Indian Journal of Radio & Space Physics Vol 43, February 2014, pp 48-56 Madden – Julian oscillations over a tropical Indian station using radar and ERA data of winds Salauddin Mohammad 1 , Gopa Dutta 1 , P Vinay Kumar 1 , E Krishna 1 , M C Ajay Kumar 2 , B V Rao 3 & P V Rao 4,$,* 1 Vignana Bharathi Institute of Technology, Ghatkesar, Hyderabad 501 301, India 2 Aurora’s Seethaiah Engineering College, Patancheru, Hyderabad 502 319, India 3 College of Engineering, JNTU (H), Hyderabad 500 085, India 4 Vasavi College of Engineering, Ibrahimbagh, Hyderabad 500 031, India $ E-mail: kishansetty@gmail.com Received 25 June 2013; revised 12 August 013; accepted 17 September 2013 The Madden – Julian oscillation (MJO) activity over Gadanki (13.5°N, 79.2°E), a low latitude station is examined using four years MST radar data of winds (4 – 20 km) between January 2006 and December 2009. Wind data obtained from European Centre for Medium Range Weather Forecasting (ECWMF) Re-Analysis (ERA) for the same latitude – longitude and for the same period have also been used to study the wave activity in the same altitude region. MJO in two period bands, 4-7 weeks and 10-13 weeks, could be clearly identified with amplitudes of ~ 4 ms -1 in zonal wind. Meridional winds also showed similar oscillations but with lesser amplitudes (~ 2.5 ms -1 ). Fast Fourier Transform (FFT) analyses were performed on yearly data sets and very clear inter-annual variability of the oscillations is found. Wavelet analyses were carried out to find the temporal variation of the oscillations. The results obtained using radar and ERA data are found to be in good agreement. Outgoing longwave radiation (OLR) data is also used to study the relation between convection and MJO activity. A significant correlation is not observed between convection and MJO activity. Keywords: Madden-Julian oscillation, Zonal wind, Meridional wind PACS Nos: 92.60.Gn; 92.60.Ry 1 Introduction Madden-Julian oscillation is the dominant mode of variability on intra-seasonal time scales (20–100 days) in the tropical atmosphere. This range of time scale is in between the typical timescales for weather (up to 15 days) and climate (from a season and beyond). MJO plays a role of bridging weather and climate and its broad impact on different components of the earth system, make it very important and worth investigating. The Madden – Julian oscillation was first reported by Madden and Julian in 1971. Since then, efforts have been made to explore the characteristics and origin of this oscillation 1-8 . Madden and Julian did not find any distinct spectral peaks in the meridional wind spectra as pronounced in the zonal wind spectra. However, there are some evidences for a role of the meridional wind in the 40-50 days oscillations 9-13 indicating that it is not solely a Kelvin wave. Therefore, study of these waves in zonal and meridional components of wind is of considerable interest. Kumar & Jain 5 confirmed that waves with period of 30-70 days are not confined to the southwest monsoon season, but are also observed in the northeast monsoon season. Over monsoon regions, particularly over India, MJO’s are characterized by poleward movement of weather anomalies including rainfall 14,15 . These MJO’s, however, show considerable intra seasonal and inter-annual variability 16 . Hendon et al. 17 studied the inter-annual variability of MJO and stated that the activity is prominent during the extended austral summer (November – March). Seasonality is one of the most fundamental characteristic of MJO 18,19 . Zhang & Dong 6 documented the salient features of seasonality in the MJO. The convective signals of MJO are clearly seen in the outgoing long wave radiation (OLR) data, and its temperature, moisture and wind structures have been well documented 20-22 . It was pointed out by a number of researchers that the 30-50 days oscillation is amplified during the northern hemisphere summer over the Indian monsoon and western pacific regions 9-11 and is intimately related to enhanced convection and rainfall 12 . Studies of the tropical intra