Sudden Stratospheric Warming: Polar Vortex Breakdown

Planetary Wave Forcing

Wave amplitude (EP flux drive) 0.00

Vortex Morphology

Simulation speed 1.0×

Stratospheric State

Vortex intact

U 60°N, 10 hPa

18.0 m/s

Vortex strength

100%

Polar cap ΔT

+0.0 K

Elapsed

0.0 d

How it works

The wintertime polar vortex is a belt of westerly stratospheric winds circling the pole near 10 hPa (~30 km). It is disturbed by planetary-scale Rossby waves (wavenumber m = 1 or 2) that propagate up from the troposphere. Where these waves break, they deposit westward momentum and decelerate the vortex — the Eliassen–Palm flux convergence mechanism:

dS/dt = -k·A(t)·S + γ·(S0 - S)
U(60°N,10hPa) = Umax·S

Here S is vortex strength (1 = full climatological strength), A(t) the wave-forcing amplitude you set below, k the deceleration rate, and γ a radiative-relaxation term that rebuilds the vortex once forcing eases. A Sudden Stratospheric Warming is formally defined as U reversing from westerly (+) to easterly (−) at 60°N/10 hPa.

As the vortex weakens, angular momentum balance forces mass to converge and sink over the pole. That descent compresses and adiabatically warms the polar stratosphere, often by 30–50 K in a matter of days — modeled here as a lagged response to the vortex's deceleration:

dT/dt = β·max(0, -dS/dt) - α·(T - T0)

Wave amplitude — stronger forcing (larger A) drives faster deceleration; sustained strong forcing pushes U through zero, triggering the SSW criterion.
m = 1 vs m = 2 — a wavenumber-1 disturbance displaces the vortex off the pole (e.g. January 2004, 2019); wavenumber-2 forcing splits it into two daughter vortices (e.g. January 2009) — both are real, observed SSW morphologies.
Radiative relaxation — with the wave amplitude returned to zero, γ slowly rebuilds the vortex over weeks, matching the post-SSW recovery seen in reanalysis data.

Real-world relevance: SSWs are the strongest documented tropospheric–stratospheric coupling event — the weakened vortex signal propagates down and shifts the surface jet stream and polar-front storm track for 4–8 weeks, a key source of sub-seasonal cold-air-outbreak predictability for forecasters.

Vortex-edge air parcels Warming polar cap

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