The dots in each quadrant of this graph show the strength of the polar vortex in past winters (December-February), based on the phase of the Quasi-biennial Oscillation (QBO) (vertical axis: positive values indicate westerly winds, negative values indicate easterly winds) and tropical sea surface temperatures in the Nino 3.4 region (horizontal axis: positive values indicate El Niño-like conditions, and negative values indicate La Niña-like conditions). The strongest polar vortex years (dark purple dots) cluster in the upper left quadrant of the graph, meaning winters with westerly QBO and La Nina. The weakest polar vortex years (dark green dots) occur in easterly QBO winters. Winters with at least one polar vortex disruption/sudden stratospheric warming (SSW) are marked with a black outline (and years with two have two circles). The red dot indicates where winter 2025 would rank based on November 2024 values only. NOAA Climate.gov image, adapted from original by Amy Butler.