bluuebunny/crossref_metadata_2025 huggingface dataset:
Atmospheric boundary layer rolls observed by the synthetic aperture radar aboard the ERS‐1 satellite
Authors: Alpers W. , Brümmer B.
Published: February 2004

Two synthetic aperture radar (SAR) images acquired by the European Remote Sensing Satellite ERS‐1 over the Jade‐Weser estuary in the German Bight of the North Sea on January 2 and 20, 1992, are analyzed. The images show sea surface manifestations of atmospheric boundary layer rolls. This is inferred from the orientation of the quasi‐periodic sea surface patterns which are aligned approximately with the wind direction, from the ratio of the wavelength of the patterns to the height of the boundary layer, and from the conditions encountered in the atmospheric boundary layer as measured quasi‐simultaneously by radiosondes. The atmospheric boundary layer rolls were generated by a dynamic instability on January 2 and by a thermal instability on January 20. For the first time, quantitative estimates of variations of the wind velocity at the sea surface associated with the atmospheric rolls are extracted from a spaceborne radar SAR image. It is shown that wind velocities derived from SAR image intensity variations are in agreement with theoretical estimates.
Evaluating models of sea state bias in satellite altimetry
Authors: Glazman Roman E. , Greysukh Alexander , Zlotnicki Victor
Published: February 2004

Investigations of the sea state bias (SSB) in altimeter measurements of sea surface height (SSH) have been reported by many authors based on aircraft, sea tower, and satellite‐borne observations. These investigations have resulted in several proposed algorithms of the form SSB = εH, where H is the significant wave height (SWH) and ε is a nondimensional function of wind speed U and SWH available from altimeter measurements. In the present work, on the basis of the full set of Geosat and an 8‐month set of TOPEX altimeter measurements, all known algorithms are examined and a conclusion is reached that the altimeter‐based U and SWH are insufficient to estimate the SSB correction with uniformly high accuracy. As a criterion of model performance we employ the value (called here the accuracy gain) by which the total variance of temporal changes in surface elevation is reduced owing to an SSB correction. This quantity is estimated for global data as well as for several selected regions of sufficiently large size. The linear geophysical model function (GMF) of the form ε = a0 + a1U is shown to yield an improvement over the simplest GMF with a constant ε. A three‐parameter linear form ε = a0 + a1U + a2H produces somewhat better results. A two‐parameter, physically based GMF relating ε to the pseudo wave age ξ (where ξ is estimated using altimeter wind and SWH) yields even higher accuracy, while the three‐parameter GMF of form ε = a0 + a1U + a2U2 yields the highest accuracy gain for global data sets. However, in terms of the SSB values, the difference between different GMFs is marginal, and the accuracy gain (as a measure of the SSB models performance) is shown to have serious deficiencies. We find that for all the SSB models, globally tuned empirical parameters often yield unacceptably poor results for certain regions in which local physical conditions differ from the global average: when the globally tuned GMFs are applied to such regions, the SSB‐related error in SSH may well exceed 5 cm.
Meteorological forcing and sea level variability in the Aegean Sea
Authors: Tsimplis M. N. , Vlahakis G. N.
Published: February 2004

The effect of atmospheric pressure and surface wind vector on sea level variability in the Aegean Sea is investigated. Daily values of sea level, atmospheric pressure, and surface wind covering the period 1984–1987 from five stations are analyzed. Principal component (PC) analysis is used to extract the spatially coherent signal from the sea level and atmospheric pressure time series, while rotary principal component analysis is used to extract the coherent part of surface wind. The first PC modes are subsequently used in multiple regression analysis in the frequency domain in order to examine the dependence of sea level fluctuations on each of the parameters. The response of sea level to atmospheric pressure is found to be frequency dependent. The response is overisostatic for lower frequencies and becomes underisostatic for higher frequencies approaching isostasy only at the limit of analysis (0.5 cycles/day). The contribution of the first principal wind component is found to be very small.
Turbulence structure over two‐dimensional bed forms: Implications for sediment transport
Authors: McLean S. R. , Nelson J. M. , Wolfe S. R.
Published: February 2004

Turbulence measurements from the flow over two‐dimensional fixed dune shapes are presented along with analysis and discussion of the ramifications of the observations for transport of sediment as bed load over bed forms. The spatial structure of the local transport rate determines the shape and stability of bed forms such as ripples and dunes, and the transport of sediment is a highly nonlinear process that is profoundly affected by the statistics of the temporal fluctuations in the near‐bed flow field. The measurements presented herein show strong spatial evolution of the joint probability distribution of the streamwise and bednormal fluctuating velocity components. Unlike measurements in uniform boundary layers, these distributions and the higher moments of the velocity do not scale with the local shear velocity, indicating that it is probably inappropriate to use the shear stress to characterize the sediment flux. This conclusion is supported by observations of sediment flux over a dune.
Sequential data assimilation with a nonlinear quasi‐geostrophic model using Monte Carlo methods to forecast error statistics
Authors: Evensen Geir
Published: February 2004

A new sequential data assimilation method is discussed. It is based on forecasting the error statistics using Monte Carlo methods, a better alternative than solving the traditional and computationally extremely demanding approximate error covariance equation used in the extended Kalman filter. The unbounded error growth found in the extended Kalman filter, which is caused by an overly simplified closure in the error covariance equation, is completely eliminated. Open boundaries can be handled as long as the ocean model is well posed. Well‐known numerical instabilities associated with the error covariance equation are avoided because storage and evolution of the error covariance matrix itself are not needed. The results are also better than what is provided by the extended Kalman filter since there is no closure problem and the quality of the forecast error statistics therefore improves. The method should be feasible also for more sophisticated primitive equation models. The computational load for reasonable accuracy is only a fraction of what is required for the extended Kalman filter and is given by the storage of, say, 100 model states for an ensemble size of 100 and thus CPU requirements of the order of the cost of 100 model integrations. The proposed method can therefore be used with realistic nonlinear ocean models on large domains on existing computers, and it is also well suited for parallel computers and clusters of workstations where each processor integrates a few members of the ensemble.
New production in the warm waters of the tropical Pacific Ocean
Authors: Peña M. Angélica , Lewis Marlon R. , Cullen John J.
Published: February 2004

The average depth‐integrated rate of new production in the tropical Pacific Ocean was estimated from a calculation of horizontal and vertical nitrate balance over the region enclosed by the climatological 26°C isotherm. The net turbulent flux of nitrate into the region was computed in terms of the climatological net surface heat flux and the nitrate‐temperature relationship at the base of the 26°C isotherm. The net advective transport of nitrate into the region was estimated using the mean nitrate distribution obtained from the analysis of historical data and previous results of a general circulation model of the tropical Pacific. The rate of new production resulting from vertical turbulent fluxes of nitrate was found to be similar in magnitude to that due to advective transport. Most (about 75%) of the advective input of nitrate was due to the horizontal transport of nutrient‐rich water from the eastern equatorial region rather than from equatorial upwelling. An average rate of new production of 14.5–16 g C m−2 yr−1 was found for the warm waters of the tropical Pacific region. These values are in good agreement with previous estimates for this region and are almost five times less than is estimated for the eastern equatorial Pacific, where most of the nutrient upwelling occurs.
Estimation of the real aperture radar modulation transfer function directly from synthetic aperture radar ocean wave image spectra without a priori knowledge of the ocean wave height spectrum
Authors: Jacobsen S. , Høgda K. A.
Published: February 2004

The phase and amplitude of the real aperture radar (RAR) modulation transfer function (MTF) are, applying both simulated and real synthetic aperture radar (SAR) image spectra, shown to strongly influence the SAR ocean wave imaging of range‐ (or near‐range) traveling wave systems. Conventionally, in situ measurement of the sea state has been used in connection with SAR estimation of the RAR MTF. In most cases, the SAR imaging has been simulated by varying the phase and amplitude of the transfer function until some criterium for best fit between the measured and simulated spectra is met. The main problem with this method is the need for in situ buoy measurements of the underlying ocean wave height spectrum. This paper proposes a new method for estimating the RAR MTF directly from the SAR ocean wave image spectrum. Hence the method differs from previously used methods in that it is independent of in situ measurements of the sea state. The only (weak) restriction is that the observed wave system is range‐ or near‐range traveling. On the basis of three range‐going profiles the RAR MTF phase and amplitude are estimated. Investigations using synthetic data reveal that the SAR image spectrum for realistic sea states is colored by the unknown transfer function to such an extent that the underlying wave spectral form is not critical. Experimentally, the phase and amplitude of the RAR modulation are computed using the Norwegian Continental Shelf Experiment 1988 data. It is shown that the phase is most important for the SAR spectral distribution. Typically, the phase is observed to be in the interval from 60° to 110° and the amplitude to be of the order of 10–18. Furthermore, it is shown from simulation studies that marked changes in real SAR image spectra crossing an atmospheric front are recreated when the measured MTF phase and amplitude are used. Eventually, the hydrodynamic modulation is also extracted from the RAR MTF data. Variations of the hydrodynamic MTF phase across the abovementioned front are focused on. The estimates confirm a consistent wind direction induced modulation on each side of the front. No marked trends are observed for the amplitude. The overall conclusion of the study is that the conformity between simulated and measured spectra is improved when measured RAR MTFs are incorporated in SAR imaging simulation procedures.
Larval dispersal via entrainment into hydrothermal vent plumes
Authors: Kim Stacy L. , Mullineaux Lauren S. , Helfrich Karl R.
Published: February 2004

One of the most intriguing ecological questions remaining unanswered about hydrothermal vents is how vent organisms disperse and persist. Because vent species are generally endemic and their habitat is patchy and ephemeral on time scales as short as decades, they must disperse frequently, presumably in a planktonic larval stage. We suggest that dispersal occurs not only in near‐bottom currents but also several hundred meters above the seafloor at the level of the laterally spreading hydrothermal plumes. Using a standard buoyant plume model and observed larval abundances near hydrothermal vents at 9°50′N along the East Pacific Rise, we estimate a mean vertical flux of approximately 100 vent larvae/h at a single black smoker. Larval abundances were extremely variable near vents, resulting in a range in estimated fluxes of at least an order of magnitude. The suitability of the plume model for these calculations was determined by releasing dyes (fluorescein and rhodamine) as larval mimics into a black smoker plume. The plume model predicted dye fluxes in the plume adequately, given the short averaging times of our measurements and the difficulty of sampling the plume centerline. Our calculations of substantial numbers of vent larvae entrained into the plume support the idea that transport in the lateral plume is an important mechanism of dispersal. Because vertical shear in flows above vents can cause larval dispersal trajectories in the plume to deviate considerably from those along the seafloor, larvae in the plume may have access to habitats that are unreachable by larvae in near‐bottom flows.
A model of ice shelf‐ocean interaction with application to the Filcher‐Ronne and Ross Ice Shelves
Authors: Nøst Ole Anders , Foldvik Arne
Published: February 2004

A simple analytical model has been developed to study the formation of Ice Shelf Water (ISW). ISW is assumed to flow as a buoyancy‐driven layer underneath the ice shelf. A relation between potential temperature and salinity in the ISW layer is calculated from the mass and energy balance. This temperature‐salinity relation is shown to depend only on the temperature and the salinity of the source water mass and to be practically independent of entrainment and melt rates. The model results are obtained without making any assumptions about entrainment and melt rates. The model is in good agreement with observations under the Ronne Ice Shelf, and it indicates that ISW in the Filchner Depression is formed from Western Shelf Water (WSW) with salinity higher than 34.75 practical salinity units. Such high‐salinity water is only observed in the Ronne Depression in the western part of the continental shelf. This implies a circulation of WSW, under the Filchner‐Ronne Ice Shelf, from the Ronne Depression into the Filchner Depression. Similarly, the model shows that the ISW observed under J9 at the Ross Ice Shelf has been formed from Low Salinity Shelf Water (LSSW) from the eastern parts of the Ross Sea continental shelf. LSSW must therefore circulate under the eastern parts of the Ross Ice Shelf.
A simple analytical model for brightness modulations caused by submarine sand waves in radar imagery
Authors: van der Kooij M. W. A. , Vogelzang J. , Calkoen C. J.
Published: February 2004

Presented here is a simple analytical model based on established physics of the magnitude of the hydrodynamic modulations caused by sand waves. The model describes the modulations of the radar backscatter when first‐order Bragg scattering is assumed. The major difference between this model and existing analytical models is that the specific shape of sand waves is incorporated. An assessment is made of the key parameters (shape, oceanographic, meteorological and radar) that influence the radar backscatter modulation. It is shown that depth, steepness of the slope, and height of the sand wave are the shape parameters that determine the radar backscatter modulation. The maximum backscatter modulation that can be found for sand waves in nature is approximately 3 dB. It is shown that sand waves in the North Sea near the Dutch coast have a linear relation between their heights and slopes. Implementation of this relation simplifies the model further. Furthermore, backscatter modulations calculated with two radar backscatter models are compared and discussed. The correlation between predictions and measurements with the airborne imaging radar of NASA's Jet Propulsion Laboratory is considered encouraging. Measurements from the images indicate a relation between sand wave height and brightness modulation similar to that predicted by the model.
Transient temperature and sea level response of a two‐dimensional ocean‐climate model to greenhouse gas increases
Authors: Harvey L. D. Danny
Published: February 2004

A two‐dimensional dynamical ocean model is coupled to an energy balance climate model and used to investigate the transient surface temperature and sea level response to greenhouse gas increases. For most experiments a step function surface‐troposphere heating perturbation of 6 W m−2 is applied. A transient reduction in the thermohaline overturning flux of 15–30% occurs in most experiments, although in some cases a near‐total circulation collapse occurs within the first 100 years and lasts 700–1000 years. The transient circulation decrease is attributed to the greater rate of downward penetration of the heating anomaly in downwelling rather than in upwelling regions and is not a result of reduced convection. For experiments in which the steady state circulation is qualitatively unchanged after a heating perturbation, the surface temperature response is roughly uniform with latitude, in the absence of ice and snow. However, in some cases a heating perturbation induces the transition from a one‐cell to a two‐cell overturning circulation, or causes a complete reversal in the direction of overturning when a single cell spans both hemispheres, causing marked latitudinal variations in the surface temperature response. The e‐folding response time scale τe for global mean surface temperature ranges from 10 to 142 years as the vertical diffusion coefficient Kν varies from 1×10−5 m2 s−1 to 5×10−4 m2 s−1, but the variation of τe with Kν is not uniform due to changes in the transient circulation response as Kν is varied. The one‐dimensional upwelling diffusion and pure diffusion models have a significantly faster surface transient response than the two‐dimensional (2‐D) model, due to transient weakening of thermohaline overturning in the 2‐D model, which dampens surface temperature warming. The presence of convection has a small effect on the transient response. Transient sea level rise is highly sensitive to Kν, with faster sea level rise associated with larger Kν and hence with slower surface warming. The steady state ocean warming is approximately uniform with depth and equal to the global mean surface warming (3°C for 6 W m−2 forcing) in most experiments, which produces a sea level rise of 2 to 3 m due to thermal expansion. The times required to reach 50% and 75% of the steady state sea level rise are 200–1100 years and 500–2400 years, respectively. However, if the unperturbed circulation consists of a single overturning cell and a second thermohaline overturning cell develops in response to a heating perturbation, vertical mean steady state deep ocean warming and associated sea level rise can be reduced by almost a factor of 10.
Particle trapping in estuarine tidal flows
Authors: Jay David A. , Musiak Jeffery D.
Published: February 2004

Particle trapping in estuarine turbidity maxima (ETM) is caused primarily by convergent mean and/or tidal fluxes of sediment. The result is an approximately bell‐shaped along‐channel distribution of vertically integrated, tidal cycle mean suspended sediment concentration. Observations from the Columbia River estuary suggest that (1) strong two‐layer or internal along‐channel residual and overtide flows are generated by time‐varying stratification and (2) correlations between the near‐bed velocity and the suspended sediment fields at these frequencies are important in landward transport of sediment. A new spatially and temporally integrated form of the sediment conservation equation has been derived to analyze this trapping process. Time changes in tidally averaged sediment concentration between two estuarine cross sections can be shown to be related to the divergence of the seaward, river flow transport; the divergence of velocity shear‐sediment stratification correlations for the mean flow and each tidal constituent; and net erosion or deposition at the bed. Vertically integrated variables other than seaward river transport are absent from this integrated balance. Analysis of sediment fluxes using this balance supports the idea that internal residual and overtide circulations are primarily responsible for the landward sediment transport on the seaward side of ETM found near the upstream limits of salinity intrusion. The balance also shows that attempts to represent fluxes causing trapping of sediment in an ETM as a product of a time‐mean, vertically integrated, along‐channel gradient and a diffusivity inevitably lead to the appearance of countergradient transport and thus a negative diffusivity on the seaward side of the ETM. This result occurs because the trapping process is inherently nonlinear and at least two‐dimensional and because a one‐dimensional representation is physically unrealistic.
On the statistics of the phase of microwave backscatter from the ocean surface
Authors: Chapman R. D. , Gotwols B. L. , Sterner R. E.
Published: February 2004

In this paper we describe what we believe is the first study of the distribution of the phase of microwave fields backscattered from the ocean surface. Scatterometry data from the Synthetic Aperture Radar and X band Ocean Nonlinearities experiment conducted on the German North Sea Research Platform, Forschungsplatform Nordsee, in November 1990 have been analyzed to reveal the distribution of phase differences as a function of time lag. A theoretical model for these statistics is presented based on modulated Gaussian fields. This theoretical model is shown to be in good agreement with the measured statistics. From this agreement we conclude that the backscattered fields have a Gaussian distribution on short time scales but are modulated in amplitude and frequency by the long surface waves. These results are of more than purely academic interest, with direct applications to the design and analysis of interferometric synthetic aperture radars, a relatively new class of instruments that may be capable of providing high‐resolution maps of ocean surface currents from aircraft or satellites.
Global low harmonic degree models of the seasonal variability and residual ocean tides from TOPEX/POSEIDON altimeter data
Authors: Knudsen Per
Published: February 2004

The large‐scale seasonal variability is estimated jointly with the mean sea surface and the ocean tide signals not recovered by the Cartwright and Ray ocean tide model. This is done in a global analysis where spherical harmonic expansions to degree 18 of the components are estimated simultaneously using 34 cycles of TOPEX/POSEIDON altimeter data. The results show that the amplitude of the annual signal at these wavelengths has a magnitude of 5.1 cm. On the northern hemisphere the annual variability is found to reach its maximum values (10–15 cm) in October. On the southern hemisphere the phases are shifted 6 months. Semiannual variability of 5 cm is found in the western part of the equatorial Pacific and has its maximum values in April and in October. The ocean tide residuals corrections are small: 3.0 cm (M2), 1.7 cm (S2), 1.8 cm (K1), and 1.0 cm (O1), all root‐mean‐square, respectively. However, M2residuals of 6–8 cm are found. A computation of formal errors shows that the accuracies of the estimated quantities are about 1 cm. Comparisons with in situ data from open ocean tide gauge stations show that the annual cycle and the M2ocean tide fit within 3 cm and that the other quantities fit the in situ data within 2 cm.
Mean sea surface height of the Antarctic Circumpolar Current from Geosat data: Method and application
Authors: Gille Sarah T.
Published: February 2004

The mean sea surface height across the Antarctic Circumpolar Current has been reconstructed from height variability measured by the Geosat altimeter without assuming prior knowledge of the geoid. For this study, an automated technique has been developed to estimate mean sea surface height for each satellite ground track using a meandering Gaussian jet model, and errors have been estimated using Monte Carlo simulation. The results are objectively mapped to produce a picture of the mean Subantarctic and Polar Fronts, which together comprise the major components of the Antarctic Circumpolar Current. The meandering jet model explains between 40% and 70% of the height variance along the jet axes. The results show that the fronts are substantially steered by topography and that the jets have an average Gaussian width of about 44 km in the meridional direction and meander about 75 km to either side of their mean locations. The average height difference across the Subantarctic Front (SAF) is 0.7 m and across the Polar Front (PF) 0.6 m. The mean widths of the fronts are correlated with the size of the baroclinic Rossby radius.
Observations and modeling of thermally induced stresses in first‐year sea ice
Authors: Lewis James K. , Tucker Walter B. , Stein Peter J.
Published: February 2004

During spring 1992, ice property, geophone, meteorological, and stress data were collected on first‐year ice southwest of Cornwallis Island within the Canadian archipelago. One of the goals of the study was to specify the average characteristics of the ice, use these characteristics in a model of thermally induced stresses in the ice, and examine the fracturing associated with the occurrence of those stresses. The results of simulations with a thermal stress model indicate that stress variations within the ice can be reasonably approximated by a viscoelastic rheology. The rheology takes into consideration thermally induced strains generated locally as well as strains generated elsewhere and then mechanically transmitted through the ice. The geophone data showed both ice‐borne and water‐borne propagation paths for individual fracturing events. The data imply a detection radius out to 500–600 m for the ice‐borne signatures of fractures. An investigation of a region after fracturing showed that (1) fracturing occurred in an area with a 10‐ to 15‐cm snow cover, (2) the snow cover had been scored down to the surface of the ice, and (3) cracks in the ice were found under each location where the snow had been scored. The cracks were 5–6 m long and at least 15 cm deep. A review of these and other experimental results draw us to the conclusion that the forces required to produce fractures in response to natural forcing is greater for first‐year floes than for multiyear floes.
Observation of melt onset on multiyear Arctic sea ice using the ERS 1 synthetic aperture radar
Authors: Winebrenner D. P. , Nelson E. D. , Colony R. , West R. D.
Published: February 2004

We present nearly coincident observations of backscattering from the ERS 1 synthetic aperture radar (SAR) and of near‐surface temperature from six drifting buoys in the Beaufort Sea, showing that the onset of melting in snow on multiyear sea ice is clearly detectable in the SAR data. Melt onset is marked by a clean, steep decrease in the backscattering cross section of multiyear ice at 5.3 GHz and VV polarization. We investigate the scattering physics responsible for the signature change and find that the cross section decrease is due solely to the appearance of liquid water in the snow cover overlying the ice. A thin layer of moist snow is sufficient to cause the observed decrease. We present a prototype algorithm to estimate the date of melt onset using the ERS 1 SAR and apply the algorithm first to the SAR data for which we have corresponding buoy temperatures. The melt onset dates estimated by the SAR algorithm agree with those obtained independently from the temperature data to within 4 days or less, with the exception of one case in which temperatures oscillated about 0°C for several weeks. Lastly, we apply the algorithm to the entire ERS 1 SAR data record acquired by the Alaska SAR Facility for the Beaufort Sea north of 73°N during the spring of 1992, to produce a map of the dates of melt onset over an area roughly 1000 km on a side. The progression of melt onset is primarily poleward but shows a weak meridional dependence at latitudes of approximately 76°–77°N. Melting begins in the southern part of the study region on June 13 and by June 20 has progressed to the northernmost part of the region.
ImprovedM2ocean tide from TOPEX/POSEIDON and Geosat altimetry
Authors: Wagner C. A. , Tai C. K. , Kuhn J. M.
Published: February 2004

TOPEX/POSEIDON (T/P) collinear differences have been used to extract errors in theM2tide model of Cartwright and Ray (1991). The fluctuation of the power of these differences clearly shows the need for corrections at a period of about 60 days, close to the aliased periods of theM2andS2tides. The level of the necessary changes at this period is about 4 cm (rms). Further detailed analysis of collinear differences in 4°×4° bins, allowing for determination of up to 6 luni‐solar and “ocean weather” tides (annual and semiannual), shows that theM2corrections are well discriminated from the others and have a global power of 3.5 cm (rms). Both in‐phase and out‐of‐phase components of the corrections have similar broad scale characteristics dominated by a few meridionally oriented bands. The T/P‐derivedM2model shows rms deviations of 3.1 cm with data at 80 selected ocean island and bottom gauges compared with 3.9 cm for the Cartwright and Ray (1991) model. A weighted combination of Geosat and T/P‐derived models achieves residuals of 2.8 cm (rms) with this global tide gauge data.
A southern hemisphere verification for the TOPEX/POSEIDON satellite altimeter mission
Authors: White Neil J. , Coleman Richard , Church John A. , Morgan P. J. , Walker S. J.
Published: February 2004

As a check on the total system accuracy of the TOPEX/POSEIDON satellite, a southern hemisphere verification study was undertaken in Bass Strait (41°S) immediately adjacent to the Southern Ocean. An acoustic tide gauge and two pressure gauges were installed on the southern side of Bass Strait near where descending pass 88 crosses the Tasmanian coast. The tide gauge benchmark height was estimated relative to the laser tracking station at Orroral Valley and the very long baseline interferometry station near Hobart using Global Positioning System surveying techniques. In the TOPEX/POSEIDON geophysical data records, flags on the altimeter data began to be set 15–20 km from land and little useful altimeter data were available within about 13 km of the coast. The acoustic tide gauge heights were corrected for changes in tidal phase and amplitude between the coast and the offshore subsatellite point using results from a numerical model. The altimeter correction for atmospheric water vapor calculated from the TOPEX microwave radiometer data and the insitu precipitable water data agreed to 2 cm RMS, with no significant bias. The RMS variability between the insitu sea level estimates and the TOPEX/POSEIDON estimates was less than 3.5 cm. As part of this variability is caused by inaccuracies in the estimates of the sea level variations between the coast and the subsatellite point, the result indicates that the precision of the TOPEX/POSEIDON sea level height measurement system, including the time dependent orbit errors, the ionospheric corrections and the wet and dry tropospheric path delays, is less than 3.5 cm over Bass Strait, considerably below the original system specifications of 13 cm. Our estimate of the altimeter bias (including geographically correlated orbit error) is −25±10 cm, consistent with the results at the NASA (Harvest Platform) and Centre National d'Etudes Spatiales (Lampedusa) calibration sites. However, accurate estimation of this total altimeter bias is confounded by the lack of altimeter data close to land and in particular, limitations in the available local geoid data. The altimeter bias shows no significant drift at this site.
Sea surface temperature variability of the Iceland‐Faeroe front: Correction
Authors: Warn‐Varnas Alex , Niiler Pearn P. , Zahorodny Malcolm , Piacsek Steve , Neuberg Lucas
Published: February 2004

In a previous study by Niiler et al. (1992), advanced very high resolution radiometer (AVHRR) images were used for locating the Iceland‐Faeroe front temperature distributions and gradients. In that study the satellite‐sensed sea surface temperatures were found to contain an error after an atmospheric correction algorithm was applied. This was determined through a regression fit of AVHRR data against in situ aircraft‐deployed expendable bathythermograph (AXBT) and conductivity‐temperature‐depth measurements. Recently, Minnett (1993) pointed out that an error occurred in that fit. As a result, we decided to repeat the comparisons of AVHRR sea surface temperature (SST) data against in situ measurements. After correcting several errors, we still find a shift of about 2°C in the scatter of AVHRR against AXBT SST data. The bias (mean difference) was 1.73°C, and the standard deviation was 0.3°C.
Estimating the sea state bias of the TOPEX and POSEIDON altimeters from crossover differences
Authors: Gaspar Philippe , Ogor Françoise , Le Traon Pierre‐Yves , Zanife Ouan‐Zan
Published: February 2004

TOPEX (Ku band) and POSEIDON altimeter measurements at crossover points are used to estimate the sea state bias (SSB) of these two instruments. Different SSB models are tested, ranging from a constant fraction of the significant wave height (SWH) to more elaborate models involving up to four adjustable parameters. For TOPEX, the data show a decrease in the magnitude of the relative bias (SSB/SWH) with SWH. This behavior is well reproduced using a simple empirical model with two adjustable parameters. The three‐parameter SSB model used in the NASA geophysical data records does well in explaining the wind‐induced variations of the bias. A model including four adjustable parameters is needed to account for both the wind‐ and SWH‐related variability of the SSB. POSEIDON data analysis reveals a significantly larger SSB than for TOPEX. This bias seems to consist of a skewness plus tracker bias of −2 to −3% of SWH superimposed on a natural EM bias whose wind‐ and SWH‐related variations are similar to those of TOPEX.
A preliminary tidal analysis of TOPEX/POSEIDON altimetry
Authors: Schrama E. J. O. , Ray R. D.
Published: February 2004

Approximately 12 months of data from the TOPEX/POSEIDON satellite altimeter mission are analyzed for the major short‐period oceanic tides. A harmonic analysis is performed on data captured within bins defined on a deep‐ocean grid, which, owing to tidal aliasing considerations, must have a relatively coarse spatial resolution. Our analysis is in terms of corrections to the Schwiderski and Cartwright‐Ray models, and it confirms many of the Schwiderski differences previously reported by Cartwright and Ray. Our differences with respect to the Geosat‐based Cartwright and Ray model form a sectorial pattern in M2with high/low differences separated roughly 180° in longitude. We suggest that these sectorial errors were most likely induced by Geosat's relatively large orbit error. Comparisons to independent data validate the improved TOPEX/POSEIDON solutions; in situ “ground truth” shows M2RMS differences of 4.10cm (Schwiderski), 3.86cm (Cartwright and Ray), 2.63cm (this paper). Global rates of energy dissipation confirm earlier estimates for M2, and show improved agreement with satellite tracking studies for K1and S2. These preliminary exercises confirm that TOPEX/POSEIDON should result in a new generation of improved global tidal models.
Spatial sampling requirements for tropical Pacific sea surface temperature variability
Authors: Nakamoto S. , Fang Z. , Matsuura T. , Kawano T. , Kashino Y. , Muneyama K. , Nakanishi T.
Published: February 2004

The Tropical Ocean and Global Atmosphere‐Tropical Atmosphere Ocean (TOGA‐TAO) array of moored buoys is a real‐time ocean‐observing system which measures wind and upper ocean temperature and currents in the equatorial Pacific Ocean. The mean square error for TOGA‐TAO sea surface temperature (SST) due to spatial sampling of 1500 km zonally and 200 km meridionaliy is expressed as the frequency‐wavenumber integral of a sampling design dependent filter multiplied by the space‐time spectral density of the SST field. Ten years of NOAA advanced very high resolution radiometer SST data, on a 1° by 1° by 1‐day grid, were used to obtain space‐time statistics. The zonal correlation length decreases with latitude. The zonal correlation length is 2000 km at the equator and 1500 km at 4°N. The temporal correlations are 90 days at all latitudes from 0°N to 5°N. Thé frequency‐wavenumber spectrum decreases as the inverse square of the wavenumber in the low‐wavenumber domain and as the inverse cube of the frequency in the low‐frequency domain. By using the sampling error formalism and the frequency‐wavenumber spectrum obtained from satellite‐derived SST, the sampling errors of the area‐ and time‐averaged SST over the averaging box of 15° zonally by 2° meridionally by 1 month were found to be about 12% of the variance of the area‐averaged SST for the TOGA‐TAO array spacing.
The response of the Irish Sea to boundary and wind forcing: Results from a three‐dimensional hydrodynamic model
Authors: Davies A. M. , Lawrence J.
Published: February 2004

A three dimensional, nonlinear hydrodynamic model of the Irish Sea is used to examine the response of the region to forcing along its open boundary and wind stress over the area. Frictional dissipation at the bed is parameterized with a quadratic law of bottom friction, which can take account of enhanced frictional effects due to wind wave/current interaction. A standard finite difference grid is used in the horizontal, with an expansion of functions through the vertical, giving a continuous current profile from sea surface to seabed. The model has a grid resolution of the order of 3 km and hence can resolve some features of the nearshore region. The model is forced with M2 tidal input along its open boundary, and time invariant wind fields, corresponding initially to spatially uniform northerly and westerly wind stresses of 1 N/m2 and subsequently to a west‐east gradient wind changing from −1 N/m2 to +1 N/m2 going from west to east, and a north‐south gradient wind changing from 1 N/m2 to −1 N/m2 going from north to south. The response to open boundary forcing through both elevations and currents is also examined, and the influence of the effects of enhanced bottom friction due to wind waves is also briefly considered.
The roles of vertical mixing, solar radiation, and wind stress in a model simulation of the sea surface temperature seasonal cycle in the tropical Pacific Ocean
Authors: Chen Dake , Busalacchi Antonio J. , Rothstein Lewis M.
Published: February 2004

The climatological seasonal cycle of sea surface temperature (SST) in the tropical Pacific is simulated using a newly developed upper ocean model. The roles of vertical mixing, solar radiation, and wind stress are investigated in a hierarchy of numerical experiments with various combinations of vertical mixing algorithms and surface‐forcing products. It is found that the large SST annual cycle in the eastern equatorial Pacific is, to a large extent, controlled by the annually varying mixed layer depth which, in turn, is mainly determined by the competing effects of solar radiation and wind forcing. With the application of our hybrid vertical mixing scheme the modelsimulated SST annual cycle is much improved in both amplitude and phase as compared to the case of a constant mixed layer depth. Beside the strong effects on vertical mixing, solar radiation is the primary heating term in the surface layer heat budget, and wind forcing influences SST by driving oceanic advective processes that redistribute heat in the upper ocean. For example, the SST seasonal cycle in the western equatorial Pacific basically follows the semiannual variation of solar heating, and the cycle in the central equatorial region is significantly affected by the zonal advective heat flux associated with the seasonally reversing South Equatorial Current. It has been shown in our experiments that the amount of heat flux modification needed to eliminate the annual mean SST errors in the model is, on average, no larger than the annual mean uncertainties among the various surface flux products used in this study. Whereas a bias correction is needed to account for remaining uncertainties in the annual mean heat flux, this study demonstrates that with proper treatment of mixed layer physics and realistic forcing functions the seasonal variability of SST is capable of being simulated successfully in response to external forcing without relying on a relaxation or damping formulation for the dominant surface heat flux contributions.
TOPEX/POSEIDON mission overview
Authors: Fu Lee‐Lueng , Christensen Edward J. , Yamarone Charles A. , Lefebvre Michel , Ménard Yves , Dorrer Michel , Escudier Philippe
Published: February 2004

TOPEX/POSEIDON is the first space mission specifically designed and conducted for studying the circulation of the world's oceans. The mission is jointly conducted by the United States and France. A state‐of‐the‐art radar altimetry system is used to measure the precise height of sea level, from which information on the ocean circulation is obtained. The satellite, launched on August 10, 1992, has been making observations of the global oceans with unprecedented accuracy since late September 1992. To meet the stringent measurement accuracy required for ocean circulation studies, a number of innovative improvements have been made to the mission design, including the first dual‐frequency space‐borne radar altimeter capable of retrieving the ionospheric delay of the radar signal, a three‐frequency microwave radiometer for retrieving the signal delay caused by the water vapor in the troposphere, an optimal model of the Earth's gravity field and multiple satellite tracking systems for precision orbit determination. Additionally, the satellite also carries two experimental instruments to demonstrate new technologies: a single‐frequency solid‐state altimeter for the technology of low‐power, low‐weight altimeter and a Global Positioning System receiver for continuous, precise satellite tracking. The performance of the mission's measurement system has been tested by numerous verification studies. The results indicate that the root‐sum‐square accuracy of a single‐pass sea level measurement is 4.7 cm for the TOPEX system and 5.1 cm for the POSEIDON system; both are more than a factor of 2 better than the requirement of 13.7 cm. This global data set is being analyzed by an international team of 200 scientists for improved understanding of the global ocean circulation as well as the ocean tides, geodesy, and geodynamics, and ocean wind and waves. The mission is designed to last for at least 3 years with a possible extension to 6 years. The multiyear global data set will go a long way toward understanding the ocean circulation and its variability in relation to climate change. A summary of the mission's systems and their performance as well as the mission's science team is presented in the paper.
Evaluation of the TOPEX/POSEIDON dual‐frequency ionosphere correction
Authors: Imel David A.
Published: February 2004

The NASA altimeter on board TOPEX/POSEIDON exploits the difference in the delays of the Ku and C band radar pulses to estimate an ionosphere correction to the range measurement. The dependence of the ionosphere correction on ocean and satellite parameters is less than 1 cm. The standard deviation of the 1‐s averaged ionosphere correction depends on the height of the ocean waves and ranges from 5 to 14 mm. The accuracy of the ionosphere correction is better than 1 cm at the 1σ confidence level. The ionosphere correction should be averaged over 140 km (20 s) along track in order to minimize its noise without sacrificing its accuracy. Ionosphere models must achieve an independent sample spacing of 900 km or less in order to allow a single‐frequency altimeter to have an ionosphere correction comparable in accuracy to that of the NASA dual‐frequency altimeter.
Aliased tidal errors in TOPEX/POSEIDON sea surface height data
Authors: Schlax Michael G. , Chelton Dudley B.
Published: February 2004

Alias periods and wavelengths for the M2, S2, N2, K1, O1, and P1 tidal constituents are calculated for TOPEX/POSEIDON. Alias wavelengths calculated in previous studies are shown to be in error, and a correct method is presented. With the exception of the K1 constituent, all of these tidal aliases for TOPEX/POSEIDON have periods shorter than 90 days and are unlikely to be confounded with long‐period sea surface height signals associated with real ocean processes. In particular, the correspondence between the periods and wavelengths of the M2 alias and annual baroclinic Rossby waves that plagued Geosat sea surface height data is avoided. The potential for aliasing residual tidal errors in smoothed estimates of sea surface height is calculated for the six tidal constituents. The potential for aliasing the lunar tidal constituents M2, N2, and O1 fluctuates with latitude and is different for estimates made at the crossovers of ascending and descending ground tracks than for estimates at points midway between crossovers. The potential for aliasing the solar tidal constituents S2, K1, and P1 varies smoothly with latitude. S2 is strongly aliased for latitudes within 50 degrees of the equator, while K1 and P1 are only weakly aliased in that range. A weighted least squares method for estimating and removing residual tidal errors from TOPEX/POSEIDON sea surface height data is presented. A clear understanding of the nature of aliased tidal error in TOPEX/POSEIDON data aids the unambiguous identification of real propagating sea surface height signals. Unequivocal evidence of annual period, westward propagating waves in the North Atlantic is presented.
Identification of diapycnal mixing through optimum multiparameter analysis: 2. Evidence for unidirectional diapycnal mixing in the front between North and South Atlantic Central Water
Authors: Klein Birgit , Tomczak Matthias
Published: February 2004

Optimum multiparameter (OMP) analysis is used to analyze mixing in the central water boundary of the tropical North Atlantic Ocean. Diapycnal mixing is found to be prevalent in the frontal region. OMP analysis shows that the mixing is unidirectional (South Atlantic Central Water is always mixed upward into North Atlantic Central Water) but cannot identify the process responsible for the observed diapycnal mixing. A histogram of stability ratios Rρ for all mixing lines shows Rρ values between unity and the value found in the parent water masses. It is suggested that this may indicate competition between isopycnal mixing and double diffusion. Double diffusive fluxes are likely to make a recognizable and significant contribution to diapycnal mixing between the Central Waters.