Tides in the atmosphere

The small white building of the Medium Frequency radar
The Medium Frequency (MF) radar measures wind speed between 80 and 100km above Davis station. (Photo: D. Murphy)

To understand our atmosphere and predict its response to change, we need an understanding of all the processes going on within it, throughout its depth, and from pole to pole. This requires international cooperation and some clever instruments.

Through atmospheric radars like the one at Davis, and international collaboration, we have just extended our understanding of atmospheric tides.

We now know the 12-hour tide is made up of a mix of waves that varies throughout the year, and that some of these waves travel all the way from the northern hemisphere.

The wind speed between 80 and 100km altitude above Antarctica changes from second to second, hour to hour and season to season. Atmospheric tides are the dominant effect and can reverse the wind direction against the action of other processes.

Until recently, the horizontal scale of the 12-hour tide (the strongest over Antarctica) has only been guessed at. But through cooperation with colleagues at the University of Colorado and numerous Antarctic research programs, we have identified a number of manifestations of the 12-hour tide and their variation with latitude and season over Antarctica.

For some years the medium frequency (MF) radar at Davis (pictured above) and other similar radars around Antarctica have been unobtrusively measuring the wind speeds at the edge of our atmosphere. But how should the data be combined?

The breakthrough came when it was noted that the way in which the tides change with latitude near the earth’s poles, could be represented with a relatively simple mathematical equation.

It had previously been assumed that a 12-hour tide has two maxima on each latitude circle and that it appears to follow the motion of the sun. New research has shown that this is not the case. A single maximum form was found and shown to be similar to one observed above the South Pole. And a ‘breathing mode’ (all longitudes moving together) was also found.

Through this international collaboration, we have been able to show that there is more going on above our Antarctic stations than previously thought. The likely cause is interactions between tides and other planet-scale waves near their birthplace in the stratosphere.

This development can now be used to test and improve global climate models.