ACOM Seminar: Stratospheric dynamics on subseasonal-to-seasonal timescales: Waves, transport, and predictability


Title: Stratospheric dynamics on subseasonal-to-seasonal timescales: Waves, transport, and predictability

Presenter: John Albers, Cooperative Institute for Research in the Environmental Sciences (CIRES), University of Colorado

Abstract: Stratospheric dynamic variability plays an important role in modulating tropospheric variability on subseasonal-to-seasonal (S2S) timescales. For example, significant MSLP anomalies occur in the weeks following major disruptions of the stratospheric polar vortex. However, turning knowledge of such anomalies into actionable S2S forecast guidance has proved difficult. Much of this difficulty stems from our incomplete understanding of stratosphere-troposphere coupling processes and inadequate representation of key physical processes in models. I will discuss three areas where progress on these issues looks promising.

First, I will explore the factors governing the strength and frequency of stratospheric ozone intrusions over the Pacific-North American region in the context of two key mechanisms: the winter season buildup of ozone abundances in the lowermost stratosphere (LMS), and North Pacific jet and wave breaking variability during late spring. This analysis suggests that the abundance of ozone in the extratropical LMS at the end of winter is a robust indicator of the amount of ozone that will mixed downwards into the troposphere during the ensuing spring and summer.

Second, I will consider how background distributions of stratospheric wind and ozone couple with wave damping to modulate the power spectra of equatorial wave activity. This is a particularly pertinent issue because of the role that these waves play in driving the quasi-biennial oscillation (QBO), which is thought to be a source of tropospheric predictive skill on S2S timescales. Our preliminary results using reanalysis data show that a linear theoretic (WKB) view of wave damping and vertical wavenumber compression can be used to explain Kelvin wave and mixed Rossby-gravity wave spectral peaks during different phases of the QBO.

And third, I will use an empirical dynamical model to explore the role of stratospheric circulation in modulating S2S tropospheric forecast skill. The empirical model skill is compared to that of the operational ECWMF and NCEP models. On average, forecast skill is low for all three models beyond week 3. However, forecasts that are initialized in the 10-25 day window following major disruptions of the stratospheric polar vortex exhibit substantially enhanced skill for the empirical model and the ECMWF model. By conducting a singular value decomposition (SVD) of the empirical model forecast operator, I will provide explicit examples of the types of circulation and heating initial states that lead to the large stratospheric vortex disruptions that are associated with enhanced tropospheric predictive skill.

Thursday, October 11, 2018, 11:00 a.m. Refreshments 10:45 a.m. NCAR Foothills Laboratory FL2-1022, Large Auditorium


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Announcement Timing: 
Monday, October 8, 2018 to Thursday, October 11, 2018
Calendar Timing: 
Thursday, October 11, 2018 - 11:00am to 12:00pm

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