On the Linkage Between Rossby Wave Phase Speed, Atmospheric Blocking, and Arctic Amplification

Abstract

It has been hypothesized that enhanced Arctic warming with respect to midlatitudes, known as Arctic amplification, had led to a deceleration of eastward propagating Rossby waves, more frequent atmospheric blocking, and extreme weather in recent decades. We employ a novel, daily climatology of Rossby wave phase speed between March 1979 and November 2018, based on upper-level wind data, to test this hypothesis and describe phase speed variability. The diagnostic distinguishes between periods of enhanced or reduced eastward wave propagation and is related to the occurrence of blocking and extreme temperatures over midlatitudes. While remaining tied to the upper-level geopotential gradient, decadal trends in phase speed did not accompany the observed reduction in the low-level temperature gradient. These results confirm the link between low phase speeds and extreme temperature events, but indicate that Arctic amplification did not play a decisive role in modulating phase speed variability in recent decades. Plain Language Summary The Arctic is warming more rapidly than midlatitudes and the temperature difference between those regions is being reduced. As a result, it has been hypothesized that the jet stream will decrease in intensity and its meanders will move more slowly eastward, leading to more persistent or even extreme weather conditions. As the persistence of weather can substantially vary within and between seasons, assessing long-term changes is not trivial. To tackle this problem, we develop a "weather speedometer" and quantify the west-east displacements of jet meanders over Northern Hemisphere midlatitudes. This metric diagnoses whether jet meanders are on average propagating eastward (positive values), stagnating, or even retrogressing westward (negative values) on each day between March 1979 and November 2018. Using this metric, we confirm that low-speed periods are related to temperature extremes over northern midlatitudes. We also assess that there has not been an overall decrease in the propagation of jet meanders despite the significant reduction of the meridional temperature difference observed in recent decades. Results suggest the need of an improved understanding of the factors determining the persistence of weather conditions and remind caution is needed when attributing recent extreme weather to an increased stagnation of jet stream meanders.

Circumglobal Rossby wave patterns during boreal winter highlighted by wavenumber/phase speed spectral analysis

“…Previous applications of this technique to the midlatitudes have mainly described the climatological characteristics of Rossby waves or their differing representation in climate models (e.g., Dell'Aquila et al, 2005). Recent work (Sussman et al, 2020;Riboldi et al, 2020;Jolly et al, 2021) employed space/time spectral analysis to model and assess the presence of trends in the properties of Rossby waves, confirming its usefulness and applicability to climate change problems.…”

Occurrence and transition probabilities of omega and high-over-low blocking in the Euro-Atlantic region

“…The 6hourly time resolution yields a larger data basis compared to daily means but potentially contains also more uninteresting variability (noise). Our identification and classification strategy starts with the well-accepted instantaneous blocked longitudes (IBLs) after Tibaldi and Molteni (1990); see also Richling et al (2015). However, the subsequent algorithm (Step 1-4; Sect.…”

“…In the first step, we use the Blocking plugin (Richling et al, 2015) of the Freva system (Freie Universität Berlin Evaluation System; see Freva, 2017;Kadow et al, 2021) to calculate the so-called instantaneous blocking index. This index is a binary 1D blocking index that determines instantaneous blocked longitudes (IBLs) for every time step of a data series.…”

“…While the CRBL is fixed to 50 • N in the original work, the Freva blocking plugin uses a modification (after Barriopedro et al, 2010) that allows for a longitudinally dependent, temporally varying CRBL in accordance with the climatological storm track. A blocking is identified if the geopotential height gradients on the northern (GHGN) and on the southern (GHGS) side of the CRBL satisfy the following criteria (following Richling et al, 2015):…”

“…For each time step, we obtain a binary value for all longitudes (1: blocked; 0: unblocked). For more details on the method see Richling et al (2015), and for the specific configurations used in the analysis see the Supplement.…”