an evaluation of the barotropic and internal tides in a high resolution global ocean circulation model 1 global comparisons of barotropic and internal tides generated in an eddy resolving ocean circulation model are made with tidal estimates obtained from altimetric sea surface heights and an altimetry constrained tide model as far as we know our hybrid coordinate ocean model hycom simulations shown here and in an earlier paper are the only published high resolution global simulations to contain barotropic tides internal tides the general circulation and mesoscale eddies concurrently comparing the model barotropic tide with a global data assimilative shallow water tide model shows that the global tidal elevation differences are approximately evenly split between discrepancies in tidal amplitude and phase both the model and observations show strong generation of internal tides at a limited number of hot spot regions with propagation of beams of energy for thousands of kilometers away from the sources the model internal tidal amplitudes compare well with observations near these energetic tidal regions averaged over these regions the model and observation internal tide amplitude estimates agree to approximately 15 for the four largest semidiurnal constituents and 23 for the four largest diurnal constituents away from the hot spots the comparison between the model and altimetric amplitude is not as good due in part to two problems errors in the model barotropic tides and overestimation of the altimetric tides in regions of strong mesoscale eddy activity examining the general energy distribution of the simulated internal tide is an important first step in the evaluation of internal tides in hycom

global wavenumber spectrum with corrections for altimeter high frequency noise the altimetry wavenumber spectra of sea surface height ssh provide a unique dataset for testing of geostrophicturbulence whilesshspectralslopesofk 211 3 andk 25 areexpectedfromtheoriesandnumerical simulations the altimetry spectra from the original unfiltered and instrument noise corrected data often are too shallow falling between k 22 and k 23 in this study the possibility that the flattened spectral slopes are partly due to contamination by unresolved high frequency 10 days motions is tested a spatiotemporal filter based on empirical orthogonal function expansion eof is used to remove the temporally incoherent signals the resulting spectral slopes are much steeper than in the previous studies over 70 of the revised global spectral estimates excluding the tropics are above k 23 moreover in high energy regions like the gulf streamandkuroshio thespectralslopesare aboutk 25 whichis consistentwiththeclassicalquasigeostrophic qg turbulence the spectral slopes are validated with the eddy kinetic energy eke spectra from shipboard acoustic doppler current profiler adcp measurements in the high and low energy regions

energy levels phase and amplitude modulation of the baroclinic tide off hawaii abstract inverted echo sounder data from station aloha north of oahu hawaii are used to determine the absolute energy levels and time varying nature of the first mode baroclinic tide north of hawaii the semidiurnal tide amplitude and phase are modulated with 60 120 day periodicity which indicates interaction between the baroclinic tide and intra annual mesoscale variability seen off hawaii close to the presumed generation site there is less phase modulation and amplitude modulation is uncorrelated with the far field variability these results are consistent with a model of phase locked generation together with phase and amplitude distortion during propagation the sense of the amplitude modulation is opposite to that previously observed at decadal timescales semidiurnal energy flux at station aloha ranges from 2 2 to 8 0 kw m 1 with a mean value of 4 4 kw m 1

open ocean validation of topex poseidon sea level in the western equatorial pacific during the verification phase of the topex poseidon radar altimeter mission a rigorous open ocean validation experiment was conducted in the western equatorial pacific ocean from august september 1992 to february march 1993 two tropical ocean and global atmosphere toga tropical atmosphere ocean moorings at 2os 156 e 1739 m depth and 2os 164 4 e 4400 m depth were outfitted with additional temperature salinity and pressure sensors to measure precisely the dynamic height from the surface to the bottom at 5 min intervals directly beneath two topex poseidon crossovers bottom pressure gauges and inverted echo sounders were deployed as well a predeployment design study using full depth conductivity temperature depth casts subsequently confirmed by postdeployment analyses indicated this suite of instruments was capable of measuring sea surface height fluctuations to within 1 2 cm the validation experiment also benefited from the comprehensive set of ocean atmosphere measurements that were made in the region during the toga coupled ocean atmosphere response experiment intensive observation period of november 1992 to february 1993 the surface relative to bottom dynamic height fluctuations observed in situ during the 6 7 month experiment had a standard deviation of 5 cm with excursions of order t 15 cm energetic steric sea level variability was found to exist on short timescales of order hours to a few days most notably the quasi permanence of strong semidiurnal internal tides such internal tides were noted to induce changes in surface dynamic height with a standard deviation of 2 dynamic centimeters at the shallower of the two sites 2 s 156 e a possible nonlinear rectification of the internal tide was observed occasionally to change the dynamic height by as much as 30 cm over less than an hour on timescales longer than the 10 day repeat of the topewposeidon satellite the low frequency fluctuations of dynamic height were related to interannual variations corresponding to the 1991 1993 ei nino southern oscillation to the seasonal cycle and to intraseasonal variations associated with the 40 to 60 day oscillations of the equatorial zonal wind field instantaneous comparisons between the 1 s topex poseidon altimeter retrievals and the 5 min dynamic height were performed with regard to several tide models the barotropic tide measured in situ european centre for medium range weather forecasts surface air pressure and the surface air pressure measured in situ depending on the choice of altimeter and of the environmental corrections applied to the altimeter data the rms differences between the satellite and the in situ measurements of sea level were as low as 3 3 cm at 2os 156 e and 3 7 cm at 2os 164 4 e when additional satellite data in the general vicinity of the mooring are included and after the use of a 30 day low pass filter the satellite and in situ data were found to be highly correlated with correlation coefficients of about 0 95 and rms differences around 1 8 cm

new altimetric estimates of mode 1 m2 internal tides in the central north pacific ocean abstract new estimates of mode 1 m2 internal tide energy flux are computed from an extended ocean topography experiment topex poseidon t p altimeter dataset that includes both the original and tandem tracks improving spatial resolution over previous estimates from o 500 km to o 250 km additionally a new technique is developed that allows separate resolution of northward and southward components half wavelength features previously seen in unseparated estimates are shown to be due to the superposition of northward and southward wave trains the new technique and higher spatial resolution afford a new view of mode 1 m2 internal tides in the central north pacific ocean as with all altimetric estimates only the coherent or phase locked signals are detectable owing to the long repeat period of the tracks emanating from specific generation sites consistent with predictions from numerical models internal tidal beams 1 are as narrow as 200 km and 2 propagate a longer distance than previously observ

internal tides around the hawaiian ridge estimated from multisatellite altimetry 1 satellite altimetric sea surface height anomaly ssha data from geosat follow on gfo and european remote sensing ers as well as topex poseidon t p are merged to estimate m2 internal tides around the hawaiian ridge with higher spatial resolution than possible with single satellite altimetry the new estimates are compared with numerical model runs along track analyses show that m2 internal tides can be resolved from both 8 years of gfo and 15 5 years of ers ssha data comparisons at crossover points reveal that the m2 estimates from t p gfo and ers agree well multisatellite altimetry improves spatial resolution due to its denser ground tracks thus m2 internal tides can be plane wave fitted in 120 km 120 km regions compared to previous single satellite estimates in 4 lon 3 lat or 250 km 250 km regions in such small fitting regions the weaker and smaller scale mode 2 m2 internal tides can also be estimated the higher spatial resolution leads to a clearer view of the m2 internal tide field around the hawaiian ridge discrete generation sites and internal tidal beams are clearly distinguishable and consistent with the numerical model runs more importantly multisatellite altimetry produces larger m2 internal tidal energy fluxes which agree better with model results than previous single satellite estimates this study confirms that previous altimetric underestimates are partly due to the more widely spaced ground tracks and consequently larger fitting region multisatellite altimetry largely overcomes this limitation

surface manifestation of internal tides in the deep ocean observations from altimetry and island gauges the sea surface height signatures of internal tides in the deep ocean amounting to a few centimeters or less are studied using two complementary measurement types satellite altimetry and island tide gauges altimetry can detect internal tides that maintain coherence with the astronomical forcing island gauges can monitor temporal variability which in some circumstances is due to internal tides varying in response to changes in the oceanic medium this latter mechanism is at work at hilo and other stations on the northern coasts of the hawaiian islands by detecting spatially coherent low frequency internal tide modulations the tide gauges along with inverted echo sounders at sea suggest that the mean internal tide is also spatially coherent satellite altimetry confirms this at hawaii and in many other places topex poseidon altimetry detects mean surface waves spatially coherent and propagating great distances 1000 km before decaying below background noise when temporal variability is small the altimetry plus information on ocean density sets useful constraints on energy fluxes into internal tides at the hawaiian ridge 15 gw of tidal power is being converted from barotropic to first mode baroclinic motion examples elsewhere warn that a simplistic interpretation of the altimetry without regard to variability noise or in situ information may be highly misleading with such uncertainties extension of the hawaiian results into a usefully realistic estimate of the global internal tide energy balance appears premature at this time

global patterns of low mode internal wave propagation part i energy and energy flux abstract extending an earlier attempt to understand long range propagation of the global internal wave field the energy e and horizontal energy flux f are computed for the two gravest baroclinic modes at 80 historical moorings around the globe with bandpass filtering the calculation is performed for the semidiurnal band emphasizing m2 internal tides generated by flow over sloping topography and for the near inertial band emphasizing wind generated waves near the coriolis frequency the time dependence of semidiurnal e and f is first examined at six locations north of the hawaiian ridge e and f typically rise and fall together and can vary by over an order of magnitude at each site this variability typically has a strong spring neap component in addition to longer time scales the observed spring tides at sites northwest of the hawaiian ridge are coherent with barotropic forcing at the ridge but lagged by times consistent with travel at the theoretical mode 1 group speed from the ridge phase c

internal tides observed at 2 s 156 e by in situ and topex poseidon data during the coupled ocean atmosphere response experiment coare during the enhanced monitoring phase of the coupled ocean atmosphere experiment coare semidiurnal baroclinic tides were observed at 2 s 156 e 1739 m depth the tropical atmosphere ocean tao mooring outfitted with additional sensors from surface to bottom provides an interesting tool for describing the internal tides m2 s2 and n2 baroclinic components were detected explaining 59 of the semidiurnal variance on surface dynamic height at the semidiurnal frequency vertical displacements of isopycnals highly variable in time can reach 30 m and are mostly a combination of the m2 s2 and n2 internal waves the two first baroclinic modes explain 70 of the variance in vertical displacement m2 and s2 internal waves have different vertical structure m2 resembles the first baroclinic mode and s2 resembles the second baroclinic mode the m2 and s2 internal waves projected on their respective baroclinic mode are highly related to the modeled barotropic tide two topex poseidon tracks cross over the mooring every 10 days and the internal wave signature can be expected in sea level along the tracks despite the weakness of the signal sea level spectra along both ascending and descending tracks show the existence of energy around the 250 km and the 120 km wavelength respectively which is in accordance with a first baroclinic wave propagating northeastward as suggested in previous studies from wavelet transform the baroclinic tide signatures on the ascending descending track are centered around 2 5 s 3 5 s with a 1 2 latitudinal extension respectively limited to the ontong java plateau we confirm the northeastward propagation of the baroclinic tides and their source could be a series of islands parallel to the kilinailau trench or the large sloping topography between the kilinailau trench and the ontong java plateau

surface manifestation of internal tides generated near hawaii analysis of topex poseidon satellite altimetry reveals short wavelength fluctuations in the ocean surface tide that are attributable to internal tides a significant fraction of the semidiurnal internal tide generated at the hawaiian ridge is evidently phase locked to the astronomical potential and can modulate the amplitude of the surface tide by 5 cm the internal tide is thus easily mapped along satellite groundtracks and it is found to be spatially coherent over great distances with waves propagating well over 1000 km from the hawaiian ridge before decaying below noise level both first and second baroclinic modes are observed in both the m 2 lunar and s 2 solar tides the high space time coherence is in sharp contrast to what is often inferred from current meter observations but it confirms recent speculations from an acoustic experiment north of hawaii

semidiurnal tides observed in the western equatorial pacific during the tropical ocean global atmosphere coupled ocean atmosphere response experiment the semidiurnal tide within a 100 km square region of the western equatorial pacific centered at 1 8 s 156 1 e is examined using shipboard survey and mooring data collected during the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare baroclinic and barotropic tidal amplitudes and phases are estimated from the survey and mooring observations in the upper 300 m of the 1800 m deep ocean by specifying their horizontal and vertical structures the barotropic tide is assumed to have zero horizontal wavenumber over the domain while a component of the baroclinic tide that is phase locked to the barotropic tide is determined by a searching method using plane wave fits to the data the estimated barotropic tidal current is in good agreement with tide models derived from topex poseidon observations the plane wave analysis indicates a dominant mode one baroclinic wave propagating toward the northeast the second vertical mode can also be detected given the phase differences between the m2 and s2 constituents in the barotropic and baroclinic tides the source of the baroclinic tidal signal is determined to be about 320 km southwest of the observing region at a series of islands and shallow ridges the combined estimates of the barotropic and baroclinic tides typically account for only 40 60 of the observed semidiurnal band current variance in the mooring data indicating the high degree of temporal and spatial variability of the baroclinic tide in this region the results of this study suggest however that coherent barotropic and baroclinic tidal signals can be successfully distinguished in the deep ocean using shipboard survey data even when the data are limited to the upper 300 m

long range propagation of the semidiurnal internal tide from the hawaiian ridge abstract the northeastward progression of the semidiurnal internal tide from french frigate shoals ffs hawaii is studied with an array of six simultaneous profiling moorings spanning 25 5 37 1 n 1400 km and 13 yr long ocean topography experiment topex poseidon t p altimeter data processed by a new technique the moorings have excellent temporal and vertical resolutions while the altimeter offers broad spatial coverage of the surface manifestation of the internal tide s coherent portion together these two approaches provide a unique view of the internal tide s long range propagation in a complex ocean environment the moored observations reveal a rich time variable and multimodal internal tide field with higher mode motions contributing significantly to velocity displacement and energy in spite of these contributions the coherent mode 1 internal tide dominates the northeastward energy flux and is detectable in both moored and altimetric data over the entire array phase and group prop

computer simulations of alaskan neotectonics ongoing deformation and fault slip neotectonics in the alaskan bering sea region were modeled with thin plate finite element methods by computing anelastic strain rate stress and velocity fields which represent time averages over several seismic cycles models included variable crust and lithosphere thickness heat flow and elevation and incorporated all faults known or thought to be active three sets of experiments totaling 46 models were computed with systematic variations of shear traction on the megathrust fault friction within north america internal friction of lithospheric blocks and mantle creep strength model quality was scored by comparing predictions to geologic data on fault slip rates the most compressive horizontal principal stress direction and geodimeter and very long baseline interferometry vlbi baseline rates best results were obtained with a driving traction of only 10 15 mpa on the aleutian megathrust and fault friction of only 0 17 within north america this is clearly a low stress model the fanning of away from the syntaxis was reproduced but does not imply high deviatoric stress magnitudes within the north american plate the aleutian forearc west of kodiak island moves westward along strike at rates which increase to 65 mm yr 1 in the komandorsky region however no crust is presently escaping westward from the alaskan syntaxis instead it is shortened and stacked by thrusting principally in the vicinity of the chugach and wrangell mountains because stresses in this model are low and are delicately balanced against topography it is likely that the present regime represents a transient response to plio pleistocene glacial mass redistribution and differs from typical tectonic flow in the tertiary

solitary waves in the western equatorial pacific ocean during the spring tides of early january and february 1993 groups of solitary intemal waves were observed propagating through the intensive flux array of the toga coare experiment the waves appear to originate near the islands of nugarba 3 s 30 s 154 30 e they travel north eastward at 2 5 3 ds closely coupled with the semi diumal baroclinic tide peak amplitudes exceed 60 m velocities are in excess of 8 ds sea surface vertical displacements of order 3 m can be inferred directly from the lateral acceleration of surface waters the equatorial undercurrent is displaced by soliton passage but apparently is unaffected otherwise the intrinsic shear of the solitary crests is small compared to ambient equatorial shears the crests while not themselves unstable are effective at triggering instabilities on the background flow the motions potentially contribute 10 15 wattdm2 to the flux of heat into the mixed layer

non stationary internal tides observed with satellite altimetry 1 temporal variability of the internal tide is inferred from a 17 year combined record of topex poseidon and jason satellite altimeters a global sampling of along track sea surface height wavenumber spectra finds that non stationary variance is generally 25 or less of the average variance at wavenumbers characteristic of mode 1 tidal internal waves with some exceptions the non stationary variance does not exceed 0 25 cm2 the mode 2 signal where detectable contains a larger fraction of non stationary variance typically 50 or more temporal subsetting of the data reveals interannual variability barely significant compared with tidal estimation error from 3 year records comparison of summer vs winter conditions shows only one region of noteworthy seasonal changes the northern south china sea implications for the anticipated swot altimeter mission are briefly discussed

the generation of internal tides at the hawaiian ridge previous analyses of altimeter and acoustic data indicate that the hawaiian ridge is a significant generation site for semidiurnal m2 period internal waves owing to tidal flow over topography such waves likely play an important role connecting the dissipation of tidal energy to ocean mixing here a primitive equation numerical model with realistic stratification bathymetry and tidal forcing is used to simulate the generation of these waves in a manner consistent with altimeter sea surface observations the highest amplitude waves originate at three generation sites located at both the island and seamount portions of the chain these sites have in common both enhanced flows across ridge shaped topography and maxima in model calculated body force which couples barotropic and baroclinic motions the model vertical structure suggests the presence of multiple dynamical modes within 100 km of the ridge and not just the lowest modes that have been detected at the sea surface

indirect evidence for substantial damping of low mode internal tides in the open ocean a global high resolution ocean circulation model forced by atmospheric fields and the m2 tidal constituent is used to explore plausible scenarios for the damping of low mode internal tides the plausibility of different damping scenarios is tested by comparing the modeled barotropic tides with tpxo8 a highly accurate satellite altimetry constrained tide model and by comparing the modeled coherent baroclinic tide amplitudes against along track altimetry five scenarios are tested 1 a topographic internal wave drag argued here to represent the breaking of unresolved high vertical modes applied to the bottom flow default configuration 2 a wave drag applied to the barotropic flow 3 absence of wave drag 4 a substantial increase in quadratic bottom friction along the continental shelves with wave drag turned off and 5 application of wave drag to the barotropic flow at the same time that quadratic bottom friction is substantially increased along the shelves of the scenarios tested here the default configuration 1 yields the most accurate tides in all other scenarios 2 5 the lack of damping on open ocean baroclinic motions yields baroclinic tides that are too energetic and travel too far from their sources despite the presence of a vigorous mesoscale eddy field which can scatter and decohere internal tides in the model the barotropic tides are also less accurate in the absence of an open ocean damping on barotropic motions that is in scenarios 3 and 4 the results presented here suggest that low mode internal tides experience substantial damping in the open ocean

altimeter and current meter observations of internal tides do they agree abstract baroclinic tides can be observed both remotely from the ocean topography experiment topex poseidon t p altimeter and in situ using current meters however it is rare that current meter moorings have high vertical resolution and are located under t p ground tracks so that a direct comparison can be made between the two methods of observations here data from a current meter mooring directly under a t p ground track off the bounty plateau new zealand are used to obtain energy estimates of the first baroclinic mode these estimates are compared with those calculated from the t p surface elevation the two methods return estimates of the internal tide that are in agreement in phase and direction but have about a factor of 2 difference in amplitude and a factor of 4 difference in energy the flux estimates are 787 and 170 w m 1 respectively uncertainties in these estimates are relatively large and there is a low but not negligible probability that the differences are entirely due to measu

estimation of nonlinear soil behavior during the 1999 chi chi taiwan earthquake the 1999 chi chi taiwan earthquake mw 7 6 was one of the strongest earthquakes in recent years recorded by a large number of strong motion devices though only surface records are available the obtained strong motion database indicates the variety of ground responses in the near fault zones in this study accelerograms of the chi chi earthquake were simulated at rock and soil sites and models of soil behavior were constructed at seven soil sites tcu065 tcu072 tcu138 chy026 chy104 chy074 and chy015 for which parameters of the soil profiles are known down to depths of at least 70 m and at 24 other soil sites for which parameters of the soil profiles are known down to 30 40 m all the sites were located within 50 km from the fault for reconstructing stresses and strains in the soil layers we used a method similar to that developed for the estimation of soil behavior based on vertical array records as input for the soil layers acceleration time histories simulated by stochastic finite fault modelling with a prescribed slip distribution over the fault plane were taken in spite of the largeness of the earthquake s magnitude and the proximity of the studied soil sites to the fault plane the soil behavior at these sites was relatively simple i e a fairly good agreement between the spectra of the observed and simulated accelerograms and between their waveforms was obtained even in cases where a single stress strain relation was used to describe the behavior of whole soil thickness down to 70 80 m during strong motion obviously this is due to homogeneity in the characteristics of soil layers in depth at all the studied sites resonant phenomena in soil layers down to 40 60 m and nonlinearity of soil response were the main factors defining soil behavior at tcu065 tcu110 tcu115 chy101 chy036 and chy039 liquefaction phenomena occurred in the upper soil layers estimated strains achieved 0 6 0 8 at other stations maximum strains in the soil layers were as high as 0 1 0 4 according to our estimates thus valuable data on the in situ soil behavior during the chi chi earthquake was obtained similarity in the behavior of similar soils during the 1995 kobe 2000 tottori japan and chi chi taiwan earthquakes was found indicating the possibility of forecasting soil behavior in future earthquakes in the near fault zones of the three earthquakes hard type soil behavior and resonant phenomena in the upper surface layers prevail both leading to high acceleration amplitudes on the surface

inferring dynamics from the wavenumber spectra of an eddying global ocean model with embedded tides 1 the slopes of the wavenumber spectra of sea surface height ssh and kinetic energy ke have been used to infer interior or surface quasi geostrophic qg or sqg dynamics of the ocean however inspection of spectral slopes for altimeter ssh in the mesoscale band of 70 to 250 km shows much flatter slopes than the qg or sqg predictions over most of the ocean comparison of altimeter wavenumber spectra with spectra from an eddy resolving global ocean circulation model the hybrid coordinate ocean model hycom at 1 12 5 equatorial resolution which has embedded tides suggests that the flatter slopes of the altimeter ssh may arise from three possible sources 1 presence of strong internal tides 2 shift of the inertial sub range to smaller scales and 3 altimeter noise artificially adding noise to the model tends to flatten the spectra for low ke regions near internal tide generating regions spectral slopes in the presence of internal waves are much flatter than qg or sqg predictions separating the variability into high and low frequency around periods of 2 days then a different pattern emerges with a flat high frequency wavenumber spectrum and a steeper low frequency wavenumber spectrum for low mesoscale ke the inertial sub range defined by the nearly flat enstrophy band moves to smaller scales and the mesoscale band of 70 to 250 km no longer represents the inertial sub range the model wavenumber spectra are consistent with qg and sqg theory when internal waves and inertial sub range shifts are taken into consideration

the effects of altimeter instrument noise on the estimation of the wavenumber spectrum of sea surface height the wavenumber spectrum of sea surface height ssh observed by satellite altimetry was analyzed by xu and fu the spectral shape in the wavelength range of 70 250 km was approximated by a power law representing a regime governed by geostrophic turbulence theories the effects of altimeter instrument noise were assumed insignificant at wavelengths longer than 70 km the authors reexamined the assumption in the study using nearly simultaneous observations made by jason 1 and jason 2 during their cross calibration phase this study found that the white noise level of altimetry measurement was best estimated from the spectral values at wavelengths from 25 to 35 km after removing a white noise level based on such estimate from the ssh spectrum the spectral slope values changed significantly over most of the oceans a keyfinding is that the spectral slopes are generally steeper than k 22 k is wavenumber poleward of the 208 latitudes where flatter spectral slopes in some regions have previously caused problems for dynamic interpretations the new results indicate that the spectral slopes in the core regions of the major ocean current systems have values between the original geostrophic turbulence theory and the surface quasigeostrophic theory the near k 24 spectrum suggests that the sea surface height variability at these wavelengths in the high eddy energy regions might be governed by frontogenesis

estimates of internal tide energy fluxes from topex poseidon altimetry central north pacific energy fluxes for first mode m2 internal tides are deduced throughout the central north pacific ocean from topex poseidon satellite altimeter data temporally coherent internal tide signals in the altimetry combined with climatological hydrographic data determine the tidal displacements pressures and currents at depth which yield power transmission rates for a variety of reasons the deduced rates should be considered lower bounds internal tides are found to emanate from several large bathymetric structures especially the hawaiian ridge where the integrated flux amounts to about 6 gigawatts internal tides are generated at the aleutian trench near 172 w and propagate southwards nearly 2000 km

validating satellite altimeter measurements of internal tides with long term tao triton buoy observations at 2 s 156 e satellite altimetry is regularly used to map the global distribution of mode 1 internal tides validating altimeter measurements of internal tides with in situ data is difficult as the mooring observations typically are too short compared with the altimetry here taking advantage of the long term since 1999 tropical atmosphere ocean triangle trans ocean buoy network tao triton buoy observations at 2 s 156 e located at a topex poseidon crossover in a region of strong internal tides direct comparisons are made between altimeter and in situ measurements over a common 15 year period the hourly buoy data are decimated every 9 9 day to emulate altimeter measurements the altimeter and decimated buoy data are highly coherent at the aliased semidiurnal period 60 day and they agree well for the stationary semidiurnal internal tide for the nonstationary internal tide which consists of a significant annual cycle the demodulated amplitude from the altimeter appears to be contaminated by nontidal signals the demodulated amplitude from the decimated buoy data on the other hand is capable of resolving the temporal variability this suggests the potential of mapping nonstationary internal tides from satellites