Geography Reference
In-Depth Information
flux in the upper troposphere, however, appears to act to maintain the jet structure.
In both cases the secondary ageostrophic circulation associated with the jet tends
to partly balance the influence of the transient eddy fluxes in order to maintain the
mean thermal and momentum balances.
10.6
LOW-FREQUENCY VARIABILITY
An understanding of the general circulation requires consideration not only of
the zonal-mean and stationary wave components and their variations with the
annual cycle, but also of irregular variability on time scales longer than that of
individual transient eddies. The term
low-frequency variability
is generally used
to describe such components of the general circulation. The observed spectrum of
low-frequency variability ranges from weather anomalies lasting only 7-10 days
to interannual variability on the scale of several years (see Section 11.1.6).
One possible cause of atmospheric low-frequency variability is forcing due to
anomalies in sea surface temperature (SST), which themselves arise from coherent
air-sea interaction. Because of the large thermal inertia of the oceanic surface
mixed layer, such anomalies tend to have time scales much longer than those
associated with subseasonal variations in the atmosphere; they are thought to be
of greatest significance on the seasonal and interannual time scales.
It is believed, however, that substantial variability on subseasonal timescales can
arise in midlatitudes in the absence of anomalous SST forcing as a result of internal
nonlinear atmospheric dynamics, although SST anomalies may tend to favor the
occurrence of certain types of variations. One example of internally generated low-
frequency variability is the forcing of large-scale anomalies by potential vorticity
fluxes of high-frequency transient waves. This process appears to be important
in the maintenance of high-amplitude quasi-stationary wave disturbances called
blocking
patterns. Some types of blocking may also be related to special nonlinear
wave patterns called
solitary waves
, in which damping by Rossby wave dispersion
is balanced by intensification due to nonlinear advection. Although most internal
mechanisms involve nonlinearity, there is some evidence that the longitudinally
dependent time-mean flow may be unstable to linear barotropic normal modes that
are stationary in space, but oscillate in time at low frequencies. Such modes, which
are global in scale, may be responsible for some observed teleconnection patterns.
10.6.1
Climate Regimes
It has long been observed that extratropical circulation appears to alternate between
a so-called high-index state, corresponding to a circulation with strong zonal flow
and weak waves, and a “low-index” state with weak zonal flow and high-amplitude
waves. This behavior suggests that more than one climate regime exists consistent
with a given external forcing and that the observed climate may switch back and
