Understanding Wavenumber-Dependent Responses of Northern Hemisphere Summer Stationary Waves to Climate Warming

Abstract Stationary waves of different wavenumbers tend to influence extreme events in different geographic regions, due to their specific locations of troughs and ridges. These different wavenumber waves are projected to respond differently to future climate change. Understanding the mechanisms behind these wavenumber-dependent responses is essential for improving our physical understanding and future projections of regional climate and extreme events. Using the initial-condition large-ensemble simulations from NCAR CESM2 as our primary tool, we find that zonal wavenumber 3 is projected to amplify at mid–high latitudes in a warmer climate. This amplification is a robust signal that appears consistently across four additional large-ensemble models, although the magnitude of the response varies among models. Sensitivity experiments with an idealized stationary wave model reveal that this amplification is primarily driven by changes in the basic state, particularly in the upper troposphere, which is characterized by an amplified tropical upper-tropospheric warming. In contrast, future projections based on these large-ensemble models show a consistent weakening of zonal wavenumber 2 in the subtropics as climate warms, contributed by changes in both the basic state and diabatic heating. Additionally, zonal wavenumber 7 is projected to amplify at midlatitudes in CESM2 Large Ensemble (CESM2-LE) under global warming, mainly due to diabatic heating changes in the Pacific sector. However, there are discrepancies in wavenumber-7 amplification across the large-ensemble models. Significance Statement Climate change is increasing the frequency and intensity of extreme events, with significant regional differences. Stationary waves, defined as the time-mean zonally asymmetric component of atmospheric circulation, play an important role in modulating regional climate extremes. These waves can be separated into different zonal wavenumbers, each affecting different geographic regions due to the specific locations of troughs and ridges. Here, we investigate the wavenumber-dependent responses of stationary waves to anthropogenic warming and further unveil their underlying mechanisms. We found a robust wavenumber-3 amplification under future warming, mainly due to jet stream changes caused by tropical upper-tropospheric warming. Our findings offer valuable insights into how climate change is influencing atmospheric circulation, which could help improve predictions and projections of regional extremes.