3.3.4 Sea-level changes
The observation requirements for GOCE did never include the object of sparating steric and non-steric sea level change. Hower, the possibility of doing this anyway have studied in this chapter. Being one of the issues og CGRACE (GRACE, 1998), it is howver, just on the border of what can be achieved with a satellite like GOCE also.
In general, the gravitational attraction from a source is a product (linear apprioximation) of the effect of source geometry (sea surface topography) and of the mass density contrast to the surroundings. A way of separating steric- and non-steric sea surface is to include the knowledge about the inducing mechanism, e.g. the seasonal heat from the sun. This is exactly the basis of the separation performed by (Stammer, 1997), where the meteorological data from ECMWF (heat flux, wind stress field and the climatological seasonal surface temperature, salinity boundary) as well as the state- of- the- art WOCE Parallel Ocean Climate Model were used. The independent measurements of the sea surface topogaphy were provided by the TOPEX/POSEIDON altimetric mission. The role of these data was to constrain the geographical extension (for different time periods) of the ocean topography anomalies associated with the steric- and the non-steric effect.
GOCE can be used to provide information about the total anomalous sea surface topography. Thus, referring to the above, the role of gravity data of GOCE mission will be similar to the information provided by the satellite altimetry data of the TOPEX/POSEIDON mission.
The investigation conducted here will make use of a stuctures chosen from Fig.1a in (Stammer, 1997) showing the Fall 1993 sea surface height anomaly with respect to a 2 years average. The structure is located in the Atlantic Ocean. The question asked will be whether the gravimetric signal generated by this anomalous sea surface topography will be visible in the gravity data of the GOCE mission at the satellite height. The structure in the Atlantic Ocean is approximately located 9oN to18oN and 300oE to 331oE. It can be approximated by a rectangular prism of height -2 cm.
Twenty realizations of noise in the gravity field caused by error in the spherical harmonic coefficients (SHC) was generated for the areafor the two types of error statistics provided by GOCE and EGM96. Moreover, the gravity signal at the height of 300 km of the gravity response from the anomalous SSH structures (see above) was computed. A mass density contrast (transition from the sea water to the free air) of 1034 kg/m3 was used.The objective of this simulation study is to investigate whether the signal from the structure will be visible above the noise level.
It was argued above that the use of gravity data alone for the separation of steric- and non-steric ocean topography is not possible. Gravimetric signal at some height consist of a combined effect of source geometry (the sea surface topography) and the mass density contrast. These effects cannot be separated aquately from the gravity signal alone. Another problem is that even if it was possible to perform a perfect separation of the geometry and the mass density contrast, this would not be diagnostic for detecting a steric effect. There are other factors in the ocean that can affect the mass density variation of the sea water at the same order of magnitude and more then the steric effect. Thus, it is not possible to diagnose a steric effect without a detailed knowledge of the other factors.
The investigation above show that the gravitational attraction of the anomalous SSH cannot be seen above the noise level, even for clearly improved gravity data of GOCE mission (as compared to EGM96), see Figs. 18.104.22.168 and 22.214.171.124. The magnitude of the gravity signal in the Atlantic Ocean at the height of 300 km generated by the anomalous SSH structure (horizontal extension; NSxEW: 1008 km x 3472 km; thickness: 2 cm) is only some 0.3 microgal, see Fig. 126.96.36.199. The standard deviation of the noise is (ensamble average for 20 realizations of noise) 141 microgal for EGM96 and some 4 microgal for GOCE. Roughly speaking, for the EGM96 gravity field the structure would be visible above the noise level if it was 9.4 m thick and for the GOCE improved gravity field model if it was 0.27 m thick.
The observation requirements for GOCE did never include the object of sparating steric and non-steric sea level change. Hower, this investigation into the possibility of doing this anyway revieled that, although the improvement in the gravity models by GOCE mission is quite remarkable, it is not (as yet) possible to detect the signals at which one could study steric/non-steric sea level changes.
Note. The corresponding figures and investigation for a similar structure in the pacific ocean can be found via the project home page. http://www.gfy.ku.dk/~cct/goce-study.htm