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Investigating the Underlying Mechanisms and Predictability of the MJO-NAM Linkage in the NMME Phase-2 Models

“Skillful weather predictions with 10- to 30-day lead times for the Northern Hemisphere (NH) extratropics remain a major challenge for the forecast community. Skillful predictions of extratropical NH subseasonal weather ultimately depend on knowledge of the position and strength of the polar jet stream, commonly represented by the Northern Annular Mode (NAM). Avenues forward to narrowing the subseasonal-to-seasonal (S2S) prediction gap with respect to the NAM seek to exploit interactions of intraseasonal modes of climate variability with the NAM. One such mode is the Madden-Julian Oscillation (MJO), the leading mode of subseasonal variability in the tropics. Another source of extended predictability for the NH extratropics is the polar stratosphere, whereby the state of the polar vortex exerts a downward influence on the tropospheric jet stream and thereby alter weather patterns and the tropospheric waveguide. Whether or not this stratospheric influence can influence MJO-related teleconnections remains unresolved.

The proposed project has three main objectives: (1) Enhance our knowledge about the dynamical links between the MJO and the NAM by considering the modulating influence of the extratropical stratosphere; (2) Evaluate these mechanisms of MJO-NH extratropical atmospheric teleconnections in the North American Multi-Model Ensemble Phase-2 (NMME-2) system; and (3) Connect and apply our findings and evaluations to predictions of atmospheric blocking and extreme weather events. Three synergistic research tasks will accomplish these goals. First, we will use observations to quantify and dynamically understand the MJO-stratosphere modulation effect. We will demonstrate how the strength of the polar vortex can modulate Rossby wave trains associated with the MJO and how this modulation affects NH wintertime blocking frequency. Next we will evaluate the MJO-NAM teleconnections in the NMME-2 models with a focus on blocking episodes and document performance as a function of (a) dynamical stratosphere-troposphere coupling and its relation to jet stream variability, and (b) the stratospheric resolution of the model. The final task will be an assessment of the MJO-stratosphere modulation effect in the NMME-2 models whereby we will quantify its ability to affect forecast skill of MJO-related blocking events.

The proposed work is submitted for consideration for the NOAA Modeling, Analysis, Predictions, and Projections (MAPP) Competition 2: Research to Advance Prediction of Subseasonal to Seasonal Phenomena. The proposed work satisfies NOAA MAPP’s mission “to enhance the Nation’s capability to understand and predict natural variability and changes in Earth’s climate system”. Quantifying the MJO-stratosphere modulation effect offers enhanced “process-level understanding” and an “understanding of predictability and the potential to advance the prediction of phenomena occurring on [subseasonal] timescales.” The described tasks collectively “address the predictability and prediction of S2S phenomena in the context of key underlying…dynamical processes (e.g., Rossby wave forcing, wave-mean flow interactions, and troposphere-stratosphere interactions)” and “explore how prediction of S2S phenomena is influenced by various aspects of the prediction system set-up, including: (i) model resolution…(ii) initialization of, and coupling between, Earth system components; [and] (iii) model physics.” Finally, narrowing the S2S predictability gap through a process-based understanding and evaluation of the simulation of the MJO-NAM link heightens societal awareness and preparation for potential extreme weather events, which contributes to NOAA’s “Weather Ready Nation” goal established in the Next Generation Strategic Plan (NGSP). Findings of this work will be shared with the proposed new NOAA S2S Task Force as well as the World Climate Research Programme (WCRP) S2S Prediction project and Working Group on Numerical Experimentation (WGNE) MJO Task Force via planned presentations, co-organized sessions at international conferences, and direct discussions with the committee members.”

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