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(a)
35
0
Figure 4.2 Percentage
frequency distribution of
mean monthly 500-mb
trough and ridge axes at
45
8
Nby10
8
deg longitude
increments for (a) January
and (b) July. (From Harman
1991
)
Trough
Ridge
0
35
30
E 0
E 0
E
120
E
150
E
180
150
W 120
W90
W60
W30
W
(b)
35
0
Trough
Ridge
0
35
30
E 0
E 0
E
120
E
150
E
180
150
W 120
W 0
W 0
W 0
W
Similarly, the preferred locations of the winter season trough axes (608W,
1508 E and 308 E) are associated with surface low pressure positioned just
downstream. Two of these trough axes (608W and 1508 E) are located above
the east coasts of major continents where relatively warm, poleward flowing
ocean currents provide additional energy to fuel cyclonic storms. The Icelandic
and Aleutian ''centers of action'' are produced in this manner. Wind patterns and
thermal advection associated with the surface cyclones again help reinforce the
preferred location of ridges and troughs in the upper tropospheric westerlies.
Winds on the backside of a cyclone bring cold polar air southward reinforcing
the trough; while southerly surface flow to the east of the low helps advect warm
air poleward reinforcing the next, downstream, ridge.
The frequency distributions of ridge and trough locations for the remaining
months (Harman
1991
) show less clustering of ridge and trough axes with
greatest variability in longwave positions occurring in July (Figure
4.2
). This
redistribution of ridge and trough locations is indicative of the breakdown of the
strong hemispheric thermal gradient of winter through the transition months
(spring and fall) to the weak thermal regime of summer (July), as illustrated in
Figure
4.3
. The orientation of the westerlies (including ridge-trough strength
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