Matthew Green-February 28

Evaluating the Joint Influences of the MJO and the Stratospheric Polar Vortex on the Northern Hemisphere Extratropical Circulation

Matthew R. Green¹

Jason C. Furtado¹

Elizabeth A. Barnes²

Michelle L’Heureux³

Laura M. Ciasto^3,4

Kirstin Harnos^3,4

¹School of Meteorology, University of Oklahoma, Norman, OK, USA

²Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA

³NOAA Climate Prediction Center, College Park, MD, USA

⁴INNOVIM, LLC, Greenbelt, MD, USA

Multiple modes of climate variability can interact and inform extended-range predictability of Northern Hemisphere (NH) wintertime weather. Two specific patterns are: (1) The Madden-Julian Oscillation (MJO) and (2) The state of the stratospheric polar vortex. The MJO and its associated convection acts as a source of Rossby waves that propagate vertically and poleward, interacting with the extratropical atmosphere. The stratospheric polar vortex is a seasonal phenomenon in the NH stratosphere, whose variability can directly and indirectly influence the NH polar jet stream. While both of these climate modes of variability have been studied separately with respect to their influence on winter weather regimes, their combined impacts on wintertime weather remains unexplored.

This study analyzes the combined interactions of the MJO and the stratospheric polar vortex on extended-range predictability of NH winter weather patterns, using the ERA-Interim reanalysis dataset. The analysis takes place through a series of conditional composites involving MJO-only, strong/weak polar vortex-only, and MJO + polar vortex composites. In general, we find that, even when considered collectively, in the Pacific basin, the MJO has a more prominent control over the tropospheric pattern, while the Atlantic tropospheric pattern is seemingly controlled by stratospheric polar vortex. This result suggests that, for example, using only the stratospheric polar vortex for Central US extended-range forecasts may lead to higher forecast errors than those forecasts also considering the state of the MJO. Wave diagnostics like Eliassen-Palm (EP) flux diagrams are also used to show that wintertime activity of the MJO and polar vortex variability leads to anomalous responses in polar jet speeds and positions, with a potential feedback between the MJO and polar vortex at work. Finally, the investigation into the path and position of the Rossby waves during these periods and subsequent impacts on extreme winter weather are further explored through the use of blocking indices.