Inter-annual Variability of Major Atmospheric Tides in the Ionosphere - Thermosphere - Mesosphere (ITM) Region

Inter-annual variability of atmospheric tides (e.g. DW1, DE3, SW2 and TW3) in the ionosphere - Thermosphere - Mesosphere (ITM) region is examined using the WACCM - X SD run (1980 - 2017). All four tidal components respond positively to solar forcing in the Mesosphere and Lower Thermosphere (MLT) region. The response in the Ionosphere - Thermosphere (IT) region is more complex. DW1’s response to solar forcing is positive whereas DE3’s response is negative. SW2’s response is positive up to 250 km, but negative in the equatorial region and positive in the mid-latitudes above 250 km. The overall response from TW3 is similar to that of SW2, but the monthly response reveals that there is hemispheric asymmetry where the response is positive in the Northern Hemisphere and negative in the Southern Hemisphere in winter and the pattern is reversed in summer. For ENSO response, the amplitude of DW1 is larger during the warm phase of ENSO, e.g. El-Nino during the majority of the months except January and February in the MLT region. The ENSO response in the IT region is generally negligible. DE3 responds positively to El-Nino except in the NH mid-latitude MLT region during November - March and in the IT region in March and April. SW2 presents a complex response with latitude, but overall generally positive. TW3’s response to ENSO is the most complicated among the four tides with positive and negative response along latitudes and heights. The strongest response is positive in the IT region during February - April. For QBO response, DW1 amplitude is generally larger during westerly QBO in the MLT region especially during March and April. The QBO response is negligible in the IT region. DE3 responds positively to westerly QBO except in December and January when negative response occurs. Both SW2 and TW3 have more complex patterns with alternating positive and negative responses to QBO with heights, latitudes and months.