Relationship between lunar tidal enhancements in the equatorial
electrojet and stratospheric wind anomalies during stratospheric sudden
A number of studies in recent years have reported about the lunar tidal
enhancements in the equatorial electrojet (EEJ) from ground- and
space-based magnetometer measurements during stratospheric sudden
warming (SSW) events. In this study, we make use of the ground
magnetometer recordings at Huancayo observatory in Peru for the years
1978 – 2013 to derive a relationship between the lunar tidal
enhancements in the EEJ and tropospheric eddy heat fluxes at 100 hPa
during the SSW events. Tropospheric eddy heat fluxes are used to
quantify the amount of wave activity entering the stratosphere.
Anomalously large upward wave activity is known to precede the polar
vortex breakdown during SSWs. We make use of the superposed epoch
analysis method to determine the temporal relations between lunar tidal
enhancements and eddy heat flux anomalies during SSWs, in order to
demonstrate the causal relationship between these two phenomena. We also
compare the lunar tidal enhancements and eddy heat flux anomalies for
vortex split and for vortex displaced SSWs. It is found that larger
lunar tidal enhancements are recorded for vortex split events, as
compared to vortex displaced events. This confirms earlier observation;
larger heat flux anomalies are recorded during vortex split SSW events
than the heat flux anomalies during vortex displaced SSW events.
Further, the temporal relations of lunar tidal enhancements in the EEJ
have been compared separately for both the QBO phases and with the
phases of the moon with respect to the central epoch of SSWs by means of
the superposed epoch analysis approach. The EEJ lunar tidal enhancements
in the east phase of QBO are found to be larger than the lunar tidal
enhancements in the west phase of QBO. The phase of moon relative to the
central SSW epoch also affects the lunar tidal enhancement in the EEJ.
It is found that the lunar tidal enhancements are significantly larger
when the day of new or full moon lies near the central SSW epoch, as
compared to cases when new or full moon occur further away from the
central SSW epoch.