Satellite Altimetry Measurements Research Articles

Amazon River Plume in the Western Tropical North Atlantic

Measurements of temperature, salinity, and currents in the Amazon River plume over a section in the open ocean of the western tropical North Atlantic (38°48′ W) are considered. The measurements were carried out using an AML Base X CTD probe in the upper layer and a flow-through system that measures salinity, turbidity, and chlorophyll-a content in seawater while a vessel is on the way. The measurements were supplemented by velocity profiling using shipborne SADCP. Additionally, archived oceanographic data from the World Ocean Database (WOD18), data on satellite altimetry measurements (AVISO), and satellite salinity data from Aquarius and SMOS were used. It is shown that the width of the Amazon River plume is about 170–400 km and the depth of desalination is from 50 to 100 m. Surface salinity decreases compared to the background (36.1) by 0.25 in February and by more than 3.0 in September during the period of maximum development of the plume, which was determined from satellite measurements of surface salinity. Lagrangian modeling of the back-in-time advection of passive markers simulating freshwater particles was carried out. It was shown that the source of freshwater in the measurement area is discharge from the Amazon River. Amazon River freshwater covered a distance of 3300 km in 60–80 days. The estimate of freshwater transport in the plume was 0.02 Sv, which is one order of magnitude smaller than the mean river discharge.

Sea level rise estimation and projection from long-term multi-mission satellite altimetry and tidal data in the Southeast Asia region

ABSTRACT The Southeast Asian (SEA) region represents a complex coastal geographical location. Most countries in SEA are encircled by the ocean, with lowlands coastal areas. The advantages of long-term satellite altimetry and tide gauge measurements have enabled climate-related research, particularly sea level rise, to be examined extensively in spatial and temporal scales. In this study, sea level anomaly (SLA) derived from 26 years of multi-mission satellite altimeter data plus approximately 28 years of tidal data have been utilized. Then, the projection for every 10 years starting from 2030 until 2100 has been analysed using machine learning regression. The projections are then evaluated based on the global model of the Intergovernmental Panel on Climate Change (IPCC). The findings presented that sea level rise trends around SEA with overall means of 4.70 ± 0.37 mm yr−1 for the satellite altimeter and 3.41 ± 0.24 mm yr−1 for the coastal tide gauge. Sea level projection from coastal tide gauge and satellite altimeter are expected to rise up to 34.87 and 33.71 cm, respectively, in 2100. These outcomes prove that the altimetry data from Radar Altimeter Database System (RADS) and the redundant tidal data are capable in measuring the sea level in SEA. These results are also expected to be valuable for multidisciplinary environmental studies in SEA, such as coastal flooding, coastal erosion, and other effect of global warming. Thus, it would make a meaningful contribution towards Sustainable Development Goal (SDG) 13, climate action, by providing the sea level rate and prediction information.

Fusion of altimetry-derived model and ship-borne data in preparation of high-resolution marine gravity determination

SUMMARY Satellite altimetry provides major data sources for marine gravity recovery, and typical altimetry derived models, for example, DTU21 and SS V32.1, were usually released with 1 arcmin × 1 arcmin gridding interval. Their true resolution is much lower than the nominal ∼2 km level. By contrast, the in situ ship-borne measurements are considered to have better short-wavelength resolution. In this paper, we aim to propose a new method to fully utilize satellite altimetry data and ship-borne measurements, namely the frequency-domain fusion method, and give certain analysis of new method along with two spatial-domain fusion methods. Comprehensive analysis is focus on four aspects: gravity signals in fusion images, numerical verifications, power spectra, as well as coherence analysis. Initial evaluation indicates that, first, the frequency-domain fusion method has advantage in flexibility, since it can autonomously select dominant bands to fuse different data sets. Secondly, the new method retains medium-long wavelength signals from altimetry-derived model and effectively incorporate dominant short-wavelength signals of in-situ measurements, while the spatial-domain methods are essentially full-wavelength fusion and inevitably diminish the role of satellite altimetry. To some extent, the new method maximize the positive contribution of satellite altimetry measurements and efficiently exploit the benefits of ship-borne data. Finally, verification experiments were similarly designed in three regions with different amount and ratio of ship-borne data to thoroughly evaluate various methods. In two regions with gridded and dense along-cruise ship-borne data, the average accuracy of this frequency-domain fusion results is improved by 0.346 and 0.613 mGal, respectively. In a region with sparse ship-borne data, we still recommend using spatial-domain fusion methods since the new method is unable to align ship-borne data with model grid. It is concluded from the above analysis that the new method effectively incorporates the short-wavelength signals from ship-borne data into the altimetry-derived gravity field model, and it is significant that the new method maximizes the application of advantageous bands from different data sources.

Dynamic Structure of Eddies of the Brazil‐Malvinas Confluence Zone Revealed by Direct Measurements and Satellite Altimetry

AbstractThe goal of this work is to study the dynamical structure of eddies of the Brazil‐Malvinas Confluence zone (BMC eddies) using direct velocity measurements carried out by Shipborne Acoustic Doppler Current Profiler during five oceanographic cruises performed in 2016–2022. In total, in situ data of 13 BMC eddies, including nine anticyclones and four cyclones are available. These data show that the orbital velocity in such eddies can reach 189 cm/s and their vertical structure is highly barotropic. In several eddies, the velocities exceeding 100 cm/s are observed down to a depth of 560 m and at a depth of 800 m they are still higher than 80 cm/s. The spatial structure of velocity and horizontal shear in the eddies is strongly asymmetric, with higher velocities in the southern part near the intense thermohaline BMC front. Altimetry data show qualitative agreement with in situ data, but underestimate the horizontal velocity shear and the maximum velocities at the periphery of the BMC eddies. We also use satellite altimetry and Argo float measurements to study these eddies, and estimate their impact on the thermohaline structure. The analysis shows that the eddies with orbital velocities exceeding 100 cm/s cause intense temperature and salinity anomalies reaching 7–9°C and 1 psu in anticyclones and −4°C and 0.8 psu in cyclones at 100–300 m depth.

Introduction

Introduction

Sea level variability at Colombo, Sri Lanka, inferred from the conflation of satellite altimetry and tide gauge measurements

Accurate long-term measurements of sea level are fundamental to evaluating coastal risks, such as the impact of sea-level rise on near-shore ecosystems, groundwater dynamics, and coastal flooding. This study examines sea-level variability at Colombo, Sri Lanka using satellite altimetry, tide gauge measurements separately and their conflated solution under a single model. Modelling of conflated satellite altimetry and tide gauge measurements shows a geocentric (absolute) local sea-level rise of 3.56 ± 0.32 mm/y without any signature of a uniform acceleration since 1981 at this locality. The measurements disclosed statistically significant periodic changes in sea level of luni-solar origin. The conflated model solution enabled the estimation of a statistically significant in-situ vertical land motion (0.58 ± 0.19 mm/yr) without the aid of global positioning measurements. The conflation model explains 98% of the sea-level variability, which makes it suitable for accurate seal level predictions for coastal risk assessments in Colombo, Sri Lanka.

Comparative analysis of significant wave height between satellite altimetry data and SWAN model simulations in the Black Sea basin

Significant wave height (Hs) data are essential in different scientific studies regarding coastal and marine environments, coastal management, various economic activities nearshore and offshore, as well as in the use of marine renewable energy, oil and gas offshore platform operation and security, safety of personnel and equipment. The current paper presents an analysis of the significant wave height parameter for the particular case of the Black Sea basin. In order to achieve this analysis, satellite altimetry measurements from five different satellites taken from the IMOS (Integrated Marine Observing System) database were used and compared with the results of SWAN model simulations in order to assess the accuracy of simulated Hs values. The analysis was performed on data available for a period of 3 years, from 2018 to 2020. The comparison of the simulated significant wave height with data measured by altimetry satellites showed a good correlation of 0.8. The obtained results bring a contribution with regards to a better insight and knowledge into the characteristics of wind waves in the Black Sea.

Convergence of the Malvinas Current branches near 44°S

Field measurements of the Malvinas Current (MC) velocity distribution were performed at the western slope of the Argentine Basin in the South Atlantic Ocean between 47° and 43°S including several sections across the current. The data were collected in January and March 2020 using a Shipborne Acoustic Current Doppler Profiler (SADCP) and allowed us to study circulation patterns in the upper 700 m of the ocean. The observations reveal substantial differences in the current structures at each location; the SADCP data suggest that two distinct current branches at 46°S converge downstream near 44°S making it difficult to distinguish the high-velocity cores farther north. A detailed comparison between the velocity structures indicates that the branches converge due to the change in the orientation and width of the continental slope, and is consistent with potential vorticity conservation. At the same time, satellite altimetry shows that the branches merge to a single high-velocity jet, suggesting that satellite altimetry does not reproduce all features of the MC internal structure and indicates the importance of further direct velocity observations in this region. Despite the above-mentioned limitations of altimetry, the satellite-derived geostrophic velocities suggest that the pathways of the MC high-velocity jets are quite stable. In addition, both satellite altimetry and direct measurements indicate that the branches tend to converge downstream near 44°S.

Epistemic uncertainty in the kinematics of global mean sea-level rise since 1993 and its dire consequences

Recent studies reported an ambiguous global sea level acceleration during the satellite altimetry (SA) era (1993–2017). New SA data created an opportunity to resolve this issue. In this study, two competing kinematic models to represent global mean sea level anomalies are compared. The first model consists of an initial velocity and uniform acceleration. The second model replaces the initial velocity and acceleration with a trend and a representation of a long periodic lunar subharmonic of period 55.8 y, which is determined to be statistically significant at globally distributed tide gauge records. The models also include parameters for the periodic effects of lunisolar origin with periods 18.6 y and 11.1 y annual, and biannual variations in 10-day average of globally SA measurements during 1993–2022. Generalized least squares solutions yielded updated statistically significant estimates for all the model parameters and their statistics for both models. However, the outcome failed to resolve the ambiguity of uniform acceleration in global mean sea level confounded with the long periodic lunar subharmonic of period 55.8 y during this period. This epistemic uncertainty will have a dire impact on climate change risk assessments as demonstrated through the prospective comparison of both kinematic models.

Seasonality of the Sub‐Mesoscale to Mesoscale Sea Surface Variability From Multi‐Year Satellite Altimetry

AbstractSea surface slope (SSS) varies in response to a range of physical processes: tides, geostrophic flows, surface and internal waves, etc. We present the sea surface variation in the form of the SSS variability using 30 years of heterogeneous satellite altimetry measurements. We apply band‐pass filters to the along‐track SSS, and derive the mean and seasonal (annual and semi‐annual) components of SSS variability in multiple wavelength sub‐bands from 10 to 1,000 km. We show that the seasonal components are generally small (<10% in amplitude) compared to the mean variability. Through correlation analysis, we show evidence that SSS variability with wavelengths less than 30 km is dominated by wave height noise. At sub‐mesoscale to mesoscale (30–100 km) wavelengths, we identify high variability over western boundary currents and regions of rough topography. In this band, the high‐latitude Northern Hemisphere and the south Indian Ocean are associated with large annual cycles. The variability is higher in local wintertime except for a few regions, for example, the Bay of Bengal, which shows high variability in the boreal spring and fall. Through power spectral density analysis of the seasonal SSS variability, we find that the energy differences between local winter and summer are stronger at smaller scales (<100 km). The Ka‐band radar interferometry instrument on the Surface Water and Ocean Topography satellite mission will allow observation of ocean surface activities down to ∼20 km at submonthly time scales, but wave‐related errors (sea state bias, aliasing, wind‐driven activities, etc.) will still be a major challenge.

Relationship of Satellite Altimetry Data, and Bathymetry Observations on the West Coast of Africa

The sea surface is variable in time and space; therefore, many researchers are currently interested in searching for dependencies and connections with the elements influencing this diversity, e.g., with the seabed topography. An important problem is combining or comparing models obtained based on different data sets with different accuracies and spatial resolutions. These studies are designed to discover the relationship that may exist between observations of the shape of the seabed and changes in sea level. The aim of this study is to investigate the relationship by checking the correlation between the observations from the point recording of satellite altimetry measurements and the bathymetric data. The object of research is the capital city of Gabon—Libreville, located on the west coast of Africa and three point in Las Palmas. The results present sea level, salinity, and potential temperature variations in selected points. The highest trends of sea level changes occur, at one point, in the open ocean (P6) and at coastal points (Libreville, P3, Pointe Noire). The study indicates that, from 1993 to 2020, the temperature trend at the assessed points is on average 0.018 ± 0.012 °C·year−1, while the average salinity trend is 0.008 ± 0.005 1e−3 year−1. The correlation coefficients for the sea water temperature trend and the sea water salinity trend—determined at 0.20 and 0.08—present a weak linear relationship. Correlation takes slightly higher values for elevation, which is determined at 0.38. Research shows that there is a relationship between satellite and bathymetric observations on the east coast of Africa; however, the strength of the correlation depends on the location.

Analysis of wave characteristics in the Black Sea Basin using satellite altimetry data and swan model simulations

The aim of this paper is to analyze the parameters of significant wave height, wave period and wave length in the Black Sea basin in the period 2017 – 2021. To achieve this objective, satellite altimetry measurements from the IMOS database were used and compared with the values of SWAN model simulations. The results obtained from the processing of these two data sets contribute to a more in-depth understanding of the characteristics of wind waves in the Black Sea due to the special importance from a scientific, economic, political-military and strategic point of view of this geographical space.

ОЦЕНКА ПЕРЕПАДОВ УРОВЕННОЙ ПОВЕРХНОСТИ МОРЯ: СКОРОСТИ ПЕРЕМЕЩЕНИЙ МОРСКИХ МАРКЕРОВ ПО ИК-ИЗОБРАЖЕНИЯМ В СРАВНЕНИИ С ДАННЫМИ СПУТНИКОВОЙ АЛЬТИМЕТРИИ

A quantitative comparison of sea level differences calculated from sea surface current velocities and satellite altimetry measurements was carried out. The velocities were calculated by the automatic method for tracing thermal inhomogeneities based оn a sequence of infrared images with a priori estimation of the calculation accuracy. Comparisons were carried out along the tracks of an altimetry satellite and at the points of the regular grid, to which the values from the tracks are interpolated. Ageostrophic components were estimated for the calculated velocities. At the distances of about 100 km, their contribution did not exceed the accuracy of altimetry measurements (2-4 cm). Along the tracks, the estimates of sea level differences calculated from the velocities were close to those computed from satellite altimetry. At the regular grid points, the estimates of sea level differences by velocities were significantly higher than the estimates of sea level differences by satellite altimetry, which is explained by the drawbacks of the optimal interpolation scheme used. It was concluded that the joint use of two types of measurements to retrieve the World Ocean circulation is reasonable.

Wet tropospheric correction for satellite altimetry using SIRGAS-CON products

Abstract The wet tropospheric correction (WTC) is a required correction to satellite altimetry measurements, mainly due to the atmospheric water vapor delay. On-board microwave radiometers (MWR) provide information for WTC estimation but fail in coastal zones and inland waters. In view to recover the WTC in these areas, the Global Navigation Satellite System (GNSS)-derived Path Delay Plus (GPD+) method, developed by the University of Porto, uses Zenith Tropospheric Delays from GNSS global and regional networks’ stations combined with other sources of information, providing a WTC solution for all along-track altimeter points. To densify the existing dataset used by GPD+, it is necessary to add new GNSS stations, mainly in the southern hemisphere, in regions such as South America, Africa and Oceania. This work aims to exploit the SIRGAS-CON data and its potential for densification of the GPD+ input dataset in Latin America and to improve GPD+ performance. The results for the three analyzed satellites (Sentinel-3A, Sentinel-3B and CryoSat-2) show that, when compared with the WTC from GNSS and radiosondes, the densified GPD+ WTC leads to a reduction in the RMS of the WTC differences with respect to the non-densified GPD+ solution, up to 2 mm for the whole region and up to 5 mm in some locations.

Second Baroclinic Mode Rossby Waves in the South Indian Ocean

Abstract Many previous studies of midlatitude Rossby waves have examined satellite altimetry data, which reflect variability near the surface above the pycnocline. Argo float observations provide hydrographic data in the upper 2000 m, which likely monitor subsurface variability below the pycnocline. This study examines the variability in meridional velocity at midlatitudes and investigates Rossby waves in the southern Indian Ocean using an ocean reanalysis generated by a 4DVAR method. The results show two modes of variability. One is trapped near the surface and propagates to the west at a phase speed close to that of first baroclinic mode Rossby waves. This mode is representative of variability detected by satellite altimetry. The other mode has a local peak in amplitude at ∼600-m depth and propagates to the west at a phase speed 3 times slower than the first baroclinic mode. Such slowly propagating signals are observed globally, but they are largest in amplitude in the southern Indian Ocean and consistent in phase speed with the second baroclinic mode. Results from numerical experiments using an OGCM show that zonal winds in the tropical Pacific Ocean related to ENSO are the primary driver of slowly propagating signals in the southern Indian Ocean. Wind forcing in the tropical Pacific Ocean drives a surface trapped jet that propagates via the Indonesian Archipelago and excites subsurface variability in meridional velocity in the southern Indian Ocean. In addition, surface heat flux and meridional winds near the west coast of Australia can drive subsurface variability. Significance Statement Many previous studies of midlatitude Rossby waves have used satellite altimetry measurements, which reflect variability in the upper few hundred meters of the ocean. Argo float observations have provided in situ hydrographic observations in the upper 2000 m, and these enable us to examine subsurface variability with high reliability. In this study, we used output from an ocean reanalysis, which assimilates in situ observations, and found that the meridional velocity below the surface (∼600-m depth) of the southern Indian Ocean propagates at a phase speed 3 times slower than that of surface variability. These slowly propagating signals can be of climatic importance because of their possible impact on meridional heat transport. We also discuss the driving force of these slowly propagating signals.

An improved algorithm for extracting crossovers of satellite ground tracks

A crossover refers to the intersection of two satellite ground tracks. Crossovers are required for performing crossover differences adjustment to remove the orbit error and for establishing the time series of elevation in surfaces, which is why crossovers are important for satellite altimetry measurements of ice sheet or ice shelf elevation changes. How to extract more crossovers precisely is the objective of this paper. On the basis of the traditional method of solving crossovers and computer graphics, this paper proposes an improved algorithm called improved rapid rejection and straddle test (IRST) for computing the position of the crossover point. This algorithm efficiently and accurately searches for two points in ascending pass and two points in descending pass that can form a crossover, and then computes crossovers’ geolocation. By using CryoSat-2 satellite altimeter data, we conducted our study on the Antarctic Ross ice shelf and Filchner–Ronne ice shelf using the fixed iteration (FI) algorithm, rapid rejection and straddle test (RST) algorithm, and the IRST algorithm. Results show that IRST is superior to the two other algorithms in terms of the number of crossovers, geolocation accuracy and computational efficiency. These advantages are more noticeable in the border areas between the ice shelf and the mountain with large terrain slope, which solves the problem that crossovers are less in these areas with poor-quality data coverage.

Ionospheric Corrections for Satellite Altimetry ‐ Impact on Global Mean Sea Level Trends

AbstractAtmospheric delay corrections for satellite altimetry measurements are essential for deriving highly accurate sea surface heights and reliable global mean sea level (GMSL) trend estimates. A commonly used method to correct for ionospheric path delays are the usage of GNSS‐based Global Ionospheric Maps (GIM). The different orbit heights of GNSS and altimeter satellites require an adaption of GIM corrections to account for free electrons in the Earth plasmasphere. This study shows that the widely used scaling approach based on the International Reference Ionosphere (IRI) is not able to accurately scale the GIM models. The impact of neglecting the plasmaspheric part of the atmosphere strongly correlates with the solar activity of about 11 years. This manifests itself as trend errors in global GMSL. For the Jason period (2002–2021) a trend error of 0.17 mm/year can be shown, which is even larger for smaller periods (e.g., 1.0 mm/year for Jason‐1 lifetime). The application of an additional constant scaling factor of 0.886 can reduce the trend differences to below 0.05 mm/year.

Re-examination of Slip Distribution of the 2004 Sumatra\u2013Andaman Earthquake (Mw 9.2) by the Inversion of Tsunami Data Using Green\u2019s Functions Corrected for Compressible Seawater Over the Elastic Earth

We re-examined the slip distribution on faults of the 2004 Sumatra–Andaman (M 9.1 according to USGS) earthquake by the inversion of tsunami data with phase-corrected Green’s functions applied to linear long waves. The correction accounts for the effects of compressibility of seawater, elasticity of solid earth, and gravitational potential variation associated with the motion of mass to reproduce the delayed arrivals and the reversed phase of the first tsunami waves. We used sea surface height (SSH) data from satellite altimetry (SA) measurements along five tracks, and the tsunami waveforms recorded at tide gauges (TGs) and ocean bottom pressure gauges (OBPGs) in and around the Indian Ocean. The inversion results for both data sets for different rupture velocities (Vr) show that the reproducibility of the spatiotemporal SSHs and tsunami waveforms is improved by the phase corrections, although the effects are not so significant within the Indian Ocean. The best slip distribution model from joint inversion of SA, TG and OBPG data with Vr of 1.3 km/s shows the largest slips of 16–25 m off Sumatra Island, large slips of 2–11 m off the Nicobar Islands, and moderate slips of 2–6 m in the Andaman Islands. The inversion results reproduce the far-field tsunami waveforms well at distant stations even more than 13,000–25,000 km from the epicenter. The total source length is about 1400 km and the seismic moment is Mw 9.2, longer and larger than that of our previous estimates based on TG records.

Meanders of the West Greenland Current near Cape Farewell

The cyclonic boundary current that circumnavigates the Labrador Sea plays an important role in modulating the strength of wintertime convection in the interior basin, as well as restratifying the newly ventilated water in spring. Modeling studies indicate that meso and sub-mesoscale processes in the boundary current flux a significant amount of heat and freshwater offshore, although observations of this small-scale variability are lacking. Using four years of data from a mooring array west of Cape Farewell, Greenland, together with satellite altimetry and sea surface temperature measurements, we present the first observations of a meandering West Greenland Current. We describe the statistics, structure, characteristics, and formation mechanism of these features. The meanders occur roughly 30% of the time and are more prevalent in winter and early spring, with an increasing trend over the four-year record. It is shown that baroclinic instability of the boundary current is the cause of the meanders, triggered by seasonal steepening of the isopycnals between the interior basin and the boundary due to offshore convection. We argue that the meandering leads to the formation of small-scale eddies, and estimate the resulting seaward heat flux. Finally, possible connections between the meanders and the production of Irminger Rings are explored.

Effect of the East Siberian Current on Water Exchange in the Bering Strait Based on Satellite Altimetry Measurements

Effect of the East Siberian Current on Water Exchange in the Bering Strait Based on Satellite Altimetry Measurements