Influence of the El Nino Southern Oscillation on the middle and upper atmosphere

Based on Whole Atmosphere Community Climate Model (WACCM) simulations, Pedatella and Liu (2012) recently demonstrated that significant interannual variability occurs in migrating and nonmigrating tides in the mesosphere and lower thermosphere (MLT) due to the El Nino Southern Oscillation (ENSO). The role of changes in tropospheric forcing, changes in the zonal mean atmosphere, and planetary wave-tide interactions on generating the tidal variability in the MLT are investigated in the present study. The ENSO-driven variability in the migrating diurnal tide (DW1) is found to be primarily due to changes in the tropospheric forcing of the DW1. Changes in tropospheric forcing are also the source of the changes in the eastward propagating nonmigrating diurnal tide with zonal wave number 3 (DE3). However, changes in the zonal mean atmosphere also contribute to interannual variability of the DE3 due to the ENSO. Variability in the eastward propagating nonmigrating diurnal tide with zonal wave number 2 (DE2) is largely due to changes in the background atmosphere, with a smaller additional contribution due to changes in tropospheric forcing. Variability in the westward propagating semidiurnal tide with zonal wave number 4 (SW4) is believed to be due to changes in planetary waves during the ENSO which will enhance generation of the SW4 through the nonlinear interaction of the migrating semidiurnal tide and stationary planetary waves with zonal wave number 2. The influence of the interannual tidal variability on the longitude structure of the low-latitude ionosphere is also investigated in the present study. Comparison of El Nino and La Nina time periods reveals that the ENSO introduces changes of similar to 2-4ms(-1) in the daytime vertical drift velocity at certain longitudes. Simulation results further illustrate that the variability in the vertical drift velocity drives interannual variability in the low-latitude daytime F region maximum electron density (NmF2). The results demonstrate that the ENSO introduces variability of similar to 10-30% in the MLT and similar to 10-15% in the ionosphere. The ENSO should therefore be considered as a potentially significant source of variability in the Earth's upper atmosphere.