Tropical climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 73

Figure 3.8 Variations in a
monthly time series of
length-of-day (computed as
deviations inmilliseconds) as
filtered recursively in the
quasi-bennial (18-35
months) band. (Adapted
from Dickey et al. 1994 )

2

0

-2

1976 1980 1985

1990

Year

Pacific tropical cyclone activity have not identified any strong QBO signal in
the occurrence or intensity of typhoons and tropical storms. However, a
recent case study of the 1994 Typhoon Orchid revealed that deep convection
from the typhoon induced a gravity wave in the lower stratosphere.

3.3.5 Tropical influences: unusual linkages
One of the most unusual influences of the QBO is its identifiable impact on
the terrestrial length-of-day. Researchers have identified the QBO as exerting
a possible effect on length-of-day because of its contribution to changes in the
angular momentum of the atmosphere (Dickey 1994 ). Several researchers
have computed the axial angular momentum linked to alternating zonal winds
of QBO and discovered that the QBO, when operating in conjunction with
momentum shifts generated by ENSO, has a small but discernable impact on
the terrestrial length-of-day (Figure 3.8 ). The QBO effect at biennial periods
is, according to these researchers, one of the clearest influences evident on the
length-of-day. But, some researchers have noted that day length variability is
also modulated by ENSO (producing triennial-quadrennial variability) and
by mantle-core interactions (producing six-year periodicity).
A second somewhat unusual influence of the QBO appears to be on the
strength of the semidiurnal pressure waves associated with normal thermal
heating and cooling. Study of the long-term variation in these semidiurnal
pressure waves indicates some QBO variability. The existence of a QBO
signal in these long-term pressure records also suggests that the QBO is not a
transitory phenomenon, but rather a fundamental and persistent characteristic
of the equatorial atmosphere.

3.3.6 The future of QBO research
Many climatologists who evaluate periodicities in various types of climatic
data around the world regard the QBO as a ubiquitous signal embedded in