Stratospheric polar warming by ENSO in winter: A statistical study

Abstract

Applying Linear Discriminant Analysis on 47 years of NCEP stratospheric temperature data from 1959 to 2005, we find that the warm‐ENSO (“El Niño”) years are significantly warmer also in the stratosphere at the Northern Hemisphere polar and midlatitudes than the cold‐ENSO (“La Niña”) 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°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°K between easterly QBO mean and the westerly QBO mean.