Gravity Wave Weakening During the 2019 Antarctic Stratospheric Sudden Warming

A rare Antarctic stratospheric sudden warming (SSW) occurred on August 30, 2019, and was a minor warming event. We investigated variations in gravity wave (GW) activity before and after this Antarctic SSW event using two satellite measurements (AIRS and CIPS) and reanalysis data (GEOS-5 FP). GW activity over the Andes decreased after August 30, although the westerly wind was 40-60 ms(-1) and cannot filter out GWs with small zonal phase speed. This decline over the Andes was probably caused by wave saturation. Zonal mean GW activity over Antarctica and the Southern Ocean likewise decreased, with a weakening of zonal wind. The zonal mean GW activity further decreased around September 8 which coincided with a reversal of the zonal mean zonal wind at 40 km. The decline in the zonal mean GW activity after August 30 was probably caused by wind filtering and polar night jet breaking. Plain Language Summary A strong westerly wind, called the polar night jet, appears in the winter polar region and typically exceeds 90 ms(-1) at its maximum. The temperature inside the jet (the polar vortex) is colder than that outside the jet. However, the polar night jet occasionally becomes highly distorted and disappears with accompanying warming in the polar stratosphere. Such events are called sudden stratospheric warmings (SSWs). SSWs drastically change the wind and temperature, which should strongly influence small-scale waves, called gravity waves (GWs). SSWs frequently occur in the Arctic, but rarely in the Antarctic. Antarctic SSWs have occurred only twice in the 21st century. The rare Antarctic SSW occurred in 2019, and we investigated GW variations before/after the SSW event. A decline in GW activity coincided with a decline in the zonal wind twice in GEOS-5 FP. The decline in GW activity was probably caused by a weak zonal wind layer. This temporal variation is the same as the Arctic GWs for the same type of SSW.

Automated summary

A rare Antarctic stratospheric sudden warming occurred on August 30, 2019, leading to a decrease in gravity wave activity over the Andes and zonal mean gravity wave activity over Antarctica and the Southern Ocean. The decline in gravity wave activity was likely caused by wave saturation and weakening zonal wind.