Last Millennium ENSO Diversity and North American Teleconnections: New Insights From Paleoclimate Data Assimilation.

El Niño‐Southern Oscillation (ENSO) variability affects year‐to‐year changes in North American hydroclimate. Extra‐tropical teleconnections are not always consistent between El Niño events due to stochastic atmospheric variability and diverse sea surface temperature anomalies, making it difficult to quantify teleconnections using only instrumentally‐based records. Here we use two paleoclimate data assimilation (DA) products spanning the Last Millennium (LM) to compare changes in amplitudes and frequencies of diverse El Niño events during the pre‐industrial period and 20th century, and to assess the stationarity of their North American hydroclimate impacts on multi‐decadal to centennial timescales. Using several definitions for Central Pacific (CP) and Eastern Pacific (EP) El Niño, we find a marked increase in 20th century EP El Niño intensity, but no significant changes in CP or EP El Niño frequencies in response to anthropogenic forcing. The associated hydroclimate anomalies indicate (a) dry conditions across the eastern‐central and northwestern U.S. during CP El Niño and wetter conditions in the same regions during EP El Niño; (b) wet conditions over the southwestern U.S. for both El Niño types. The magnitude of regional hydroclimate teleconnections also shows large natural variability on multi‐decadal to centennial timescales. However, when the entire LM is considered, mean hydroclimate anomalies in North America during CP or EP El Niño are consistent in terms of sign (wet vs. dry). Results are sensitive to proxy data and model priors used in DA products. Inconsistencies between El Niño classification methods underscore the need for improved ENSO diversity classification when assessing precipitation teleconnections. Key Points: Last Millennium data assimilation products show intensification of Eastern Pacific (EP) El Niño events in the 20th century, but no change in Central Pacific (CP) frequencyShifts in El Niño hydroclimate teleconnections over North America are variable in magnitude, but consistent in sign over the Last MillenniumInconsistent results between CP and EP El Niño definitions suggest a need for more robust, spatially dependent classification schemes [ABSTRACT FROM AUTHOR]