Hybrid Mean Dynamic Topography - HYBRID_MDT_CNES_CLS22_CMEMS2020

The Mean Dynamic Topography (MDT) is the temporal mean of the Sea Surface Height above the Geoid over a reference period (here 1993-2012). The Hybrid Mean Dynamic Topography contains the MDT-CNES-CLS18 for the global coverage and the MDT CMEMS 2020 for the Black Sea (MDT-CMEMS2020-BLK) and the Mediterranean Sea (MDT-CMEMS2020-MED).

The hybrid MDT-HYBRID-CNES-CLS18-CMEMS2020 is a combined product using data from the full GOCE mission (Nov. 2009-Oct. 2013), 10.5 years of GRACE mission data, as well as 25 years of altimetry data (1993-2018) and 23 years of in-situ data (hydrology 1993-2016, drifters 1997-2016).

The method used to calculate the Mean Dynamic Topography is similar to the one used in previous CNES-CLS MDT versions. This is a three step approach:

1/ In a first step, a first guess MDT solution is calculated from the optimally filtered differences between an altimeter Mean Sea Surface (MSS) and a geoid model. The effective resolution of the obtained field depends on the level of noise of the raw difference between MSS and geoid height.

2/ In the second step of the method, synthetic estimates of the MDT and mean geostrophic velocities are calculated using in-situ measurements of the ocean dynamic heights and surface velocities. First the insitu measurements are processed so as to extract the geostrophic component only from the drifting buoy total velocities, and to complete the dynamic heights with the missing barotropic and deep baroclinic components. The temporal variability of the measured heights and velocities is further removed by subtracting the altimeter sea level and geostrophic velocity anomalies respectively. The processed in-situ measurements are further averaged into boxes to obtain respectively the synthetic mean heights and velocities.

3/ Then, using a multivariate objective analysis, the synthetic estimates of mean heights and mean velocities are combined to improve the first guess of the MDT.