On the relationship between weakening of the northern polar vortex and the lunar tidal amplification in the equatorial electrojet

Abstract

Enhanced lunar tidal effects in the equatorial electrojet (EEJ) during northern winters in the form of "big L" days have been known for a long time. Recent studies suggest that the changes in the tidal propagation conditions due to stratospheric sudden warmings could be responsible for this phenomenon. In this work we have used the H component of the magnetic field recorded at Huancayo from 1997 to 2013 to study the relation between the timing and magnitude of the semimonthly lunar tide in the EEJ and the stratospheric polar vortex weakening (PVW). We prefer a definition of PVW by taking into account the atmospheric conditions from December to February for each winter. Our results indicate that the semimonthly lunar tide in the EEJ gets enhanced during northern winters when a significant PVW occurs and its peak timing and magnitude is correlated with the timing and intensity of PVW. The timing of lunar tidal peaks and PVW correlate better than their respective magnitudes. Our results suggest that the initiation of the lunar tidal enhancement in most of the cases is closely related to a PVW event. Furthermore, we discuss events where the semimonthly lunar tidal enhancements are not well timed with respect to PVW. We also suggest that the amount of tropospheric forcing into the stratosphere plays a major role in the enhancement of the lunar tides in the EEJ.