Effects Of Quasi-biennial Oscillation On Tropical Deep Convective Activities And Stratospheric Water Vapor

Using NCEP/NCAR, ERA-Interim reanalysis data and satellite observation data from AURA MLS combined with WACCM climate model, the effects of the quasi-biennial oscillation (QBO) on tropical tropopause, deep convection, Brewer-Dobson circulation and water vapor in stratosphere are analyzed,the main conclusions as following:1.The tropical tropopause anomalies associated with the QBO exhibit a belted distribution along the equator. In the tropics, the tropopause height (temperature) anomalies are higher (lower) during QBO east phase than that during west phase, while in the extra-tropics, the tropopause height (temperature) anomalies are higher (lower) during QBO west phase than that during east phase. The effects of QBO on tropical tropopause have significant seasonal and spatial variations. The effects of QBO on OLR also have significant seasonal and spatial variations. The effects are most significant in winter and autumn while relatively insignificant in spring and summer. And in Indonesia and tropical western pacific regions, in particular, the convective activities are stronger during the QBO east phase than that during the QBO west phase. Compared tropical tropopause anomalies associated with the QBO with OLR anomalies in this area, we can claim that QBO can indirectly influence tropopause via modulating tropical convective activities. But the tropical tropopause height and temperature anomalies associated with the QBO exhibit a belted distribution in the tropics while the OLR anomalies have both positive and negative anomalies along the equator. This implies that the effect of the QBO on tropical tropopause is mainly due to the downward propagation of zonal wind shear anomalies induced by the QBO and the indirect QBO effect on tropopause via modulating tropical convective activities is a secondary factor. Further research reveals that the buoyancy frequency anomalies and convective potential energy (CAPE) anomalies associated with the QBO are spatially in accordance with the QBO-induced OLR anomalies. It suggests that the QBO can affect tropical convective activities through modulating the static stability and the CAPE in the tropical troposphere.2. The Brewer-Dobson circulation vertical velocity (w*) anomalies associated with QBO have two maxima located at50-70hPa and10-30hPa and these two maxima have opposite sign. In the lower stratosphere (50-70hPa), the Brewer-Dobson circulation become stronger (weaker) during the QBO west (east) phase. At the middle stratosphere (10-30hPa), the Brewer-Dobson circulation is stronger (weaker) during QBO the west (east) phase. Moreover, the effects of QBO on the Brewer-Dobson circulation have significant seasonal variations. Vertical velocity anomalies in the winter hemisphere are more stronger than that in the summer hemisphere. The QBO-induced Brewer-Dobson circulation vertical velocity anomalies (w*’), zonal wind shear anomalies and temperature anomalies all exhibit significant two maxima structure and seasonal variations in the vertical direction with stronger anomalies in winter and autumn. The net effect of QBO on the lower stratospheric Brewer-Dobson circulation is weaker in all seasons, while at the middle stratosphere, this effect become smaller. Further analysis of the effect of QBO on the cross-tropopause mass flux reveals that the tropical upward mass flux increases during the QBO east phase while the downward mass flux decrease during the QBO west phase in all seasons. In autumn and winter, the tropical upward mass flux decreases significantly during the QBO west phase. While in summer and winter, the tropical upward cross tropopause mass flux increases significantly during the QBO east phase.3. The effects of the QBO on water vapor in the upper troposphere and lower stratosphere (UTLS) are mainly though modulating tropical deep convective activities, lower stratospheric temperature and Brewer-Dobson circulation. The effect of QBO on the tropical upper troposphere (100-200hPa) is mainly via modulating tropical deep convective activities and this effect is significant. The horizontal distributions of the QBO-induced OLR anomalies and tropical upper tropospheric water vapor anomalies are in accordance with each other in all seasons except for summer. The effect of QBO on the tropical lower stratospheric (50-100hPa) water vapor is mainly via modulating the lower stratospheric temperature and Brewer-Dobson circulation. In winter and autumn, as the tropical tropopause temperature is higher during the QBO west phase, more water vapor of tropospheric origin could enter into the stratosphere. Meanwhile, in the lower stratosphere, QBO-induced anomalous downwelling tends to transport water vapor in upper stratosphere into lower stratosphere. Conversely, the lower tropical tropopause temperature during the QBO east phase leads to less water vapor getting into the stratosphere and QBO-induced anomalous upwelling transport the water vapor in the lower stratosphere upward. The effect of QBO on the tropical middle stratosphere (10-30hPa) is mainly via modulating lower stratospheric Brewer-Dobson circulation, temperature and the chemical process associated with the lower stratospheric temperatures. In winter, autumn and summer, the effect of QBO on water vapor in the north hemisphere and equator is mainly via modulating Brewer-Dobson circulation and temperature while water vapor anomalies in the south hemisphere is mainly influenced by Brewer-Dobson circulation anomalies. Therefore, the distributions of QBO-induce water vapor anomalies in the north hemisphere and south hemisphere are different during different QBO phase. Moreover, the water vapor in spring is much smaller than in other season. Further analysis of the net effect of QBO on water vapor shows that the net effect of QBO is moistening the upper troposphere and lower stratosphere (UTLS). In winter the moistening effect is most significant and in spring this effect is relatively insignificant. At50-160hPa,the effects of the QBO on water vapor have significant seasonal variations. But at10-50hPa this seasonal variations is insignificant.