Role of stratospheric processes in predicting ENSO-NAO connections on subseasonal time scale

“There is increasing evidence that stratospheric processes and stratospheric-tropospheric coupling contribute to an enhanced predictive skill of tropospheric phenomena, including El Niño/Southern Oscillation-North Atlantic Oscillation (ENSO-NAO) connections on seasonal time scale. The role of the stratosphere on the predictive skill on the subseasonal time scale has not been systematically explored, and the available data and model studies are not conducive to understanding the responsible processes. This joint proposal between CIRES-University of Colorado/NOAA-ESRL-Physical Sciences Division and CGD-NCAR aims to improve our understanding of the role of the stratosphere on the predictability of the NAO and related extremes and to quantify to what extend NAO predictability can be improved by including a well-resolved stratosphere in a subseasonal modeling framework.

This proposed project has three main objectives:
1. To improve our understanding of the role of the stratosphere on the predictability of the NAO and related extremes on subseasonal time scale in the context of ENSO-NAO connections,
2. To quantify changes in predictive skill of a model with and without a well-resolved
stratosphere,
3. To provide the scientific community with a thoroughly tested and evaluated stratospheric
resolving subseasonal to seasonal (S2S) forecast system and dataset based on the Community Earth System Model (CESM).

We hypothesize that the skill of predicting the NAO phase and related climate extremes on a subseasonal time scale can be advanced during periods of extreme stratospheric vortex events, and the knowledge of the phase of the tropical stratospheric Quasi-biennial Oscillation (QBO) specifically when also taking into account the phase of ENSO. This project will utilize the default 30-level, as well as the newly developed 46-level version of CESM in subseasonal forecasting and reforecasting mode. The detailed comparison of reforecasts performed with two models with the same tropospheric physics and model resolution but a poorly and well-resolved stratosphere, will allow for the examination of the processes responsible for potentially enhanced predictive skill. Predictive skill of the 46-level CESM subseasonal to seasonal forecasting system (46LCESM-S2S) will be compared to the predictive skill of other S2S systems from the international modeling community participating in the Subseasonal to Seasonal Prediction Project, a joint research program between the World Weather Research Programme (WWRP) and the World Climate Research Programme (WCRP). All datasets generated as part of this project will follow the S2S protocol and will be provided for public use. The proposed project directly addresses MAPP competition 2 on research to advance prediction of phenomena on S2S time scale in the context of troposphere-stratosphere coupling and NAO predictability. This project addresses NOAA’s long-term goal: “Weather-Ready Nation Society is prepared for and responds to weather-related events” by carrying out research on and providing tools for the prediction of extreme events on the subseasonal time scale.”