Observations of equatorial Kelvin wave modes in FORMOSAT-3/COSMIC GPS RO temperature profiles

Abstract

In this study, we analyze FORMOSAT-3/COSMIC (F3/C) GPS radio occultation (RO) derived temperature components for the period September 2006 to February 2008. Results show the presence of slow Kelvin waves (wave period > 10 days) with higher zonal wavenumbers (either one or two) in the upper troposphere and lower stratosphere (UTLS). The vertical wavelengths of these waves are found to be in the range of 5-12 km. The predominant Kelvin waves observed in the temperature fluctuations are in the altitude range between 15 and 28 km and centered on the tropical tropopause. The downward phase progression of these waves suggests that the derived waves are propagating upward, with the source region located at lower altitudes possibly due to tropical convective heating. The zonal winds retrieved using radiosonde observations over Singapore (1°N, 104°E) during this period show a periodicity of ∼24-26 months in the stratosphere, and quasi-biennial oscillation (QBO) characteristics with eastward zonal winds from March 2006 to May 2007 and westward winds from June 2007 to July 2008 respectively. Our results further show that the Kelvin wave characteristics are enhanced during the westward phase of QBO and diminish during the eastward phase, in line with the previous reported results. Furthermore, an examination of outgoing longwave radiation (OLR) data shows that deep convection activity is developed episodically over the Indonesian archipelago during the observation period, thereby indicating that the Kelvin wave events observed in temperature fluctuations are either driven by convective activity (convectively coupled waves) or by a broad spectrum of convective variability (free waves) over the Indonesian region.