Geography Reference
In-Depth Information
10.5.2
Jetstream and Storm Tracks
When the longitudinally asymmetric geopotential anomalies associated with sta-
tionary waves are added to the zonal-mean geopotential distribution, the resulting
time mean field includes local regions of enhanced meridional geopotential gradi-
ent that are manifested in the wind field of the Northern Hemisphere by the Asian
and North American jetstreams. The existence of these two jets can be inferred
from the January mean 500-hPa geopotential height field shown in Fig. 6.3. Note
the strong meridional gradients in height associated with the troughs centered just
east of the Asian and North American continents (the same features can be seen in
annual mean charts, although with somewhat reduced intensity). The zonal flow
associated with these semipermanent troughs is illustrated in Fig. 6.2. In addition
to the two intense jet cores in the western Pacific and western Atlantic, there is a
third weaker jet centered over North Africa and the Middle East. Figure 6.2 shows
dramatically the large deviations from zonal symmetry in the jetstream structure.
In midlatitudes the zonal wind speed varies by nearly a factor of three between
the core of the Asian jet and the low wind speed area in western North America.
Although, as was argued earlier, the climatological stationary wave distribution
on which the Asian and North American jets are superposed is apparently forced
primarily by orography, the structure of the jets also appears to be influenced
by continent-ocean heating contrasts. Thus, the strong vertical shear in Asian and
North American jets reflects a thermal wind balance consistent with the very strong
meridional temperature gradients that occur in winter near the eastern edges of the
Asian and North American continents due to the contrast between warm water
to the southeast and cold land to the northwest. A satisfactory description of the
jetstreams must account, however, not only for their thermal structure, but for the
westerly acceleration that air parcels must experience as they enter the jet, and
the deceleration as they leave the jet core.
To understand the momentum budget in the jetstreams and its relationship to
the observed distribution of weather, we consider the zonal component of the
momentum equation, which (if we neglect the β effect) may be written in the form
of (6.38):
f
0
v
v
g
≡
D
g
u
g
Dt
=
−
f
0
v
a
(10.70)
where v
a
is the meridional component of the ageostrophic wind. This equation indi-
cates that the westerly acceleration (Du
g
/Dt) > 0) that air parcels experience as
they enter the jet can only be provided by a poleward ageostrophic wind compo-
nent (v
a
> 0), and conversely, the easterly acceleration that air parcels experience
as they leave the jet requires an equatorward ageostrophic motion. This meridional
flow, together with its accompanying vertical circulation, is illustrated in Fig. 10.16.
Note that this secondary circulation is thermally direct upstream of the jet core. A
magnitude of v
a
∼
3ms
−
1
is required to account for the observed zonal wind
2
−
