estimation of a global mean dynamic topography ...

ESTIMATION OF A GLOBAL MEAN DYNAMIC TOPOGRAPHY USING GRAVIMETRY, ALTIMETRY AND IN-SITU DATA M-H Rio1, F.Hernandez1, J-M Lemoine2 1,

CLS/Space Oceanography Division, 8-10 rue Hermes, Parc Technologique du canal, 31526 Ramonville,France 2, LEGOS/GRGS 18 av. E. Belin, 31401, Toulouse Cedex, France

The issue of estimating a global mean dynamic topography (MDT) is to reference the altimetric Sea Level Anomalies (computed relative to a 7 year (1993-1999) mean profile) in order to obtain absolute measurements of the ocean dynamic topography. Required MDT has to be consistent with altimetric physical content and shall therefore correspond to: The mean over 1993-1999 of the geostrophic, barotropic and baroclinic oceanic circulation

Direct Method h 93−99

The direct method is presently limited by the lack of an accurate geoid. However it will be more and more relevant in the future with the exploitation of GRACE data and the launch in 2005 of GOCE satellite.

η ’= h ’ h

G

h 93−99 = η93−99 − G

Absolute dynamic topography MSS Degree 30

We used the Mean Sea Surface computed at CLS by Hernandez and Schaeffer (2001) and the EIGEN2 geoid model computed at GRGS and GFZ including four months of CHAMP data (September-December 2001).

Mean dynamic topography

1

The method consists in subtracting the variable part of the dynamic topography as measured by altimetry to the absolute dynamic topography as measured by in-situ measurements.

h 93−99

Sea level anomaly Altimetric measurement Mean sea level η h Geoid h 93 −99 η93−99 Instantaneous sea level Reference ellipsoid

h 93−99 = h − η'

η’

Estimates of the MDT EIGEN 2 - Degree 30

Synthetic Method

This work combines two methods

η93−99

Implementation

In situ data processing to achieve coherency with SLA

Dynamic heights referenced to 700m from 1993 to 1999 A coefficient (deduced from OCCAM model outputs) is applied to compensate for the lack of the dynamic topography at 700m.

15 m drogued buoy velocities from 1993 to 1999 Ekman currents are modelled (Rio et al, 2002) and removed. Other ageostrophic phenomena are filtered (3 days filter)

EIGEN2 Degree 30 cumulated error

2 Interpolation of SLA (ERS 1-2 and T/P) to the buoys and dynamic heights day and position h93− 99 = h − h' (Multivariate Objective Analysis) alti u93 − 99 = u − u'alti v 93− 99 = v − v'alti 3

Before subtracting the geoid height to the altimetric MSS, spherical harmonic expansion method is used to extract from both surfaces only the wavelengths at which the geoid error is consistent with the oceanographic signal. Degree 30 of harmonic expansion is chosen, corresponding to a spatial resolution of 660 km.

Computation of 1° by 0.5° box means - Error associated: σ²/N where N is the number of independent data Efficiency

Estimates of the Mean Zonal Velocity

σ (uinsitu-u ’alti)

σ (hinsitu-h ’alti) σ (h ) insitu

σ (uinsitu)

σ (vinsitu-v ’alti)

σ (vinsitu)

Mean Dynamic Topography - Degree 30

Direct method allows to obtain information about the long wavelengths of the MDT, with higher confidence at high latitudes, where geoid errors are less than 10 cm.

Set of estimates of the MDT

Estimates of the Mean Meridian Velocity cm

Multivariate Objective Analysis

First Guess

m

m

This information is then used to improve the long wavelengths of Levitus climatology referenced to 700m so as to obtain a first guess of the MDT : To validate the SMDT, we compare it to other existing solutions: OCCAM: Three years (1993-1995) average of the XBT assimilating (Troccoli and Haines, 1999) OCCAM model outputs LEGRAND: Solution obtained using an inverse model (Legrand et al, 2002) - LEVITUS: climatology referenced to 700m (Levitus et al, 2001).

SMDT

U:17.9 cm/s V: 17.3 cm/s

ratio

Applications 1

OCCAM

Understanding better the general circulation

Contourne le Flemish Cap

North West corner Worthington,1976 Newfoundland basin ’s anticyclonic recirculation Rossby,1996 Caniaux et al,2001

Mann eddy Mann,1967

LEVITUS

LEGRAND

cm

Year 2001: 38429 data The Gulfstream Cap Hatteras detachment

Validation is made comparing the absolute velocities measured by independent drifting buoys (for year 2001) to absolute altimetric velocities obtained referencing the SLA to the various Mean Dynamic Topographies available. For each solution, Root Mean Square differences between the two data sets are computed (for the zonal and meridian velocity components). The use of the SMDT to reference the altimetric SLA allows to reduce the rms differences to the observations.

SMDT

U:15.5 cm/s V: 15.7 cm/s

OCCAM

U:17.5 cm/s

V: 19.4 cm/s

re-circulation cells Rossby et Gottlieb,1998

Cyclonic circulation

Labrador current ’s Rétroflexion Mann,1967 Pickart et al, 1999

Inter-annual variability of NAC main front is correlated to NAO index. When NAO index is high (1994-1995-1999), detected main fronts pass through the North West corner while they follow a straight path from south west to north east during low NAO years (1996-19971998)I

Year 2001: 40802 data

LEVITUS

U:16.6 cm/s V: 16.1 cm/s

Analysing main front’s seasonal and inter-annual variability

NAC front inter-annual variability

Northern bifurcation current ’s widening velocities decrease Fratantoni,2001 Reverdin,2001

Gulfstream front inter-annual variability

LEGRAND

2

SMDT is used together with CLS SLA maps produced as part of ENACT project to compute absolute dynamic topography maps from 1993 to 1999. These maps are then averaged monthly or yearly. Fronts are detected using the SACSO software (Mertz,2001)

Florida current - velocities 60 cm/s

U:16.2 cm/sV: 16.3 cm/s

Subtracting variability to observations avoids taking into account observations over several decades (climatologies…)

The North Atlantic current

U:18.5 cm/s V: 17.6 cm/s

U:19.0 cm/s V: 17.5 cm/s U:19.2 cm/s V: 17.5 cm/s

ratio

Without applying the synthetic method, an equivalent error would have necessitate 4 times more observations

Synthetic method allows to consider in situ data outside the 1993-1999 period (as long as a simultaneous altimetric information is available)

Synthetic Mean DynamicTopography (SMDT)

Validation

ratio

Ratio=0.5

Gulfstream front features a Northern position during high NAO years (1993-1994-1995 and 1999) and a southern position during low NAO years (1996-1997-1998). This is in good agreement with previous results (Rossby and Gottlieb, 1998). Reversal between 1995 (northern position) and 1996 (southern position) was also mentioned in a study by Chao (2001) using SST data.

Gulfstream front seasonal variability Gulfstream front was observed to present a relatively northern position in Autumn and a southern position in Spring (Tracey and Watts, 1986; Kelly and al, 1999; Guinehut, 2002). A very clear cycle is obtained in our study at 69°W with a northern position in October and a southern position in June.

Conclusions Red line corresponds to the Synthetic Mean Dynamic topography front. Gulfstream interannual and seasonal variations intensify east of 65°W

CONCLUSIONS: A global Mean Dynamic Topography was estimated combining complementary information from gravimetric, altimetric and in-situ measurements. Comparison to independent observations suggests that the obtained SMDT is more appropriate than other existing solutions to reference altimetric anomalies. Implemented methods allow continuous improvement of the solution (using new in-situ data or gravimetric data from GRACE or GOCE missions). Major application of the SMDT is the assimilation of absolute altimetric data into operational ocean forecasting systems as Mercator or ENACT (Enhanced ocean data Assimilation and Climate prediction). SMDT is already routinely used in Mercator PSY2 prototype. Impact in PSY1 prototype was studied by F. Davidson, 2001. (Sea the poster ‘Impact of an Observed MSSH on the MERCATOR operational ocean model ‘ from session 0S9 - Operational Oceanography) .

69°W

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