Tropical climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 72

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Figure 3.7 Relationship
between 30-millibar (mb)
stratospheric wind direction
(West phase - black bars;
East phase - white bars) and
seasonal number of
hurricane days from 1949 to
1982. Years with no
observations are those in
which the 30 mb zonal wind
is changing direction or is
very weak. (Adapted from
Gray 1984 : permission from
American Meterological
Society)

50

40

30

20

10

0

1949 1956 1961 1967 1973 1979

identified that strong Atlantic hurricanes tend to occur more frequently when the
QBO is transitioning into or is already in its westerly phase. Conversely, when
the QBO is in its easterly phase, the Atlantic hurricane season tends to be milder.
Even today the QBO remains one of the main statistical variables in Gray's
seasonal predictions of tropical storm activity in the Atlantic Basin.
Two major physical explanations have been postulated to account for
a QBO-tropical cyclone. The first suggests that lower stratospheric vertical
wind shear restricts the penetrative convection associated with strong storms.
Following this line of thinking, the easterly phase of the QBO would have
inhibited tropical cyclone activity because of the extra lower stratospheric
wind ventilation and increased tropospheric to stratospheric wind shear.
The second explanation involves the effect of the QBO on the position of
critical levels of propagation for tropical easterly waves. Shapiro ( 1989 ) noted
that slower tropical waves have their propagation speeds comparable to winds
at the 85 hPa level while stronger system (hurricanes and tropical storms) move
at speeds closer to 50 hPa wind speeds. These differences are accented during
different phases of the QBO. During the east phase of the QBO, the relative
advection (the rate of airflow into the moving tropical system) is smaller than
during the west phase. Consequently, the shearing associated with the east
phase would advect air away from the developing system and thereby inhibit
its intensification. The reverse would hold true for the west phase of the
QBO. Ultimately, no matter the physical explanation, the statistical relation-
ship between the QBO and Atlantic tropical cyclone activity has led to the
QBO's incorporation into Atlantic seasonal tropical cyclone algorithms.
Unfortunately, the influence of the QBO on typhoon activity in the Pacific
basin is less clear. Some climatologies and prediction schemes of North