20 Mar 2023
 | 20 Mar 2023
Status: this preprint is currently under review for the journal ACP.

The Impact of Background ENSO and NAO Conditions and Anomalies on the Modeled Response to Pinatubo-Sized Volcanic Forcing

Helen Weierbach, Allegra N. LeGrande, and Kostas Tsigaridis

Abstract. Strong, strato-volcanic eruptions are a substantial, intermittent source of natural climate variability. Background atmospheric and oceanic conditions such as El Niño Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) also naturally impact climate on regular time scales. We examine how background conditions of ENSO and NAO impact the climate's response to a Pinatubo-type eruption using a large (81-member) ensemble of model simulations in GISS Model E2.1-G. Simulations are sampled from possible background conditions under the protocol of the coordinated CMIP6 Volcanic Model Intercomparison Project (VolMIP) – where aerosol forcing with time, latitude, and height. We analyze paired paired anomalous variation (perturbed - control) to understand changes in global and regional climate responses under positive, negative, and neutral ENSO and NAO conditions. In particular, we find that for paired anomalies there is a high probability of strong (~1.5 °C) post-eruptive winter warming for negative NAO ensembles with analysis coincident with decreased lower stratospheric temperature at the poles, decreased geopotential height, and strengthening of the stratospheric polar vortex. Historical anomalies (relative to climatology) show no mean warming and suggest that strength of this response is impacted by control conditions. Again using paired anomalies, we also observe that positive and negative ENSO ensembles relax the ENSO anomaly in the first post-eruptive Boreal Winter while neutral-phase ensembles are variable and show no clear response. In general, paired anomalies give insight into the evolution of the climate response to volcanic forcing, but are significantly impacted by background climate conditions present in control conditions.

Helen Weierbach et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Review of acp-2023-54 by Alan Robock', Alan Robock, 31 Mar 2023
  • RC1: 'Comment on acp-2023-54', Davide Zanchettin, 07 Apr 2023
  • RC2: 'Comment on acp-2023-54', Anonymous Referee #2, 11 Apr 2023

Helen Weierbach et al.

Helen Weierbach et al.


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Short summary
Volcanic aerosols impact global and regional climate conditions but can vary depending on pre-existing background climate conditions. We run an ensemble of volcanic aerosol simulations under varying ENSO and NAO background conditions to understand how background state impacts the modeled response. Overall we find that background NAO conditions can impact the strength of the first winter post-eruptive response, but are also affected by the choice of anomaly and sampling routine.