[A-CG38] Dynamics of Oceanic and Atmospheric Waves, Vortices, and Circulations

[ACG38-11] Precursory analysis ensemble spread signals that foreshadow stratospheric sudden warmings

*Akira Yamazaki¹, Shunsuke Noguchi² (1.Application Laboratory, Japan Agency for Marine-Earth Science and Technology, 2.Kyushu University)

Keywords:Stratospheric sudden warming, ensemble Kalman filter, Atmospheric reanalysis

Behaviors of analysis ensemble spreads were investigated during stratospheric sudden warming (SSW) events in an atmospheric general circulation model—ensemble Kalman filter (LETKF; the local ensemble transform Kalman filter) reanalysis system, in which the model resolves the stratospheric circulations. Precursory signals of the increased spreads in the reanalysis system were found a week to a few days prior to stratospheric sudden warming events occurred during December 2018 and August–September 2019 in both hemispheres. The signals did appear in upper and middle stratosphere and did not below the lower stratosphere. When the signals appeared analysis increments became substantial against forecast updates in the forecast-analysis cycles; it is found that the precursory signals help to reproduce SSWs accordingly. An empirical orthogonal function analysis showed that the dominant structures of the precursory signals were equivalent barotropic and were 90° out of phase with the analysis ensemble-mean field. In the same timing the meridional gradients of absolute or potential vorticity in the middle stratosphere became temporally reversed. Hence, the spread signals can be equivalent to barotropic instability modes in the ensemble perturbation fields associated with SSWs.