The Merging Storms

Jet streams vary in latitude, width, and depth on timescales from days to decades. The sensitivity of extreme weather to these variations has been underexplored — most jet stream research focuses on the upper-level dynamics of baroclinic waves, not on the extreme-generating phenomena near the surface.

Using dry, adiabatic idealized experiments, researchers systematically varied jet structure and found three distinct connections. Poleward-shifted jets accelerate initial cyclone intensification and favor anticyclonic Rossby wave breaking — consistent with established theory. But jet width regulates something less expected: the likelihood of surface cyclone merging. Poleward-shifted, broader, higher jets produce more frequent cyclone merging, generating intense wind extremes that neither cyclone would produce alone.

And the third finding: poleward-shifted, broad, deep jets dynamically precondition the flow for persistent stationary anticyclones without any diabatic contribution. No moisture, no condensation, no latent heat release. Pure dry dynamics creates the conditions for blocking — high-pressure systems that stall weather patterns for weeks.

The structural insight: the jet stream doesn’t just steer storms. It determines whether they merge. The merging is the amplification mechanism — two moderate cyclones combining into one extreme event. The jet width is the control variable, not the individual cyclone intensity. This means that changes in jet structure (from climate change, from natural variability) don’t just shift storms north or south. They change the probability that storms interact, and interaction is where the extremes live.

The through-claim: weather extremes are a property of storm interactions, not individual storms, and those interactions are controlled by the large-scale structure that contains them.