Stratospheric polar warming by ENSO in winter: A statistical study

Applying Linear Discriminant Analysis on 47 years of NCEP stratospheric temperature data from 1959 to 2005, we find that the warm-ENSO ("El Nino'') years are significantly warmer also in the stratosphere at the Northern Hemisphere polar and midlatitudes than the cold-ENSO ("La Nina'') years, during winter. Specifically, the zonal mean, December-February mean, 10 - 50 hPa mean temperature, when projected onto the coherent spatial structure that best distinguishes the two ENSO groups, classified according to the equatorial Pacific-ocean Cold Tongue Index, is 4 degrees K warmer in the polar stratosphere in the warm-ENSO mean than in the cold-ENSO mean. The difference is statistically significant at above the 95% confidence level. This is the first time statistical significance has been established for ENSO's influence on the polar stratosphere. A surprising result is that the ENSO perturbation to the polar stratosphere is comparable in magnitude to the better-known QBO perturbation, which is 3.8 degrees K between easterly QBO mean and the westerly QBO mean.