Summer polar mesosphere and lower thermosphere response to El Niño-Southern Oscillation

We investigate the influence of El Niño-Southern Oscillation (ENSO) on the summer mesosphere and lower thermosphere (MLT) region of the Southern Hemisphere (SH). Analyses use the observed satellite temperature datasets from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument between 2002 and 2013, the Microwave Limb Sounder (MLS) between 2004 and 2013, and an ensemble of transient simulations from the Whole Atmospheric Community Climate Model version 3.5 (WACCM3.5) between 1953 and 2005. Significant temperature responses to ENSO are observed by SABER and MLS in the SH summer polar MLT region, characterized by an anomalous cooling in the upper mesosphere and an anomalous warming in the lower thermosphere in December and January during warm ENSO events. The observational patterns agree well with those in the WACCM simulations except that the observational magnitudes are clearly ~5 times stronger than model simulations. Analysis of the WACCM simulations indicates the following mechanism. The cooler temperatures in the tropical stratosphere associated with warm ENSO events affect the temperature gradient in the summer hemisphere leading to anomalous stratospheric easterly zonal wind in the SH high latitudes. The anomalous filtering of gravity waves (GWs) by the enhanced stratospheric easterly zonal wind in the SH high latitudes contributes to an anomalous eastward GW forcing in the SH upper mesosphere during warm ENSO events. Stronger forcing of the residual circulation enhances upwelling in the SH polar upper mesosphere and therefore cooler temperatures. This temperature anomaly in the SH summer upper mesosphere may impact the inter-annual variability in the formation of polar mesospheric clouds.