ENSO Normals: A New Normals Product Conditioned by ENSO Phase and Intensity and Accounting for Secular Trends

Climate normals have traditionally been calculated every decade or so as the average values over a long period of time, often 30 years. Such an approach assumes a stationary climate, so several so-called alternative normals have been recently introduced. These typically attempt to account for trends associated with global climate change by using a shorter averaging period, more frequent updates, and/or extrapolating the linear trend. While such approaches account for monotonic climate change, they fail to harness known interannual climate variability such as that associated with the El Niño-Southern Oscillation (ENSO). Similar to climate change, ENSO systematically alters the background state of the climate. These effects and their uncertainties are relatively well established, but they are not reflected in any readily available climate normals datasets.

In this presentation, we derive a new set of normals for the continental United States aimed at accounting for both ENSO and climate change. Climate change is accounted for by using the optimal climate normal (OCN): an 11-year running average for temperature and 15-year average for precipitation. Anomalies are calculated by subtracting these running means and are then composited by ENSO phase. ENSO is broken into five phases for every month since 1950: Strong La Nina, Weak La Nina, Neutral, Weak El Nino, and Strong El Nino. Composites are produced for each 3-month season and each of the five phases. The ENSO normals are the sum of these composites with the OCN for a given month. The result is five sets of normals, one for each phase, that users may consult with respect to anticipated ENSO outcomes.