Polar jet streams are huge rivers of air flowing from west to east at 10km height with a maximum speed of around 150 km/h. The jet streams carry with them the ‘weather’ (that is, the cyclones).

Northern hemisphere jet stream and cyclones.

The jets and the cyclones interact with each other and are in a delicate balance. The characteristics of this balance affect the climate on a regional scale. For example, storms brewing over the Southern Ocean gather moisture as they are carried by the jet stream and then shed it as rain over Tasmania and Southern Australia.

Numerical simulations of simplified mathematical models for the atmosphere revealed two striking features regarding this interaction of the jet and the cyclones.

First, cyclones can collectively organise to create and reinforce jets.

Secondly there can be multiple climate states. That is, for the same parameters, there can exist atmospheric flows with different mean jet position and also different statistics for the accompanying cyclones. For example, in one climate state the jet can be in its current position flowing South of Tasmania, between latitudes 40S and 60S, while in a different state the jet stream may flow between Tasmania and Australia.

If, for some reason, the climate switches from its current state to a different state the regional climate will be significantly affected. Unfortunately, due to the complex turbulent motions that take place, scientists do not have a firm analytic understanding of how these processes come about.

A recently outlined and quite radical viewpoint has proven useful in identifying the underlying physics of jet—cyclone interactions. The novelty of this viewpoint lies in a change of framework: rather than simulating the turbulent complex flows using the usual fluid equations, a new set of equations were obtained for the evolution of the flow statistics. The advantage is the new set of equations is simpler to analyse.

Using this new framework, CLEX researchers gained an analytical understanding of this collective interaction of the cyclones with the jet. They found jets form out of a chaos of cyclones in a manner similar to phase transitions in materials (e.g., water becoming ice). The jet streams formed due to instability that lies in the interaction of the jets and the collective action of its surrounding cyclones.

Using the same viewpoint, the researchers studied how this jet-forming instability is halted, which leads to the jet attaining a particular speed. Analysing the processes that limit the speed of a jet, the researchers found two different climate states: a climate with a weak jet and one with a stronger jet. Furthermore, they found abrupt switches between these climates as the strength of the cyclones varied.

These findings can help us understand how the turbulent atmosphere is organised in coherent motions via the jet stream. The mathematical tools devised here are also the first step towards an analytical understanding of the sensitivity of the jet organisation (and thus of the sensitivity of the climate state) to changes in external parameters, such as anthropogenic forcing.

  • Paper: Bakas, N. A., Constantinou, N. C., and Ioannou, P. J. (2019). Statistical state dynamics of weak jets in barotropic beta-plane turbulence. J. Atmos. Sci., 76 (3), 919-945. doi:10.1175/JAS-D-18-0148.1