Hausse du niveau de la mer et impact du changement climatique global

Au cours du 20ème} siècle, les enregistrements marégraphiques suggèrent une hausse du niveau de la mer de 1.8 mm/an. Plus récemment, les observations spatiales indiquent une hausse de 3.3 mm/an sur la période 1993-2009. Cette augmentation au cours du temps est attribuée au réchauffement global de la planète enregistré depuis plusieurs années maintenant. Durant cette thèse, nous analysons les observations et les causes de la hausse moyenne globale du niveau de la mer. Nous estimons les variations stériques du niveau de la mer grâce aux données du projet international Argo et, les variations de masse des océans liés aux apports d'eau des continents à l'aide des mesures de la mission GRACE. Une autre étude se concentre sur la variabilité du stock d'eaux continentales des plus grands bassins hydrologiques de la planète, à l'aide des données GRACE, et l'impact de cette composante à la hausse du niveau de la mer. Puis, nous analysons l'impact de la variabilité interannuelle du stock d'eaux continentales aux variations du niveau de la mer sur diverses périodes. Enfin, nous étudions le bilan des contributions climatiques à la hausse observée du niveau de la mer sur la période altimétrique totale et pour les années récentes. Dans une deuxième partie, nous étudions les variations régionales du niveau de la mer. Nous établissons dans un premier temps, les causes de la variabilité régionale du niveau de la mer et nous interprétons le signal résiduel issu de la différence entre le niveau de la mer observé et l'expansion thermique des océans. Nous étudions ensuite les variations passées des structures régionales du niveau de la mer sur les dernières décennies (1950-2003). Le but est de reconstruire les variations passées du niveau de la mer en 2-D et ainsi d'avoir une connaissance plus approfondie de l'évolution régionale du niveau moyen des mers. Ce travail a pour objectif ultime de contraindre les modèles climatiques couplés utilisés par l'IPCC (Intergovernmental Panel on Climate Change) pour prédire l'évolution future du niveau de la mer au cours du 21ème siècle. Cette analyse nous permet de détecter un signal basse fréquence dans la variabilité régionale du niveau de la mer que nous observons non seulement dans les données in situ mais aussi dans les modèles climatiques couplés.Tide gauge records suggest a rise in sea level rise of ~1.8 mm/yr over the 20st century. More recently, satellite altimetry data reveal a global mean sea level rise of 3.3 mm/yr over 1993-2010. This rise is attributed to Earth's global warming observed since several decades. In this thesis, we analyze observed global mean sea level and its causes over the entire altimetry era (since 1993). Over the recent years (2002-2009), we estimate the effects of ocean thermal expansion and salinity (called steric effects) on sea level, as well as ocean mass change due to land ice and land waters, using Argo and GRACE space gravimetry data. We discuss the regional variability by comparing several datasets for thermal expansion and ocean mass signal. In another study, we investigate terrestrial land water storage variability of the 33 largest river basins worldwide, using GRACE space gravimetry data. We analyze this contribution to the observed global mean sea level inferred by satellite altimetry. In an extension of this study, we analyze the interannual variability of terrestrial land water storage and its impact on sea level variations over the altimetry era and tide gauge era. Finally, we conclude this chapter by studying the sea level budget over the entire altimetry era and the recent years. In a second part, we study the regional patterns in sea level trends. First, we discuss causes of regional variability, mainly non-uniform ocean warming. We then interpret the residual signal (i.e., observed sea level corrected for thermal effects) for the altimetry era. Thereafter, we analyze regional patterns of past sea level over the last decades (1950-2003). The purpose of this study is to provide 2-D regional past sea level reconstruction and obtain some insight on spatial trend patterns and their dominant modes of variability. The ultimate goal is to constrain coupled climate models used by the IPCC (Intergovernment Panel on Climate Change) to predict sea level rise over the 21st century. Moreover, this study highlights a long term signal detected in the reconstructed sea level. This signal is also observed in in situ data and in coupled climate models

Regional distribution of steric and mass contributions to sea level changes

Póster presentado en EGU General Assembly 2010, celebrada en Viena (Austria), del 2 al 7 de mayo de 2010Peer Reviewe

Measuring global ocean heat content to estimate the earth energy imbalance

The energy radiated by the Earth toward space does not compensate the incoming radiation from the Sun leading to a small positive energy imbalance at the top of the atmosphere (0.4–1 Wm–2). This imbalance is coined Earth’s Energy Imbalance (EEI). It is mostly caused by anthropogenic greenhouse gas emissions and is driving the current warming of the planet. Precise monitoring of EEI is critical to assess the current status of climate change and the future evolution of climate. But the monitoring of EEI is challenging as EEI is two orders of magnitude smaller than the radiation fluxes in and out of the Earth system. Over 93% of the excess energy that is gained by the Earth in response to the positive EEI accumulates into the ocean in the form of heat. This accumulation of heat can be tracked with the ocean observing system such that today, the monitoring of Ocean Heat Content (OHC) and its long-term change provide the most efficient approach to estimate EEI. In this community paper we review the current four state-of-the-art methods to estimate global OHC changes and evaluate their relevance to derive EEI estimates on different time scales. These four methods make use of: (1) direct observations of in situ temperature; (2) satellite-based measurements of the ocean surface net heat fluxes; (3) satellite-based estimates of the thermal expansion of the ocean and (4) ocean reanalyses that assimilate observations from both satellite and in situ instruments. For each method we review the potential and the uncertainty of the method to estimate global OHC changes. We also analyze gaps in the current capability of each method and identify ways of progress for the future to fulfill the requirements of EEI monitoring. Achieving the observation of EEI with sufficient accuracy will depend on merging the remote sensing techniques with in situ measurements of key variables as an integral part of the Ocean Observing System

Altimetry for the future: Building on 25 years of progress

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the ‘‘Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion

Altimetry for the future: building on 25 years of progress

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the “Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion

Sea level trends over 1993-2015 and 2005-2015 from the OCCIPUT ensemble simulation

Contributions of atmospheric forcing and chaotic ocean variability to regional sea level trends over 1993-2015 William Llovel, Thierry Penduff, Benoit Meyssignac, Jean-Marc Molines, Laurent Terray, Laurent Bessières and Bernard Barnier This data set contains 50 sea level trend fields computed globally over 1993-2015 and 2005-2015 from the OCCIPUT ensemble hindcast. These fields are studied in the paper “Contribution of atmospheric forcing and chaotic ocean variability to regional sea level trends over 1993-2015” in revision in Geophysical Research Letters. These sea level trends come from the OceaniC Chaos – ImPacts, structure, predictability (OCCIPUT) ensemble of 1/4° ocean/sea-ice simulations (Penduff et al, 2014; Bessières et al., 2017). This ensemble consists of 50 global hindcasts at ¼° horizontal resolution performed over 1960-2015. The configuration is based on the NEMO 3.5 model and implemented on an eddy-permitting quasi-isotropic horizontal mesh whose grid spacing is about 27 km at the equator and decreases poleward. The 50 members are initialized on January 1st 1960 from the final state of a 21-year one-member spinup. A small stochastic perturbation is applied within each ensemble member during the first year (1960) and switched off at the end of 1960, yielding 50 different oceanic states on January 1st 1961. Each member is then integrated until the end of 2015 with the same atmospheric forcing (DSF5.2) based on the ERA-Interim atmospheric reanalysis. We therefore obtain an ensemble of 50 simulations with the same numerical model and forcing, but different initial conditions. A one-member 327-year climatological simulation based the exact same code and setup is used to estimate the impact of spurious model drift on sea level trends. This simulation was forced each year with the same annual atmospheric cycle derived from DFS5.2. The spurious trends of simulated sea level was estimated at every grid point by computing sea level trends in the climatological simulation over the corresponding years of the 1993-2015 OCCIPUT simulations. This spurious trend map was then removed from the 50 trend maps derived from the ensemble simulation (as also done in Penduff et al., 2018). As NEMO conserves volume rather than mass, the global mean steric effect is missing and the global mean sea level is not properly computed (Greatbatch, 1994). The global mean sea level trends were thus subtracted from the regional sea level trends within each member: the trend provided in the present dataset are anomalies respective to their global mean sea level trend (and corrected for the model drift). Here is an example of the file header. dimensions: y = 1021 ; x = 1442 ; variables: float nav_lat(y, x) ; nav_lat:axis = "Y" ; nav_lat:standard_name = "latitude" ; nav_lat:long_name = "Latitude" ; nav_lat:units = "degrees_north" ; nav_lat:nav_model = "grid_T" ; float nav_lon(y, x) ; nav_lon:axis = "X" ; nav_lon:standard_name = "longitude" ; nav_lon:long_name = "Longitude" ; nav_lon:units = "degrees_east" ; nav_lon:nav_model = "grid_T" ; double trend(y, x) ; trend:units = "m/yr" ; trend:_FillValue = 0. ; // global attributes: :_NCProperties = "version=1|netcdflibversion=4.4.1|hdf5libversion=1.8.14" ; :Conventions = "CF" ; :title = "NCL Efficient Approach by W. Llovel" ; nav_lat and nav_lon represent the latitude and longitude of the NEMO model whereas trend represents the trend anomalies. The trend anomaly estimates are in m.yr-1. References Bessières, L., Leroux, S., Brankart, J.-M., Molines, J.-M., Moine, M.-P., Bouttier, P.-A., Penduff, T., Terray, L., Barnier, B., and Sérazin, G., 2017: Development of a probabilistic ocean modelling system based on NEMO 3.5: application at eddying resolution, Geosci. Model Dev., 10, 1091-1106, doi:10.5194/gmd-10-1091-2017 Greatbatch, R. J. , 1994: A note on the representation of steric sea level in models that conserve volume rather than mass, J. Geophys. Res., 99(C6), 12767–12771. Penduff T, Juza M, Barnier B, Zika J, Dewar WK, Treguier A-M, Molines JM, Audiffren N (2011) Sea-level expression of intrinsic and forced ocean variabilities at interannual time scales. J Clim 24:5652–5670. Penduff, T., Barnier, B., Terray, L., Bessières, L., Sérazin, G., Gregorio, S., Brankart, J., Moine, M., Molines, J., and Brasseur, P.: Ensembles of eddying ocean simulations for climate, CLIVAR Exchanges, Special Issue on High Resolution Ocean Climate Modelling, 19, 2014 Penduff, T., W. Llovel, S. Close, B.-I. Garcia-Gomez, G. Sérazin, L. Bessières, S. Leroux, Trends of coastal sea level between 1993 and 2015: roles of atmospheric forcing and oceanic chaos, in prep

Hausse du niveau de la mer et impact du changement climatique global

TOULOUSE3-BU Sciences (315552104) / SudocTOULOUSE-Observ. Midi Pyréné (315552299) / SudocSudocFranceF

Observed southern upper-ocean warming over 2005-2014 and associated mechanisms

International audienceThe climate system is gaining heat owing to increasing concentration of greenhouse gases due to human activities. As the world’s oceans are the dominant reservoir of heat in the climate system, an accurate estimation of the ocean heat content change is essential to quantify the Earth’s energy budget and global mean sea level rise. Based on the mean estimate of the three Argo gridded products considered, we provide a decadal ocean heat content estimate (over 2005-2014), down to 2000 m, of 0.76 ± 0.14 W m-2 and its spatial pattern since 2005 with unprecedented data coverage. We find that the southern hemisphere explains 90% of the net ocean heat uptake located around 40°S mainly for the Indian and Pacific oceans that corresponds to the center of their subtropical gyres. We find that this rapid upper ocean warming is linked to a poleward shift of mean wind stress curl enhancing Ekman pumping for the 45°S-60°S band. Therefore, the increase of Ekman pumping steepens the isopycnal surface and can enhance heat penetration into the deeper layers of the ocean. We also highlight a relative consistency between the year-to-year net top-of-the-atmosphere flux inferred by satellite measurements and the ocean heating rates (correlation coefficient of 0.53). We conclude that there is no strong evidence of missing energy in the climate system because of remaining large uncertainties in the observing system

Cause of Substantial Global Mean Sea Level Rise Over 2014–2016

Global mean sea level rose by 15 mm over June 2014 – May 2016. This rise is 7 mm larger than the 8 mm increase associated with the long-term trend of 4 mm/yr estimated over 2006–2016. Using a combination of satellite gravimetry data and in situ measurements, we find that 20% of this rise is explained by ocean thermal expansion and 80% by an ocean mass increase, the latter being largely correlated with an equivalent terrestrial water storage (TWS) decrease. Half of the global ocean mass increase during that period can be attributed to the South American continent where we find a significant contribution of the TWS over the Amazon basin (5 mm). This TWS change between oceans and continents occurred during two El Nino events: one aborted in 2014–2015 and an extreme event in 2015–2016 which affected precipitation patterns, especially over the equatorial Pacific ocean and over South America. Key Points Global mean sea level rose by 15 mm over June 2014 – May 2016 80% of this rise had a mass origin (12 mm) and 20% had an ocean warming origin (3 mm) The terrestrial water storage change in the Amazon basin (5 mm) contributed to one third of the substantial sea level rise Plain Language Summary Interannual variability of global mean sea level (GMSL) change is linked to natural climate modes of variability such as El Nino Southern Oscillation (ENSO). Over May 2014–June 2016, two consecutive El Nino events (warm phase of ENSO) occurred in the tropical Pacific ocean: one aborted in 2014–2015 and an extreme event in 2015–2016. At the same time, satellite altimetry recorded a GMSL increase of 15 mm. 80% of the rise was due to a global ocean mass increase. Half of the global ocean mass increase is attributed to the South American continent with an exceptional contribution from the Amazon basin (5 mm). Those unusual El Nino events affected the precipitation pattern worldwide, decreasing the TWS in the Amazon basin and therefore leading to an increase of the global mean ocean mass. Our results suggest the importance of TWS changes, especially in the tropics, to explain interannual variability of GMSL recorded by satellite altimetry