Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, 3173-25 Showamachi, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan.
Nature. 2013 May 23;497(7450):478-81. doi: 10.1038/nature12140.
The zonal wind in the tropical stratosphere switches between prevailing easterlies and westerlies with a period of about 28 months. In the lowermost stratosphere, the vertical structure of this quasibiennial oscillation (QBO) is linked to the mean upwelling, which itself is a key factor in determining stratospheric composition. Evidence for changes in the QBO have until now been equivocal, raising questions as to the extent of stratospheric circulation changes in a global warming context. Here we report an analysis of near-equatorial radiosonde observations for 1953-2012, and reveal a long-term trend of weakening amplitude in the zonal wind QBO in the tropical lower stratosphere. The trend is particularly notable at the 70-hectopascal pressure level (an altitude of about 19 kilometres), where the QBO amplitudes dropped by roughly one-third over the period. This trend is also apparent in the global warming simulations of the four models in the Coupled Model Intercomparison Project Phase 5 (CMIP5) that realistically simulate the QBO. The weakening is most reasonably explained as resulting from a trend of increased mean tropical upwelling in the lower stratosphere. Almost all comprehensive climate models have projected an intensifying tropical upwelling in global warming scenarios, but attempts to estimate changes in the upwelling by using observational data have yielded ambiguous, inconclusive or contradictory results. Our discovery of a weakening trend in the lower-stratosphere QBO amplitude provides strong support for the existence of a long-term trend of enhanced upwelling near the tropical tropopause.
热带平流层中的纬向风以约 28 个月的周期在盛行东风和西风之间切换。在最底层平流层中，这种准两年振荡(QBO)的垂直结构与平均上升流有关，而上升流本身是决定平流层成分的关键因素。到目前为止，QBO 变化的证据一直存在争议，这引发了人们对全球变暖背景下平流层环流变化程度的质疑。在这里，我们报告了对 1953 年至 2012 年近赤道无线电探空仪观测的分析结果，并揭示了热带低层平流层中纬向风 QBO 振幅长期减弱的趋势。在 70 毫巴（约 19 公里的高度）的压力水平上，这种趋势尤为显著，在这段时间内，QBO 振幅下降了约三分之一。在耦合模式比较计划第 5 阶段（CMIP5）的四个模型的全球变暖模拟中也明显存在这种趋势，这些模型真实地模拟了 QBO。这种减弱最合理的解释是由于低层平流层中热带上升流的平均趋势增强。几乎所有的综合气候模型都预测在全球变暖情景下，热带上升流会加剧，但尝试利用观测数据来估计上升流的变化，得到的结果却模棱两可、没有定论或相互矛盾。我们发现低层平流层 QBO 振幅减弱的趋势，为热带对流层顶附近上升流长期增强的趋势提供了有力支持。
