33
because of the constraint of geostrophy, pressure changes are invariably
accompanied by wind changes somewhere in the atmosphere, but a horizontally uniform warming or cooling could occur without affecting the wind
field.
Because enhanced ascent in one region must be accompanied by enhanced subsidence over the remainder of the globe, the field of precipitation anomalies exhibits some tendency for compensation. However, since
precipitation rates depend not only upon vertical motion, but also upon
the available moisture, the compensation is not as complete as in the case
of sea-level pressure. Because vertical velocity is closely related to horizontal divergence which, in turn, is related to the Laplacian of the pressure
field, rainfall anomalies tend to occur on smaller horizontal scales than the
concomitant pressure or geopotential height anomalies.
2.2 Geometrical considerations
In contrast to the categorization of temporal variability, which is inherently
one-dimensional, the description of spatial variability involves consideration of two or three dimensions, though in practice, the task is usually
simplified by reducing the dimensionality of the space domain. Perhaps
the most common simplification is the use of zonal averaging to eliminate
the longitude dimension so that fields such as surface temperature can be
displayed as functions of latitude only and three dimensional fields can be
displayed in the form of latitude-height sections. This approach is deeply
ingrained in the general circulation literature. If the phenomenon of interest is well represented by zonal-mean data at a particular latitude, it may
be possible to collapse such displays to one spatial dimension, as in Fig. 7
of chapter l.
Since extratropicallow frequency circulation anomalies tend to be equivalent barotropic (i.e., similar in shape throughout the depth of the troposphere, with amplitude increasing with height), they are often displayed by
patterns at a single level such as 500-mb, which is viewed as representative
of vertically averaged conditions. The more significant tropical circulation anomalies, on the other hand, project more strongly onto the first
baroclinic mode, which is characterized by wind and pressure anomalies
of opposing sign in upper and lower troposphere, with large temperature
anomalies in the intervening layer (e.g., see Gi1l1980). Hence, display of
anyone of these variables reveals a considerable amount of information
because of the constraint of geostrophy, pressure changes are invariably
accompanied by wind changes somewhere in the atmosphere, but a horizontally uniform warming or cooling could occur without affecting the wind
field.
Because enhanced ascent in one region must be accompanied by enhanced subsidence over the remainder of the globe, the field of precipitation anomalies exhibits some tendency for compensation. However, since
precipitation rates depend not only upon vertical motion, but also upon
the available moisture, the compensation is not as complete as in the case
of sea-level pressure. Because vertical velocity is closely related to horizontal divergence which, in turn, is related to the Laplacian of the pressure
field, rainfall anomalies tend to occur on smaller horizontal scales than the
concomitant pressure or geopotential height anomalies.
2.2 Geometrical considerations
In contrast to the categorization of temporal variability, which is inherently
one-dimensional, the description of spatial variability involves consideration of two or three dimensions, though in practice, the task is usually
simplified by reducing the dimensionality of the space domain. Perhaps
the most common simplification is the use of zonal averaging to eliminate
the longitude dimension so that fields such as surface temperature can be
displayed as functions of latitude only and three dimensional fields can be
displayed in the form of latitude-height sections. This approach is deeply
ingrained in the general circulation literature. If the phenomenon of interest is well represented by zonal-mean data at a particular latitude, it may
be possible to collapse such displays to one spatial dimension, as in Fig. 7
of chapter l.
Since extratropicallow frequency circulation anomalies tend to be equivalent barotropic (i.e., similar in shape throughout the depth of the troposphere, with amplitude increasing with height), they are often displayed by
patterns at a single level such as 500-mb, which is viewed as representative
of vertically averaged conditions. The more significant tropical circulation anomalies, on the other hand, project more strongly onto the first
baroclinic mode, which is characterized by wind and pressure anomalies
of opposing sign in upper and lower troposphere, with large temperature
anomalies in the intervening layer (e.g., see Gi1l1980). Hence, display of
anyone of these variables reveals a considerable amount of information