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time series no significant trend becomes evident, although other studies (Gruber et al.
2009 ) show changes of the geocenter up to 2.6mm per year in x-, 0.8 in y- and 6.7
in z-direction.
2.2 Variation of the Atmospheric Pressure
Although the change in the global mean of the atmospheric mass is small (the range
for the 30 years in Fig. 4 is 5.5hPa), the changes within smaller temporal and spacial
scales can be of several tens of hPa. Such changes in the atmosphere are responsi-
ble for one of the biggest signals in the time variable gravity field (Warburton and
Goodkind 1977 ). Typically, the surface pressure variations are larger in mid and high
latitudes, whereas in tropical regions they are rather small. Figure 7 shows the RMS
pressure variability using ECMWF surface pressure data for 2008.
Generally atmospheric phenomena can be categorized based on their time vari-
ability. One class consists of long term variations, such as seasonal, annual and
inter-annual variations. The global atmospheric pressure cycle (Fig. 4 ), the seasonal
variation between northern and southern hemisphere or mass transport between land
and ocean such as the monsoon fall into this category. The El NiƱo southern oscil-
lation (ENSO) classifies for inter-annual variations (McPhaden 2002 ), just as the
North-Atlantic Oscillation (NAO) (Hurrell and van Loon 1997 ).
The second class contains short term variations, containing signals like the diurnal
and semi-diurnal solar tides (see Fig. 8 a, b). But the most prominent variations are
introduced by (anti-) cyclones, which can reach 50-60hPa in central Europe (Rabbel
and Zschau 1985 ) within a few days, in Scandinavia even 80hPa. But also extreme
events such as hurricanes fall into this class. At the center of a hurricane the pressure
can drop by 100hPa within hours.
Fig. 7 RMS of ECMWF surface pressure over the year 2008
 
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