Last Millennium ENSO Diversity and North American Teleconnections: New Insignts from Paleoclimate Data Assimilation

El Nino-Southern Oscillation (ENSO) variability affects year-to-year changes in North American hydroclimate. Extra-tropical teleconnections are not always consistent between El Nino 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 Nino 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 Nino, we find a marked increase in 20th century EP El Nino intensity, but no significant changes in CP or EP El Nino 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 Nino and wetter conditions in the same regions during EP El Nino; (b) wet conditions over the southwestern U.S. for both El Nino 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 Nino 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 Nino classification methods underscore the need for improved ENSO diversity classification when assessing precipitation teleconnections.