Effect of sudden stratospheric warming on lunar tidal modulation of the equatorial electrojet

Using the equatorial electrojet (EEJ) peak current intensity as deduced from CHAMP magnetic observations from the years 2001 through 2009, we investigated the relationship between sudden stratospheric warming (SSW) and lunitidal signatures in the tropical ionosphere. There is a practically one-to-one correspondence between midwinter SSW periods and the strongest 13 day modulation of the EEJ strength as observed by CHAMP. That is, all the midwinter SSW periods from December 2001 to August 2009 were accompanied by an enhanced 13 day modulation of the EEJ strength. No other geophysical phenomenon brought about as strong a 13 day modulation as those of the midwinter SSW periods. During each midwinter SSW period the amplified 13 day modulation of the EEJ strengths starts roughly within +/- 1 week around the first peak in stratospheric temperature difference. An oscillation with a period of 13.26 days is predicted by the lunitidal equation when considering the precession of the CHAMP orbit. When fitting the lunitidal equation to the EEJ modulations during the midwinter SSW periods, consistent phase delays of 4.4 +/- 0.3 days of the tidal signal emerge for all the cases. The results suggest that the pronounced 13 day modulation of the EEJ strength is related to an enhancement of the lunar tide in the ionosphere by the SSW effect.