ENSO Normals: A New U.S. Climate Normals Product Conditioned by ENSO Phase and Intensity and Accounting for Secular Trends

Climate normals are traditionally calculated every decade 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 to account for climate change. However, these still fail to harness known interannual climate variability such as 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 well established, but they are not reflected in any readily available climate normals datasets.

To account for ENSO and climate change, a new high-resolution set of normals is derived for the continental United States (CONUS). This new product uses the optimal climate normal (OCN): a 10-year running average for temperature and 15-year average for precipitation to account for climate change. Anomalies are calculated by subtracting the running means and then partitioned by ENSO phase. Using a quantiles approach for every 3-month season since 1950, ENSO is broken into five phases: Strong La Niña, Weak La Niña, Neutral, Weak El Niño, and Strong El Niño. Seasonal averages are produced for each of the five phases. The ENSO Normals are the sum of these composite anomalies with the OCN for a given month. The result is five sets of normals, one for each phase, which users may consult with respect to anticipated ENSO outcomes.

Our key findings include the following:

• ENSO impacts over the US vary significantly based on intensity of ENSO events.
• While well-established ENSO patterns are found in most cases, a distinct East-West temperature anomaly pattern emerges for wintertime Weak El Niño events.
• ENSO-conditioned temperature and precipitation quantile values (10%, 25%, 75%, 90%) exhibit a broad spread, indicating users must be prepared for a wide range of possible outcomes, even when the mean composite is significantly different from zero.
• ENSO impacts are physically consistent with changes in large-scale dynamics.
• In general, Weak El Niño and La Niña events feature more robust precipitation signals in California whereas Strong El Niño and La Niña events tend to bring more significant impacts to Texas and Florida. This helps explain the lack of extremely wet wintertime conditions in California during the Strong El Niño event of 2015/2016.