Climate dynamics

Abstract:

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 15 that characterises 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 20 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 10-70hPa. 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 hPa 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 25 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 West 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 Intertropical Convergence Zone (ITCZ) shifts southward under QBO westerly conditions. The EOF-based analysis suggests that 30 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.