04 Oct 2023
 | 04 Oct 2023

Changing effects of external forcing on Atlantic-Pacific interactions

Soufiane Karmouche, Evgenia Galytska, Gerald A. Meehl, Jakob Runge, Katja Weigel, and Veronika Eyring

Abstract. Recent studies have highlighted the increasingly dominant role of external forcing in driving Atlantic and Pacific Ocean variability during the second half of the 20th century. This paper provides insights into the underlying mechanisms driving interactions between modes of variability over the two basins. We define a set of possible drivers of these interactions and apply causal discovery to reanalysis data, an ensemble of pacemaker simulations where the observed El Niño Southern Oscillation (ENSO) is prescribed, and a pre-industrial control simulation. We also utilize large ensemble means of historical simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to quantify the effect of external forcing and improve the understanding of its impact. By conducting a causal analysis of the historical time series, a regime switch is identified in the interactions between major modes of Atlantic and Pacific climate variability. Causal networks derived from pacemaker simulations support this finding and further demonstrate that the effect of external forcing could favor an Atlantic-driven regime between 1985 and 2014 where warming tropical North Atlantic sea surface temperatures induce a La Niña-like cooling in the equatorial Pacific during the following season through a strengthening of the Pacific Walker Circulation. This negative sign effect was not detected when the historical external forcing signal is removed in the pacemaker ensemble. The analysis of the pre-industrial control run further supports the notion that the Atlantic and Pacific modes of natural climate variability exert contrasting impacts on each other even in the absence of external forcing. We show that causal discovery can quantify previously unknown connections and thus provides important potential to contribute to a deeper understanding of the mechanisms driving changes in regional and global climate variability.

Soufiane Karmouche et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-1861', Anonymous Referee #1, 26 Oct 2023
  • RC2: 'Comment on egusphere-2023-1861', Anonymous Referee #2, 15 Nov 2023

Soufiane Karmouche et al.

Soufiane Karmouche et al.


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Short summary
This study explores Atlantic-Pacific interactions and their response to external factors. Using advanced causal analysis, we analyzed climate data from 1950–2014, revealing a shift from a Pacific- to an Atlantic-driven regime. The contrasting impacts between El Niño and Tropical North Atlantic temperatures were highlighted, along with different pathways connecting the two oceans. These findings have the potential to enhance understanding of the climate system and guide model improvements.