Equatorial vertical drift modulation by the lunar and solar semidiurnal tides during the 2013 sudden stratospheric warming

During the 2013 stratospheric sudden warming (SSW) period the Jicamarca Unattended Long-term Investigations of the Ionosphere and Atmosphere (JULIA) radar at Jicarmarca, Peru, observed low-latitude vertical drift modulation with lows of 0-12m/s daytime maximum drifts between 6-13 and 22-25 January and enhanced drifts up to 43m/s between 15 snd 19 January. The NCAR thermosphere-ionosphere-mesosphere-electrodynamics general circulation model reproduces the prevailing vertical drift feature and is used to examine possible causes. The simulations indicate that the modulation of the vertical drift is generated by the beating of the semidiurnal solar SW2 and lunar M-2 tides. During the SSW period the beating is observable since the magnitudes of lunar and solar semidiurnal tidal amplitudes are comparable. The theoretical beating frequency between SW2 and M-2 is 1/(15.13day) which may be modified due to phase changes. This study highlights the importance of the lunar tide during SSW periods and indicates that the equatorial vertical drift modulation should be observable at other longitudes as well.