Content area
Full Text
ABSTRACT
In the Southern Hemisphere (SH) polar region, satellite observations reveal a significant upper-mesosphere cooling and a lower-thermosphere warming during warm ENSO events in December. An opposite pattern is observed in the tropical mesopause region. The observed upper-mesosphere cooling agrees with a climate model simulation. Analysis of the simulation suggests that enhanced planetary wave (PW) dissipation in the Northern Hemisphere (NH) high-latitude stratosphere during El Niño strengthens the Brewer-Dobson circulation and cools the equatorial stratosphere. This increases the magnitude of the SH stratosphere meridional temperature gradient and thus causes the anomalous stratospheric easterly zonal wind and early breakdown of the SH stratospheric polar vortex. The resulting perturbation to gravity wave (GW) filtering causes anomalous SH mesospheric eastward GW forcing and polar upwelling and cooling. In addition, constructive inference of ENSO and quasi-biennial oscillation (QBO) could lead to stronger stratospheric easterly zonal wind anomalies at the SH high latitudes in November and December and early breakdown of the SH stratospheric polar vortex during warm ENSO events in the easterly QBO phase (defined by the equatorial zonal wind at ~25 hPa). This would in turn cause much more SH mesospheric eastward GW forcing and much colder polar temperatures, and hence it would induce an early onset time of SH summer polar mesospheric clouds (PMCs). The opposite mechanism occurs during cold ENSO events in the westerly QBO phase. This implies that ENSO together with QBO could significantly modulate the breakdown time of SH stratospheric polar vortex and the onset time of SH PMC.
1. Introduction
As a result of sea surface temperature variation in the tropical east-central Pacific Ocean, El Niño-Southern Oscillation (ENSO) strongly impacts interannual variability not only in the tropical troposphere (Yulaeva and Wallace 1994) but also in the global middle atmosphere (e.g., the stratosphere andmesosphere;Randel et al. 2009; Calvo et al. 2010; Li et al. 2008, 2013; Pedatella and Liu 2013). Previous observations and model simulations have suggested significant middle atmosphere temperature anomalies during El Niño, with an equatorial stratosphere cooling and a Northern Hemisphere (NH) high-latitude warming (Garcia-Herrera et al. 2006). This is induced by the anomalously increased planetary wave (PW) propagation (from the troposphere) and PW dissipation in the stratosphere of NH high latitudes, leading to anomalous wave dissipation, decelerated stratospheric westerly...