Summertime Planetary Wave Resonance in the Northern and Southern Hemispheres

Abstract

Slow-moving planetary waves of high amplitudes are often associated with persistent surface weather conditions. This persistence can lead to extreme weather events with potentially serious implications for society and nature. Quasi-resonant amplification (QRA) of planetary waves has been proposed as a mechanism to generate high-amplitude hemisphere-wide patterns of wavenumbers 6–8 in the Northern Hemisphere (NH) summer. Here this mechanism is studied in both hemispheres. Analyzing 1979–2015 reanalysis data, evidence for QRA in the Southern Hemisphere (SH) is found for wavenumbers 4 and 5. It is shown that the difference in resonating wavenumbers between hemispheres stems from the different magnitude and latitudinal shape of the respective zonal-mean zonal winds. During resonance events a strong and narrow jet is observed in both hemispheres, whereas the emergence of a second zonal mean jet at high latitudes (i.e., a “double jet”) is seen in the NH only. Strong and narrow jets can form stable, zonally oriented waveguides, which are an essential prerequisite for resonance. After investigating the waves’ preferred phase positions during QRA, a bimodal behavior is identified for wavenumbers 4 and 5 in the SH and for wavenumber 6 in the NH, whereas wavenumbers 7 and 8 exhibit a single preferred phase position in the NH. Composite plots of meridional wind and temperature anomalies during periods when the resonating wave is in the preferred phase position indicate those regions that are most likely to experience heat extremes. These regions include western North America, western Europe, and central Eurasia in the NH and Patagonia and Australia in the SH.

Extreme weather events in early summer 2018 connected by a recurrent hemispheric wave-7 pattern

“…Meanders in the jet are referred to as Rossby-or planetary waves and previous studies have pointed out that slow moving amplified Rossby waves favor the occurrence of extreme weather conditions at mid-latitudes [10][11][12]. Such quasi-stationary Rossby waves can form (fully or nearly) circum-global teleconnections leading to the cooccurrence of unusual weather across the mid-latitudes [13][14][15][16][17][18]. Moreover, the extreme heatwaves of 2003, 2010 and 2015 have been linked to high amplitude Rossby waves.…”

Quasi‐stationary waves and their impact on European weather and extreme events

“…In summer, extreme heat waves and droughts can be linked to strong blocking anticyclones that can be part of a wider wave signal (Kornhuber et al. ).…”

“…On a more climatological level, very persistent or recurrent QSWs with the same phasing can lead to weather extremes and dominate the flow regime of a complete season (Trenberth and Guillemot, 1996;Pan et al, 1999). In summer, extreme heat waves and droughts can be linked to strong blocking anticyclones that can be part of a wider wave signal (Kornhuber et al, 2017). Petoukhov et al (2013) introduced the idea of quasi-resonant amplification as a specific class of QSWs, based on the model studies of circumglobal teleconnections of QSWs (Branstator, 2002;Ding and Wang, 2005).…”

Assessing changes in risk of amplified planetary waves in a warming world

“…Additionally, with QRA dependent on the strength and shape of the zonal mean of the zonal wind (see eq. 9 of Kornhuber et al, 2017a) and related meridional gradients in that quantity, we show the mean and spread of daily values of this wind diagnostic in Supporting Information Figure S1. Figure S1 is also for the JJA period, at 300 mb height, and for both CPDN and ERA-Interim.…”

“…Particular to mid-latitudes, persistent weather patterns are often associated with near stationary and highly amplified Rossby waves (Francis and Vavrus, 2012). Stationary and potentially stationary waves, with high amplitude ridges and troughs, have caused dangerous heat waves or flooding (Woollings, 2010;Coumou et al, 2014;Huntingford et al, 2014;Petoukhov et al, 2016;Stadtherr et al, 2016), and more specifically in the Central United States, Western United States and Central Eurasia (Kornhuber et al, 2017a). One concern is whether, within a GHG-enriched atmosphere, such amplification will occur more often.…”