Research interests
Applications of Earth observations from space to:
Satellite geodesy and solid Earth geophysics: Gravity field; Earth rotation and polar motion; tides; precise positioning by space geodesy techniques and applications to tectonic motions, vertical crustal motions, geocenter motions; marine geophysics, lithosphere & mantle properties.
Climate research: Sea level variations at global, regional and local scales (observations from satellite altimetry and tide gauges; study of climatic causes: ocean thermal expansion, land ice melt, land water storage change, projections); global water cycle and land hydrology from space.
Reseach accomplishments
Anny Cazenave’s research deals with the applications of space techniques to Earth sciences. It can be divided into 3 main topics (roughly in chronological order, although with some overlap in between) : (1) Satellite geodesy and applications to geodetic problems (Earth gravity field, tides, global Earth deformations, tectonic and geocenter motions, etc.), (2) Marine geophysics, mainly from satellite altimetry and (3) Sea level changes and its relation with climate, and Hydrology from space.
After a ‘Thèse de 3eme cycle’ in astronomy at the Observatoire de Paris, Anny Cazenave (AC) prepared a PhD thesis (Doctorat d’Etat) on the rotation of the Earth under supervision of Kurt Lambeck (Australian National University). With the wind data available at that time, she could quantify for the first time the effects of the atmospheric circulation on the seasonal and interannual fluctuations of the length of day. Meanwhile, she started to work in the field of satellite geodesy at the Centre National d’Etudes Spatiales (CNES; the French Space Center), focusing on the determination of the long-wavelength anomalies of the Earth’s gravity field, as well as tidal deformations of the Earth, from the analysis of orbits of geodetic satellites (including the passive laser satellite Starlette launched by France in 1975). With the development, during the 1980s and 1990s, of precise positioning techniques from satellite laser ranging and Doppler systems such as DORIS (Doppler Orbitography and Radio-positioning Integrated by Satellite), she moved to new problems such as the determination of present-day large-scale tectonic motions, crustal deformations, and motions of the Earth’s centre of mass at seasonal and interannual time scales. In addition to measuring these phenomena, she also worked at explaining the causes of the observed motions (e.g., for the Earth’s centre of mass and crustal deformations). For that purpose, she used different environmental data (of atmospheric, oceanic and hydrological origin), in addition to the space geodetic techniques.
With the launch of the very first satellite altimetry missions (Geos 3 and Seasat in the mid-to-late 1970s, Geosat in the mid-1980s, etc.), AC started to work on the geophysical applications of satellite altimetry. Although satellite altimetry was developed to study ocean dynamics, the early missions were not accurate enough to measure the tiny oceanic signal, mostly because of large satellite orbit errors caused by insufficient knowledge of the Earth gravity field at that time. However, these early altimetry missions provided invaluable new information on the marine gravity field through measurements of the ‘mean sea surface’ (or equivalently, the marine geoid). AC’s early contribution to this domain was a very first determination of the ‘mean sea surface’, revealing that the permanent sea surface topography is not flat but mimics the ocean floor topography. With her PhD students, she further developed research to understand the causes of the observed anomalies of the marine geoid, which led her to get involved in various areas of marine geophysics. Her contributions concerned the mechanical properties (elastic thickness) and thermal evolution of the oceanic lithosphere, detection of small-scale convection in the upper mantle from its signature in the marine geoid, deep structure of hotspot swells, mid-ocean ridges and fracture zones properties derived from geoid anomalies, etc.). At this epoch (1980s and 1990s), she also worked on the origin of the large-scale anomalies of the Earth’s gravity field and their link with deep mantle convection, as well as on simultaneous inversion of gravity and seismic data to constrain the viscosity profile of the Earth’s mantle and the large-scale convective flow.
During the 1980s, AC also worked in planetary science. Her contributions in this domain concerned the origin of the orbital inclination the Martian satellites, Phobos and Deimos, the long-term evolution of the rotation of Venus, the dynamical origin of comets. She also developed numerical models of telluric planets formation and of initial planet rotations.
During the 1990s, the improvement of the performances of space geodesy techniques allowed the very first determinations of temporal variations of low-degree harmonics of the Earth’s gravity field. AC was able to determine the seasonal variations of the long-wavelength geoid and to quantify the respective contributions of atmospheric loading and land hydrology to the observed geoid variations. This represented very preliminary attempts to study temporal variations of the gravity field (limited at that time to the longest wavelengths), prior to the revolutionary GRACE (Gravity Recovery and Climate Experiment) space gravimetry mission launched in 2002.
During the 1990s also, AC moved to climate research with focus on two main topics: present-day sea level variations and continental water cycle, and their causes. She decided to work on these topics in view of the spectacular increase through time of the precision of the altimetry missions. Improved models of the Earth’s gravity field and of other forces acting on the satellite orbits, as well as improved geophysical corrections that need to be applied to the altimetry measurements, together with increased instrumental performances, opened the era of high-precision altimetry in the early 1990s. This started with the launch of the American-French mission Topex/Poseidon in 1992 (followed by several other high-precision altimetry missions such as Jason-1,2,3, Sentinel 6, and now SWOT, plus additional altimeter satellites from of the European Space Agengy). AC became quickly involved in the precise determination of sea level changes by satellite altimetry at regional and global scales. Her very first contribution in this area concerned the annual mean sea level (poorly known globally at that time) and its causes (quantification of the respective contributions of ocean thermal expansion and land hydrology). With her PhD students, she further worked at improving the sea level measurements from multi satellite altimetry missions and at quantifying the causes of the global mean sea level rise: ocean thermal expansion using in situ hydrographic data, land ice melt and land water storage changes using GRACE space gravimetry data and models. She performed early investigations of the sea level budget and its closure over the altimetry era, combining altimetry, GRACE and in situ hydrographic data -including data from the Argo autonomous floats (as of 2005)-, an approach now widely applied by various groups worldwide. AC also studied the causes of the interannual variability of the global mean sea level, in particular the link with ENSO (El Nino-Southern Oscillation) events and showed that during El Niño/La Niña, the global mean sea level displays temporary positive/negative anomalies due to exchanges of water between tropical river basins on land and the tropical Pacific Ocean. Account of the effects of internal climate variability on the global mean sea level is very important to accurately estimate the evolution of rate of sea level rise due to external forcing factors, including anthropogenic forcing. AC and her team also studied the causes of the regional variability in sea level trends, developed 2-dimensional past sea level reconstructions to derive spatial trend patterns prior to the altimetry era (back to 1950) and determined the dominant modes of variability of these spatial patterns (found to be mainly driven by ENSO as well as other modes of natural –internal- variability of the ocean-atmosphere system, such as the Pacific Decadal Oscillation, the North Atlantic Oscillation). They addressed the problem of detection/attribution, i.e., tried to detect the anthropogenic signal in the spatial sea level patterns (comparing satellite altimetry data, past sea level reconstructions and outputs of coupled climate models with and without anthropogenic forcing) and concluded that spatial trend patterns revealed by satellite altimetry still mostly result from internal climate variability. However, most recent results suggest the anthropogenic forcing starts to be detectable in some oceanic regions.
AC and her team also studied the regional/local variability of the ‘relative’ sea level rise (i.e., with respect to the crust) over the past few decades (considering the total –global mean rise plus regional variability, plus vertical crustal motions) in different regions, in particular the low-lying Pacific islands, the islands of the Indian Ocean, and of the Caribbean region.
In the early 2000s, AC initiated with her research group a new research topic: ‘land hydrology from space’. They showed that satellite altimetry can also be used to monitor temporal variations of surface water levels (lakes, rivers, man-made reservoirs) and developed a database of satellite altimetry-derived water level time series over thousands of lakes and rivers sites (called HYDROWEB). Altimetry-based water level time series over rivers are particularly useful in regions where in situ gauge networks are declining or are even inexistent (e.g., in Africa). They used this data set to study the hydrological behaviour of lakes and rivers under climate change and natural climate variability in different environments, as well as under direct human forcing (dam building on rivers, irrigation, land use, etc.). With the launch of the GRACE space gravimetry mission in 2002, they also conducted a series of studies dedicated to estimate the total water storage change in large river basins, determined ground water storage variations after accounting for the surface waters and soil moisture contribution, and more generally tried to use in synergy observations from different satellite sensors (altimetry, GRACE) together with model outputs, to determine different hydrological parameters of the continental water cycle.
Since a few years, Anny Cazenave’s research is devoted to two subjects: (1) sea level changes under different aspects: improvement of satellite measurements of sea level and quantification of uncertainties, estimates of the causes of sea level variations from global to local scales, and study of sea level impacts in coastal zones, and (2) the detection and origin of a 6-year cycle in the Earth system.
Since 2011, in the context of the European Space Agency (ESA) Climate Change Initiative (CCI) programme, AC is the science leader of a project on sea level involving several European partners, dedicated to improve the now 30+ year-long sea level record with high-precision satellite altimetry data. In the first phase of this project, AC and her partners worked at a complete reprocessing of altimetry data from nine different space missions, developing new algorithms and improved geophysical corrections, which led to the production of a new sea level data set (global mean and gridded time series) available to the international community. In parallel, AC and her team pursued their studies on the sea level budget using data from Argo, GRACE, etc., to estimate the different contributions to present-day sea level rise. Studying closure of the sea level budget has a number of potential applications such as the determination of unknown (or poorly known) components of the global mean sea level, e.g., deep ocean warming, not sampled by Argo (and its contribution to the Earth energy imbalance), or terrestrial water storage, the least well-known component of the sea level budget (sign even controversial). This approach can also serve to detect instrumental drifts of the observing systems, for example the detection of the instrumental drift that affected the Topex A radar instrument onboard the Topex/Poseidon satellite over 1993-1998 (a problem now corrected).
Recently, AC also initiated new research (still in the context of the ESA CCI sea level project), consisting of estimating sea level changes in coastal zones (classical nadir altimetry does not work well within 10-20 km to the coast because of parasitic reflections from land; however, dedicated data reprocessing can be implemented to retrieve coastal sea level data that otherwise would be dropped). This is an important topic because, up to recently, one was unable to answer the question: does sea level at the coast rise at the same rate as in the open ocean? For a number of reasons due to small-scale processes occurring near the coast (coastal currents, fresh water input from rivers in estuaries, trends in wind & wave regimes, etc.), it was suspected that coastal sea level rise could be significantly different from offshore. However, novel, unexpected results obtained recently by AC and her group, along the world coastal zones indicate that in 90% of the thousand+ studied sites, coastal sea level trends (at distances < 2-3 km from the shoreline) are do not differ much from nearby open ocean trends. But this is not always the case. In some instances, higher or lower rates are observed at the coast compared to offshore. Providing long coastal sea level time series worldwide and understanding the causes of the observed coastal sea level trends is an important goal for studying coastal impacts of sea level rise and determining the relative contribution of natural processes and human-induced factors to coastal erosion and shoreline retreat. This will represent invaluable information for decision-making and adaptation purposes.
Since 2020, AC is also one of the three PIs of a European Research Council (ERC) ‘Synergy’ project (named GRACEFUL; the other two PIs being Mioara Mandea from CNES and Veronique Dehant from the Royal Observatory of Belgium), whose objective is to study deep Earth processes occurring in the fluid core and at the core-mantle boundary, by analysing in synergy three global observables: the rotation, the internal magnetic field and the gravity field of the Earth. In the context of this project, AC and her team have recently discovered a 6-year cycle in the climate system that appears to be in phase with the already known 6-year cycle in the rotation of the Earth and in the internal magnetic field. AC’s current research now focusses on the interactions between the deep Earth and climate system at this particular frequency and tries to answer the questions : (1) are core processes driving the observed climate oscillations at the 6-year period?, (2) is it is the opposite direction (climate à core)? or (3) are core processes and climate responding independently to external driving mechanisms (e.g., the high-frequency Milankovitch orbital forcing that includes a clear 6-year frequency)? With her co-PIs of this ERC Synergy project, she is currently developing multi-disciplinary collaborations with different groups worldwide to shed light on this (so far unknown) puzzling property of the Earth System.
