Robin Fraudeau

Magellium (France)

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Michaël Ablain

Magellium (France)

Alejandro Blazquez

Laboratoire d’Études en Géophysique et Océanographie Spatiales

Benoît Meyssignac

Université Toulouse III - Paul Sabatier

Anne Barnoud

Magellium (France)

Marco Restano

European Space Research Institute

Florence Marti

Magellium (France)

Victor Rousseau

Magellium (France)

Jérôme Benveniste

European Space Research Institute

William Llovel

Université de Bretagne Occidentale

Gilles Larnicol

Magellium (France)

Cooperative Institutions

Centre National de la Recherche Scientifique

Institut de Recherche pour le Développement

Laboratoire d’Études en Géophysique et Océanographie Spatiales

Magellium (France)

Université de Toulouse

Centre National d'Études Spatiales

Université Toulouse III - Paul Sabatier

Ifremer

European Space Research Institute

Université de Bretagne Occidentale

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High-frequency sea level temporal variability estimate from SWOT KaRIn and Sentinel-3A/B crossovers

Anne Barnoud Robin Fraudeau Noémie Lalau Marie Bouih Michaël Ablain

The observation of sea level variability on very small time scales ranging from less than an hour to a few days is currently very limited with the constellation of nadir altimeter satellites. Calculating sea surface height at crossovers between a single mission or several nadir altimeter missions makes it possible to analyse the sea level variability on these very short timescales for a small number of measurements (a few hundred). Moreover, these observations are spatially sparsely distributed, most often at very high latitudes for crossovers of less than a few hours of time difference. Thanks to the launch of the joint CNES/NASA SWOT mission in December 2022 with swath measurements, a new paradigm for observing high-frequency temporal variability in sea level is now possible. SWOT KaRIn instrument offers 2D observations of the oceans with an unprecedented coverage and resolution. We take advantage of this new high quality dataset to estimate the sea level variability over short time scales. The crossovers of SWOT KaRIn during the 1-day orbit phase with itself and with Sentinel-3A/B nadir observations less than 1 hour to 72 hours of time difference from 17/04/2023 to 07/07/2023 are analysed. The crossovers with Sentinel-3A/B offer a very good spatial sampling of the oceans which cannot be reached with SWOT crossovers only. We analyse the variance of the sea surface height differences at the crossovers, as a function of time difference and of latitude. The variability of the sea surface height differences at the crossovers contains both the sea level variability and the instrumental errors. We attempt to disentangle the errors from the sea level variability. Sea surface height differences variance with time differences tending to 0 hours, hence free of oceanic variability, amount to ~3.2-3.5 cm. This value increases fastly with time difference up to about 4 hours as the contribution of sea level variability and errors increase. With time differences of 4 hours, the sea surface height differences variance reaches 4 cm. Then, the increase of the sea surface height differences variance slows down, as some phenomena are not correlated anymore, and roughly linearly increases to reach 5 cm for time differences of 72 hours.

SWOT Analysis

10.5194/egusphere-egu24-11687

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On the uncertainty associated with detecting global and local mean sea level drifts on Sentinel-3A and Sentinel-3B altimetry missions

Rémi Jugier Michaël Ablain Robin Fraudeau Adrien Guérou Pierre Féménias

Abstract. An instrumental drift in the Point Target Response (PTR) parameters has been detected on the Copernicus Sentinel-3A (S3A) altimetry mission. It could have an impact on sea level rise of a few tenths of mm yr−1. In order to accurately evaluate this drift, a method for detecting global and local mean sea level relative drifts between two altimetry missions is implemented. Associated uncertainties are also accurately calculated thanks to a detailed error budget analysis. A drift on both S3A and S3B GMSL is detected with values significantly higher than expected. For S3A, the relative GMSL drift detected is 1.0 mm yr−1 with Jason-3 and 1.3 mm yr−1 with SARAL/AltiKa. For S3B, the relative GMSL drift detected is −2.2 mm yr−1 with SARAL/AltiKa and −3.4 mm yr−1 with Jason-3. The drift detected at global level does not show detectable regional variations above the uncertainty level of the proposed method. The investigations led by the altimeter experts can now explain the origin of this drift for S3A, while it is still under investigation for S3B. The ability of the implemented method to detect a sea level drift with respect to the length of the common period is also analysed. We find that the maximum detectable sea-level drift over a 5 years period is 0.3 mm yr−1 at the global scale, and 1.5 mm yr−1 at local scales (2400 km). However, these levels of uncertainty do not meet the sea-level stability requirements for climate change studies.

Satellite Altimetry

10.5194/os-2021-106

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Regional sea level budget over 2004–2022

Marie Bouih Anne Barnoud Chunxue Yang Andrea Storto Alejandro Blazquez

Abstract. Closure of the regional sea level budget is investigated over the 2004–2022 time span by comparing trend patterns from the satellite altimetry-based sea level with the sum of contributions, i.e. the thermosteric, halosteric, manometric and GRD (Gravitational, Rotational, and Deformational fingerprints due to past and ongoing land ice melt) components. The thermosteric and halosteric components are based on Argo data. For the manometric component, two approaches are considered: one using GRACE/GRACE-Follow On satellite gravimetry data, and the other using ocean reanalyses-based sterodynamic sea level data corrected for local steric effects. For the latter, six different ocean reanalyses are considered, including two reanalyses that do not assimilate satellite altimetry data. The results show significantly high residuals in the North Atlantic for both approaches. In other regions, small-scale residuals of smaller amplitude are observed and attributed to the finer resolution of altimetry data compared to the coarser resolution of data sets used for the components. Focus on the strong residual signal seen in the North Atlantic suggests Argo-based salinity errors in this region. However, it is not excluded that other factors also contribute to the non-closure of the budget in this region.

10.5194/egusphere-2024-3945

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Revisiting the global mean ocean mass budget over 2005-202

Anne Barnoud Julia Pfeffer Anny Cazenave Robin Fraudeau Victor Rousseau

We investigate the performances of GRACE and GRACE Follow-On satellite gravimetry missions in assessing the ocean mass budget at global scale over 2005-2020. For that purpose, we focus on the last years of the record (2015-2020) when GRACE and GRACE Follow-On faced instrumental problems. We compare the global mean ocean mass estimates from GRACE and GRACE Follow-On to the sum of its contributions from Greenland, Antarctica, land glaciers, terrestrial water storage and atmospheric water content estimated with independent observations. Significant residuals are observed in the global mean ocean mass budget at interannual time scales. Our analyses suggest that the terrestrial water storage variations based on global hydrological model likely contributes to a large part to the misclosure of the global mean ocean mass budget at interannual time scales. We also compare the GRACE-based global mean ocean mass with the altimetry-based global mean sea level corrected for the Argo-based thermosteric contribution (an equivalent of global mean ocean mass). After correcting for the wet troposphere drift of the radiometer on-board the Jason-3 altimeter satellite, we find that mass budget misclosure is reduced but still significant. However, replacing the Argo-based thermosteric component by the ORAS5 ocean reanlaysis or from CERES top of the atmosphere observations leads to closure of the mass budget over the 2015-2020 time span. We conclude that the two most likely sources of error in the global mean ocean mass budget are the thermosteric component based on Argo and the terrestrial water storage contribution based on global hydrological models. The GRACE and GRACE Follow-On data are unlikely to be responsible on their own for the non-closure of the global mean ocean mass budget.

Argo

10.5194/egusphere-egu23-8001

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Estimate of the global and regional Ocean Heat Content changes from space gravimetry and altimetry observations to assess the Earth Energy Imbalance variations and trend

Robin Fraudeau Florence Marti Benoît Meyssignac Alejandro Blazquez Sébastien Fourest

The Earth energy imbalance (EEI) at the top of the atmosphere (TOA) is the cause of the energy accumulation in the climate system. Measuring the EEI is challenging because it is a globally integrated variable whose variations are small (0.5-1 W.m−2) compared to the amount of energy entering and leaving the climate system (~ 340 W.m-2). 91% of the excess of energy stored by the planet in response to the EEI is accumulated in the ocean in the form of heat making the ocean heat content (OHC) change an accurate proxy of EEI. In this work, we adopt the space geodetic approach which relies on the sea level budget equation to estimate the OHC changes. The thermosteric sea level change is derived at regional scale from a combination of space altimetry and space gravimetry observations, and divided by the integrated expansion efficiency of heat  to estimate the OHC changes. The global OHC (GOHC) change is then estimated by a spatial integration of the regional OHC changes. The uncertainty in GOHC is estimated by propagation of the uncertainty of input data using the input data error variance-covariance matrix to account for the instrumental and post-processing errors and for the time correlation in errors. Regional estimates of the OHC changes are validated over the Atlantic Ocean directly against data from in-situ Argo profiles and indirectly by an energy budget approach. In the energy budget approach, surface heat flux derived from ERA5 and CERES TOA radiation budget are combined with regional OHC changes to estimate the north Atlantic meridional heat transport which is then validated against in-situ RAPID and OSNAP estimates. Both validations show good agreement in terms of signal amplitudes and variability with time correlations above 0.6.    Over the period 1993-2022, the GOHC shows a significant positive trend of 0.75 W m-2 [0.61, 1.04] at the 90% confidence level, indicating a positive mean ocean heat uptake or EEI. Comparisons with GOHC estimates based on in-situ ocean temperature measurements over the full ocean depth show good agreement over 2005-2019 (Marti et al. 2023, in review). Over 2000-2020, the ocean heat uptake presents a positive trend of 0.33 W/m²/decade, significant at the 90% confidence level and in agreement with CERES estimate. This EEI trend  reflects an acceleration in ocean warming.   The two space geodetic products based on space altimetry and space gravimetry are freely available on the AVISO website. One estimating the GOHC and EEI (https://doi.org/10.24400/527896/a01-2020.003), the other estimating regional OHC over the Atlantic Ocean (https://doi.org/10.24400/527896/a01-2022.012).

Gravimetry

Satellite Altimetry

10.5194/egusphere-egu24-16132

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Monitoring local Ocean Heat Content changes with satellite altimetry and space gravimetry to assess the variability of the Meridional Heat Transport in the North Atlantic: the 4DATLANTIC-OHC Project

Robin Fraudeau

Given the major role of the Atlantic Ocean meridional heat transport in the climate system, it is essential to characterize its temporal variations at different locations. The 4DATLANTIC-OHC Project (https://eo4society.esa.int/projects/4datlantic-ohc/) aims at developing and testing space geodetic methods based on satellite altimetry and space gravimetry to estimate the local ocean heat content (OHC) changes over the Atlantic Ocean. Combined with independent estimates of the surface heat fluxes this approach holds promise to estimate the Atlantic Meridional Heat Transport (MHT) at any section across the Atlantic basin. The official version 1.0 of the 4DAtlantic-OHC product has been released, it provides estimates of local changes in OHC with their uncertainties. This product is accessible with DOI https://doi.org/10.24400/527896/A01-2022.012  and can be downloaded on AVISO portal. At two test sites, OHC changes derived from in situ data (RAPID and OVIDE) are used to evaluate the accuracy and reliability of the new space geodetic based OHC change estimate. Combined with ERA5 estimate of the surface heat fluxes, the Atlantic OHC product will be used to derive an energy budget of the North Atlantic basin and estimate the associated divergence in ocean heat transport. From the divergence field we will derive at the end of the project new estimates of the Atlantic meridional heat transport at different sections in the North Atlantic basin (RAPID and OSNAP sections) and compare it with in situ estimates. The V1.0 of 4DAtlantic-OHC products over the Atlantic Ocean, the evaluation results of the OHC against in situ data and preliminary results of MHT estimation will be presented.

10.5194/egusphere-egu23-12244

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Revisiting the global mean ocean mass budget over 2005–2020

Ocean science (2023)

Anne Barnoud Julia Pfeffer Anny Cazenave Robin Fraudeau Victor Rousseau

Abstract. We investigate the performances of Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) satellite gravimetry missions in assessing the ocean mass budget at the global scale over 2005–2020. For that purpose, we focus on the last years of the record (2015–2020) when GRACE and GRACE Follow-On faced instrumental problems. We compare the global mean ocean mass estimates from GRACE and GRACE Follow-On to the sum of its contributions from Greenland, Antarctica, land glaciers, terrestrial water storage and atmospheric water content estimated with independent observations. Significant residuals are observed in the global mean ocean mass budget at interannual timescales. Our analyses suggest that the terrestrial water storage variations based on global hydrological models likely contribute in large part to the misclosure of the global mean ocean mass budget at interannual timescales. We also compare the GRACE-based global mean ocean mass with the altimetry-based global mean sea level corrected for the Argo-based thermosteric contribution (an equivalent of global mean ocean mass). After correcting for the wet troposphere drift of the radiometer on board the Jason-3 altimeter satellite, we find that mass budget misclosure is reduced but still significant. However, replacing the Argo-based thermosteric component by the Ocean Reanalysis System 5 (ORAS5) or from the Clouds and the Earth's Radiant Energy System (CERES) top of the atmosphere observations significantly reduces the residuals of the mass budget over the 2015–2020 time span. We conclude that the two most likely sources of error in the global mean ocean mass budget are the thermosteric component based on Argo and the terrestrial water storage contribution based on global hydrological models. The GRACE and GRACE Follow-On data are unlikely to be responsible on their own for the non-closure of the global mean ocean mass budget.

Argo

10.5194/os-19-321-2023

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Monitoring the ocean heat content change and the Earth energy imbalance from space altimetry and space gravimetry

Earth system science data (2022)

Florence Marti Alejandro Blazquez Benoît Meyssignac Michaël Ablain Anne Barnoud

Abstract. The Earth energy imbalance (EEI) at the top of the atmosphere is responsible for the accumulation of heat in the climate system. Monitoring the EEI is therefore necessary to better understand the Earth's warming climate. Measuring the EEI is challenging as it is a globally integrated variable whose variations are small (0.5–1 W m−2) compared to the amount of energy entering and leaving the climate system (∼340 W m−2). Since the ocean absorbs more than 90 % of the excess energy stored by the Earth system, estimating the ocean heat content (OHC) change provides an accurate proxy of the EEI. This study provides a space geodetic estimation of the OHC changes at global and regional scales based on the combination of space altimetry and space gravimetry measurements. From this estimate, the global variations in the EEI are derived with realistic estimates of its uncertainty. The mean EEI value is estimated at +0.74±0.22 W m−2 (90 % confidence level) between August 2002 and August 2016. Comparisons against estimates based on Argo data and on CERES measurements show good agreement within the error bars of the global mean and the time variations in EEI. Further improvements are needed to reduce uncertainties and to improve the time series, especially at interannual timescales. The space geodetic OHC-EEI product (version 2.1) is freely available at https://doi.org/10.24400/527896/a01-2020.003 (Magellium/LEGOS, 2020).

Gravimetry

10.5194/essd-14-229-2022

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Citations (40)

Monitoring the ocean heat content change and the Earth energy imbalance from space altimetry and space gravimetry

Florence Marti Alejandro Blazquez Benoît Meyssignac Michaël Ablain Anne Barnoud

Abstract. The Earth energy imbalance (EEI) at the top of the atmosphere is responsible for the accumulation of heat in the climate system. Monitoring the EEI is therefore necessary to better understand the Earth’s warming climate. Measuring the EEI is challenging as it is a globally integrated variable whose variations are small (0.5–1 W m−2) compared to the amount of energy entering and leaving the climate system (~ 340 W m−2). Since the ocean absorbs more than 90 % of the excess energy stored by the Earth system, estimating the ocean heat content (OHC) provides an accurate proxy of the EEI. This study provides a space geodetic estimation of the OHC changes at global and regional scales based on the combination of space altimetry and space gravimetry measurements. From this estimate, the global variations in the EEI are derived with realistic estimates of its uncertainty. The mean EEI value is estimated at +0.74 ± 0.22 W m−2 (90 % confidence level) between August 2002 and August 2016. Comparisons against independent estimates based on Argo data and on CERES measurements show good agreement within the error bars of the global mean and the time variations in EEI. Further improvements are needed to reduce uncertainties and to improve the time series especially at interannual and smaller time scales. The space geodetic OHC-EEI product is freely available at https://doi.org/10.24400/527896/a01-2020.003.

Gravimetry

10.5194/essd-2021-220

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Citations (2)

Assessing Spatial and Temporal Variations in the Ocean Heat Content and Earth Energy Imbalance from Space Geodetic Data 

Julia Pfeffer Robin Fraudeau Benoît Meyssignac Alejandro Blazquez Sébastien Fourest

The GRACE (Gravity Recovery and Climate Experiment) and GRACE-FO (Gravity Recovery and Climate Experiment - Follow On) satellite missions have enabled global monitoring of mass transport within the Earth's system, leading to unprecedented advances in understanding the global water cycle in the context of a changing climate. Over the past two decades (2002–2022), the combination of satellite gravimetry with altimetry data has allowed us to monitor the change in the ocean heat content (OHC) which is the main reservoir to store the excess of energy accumulated in the Earth system due to human activities. From the global OHC estimate, the Earth's energy imbalance (EEI: +0.8 W/m²) was derived with high accuracy (±0.16 W/m² within a 90% confidence interval). The primary source of uncertainty in our geodetic EEI estimate arises from the post-processing of satellite gravity measurements, which has been rigorously evaluated using an ensemble approach. Independent comparisons with the Clouds and the Earth's Radiant Energy System (CERES) mission further highlighted the critical importance of post-processing corrections for the detection of realistic changes in EEI at interannual and decadal time scales. These findings underscore the need to enhance consistency between satellite altimetry and gravimetry measurements, particularly concerning geocenter corrections. Additionally, investigations into the spatial and temporal variations in OHC have shown that post-processing corrections applied to satellite gravimetry data, such as leakage correction, are crucial for accurately capturing temporal variations in EEI. Better-informed decisions regarding the selection of post-processing corrections can be made by leveraging the redundancy of the ocean and climate monitoring system to ensure the closure of the energy and sea level budgets at the global and regional scales.

Content (measure theory)

10.5194/gstm2024-56

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