TY - JOUR
T1 - Large Internal Variability Dominates over Global Warming Signal in Observed Lower Stratospheric QBO Amplitude
AU - Match, Aaron
AU - Fueglistaler, Stephan
N1 - Publisher Copyright:
© 2021 American Meteorological Society. All rights reserved.
PY - 2021/12/15
Y1 - 2021/12/15
N2 - 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 hPaK21 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 upward by 6 hPa per kelvin of global warming. In global warming simulations, QBO amplitude in the lower to middle stratosphere shifts upward as predicted by tropospheric expansion. Applied to observations, the tropospheric expansion framework suggests a historical weakening of QBO amplitude at 70 hPa of 3% decade21 from 1953 to 2020. This expected weakening trend is half of the 6% decade21 from 1953 to 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.
AB - 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 hPaK21 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 upward by 6 hPa per kelvin of global warming. In global warming simulations, QBO amplitude in the lower to middle stratosphere shifts upward as predicted by tropospheric expansion. Applied to observations, the tropospheric expansion framework suggests a historical weakening of QBO amplitude at 70 hPa of 3% decade21 from 1953 to 2020. This expected weakening trend is half of the 6% decade21 from 1953 to 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.
KW - Climate change
KW - Climate models
KW - Internal variability
KW - Quasibiennial oscillation
KW - Tropopause
UR - http://www.scopus.com/inward/record.url?scp=85119961457&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85119961457&partnerID=8YFLogxK
U2 - 10.1175/JCLI-D-21-0270.1
DO - 10.1175/JCLI-D-21-0270.1
M3 - Article
AN - SCOPUS:85119961457
SN - 0894-8755
VL - 34
SP - 9823
EP - 9836
JO - Journal of Climate
JF - Journal of Climate
IS - 24
ER -