On the tropospheric origin of Mesosphere Lower Thermosphere region intraseasonal wind variability

Abstract

Continuous medium frequency radar observations of mesosphere lower thermosphere (MLT) region winds during February 2004–May 2005 (486 days) over Tirunelveli (8.7°N, 77.8°E) revealed intraseasonal oscillations (ISOs) in the 82‐ to 94‐km height region. Two distinct oscillations with periods 50–70 and 20–40 days are noticed predominantly in zonal winds. As it is well established that these oscillations are nonstationary and localized in time, wavelet analysis has been employed to study the time evolution of these oscillations. The analysis showed that 50‐ to 70‐day oscillation peaks during June‐October and 20‐ to 40‐day oscillation peaks during January‐March in the MLT region. To trace back the origin of these oscillations, the tropospheric ISO has been studied for the same period using outgoing long‐wave radiation (OLR) observations. The OLR, which is the proxy for convective activity in the lower atmosphere, around the radar site 5–10°N, 70–80°E is used for the present analysis. The wavelet analysis of OLR showed the 50‐ to 70‐day oscillation peaking at the same time as in the MLT region. The shorter period oscillation (20–40 days), which showed its peak during January‐March in the MLT region, is not observed in the OLR data during these months. However, the analysis of water vapor, which is the prime candidate for excitation of tides, showed the 20‐ to 40‐day oscillation during the same time as in the MLT region. In the present study, the lower atmospheric convective activity through gravity wave excitation and water vapor through tidal forcing are accounted for the observed ISO in the MLT region. The significance of present results lies in showing the highly coherent oscillations in OLR and MLT region zonal winds. The coupling between the lower and middle atmospheric ISO is extensively discussed in the light of existing mechanisms.