Blocking Statistics in a Varying Climate: Lessons from a “Traffic Jam” Model with Pseudostochastic Forcing

Abstract

Recently Nakamura and Huang proposed a semiempirical, one-dimensional model of atmospheric blocking based on the observed budget of local wave activity in the boreal winter. The model dynamics is akin to that of traffic flow, wherein blocking manifests as traffic jams when the streamwise flux of local wave activity reaches capacity. Stationary waves modulate the jet stream’s capacity to transmit transient waves and thereby localize block formation. Since the model is inexpensive to run numerically, it is suited for computing blocking statistics as a function of climate variables from large-ensemble, parameter sweep experiments. We explore sensitivity of blocking statistics to (i) stationary wave amplitude, (ii) background jet speed, and (iii) transient eddy forcing, using frequency, persistence, and prevalence as metrics. For each combination of parameters we perform 240 runs of 180-day simulations with aperiodic transient eddy forcing, each time randomizing the phase relations in forcing. The model climate shifts rapidly from a block-free state to a block-dominant state as the stationary wave amplitude is increased and/or the jet speed is decreased. When eddy forcing is increased, prevalence increases similarly but frequency decreases as blocks merge and become more persistent. It is argued that the present-day climate lies close to the boundary of the two states and hence its blocking statistics are sensitive to climate perturbations. The result underscores the low confidence in GCM-based assessment of the future trend of blocking under a changing climate, while it also provides a theoretical basis for evaluating model biases and understanding trends in reanalysis data.

Why Are Stratospheric Sudden Warmings Sudden (and Intermittent)?

“…Mathematically this process is analogous to a traffic jam on a highway, where the jam expands backward by absorbing the incoming traffic. The resultant 1D model is akin to the traffic jam model of atmospheric blocking considered by Huang (2017, 2018) and Paradise et al (2019). Further simplified from previous theoretical models (e.g., Holton and Mass 1976;Plumb and Semeniuk 2003), this model permits certain analytical predictions besides the threshold, including the growth rate of SSW [Eq.…”

“…This is akin to a traffic jam on a highway, where the jam expands backward by absorbing the traffic from behind (Lighthill and Whitham 1955;Richards 1956). Huang (2017, 2018) and Paradise et al (2019) demonstrate that a similar dynamics is at play in the formation of atmospheric blocking.…”

A Nonlinear Multiscale Theory of Atmospheric Blocking: Dynamical and Thermodynamic Effects of Meridional Potential Vorticity Gradient

“…Atmospheric blocking is a large-scale, quasi-stationary, low-frequency circulation pattern with the lifetime of 10-20 days occurring in midlatitudes (Berggren et al 1949;Rex 1950;Shukla and Mo 1983). Because the extreme cold spells in winter (Buehler et al 2011), European temperature extremes in spring (Brunner et al 2017) and heat waves in summer (Della-Marta et al 2007;Schaller et al 2018) are often related to the establishment and maintenance of mid-high-latitude blocking, the formation and maintenance mechanism of atmospheric blocking has been an important research topic in past decades (Yeh 1949;Shutts 1983;Colucci 1985;Haines and Marshall 1987;Holopainen and Fortelius 1987;Luo 2000Luo , 2005Luo et al 2014Luo et al , 2019Zhang and Luo 2020;Huang 2017, 2018;Aikawa et al 2019;Paradise et al 2019).…”

Hilbert problems for the geosciences in the 21st century – 20 years later

“…1 is a sketch the linear point of view that persistent anomalies in mid-latitude atmospheric flows on 10-100-day time scales are just due to the slowing down of Rossby waves or to their linear interference (Lindzen, 1986). An interesting extension of this approach into the nonlinear realm is due to N. Nakamura and associates (Nakamura and Huang, 2018;Paradise et al, 2019). The traffic jam analogy for blocking in this work is somewhat similar to the hydraulic jump analogy of C. G. Rossby and collaborators (1939); see also Malone et al (1951Malone et al ( /1955Malone et al ( /2016.…”