The Asymmetric Impacts of El Niño-Southern Oscillation Modulation on Daily Temperature Variability over North America

While the impacts of the El Niño-Southern Oscillation (ENSO) on the overall winter temperature pattern across North America are well known, ENSO’s impact on day-to-day variability in winter temperature is more obscure. One of the two primary inhibitors to our understanding of this modulation by ENSO is the relatively short observational record. The other inhibiting factor is isolating a true ENSO neutral state, as ENSO conditions can have a prolonged impact on the climate system that extend well into the following years. Recent coupled model experiments prove useful in remediating these issues. In this investigation, we study how ENSO modulates daily temperature variability over North America using two Community Earth Systems Model version 2 (CESM2) experiments in which ENSO and non-ENSO-related variability can be differentiated without the aforementioned caveats. In the first experiment, a fully coupled control version (FC) of CESM2 is used to examine the temperature variability, including that driven by ENSO. In the second experiment, ENSO variability is dynamically suppressed in the tropical Pacific, and thus is absent (NoENSO). As a result, the NoENSO simulation lacks ENSO-forced temperature variability in the extratropics. This provides a base state that is not biased towards either phase of ENSO and effectively removes ENSO-related variability without disrupting other important ocean-atmosphere connections and related responses in the other basins. We use boreal winter (DJF) months to calculate the seasonal, subseasonal, and total components of daily temperature variability and examine spatial patterns of the differences in variability between the two models.

When first comparing temperature variance for El Niño, La Niña, and Neutral ENSO phases as derived from the FC experiment, our results show that variations in the subseasonal daily temperature are the dominant driver of overall daily temperature variability. Over North America, El Niño leads to a reduction in subseasonal daily temperature variability across most of the continent compared to ENSO Neutral conditions. The seasonal component of El Niño drives higher daily temperature variability when compared to ENSO Neutral conditions across the Pacific Northwest and parts of northeastern Canada. Contrastingly, La Niña drives higher subseasonal daily temperature variability across much of the northern and central portions of North America when compared to ENSO Neutral conditions. Reductions in La Niña subseasonal daily temperature variability exist across the Gulf of Mexico and portions of the Pacific that are at a similar latitude when compared to ENSO Neutral conditions. Meanwhile, the seasonal component of La Niña daily temperature variability is elevated over the Pacific Northwest and southeastern United States, and reduced across the central portion of the continent when compared to the ENSO Neutral state from the FC simulation. Next, we repeat these temperature variance comparisons but instead replace the ENSO Neutral variance with the NoENSO simulation, which is definitively ENSO Neutral. Differences in our results when using the ENSO Neutral variance from the FC versus the NoENSO variance highlight the impacts of isolating ENSO-driven atmospheric effects from other climate system variation mechanisms.