Satellite observations of large atmospheric gravity waves in the Mozambique Channel

Abstract

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.

Acknowledgements

Envisat‐MERIS and ASAR image data provided by the European Space Agency (ESA) under project number AOPT‐2423. The work was also undertaken as part of FCT project AMAZING (project code PDCTE/CTA/49953/2003), SPOTIWAVE‐II (POCI/MAR/57836/2004) and INTAS/EU project SIMP (Slicks as Indicators of Marine Processes). One of us is grateful to the FCT support for a Sabbatical Leave (BSAB/610/2006). MODIS data provided by NASA, DAAC archives. The atmospheric soundings were obtained from University of Wyoming official web page. We thank Professor Werner Alpers for bringing our attention to the existence of large‐scale cloud bands in the form of solitary waves in the Mozambique Channel. We would like to thank Heidi Marsella for improving the English style of an early version of this manuscript.