Abstract. Favored occurrences of Equatorial Counter Electrojets (CEJs) with a quasi 16-day periodicity over Trivandrum (8.5 • N, 76.5 • E, 0.5 • N diplat.) in association with the polar Stratospheric Sudden Warming (SSW) events are presented. It is observed that, the stratospheric temperature at ∼30 km over Trivandrum shows a sudden cooling prior to the SSWs and the CEJs of maximum intensity which occurs around this time. In general stronger CEJs are associated with more intense SSW events. The stratospheric zonal mean zonal wind over Trivandrum also exhibits a distinctly different pattern during the SSW period. These circulation changes are proposed to be conducive for the upward propagation of the lower atmospheric waves over the equatorial latitudes. The interaction of such waves with the tidal components at the upper mesosphere and its subsequent modification are suggested to be responsible for the occurrence of CEJs having planetary wave periods.

[1] The present paper analyses the variations of daytime mesopause temperature and the Equatorial electrojet strength over the dip equator during December 2005 -March 2006 period, indicating a possible strong dynamical coupling between the two region through the intensification of planetary wave activity. The wave signatures (quasi 16-day period) are seen in the mesopause temperature and the electrojet-induced surface magnetic field, measured from Trivandrum (8.5°N, 76.5°E, 0.5°N diplat.), a geomagnetic dip equatorial station in India. This investigation reveals (1) amplification of the quasi 16-day wave in the equatorial mesopause temperature and the EEJ induced magnetic field (2) regular occurrence of Counter Electrojet (CEJ), with a periodicity of $16 days (3)

The first observations of lowering of mesopause temperature during Counter Electrojet (CEJ) events over a narrow region of ∼ ±150 km centered at around magnetic equator are presented. The daytime mesopause temperature is measured over Trivandrum (8.5°N; 77°E; dip lat. 0.5°N), India using the ground based Multiwavelength Dayglow Photometer. The unique meridional scanning capability of the instrument is extensively used in this study. A lowering of temperature by as much as ∼25 K has been observed during certain CEJ events, which includes a few partial CEJs. The gravity wave tidal interaction through vertically upward wind is proposed to be manifesting as lowering in the mesopause temperature and also as CEJ. These observations are ‘new’ and address to the issues concerning the vertical coupling processes prevailing in the equatorial Mesosphere Lower Thermosphere Ionosphere (MLTI) region.

Abstract. The Planetary Waves (PWs) are believed to have significant role in generating the wintertime warming over the polar stratosphere, known as Stratospheric Sudden Warming (SSW). However, the origin, characteristics and evolution of these waves are still speculative. The possibility that the PWs over the polar stratosphere, which play an important role in the generation of SSW, could also have contribution from the tropics has been indicated through many numerical simulations in the past, but due to the paucity of global measurements it could not be established unequivocally. The earlier numerical studies also indicated the presence of a zero-wind line (more general the critical layer, where the zonal wind amplitude becomes zero) whose real counterparts were not observed in the atmosphere. The present study based on the NCEP/NCAR reanalysis of stratospheric wind and temperatures of recent years clearly shows that (i) the zero-wind line appears over the tropics ∼60 days prior to the major SSWs and progresses towards the Pole and (ii) an enhanced PW activity of quasi periodicity 16-days, which is also seen almost simultaneously with the zero-wind line, shows a propagation from equator to the Pole. This result is significant as it presents for the first time the connection between the tropics during the SSW events and the pole, through the quasi 16-day wave.