PNNL

Abstract

While a large meridional meandering of the jet stream such as atmospheric blocking brings about disproportionate socioeconomic impacts, it remains uncertain as to whether the Northern Hemisphere winter circulation will become wavier or less under climate change (1–6). Previous studies have hypothesized that a reduced westerly jet speed under Arctic warming leads to increased waviness and associated extremes (7–10), but jet speed changes in observations or models cannot predict midlatitude waviness changes on multidecadal timescales (6). Using observations and large ensembles of climate model simulations of the 21st century, we show that changes to winter circulation waviness under climate change are, to a large extent, estimated from changes to the waviness of a passive tracer, when the corresponding climate response in zonal wind is used for tracer advection. Remarkably, changes in waviness and zonal wind are dominated by the optimal modes for wavier circulation, and future changes in the optimal modes are attributed not to simple jet speed changes but to a poleward shift of the North Pacific jet and a northeastward extension of the North Atlantic jet. Changes in jet streams further suggest that future changes to the North Pacific wave activity are obscured by model uncertainty rather than internal variability of the climate system, while the North Atlantic wave activity will experience a robust decline in a warmer climate.