Geoscience Reference
In-Depth Information
confines of warm oceans. The cold-cored tropical
storm is transformed into a warm-cored hurri-
cane 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 transformation 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:
hurricanes, 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.
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.
Recent studies suggest that there has been an
increase in the number and proportion of category
4-5 hurricanes over the past 30 years. The largest
increases took place in the North Pacific, Indian
and southwest Pacific oceans, and the smallest
increase occurred in the North Atlantic Ocean. At
the same time the number of cyclones and cyclone
days has decreased in all basins except the North
Atlantic. The reported increase in tropical cyclone
energy, numbers and wind speeds in some regions
during the past few decades has been attributed to
higher sea surface temperatures. However, other
studies consider that changes in observational
techniques and instrumentation can account for
these changes.
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 (50mb)
winds and the lower stratospheric (30mb)
winds. The onset of such an oscillation can be
predicted with some confidence almost a year
in advance. The east phase of the QBO is
associated with strong easterly winds in the
lower stratosphere between latitudes 10
°
N and
Other tropical depressions
Not all low pressure systems in the tropics are of
the intense tropical cyclone variety. There are two
other major types of cyclonic vortex. One is the
monsoon depression that affects South Asia
during the summer. This disturbance is somewhat
unusual in that the flow is westerly at low levels
and easterly in the upper troposphere (see F
igure
11.27
). It is more fully described in C.4, this
chapter. (p. 354).
The second type of system is usually relatively
weak near the surface, but well developed in the
mid-troposphere. In the eastern North Pacific and
northern Indian Ocean, such lows are referred to
as subtropical cyclones. Some develop from the
cutting off in low latitudes of a cold upper-level
wave in the westerlies (cf. Chapter 9H.4).
They possess a broad eye, 150km in radius with
little cloud, surrounded by a belt of cloud and
precipitation about 300km wide. In late winter
N, producing a large vertical wind shear.
This phase usually persists for 12 to 15 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 13 to 16
months, is associated with 50 percent more
named storms, 60 percent more hurricanes
and 200 percent 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 percent of
major hurricane activity, the latter for only 5
percent. Between the late 1960s and 1980s, the
Sahel drought was associated with a marked
decrease in Atlantic tropical cyclones and
15
°
