The AO index measures the strength of the polar vortex — a belt of westerly winds circling the Arctic. It controls how cold Arctic air spills into mid-latitudes.
AO+: Polar vortex strong → jet stream fast and zonal → Arctic air contained → mild winters for US/Europe. AO−: Vortex weakens → jet stream meanders → Arctic air outbreaks → polar vortex "collapse". Sudden Stratospheric Warming (SSW) events can flip AO from + to − in days.