New insights on ENSO teleconnection asymmetry and ENSO forced atmospheric circulation variability over North America

El Niño-Southern Oscillation (ENSO)-related sea surface temperature variability in the eastern equatorial Pacific drives an extratropical large-scale atmospheric response. The atmospheric response is a key driver of global climate variability, with the strongest impact occurring during Northern Hemisphere winter. The degree to which atmospheric circulation variability is altered during ENSO events, in comparison with atmospheric circulation variability during ENSO-neutral conditions, is the focus of this study. Two multi-century, CESM1-CAM4 simulations are compared: a fully coupled experiment (CTRL), and a partially decoupled experiment in which ENSO is dynamically suppressed (NoENSO) so that the mean state is not biased towards a particular ENSO phase. We present evidence that the rectification of ENSO and its teleconnections onto the mean state lead to an underestimation of the asymmetry of ENSO teleconnections in this model. Analyses also show that ENSO displaces 500 hPa geopotential height (Z500) variability away from the central northern U.S. and southern Canada, resulting in less variability during ENSO years than ENSO-neutral years. Additionally, we find that estimating the ENSO-forced change in Z500 variance compared to ENSO-neutral years requires a surprisingly large sample of ENSO-neutral years. The results imply that a substantially longer record–roughly an order of magnitude longer in length–is needed to fully capture the statistics of ENSO’s teleconnected impacts over North America than suggested in previous studies.