Pierre-Yves Le Traon

Mean sea level and surface circulation variability of the Mediterranean Sea from 2 years of TOPEX/POSEIDON altimetry

We describe the circulation and mean sea level variations of the Mediterranean Sea from 2 years of TOPEX/POSEIDON altimetric data. It is first shown that the response of the Mediterranean Sea to atmospheric pressure forcing is close to an inverse barometer (except at high frequencies) which means that the adjustment is accompanied by a flow through the Straits of Sicily and Gibraltar. We then use TOPEX/POSEIDON to study the mean sea level variations, representing steric effects and integrated large-scale changes of the mass of the Mediterranean Sea. We observe an annual cycle with a fast drop during winter. Steric effects account for about half of the observed variations. The remaining signal is believed to be driven by evaporation minus precipitation (E - P) forcing and internal hydraulic control in the Straits of Gibraltar. Using suboptimal space-time objective analysis, the classic components of the Mediterranean surface circulation are recovered, despite low signal-to-noise ratio (the rms of sea level variability is less than 10 cm). The variable Mediterranean circulation is seen as a complex combination of mesoscale and large-scale variations. The surface circulation is more complex in the eastern basin than in the western basin. In the east it is composed of subbasin-scale gyres, such as the so-called Mersa-Matruh and Shikmona gyres, which do not have an obvious recurrence period. We also observe an intensification of the large-scale cyclonic winter circulation in the western and in the Ionian basins. Several mesoscale structures, such as the Alboran gyres and the Ierepetra gyre, show a clear seasonal cycle, with a maximum in summer. The good qualitative and quantitative agreement of the results with previous data from the Mediterranean illustrates the improved accurary of TOPEX/POSEIDON over its predecessors.

Sea level variations and their dependency on meteorological and hydrological forcing: Analysis of altimeter and surface data for the Black Sea

TOPEX/Poseidon (T/P) altimeter data in the Black Sea are analyzed for almost 5 years in parallel with available hydrological and meteorological data with the aim of studying the water balance and the dependency of sea level oscillations on meteorological and hydrological forcing. This forcing induces seasonal variations of mean sea level with oscillations of ∼10–15 cm. The consistency between satellite and tidal gauge data is demonstrated in several coastal locations, and a mean ascending trend of ∼3 cm yr−1 is found in the two data sets. The variability in all components of water balance, including the Bosphorus outflow calculated as the difference between the fresh water flux and the time rate of sea level change estimated from altimeter data, is analyzed. The T/P data give very clear signals in the patterns of amplitudes of oscillations at intraannual, seasonal, and interannual timescales that help in understanding the variability of circulation. The intraannual variations are well pronounced on the continental slope and shelf and reach highest amplitudes in the areas of Sevastopol and Batumi quasi-permanent eddies. The clearest representation of oscillations with seasonal periodicity exists in the area of Batumi Eddy. This variability is associated with the transition between states with intense cyclonic circulation in winter and weaker (sometimes anticyclonic) circulation in summer-fall period. The Sevastopol Eddy is not clearly resolved in the seasonal variability. The interannual variability has the strongest signature in the area of western gyre and southeastern Black Sea. The analysis of satellite data supports some earlier studies on the circulation based on dynamic computations and numerical modeling. They make it possible to estimate the amount of water exchanged between coastal and open ocean areas caused by the time variability in the Ekman drift. The good quality of altimeter data and the high level of signals could ensure more accurate numerical simulations by means of data assimilation.

On the Use of Satellite Altimeter Data in Argo Quality Control

Abstract Satellite altimeter measurements are used to check the quality of the Argo profiling floats time series. The method compares collocated sea level anomalies from altimeter measurements and dynamic height anomalies calculated from Argo temperature and salinity profiles for each Argo float time series. Different kinds of anomalies (sensor drift, bias, spikes, etc.) have been identified on some real-time but also delayed-mode Argo floats. About 4% of the floats should probably not be used until they are carefully checked and reprocessed by the principal investigators (PIs). The method appears to be very complementary to the existing quality control checks performed in real time or delayed mode. It could also be used to quantify the impact of the adjustments made in delayed mode on the pressure, temperature, and salinity fields.

Western Mediterranean sea‐level rise: Changing exchange flow through the Strait of Gibraltar

Sea-level data from tide gauges and satellite altimetry show a decrease of nearly 40% in the sea-level drop between the Atlantic and the Mediterranean from 1994 to 1997, coming mainly from a rapid rise of western Mediterranean sea-level by more than 10 mm/year. A decrease in the sea-level difference across the Strait, coincident with this Mediterranean rise, indicates that the surface inflow is reduced by an amount dynamically consistent with the change along the strait. These secular changes are accompanied by a seasonal cycle in the sea-level drop between the Atlantic and the Mediterranean, which indicates a seasonal flipping of hydraulic exchange states in the Strait of Gibraltar. Thus, we suggest the sea-level rise in the Mediterranean is a consequence of a changing exchange flow through the Strait of Gibraltar, driven indirectly by changing conditions in the Mediterranean.

On the Quality of Real-Time Altimeter Gridded Fields: Comparison with In Situ Data

Abstract The timeliness of satellite altimeter measurements has a significant effect on their value for operational oceanography. In this paper, an Observing System Experiment (OSE) approach is used to assess the quality of real-time altimeter products, a key issue for robust monitoring and forecasting of the ocean state. In addition, the effect of two improved geophysical corrections and the number of missions that are combined in the altimeter products are also analyzed. The improved tidal and atmospheric corrections have a significant effect in coastal areas (0–100 km from the shore), and a comparison with tide gauge observations shows a slightly better agreement with the gridded delayed-time sea level anomalies (SLAs) with two altimeters [Jason-1 and European Remote Sensing Satellite-2 (ERS-2)/Envisat] using the new geophysical corrections (mean square differences in percent of tide gauge variance of 35.3%) than those with four missions [Jason-1, ERS/Envisat, Ocean Topography Experiment (TOPEX)/Poseid...

Diagnosing Surface Mixed Layer Dynamics from High-Resolution Satellite Observations: Numerical Insights

High-resolution numerical experiments of ocean mesoscale eddy turbulence show that the wind-driven mixed layer (ML) dynamics affects mesoscale motions in the surface layers at scales lower than O(60 km). At these scales, surface horizontal currents are still coherent to, but weaker than, those derived from sea surface height using geostrophy. Vertical motions, on the other hand, are stronger than those diagnosed using the adiabatic quasigeotrophic (QG) framework. An analytical model, based on a scaling analysis and on simple dynamical arguments, provides a physical understanding and leads to a parameterization of these features in terms of vertical mixing. These results are valid when the wind-driven velocity scale is much smaller than that associated with eddies and the Ekman number (related to the ratio between the Ekman and ML depth) is not small. This suggests that, in these specific situations, three-dimensional ML motions (including the vertical velocity) can be diagnosed from high-resolution satellite observations combined with a climatological knowledge of ML conditions and interior stratification.

The Copernicus Marine Environment Monitoring Service Ocean State Report

ABSTRACT The Copernicus Marine Environment Monitoring Service (CMEMS) Ocean State Report (OSR) provides an annual report of the state of the global ocean and European regional seas for policy and decision-makers with the additional aim of increasing general public awareness about the status of, and changes in, the marine environment. The CMEMS OSR draws on expert analysis and provides a 3-D view (through reanalysis systems), a view from above (through remote-sensing data) and a direct view of the interior (through in situ measurements) of the global ocean and the European regional seas. The report is based on the unique CMEMS monitoring capabilities of the blue (hydrography, currents), white (sea ice) and green (e.g. Chlorophyll) marine environment. This first issue of the CMEMS OSR provides guidance on Essential Variables, large-scale changes and specific events related to the physical ocean state over the period 1993–2015. Principal findings of this first CMEMS OSR show a significant increase in global and regional sea levels, thermosteric expansion, ocean heat content, sea surface temperature and Antarctic sea ice extent and conversely a decrease in Arctic sea ice extent during the 1993–2015 period. During the year 2015 exceptionally strong large-scale changes were monitored such as, for example, a strong El Niño Southern Oscillation, a high frequency of extreme storms and sea level events in specific regions in addition to areas of high sea level and harmful algae blooms. At the same time, some areas in the Arctic Ocean experienced exceptionally low sea ice extent and temperatures below average were observed in the North Atlantic Ocean.

Satellites work in tandem to improve accuracy of data

Global observation of the oceans by satellite altimetry entered a new era with the European Space Agency ERS-1 and U.S./French TOPEX/POSEIDON missions, which were launched in July 1991 and August 1992, respectively. For the first time, two altimetric satellites were flying simultaneously. TOPEX/POSEIDON (TP) is an optimized altimetric mission dedicated to the precise observation of the large-scale oceanic circulation. ERS-1 is a multimission satellite. It carries a Synthetic Aperture Radar (SAR), a wind scatterometer, an along-track scanning radiometer and a radar altimeter.

Satellites and Operational Oceanography

The chapter starts with an overview of satellite oceanography, its role and use for operational oceanography. Main principles of satellite oceanography techniques are then summarized. We then describe key techniques of radar altimetry, sea surface temperature, ocean colour satellite measurements. This includes measurement principles, data processing issues and the use of these data for operational oceanography. SAR, scatterometry, sea ice and sea surface salinity measurements are also briefly described. Main prospects are given in the conclusion.

Contribution of a Wide-Swath Altimeter in a Shelf Seas Assimilation System: Impact of the Satellite Roll Errors

Abstract The authors investigate the potential qualitative improvement brought by wide-swath, interferometry-based ocean altimetry measurements with respect to classical nadir altimeters in a coastal/shelf data assimilation system. In addition, particular attention is paid to roll errors, which could significantly reduce the expected benefits of wide-swath altimetry. A barotropic, nonlinear free-surface model is set up over the European shelf as part of an ensemble Kalman filter. Experiments assimilating simulated data are performed over the North Sea to test the ability of altimeter configurations to reduce model errors due to the action of meteorological forcing in the presence of bathymetric uncertainties. A simplified wide-swath observation scheme is used, composed of nadir altimeter height plus a nadir-centered cross-track sea level slope measurement. The simplified wide-swath measurements are found to be able to constrain events unsampled by a single nadir altimeter owing to a wider domain of influe...