Implications of the permanent El Niño teleconnection "blueprint" for past global and North American hydroclimatology
1Earth and Atmospheric Sciences, Purdue University, USA
2School Earth of Sciences, Stanford University, USA
3Meteorology and Climate Centre, School of Mathematical Sciences, University College Dublin, Dublin, Ireland
Abstract. Substantial evidence exists for wetter-than-modern continental conditions in past warm climates. This is in apparent conflict with the robust global prediction for future climate change of a northward expansion of the subtropical dry zones that should drive aridification of many semiarid regions. Areas of expected aridification include much of Western North America, where extensive paleoenvironmental records from North America point to wetter conditions before the onset of Quaternary ice ages. It has been proposed that climates previous to the early Pliocene may have been characterized as being in a state with warmer-than-modern eastern equatorial sea surface temperatures (SSTs). Because Equatorial Pacific SSTs exert strong controls on midlatitude atmospheric circulation and the global hydrologic cycle, the teleconnected response from this permanent El Niño-like mean state has been proposed as a useful analogue model, or "blueprint", for understanding global climatological and hydrological anomalies in the past. The present study quantitatively explores the implications of this blueprint for past climates, using a global climate model (CAM3.0) and a nested high-resolution climate model (RegCM3) to study the hydrologic impacts of a permanent El Niño on global and North American climate. We find that the global circulation response to a permanent El Niño resembles a large, long El Niño event. However, this state also exhibits equatorial super-rotation, which would represent a fundamental change to the tropical circulations. We also find intensification and southward drift in winter storm tracks in the Pacific, which affects precipitation and temperature over the mid-latitudes via large shifts in atmospheric circulation. In addition, summertime precipitation increases over the majority of the continental United States, with these increases likely controlled by shifts in the subtropical jet and secondary atmospheric feedbacks. Based on these results, we conclude that a permanent El Niño is a good explanation of the Pre-Quaternary wetter-than-modern conditions observed in paleo proxy records, particularly over the Western United States.