Abstract. The present study demonstrates the relationship between the high latitude northern hemispheric major sudden stratospheric warming (SSW) events and the reversal in the afternoon equatorial electrojet (EEJ), often called the counter-electrojet (CEJ), during the winter months of 1998-1999, 2001-2002, 2003-2004 and 2005-2006. As the EEJ current system is driven by tidal winds, an investigation of tidal variabilities in the MF radar observed zonal winds during the winters of 1998-1999 and 2005-2006 at 88 km over Tirunelveli, a site close to the magnetic equator, shows that there is an enhancement of semi-diurnal tidal amplitude during the days of a major SSW event and a suppression of the same immediately after the event. The significance of the present results lies in demonstrating the latitudinal coupling between the high latitude SSW phenomenon and the equatorial ionospheric current system with clear evidence for major SSW events influencing the day-to-day variability of the CEJ.

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are used to study long-term variability of equatorial mesosphere and lower thermosphere (MLT) winds. The monthly mean zonal wind exhibits dominant semiannual variability with westward winds during equinox and eastward winds during solstice. The first westward phase, which occurs during spring equinox, undergoes interannual variability with larger westward winds during the years 1993, 1995 and 1997. This interannual variability has been interpreted as quasibiennial oscillation (QBO) in the MLT winds. However, this QBO is absent during the year 1999, as the next large westward wind phase occurs during the year 2000. Thus the period of QBO is extended from nearly 2 to 3 years (a). During these years, the period of stratospheric QBO winds is also extended to 3 a. Besides SAO and QBO, as the zonal winds undergo intraseasonal oscillations with periodicities resembling those of Madden-Julian Oscillation (MJO) in the troposphere, the relation between the two is examined on the basis of the existing hypothesis. The present study reveals long-term variation in the amplitude of the annual oscillation noticed in monthly mean meridional winds. A decreasing trend is observed in the annual mean northward winds during the years 1993-2006 in contrast to decreasing trend in the annual mean southward winds reported from midlatitudes. These observations are discussed in the context of our current understanding of long-term variability of MLT winds.

[1] Mesospheric wind observations by the medium frequency radar and geomagnetic field observations at Tirunelveli (8.7 N, 77.8 E, 1.75 N dip angle) are used to study the relative importance of solar and lunar influences in the variabilities of mesospheric tides and equatorial electrojet (EEJ) strength during the unprecedented major stratospheric sudden warming (SSW) of 2009. It is observed that the afternoon reversal in the EEJ, popularly known as counter electrojet, occurs consecutively for several days during the SSW event, when there is an enhancement of solar semidiurnal tide in both zonal wind at 90 km and EEJ strength over Tirunelveli. Although the amplitude of lunar tides also shows enhancement, it is much less than that of solar. The diurnal tidal amplitude in zonal wind and EEJ strength also shows large enhancement just before the onset of SSW. However, solar semidiurnal tide dominates diurnal tide during the SSW. The diurnal tidal phase in zonal wind shifts to a few hours earlier during the SSW. The lunar semidiurnal tidal phase shifts to later hours in both zonal wind and EEJ strength. The main observation of the present study is that the large semidiurnal tide observed during the SSW 2009 is mostly solar driven and only partly lunar driven, although tidal planetary wave interaction also may play a vital role. Although a similar behavior is noticed during the SSW 2006 also, the large lunar semidiurnal tide observed in the EEJ strength without having large lunar semidiurnal tide in the underlying mesospheric winds needs further investigation.Citation: Sathishkumar, S., and S. Sridharan (2013), Lunar and solar tidal variabilities in mesospheric winds and EEJ strength over Tirunelveli (8.7 N, 77.8 E) during the 2009 major stratospheric warming,

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Long-term variability of middle atmosphere temperature (T) and zonal wind (U) is investigated using a three-member ensemble of historical simulations of NCAR's Whole Atmospheric Community Climate Model latest version 6 (WACCM6) for 1850-2014 (165 years). The model reproduces the climatological features of T and U. The contributions of Quasi Biennial Oscillation (QBO) at 10 and 30 hPa, solar cycle (SC), El Niño-Southern Oscillation (ENSO), ozone depleting substances (ODS), carbon dioxide (CO 2), and stratospheric sulfate aerosol (volcanic eruptions) to change in monthly zonal mean T and U are analyzed using multiple linear regression. The signal due to CO 2 increase dominates as a predictor of the net multidecadal global annual mean temperature change at all levels in the middle atmosphere. Contributions from ODS also affect the net multidecadal global mean temperature trend in the stratosphere. Because of similarities in the time evolution of the emissions of CO 2 and ODS, the analysis of existing model output cannot accurately separate the attributions of cooling to these two dominant forcing processes. On shorter time scales, solar flux variations are the largest source of variability in the mesosphere while volcanic eruptions are the largest in the stratosphere. In the stratosphere and mesosphere, both QBO and ENSO can significantly impact zonal mean temperature and zonal-mean zonal wind depending on latitudes, but their impact on the multidecadal global mean temperature trend is very small.

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[1] The present study examines changes in monthly mean winds and tides observed by MF radar at Tirunelveli (8.7°N, 77.8°E) for the years 1993-2007 with respect to long-term variabilities, namely, the stratospheric quasi-biennial oscillation (QBO), the El Niño-Southern Oscillation (ENSO), the solar cycle (SC), and the long-term trends, using regression analysis. Both zonal and meridional winds show negative QBO response in the altitude region 84-94 km and negative ENSO response (∼1 m s −1 per Southern Oscillation Index) above 90 km. However, only the latter show a notable positive SC response. The response of meridional diurnal tide is positive to ENSO (above 90 km) and stratospheric QBO and is negative to SC. The tides in both wind components show positive trends. The meridional winds show a significant negative trend (0.3 m s −1 yr −1 ). Further, monthly variations of the responses of mean winds and tides to QBO, ENSO, and SC are presented and discussed.Citation: Sridharan, S., T. Tsuda, and S. Gurubaran (2010), Long-term tendencies in the mesosphere/lower thermosphere mean winds and tides as observed by medium-frequency radar at