Gravity waves in the winter stratosphere over the Southern Ocean: high-resolution satellite observations and 3-D spectral analysis

Abstract

Abstract. Atmospheric gravity waves play a key role in the transfer of energy and momentum between layers of the Earth's atmosphere. However, nearly all general circulation models (GCMs) seriously under-represent the momentum fluxes of gravity waves at latitudes near 60∘ S, which can lead to significant biases. A prominent example of this is the “cold pole problem”, where modelled winter stratospheres are unrealistically cold. There is thus a need for large-scale measurements of gravity wave fluxes near 60∘ S, and indeed globally, to test and constrain GCMs. Such measurements are notoriously difficult, because they require 3-D observations of wave properties if the fluxes are to be estimated without using significant limiting assumptions. Here we use 3-D satellite measurements of stratospheric gravity waves from NASA's Atmospheric Infrared Sounder (AIRS) Aqua instrument. We present the first extended application of a 3-D Stockwell transform (3DST) method to determine localised gravity wave amplitudes, wavelengths and directions of propagation around the entire region of the Southern Ocean near 60∘ S during austral winter 2010. We first validate our method using a synthetic wavefield and two case studies of real gravity waves over the southern Andes and the island of South Georgia. A new technique to overcome wave amplitude attenuation problems in previous methods is also presented. We then characterise large-scale gravity wave occurrence frequencies, directional momentum fluxes and short-timescale intermittency over the entire Southern Ocean. Our results show that highest wave occurrence frequencies, amplitudes and momentum fluxes are observed in the stratosphere over the mountains of the southern Andes and Antarctic Peninsula. However, we find that around 60 %–80 % of total zonal-mean momentum flux is located over the open Southern Ocean during June–August, where a large “belt” of increased wave occurrence frequencies, amplitudes and fluxes is observed. Our results also suggest significant short-timescale variability of fluxes from both orographic and non-orographic sources in the region. A particularly striking result is a widespread convergence of gravity wave momentum fluxes towards latitudes around 60∘ S from the north and south. We propose that this convergence, which is observed at nearly all longitudes during winter, could account for a significant part of the under-represented flux in GCMs at these latitudes.

An 18‐Year Climatology of Directional Stratospheric Gravity Wave Momentum Flux From 3‐D Satellite Observations

“…Retrieval noise and vertical resolution are balanced to give an effective vertical resolution varying between 7 km at 20 km altitude and 15 km at 60 km altitude (Ern et al, 2017;Hoffmann & Alexander, 2009;Sato et al, 2016). Noise is lowest between 20 and 60 km, at approximately 1.4 to 2 K (Ern et al, 2017;Hindley et al, 2019;Hoffmann & Alexander, 2009). Sensitivity of the retrieval to GWs is discussed in Meyer and Hoffmann (2014), Hindley et al (2019), and Ern et al (2017).…”

“…In the vertical, we interpolate onto a regular 1.5 km altitude grid. Measurements above 60 km and below 20 km are set to zero, with a smooth half-bell taper applied to reduce edge effects near the boundaries as described by Hindley et al (2019). This limits our visibility to vertical wavelengths greater than around 40 km.…”

Lidar observations of large-amplitude mountain waves in the stratosphere above Tierra del Fuego, Argentina

“…In austral winter, the westerlies of the polar night jet (PNJ) allow these mountain waves to propagate deeply into the middle atmosphere where they deposit their momentum and decelerate the mean stratospheric flow [2][3][4] . So far, mainly satellite-based instruments are utilized for estimations of the stratospheric gravity wave momentum that is transported vertically [5][6][7][8][9][10][11][12][13][14][15][16] . Due to the observational filter of these instruments, only a part of the full wave spectrum is included.…”

Authors' response to Referee comments

“…AIRS is not the focus of this study, but for information I find that in practice the vast majority of vertical wavelengths measured in AIRS are generally between 15 and 25km (e.g. Hindley et al, 2019). Shorter vertical wavelengths tend to be lost below the noise threshold, and longer vertical wavelengths tend to be underestimated due to only a few cycles being present in the vertical, which reduces the signal to noise.…”

“…Shorter vertical wavelengths tend to be lost below the noise threshold, and longer vertical wavelengths tend to be underestimated due to only a few cycles being present in the vertical, which reduces the signal to noise. We did not apply a correction in the measured vertical wavelength in Hindley et al (2019) because I found that the measurement error in the vertical wavelength of around 10-25% was comparable to the correction factor, so I didn't want to C4 incorrectly apply it and introduce a further source of error. Here, error in the vertical wavelength measurement of the S3D method could be similar, so they may have the same problem, but the authors do not discuss or quantify that here.…”