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

Abstract

Climate normals are traditionally calculated every decade as the average values over a period of time, often 30 years. Such an approach assumes a stationary climate, with several alternatives recently introduced to account for monotonic climate change. However, these methods fail to account for interannual climate variability [e.g., El Niño–Southern Oscillation (ENSO)] that systematically alters the background state of the climate similar to climate change. These effects and their uncertainties are well established, but they are not reflected in any readily available climate normals datasets. A new high-resolution set of normals is derived for the contiguous United States that accounts for ENSO and uses the optimal climate normal (OCN)—a 10-yr (15 yr) running average for temperature (precipitation)—to account for climate change. Anomalies are calculated by subtracting the running means and then compositing into 5 ENSO phase and intensity categories: Strong La Niña, Weak La Niña, Neutral, Weak El Niño, and Strong El Niño. Seasonal composites are produced for 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, which users may consult with respect to anticipated ENSO outcomes. While well-established ENSO patterns are found in most cases, a distinct east–west temperature anomaly pattern emerges for Weak El Niño events. This new product can assist stakeholders in planning for a broad array of possible ENSO impacts in a changing climate.

Exploring ENSO-Induced Anomalies over North America in Historical and Future Climate Simulations That Use HadGEM2-ESM Output to Drive WRF

“…In the evaluation of the HadGEM2 and HadGEM2-WRF output, we used as the reference dataset the NOAA ENSO Climate Normals [10] computed from temperature and precipitation data in the nClimGrid dataset [38]. This dataset has a horizontal resolution of ~5 km × 5 km and describes anomalies in minimum and maximum 2-m air temperature and monthly total precipitation anomalies (expressed as percent deviation from normal) as a function of the sign and magnitude of ENSO phase.…”

“…No new ESM simulations are performed but direct output from the HadGEM2 RCP8.5 r1i1p1 experiment is also presented to contextualize that from the HadGEM2-WRF model chain. Naturally, inferences drawn from the analyses presented herein must be viewed with caution, since we consider here only two exemplar periods with different ENSO phase in the historical and future climate, and past research has shown a diversity of ENSO global teleconnections in the historical climate [10]. Nevertheless, as illustrated herein, the temperature and precipitation anomalies directly from HadGEM2 for these specific periods are similar to the mean of all events of these phases, as simulated by HadGEM2 for 1979-2009.…”

“…Because of the global impact of ENSO teleconnections, it is the leading source of forecast skill on seasonal to interannual time scales [5]. Past research has established relationships between ENSO phase and temperature and precipitation patterns for countries such as the United States of America (US) [6][7][8][9][10][11]. Previous research has also sought to advance understanding of the relationships between ENSO and global climate anomalies, and how these teleconnections have changed over the last 30-40 years [9] and may evolve in the future.…”

“…There is emerging evidence that the ENSO-forced Pacific-North American climate mode may shift east [15] and that ENSO-derived precipitation variability may increase in many regions [16]. Recently, a new dataset (NOAA ENSO Climate Normals) of temperature and precipitation anomalies over the contiguous United States (CONUS) that accounts for background climate change trends has been developed [10].…”

Comparison of the Madden–Julian Oscillation-Related Tropical Cyclone Genesis over the South China Sea and Western North Pacific under Different El Niño-Southern Oscillation Conditions

“…The different ENSO conditions are defined by using the ONI. If more than five consecutive ONI values for the active TC season were > 0.5 • C (< −0.5 • C), the year was defined as an El Niño (La Niña) year [37]. On the basis of this definition, 1982, 1987, 1991, 1997, 2002, 2004, 2009, and 2015 were defined as the El Niño years, and 1988, 1998, 1999, 2000, 2007, 2010, and 2011 were defined as the La Niña years; the 23 remaining years were considered as neutral years.…”