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(a)
J
300
500
cold
Lower
sea-level pressure
Higher
sea-level pressure
Weak surface winds
1000
(b)
J
Strong convergence
of eddy flux of
westerly momentum
300
500
cool
Strong eddy
heat flux
Oceanic
lows
Subtropical
highs
Strong westerly surface winds
1000
60¡
30¡
40¡
50¡
Latitude
Fig. 10.16
Meridional cross sections showing the relationship between the time mean secondary
meridional circulation (continuous thin lines with arrows) and the jet stream (denoted
by J ) at locations (a) upstream and (b) downstream from the jet stream cores. (After
Blackmon et al., 1977. Reproduced with permission of the American Meteorological
Society.)
acceleration. This is an order of magnitude stronger than the zonal-mean indirect
cell (Ferrel cell) that prevails in midlatitudes. Downstream of the jet core, however,
the secondary circulation is thermally indirect, but much stronger than the zonally
averaged Ferrel cell. It is interesting to note that the vertical motion pattern on the
poleward (cyclonic shear) side of the jet is similar to that associated with deep
transient baroclinic eddies in the sense that subsidence occurs to the west of the
stationary trough associated with the jet, and ascent occurs east of the trough (see,
e.g., Fig. 6.12).
Because the growth rate of baroclinically unstable synoptic scale disturbances
is proportional to the strength of the basic state thermal wind, it is not surprising
that the Pacific and Atlantic jet streams are important source regions for storm
development. Typically, transient baroclinic waves develop in the jet entrance
region, grow as they are advected downstream, and decay in the jet exit region. The
role of these transient eddies in maintenance of the jetstream structure is rather
complex. Transient eddy heat fluxes, which are strong and poleward in the storm
tracks, appear to act to weaken the climatological jets. The transient eddy vorticity
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