[1] An accurate knowledge of the ocean mean dynamic topography (MDT) is mandatory for the optimal use of altimetric data, including their assimilation into operational ocean forecasting systems. A new global 1/4°resolution MDT was computed for the 1993-1999 time period with improved data and methodology compared to the previous RIO05 MDT field. First, a large-scale MDT is obtained from the CLS01 altimetric Mean Sea Surface and a recent geoid model computed from 4.5 years of GRACE (Gravity Recovery and Climate Experiment) data. Altimetric sea level anomalies and in situ measurements are then combined to compute synthetic estimates of the MDT and the corresponding mean currents. While the RIO05 MDT was based on 10 years of in situ dynamic heights and drifting buoy velocities, the new field benefits from an enlarged data set of in situ measurements ranging from 1993 to 2008 and includes all hydrological profiles from the Argo array. Moreover, the processing of the in situ data has been updated. A new Ekman model was developed to extract the geostrophic velocity component from the drifting buoy measurements. The handling of hydrologic measurements has also been revisited. Compared to the previous RIO05 solution, the new global MDT resolves much stronger gradients in western boundary currents, with mean velocities being doubled in some places. Moreover, in comparison to several other recent MDT estimates, we find that the new CNES-CLS09 MDT is in better agreement with independent in situ observations. Citation: Rio, M. H., S. Guinehut, and G. Larnicol (2011), New CNES-CLS09 global mean dynamic topography computed from the combination of GRACE data, altimetry, and in situ measurements,

[1] The lack of an accurate geoid still prevents precise computation of the ocean absolute dynamic topography from satellite altimetry and only sea level anomalies (SLA) can be accurately deduced. In the new context of Global Ocean Data Assimilation Experiment (GODAE) where models are assimilating satellite altimetry, the estimation of a realistic mean dynamic topography (MDT) consistent with SLA is a crucial issue. In a first ''direct'' approach, a MDT is computed by subtracting the geoid model EIGEN-2 from the Mean Sea Surface Height CLS01, determined from 7 years of altimetric data (TOPEX and ERS1,2) at spherical harmonic degree 30. To provide the scales shorter than 660 km, the Levitus climatology is merged with the resulting MDT, both weighted by their respective errors. This solution provides a ''first guess'' for the computation of a global and higher resolution MDT. Then, a ''synthetic'' technique is used to combine in situ measurements and altimetric data: TOPEX and ERS1,2 altimetric anomalies are subtracted from in situ measurements of the full dynamical signal (based on buoy velocities from the WOCE-TOGA program and XBT, CTD casts). The resulting values provide local estimates of the mean field, in terms of currents or dynamic topography, which are used to improve the first guess using an inverse technique. The MDT obtained is compared to other mean dynamic fields, and a verification using independent in situ data shows improvements in most areas. It exhibits a more energetic representation of the subtropical and subpolar gyres; sea level gradients associated with the main currents are strongly enhanced. Differences with independent velocity observations are globally lower than 13 cm/s rms.

Accurate estimate of ocean surface currents is both a challenging issue and a growing end-users requirement. In this paper ocean currents are calculated at two levels (surface and 15 m depth) as the sum of the geostrophic and Ekman components. First, a new, global, 1 4°M ean Dynamic Topography, called the CNES-CLS13 MDT, has been calculated and is now available for use by the oceanographic community. By exploiting information from surface drifters and Argo floats, the new MDT resolves spatial scales beyond the resolution permitted by the recent Gravity and Ocean Circulation Experiment (GOCE) geoid models (125 km). Associated mean geostrophic speeds in strong currents are increased by 200% on average compared to GOCE-based mean currents. In addition, for the first time, a two-level, monthly, empirical Ekman model that samples a spiral-like behavior is estimated. We show that combining both pieces of information leads to improved ocean currents compared to other existing observed products.

Presented here are three mean dynamic topography maps derived with different methodologies. The first method combines sea level observed by the high-accuracy satellite radar altimetry with the geoid model of the Gravity Recovery and Climate Experiment (GRACE), which has recently measured the earth’s gravity with unprecedented spatial resolution and accuracy. The second one synthesizes near-surface velocities from a network of ocean drifters, hydrographic profiles, and ocean winds sorted according to the horizontal scales. In the third method, these global datasets are used in the context of the ocean surface momentum balance. The second and third methods are used to improve accuracy of the dynamic topography on fine space scales poorly resolved in the first method. When they are used to compute a multiyear time-mean global ocean surface circulation on a 0.5° horizontal resolution, both contain very similar, new small-scale midocean current patterns. In particular, extensions of western boundary currents appear narrow and strong despite temporal variability and exhibit persistent meanders and multiple branching. Also, the locations of the velocity concentrations in the Antarctic Circumpolar Current become well defined. Ageostrophic velocities reveal convergent zones in each subtropical basin. These maps present a new context in which to view the continued ocean monitoring with in situ instruments and satellites.