Large internal variability dominates over global warming signal in observed lower stratospheric QBO amplitude
Large internal variability dominates over global warming signal in observed lower stratospheric QBO amplitude
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在观察到的较低平流层 QBO 振幅中，较大的内部变异性主导了全球变暖信号
作者：
Aaron Match;S. Fueglistaler
动力学的全球变暖预估不如热力学的预估可靠。然而，热力学的健壮方面可以用来约束一些动力学方面。本文认为，全球变暖背景下的对流层扩张（一个热力过程）解释了平流层中下层准两年振荡（QBO）振幅的变化（一个动力过程）。本文推导了对流层膨胀约6 hPa K−1的理论尺度，与全球气候模式（GCM）实验结果吻合较好。利用这一理论尺度，通过将QBO气候振幅剖面向上移动6 hPa / Kelvin来预测QBO振幅对全球变暖的响应。在全球变暖模拟中，对流层膨胀预测平流层中下层的QBO振幅向上移动。应用于观测，对流层膨胀框架表明，从1953-2020年，QBO在70 hPa的振幅在历史上减弱了3%的十年−1。这一预期的减弱趋势是最近一项研究发现并归因于全球变暖的1953-2012年10 - 1年6%的一半。在2005年前后，QBO的振幅达到了创纪录的低点，这进一步强化了之前报告的趋势，此后QBO已从低点回升。鉴于基于物理原因的温和减弱预期，QBO振幅过去的年代际调制被重新解释为迄今尚未认识到的内部变率来源。这种巨大的内部变率支配着全球变暖信号，因此，尽管有65年的观测，在统计上还没有显著的减弱趋势。
Global warming projections of dynamics are less robust than projections of thermodynamics. However, robust aspects of the thermodynamics can be used to constrain some dynamical aspects. This paper argues that tropospheric expansion under global warming (a thermodynamical process) explains changes in the amplitude of the Quasi-Biennial Oscillation (QBO) in the lower and middle stratosphere (a dynamical process). A theoretical scaling for tropospheric expansion of approximately 6 hPa K−1 is derived, which agrees well with global climate model (GCM) experiments. Using this theoretical scaling, the response of QBO amplitude to global warming is predicted by shifting the climatological QBO amplitude profile upwards by 6 hPa per Kelvin of global warming. In global warming simulations, QBO amplitude in the lower- to mid-stratosphere shifts upwards as predicted by tropospheric expansion. Applied to observations, the tropospheric expansion framework suggests a historical weakening of QBO amplitude at 70 hPa of 3% decade−1 from 1953-2020. This expected weakening trend is half of the 6% decade−1 from 1953-2012 detected and attributed to global warming in a recent study. The previously reported trend was reinforced by record low QBO amplitudes during the mid-2000s, from which the QBO has since recovered. Given the modest weakening expected on physical grounds, past decadal modulations of QBO amplitude are reinterpreted as a hitherto unrecognized source of internal variability. This large internal variability dominates over the global warming signal, such that despite 65 years of observations, there is not yet a statistically significant weakening trend.