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

Abstract

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.

Influence of Western Pacific Madden–Julian Oscillation on New York City's Record‐Breaking Air Pollution in Early June 2023

“…The regressed geopotential height fields also revealed a dipole structure (Figure 2g), with amplitudes and spatial extents significantly larger than those associated with the Arctic SAT anomaly (Figure 2f). The observed pattern of high‐pressure anomalies over Canada and the Atlantic, juxtaposed with low‐pressure anomalies over the eastern United States, resembles a negative NAO pattern, which is associated with elevated SSTs to the north and equator of the North Atlantic (Czaja & Frankignoul, 1999; Frankignoul & Kestenare, 2005; Johnstone & Curry, 2023). Notably, the high‐pressure anomaly extended westward from the northern North Atlantic and covered the Canadian area (Figure 2g), potentially contributing to the catastrophic wildfire conditions experienced in the summer of 2023.…”