The South Georgia Wave Experiment (SG-WEX): Radiosonde observations of gravity waves in the lower stratosphere. Part 1: Energy density, momentum flux and wave propagation direction

Gravity waves play a critical role in the transport of energy and momentum throughout the atmosphere. It has been suggested that small mountainous islands located in regions of strong winds may generate significant fluxes of these waves. Such fluxes would be important because these islands are not well resolved in global circulation models. Thus, there is a need to determine the magnitude and variability of gravity wave generated from such islands: South Georgia (54°S, 37°W) has the highest mountains of these islands. Here, we present the first report of gravity waves measured by radiosondes over South Georgia. The measurements were made in two intensive campaigns as part of the South Georgia Wave EXperiment (SG-WEX), a multi-instrument and modelling campaign investigating gravity waves above South Georgia. The two intensive radiosonde campaigns were held in 2015, one in January and one in June/July, totalling 89 successful launches. We use these new observations to determine gravity wave properties in the lower stratosphere. The summer campaign observed an average wave energy density (kinetic+potential+vertical) of 3.6 JKg-1 and an average pseudo-momentum flux of 2.3 mPa . In the winter campaign the values observed were larger; an average wave energy density of 8.4 JKg-1 and an average pseudo-momentum flux of 8.7 mPa . Strikingly, analysis reveals that in winter 66% of waves were propagating downwards, in summer only 8% did so. These results suggest that there may be additional sources of waves in the winter stratosphere. We propose that the differences between wave properties observed during the summer and winter campaigns are due to a complex combination of factors including differences in surface wind conditions (linked to orographic wave generation), frequency of storms and the proximity of the Polar stratospheric jet. These results demonstrate a large increase in gravity wave activity in winter above South Georgia.

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