24 Feb 2023
 | 24 Feb 2023
Status: this preprint is open for discussion and under review for Weather and Climate Dynamics (WCD).

Predictable Decadal Forcing of the North Atlantic Jet Stream by Sub-Polar North Atlantic Sea Surface Temperatures

Kristian Strommen, Tim Woollings, Paolo Davini, Paolo Ruggieri, and Isla R. Simpson

Abstract. It has been demonstrated that decadal variations in the North Atlantic Oscillation (NAO) can be predicted by current forecast models. While Atlantic Multidecadal Variability (AMV) in sea surface temperatures (SSTs) has been hypothesised as the source of this skill, the validity of this hypothesis and the pathways involved remain unclear. We show, using reanalysis and data from two forecast models, that the decadal predictability of the NAO can be entirely accounted for by the predictability of decadal variations in the speed of the North Atlantic eddy-driven jet, with no predictability of decadal variations in the jet latitude. The sub-polar North Atlantic (SPNA) is identified as the only potential source of an SST-based signal across the models and reanalysis, and the predictability of the jet speed is shown to be consistent with a forcing from the SPNA visible already within a single season. The pathway is argued to be tropospheric in nature, with the SPNA-induced heating extending up to the mid-troposphere, which alters the meridional temperature gradient around the climatological jet position. The link between SSTs and heatfluxes, which mediates the predictability, is shown to be underestimated in the forecast models by approximately a factor of two, with potential implications for the `signal-to-noise paradox'. The relative roles of anthropogenic aerosol emissions and the AMOC at generating predictable SPNA variability are also discussed. The analysis is extensively supported by the novel use of a set of seasonal hindcasts spanning the 20th century and forced with prescribed SSTs.

Kristian Strommen et al.

Status: open (until 22 Apr 2023)

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Kristian Strommen et al.

Kristian Strommen et al.


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Short summary
We present evidence which strongly suggest that decadal variations in the intensity of the North Atlantic winter jetstream can be predicted by current forecast models, but that decadal variations in its position appear to be unpredictable. It is argued that this skill at predicting jet intensity originates from the slow, predictable variability of sea surface temperatures in the sub-polar North Atlantic.