Nonlinear forced change and nonergodicity: The case of ENSO-Indian monsoon and global precipitation teleconnections

TL;DR

This study investigates the nonlinear and nonergodic forced changes in the ENSO-Indian monsoon teleconnection using ensemble data, revealing a consistent strengthening of this link under climate change scenarios and demonstrating its nonlinear nature.

Contribution

It introduces a novel analysis of the nonlinear, nonergodic forced response of ENSO-Indian monsoon teleconnection using ensemble-based correlation measures and multiple analytical methods.

Findings

Strengthening of ENSO-IM teleconnection is consistent across different representations.

The forced change in teleconnection is typically nonlinear and nonergodic.

Global maps show the spatial distribution of nonlinearity in teleconnection changes.

Abstract

We study the forced response of the teleconnection between the El Nino-Southern Oscillation (ENSO) and global precipitation in general and the Indian summer monsoon (IM) in particular in the Max Planck Institute Grand Ensemble. The forced response of the teleconnection is defined as the time-dependence of a correlation coefficient evaluated over the ensemble. The ensemble-wise variability is taken either wrt. spatial averages or dominant spatial modes in the sense of Maximal Covariance Analysis or Canonical Correlation Analysis or EOF analysis. We find that the strengthening of the ENSO-IM teleconnection is robustly or consistently featured in view of all four teleconnection representations, whether sea surface temperature (SST) or sea level pressure (SLP) is used to characterise ENSO, and both in the historical period and under the RCP8.5 forcing scenario. The main contributor to this strengthening in terms of a linear regression model is the regression coefficient, which can outcompete even a declining ENSO variability in view of using the SLP. We also find that the forced change of the teleconnection is typically nonlinear by (1) formally rejecting the hypothesis that ergodicity holds, i.e., that expected values of temporal correlation coefficients with respect to the ensemble equal the ensemble-wise correlation coefficient itself, and also showing that (2) the trivial contributions of the forced changes of e.g. the mean SST and/or precipitation to temporal correlations are insignificant here. We also provide, in terms of the test statistics, global maps of the degree of nonlinearity/nonergodicity of the forced change of the teleconnection between local precipitation and ENSO.