Strong Kelvin wave activity observed during the westerly phase of QBO – a case study

Abstract. Temperature data from Global Positioning System based Radio Occultation (GPS RO) soundings of the Formosa Satellite mission 3/Constellation Observing System for Meteorology, Ionosphere and Climate (FORMOSAT-3/COSMIC or F-3/C) micro satellites have been investigated in detail to study the Kelvin wave (KW) properties during September 2008 to February 2009 using the two-dimensional Fourier transform. It is observed that there was strong KW activity during November and December 2008; large wave amplitudes are observed from above the tropopause to 40 km – the data limit of F-3/C. KW of wavenumbers E1 and E2 with time periods 7.5 and 13 days, dominated during this period and the vertical wavelengths of these waves varied from 12 to 18 km. This event is very interesting as the QBO during this period was westerly in the lower stratosphere (up to ~ 26 km) and easterly above, whereas, climatological studies show that KW get attenuated during westerlies and their amplitudes maximise during easterlies and westerly shears. In the present study, however, the eastward propagating KW crossed the westerly lower stratosphere as the vertical extent of the westerly wind regime was less than the vertical wavelengths of the KW. The waves might have deposited eastward momentum in the upper stratosphere at 26–40 km, thereby reducing the magnitude of the easterly wind by as much as 10 m s⁻¹. The outgoing long wave radiation (OLR) is also investigated and it is found that these KW are produced due to deep convections in the lower atmosphere.