Middle Atmosphere Response to ENSO Events in Northern Hemisphere Winter by the Whole Atmosphere Community Climate Model

Abstract

The influences of El Niño and La Niña sea surface temperature anomalies on the middle atmosphere are analyzed using a chemistry-climate model, the Whole Atmosphere Community Climate Model, version 3 (WACCM3), and the 40-year reanalysis dataset from the European Centre for Medium-range Weather Forecasts (ERA-40). Monthly mean data are used to perform statistical and dynamical analyses. Temperature anomalies are found in the stratosphere during both El Niño and La Niña years. These anomalies exhibit diverse distribution patterns, which we ascribe to the different scales of the planetary waves. During an El Niño winter, planetary waves are most active in December and January, leading to more disturbances of the stratospheric polar vortex and a higher frequency of stratospheric sudden warming (SSW) events. Moreover, on the basis of the three-dimensional Eliassen-Palm (E-P) flux, we find that the main wave action is located in the eastern hemisphere and displaces the polar vortex to the western hemisphere. These vortex displacements are closely connected to the weather and climate in the corresponding areas. In contrast, during a La Niña winter, planetary waves are most active in February and March, and most SSW events occur at the same time. The distribution of the E-P flux indicates that planetary wave fluctuations are located between 90°E and 180°E, resulting in a displacement of the polar vortex with some associated changes in the related atmospheric circulation. In addition, the mesospheric residual circulation (RC) reverses between February and March in both El Niño and La Niña years. Dynamical analyses using parameterized gravity waves show that the mesospheric meridional RC is closely connected to gravity-wave drag. This, in turn, is directly influenced by temperature and zonal wind patterns in the early and late winter of El Niño and La Niña years. These anomalies in the RC may influence the distribution of atmospheric constituents and should be explored further.