emissions measured

for recent eruptions by remote sensing techniqueshas led to a conundrum, known as the “excess”sulfur problem,concerning S mass balance dur-ing volcanic eruptions [Andres et al., 1991].

Concentrations of dissolved S in magmasbefore eruption, as estimated from melt inclu-sion data, are commonly far too low (by 1 to 2orders of magnitude) to account for the totalmass of SO22 released during the eruption,asmeasured by remote sensing techniques (Fig-ure 3). Excess S is observed in most eruptionsfor which remote sensing and melt inclusiondata are available, and in particular, is charac-teristic of explosive eruptions of intermediateand silicic magma in subduction zone settings[Andres et al., 1991]. In contrast, basaltic erup-tions from divergent plate boundaries and hotspots, such as Hawaiian and Icelandic volca-noes, often do not show evidence of excess Semissions (Figure 3).

It has commonly been assumed that magmasof intermediate-to-silicic composition onlybecome vapor-saturated during shallow ascentand emplacement, during eruptive decompres-sion,or during advanced (pegmatitic) stages ofplutonic crystallization. However, numerouslines of evidence from petrologic, remotesensing, and volcanic gas data suggest thatthe “excess”S problem is caused by the pres-ence of an exsolved C-O-H-S vapor phase inthe magma before eruption [Luhr et al., 1984;Andres et al., 1991; Westrich and Gerlach, 1992;Wallace,2003].That is, the concentrations ofdissolved volatiles are sufficient to cause satu-ration of the magma with a multi-componentvapor phase, which would presumably bepresent as bubbles dispersed throughout mag-ma in crustal storage reservoirs.As a result,eruptions of silicic magma can release largeamounts of SO22 derived from pre-eruptiveexsolved vapor, despite the fact that such mag-mas generally have very low concentrationsof dissolved S.Techniques for quantifying themass fraction of exsolved vapor in magmabefore eruption are thus important for under-standing SO22 flux data used for forecasting andmonitoring eruptions, and for assessing thepotential climatic impacts of volcanic erup-tions.

The large amounts of volatiles implied bythese estimates suggest that exsolved vaporaccumulates in the apical regions of magmabodies during repose periods between erup-tions, or may get trapped below hydrothermalcaps above the body.The presence of an exsolvedvapor phase in crystallizing magma bodiesmay also play an important role in triggeringvolcanic eruptions. Remote sensing data fre-quently observe spatial separation of SO22 andash in explosive eruptions, and improvedfrequency of IR satellite data is likely to clarifywhether a gas-rich phase is released early inexplosive events, an idea consistent withobservations [Rose et al., 2003].

Sources of Volatiles in Crustal Magmatic Systems

Isotopic studies of S and CO2 in intermedi-ate-to-silicic magmas have demonstrated thata large component of these volatiles is

Fig.2. (A) Location map with GPS flight path for a 21-orbit airborne survey of CO2 and H2S at Mount Baker,Washington on 13 September 2000.Upper elevations of Mount Baker arecontoured.Solid triangle marks the location of Sherman Crater, a site of several large fumaroles.Arrow shows wind direction.Dark portions of flight paths indicate where the plume was intersect-ed downwind during the survey.Contoured cross-sections of plume CO2 (B) and H2S (C) are 3km downwind of Mount Baker and corrected for orbital curvature.The cross-section views areupwind toward the volcano.Contours indicate ppm concentrations above background,which areapproximately 368 ppm for CO2 and essentially zero for H2S.The CO2 emission rate for this surveyis about 190 tons/day; that for H2S is about 6 tons/day [McGee et al., 2001].

Eos,Vol. 84, No. 42, 21 October 2003

mantle-derived.The ultimate source of S andCO22 must therefore be from mafic magma,because it is well established that silicic mag-ma reservoirs are created and sustainedthrough long-term intrusion of mantle-derivedbasaltic magma into the crust. It should be not-ed, however, that in the case of subductionzones, some portion of the mantle S and C(and almost all H22O) may ultimately havebeen recycled from subducted sediments andaltered oceanic crust.

Andesitic, dacitic, and rhyolitic magmas incrustal reservoirs are probably vapor-saturateddue to recharge and underplating by basalticmagma that is saturated with multi-componentC-O-H-S vapor. In this regard, vapor saturationof intermediate to silicic magmas can beviewed as the inevitable consequence of therelatively high CO22 concentration of basalticmagmas coupled with relatively low CO22

solubility in silicate melts at crustal pressures.Vapor transfer from underplated basaltic mag-ma into an overlying silicic magma body isprobably a slow process that occurs duringthe long repose periods that separate eruptiveepisodes. In systems where intrusion of basalticmagma into the lower crust generates silicicmagma by partial melting and/or fractionalcrystallization, transfer of volatiles into mid- toupper-crustal silicic magma chambers may bea very complex process compared to systems inwhich basaltic magma intrudes directly into asilicic magma reservoir. However, in eithercase,if the S and CO22 in crustal magmatic sys-tems are largely derived from the mantle viabasaltic magma, then the flux of these volatilesshould be directly related to the basaltic mag-ma flux.

The importance of mantle-derived basalt asa source of volatiles is exemplified by mafic

cinder cones surrounding Popocatépetl(“Popo”) volcano in central Mexico. Magne-sian andesites erupted explosively from Popoduring the current eruption (December 1994to 2002) have phenocryst assemblages andmajor- and trace-element systematics thatindicate mixing of mafic and silicic magma,and SO22 and CO22 emissions have both beenvery high [see references in Wallace, 2003].Quaternary mafic cinder cones in the regionsurrounding Popo provide samples of possiblemafic end members involved in mixing.Olivine-hosted melt inclusions from thesecones contain 1000–6000 ppm S, 250–2100ppm CO22, and 1.3–5.2 wt% H2O [Cervantes andWallace, 2003].The high S contents suggestthat mafic magma recharging into the Poposystem provides an abundant source of S thatmay explain the very large SO22 emissions fromthe current eruption.The H22O and CO22

contents are sufficiently high that these maficmagmas would be vapor-saturated at thedepths of a shallow crustal magma chamberbeneath the volcano, so recharging maficmagmas could transfer significant volatilesduring recharge events.

Future Directions

Future directions in remote sensing of volcanicclouds feature multi-spectral retrievals to examinethe interactions and removal rates of differentspecies in the clouds,and to provide more com-prehensive evaluations of emission fluxes through-out various phases (e.g.,explosive versus effusive)of volcanic activity. Progress in understandingbudgets of the major magmatic volatilesrequires integration of remote sensing andvolcanic gas data on fluxes of CO22, SO22, H22S,H22O, and HCl with petrologic data on tempera-ture, pressure, oxygen fugacity, and dissolvedvolatiles in both differentiated magma stored incrustal reservoirs and mafic magma rechargingthese systems.

Experimental studies of vapor-melt partitioningof volatiles are also needed, particularly for S. Such studies are crucial for developing andcalibrating thermodynamic models for predict-ing vapor saturation and degassing of a multi-component vapor phase over a wide range ofconditions. Seismic studies of sub-volcanicmagma bodies can also potentially play animportant role in the problem of determiningvolatile budgets, by providing evidence for thepresence and abundance of exsolved vapor inmagma stored within the Earth’s crust.

Acknowledgments

We thank Paul Renne for inviting this reviewand Bruno Scaillet for his helpful commentson the manuscript. S. Carn acknowledges sup-port from NASA funds,and W.Rose and G.Bluthacknowledge support from both the U.S.National Science Foundation and NASA.

References

Andres, R. J.,W. I. Rose, P. R. Kyle, S. deSilva, P. Francis,M. Gardeweg, and H. Moreno Roa, Excessive sulfur

Fig.3.Volcanic SO2 emissions, in megatons (1 Mt = 101122g),versus total volume of erupted magma.SO22 emissions were measured using remote sensing methods except for the eruption of the BishopTuff (BT; see references in Wallace, 2003).Most of the remote sensing data are from TOMS,exceptfor the data for Fuego (determined by COSPEC),Agung (estimated from stratospheric opticaldepth measurements),and Laki (estimated from information on atmospheric turbidity).Uncertain-ties of ± 50% for both SO2 emission and eruptive volume are less than to slightly greater than thesize of the symbols.Uncertainties in SO22 emission data are generally considered to be about ±30% for the TOMS data and ± 20% to 50% for COSPEC.Data are shown for the followingeruptions: (A) Agung,1963, (EC) El Chichón,1982, (Et) Etna,1975, (F) Fuego,1974, (H) CerroHudson,1991, (K) Kilauea,annual average, (L89) Láscar, 1989, (L93) Láscar, 1993, (Lk) Laki,1783–1784, (Lq) Lonquimay,1989, (ML) Mauna Loa,1984, (M) Mount St.Helens,1980, (Pc)Pacaya,1972, (P) Pinatubo,1991, (Rb) Rabaul, 1994, (R) Redoubt, 1989–1990, (Rz) Ruiz,1985,(S) Spurr, 1992,and (St) Stromboli, annual average.References to data sources can be found inWallace [2003]. Shown for comparison are predicted relationships between SO22 emission anderuptive volume for rhyolitic (dashed line),andesitic (solid line),and basaltic (short dashed line)melts calculated by assuming that the only SO22 released during the eruption is from S that is orig-inally dissolved in silicate melt.Note that the SO22 emissions for all eruptions,with the exceptionof Mauna Loa,Kilauea,and Laki, are at least 1 order of magnitude greater than predicted for theappropriate bulk composition by syneruptive degassing of dissolved S only.This clear discrepancyover many orders of magnitude in eruptive volume indicates that a major additional source of Smust be involved in all of these eruptions.

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Burton, M. R., C. Oppenheimer, L.A. Horrocks, and P.W. Francis, Remote sensing of CO22 and H22O emission rates from Masaya volcano, Nicaragua,Geology,28, 915–918, 2000.

Cervantes, P. and P. J.Wallace,.The role of H22O in sub-duction zone magmatism: New insights from meltinclusions in high-Mg basalts from central Mexico,Geology,31, 235–238, 2003.

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fluxes: a new tool for volcano surveillance,J.Volcanol.Geotherm.Res., 119, 241–254, 2003.

Gerlach,T. M., K.A. McGee,T. Elias,A. J. Sutton, and M. P. Doukas, Carbon dioxide emission rate ofKilauea volcano: Implications for primary magmaand the summit reservoir, J.Geophys.Res.,107(B9), 2189, doi:10.1029/2001JB000407, 2002.

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edited by P. J. Mouginis-Mark, J.A. Crisp, and J. H. Fink, pp. 25-43, Geophysical Monograph 116,AGU,Washington, D.C., 2000.

Luhr,J.F.,I.S.E.Carmichael,and J.C.Varekamp,The 1982eruptions of El Chichón,Chiapas,Mexico:Mineralogyand petrology of the anhydrite-bearing pumices,J.Volcanol.Geotherm.Res., 23, 69–108, 1984.

McGee, K.A., M. P. Doukas, and T. M. Gerlach, Quies-cent hydrogen sulfide and carbon dioxidedegassing from Mount Baker,Washington,Geophys.Res. Lett., 28, 4479–4482, 2001.

Prata A. J.,W. I. Rose, S. Self, and D. O’Brien, Global,long-term sulphur dioxide measurements fromTOVS data: A new tool for studying explosive vol-canism and climate, in Volcanism and the Earth’sAtmosphere, edited by A. Robock and C. Oppen-heimer,AGU,Washington, D.C., in press.

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Author Information

Paul J.Wallace, University of Oregon, Eugene;Simon A.Carn,Joint Center for Earth Systems Technology(NASA/UMBC),University of Maryland,Baltimore County;William I.Rose,and Gregg J.S.Bluth,Michigan Techno-logical University,Houghton; and Terry Gerlach,USGS,Cascades Volcano Observatory, Vancouver,Washington

Coastal circulation along the southeast Floridashelf is strongly related to the dynamics of theFlorida Current.The current follows the steepbottom terrain along the shelf break separatingthe deep ocean from the coastal zone (Figure1 a, b). Energetic and complex flow regimesarise along the current’s western edge, caus-ing transient features such as sub-meso-scaleeddies [Lee and Mayer, 1977; Shay et al., 2000]and energetic internal oscillations [Mooers,1975; Soloviev et al., 2003], which affect mix-ing between the shelf and deeper oceanwaters. Understanding of these regimes isimportant for modeling and prediction of thecoastal circulation on the southeast Floridashelf and description of western boundarycurrent/continental shelf interactions.

A striking feature of the circulation on theshelf off southeast Florida is the energeticbaroclinic supertidal oscillation,which produces10-hr period current velocity oscillations witha 0.5-m s--11 amplitude.At the same time, currentvelocity oscillations induced by the barotropictide usually do no exceed about 0.05-m s--11

amplitude. Spectral analysis of the sea levelsignal reveals the expected semi-diurnal tidalpeak close to the 12-hr time period (Figure1c).The corresponding peak, however, is notseen in the velocity spectrum; instead, a 10-hrpeak is prominent (Figure 1d).

Two years of continuous observations on thesoutheast Florida shelf reported in Soloviev etal. [2003] suggested that this oscillation isobserved primarily during summer months.A4-year data set collected at the same location,which is now available for analysis, clearlydemonstrates that this phenomenon has aseasonal cycle, which is the subject of interan-nual variability. On average, the kinetic energy

associated with the oscillation appears muchhigher in the summer season than at othertimes throughout the year (Figure 2); there isconsiderable variation between years. Interan-nual differences can be seen in Figure 3,where the squared current velocity magnitudeswithin the 8- to 12.5-hour band for each summerfrom 1999–2002 are shown.

Variability of the coastal currents affect soci-etally relevant issues such as navigation,acoustics,search and rescue operations,beach erosion,and coastal pollution from sewage plants,harmful algal blooms, and oil spills [Peters etal., 2002].These important practical issuescould require that the baroclinic current oscil-lation on the southeast Florida shelf becomemore fully understood and its frequency andimportance in nature be assessed.

Project Background

The South Florida Ocean Measurement Center(SFOMC) was established in 1998 through apartnership involving government and acade-mia.A congressional grant, along with a

Coastal Observatory InvestigatesEnergetic Current Oscillations on Southeast Florida Shelf PAGES 441, 448–449

Fig.1. (a) Map indicating location of three mooring buoys in the SFOMC range. (b) Cross-sectionof Florida Straits. Sea level (c) and velocity (d) spectra in the SFOMC range.Note a 2-hr “blue”shift of the tidal peak in the velocity spectra compared to the sea-level spectrum. Inertial frequencyfii (27.34 hrs) and tidal constituents O11 (25.82 hrs),K11 (23.93 hrs),N22 (12.66 hrs),M22 (12.42 hrs),and S22 (12 hrs) are marked by vertical dashes.The spectral curves in subplots (c) and (d) arecolor-coded according to the color of the mooring symbols in subplot (a).

a)

b)

d)

c)