Role of quasiresonant planetary wave dynamics in recent boreal spring-to-autumn extreme events

Abstract

In boreal spring-to-autumn (May-to-September) 2012 and 2013, the Northern Hemisphere (NH) has experienced a large number of severe midlatitude regional weather extremes. Here we show that a considerable part of these extremes were accompanied by highly magnified quasistationary midlatitude planetary waves with zonal wave numbers m = 6, 7, and 8. We further show that resonance conditions for these planetary waves were, in many cases, present before the onset of high-amplitude wave events, with a lead time up to 2 wk, suggesting that quasiresonant amplification (QRA) of these waves had occurred. Our results support earlier findings of an important role of the QRA mechanism in amplifying planetary waves, favoring recent NH weather extremes.

Keywords: atmospheric dynamics; heat waves; planetary waves; waveguides; weather extremes.

Fig. 1.

Time series of the observed amplitudes (in meters per second) of zonal wave numbers m = 6 (black), m = 7 (red), and m = 8 (blue) for the 15-d running means of the meridional wind velocity at 300 hPa averaged over 37.5°N−57.5°N for May−September 2012 and 2013, based on daily reanalysis data (41). The filled circles designate the observed HPA events exceeding the 1.5 SD (dashed horizontal lines) above the 1980–2013 climatology (solid horizontal lines), when the QRA mechanism was at work within up to 1.5–2 wk time offset relative to respective HPA event. The amplitudes of these QRA events are marked by colored filled squares. The open circles denote the observed HPA events when the QRA mechanism was not in action. The open square marks the high amplitude of the QRA event for m = 7, when the corresponding QRA-predicted HPA event did not happen.

Fig. 2.

The QRA event with 10 August 2012 as the central date, followed, with a lag of 5 d, by the observed HPA event during heat waves in the western and eastern United States and severe flooding in the central United States, accompanied by flooding in central China and droughts in eastern and western China (44). (A) Map (in meters per second) of the HPA event. (B) Nondimensional stationary wave number squared (the curve, left y axis) and resonance zonal wave number k (the straight line, right y axis) at the QRA event. All of the necessary conditions for the QRA event were met for the free wave with zonal wave number $k\approx 7.05$ within the midlatitude waveguide whose boundaries are marked by the vertical solid lines in B. The QRA amplitude ${\tilde{A}}_7\approx (5.6\pm 0.8)$ m⋅s⁻¹ of the forced m = 7 component matches well the observed ${\tilde{A}}_{7,obs}\approx (5.4\pm 1.3)$ m⋅s⁻¹ for the HPA event lagged by 5 d (see Fig. 1).

Fig. 3.

The QRA event with 2 May 2013 as the central date during the catastrophic flood in central Europe. (A) Map (in meters per second) of the following observed HPA event for m = 7 with the central date shifted 1 d later. (B) Nondimensional stationary wave number squared (the curve, left y axis) and resonance zonal wave number k (the straight line, right y axis) at the QRA event. All of the necessary conditions for the QRA event were met for the free wave with zonal wave number $k\approx 6.8$ within the midlatitude waveguide whose boundaries are shown by the vertical solid lines in B. The QRA amplitude ${\tilde{A}}_7\approx (4.4\pm 0.6)$ m s⁻¹ matches well the observed ${\tilde{A}}_{7,obs}\approx (4.5\pm 1.3)$ m⋅s⁻¹ of the following HPA event (see Fig. 1).