Geography Reference
In-Depth Information
This oscillation is best depicted by means of a time-height section of the zonal
wind speed at the equator as shown in Fig. 12.15. It is apparent from Fig. 12.15
that the vertical shear of the wind is quite strong at the level where one regime
is replacing the other. Because the QBO is zonally symmetric and causes only
very small mean meridional and vertical motions, the QBO mean zonal wind and
temperature fields satisfy the thermal wind balance equation. For the equatorial
β-plane this has the form [compare (10.13)]
RH
−
1
∂T/∂y
βy∂u/∂z
=−
For equatorial symmetry ∂T/∂y
=
0aty
=
0, and by L'Hopital's rule thermal
wind balance at the equator has the form
R (Hβ)
−
1
∂
2
T/∂y
2
=−
∂u/∂z
(12.45)
Equation (12.45) can be used to estimate the magnitude of the QBO tempera-
ture perturbation at the equator. The observed magnitude of vertical shear of the
mean zonal wind at the equator is
5ms
−
1
km
−
1
, and the meridional scale is
∼
∼
1200 km, from which (12.45) shows that the temperature perturbation has an
amplitude
3 K at the equator. Because the second derivative of temperature has
the opposite sign to that of the temperature at the equator, the westerly and easterly
shear zones have warm and cold equatorial temperature anomalies, respectively.
The main factors that a theoretical model of the QBO must explain are the
approximate biennial periodicity, the downward propagation without loss of ampli-
tude, and the occurrence of zonally symmetric westerlies at the equator. Because
a zonal ring of air in westerly motion at the equator has an angular momentum
per unit mass greater than that of the earth, no plausible zonally symmetric advec-
tion process could account for the westerly phase of the oscillation. Therefore,
there must be a vertical transfer of momentum by eddies to produce the westerly
accelerations in the downward-propagating shear zone of the QBO.
Observational and theoretical studies have confirmed that vertically propagating
equatorial Kelvin and Rossby-gravity waves provide a significant fraction of the
zonal momentum sources necessary to drive the QBO. From Fig. 12.12 it is clear
that Kelvin waves with upward energy propagation transfer westerly momentum
upward (i.e., u
and w
are positively correlated so that u
w
> 0). Thus, the Kelvin
waves can provide a source of westerly momentum for the QBO.
Vertical momentum transfer by the Rossby-gra
vity
mode requires special con-
sideration. Examination of Fig. 12.13 shows that u
w
> 0 also for the Rossby-
gravity mode. The total effect of this wave on the mean flow cannot be ascertained
from the vertical momentum flux alone, but rather the complet
e ver
tical EP flux
must be considered. This mode has a strong poleward heat flux (v
T
> 0), which
∼
