Effect of Gravity Waves From Small Islands in the Southern Ocean on the Southern Hemisphere Atmospheric Circulation

Garfinkel, Chaim I.; Oman, Luke D.

doi:10.1002/2017jd027576

Cited by 19 publications

(16 citation statements)

References 32 publications

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“…It has mountains ranges nearly 3 km high, lies in a region of very strong tropospheric winds, and is an intense source of stratospheric gravity wave activity (e.g. Alexander and Grimsdell, 2013;Hoffmann et al, 2013Hoffmann et al, , 2016Hindley et al, 2016;Jackson et al, 2018;Garfinkel and Oman, 2018). Figure 5a shows AIRS measured temperature perturbations over this the island in the region specified by dashed lines in Fig.…”

Section: Case Study 2: 3-d Gravity-wave Measurements Over South Georgmentioning

confidence: 99%

“…1, this region is important due to the long-standing "cold-pole problem", where unresolved gravity-wave drag is a leading candidate for the strong wind and temperature biases found in nearly all modern GCMs (e.g. McLandress et al, 2012;Garfinkel and Oman, 2018). Understanding the nature of gravity waves around 60 • S, and measuring their momentum fluxes, is key to solving this problem.…”

Section: Wintertime Gravity-wave Characteristics Over the Southern Oceanmentioning

confidence: 99%

“…The contribution of small islands to the "missing momentum" problem during winter around the latitude belt of 60 • S could be significant (McLandress et al, 2012). Garfinkel and Oman (2018) found that, for several GCMs, springtime stratospheric temperature biases could be reduced by up to 30 − 50% by increasing by 5× the topographic variance from the islands. They also found however that further increases did not lead to much more significant improvement, suggesting that fluxes from small islands may only form part of the solution.…”

Section: The Role Of Small Islandsmentioning

confidence: 99%

“…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). It has been hypothesised that the coldpole problem arises due to an under-representation of the momentum fluxes of these waves in models at latitudes around 60 • S (McLandress et al, 2012;Geller et al, 2013;Garfinkel and Oman, 2018). Determining the true sources of these waves that are "missing" in models, such that they can be accurately simulated or parameterised, is thus an important step to solving the "cold-pole problem" in GCMs.…”

Section: Introductionmentioning

confidence: 99%

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

Hindley¹,

Smith²,

Hoffmann³

et al. 2019

Preprint

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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 Global Circulation Models (GCMs) seriously under-represent the momentum fluxes of gravity waves at latitudes near 60&#176;&#8201;S. This can result in modelled winter stratospheres that are unrealistically cold &#8211; a significant bias known as the "cold-pole problem". There is thus a need for measurements of gravity-wave fluxes near 60S 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 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&#176;&#8201;S during austral winter 2010. We first validate our method using a synthetic wave field 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&#8211;80&#8201;% of total zonal-mean momentum flux is located over the open Southern Ocean during June&#8211;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&#176;&#8201;S from the north and south. We propose that this convergence, which is observed at nearly all longitudes during winter, accounts for a significant part of the under-represented flux in GCMs at these latitudes.

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Section: Case Study 2: 3-d Gravity-wave Measurements Over South Georgmentioning

confidence: 99%

Section: Wintertime Gravity-wave Characteristics Over the Southern Oceanmentioning

confidence: 99%

Section: The Role Of Small Islandsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Hindley¹,

Smith²,

Hoffmann³

et al. 2019

Preprint

View full text Add to dashboard Cite

show abstract

“…Lower correlations are observed in all cases at the summer pole, falling to near-zero correlation for the southern summer pole in mesospheric SABER comparisons. This is presumably related to the well-known cold-pole problem (e.g McLandress et al, 2012;Hindley et al, 2015;Wright et al, 2017;Garfinkel and Oman, 2018). which affects model temperatures over the winter pole due to issues resoving gravity-wave fluxes, compounded by a limited volume of assimilative constraints.…”

mentioning

confidence: 99%

How well do stratospheric reanalyses reproduce high-resolution satellite temperature measurements?

Hindley

2018

Preprint

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Abstract. Atmospheric reanalyses are data-assimilating weather models which are widely used as proxies for the true state of the atmosphere in the recent past, particularly for the stratosphere, where historical observations are sparse. But how realistic are these stratospheric reanalyses? Here, we resample stratospheric temperature data from six modern reanalyses (CFSR, ERA-5, ERA-Interim, JRA-55, JRA-55C and MERRA-2) to produce synthetic satellite observations, which we directly compare to retrieved temperatures from the COSMIC, HIRDLS and SABER instruments and to brightness temperatures from the AIRS instrument for the ten-year period 2003&#8211;2012. We explicitly sample standard public-release products in order to best assess their suitability for typical use cases. We find that all-time all-latitude correlations between limb sounder observations and synthetic observations from full-input reanalyses are 0.97&#8211;0.99 at 30&#8201;km altitude, falling to 0.84&#8211;0.94 at 50&#8201;km. The highest correlations are seen at high latitudes and the lowest in the sub-tropics, but root-mean-square (RMS) differences are highest (10&#8201;K or greater) in high-latitude winter. At all latitudes, differences increase with increasing height. High-altitude differences become especially large during disrupted periods such as the post-sudden stratospheric warming recovery phase, where zonal-mean differences can be as high as 18&#8201;K between different datasets. We further show that, for the current generation of reanalysis products, a full-3D sampling approach is always required to produce realistic synthetic AIRS observations, but is almost never required to produce realistic synthetic HIRDLS observations. For synthetic SABER and COSMIC observations full-3D sampling is required in equatorial regions and regions of high gravity-wave activity but not otherwise. Finally, we use cluster-analyses to show that full-input reanalyses are more tightly correlated with each other than with observations, even observations which they assimilate. This may suggest that these reanalyses are over-tuned to match their comparators. If so, this could have significant implications for future reanalysis development.

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

Hindley

Wright

Hoffmann

et al. 2020

Geophysical Research Letters

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Atmospheric gravity waves (GWs) are key drivers of the atmospheric circulation, but their representation in general circulation models (GCMs) is challenging, leading to significant biases in middle atmospheric circulations. Unresolved GW momentum transport in GCMs must be parameterized, but global directional GW observations are needed to constrain this. Here we present an 18-year climatology of directional stratospheric GW momentum flux (GWMF) from global AIRS/Aqua 3-D satellite observations during 2002 to 2019. Striking hemispheric asymmetries are found at high latitudes, including dramatic reductions and reversals of GWMF during sudden stratospheric warmings. During Southern Hemisphere winter, a lateral convergence of GWMF toward 60°S is found that has no Northern Hemisphere counterpart. In the tropics, we find that zonal GWMF in AIRS measurements is strongly modulated by the semiannual oscillation (SAO) but not the quasi-biennial oscillation (QBO). Our results provide guidance for future GW parameterizations needed to resolve long-standing biases in GCMs. Plain Language Summary Gravity waves (GWs) are traveling waves that can occur in geophysical fluid environments subject to the gravitational force. In the Earth's atmosphere, GWs transport momentum that helps to drive the atmospheric circulation, especially in the middle atmosphere. But for numerical weather and climate models, accurately simulating GWs has proven very challenging because of a lack of GW observations, leading to significant model biases. Here we present an 18-year climatology of GW observations near 40-km altitude for 2002 to 2019. For the first time, we present multidecadal global measurements of directional GW momentum flux derived from 3-D satellite observations from National Aeronautics and Space Administration's (NASA's) AIRS instrument. We find that during Southern Hemisphere winter, GWs travel laterally toward 60°S each year. This is significant because most models underestimate GW momentum near 60°S and do not typically include lateral propagation in their parameterizations. In the tropical stratosphere, we find no modulation of GWs in AIRS observations by the quasi-biennial oscillation (QBO). This is consistent with the hypothesis that GW-QBO interactions occur at shorter vertical wavelengths, which are invisible to AIRS. Our results will help to guide future GW parameterizations, leading to more accurate atmospheric simulations and ultimately better forecasts of weather and climate.

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Effect of Gravity Waves From Small Islands in the Southern Ocean on the Southern Hemisphere Atmospheric Circulation

Cited by 19 publications

References 32 publications

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

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

How well do stratospheric reanalyses reproduce high-resolution satellite temperature measurements?

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

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