32
have considered all seasons. The differing research priorities are rooted in
differing paradigms concerning the spatial structure and seasonality of climate variability. In this chapter we will examine these contrasting ways of
conceptualizing and analyzing climate variability and attempt to reconcile
them.
2 Background
2.1 Some dynamical considerations
The sea-level pressure field is unique among climatic variables in the sense
that in the global average, the anomalies at any given time must be very
close to zero: i.e., there must be almost perfect compensation between
positive and negative anomalies. This constraint is a consequence of the
fact that the mass of the atmosphere does not change appreciably on the
interdecadal time scale. Strictly speaking, it applies to surface pressure as
opposed to sea-level pressure, but the two are very closely related over most
of the globe (Trenberth and Christy, 1985). Because ofthis constraint, the
anomalies in the sea-level pressure field tend to assume the form of dipole
structures or wavelike patterns. In contrast, there is no reason why positive
temperature anomalies or trends in one region need to be compensated by
negative anomalies or trends somewhere else. This structural distinction
between the sea-level pressure and temperature fields is clearly reflected in
the statistical correlation patterns for the Southern Oscillation, as defined
by Walker and Bliss (1932), in which the pressure pattern is characterized as an east-west 'seesaw' between the eastern and western sides of the
tropical South Pacific, whereas the corresponding temperature pattern is
characterized by fluctuations of the same polarity throughout virtually the
entire tropics.
Even though the anomaly patterns in the temperature and pressure
fields are capable of assuming quite different forms, their horizontal structures are related through the hypsometric equation. Hence, temperature
anomalies are bound to be reflected in the pressure (or geopotential height)
field. For example, a spatially uniform warming would be reflected in rising heights aloft, whereas a localized warming of the lower troposphere
might be reflected in a local drop in sea-level pressure. However, it should
be noted that pressure changes can occur in the absence of temperature
changes, provided that they are purely barotropic. In a similar manner,
have considered all seasons. The differing research priorities are rooted in
differing paradigms concerning the spatial structure and seasonality of climate variability. In this chapter we will examine these contrasting ways of
conceptualizing and analyzing climate variability and attempt to reconcile
them.
2 Background
2.1 Some dynamical considerations
The sea-level pressure field is unique among climatic variables in the sense
that in the global average, the anomalies at any given time must be very
close to zero: i.e., there must be almost perfect compensation between
positive and negative anomalies. This constraint is a consequence of the
fact that the mass of the atmosphere does not change appreciably on the
interdecadal time scale. Strictly speaking, it applies to surface pressure as
opposed to sea-level pressure, but the two are very closely related over most
of the globe (Trenberth and Christy, 1985). Because ofthis constraint, the
anomalies in the sea-level pressure field tend to assume the form of dipole
structures or wavelike patterns. In contrast, there is no reason why positive
temperature anomalies or trends in one region need to be compensated by
negative anomalies or trends somewhere else. This structural distinction
between the sea-level pressure and temperature fields is clearly reflected in
the statistical correlation patterns for the Southern Oscillation, as defined
by Walker and Bliss (1932), in which the pressure pattern is characterized as an east-west 'seesaw' between the eastern and western sides of the
tropical South Pacific, whereas the corresponding temperature pattern is
characterized by fluctuations of the same polarity throughout virtually the
entire tropics.
Even though the anomaly patterns in the temperature and pressure
fields are capable of assuming quite different forms, their horizontal structures are related through the hypsometric equation. Hence, temperature
anomalies are bound to be reflected in the pressure (or geopotential height)
field. For example, a spatially uniform warming would be reflected in rising heights aloft, whereas a localized warming of the lower troposphere
might be reflected in a local drop in sea-level pressure. However, it should
be noted that pressure changes can occur in the absence of temperature
changes, provided that they are purely barotropic. In a similar manner,