Geoscience Reference
In-Depth Information
Figure 11.10
A schematic model
of the conditions conducive (left) or
detrimental (right) to the growth of a
tropical storm in an easterly wave;
U
is the mean upper-level wind speed
and c is the rate of propagation of the
system. The warm vortex creates a
thermal gradient that intensifies both
the radial motion around it and the
ascending air currents, termed
the
solenoidal effect
.
Source
: From Kurihara (1985), copyright
© Academic Press. Reproduced by
permission.
conditions, grows first into a tropical depression and
then into a tropical storm. The tropical storm stage may
persist for four to five days, whereas the cyclone stage
usually lasts for only two to three days (four to five days
in the western Pacific). The main energy source is latent
heat derived from condensed water vapour, and for this
reason hurricanes are generated and continue to gather
strength only within the confines of warm oceans. The
cold-cored tropical storm is transformed into a warm-
cored hurricane in association with the release of latent
heat in cumulonimbus towers, and this establishes or
intensifies an upper tropospheric anticyclonic cell. Thus
high-level outflow maintains the ascent and low-level
inflow in order to provide a continuous generation of
potential energy (from latent heat) and the transforma-
tion of this into kinetic energy. The inner eye that forms
by sinking air is an essential element in the life cycle.
Hurricane forecasting is a complex science. Recent
studies of annual North Atlantic/Caribbean hurricane
frequencies suggest that three major factors are
involved:
of the QBO is associated with strong easterly winds
in the lower stratosphere between latitudes 10°N
and 15°N, producing a large vertical wind shear.
This phase usually persists for twelve to fifteen
months and inhibits hurricane formation. The west
QBO phase exhibits weak easterly winds in the lower
stratosphere and small vertical wind shear. This
phase, typically lasting thirteen to sixteen months,
is associated with 50 per cent more named storms,
60 per cent more hurricanes and 200 per cent more
major hurricanes than is the east phase.
2
West African precipitation during the previous year
along the Gulf of Guinea (August to November) and
in the western Sahel (August to September). The
former moisture source appears to account for some
40 per cent of major hurricane activity, the latter
for only 5 per cent. Between the late 1960s and 1980s
the Sahel drought was associated with a marked
decrease in Atlantic tropical cyclones and hurri-
canes, mainly through strong upper-level shearing
winds over the tropical North Atlantic and a decrease
in the propagation of easterly waves over Africa in
August and September.
1
The west phase of the Atlantic Quasi-Biennial
Oscillation (QBO). The QBO involves periodic
changes in the velocities of, and vertical shear
between, the zonal upper tropospheric (50 mb) winds
and the lower stratospheric (30 mb) winds. The onset
of such an oscillation can be predicted with some
confidence almost a year in advance. The east phase
3
ENSO predictions for the following year (see G, this
chapter). There is an inverse correlation between
the frequency of El Niños and that of Atlantic
hurricanes.
