Solar and QBO Influences on the Timing of Stratospheric Sudden Warmings

Abstract

The interaction of the 11-yr solar cycle (SC) and the quasi-biennial oscillation (QBO) and their influence on the Northern Hemisphere (NH) polar vortex are studied using idealized model experiments and ECMWF Re-Analysis (ERA-40). In the model experiments, the sensitivity of the NH polar vortex to imposed easterlies at equatorial/subtropical latitudes over various height ranges is tested to explore the possible influence from zonal wind anomalies associated with the QBO and the 11-yr SC in those regions. The experiments show that the timing of the modeled stratospheric sudden warmings (SSWs) is sensitive to the imposed easterlies at the equator/subtropics. When easterlies are imposed in the equatorial or subtropical upper stratosphere, the onset of the SSWs is earlier. A mechanism is proposed in which zonal wind anomalies in the equatorial/subtropical upper stratosphere associated with the QBO and 11-yr SC either reinforce each other or cancel each other out. When they reinforce, as in Smin–QBO-east (Smin/E) and Smax–QBO-west (Smax/W), it is suggested that the resulting anomaly is large enough to influence the development of the Aleutian high and hence the time of onset of the SSWs. Although highly speculative, this mechanism may help to understand the puzzling observations that major warmings often occur in Smax/W years even though there is no strong waveguide provided by the QBO winds in the lower equatorial stratosphere. The ERA-40 data are used to investigate the QBO and solar signals and to determine whether the observations support the proposed mechanism. Composites of ERA-40 zonally averaged zonal winds based on the QBO (E/W), the SC (min/max), and both (Smin/E, Smin/W, Smax/E, Smax/W) are examined, with emphasis on the Northern Hemisphere winter vortex evolution. The major findings are that QBO/E years are more disturbed than QBO/W years, primarily during early winter. Sudden warmings in Smax years tend to occur later than in Smin years. Midwinter warmings are more likely during Smin/E and Smax/W years, although the latter result is only barely statistically significant at the 75% level. The data show some support for the proposed mechanism, but many more years are required before it can be fully tested.

The influence of solar variability and the quasi-biennial oscillation on lower atmospheric temperatures and sea level pressure

“…Other studies have shown that in the Northern Hemisphere winter extratropics a solar signal in polar winter temperature and wind is QBOphase dependent, moving poleward and downward as winter progresses, taking about 1 month to move from the upper to the lower stratosphere with a faster descent rate under wQBO than eQBO (Matthes et al, 2004;Gray et al, 2004;Lu et al, 2009;Haigh and Roscoe, 2009) showed a similar progression in the Southern Hemisphere late winter.…”

“…We note that none of these statistical studies provides any mechanism for the apparent relationships but that the different responses might indicate where causal links occur. For example, Gray et al (2004) propose an effect of solar/QBO variability whereby a perturbation to the sub-tropical upper stratosphere might affect the polar lower stratosphere after 3-4 months through influencing the timing of sudden stratospheric warmings. Over this period the QBO phase would have descended 3-4 km in the tropics so that an instantaneous correlation between polar temperature and QBO would appear highest at lower altitudes in the tropics.…”

The 11-year solar cycle in current reanalyses: a (non)linear attribution study of the middle atmosphere

“…The negative zonal wind response in late northern winter may be caused by an increased likelihood of major stratospheric warmings later in the winter under solar maximum conditions when the polar vortex in early winter is stronger, on average, and therefore less susceptible to disruption (e.g. Gray et al, 2004). Since GCMs have not yet successfully simulated this pattern (e.g.…”

Solar Influence on Global and Regional Climates

“…The latitudes of tropospheric jet streams have been shown to be sensitive to the solar forcing of stratospheric temperatures (Haigh 1994(Haigh , 2003Poore et al 2004). This could occur through disturbances to the stratospheric polar vortex (Gray et al 2004), which can propagate downwards to affect the tropospheric jets (Plumb and Semeniuk 2003) or through the effects of stratospheric temperature changes on stratospheric winds and the refraction of tropospheric eddies (Kushner and Polvani 2004;Simpson et al 2009). Models (e.g., Shindell et al 2001a, b) predict that perturbations can descend from the stratosphere to the surface by altering the propagation of planetary waves coming up from the surface, an effect that has been observed in data (Perlwitz and Harnik 2003).…”