Summer warming in the Northern Hemisphere midlatitudes amplified by tropical-extratropical interactions

The Northern Hemisphere (NH) midlatitudes have exhibited intensified summer heat extremes over the past decades and growing evidence suggests that this reflects not only the thermodynamic background warming but also dynamical variability that promotes persistent ridging and land-atmosphere feedback. Here we assess the extent to which tropical-extratropical interactions, and in particular ENSO-like tropical Pacific variability, modulate NH summer circulation and eddy-mean flow feedbacks in ways that amplify midlatitude warming and extremes in addition to studying how the dynamical contribution may evolve under anthropogenic forcing. The analysis is motivated by an observed shift in the distribution of the summer daily surface temperatures across the midlatitudes towards more extreme warm conditions during years when tropical Pacific sea surface temperatures (SST) are anomalously cold over the period 1979-2024. The La Niña-like conditions in the tropics are accompanied by a coherent upper-tropospheric response characterized by enhanced ridging and meridional convergence of eddy momentum flux around 40°N. However, trends in eddy momentum flux convergence over the same period show opposite sign changes relative to the La Niña composite, despite tropical Pacific SST trends that appear La Niña-like, emphasizing that ENSO-like SST patterns in trends do not necessarily imply ENSO-like eddy-mean flow feedbacks and highlighting the role of the evolving mean state conditions. To isolate the role of radiative forcing versus SST changes, we analyze two sets of tropical Pacific pacemaker simulations conducted with the fully-coupled Community Earth System Model v.2, in which reanalysis SST anomalies are prescribed while radiative forcing is either held fixed or allowed to evolve. This design allows us to study how the evolving forced mean state alters the tropical precipitation/divergence response to SST and the midlatitude waveguide and eddy momentum convergence. We find that the observed shift towards more extreme warm summers during La Niña years emerges only when radiative forcing is fixed despite identical tropical Pacific SST nudging. We interpret this contrast through CO2-driven “fast” atmospheric adjustments (reduced radiative cooling) that weaken tropical vertical motions independent of SST warming, thereby altering the effective ENSO heating anomalies that drive teleconnections. Implications for the dynamical modulation of NH summer hot extremes by ENSO under continued anthropogenic forcing are discussed. Lastly, we show that a composite conditioned on capturing the observed trends in summer heat extremes in the CESM2 Large Ensemble also shows a La Niña-like tropical Pacific cooling and a chain of high-pressure trends across the NH midlatitudes, which suggests that tropical-extratropical interactions can amplify midlatitude summer warming albeit with a likely mean state-dependent response.