Satellite observations of large atmospheric gravity waves in the Mozambique Channel

Satellite imagery (MODIS-Terra and Aqua, ASAR and MERIS-Envisat) has revealed signatures consistent with horizontally propagating large-scale atmospheric gravity waves which are frequently observed during the winter season in the Mozambique Channel. We examine and characterize statistically the full horizontal structure of these atmospheric gravity waves. The waves were found to have dispersive average wavelengths ranging from 8.5 to 3.5 km and a mean along-crest 'coherence' length of 443 km. The propagation characteristics of these waves are examined using in situ and remote sensing measurements. The propagation speeds are estimated using two consecutive satellite images of the same wave-packet and are used as a proxy for the waves' phase speeds. The Scorer parameter vertical profiles, calculated from available atmospheric soundings nearest to the satellite observations, were found to support wave propagation conditions. The vertical structure of stability and wind determine the favourable conditions for wave propagation. Air subsidence associated with high-pressure systems is an important factor setting the pre-conditions for wave propagation. Some suggestions are presented regarding the influence of Moving Polar Highs in the generation and propagation of the observed waves.