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The ECMWF ranalyses (1979-1993) were used to study low-frequency variabilities of the tropical CPT. Stratospheric zonal wind shear at 50 mbar over Singapore, and Sea Surface Temperature Anomalies (SSTA) in the Niño3.4 region are used as indices for the stratospheric QBO and tropospheric ENSO, respectively. Correlation calculations indicated that westerly wind shear at 50 mbar leads positive temperature anomaly at the tropical CPT by about 6 months and SSTA in the Niño3.4 region is simultaneously correlated with CPT temperatures. We used bivariate regression to filter QBO and ENSO signatures in the tropical CPT, then performed composite analysis using regressed time series.
Composite analysis of the CPT temperatures for the westerly and easterly QBO wind shears shows zonally symmetric features of the QBO in CPT properties. During the westerly shear period, the composited tropical CPT is warmer by 0.2 to 0.3 K, and during easterly shear conditions it is colder by 0.2 to 0.4 K.
It takes about 3-4 months for the westerly shear at 50mb to reach 100mb and takes about 7 months for the easterly shear to propagate from 50mb to 100mb (Naujoket 1986), which gives an average of about 5-6 months. This time lag is very consistent with our analyses. The amplitude of QBO temperature perturbation can be estimated using (Andrews et al., 1987)
Given a wind change over a scale height of 10 m/s and a latitudinal
scale of 1000 km, the above equation gives an estimate of about
0.79 K for temperature change over the latitudinal scale. Amplitude
of the CPT temperature anomalies associated with the QBO is about
0.3-0.5 K, according to the time series of QBO signatures in the
tropical CPT. Consistency in time lag and amplitude suggests that
the QBO signature in the tropical CPT temperatures is probably
due to the stratospheric QBO temperature anomalies that accompany
the downward-propagating QBO meridional circulation.
The temperature anomalies associated with ENSO are larger than
those associated with the QBO. They can be as low as -0.6 K during
El Niño and as high as 1.0 K during La Niña over the eastern Pacific.
The temperature anomalies over the western Pacific are about 0.4
K (-0.4 K) during El Niño (La Niña) events. Composites of the
CPT temperature anomalies for ENSO shows three distinct features.
First, there is a East-West (E-W) dipole over the tropical Pacific.
The second feature is that there are three North-South (N-S) dumbbells
in the tropics. The strongest dumbbell is over the central to
eastern Pacific. One dumbbell pattern is clear over the Atlantic,
and similar features are also seen over the eastern Indian Ocean
and the western Pacific, though less clearly. The third feature
is that there is a maximum (in absolute value) over the equatorial
western Pacific. These three features can be explained by changes
of tropical convection activity associated with ENSO.
The annual cycle of the zonal mean tropical tropopause is driven by extratropical stratospheric wave forcing (Yulaeva et al., 1994), but the zonal asymmetrics in the tropical tropopause can be attributed to the direct response of the atmosphere to a large-scale region of tropospheric diabatic heating (Highwood and Hoskins, 1998). The mechanism proposed by Highwood and Hoskins (1998) can be extended to explain the simultaneous anti-correlation between the SSTA and the CPT temperatures over the eastern Pacific. During El Niño events, the active convection center shifts to the central to eastern Pacific. As a result, there is more precipitation in the central to eastern Pacific and so more diabatic heating there. Thus during El Niño events the tropopause is colder over the eastern Pacific. During La Niña events, the situation is reversed.
Yulaeva and Wallace (1994) investigated the QBO and ENSO signatures in global temperature
and precipitation fields derived from the Microwave Sounding Unit
(MSU). The ENSO associated temperature anomalies in the troposphere
(MSU-2) and the lower stratosphere (MSU-4) are very consistent
with the ENSO signature in the tropical CPT, in respect of the
shape and positions of the E-W dipole and N-S dumbbells and the
maximum anomaly over the western Pacific. Zhou and Sun (1994) extended the Gill (1980) model including a cooling over the western domain and
a heating over the eastern domain, which is consistent with the
distribution of diabatic heating anomalies during El Niño (Yulaeva and Wallace, 1994), to study the tropical troposphereic nonlinear steady
response to two heating sources of contrasting nature. The steady
geopotential height response at the top of model domain to these
two heat sources of contrasting nature shows that there is positive
dumbbell over the eastern half domain and a negative dumbbell
over the western half domain. These two dumbbells form a E-W (positive-negative)
dipole. Additionally, there is a negative maximum geopotential
height anomaly to the east of the western cooling source and this
maximum anomaly is a result of wave-wave interaction.
We expected low-frequency variabilities in the tropical CPT temperatures
to be recorded in tropical stratospheric water vapor in a similar
manner as the annual cycle of the tropical CPT temperatures is
recorded (Mote et al. 1996). Simulations using the SUNY-SPb two-dimensional chemistry-transport
model indicated that the interannual anomalies associated with
the QBO circulation can explain a good deal of the observed interannual
anomalies of stratospheric methane. However, it can interpret
only part of the observed interannual variation of stratospheric
water vapor (Randel et al. 1998). Interannual variation of stratospheric water vapor is
affected by the low-frequency variability in tropical CPT temperatures,
in addition to the QBO circulation. Water vapor anomalies due
to the QBO circulation show large values in the middle stratosphere,
consistent with the vertical structure of observations (Randel et al., 1998). However, they are too small to explain the observed interannual
anomalies of water vapor in the tropical lower stratosphere. The
influences of low-frequency thermal effects in CPT temperatures
associated with the stratospheric QBO and the tropospheric ENSO
produce low frequency ``tape recorder" signals.
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