Understanding Future Climate Trends of Northern Hemisphere Meridional Winds through the Lens of Subseasonal Teleconnections

Large-scale atmospheric circulation is expected to undergo various changes in the upcoming decades, affecting local weather regimes and extreme events. A prominent part of this process will be manifested in the interaction between midlatitude upper-tropospheric Rossby waves and the jet stream, which acts as their waveguide. Quasi-stationary wavenumber 5 zonal wave packets (sometimes referred to as CTP - Circumglobal Teleconnection Pattern) are a common feature of upper troposphere internal variability in the winter (Branstator & Teng, 2017). They have been previously associated with weather and climate extremes, such as cold spells and droughts over North America (Harnik et al., 2016; Teng & Branstator, 2017). Additionally, their likeness can be seen in the northern hemisphere meridional wind trend projections by the CMIP5 ensemble (Simpson et al., 2015), with considerable model spread in magnitude.
In this study, we explore the representation of these transient subseasonal-scale waves by the CMIP5 models, as well as their connection to the long-term climate trend. We show that, on a monthly time scale, most models under RCP8.5 runs develop a gradual preference for waves with certain phases. We then relate this to future changes on daily time scales, in which we track localized eastward-propagating wave packets with near-zero phase speed. We thus identify an increasing number of wave packets per winter that share this preferred phasing. In that respect, the model spread in the climate trend and its spatial complexity can be partially explained by differences in wave dynamics. Essentially, different models have different longitudinal sectors which show increased preferably-phased wave excitation (either over the Atlantic or the Pacific Ocean). Given the association of the CTP with extreme events, we also expect this range of dynamical patterns to be further expressed in temperature and precipitation trends in the future.