Enhanced North American ENSO teleconnection during the Little Ice Age revealed by Paleoclimate Data Assimilation

Pronounced seasonal changes in U.S. rainfall occur in association with the El Niño-Southern Oscillation (ENSO) via atmospheric teleconnections. Recent climate modeling studies show that teleconnection rainfall over North America may be sensitive to and systematically altered by mean state temperature changes in the tropical Pacific; however, characterizing teleconnection rainfall and improving prediction requires a large number of realizations of El Niño/La Niña events, and 20th century instrumental data are temporally limited. Given the immense bearing of these model projections on future U.S. hydroclimate risk, this study uses complementary high-resolution paleoclimate reconstructions to expand the statistics of how mean state changes in climate may affect teleconnection rainfall, providing validation from independent data provenance. ENSO variations spanning the Common Era are evaluated in new paleoclimate data assimilation (DA) products: the Last Millennium Reanalysis (LMR) and the Paleo Hydrodynamics Data Assimilation (PHYDA). The resulting synthesis is used to deduce how mean state changes in tropical Pacific temperature affect the magnitude/extent of ENSO-driven rainfall, using the Little Ice Age (LIA) and Medieval Climate Anomaly (MCA) as test periods. Despite widespread cooling during the LIA, the central-eastern tropical Pacific warms relative to the MCA and shifts El Niño teleconnections to the east, generally enhancing ENSO rainfall anomalies over the southwestern U.S. and drying the east coast and midwest. Teleconnections strengthen independently of ENSO amplitude, and we suggest caution in the use of paleoclimate reconstructions of ENSO amplitude based on teleconnection strength. The spatial signature of the LIA minus MCA differences in ENSO rainfall is strikingly similar to the predicted 2080-2100 change in ENSO rainfall for the U.S. in the Community Earth System Model Large Ensemble (RCP8.5). Taken together, our results underscore the importance of reducing uncertainties in the global warming pattern and mean temperature changes in the tropical Pacific, and harbor implications for the predictability of teleconnection rainfall in the U.S.