Comment on "Influence of the Southern Oscillation on tropospheric temperature" by J. D. McLean, C. R. de Freitas, and R. M. Carter

TL;DR

This paper presents an alternative correlation between ENSO and global temperature, highlighting the role of accumulated ENSO effects in multi-decadal warming trends and suggesting ENSO's significant contribution to temperature increases since 1960.

Contribution

It introduces a novel cumulative SOI term that better explains temperature variation and supports the hypothesis that ENSO accumulation influences long-term warming trends.

Findings

50% of temperature variation explained by cSOI in regression

Accumulation of ENSO effects can produce multi-decadal warming

ENSO may significantly contribute to temperature trends since 1960

Abstract

We demonstrate an alternative correlation between the El Ni\~no Southern Oscillation (ENSO) and global temperature variation to that shown by McLean et al. [2009]. We show 50% of the variation in RATPAC-A tropospheric temperature (and 54% of HadCRUT3) is explained by a novel cumulative Southern Oscillation Index (cSOI) term in a simple linear regression. We review evidence from physical and statistical research in support of the hypothesis that accumulation of the effects of ENSO can produce natural multi-decadal warming trends. Although it is not possible to reliably determine the relative contribution of anthropogenic forcing and SOI accumulation from multiple regression models due to collinearity, these analyses suggest that an accumulation ratio cSOI/SOI of $4.8\pm1.5%$ and up to $9\pm2%$ is sufficient for ENSO to play a large part in the global mean temperature trend since 1960.