Wave Energy Associated with the Variability of the Stratospheric Polar Vortex

Abstract

A study is performed on the energetics of planetary wave forcing associated with the variability of the northern winter polar vortex. The analysis relies on a three-dimensional normal mode expansion of the atmospheric general circulation that allows partitioning the total (i.e., kinetic ϩ available potential) atmospheric energy into the energy associated with Rossby and inertio-gravity modes with barotropic and baroclinic vertical structures. The analysis mainly departs from traditional ones in respect to the wave forcing, which is here assessed in terms of total energy amounts associated with the waves instead of heat and momentum fluxes. Such an approach provides a sounder framework than traditional ones based on Eliassen-Palm (EP) flux diagnostics of wave propagation and related concepts of refractive indices and critical lines, which are strictly valid only in the cases of small-amplitude waves and in the context of the Wentzel-Kramers-Brillouin-Jeffries (WKBJ) approximation.Positive (negative) anomalies of the energy associated with the first two baroclinic modes of the planetary Rossby wave with zonal wavenumber 1 are followed by a downward progression of negative (positive) anomalies of the vortex strength. A signature of the vortex vacillation is also well apparent in the lagged correlation curves between the wave energy and the vortex strength. The analysis of the correlations between individual Rossby modes and the vortex strength further confirmed the result from linear theory that the waves that force the vortex are those associated with the largest zonal and meridional scales.The two composite analyses of displacement-and split-type stratospheric sudden warming (SSW) events have revealed different dynamics. Displacement-type SSWs are forced by positive anomalies of the energy associated with the first two baroclinic modes of planetary Rossby waves with zonal wavenumber 1; split-type SSWs are in turn forced by positive anomalies of the energy associated with the planetary Rossby wave with zonal wavenumber 2, and the barotropic mode appears as the most important component. In respect to stratospheric final warming (SFW) events, obtained results suggest that the wave dynamics is similar to the one in displacement-type SSW events.

Observed and simulated precursors of stratospheric polar vortex anomalies in the Northern Hemisphere

“…This is as expected and consistent with previous studies, such as Liberato et al (2007), who found a strong lagged correlation between the stratospheric NAM index and wave-number one baroclinic waves. As mentioned earlier, we found that the geopotential height anomaly precursor of WVMs in Fig.…”

“…It is known that stratospheric vortex anomalies are preceded by positive poleward anomalies in the zonal-mean heat fluxes in the lower stratosphere (Polvani and Waugh 2004) and increased baroclinic wave energy associated with planetary-scale waves (Liberato et al 2007;Castanheira et al 2009). However, the relationship between anomalous tropospheric planetary wave amplitudes and subsequent stratospheric variability is complex and dependent on the state of the stratosphere-troposphere system at the time of the generation of planetary wave anomalies (Reichler et al 2005).…”

Vertical and interhemispheric links in the stratosphere-mesosphere as revealed by the day-to-day variability of Aura-MLS temperature data

“…1c) are also large near 1 and 25 February, but not all disturbances in the heat flux are exhibited in the stratospheric PW1 since the heat flux includes contributions from all waves. There is considerable evidence that amplified planetary scale disturbances in the winter stratosphere can be traced into the troposphere (Quiroz, 1979;Randel, 1987;Liberato et al, 2007).…”

Increase of upper troposphere/lower stratosphere wave baroclinicity during the second half of the 20th century

“…We analyzed the NH cool season (November to April) daily means of the horizontal wind components (u,v) and of the geopotential height, available on 17 standard pressure levels from 1000 to 10 hPa, on a 2.5 • ×2.5 • horizontal grid. The global horizontal wind (u,v) and geopotential(φ) fields were expanded in terms of the normal modes of the NCEP/NCAR reference atmosphere (see Liberato et al, 2007, and references therein for details), i.e.,…”

“…kinetic + available potential) energy per unit area associated with a given mode is proportional to the squared norm of the respective complex amplitude, w α msl (Liberato et al, 2007, and references therein)…”