Modulation of tropical stratospheric gravity wave activity and the ITCZ position by modes of climate variability using radio occultation and reanalysis data

Abstract

The Intertropical Convergence Zone (ITCZ) is a dominant feature of tropical climate characterized by intense convection that influences global atmospheric circulation and serves as a primary source of stratospheric gravity waves (GWs), which transport energy and momentum vertically through the atmosphere. This study investigates the modulation of the tropical ITCZ position and stratospheric gravity wave activity by the El Niño-Southern Oscillation (ENSO), the Madden-Julian Oscillation (MJO), and the Quasi-Biennial Oscillation (QBO) using 11 years (2011-2021) of radio occultation and reanalysis data. ITCZ latitude (from 850 hPa refractivity) and gravity wave potential energy maxima (from stratospheric temperatures) were identified via Gaussian fitting. Both ITCZ and gravity wave potential energy maxima exhibit coherent seasonal migration (∼10 and ∼5° latitudinal shifts, respectively), with potential energy maxima typically equatorward of the ITCZ. ENSO is the primary modulator: El Niño conditions shift the ITCZ northward in the American sector but southward in the African and Asian sectors. For gravity wave potential energy maxima, El Niño induces southward shifts in the American sector but northward shifts in the Asian sector, while enhancing overall GW activity. The MJO prompts regionally complex southward shifts in the ITCZ/potential energy maxima. The QBO predominantly influences gravity wave potential energy, with westerly phases associated with southward shifts in the potential energy maxima in the African and Asian sectors. While long-term latitudinal trends are weak, climate modes significantly impact ITCZ/GW peak values. The radio occultation data captured finer-scale features than reanalysis products, highlighting the importance of observational constraints in understanding troposphere-stratosphere coupling mechanisms.