Do Moving Polar Highs play a significant role on the generation of Internal Solitary Waves in the Atmosphere?
José da Silva(1) and J. Magalhaes(1)
(1) University of Lisbon, Campo Grande, 1749-016 Lisbon, Portugal
It has been recently demonstrated that large-scale Atmospheric Gravity Waves (AGWs) are frequently observed during the winter season in the Mozambique Channel. Satellite imagery (ASAR and MERIS-Envisat, MODIS-Terra and Aqua) revealed the full horizontal structure of these “giant” AGWs, which have dispersive average wavelengths ranging from 8.5 to 3.5 km, and a mean along-crest “coherence” length of 443 km. We have established the correct pre-conditions for wave-trapping and consequent horizontal propagation of the observed AGWs using the Scorer parameter and considering the most prominent trapping mechanisms referred to in the literature. The Scorer parameter, calculated from available atmospheric soundings nearest to the satellite observations, was found to support wave propagation. In the study region, the main characteristics of the waveguides (height and width) are predominantly determined by stratification (N), and only to a minor extent by the wind profiles. Air subsidence associated with high pressure systems is an important factor setting the pre-conditions for wave propagation. It is suggested that Moving Polar Highs (MPHs), born in the Polar Regions, are associated with subsiding air in the Mozambique Channel during the winter season possibly providing the necessary conditions for wave propagation. More importantly, we investigate the influence of MPHs in the generation mechanism of these waves. MPHs can be viewed as large scale gravity currents and, since suitable waveguides are available, could generate the AGWs. Generation can occur if the speed of the decelerating disturbance is close to, and falling below, the linear long wave speeds for one of the long wave modes allowed by an existing waveguide. We also show possible examples of this generation mechanism for other regions over the world’s oceans, indicating that this might be a major global generation mechanism.
Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,