Reconsidering the relationship of the El Ni\~no--Southern Oscillation and the Indian monsoon using ensembles in Earth system models

TL;DR

This study investigates how the connection between ENSO and the Indian monsoon has changed over time using ensemble climate model simulations, revealing increased teleconnection strength historically and complex relationships in climate means.

Contribution

It demonstrates the nonstationarity of ENSO-monsoon teleconnection over the 20th century and highlights the seasonal variability of their climatic mean relationship using advanced Earth system models.

Findings

Increased teleconnection strength from 1890 to 2005 in MPI-ESM.

No significant change in teleconnection post-2005 under RCP8.5.

Linear displacement of climatic means with strong seasonal slope variation.

Abstract

We study the relationship between the El Ni\~no--Southern Oscillation (ENSO) and the Indian summer monsoon in ensemble simulations from state-of-the-art climate models, the Max Planck Institute Earth System Model (MPI-ESM) and the Community Earth System Model (CESM). We consider two simple variables: the Tahiti--Darwin sea-level pressure difference and the Northern Indian precipitation. We utilize ensembles converged to the system's snapshot attractor for analyzing possible changes (i) in the teleconnection between the fluctuations of the two variables, and (ii) in their climatic means. (i) With very high confidence, we detect an increase in the strength of the teleconnection, as a response to the forcing, in the MPI-ESM under historical forcing between 1890 and 2005, concentrated to the end of this period. We explain that our finding does not contradict instrumental observations, since their existing analyses regarding the nonstationarity of the teleconnection are either methodologically unreliable, or consider an ill-defined teleconnection concept. In the MPI-ESM we cannot reject stationarity between 2006 and 2099 under the Representative Concentration Pathway 8.5 (RCP8.5), and in a 110-year-long 1-percent pure CO2 scenario; neither can we in the CESM between 1960 and 2100 with historical forcing and RCP8.5. (ii) In the latter ensembles, the climatic mean is strongly displaced in the phase space projection spanned by the two variables. This displacement is nevertheless linear. However, the slope exhibits a strong seasonality, falsifying a hypothesis of a universal, emergent relationship between these two climatic means, excluding applicability in an emergent constraint.