What’s so hard about making theories of atmospheric blocking?

Atmospheric blocking and its impact on regional weather have been well-recognized for almost as long as the existence of jet streams. While the short-term forecast skills for blocking events have improved considerably over the years, blocking still remains a challenge to numerical weather prediction and causes occasional forecast busts in the midlatitudes. There are also significant uncertainties in the future trends of blocking frequencies under warming climate. What underlies all this is the lack of an accepted theory for the formation of atmospheric blocking. Given that theory has been successful at providing a first-order explanation for many large-scale phenomena in the Earth’s midlatitude atmosphere, the absence of theory for blocking appears quite stark to me.

Although there are different ways to define a blocking event, there is little dispute (I hope) that blocking entails a regional amplification of Rossby waves (or the meandering of the jet stream) that persists longer than the typical timescale of synoptic eddies. In this presentation, I will review different pathways to Rossby wave amplification. These include: (i) internal instabilities such as baroclinic and barotropic instabilities, (ii) interference of multiple wave components, (iii) resonance between external forcing and freely traveling Rossby waves, and (iv) inhomogeneity in the flow. These processes can operate in linear dynamics, but as the wave amplitude grows, interactions between finite-amplitude waves and mean flows, as well as between high-frequency and low-frequency eddies, will become increasingly important. I will use simple models to highlight these processes, and offer certain strategy for further theoretical developments or diagnosis using meteorological data. I will also touch on the two (seemingly) orthogonal approaches, namely global and local, in characterizing a transition between two distinct atmospheric states (e.g. high-index versus low-index states) and their relevance to the blocking dynamics.