A226-0009
Attribution of NAO predictability beyond two weeks

Wednesday, 16 December 2020
Poster
Lantao Sun, Colorado State University, Department of Atmospheric Science, Fort Collins, CO, US, Judith Perlwitz, NOAA Physical Sciences Laboratory, Boulder, CO, United States, Jadwiga Richter, National Center for Atmospheric Research, Climate and Global Dynamics Laboratory, Boulder, CO, United States, Martin P Hoerling, NOAA, Physical Sciences Laboratory, Boulder, CO, United States and Jim Hurrel, Colorado State University, Department of Atmospheric Science, Fort Collins, United States
Abstract:
Winter climate over Europe and eastern North America is significantly affected by variability of the North Atlantic Oscillation (NAO). There is increasing evidence that the stratosphere provides a source of predictive skill for the NAO on subseasonal time scales. In this study, we use 20-member ensembles of 45-day reforecasts over the period of 1999-2015 with the Community Earth System Model, version 1 (CESM1) to quantify NAO predictive skill beyond two weeks and attribute that skill to two principal sources: namely, upper and lower boundary constraints linked to the stratosphere and El Niño-Southern Oscillation (ENSO), respectively. We find that the winter NAO skill for forecast lead times out to six weeks is higher following extreme stratospheric polar vortex conditions (weak and strong vortex events) compared to neutral states. Enhanced NAO predictive skill for weak vortex events results from stratospheric downward coupling, while enhanced skill for strong vortex events results also from lower boundary forcing related to ENSO. Implications for forecast system development and improvement are discussed.
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