The Major Stratospheric Sudden Warming of January 2013: Analyses and Forecasts in the GEOS-5 Data Assimilation System

We present diagnostics of the January 2013 major stratospheric sudden warming (SSW) event based on real-time analyses and 5-day forecasts produced by the high-resolution (0.25 degree) GEOS-5 data assimilation system (DAS) with an emphasis on coupling between the troposphere, stratosphere, and lower mesosphere. This SSW event began with a strong wavenumber 1 pattern (late Dec 2012) followed by a splitting of the vortex into a wavenumber 2 pattern (6 Jan 2013). After the SSW the polar middle stratosphere circulation remained weak, with several significant intrusions of low EPV air, until it reformed in late February. We examine the middle stratosphere evolution before, during, and after the SSW event using several metrics, including EPV maps and 3D wave activity flux vectors. The 3D wave activity fluxes highlight the importance of specific tropospheric ridge patterns to the forcing of the SSW. The high horizontal resolution of GEOS-5 enables the detailed examination of resolved gravity wave breaking in the middle stratosphere during and after the SSW associated with tropospheric topography, and reveals the advection of narrow, low EPV, filaments into the polar region. Our results highlight the usefulness of high horizontal resolution data assimilation in understanding the complex tropospheric forcing and subsequent evolution of SSW events.