Jet streams are narrow, fast-flowing westerly air currents located near the tropopause, the tropospheric boundary with the higher stratosphere. These jets play a major role in driving the large-scale atmospheric circulation and modulate the frequency, severity and persistence of weather events at middle latitudes. Although these jets have two broad sub-types, a single, spiral-shaped jet structure emerges when considering monthly or longer time averages. This mostly westerly flow also meanders over longer times, as the local flow becomes strongly meridional, or the jet splits temporarily. These important events lead to temperature and precipitation extremes, yet researchers still lack a good understanding of their dynamics.
In a new paper, a team of researchers including LML External Fellows Davide Faranda and Yuzuru Sato and LML Fellow Nicholas Moloney try to advance the understanding of jet dynamics by employing a low-dimensional dynamical systems model derived from reanalysis data. The best-known example of a low-dimensional model for atmospheric phenomena is Lorenz’ simple system reflecting Rayleigh–Bénard convection, but similar models have also been devised to study El Niño, ocean–atmosphere interactions and climate tipping points. The aim in each case is to capture key emergent behaviors such as chaos, intermittency or multi-stability. As the authors note, there was until very recently a strong case against the use of embedding techniques to derive low-dimensional models from experimental data. The main problem was a weak connection between models and real data due to the scarcity of observations and theoretical limitations. But progress in data quality and availability and the advent of stochastic dynamical systems have renewed the attention for data embedding.
In earlier work, Faranda and others showed that embedding techniques can now be used to yield effective low-dimensional dynamics, as long as the observables chosen reflect key symmetries of the system and one treats small-scale (sub-grid) dynamics as stochastic perturbations. In the new paper, the authors use these results to develop a minimal model of the effective dynamics of the mid-latitude jet. The jet model is based on a coupled map lattice, each element of which reflects the dynamics of the jet at a given longitude. As the researchers demonstrate, this model is useful for exploring a range of possible behaviours which may have appeared in past climates, and could appear again in future. Using the model, they offer an evaluation of key dynamical features of the jet, namely its stability, the statistics of splitting or breaking and the response to topographical features. They hope the model will help in understanding changes in the mid-latitude atmospheric circulation under climate change. In order to do so, they will extend the model to include information about tropics-to-poles temperature gradients, a quantity directly modified by anthropogenic emissions.
The paper is available at https://www.earth-syst-dynam.net/10/555/2019/