NAO predictability from external forcing in the late 20th century

Jeremy M Klavans,Amy C Clement,Mark A Cane,Lisa N Murphy

doi:10.1038/s41612-021-00177-8

Abstract

The North Atlantic Oscillation (NAO) is predictable in climate models at near-decadal timescales. Predictive skill derives from ocean initialization, which can capture variability internal to the climate system, and from external radiative forcing. Herein, we show that predictive skill for the NAO in a very large uninitialized multi-model ensemble is commensurate with previously reported skill from a state-of-the-art initialized prediction system. The uninitialized ensemble and initialized prediction system produce similar levels of skill for northern European precipitation and North Atlantic SSTs. Identifying these predictable components becomes possible in a very large ensemble, confirming the erroneously low signal-to-noise ratio previously identified in both initialized and uninitialized climate models. Though the results here imply that external radiative forcing is a major source of predictive skill for the NAO, they also indicate that ocean initialization may be important for particular NAO events (the mid-1990s strong positive NAO), and, as previously suggested, in certain ocean regions such as the subpolar North Atlantic ocean. Overall, we suggest that improving climate models’ response to external radiative forcing may help resolve the known signal-to-noise error in climate models.

Highlights

Groundbreaking work over the last several years demonstrates that climate models can predict the North Atlantic Oscillation (NAO) out to near-decadal timescales[1,2,3,4]
In our very large uninitialized ensemble, we find that predictive skill for the NAO is at least equal to that from S20’s initialized forecast system
As in S20, we find a very large ratio of predictable components (RPC) (11.61), consistent with Scaife and Smith’s6 finding that the signalto-noise ratio is too low in climate models

Summary

INTRODUCTION

Groundbreaking work over the last several years demonstrates that climate models can predict the North Atlantic Oscillation (NAO) out to near-decadal timescales[1,2,3,4]. The key insight offered by Smith et al (2020; S20) is that decadal predictive skill for the NAO only emerges in very large multi-model ensembles[4]. There are hints that external forcing can be a source of predictive skill for the NAO at longer lead times In both models and observations, tropical volcanic eruptions instigate the NAO to move towards its positive phase[19,20]. On decadal timescales, in the Community Earth System Model (CESM), an uninitialized ensemble produces skill commensurate with an initialized prediction system for two NAO impacts: North American and western European summertime precipitation (as estimated via field significance in Yeager et al 2018s Fig. 5)[8]. Using a 269-member uninitialized multi-model large ensemble, in this paper we show that external forcing is the larger component of NAO predictability.

RESULTS

DISCUSSION

METHODS

CODE AVAILABILITY