M-H Rio 1 , F.Hernandez 2 , J-M Lemoine 3 , R. Schmidt 4 , Ch. Reigber 4

1. (u,v) (u’a,v’a) u - u’a = v - v’a geoid =  ’= h ’ h h - h’a = cm AN IMPROVED MEAN DYNAMIC TOPOGRAPHY COMPUTED GLOBALLY COMBINING GRACE DATA, ALTIMETRY AND IN-SITU MEASUREMENTS • M-H Rio1, F.Hernandez2, J-M Lemoine3, R. Schmidt4, Ch. Reigber4 1, ISAC/ Gruppo di Oceanografia da satellite, Via Fosso del Cavaliere 100, 00133 Rome, Italy - MH.rio@isac.cnr.it2, CLS/Space Oceanography Division, 8-10 rue Hermes, Parc Technologique du canal, 31526 Ramonville,France - fabrice.hernandez@cls.fr 3, LEGOS/GRGS 18 av. E. Belin, 31401, Toulouse Cedex, France, Jean-Michel.Lemoine@cnes.fr - 4, GeoForschungsZentrum (GFZ), Potsdam, Germany The main objective of estimating a global and accurate Mean Dynamic Topography (MDT) is to compute absolute sea level heights from altimetric data. This is a key issue if the altimetric information has to be assimilated in operationnal forecasting systems (GODAE, MERCATOR, MFSTEP). Strong improvements in large scale geoid accuracy have been allowed by recent GRACE data. But if the shortest scales of the MDT have to be resolved, GRACE data have to be combined to other types of data. A method is developped to estimate a global,“full scale”, high resolution (1/4°) Mean Dynamic Topography (MDT) of the ocean. The method is made of three steps and allows to combine the recent geoid information from GRACE data to in-situ measurements of the ocean dynamic and altimetric data. The ocean Mean Dynamic Topography as deduced from the direct use of most recent global geoid models :The Direct Method Geoid models issued from GRACE data analysis are now available. We use here the EIGEN-GRACE02S geoid field computed at GFZ from 110 days of GRACE data and an altimetric Mean Sea Surface (MSS CLS01) to estimate the ocean Mean Dynamic Topography (hereafter GR2-60 MDT) at degree 60 of spherical harmonic expansion, corresponding to spatial scales greater than 333 km. For comparison similar MDT are derived using the EIGEN-GRACE01S geoid (hereafter GR-60 MDT) and the EIGEN2 geoid (hereafter EIG-60 MDT). 1 The Synthetic Method GR2-60 MDT GR2-60 MDT The variable part of the dynamic topography h’ and the corresponding geostrophic circulation u’,v’ as measured by altimetry (T/P, ERS1-2) is subtracted from the absolute dynamic topography h and the associated geostrophic circulation (u,v) as given by in-situ measurements to obtain synthetic estimates of the mean dynamic topography and the corresponding mean geostrophic circulation , EIG-60 MDT GR-60 MDT 2 Construction of a first guess 15 m drogued buoy velocities from 1993 to 2002. Ekman currents are modelled (Rio and Hernandez, 2003) and removed. Other ageostrophic phenomena are filtered (3 days filter). RMS differences to independent observations computed using the GR2-60 MDT and the Levitus climatology referenced at 1500m are used as weights to merge both surfaces. A FIRST GUESS of the MDT is obtained, which is very close to the Levitus climatology at low and mid-latitudes but which features at high latitudes and in Western Boundary Currents the strong and realistic gradients contained in the GR2-60 solution. cm Set of synthetic velocity estimates At scales larger than 333 km, the EIGEN-GRACE02S geoid provides a proxy of the ocean mean circulation that is globally more realistic (example above in the Gulfstream area), as proved using totally independent observations of the surface ocean currents over a decade. V (cm/s) U (cm/s) Zonal RMS differences Meridional RMS differences Hydrological profiles at various depths from 1993 to 2002. MDT at reference depth is approximated removing the Levitus climatology at reference depth from the chosen first guess. Set of synthetic MDT estimates RMS differences are computed globally between the geostrophic velocities as measured by drifting buoys available from 1993 to 2003 (more than 6000000 data) and absolute altimetric velocities obtained referencing the altimetric SLA interpolated along the buoy trajectories to the various MDTs (EIG-60, GR-60 and GR2-60). cm 3 cm The values obtained in 20° by 20° boxes using the GR2-60 MDT can be used to evaluate the accuracy of the EIGEN-GRACE02S geoid model: These values account for errors on GRACE geoid model but also for errors on in situ geostrophic velocities (measurement and processing errors) and errors on interpolated altimetric velocity anomalies. For both velocity components, strongest differences (from 15 to 20 cm/s) are obtained in the equatorial band as well as in Western Boundary Currents (WBC). Elsewhere, differences are less than 15 cm/s. Lowest differences (5-8 cm/s) are obtained at mid latitudes in the central and eastern Pacific. This set of synthetic estimates and the associated errors are used to improve the First Guess through a Multivariate Objective Analysis and map the MDT on a global ¼° regular grid. However, mainly at low and mid latitudes (see black dots on the plots), reduced RMS differences to in-situ velocities are obtained using the Levitus climatology (in respect to 1500m) to reference the altimetric anomalies. CMDT RIO04 The Mediterranean Sea A similar method is applied to estimate the Mediterranean MDT. In the particular case of the Mediterranean Sea, which is characterized by shorter scales than the open ocean, the GR2-60 MDT, too smooth, can not be used as MDT first guess. We rather use the mean issued for the period 1993-1999 from MFSTEP model. The synthetic method is applied on all drifting buoys available in the area for the period 1993-1999. Data were initially processed at OGS, Trieste and low-pass filteres (36h) to remove ageostrophic components as tides, inertial oscillations…Furthermore, the Ekman component was modelled (Mauri et al., 2004) and removed. cm The Synthetic Mediterranean Mean Dynamic Topography (1/8° grid) Number of drifting buoys used in the estimation (in 1/8° boxes) Validation of the CMDT RIO04Comparison to other existing solutions: 1- CMDT RIO03 computed using the same method but using information from the EIGEN2 geoid model to compute the first guess instead of EIGEN-GRACE02S and a reduced synthetic dataset (velocities from 1993 to 1999 and dynamic heights from 1993 to 2000). See Rio and Hernandez (2004) for details. 2- Ten years (1993-2002) average of ECCO (Estimating the Circulation and Climate of the Ocean) model outputs, adjusted to the period 1993-1999. 3- Solution based on drifting buoy velocities (Niiler et al, 2003) computed for the period 1993-2002 and adjusted to the period 1993-1999. NIILER1: full dataset was used.NIILER2: only data for winds observed by drifters less than 8 cm/s were used. ECCO NIILER2 cm A validation of the SMDT is given in the Bonifacio gyre area, where CTD profiles were measured during the NORBAL2 experiment. Dynamic heights relative to 500m (circles), 1000m (squares) and 1500m (triangles) are superimposed on altimetric maps. When the SMDT is added to the SLA, absolute altimetric heights get closer to in-situ observations (the cyclonic Bonifacio gyre gets deeper, in consistancy with observations). 49 cm 54 cm SLA SLA + MFSTEP mean SLA + SMDT cm cm Method: Absolute velocities measured by independent drifting buoys and dynamic heights relative to 1500m from independent in-situ profiles (year 2003) are compared to absolute altimetric heights and velocities obtained referencing the SLA to the various Mean Dynamic Topographies. For each solution, Root Mean Square (RMS) differences between the two data sets are computed These high values are due to the strong mean height difference between the Pacific and Atlantic oceans (49 cm for ECCO, 54 cm for NIILER2 and 1m for NIILER1) compare to what is measured by hydrology (39 cm). On the contrary, consistent values are obtained in CMDT RIO03-04 (38cm) and in GR2-60 MDT (42 cm) cm Best results are obtained using CMDT RIO04to reference the altimetric anomalies Conclusions This work highlighted the strong improvements of recent geoid models issued from GRACE data to estimate the ocean Mean Dynamic Topography at large scale. Major contribution is at high latitudes, where in-situ observations of the ocean dynamics are scarse. A method was presented allowing to combine this large scale information to in-situ measurements and altimetric data and compute a full scale Mean Dynamic Topography on a 1/4° resolution grid. The solution obtained (CMDT RIO04) was shown to be more efficient to reference altimetric anomalies than other existing solutions. Also, a Synthetic Mean Dynamic Topography was computed in the Mediterranean Sea. The accurate determination of the MDT is a key issue if altimetric signal has to be assimilated into operational forecasting systems (GODAE, MERCATOR, ENACT,MFSTEP…) Observations available in 2003 (= independent data) used to validate the CMDT RIO04 ! This dataset contains only few data at high latitudes. The RMS values obtained do not entirely reflect the contribution in the computation of CMDT RIO04 of EIGEN-GRACE02S geoid, which is expected to be important mostly at high latitudes! References: * Rio, M.-H. and Hernandez, F., 2003. High-frequency response of wind-driven currents measured by drifting buoys and altimetry over the world ocean. Journal of Geophysical Research, 108(C8): 3283-3301.* Rio, M.-H. and Hernandez, F., 2004. A Mean Dynamic Topography computed over the world ocean from altimetry, in-situ measurements and a geoid model. Journal of Geophysical Research (accepted). * Niiler, P., Maximenko, N.A. and McWilliams, J.C., 2003. Dynamically balanced absolute sea level of the global ocean derived from near-surface velocity observations. Geophysical Research Letters, 30(22): 2164-2167.*Mauri, E.and P.-M.Poulain (2004) Wind-driven current in Mediterranean drifter data, OGS Tech. Report 1/2004 OGA-1, OGS, Trieste, Italy, 25pp. Hydrological profiles Drifting buoy trajectories