Geography Reference
In-Depth Information
annular modes exist year round in the troposphere, but are strongest in the winter
when they extend well into the stratosphere, especially in the Northern Hemi-
sphere. The zonally symmetric mean-flow anomalies associated with the annular
modes are apparently maintained by anomalous eddy momentum fluxes, which are
themselves influenced by the zonally symmetric flow anomalies. Because these
modes have their greatest influence at high latitudes, the northern and southern
annular modes are sometimes referred to as the Arctic and Antarctic oscillations,
respectively; it should be stressed, however, that they are not periodic oscillations,
but rather represent two extremes of a broad distribution of climate states, with a
wide range of associated time scales.
There is some evidence for downward propagation of the wintertime annular
modes, suggesting that circulation changes in the stratosphere may precede annular
mode changes in the troposphere. Dynamical linkages between the stratosphere
and the troposphere are discussed further in Chapter 12.
10.6.3
Sea Surface Temperature Anomalies
Sea surface temperature anomalies influence the atmosphere through altering the
flux of latent and sensible heat from the ocean, and thus providing anomalous
heating patterns. The efficacy of such anomalies in exciting global scale responses
depends on their ability to generate Rossby waves. A thermal anomaly can generate
a Rossby wave response only by perturbing the vorticity field. This requires that
the thermal anomaly produces an anomalous vertical motion field, which in turn
produces anomalous vortex tube stretching.
For low-frequency disturbances the thermodynamic energy equation (10.5) may
be approximated as
wN
2
HR
−
1
·∇
+
≈
V
T
J/c
p
(10.74)
Thus, diabatic heating can be balanced by horizontal temperature advection or by
adiabatic cooling due to vertical motion. The ability of diabatic heating produced
by a sea surface temperature anomaly to generate Rossby waves depends on which
of these processes dominates. In the extratropics SST anomalies primarily generate
low-level heating, which is balanced mainly by horizontal temperature advection.
In the tropics, positive SST anomalies are associated with enhanced convection,
and the resulting diabatic heating is balanced by adiabatic cooling. Tropical anoma-
lies have their greatest effect in the Western Pacific where the average sea surface
temperature is very high so that even a small positive anomaly can generate large
increases in evaporation due to the exponential increase of saturation vapor pres-
sure with temperature. By continuity of mass the upward motion in cumulonimbus
convection requires convergence at low levels and divergence in the upper tropo-
sphere. The low-level convergence acts to sustain the convection by moistening
and destabilizing the environment, whereas the upper-level divergence generates
