Abstract
Abstract. The quasi-biennial oscillation (QBO) is quasi-periodic oscillation of the tropical zonal wind in the stratosphere. When the tropical lower stratospheric wind is easterly (westerly), the winter Northern Hemisphere (NH) stratospheric polar vortex tends to be weak (strong). This relation is known as the Holton–Tan relationship. Several mechanisms for this relationship have been proposed, especially linking the tropics with high latitudes through stratospheric pathway. Although QBO impacts on the troposphere have been extensively discussed, a tropospheric pathway of the Holton–Tan relationship has not been explored previously. Here, we propose a tropospheric pathway of the QBO impact, which may partly account for the Holton–Tan relationship in early winter, especially in the November–December period. The study is based on analyses of observational data and results from a simple linear model and atmospheric general circulation model (AGCM) simulations. The mechanism is summarized as follows: the easterly phase of the QBO is accompanied with colder temperature in the tropical tropopause layer, which enhances convective activity over the tropical western Pacific and suppresses it over the Indian Ocean, thus enhancing the Walker circulation. This convection anomaly generates a Rossby wave train, propagating into the midlatitude troposphere, which constructively interferences with the climatological stationary waves, especially in wavenumber 1, resulting in enhanced upward propagation of the planetary wave and a weakened polar vortex.
Highlights
The stratospheric quasi-biennial oscillation (QBO) is dominant interannual oscillation of the zonal wind in the stratospheric tropics with an approximate 28-month period (Veryard and Ebdon, 1961; Reed et al, 1961; Baldwin et al, 2001)
Why does the tropospheric mechanism not seem to occur from January onward? Is this because QBO-induced tropical convection anomalies disappear in midwinter? We examined the observed tropical convection difference between EQBO and WQBO for each month and found that it does not disappear but shifts slowly eastward
From the composite analysis of the observed data, we found that convective activity over the tropical western Pacific is enhanced and that over the Indian Ocean is suppressed in the EQBO compared with WQBO winters
Summary
The stratospheric quasi-biennial oscillation (QBO) is dominant interannual oscillation of the zonal wind in the stratospheric tropics with an approximate 28-month period (Veryard and Ebdon, 1961; Reed et al, 1961; Baldwin et al, 2001). When the tropical lower stratospheric wind is easterly (EQBO), the winter NH polar vortex tends to be weak, and the vortex tends to be strong when the tropical lower stratospheric wind is westerly (WQBO). This relation is called the Holton–Tan relationship (Holton and Tan, 1980, 1982), for which several mechanisms have been proposed in terms of the stratospheric linkages between the tropics and high latitudes (Anstey and Shepherd, 2014). In the WQBO winters, the westerly region extends more to the tropics, and planetary waves tend to propagate more equatorward. Naoe and Shibata (2010) analyzed the Holton–Tan relationship by a QBO-producing
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