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Modeling and mechanisms of polar winter upper stratosphere/lower mesosphere disturbances in WACCM
Author(s) -
Greer K. R.,
Thayer J. P.,
Harvey V. L.,
Peck E. D.
Publication year - 2015
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
eISSN - 2169-8996
pISSN - 2169-897X
DOI - 10.1002/2015jd023471
Subject(s) - baroclinity , stratosphere , stratopause , atmospheric sciences , climatology , mesosphere , environmental science , polar night , longitude , atmosphere (unit) , extratropical cyclone , sudden stratospheric warming , latitude , polar vortex , meteorology , geology , physics , geodesy
This work focuses on the characteristics and mechanisms of upper stratosphere/lower mesosphere (USLM) disturbances in the polar winter as simulated by a free‐running 40 year run of the Whole Atmosphere Community Climate Model (WACCM) version 4. USLM disturbances have been shown to precede the development of every major sudden stratospheric warming (SSW), thus potentially increasing the predictability of SSWs. WACCM4 is shown to internally and spontaneously generate polar USLM disturbances that are consistent with an established USLM climatology based on the UK Meteorological Office stratospheric assimilation. Arctic USLM disturbances in WACCM4 occur on average 2.65 times per winter season; are most frequently generated in the months of December, January, and February; and are preferentially located in the longitude band between 30°E and 120°E, poleward of 40° latitude, all in good agreement with observations. Analysis of composite USLM events corroborates the underlying mechanism responsible for their formation as planetary wave breaking between ~45 km and 65 km that decelerates the mean flow, induces vertical air motion, and causes regions of adiabatic heating in a limited longitude band. These conditions are suitable for the growth of a baroclinic instability at the stratopause level. Using the Trenberth localized three‐dimensional Eliassen‐Palm flux along with the Charney‐Stern criteria, a WACCM4 case study of an independent USLM event implicates baroclinic instability as a critical process in the development of the USLM thermal structure.