Stratospheric gravity waves at Southern Hemisphere orographic hotspots: 2003–2014 AIRS/Aqua observations

Abstract

Abstract. Stratospheric gravity waves from small-scale orographic sources are currently not well-represented in general circulation models. This may be a reason why many simulations have difficulty reproducing the dynamical behavior of the Southern Hemisphere polar vortex in a realistic manner. Here we discuss a 12-year record (2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014) of stratospheric gravity wave activity at Southern Hemisphere orographic hotspots as observed by the Atmospheric InfraRed Sounder (AIRS) aboard the National Aeronautics and Space Administration's (NASA) Aqua satellite. We introduce a simple and effective approach, referred to as the "two-box method", to detect gravity wave activity from infrared nadir sounder measurements and to discriminate between gravity waves from orographic and other sources. From austral midfall to mid-spring (April-October) the contributions of orographic sources to the observed gravity wave occurrence frequencies were found to be largest for the Andes (90 %), followed by the Antarctic Peninsula (76 %), Kerguelen Islands (73 %), Tasmania (70 %), New Zealand (67 %), Heard Island (60 %), and other hotspots (24-54 %). Mountain wave activity was found to be closely correlated with peak terrain altitudes, and with zonal winds in the lower troposphere and mid-stratosphere. We propose a simple model to predict the occurrence of mountain wave events in the AIRS observations using zonal wind thresholds at 3 and 750 hPa. The model has significant predictive skill for hotspots where gravity wave activity is primarily due to orographic sources. It typically reproduces seasonal variations of the mountain wave occurrence frequencies at the Antarctic Peninsula and Kerguelen Islands from near zero to over 60 % with mean absolute errors of 4-5 percentage points. The prediction model can be used to disentangle upper level wind effects on observed occurrence frequencies from low-level source and other influences. The data and methods presented here can help to identify interesting case studies in the vast amount of AIRS data, which could then be further explored to study the specific characteristics of stratospheric gravity waves from orographic sources and to support model validation.

A decadal satellite record of gravity wave activity in the lower stratosphere to study polar stratospheric cloud formation

“…We found standard errors of 29-39 % for November to February for the Northern Hemisphere and 18-21 % for May to October for the Southern Hemisphere. The spatial patterns of gravity wave activity in the lower stratosphere found here agree well with those identified by Gong et al (2012) and Hoffmann et al (2013Hoffmann et al ( , 2014Hoffmann et al ( , 2016a for the mid-and upper stratosphere.…”

“…NAT particles are observed further downstream of the Antarctic Peninsula, which is consistent with the ice particles serving as condensation nuclei for NAT formation (Carslaw et al, 1998a;Höpfner et al, 2006b;Eckermann et al, 2009). Although mountain wave activity is most frequent at the Antarctic Peninsula (Hoffmann et al, 2016a), other orographic features in Antarctica can also play a role in PSC formation. The AIRS and MIPAS observations on 25 July and 29 August 2008 show cases of the mountainwave-induced formation of ice PSCs over the Transantarctic Mountains and near Enderby Land and Mac.…”

“…Conversely, weak background winds are associated with gravity waves with short vertical wavelengths, which are generally not detectable by AIRS. This combined effect of the stratospheric background winds and the vertical wavelength sensitivity of AIRS is referred to as an "observational filter" (Alexander and Barnet, 2007;Hoffmann et al, 2014Hoffmann et al, , 2016a. Figures 7 and 8 also show the contours of ERA-Interim monthly mean zonal winds at 30 hPa.…”

“…Robertson Land. Previous climatological studies using AIRS data showed that these regions are in fact hot spots of stratospheric gravity wave activity (Gong et al, 2012;Hoffmann et al, 2013Hoffmann et al, , 2016a. AIRS measurements in the spectral window region at 8.1 µm can be used to detect storm systems Hoffmann and Alexander, 2010).…”

Intercomparison of AIRS and HIRDLS stratospheric gravity wave observations

“…For an overview of gravity wave activity in the Northern Hemisphere polar region during recent winters see Ern et al (2016). Hoffmann et al (2016) discussed gravity wave activity located at Southern Hemisphere orographic hotspots and their correlation with background winds in more detail.…”