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.
Highlights
Gravity waves (GWs) are a key component in the dynamics of the Earth’s atmosphere
While some increased wave amplitudes and momentum fluxes were observed over these Antarctic mountain ranges in Figs. 7 and 8, these results suggest that, during June and August, the total monthly flux was unevenly distributed into relatively few, high-flux wave events, something that was hidden in the monthly mean analysis
The spatial distribution of our intermittency results are in good general agreement with the results of Plougonven et al (2013), who used the Gini coefficient to characterise intermittency of absolute momentum flux in stratospheric balloon observations from the Vorcore campaign around the Southern Ocean. They found increased regions of intermittency over the mountains of the Antarctic Peninsula, with generally lower values over the Southern Ocean, we note that the Vorcore campaign took place later in the year between October and December, whereas we focus on June, July and August here
Summary
Gravity waves (GWs) are a key component in the dynamics of the Earth’s atmosphere. Through the transportation and deposition of energy and momentum, these waves act as the primary coupling mechanism between atmospheric layers (e.g. Fritts and Alexander, 2003, and citations therein). Inaccurate projections of future change in winds over the Southern Ocean, which is the major region of additional heat and carbon uptake in the global ocean (Froelicher et al, 2015), can result in unrealistically low predictions of the Antarctic ozone, which is a principal driver of recent Antarctic climate change (Garcia et al, 2017) These biases have been identified as a serious impediment to progress in understanding the dynamics of the stratosphere and to developing GCMs. During austral winter, observations have revealed the Southern Hemisphere stratosphere to be home to some of the most intense gravity wave activity on Earth (see Hindley et al, 2015, and citations therein). The observational filter (e.g. Preusse et al, 2002; Alexander et al, 2010) of the 3-D AIRS retrieval is sensitive gravity waves with relatively long vertical wavelengths (λZ 10–15 km) and relatively short horizontal wavelengths (λH 500–1000 km) This spectral portion of gravity waves is associated with high momentum fluxes via the relation in Eq (8), derived in Ern et al (2004). Since we focus on mid-latitudes to high latitudes during winter in this study, there are likely to be far more observations made during nighttime conditions than during daytime
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