Abstract. TOPEX/Poseidon (T/P) altimeter data in the Black

TOPEX (Ku band) and POSEIDON altimeter measurements at crossover points are used to estimate the sea state bias (SSB) of these two instruments. Different SSB models are tested, ranging from a constant fraction of the significant wave height (SWH) to more elaborate models involving up to four adjustable parameters. For TOPEX, the data show a decrease in the magnitude of the relative bias (SSB/SWH) with SWH. This behavior is well reproduced using a simple empirical model with two adjustable parameters. The three‐parameter SSB model used in the NASA geophysical data records does well in explaining the wind‐induced variations of the bias. A model including four adjustable parameters is needed to account for both the wind‐ and SWH‐related variability of the SSB. POSEIDON data analysis reveals a significantly larger SSB than for TOPEX. This bias seems to consist of a skewness plus tracker bias of −2 to −3% of SWH superimposed on a natural EM bias whose wind‐ and SWH‐related variations are similar to those of TOPEX.

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.

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16Since October 19, 2016, and

The future Surface Water Ocean Topography (SWOT) mission due to be launched in 2022 will extend the capability of existing nadir altimeters to enable two-dimensional mapping at a much higher effective resolution. A significant challenge will be to assimilate this kind of data in high-resolution models. In this context, Observing System Simulation Experiments (OSSEs) have been performed to assess the impact of SWOT on the Mercator Ocean and Copernicus Marine Environment Monitoring Service (CMEMS) global, high-resolution analysis and forecasting system. This paper focusses on the design of these OSSEs, in terms of simulated observations and assimilation systems (ocean model and data assimilation schemes). The main results are discussed in a companion paper. Two main updates of the current Mercator Ocean data assimilation scheme have been made to improve the assimilation of information from SWOT data. The first one is related to a different parametrisation of the model error covariance, and the second to the use of a four-dimensional (4D) version of the data assimilation scheme. These improvements are described in detail and their contribution is quantified. The Nature Run (NR) used to represent the “truth ocean” is validated by comparing it with altimeter observations, and is then used to simulate pseudo-observations required for the OSSEs. Finally, the design of the OSSEs is evaluated by ensuring that the differences between the assimilation system and the NR are statistically consistent with the misfits between real ocean observations and real-time operational systems.