Drivers of Atlantic-African jet merging: A localized-building-blocks view of dynamical regimes

A winter-long or month-long merging of the Atlantic and African jet streams, as occured during Northern Hemisphere winter 2009-10, represents a rare dynamical change in the jet stream, with significant large-scale changes in synoptic storms and the distribution of weather extremes. Previous reanalysis-based studies showed that these rare jet-merging occurred along with anomalously strong tropical-Pacific heating and weak mid-latitude eddies. In idealized models, externally imposed stronger tropical heating and weaker mid-latitude baroclinicity will result in a shift of an eddy-driven jet to a merged-jet. In this talk we examine how this idealized-model picture can be extended to the complex atmosphere, to explain the observed and possible future Atlantic-African jet merging.

Using ERA5 reanalysis and CESM2-LENS large ensemble simulations, we identify a few "external forcing building blocks" that emerge 1-2 months before month-long jet merging during winter. These include Central Pacific and Eastern Pacific El Ninos, and several dynamically linked but distinct recurring large-scale mid-latitude anomaly patterns, which act to weaken the synoptic eddies. These include anomalies in surface temperature, lower tropospheric moisture, upper level geopotential height, and the stratospheric polar vortex.We find several distinct combinations of these external forcing building blocks which lead to jet merging, depending on the type of tropical heating anomaly. We will conclude with discussing possible implications for climate change.