FIGURE 8.1 The degree amplitudes in mass, expressed as the thickness of a water layer, for the annually-varying terms in the atmosphere. The atmospheric results are inferred from ECMWF pressure data from 1992-1993. The results for the satellite errors assume a 5-year satellite mission. The units on the y-axis are approximately equivalent to mbar of pressure.

Southern and Northern Hemispheres, and (3) between high elevations, such as the Tibetan Plateau, and lower elevations. These mass variations are caused by differential heating of the Northern and Southern Hemispheres and the land and the ocean (Van den Dool and Saha, 1993; Saha et al., 1994) and, as such, they are the mass counterparts of the monsoons. The mass redistributions are sufficiently large, both in terms of the pressure difference (20 mbar = ~200 mm of water) and the spatial scale, and they move sufficiently slowly to be detectable from space.

Other prominent regular features in the atmosphere are the diurnal and semi-diurnal solar tides. The departure of the surface pressure from the daily mean at a specific universal time is an expression of propagating tides (Figure 8.4). The tides are up to 2.5 mbar but exhibit both regular changes (April and October appear to have the strongest tides) and irregular changes. Although the tides move too quickly to be observed directly by gravity measurements, they are an important source of noise and, through their near-resonances with some orbits, can produce long-period (observable) perturbations to orbital elements. In Sun-synchronous measurement systems like the proposed gravity missions, it is important to know how the tides alias into lower frequencies.

Figures 8.3 and 8.4 display "climatology" or normal variations. Departures from normal variations can be large (up to about 50 mbar) and can occur over a wide spectrum of temporal and spatial scales, particularly at high and mid-latitudes (Figure 8.5). These anomalies drift around, decay, develop, and merge, usually with a slight net eastward phase speed at mid-latitudes, and a net westward phase speed at low lati-