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Good question!  I'll quote Amy Butler's ENSO blog post on this very subject: 

El Niño, with its modified planetary waves, tends to promote the breakdown of the polar vortex, especially in late winter, which in turn modifies the atmospheric circulation pattern associated with the El Niño.

All the details here:  https://www.climate.gov/news-features/blogs/enso/el-niño-and-stratospheric-polar-vortex