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

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 Niño and La Niña time periods reveals that the ENSO introduces changes of ~2–4 ms−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 ~10–30% in the MLT and ~10–15% in the ionosphere. The ENSO should therefore be considered as a potentially significant source of variability in the Earth's upper atmosphere.