Behavior of Planetary Waves before and after Stratospheric Sudden Warming Events in Several Phases of the Equatorial QBO

Abstract

Almost a thousand stratospheric sudden warming (SSW) events are obtained through long time integrations with a simple global circulation model, and a statistical analysis based on such a large number of samples is made to investigate behavior of planetary waves before and after SSW events depending on the phase of the equatorial quasi-biennial oscillation (QBO). An idealized zonal momentum forcing to mimic a phase of the QBO is imposed under a perpetual winter condition, and eight phases of the QBO-wind forcing are examined for 8 × 10 800-day datasets. Some systematic dependence on the phase of the QBO-wind forcing is seen in the anomaly of the Eliassen–Palm (EP) flux in the winter hemisphere, both in the 10 800-day average and in the composites for SSW events. The composite analysis shows that before SSW events, the upward EP flux in the troposphere and midlatitude lower stratosphere as well as the equatorward flux above the tropopause is larger in the westerly forcing runs than in the easterly forcing runs. After SSW events, the upward EP flux in the troposphere is still larger in the westerly forcing runs. Correlation associated with the differences among SSW events that occurred in each run is significantly positive between the magnitude of the warming and the planetary wave activity flux before all the events in QBO-wind forcing in the stratosphere, but only in the easterly forcing runs in the troposphere.

Dynamical Response to the QBO in the Northern Winter Stratosphere: Signatures in Wave Forcing and Eddy Fluxes of Potential Vorticity

“…The HTE also holds when the QBO is defined by the vertical wind shear between 50 and 70 hPa, but it tends to be weaker for the QBO defined at other levels (Hitchman and Huesmann 2009, hereafter HH09). The HTE has been confirmed by observational studies (e.g., Dunkerton and Baldwin 1991;Naito and Hirota 1997;Lu et al 2008Lu et al , 2014 as well as by both mechanistic (e.g., O'Sullivan and Young 1992; Naito and Yoden 2006;Pascoe et al 2006) and complex general circulation models (e.g., Calvo et al 2009;Marshall and Scaife 2009). However, the mechanism behind the HTE is still under debate (e.g., Naoe and Shibata 2010;Yamashita et al 2011;Garfinkel et al 2012;Lu et al 2014).…”

“…In extreme cases, these wave disturbances can cause rapid breakdowns of the vortex and rising temperatures in the polar stratosphere, a phenomenon known as a stratospheric sudden warming (SSW; Labitzke 1982). It has been found that SSWs tend to occur more frequently under QBOe than under QBOw conditions (e.g., Dunkerton and Baldwin 1991;Naito and Hirota 1997;Lu et al 2008); thus, it is natural to suspect that a QBO modulation of planetary waves is responsible for the changes in the frequency of SSWs and the overall polar vortex strength (Holton and Tan 1980;Calvo et al 2009).…”

“…Dunkerton and Baldwin (1991) found that the QBO modulation of the EP flux was consistent with the observed wind anomaly but that there were only marginal differences in planetary wave amplitudes and the number of planetary wave events between the two QBO phases. Modeling studies have suggested that the QBO does cause changes in planetary wave amplitude in addition to changes in the EP flux and EP flux divergence (e.g., Holton and Austin 1991;Naito and Yoden 2006;Calvo et al 2009;Watson and Gray 2014), although exactly how the QBO induces these wave changes remains unclear (Anstey and Shepherd 2014).…”

The impact of planetary waves on the latitudinal displacement of sudden stratospheric warmings

“…Labitzke (2004) even shows a statistical relation of SSW on the QBO and on the solar cycle, but the results for the SSW of 2009 did not fit with this statistical relation (Labitzke and Kunze, 2009) phase of the QBO while the other displaced SSWs lie on the easterly phase in the hight region between 25 and 50 km where the continuous westward wind band occurs (see http: //www.geo.fu-berlin.de/en/met/ag/strat/produkte/qbo). Naito and Yoden (2006) studied the PW activity before and after SSWs depending on the phase of the QBO by a numerical model. They found that the dominant zone of the upward and equatorward Eliassen-Palm flux in the lower stratosphere is shifted southward in the westerly phase and poleward in the easterly phase of the QBO during a SSW.…”

Does the Holton–Tan Mechanism Explain How the Quasi-Biennial Oscillation Modulates the Arctic Polar Vortex?

“…Arctic polar cap temperatures are significantly colder, and polar stratospheric heights significantly lower, when lower stratospheric QBO winds (e.g., winds near 50 hPa) are westerly than when they are easterly. Both modelingbased studies (e.g., Hampson and Haynes 2006;Pascoe et al 2006;Naito and Yoden 2006;Kinnersley and Tung 1999;Naoe and Shibata 2010;Calvo et al 2007;O'Sullivan and Young 1992, hereafter OY92;Niwano and Takahashi 1998) and reanalysis-based studies (e.g., Hu and Tung 2002;Hitchman and Huesmann 2009;Ruzmaikin et al 2005;Garfinkel and Hartmann 2007) have shown a robust effect of the QBO in the polar stratosphere. This effect is referred to as the Holton-Tan (HT) effect in the rest of this paper.…”