Teleconnections between the Quasi Biennial Oscillation (QBO) and the Northern
Hemisphere zonally averaged zonal winds, mean sea level pressure (mslp) and
tropical precipitation are explored. The ...standard approach that defines the
QBO using the equatorial zonal winds at a single pressure level is compared
with the empirical orthogonal function approach that
characterizes the vertical
profile of the equatorial winds. Results are interpreted in terms of three
potential routes of influence, referred to as the tropical, subtropical and
polar routes. A novel technique is introduced to separate responses via the
polar route that are associated with the stratospheric polar vortex, from the
other two routes. A previously reported mslp response in January, with a
pattern that resembles the positive phase of the North Atlantic Oscillation
under QBO westerly conditions, is confirmed and found to be primarily
associated with a QBO modulation of the stratospheric polar vortex. This
mid-winter response is relatively insensitive to the exact height of the
maximum QBO westerlies and a maximum positive response occurs with westerlies
over a relatively deep range between 10 and 70 hPa. Two additional mslp
responses are reported, in early winter (December) and late winter
(February/March). In contrast to the January response the early and late
winter responses show maximum sensitivity to the QBO winds at ∼ 20 and
∼ 70 hPa respectively, but are relatively insensitive to the QBO winds
in between (∼ 50 hPa). The late winter response is centred over the
North Pacific and is associated with QBO influence from the lowermost
stratosphere at tropical/subtropical latitudes in the Pacific sector. The
early winter response consists of anomalies over both the North Pacific and
Europe, but the mechanism for this response is unclear. Increased
precipitation occurs over the tropical western Pacific under westerly QBO
conditions, particularly during boreal summer, with maximum sensitivity to
the QBO winds at 70 hPa. The band of precipitation across the Pacific
associated with the Inter-tropical Convergence Zone (ITCZ) shifts southward
under QBO westerly conditions. The empirical orthogonal function (EOF)-based
analysis suggests that this ITCZ precipitation response may be particularly
sensitive to the vertical wind shear in the vicinity of 70 hPa and hence the
tropical tropopause temperatures.
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