Tropical climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 70

evaluated using datasets involving measures of tropical convection (i.e. out-
going longwave radiation, OLR; and an index of highly reflective clouds,
HRC). Researchers identified oscillations in both equatorial OLR and HRC
values that appear to coincide with the fundamental periods of the QBO.
Their findings indicate that the QBO modulation of the lower stratospheric
vertical wind gradient may result in cloud tops more likely being ''sheared
off '' in some years than in others.
Such linkages between tropical deep convection and the QBO led hurri-
cane expert William Gray and colleagues to suggest that the timing of
El Ni˜o-Southern Oscillation (ENSO) may be modulated by the QBO.
ENSO is the set of phenomena associated with oscillating shifts in water
(El Ni˜o/La Ni˜a) and atmosphere (the Southern Oscillation) in the Pacific
Ocean. Gray and colleagues theorized that since ENSO is strongly linked to
the creation of deep convective activity in the tropical Pacific Ocean, the
relative phase (east/west) of the QBO should play a large role in modulating
the character of ENSO. During the easterly phase of the QBO, the vertical
wind shear favors deep convective activity near the equator while inhibiting
such activity in the subtropics. Conversely, during the westerly shear phase of
the QBO, deep equatorial convection is suppressed while subtropical mon-
soonal convection is enhanced. This may be part of the physical linkage
between the QBO and Indonesia sea-level pressure oscillations that appear to
display some QBO periodicity (Gray, 1984 ).
However, the direct linkage between the QBO and the tropics is unclear
because the tropical troposphere itself does have an identifiable quasi-
biennial oscillation. That tropical QBO is apparently unrelated to the strato-
spheric QBO. Instead it appears that the ''tropospheric QBO'' is directly
linked to the Southern Oscillation. This tropospheric biennial variability is
less regular than the stratospheric QBO, more asymmetric in its longitudinal
structure and has its greatest amplitude over Indonesia.
3.3.3 Tropical influences: ozone and aerosols
One easily identifiable - but not surprising - influence of the stratospheric
QBO is seen in the interannual variability of subtropical stratospheric ozone
(Randel and Wu 1996 ). Annual variations matching those seen in the QBO
are evident in ozone concentrations (Figure 3.6 ). The likely mechanism in
accounting for the QBO influence on ozone involves the synchronicity of the
maximum westerly vertical wind shear and the maximum diabatic cooling,
which fundamentally leads to the descent of air parcels and their constituents.
One interesting aspect of the ozone space/time variability is the dichotomy
(evident equatorward and poleward of 158 latitude) in the signal between the
tropics and high latitudes. Generally, this variability coincides with ascent