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North Pacific Internal Tides from the Aleutian Ridge

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North Pacific Internal Tides from the Aleutian Ridge Journalof Marine Research, 59, 167–191, 2001 Journal of MARINE RESEARCH Volume59, Number 2 NorthPaci c internaltides fromthe Aleutian Ridge: Altimeterobservations andmodeling byPatrickF. Cummins 1,JosefY. Cherniawsky 1 andMichael G. G.Foreman 1 ABSTRACT Internaltides radiating into the North Paci c fromthe Aleutian Ridge near Amukta Pass are examinedusing 7 yearsof Topex/ Poseidonaltimeter data. The observations show coherent southwardphase propagation at the M2 frequencyover a distanceof atleast1100 km intothe central Pacic. Barotropicand baroclinic models are applied to study this internal tidal signal. Results from thebarotropic model show that the strongest cross-slope volume and energy uxesoccur in the vicinityof Amukta Pass, helping to establish this region as an important site for baroclinic energy conversionalong the eastern half of theridge. Atwo-dimensionalversion of the Princeton Ocean Model is used to simulate internal tide generationand propagation. A comparisonbetween the altimeter data south of the ridge and the sea-surfacesignature of theinternal tide signal of the model shows good agreement for the phase, bothclose to thesourceand well into the far eld.Comparison of thephase between model and data alsoprovides evidence for wave refraction. This occurs due to the slow modulation of wavelength associatedwith the variation in the Coriolis parameter encountered as the internal tide propagates southward.The model results suggest that the net rate of conversion of barotropic to baroclinic energyis about 1.8 GW inthe vicinity of Amukta Pass. This represents about 6% of the local barotropicenergy uxacrossthe ridge and perhaps 1% ofglobalbaroclinic conversion. 1.Introduction Prior tothelaunch of the Topex-Poseidon (T/ P)missionin August,1992, it was widely heldthat oceanic internal tides generally did not propagate more than short distance away from asourceregion before becoming ‘ incoherent,’that is, before losing their phase relationwith the generating surface tide. For example,in observations from moored 1.Institute of Ocean Sciences, Sidney,BC, Canada V8L4B2. email:cumminsp@ pac.dfo-mpo.gc.ca 167 168 Journalof MarineResearch [59, 2 sensors,internal tides often appear intermittently (Wunsch, 1975), due perhaps to spatial andtemporal variability of thestrati ed medium,but perhaps also to auctuatingsignal to noiselevel. However, data since acquired from theT/Paltimeterhave clearly demonstrated thatenergetic, phase-locked internal tides are indeed present in the open ocean and may propagateover thousands of kilometersbefore being dissipated. The best studied case is theinternal tide generated along the Hawaiian Ridge. Using three years of T/ Paltimetric data,Ray and Mitchum (1996) showed that coherent propagation occurs from bothsides of theridgewith an e-foldingdecay scale of about 1000 km. The netenergy transfer rate from thebarotropic to baroclinic tides along the ridge was estimatedto be15GW, representing perhaps7– 8% ofthe200 GW suggestedby Munk(1997) as the global rate of baroclinic conversionfor the M2 tide. Kang et al. (2000)and Merri eld et al. (2001)have presented resultsfrom numericalmodels of internaltide generation at theHawaiian Ridge. Thesatellite altimetry has also indicated the existence of numerousadditional genera- tionregions for coherentbaroclinic tides. Kantha and Tierney (1997) present a global surveyusing data smoothed over 2° 3 2°bins and show that phase-locked signals typically appearin thevicinity of undersea ridges. On the other hand, there are certain regions where largeinternal tides have been documented (e.g., southwest of theEuropean shelf) that fail toshow a signicant signal in the T/ Pdata.Ray and Mitchum (1997) suggest loss of temporalcoherence with the barotropic tide as a plausibleexplanation. Cherniawsky et al. (2001)recently discussed a techniquefor harmonicanalysis of the T/Pdatausing singular value decomposition (SVD) andapplied the method to over 5 years (1992–97) of T/Paltimetricdata collected over the northeast Paci c. Theirresults permit identication of small-scalefeatures and, in particular,they mention an internal tide signal apparentlyemanating from theAleutian Island chain. Evidence for thiswas givenin the residualamplitude and phase along T/ Ptrack117. A localizedsource near the Aleutian Islandsmay also be discerned in thesmoothed results of Kanthaand Tierney (1997). Thispaper further discusses the internal tide propagating south from theAleutian Ridge intothe central North Paci c usingan updated analysis of the T/ Pdata,combined with resultsof numericalsimulations from barotropicand baroclinic models. Results from the T/Pdatashow that the internal tide from theAleutian Ridge appears as adirectedbeam of elevatedamplitudes with a robustphase that can be tracedfor over10° of latitudeto the south.In the barotropic model, Amukta Pass isthedominantlocation for semi-diurnaltidal energyto crossthe eastern half of theAleutian Ridge and enter into the Bering Sea. The strongestcross-slope barotropic tidal volume uxesalong the eastern Aleutian Ridge occur inthe vicinity of thePass, providing the forcing for internaltides. Ahighresolution, two-dimensional ( x2z)baroclinicmodel is applied to simulate the M2 baroclinictide generated at AmuktaPass. As expected,the model response includes an energeticinternal tide propagating to the south away from theridge topography. A comparisonof thebaroclinic component of themodel sea-level response with along-track altimetricdata shows that the phase of the sea-surface signature is well simulated both closeto thesource and in the far eld.The results also demonstrate that the observedphase 2001] Cumminset al.:North Paci c internaltides 169 Figure1. Map of the eastern Aleutian Island chain with place names and contours of bottom topography. issignicantly affected by the varying rotation rate encountered by the internal tide as it propagatessouthward into the central Paci c. Inthe next section, the T/ Paltimeterobservations of internal tides from theAleutian Ridgearepresented. Sections 3 and4 discussresults of barotropicand baroclinic numerical models,respectively, including a comparisonwith the altimeter observations. The last sectionsummarizes. For reference,an area map of the eastern Aleutian Island chain is includedas Figure 1. 2.Observations from the T/Paltimeter Theresults presented here are derived from sea-leveldata obtained over the northeast Pacic Oceanby theT/ Paltimeterduring cycles 1– 250, representing nearly seven years of datafrom September,1992 to July,1999. Standard instrumental and geophysical correc- tionsto thedata, except for tidalanalysis, were madeat theGoddard Space Flight Center. Detailson these algorithms may be foundin Koblinsky et al. (1999).Tidal constants for 21 constituentswere determinedby aleast-squaresharmonic analysis method. This leads to anoverdeterminedmatrix system which is solvedusing SVD (Cherniawsky et al., 2001). Themethodalso provides an estimate of theexpectederror, whichscales with the standard deviationof thenontidal residual. Sincethe T/Psamplingrate of onceevery 9.9156 days is longer than the semidiurnal and diurnalperiods, aliasing occurs between tidal constituents. A revisedRayleigh criterion (Parke et al., 1987)gives the length of time series required to separate individual tidal 170 Journalof MarineResearch [59, 2 constituents.The time series of nearly2500 days used in thisstudy is easilyof suf cient lengthto decouple M2 (theonly constituent considered below) from othertidal constitu- ents.For example,separation of M2 from S2 requiresabout 1100 days of data. The principalsource of aliasing error thusarises from broadbandmesoscale eddies. Such variabilityis relatively weak over the central north Paci c, except perhaps immediately southof the Aleutian chain, where the Alaskan Stream, a swift (100cm s 21), narrow (50km width), western boundary current is found. Even there mesoscale activity in the Streamis relatively weak in comparison to other western boundary currents like the Kuroshioor Gulf Stream (Reed et al., 1991).A preprocessingof the data involving selectivedata trimming helps to reduce the projection of strongsea-level anomalies into thetidal signal. The estimated expected error for M2 isabout1.8 cm in the vicinity of the AlaskanStream, decreasing sharply to about0.8 cm overmost of theregion of interest. From resultsof theSVD analysis,a residualfor M2 isobtained by takingthe difference betweenthe observed harmonic and a smoothedversion with high wavenumber variability removed.This procedure is fairly standard, but for completeness,a concisesummary is givenin the Appendix. The smoothed eld,taken to represent the barotropic tide, is obtainedfrom aneighth order polynomial ttothe alongtrack T/ Pdata.The residual is interpretedas an internaltide signal plus background noise. Figure2 displaysthe amplitude of the resulting residual over the northeast Paci c. Exceptfor afew distinctsignals, amplitudes over most of theregionare near a background levelof 0.5–0.6 cm. This is theeffective noise level and it appearsto besomewhatlower thanthe formal estimate provided by theSVD analysis.Before turning our attentionto the signalemanating from theAleutianRidge, we rst notethe other major sources of coherent internaltides over theregion. The broad area of elevatedamplitudes near the lower left side ofFigure2 isthe far eldexpression of theinternal tide originating from thenorthern ank oftheHawaiianRidge (Ray and Mitchum, 1996). Also noteworthy are internal tide signals evidenton either side of
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