Multivariate Objective Analysis Research Articles

Comparison of Mean Dynamic Topography Modeling from Multivariate Objective Analysis and Rigorous Least Squares Method

Filtering methods are usually used to combine the mean sea surface (MSS) and geoid (computable by global geopotential model (GGM)) into a common subspace, to model mean dynamic topography (MDT), which may lead to signal leakage and distortion problems. The use of the rigorous least squares (LS) method and multivariate objective analysis (MOA) alleviates these problems, and the derived MDTs from these two methods show better performance than MDTs derived from filtering methods. However, the advantages and disadvantages of these two methods have not been evaluated, and no direct comparison has yet been conducted between these two approaches regarding the performances in MDT recovery. In this study, we compare the performances of the MOA method with the LS method, providing information with respect to the usability of different methods in MDT modeling over regions with heterogeneous ocean states and hydrological conditions. We combined a recently published mean sea surface called DTU21MSS, and a satellite-only GGM named GO_CONS_GCF_2_DIR_R6, for MDT computation over four typical study areas. The results showed that the MDTs derived from the LS method outperformed the MOA method, especially over coastal regions and ocean current areas. The root mean square (RMS) of the discrepancies between the LS-derived MDT and the ocean reanalysis data was lower than the RMS of the discrepancies computed from the MOA method, by a magnitude of 1–2 cm. The formal error of the MDT estimated by the LS method was more reasonable than that derived from the MOA method. Moreover, the geostrophic velocities calculated by the LS-derived MDT were more consistent with buoy data than those calculated by the MOA-derived solution, by a magnitude of approximately 1 cm/s. The reason can be attributed to the fact that the LS method forms the design matrix segmentally, based on the error characteristics of the GGM, and suppresses high-frequency noise by applying constraints in different frequency bands, which improves the quality of the computed MDT. Our studies highlight the superiority of the LS-derived method versus the MOA method in MDT modeling.

Coastal Mean Dynamic Topography Recovery Based on Multivariate Objective Analysis by Combining Data from Synthetic Aperture Radar Altimeter

MDT recovery over coastal regions is challenging, as the mean sea surface (MSS) and geoid/quasi-geoid models are of low quality. The altimetry satellites equipped with the synthetic aperture radar (SAR) altimeters provide more accurate sea surface heights than traditional ones close to the coast. We investigate the role of using the SAR-based MSS in coastal MDT recovery, and the effects introduced by the SAR altimetry data are quantified and assessed. We model MDTs based on the multivariate objective analysis, where the MSS and the recently released satellite-only global geopotential model are combined. The numerical experiments over the coast of Japan and southeastern China show that the use of the SAR-based MSS improves the local MDT. The root mean square (RMS) of the misfits between MDT-modeled with SAR altimetry data and the ocean data is lower than that derived from MDT computed without SAR data—by a magnitude of 4–8 mm. Moreover, the geostrophic velocities derived from MDT modeled with the SAR altimetry data have better fits with buoy data than those derived from MDT modeled without SAR data. In total, our studies highlight the use of SAR altimetry data in coastal MDT recovery.

Synergy between surface drifters and altimetry to increase the accuracy of sea level anomaly and geostrophic current maps in the Gulf of Mexico

Abstract Knowledge of ocean surface geostrophic circulation has been greatly improved due to satellite observations. However, it is essential to use different observation sources (satellites and in situ data) in order to improve accuracy and resolution. This study combines along-track Sea Level Anomalies (SLA) with geostrophic velocity estimated from surface drifters to map SLA and associated geostrophic current anomalies in the Gulf of Mexico. Firstly, substantial pre-processing is needed on the drifter data to extract the geostrophic component of the signal in order to be consistent with the physical content provided by altimetry. This step includes estimating and removing the Ekman current, Stokes drift and wind slippage. Three kinds of drifters are used: – Drifters belonging to Woods Hole Group, a CLS Group company that launches their own drifters in the Gulf of Mexico. – Drifters launched in the framework of the Lagrangian Submesoscale ExpeRiment (LASER) campaign (January-April 2016). – Drifters from Atlantic Oceanographic and Meteorological Laboratory (AOML). Secondly, drifters and along-track SLA from Jason2, HY2, SARAL and Cryosat-2 are combined through multivariate objective analysis to map a daily time series of SLA and associated geostrophic current anomalies from 01/09/2015 to 30/04/2016. Finally, comparisons with independent data reveal the improved agreement of maps combining both altimetry and drifter data, especially for the meridional component of geostrophic current.

Near-Surface Currents in DeSoto Canyon (1997–99): Comparison of Current Meters, Satellite Observation, and Model Simulation

This study evaluates a data-assimilated model simulation of near-surface circulation in DeSoto Canyon (DSC), Gulf of Mexico, with emphasis on analyzing moored current-meter observations and comparing them with satellite data and model results. The study period is for two years from April 1997 to April 1999. The model results are from a high-resolution Gulf of Mexico model forced by analyzed wind and surface heat flux. Two types of data are used to deduce near-surface circulation: moored current meters at 13 locations in the DSC, and satellite sea level anomaly. The moored currents are mapped through multivariate objective analysis to produce surface currents and surface geopotentials, against which satellite- and model-derived sea surface heights and geostrophic currents are compared. Coupled patterns between the observations, model results, and satellite data are obtained using the singular value decomposition (SVD) analysis. There are two dominant modes: a ‘‘single-eddy’’ mode, in which currents are concentrated at the foot of the canyon, and an ‘‘eddy-pair’’ mode, in which one eddy is at the foot of the canyon and the other, a counterrotating eddy, is over the head of canyon. Mode 1 appears to be associated with the mesoscale eddy traveling around the Loop Current crest and trough, and mode 2 is associated with the intrusion of Loop Current crest and trough over the west Florida shelf. The observed and model currents are in good agreement about the means and variances. The model currents also appear to be well constrained by the steep topography. However, the model velocity field contains only the first mode. The satellite-derived velocity field, on the other hand, contains both the first and second modes; though, the satellite field does not adequately resolve the velocity structures over the slope.

Multivariate objective analysis of the coastal circulation of Barbados, West Indies: implication for larval transport

A multivariate spatial objective analysis (MVOA) assimilating high spatio-temporal resolution of hydrographic (CTD) and acoustic (ADCP) observations near Barbados provided a comprehensive view of the local surface circulation (0–100 m) during early spring of two consecutive years (1996 and 1997). Significant submesoscale fluctuations of the velocity and salinity fields exhibit a very dynamic environment. In the middle of each cruise, low-salinity water originating from the Amazon and entrained by a North Brazil Current Ring (NBCR) intruded from offshore and persisted on the west coast of Barbados throughout the rest of the survey. Principal component analysis (PCA) of velocity relative to the vertical structure and temporal factors in the study area demonstrated that the local circulation was mostly baroclinic and was dominated by a strong salinity front impinging on the island and large amplitude current reversals with a periodicity of ca. 20 d. During transition times, indicated by a change of the sign of the amplitude of the empirical orthogonal function (EOF), the flow became barotropic. This situation produced strong southward currents followed by the onset of vertical velocity shear. Most of the flow variability occurred in the upper 40 m of the water column, which was also found to be the depth of penetration of the low-salinity lenses. These results indicate that the NBCR structure was retained during both intrusions. Lagrangian trajectories using the MVOA currents were found to be consistent with in situ drifter trajectories, suggesting that the analyzed flow field is representative of the near-shore circulation. Tracking of particles released in the surface layer (0–20 m) from the reef shows a maximum residence time of 18 d indicating the possibility of larval retention within the island-scale flow field. Finally, our results suggest that MVOA, within its limitations, is a powerful tool that can be applied elsewhere to infer circulation and larval transport, even in situations when forcing is unknown.

Multivariate objective analysis of wind and height fields in the tropics

The commonly used objective analysis scheme (Scheme-A) for the analysis of wind and geopotential height smoothen the divergent component of the wind which is rather important in the tropics, specifically over convective regions. To overcome this deficiency, a new analysis scheme in which divergent component is included in the statisti-cal model of the wind forecast errors, has been proposed by Daley (1985). Following this scheme, a new set of correla-tion functions of forecast errors for the Indian region during monsoon season which are suitable for analysing the tropical wind are obtained. This analysis scheme (Scheme-B) as well as Scheme-A were used to make analyses for the period from 4 July to 8 July 1979 (12 GMT) at 850, 700 and 200 hPa levels over an area bounded by 1.875°N to 39.375°N and 41.250°E to 108.750°E and subsequently divergent component, velocity potential are computed for both schemes. Results from both these schemes show that in the monsoon depression region the velocity potential and divergence have increased in the later case (Scheme-B). This suggests that the divergent component has been en-hanced in Scheme-B and that the objective of this study is realized to some extent.

Some experiments with multivariate objective analysis scheme of heights and winds using optimum interpolation

A two-dimensional, multitvariate objective analysis scheme for simultaneous analysis of geopotential height and wind fields has been developed over Indian and adjoining region for use in numerical weather prediction. The height-height correlations calculated using daily data of four July months (1976–1979), are used to derive the other autocorrelations and cross-correlations assuming geostropic relationship. A Gaussian function is used to model the autocorrelation function. Since the scheme is multivariate the regression coefficients (weights) are matrix.

Mapping the Oceanic Mesoscale Circulation: Validation of Satellite Altimetry Using Surface Drifters

Abstract This study aims to show that Lagrangian surface drifters are a suitable means of validating the mapping of oceanic mesoscale circulation by satellite altimetry. Tests are done using Geosat data to simulate drifter trajectories in the Azores-Madeira area. Multivariate objective analysis is then done to estimate the dynamic topography and its associated formal error using the velocity measurements obtained along drifter trajectories. This dynamic-topography field is compared with the reference field as given by Geosat data. Sensitivity to drifter number and energy level is studied. It is shown that with 25 drifters in a 500-km × 500-km area, the dynamic topography is obtained to within a formal accuracy of around 10%–20%. The difference between the estimated and reference fields is below 2 cm rms. These errors are smaller than the mapping errors induced by the space-time sampling of ERS-1 or TOPEX-POSEIDON satellites. According to these preliminary results, surface drifters are an efficient tool fo...

Experiments with Correlation Representations for Objective Analysis

Abstract The Gandin optimal analysis scheme leaves open the problem of determining the covariance matrix on which it depends. The choice of an autocorrelation function for geopotential, from which geostrophically consistent cross-correlations—and thus covariances—for heights and winds may be derived, influences the performance of the analysis. Four related functions are considered for representation of height field autocorrelations. They are compared on the basis of spectral behavior, capacity to conform with geostrophy, and accuracy of analyses employing them. A multivariate objective analysis test, with both regular and irregular configurations of predictor stations, compares performance of analyses based on modeled covariances with performance of an analysis using observed sample covariances.