Satellite Observations of Gravity Waves at the Stratospheric Speed Limit from the Hunga-Tonga Hunga Ha'apai Volcanic Eruption
Résumé
On 15th January 2022, a major volcanic eruption occurred between the islands of Hunga Tonga and Hunga Ha’apai (175.4W, 20.5S). Located under only a shallow depth of water, this submarine volcano launched an explosive plume of ash and flash-boiled water up through the ocean upwards into the atmosphere, with an explosive energy comparable to Krakatau in 1883. The explosion generated global-scale atmospheric waves that were detectable from the surface to the edge of space. This single event also sent shockwaves through the global research community, triggering upwards of several thousands of scientific studies. In this presentation, we focus on ultra-fast stratospheric gravity waves generated by the initial explosion that propagated over the entire Pacific region detected by satellite. We analyse brightness temperature perturbations in the 4.3 and 15 micron bands of the AIRS/Aqua, CrIS/Suomi-NPP, CrIS/JPSS-1, IASI/MetOp-B and IASI/MetOp-C instruments, supported by GOES radiance observations. An atmospheric "explosion time" of 04:28:48 UTC is calculated using surface pressure station anomalies and allows us to measure lower-bound propagation speeds of the leading atmospheric waves. Strikingly, we find not only a clear signal of the surface Lamb wave throughout the stratosphere travelling near the sound speed at 318 m/s, but also a leading gravity wave packet travelling at up to 275 m/s, with an apparent vertical depth greater than the depth of the atmosphere. This is, to our knowledge, one of the fastest gravity wave packets ever observed. These results are combined with airglow observations over Hawaii in the lower thermosphere to provide an independent estimate of phase speeds. Finally, analysis of small-scale gravity waves propagating in the ash plume reveal wave periods close to fastest possible oscillation speed near 5 mins. This event triggered gravity waves with speeds, scales and extents that are unprecedented in nearly 20 years of satellite observations, and will likely keep scientists busy for many years to come as we seek to understand the atmospheric response to this unique eruption.