Equatorial waves in the lower stratosphere. II: Annual and interannual variability

Tindall, Julia; Thuburn, John; Highwood, Eleanor J.

doi:10.1256/qj.04.153

Cited by 30 publications

(32 citation statements)

References 46 publications

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“…This means that there is an overall agreement between our estimates and the previous findings by Hitchman and Leovy (1988); Canziani and Holton (1998) and Tindall et al (2006b). Also the relative contribution of the Kelvin waves to the total eastward wave forcing is similar.…”

Section: Comparison With Previous Estimatessupporting

confidence: 89%

“…Also Tindall et al (2006b) derived accelerations due to Kelvin waves from ERA-15 data for the 70-50 hPa layer (about 16.2-18.7 km altitude). Their minimum and maximum values are about 0.02 and 0.06 m/s/day, respectively.…”

Section: Comparison With Previous Estimatesmentioning

confidence: 99%

“…During periods of easterly winds there is only little activity of westward propagating equatorial wave modes like Rossby-gravity or equatorial Rossby waves (Ern et al, 2008). In addition, forcing by other eastward propagating global scale wave modes should be comparably low (Tindall et al, 2006b). This means that a considerable part of the wave forcing would have to be attributed to mesoscale gravity waves or intermediate scale waves.…”

Section: The Importance Of Other Wave Modes For the Qbo And Saomentioning

confidence: 99%

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Wave fluxes of equatorial Kelvin waves and QBO zonal wind forcing derived from SABER and ECMWF temperature space-time spectra

Ern¹,

Preusse²

2009

Atmos. Chem. Phys.

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Abstract. The quasi-biennial oscillation (QBO) of the zonal mean zonal wind is a dynamical phenomenon of the tropical middle atmosphere. Influences of the QBO can even be found at mid and high latitudes. It is widely accepted that the phase descent of alternating tropical easterlies and westerlies is driven by atmospheric waves of both global scale (equatorial wave modes like Kelvin, equatorial Rossby, Rossbygravity, or inertia-gravity waves), as well as mesoscale gravity waves. However, the relative distribution of the different types of waves to the forcing of the QBO winds is highly uncertain. This is the case because until recently there were no high resolution long-term global measurements in the stratosphere. In our study we estimate Kelvin wave momentum flux and the contribution of zonal wind forcing by Kelvin waves based on space-time spectra determined from both Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature measurements as well as temperatures from European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses. Peak values of total Kelvin wave zonal wind forcing found are about 0.2 m/s/day. There is good agreement between SABER and ECMWF results. Altitude-time cross sections are shown and the results are compared to the total wave forcing required to balance the background atmosphere. Sometimes Kelvin wave forcing is sufficient to explain almost the whole total wave forcing required for the momentum balance during the transition from QBO easterly to westerly winds. This is especially the case during the periods of strong westerly wind shear when the zonal wind is between −20 and 10 m/s at the equator in the altitude range 20 to 35 km. During other parts of the phases of strong westerly wind shear, however, the contribution of Kelvin waves can be comparably low and the missing wave forcing, which is often attributed Correspondence to: M. Ern (m.ern@fz-juelich.de) to mesoscale gravity waves or intermediate scale waves, can be the by far dominant contribution of the QBO forcing. It is also found that seasonal variations of Kelvin wave accelerations could play an important role for the maintenance of the QBO westerly wind jets in the lower stratosphere.

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Section: Comparison With Previous Estimatessupporting

confidence: 89%

Section: Comparison With Previous Estimatesmentioning

confidence: 99%

Section: The Importance Of Other Wave Modes For the Qbo And Saomentioning

confidence: 99%

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Wave fluxes of equatorial Kelvin waves and QBO zonal wind forcing derived from SABER and ECMWF temperature space-time spectra

Ern¹,

Preusse²

2009

Atmos. Chem. Phys.

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“…Other theory-based estimates of momentum fluxes have been derived for use with satellite irradiances (Hitchman and Leovy, 1988;Ern and Preusse, 2009) and have found similar magnitudes of the momentum fluxes. Together with estimations using reanalysis products (e.g., Tindall et al, 2006), qualitatively consistent bounds to the momentum flux amplitudes have been determined.…”

Section: Introductionmentioning

confidence: 74%

Intraseasonal to interannual variability of Kelvin wave momentum fluxes as derived from high-resolution radiosonde data

2017

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Abstract. Observational estimates of Kelvin wave momentum fluxes in the tropical lower stratosphere remain challenging. Here we extend a method based on linear wave theory to estimate daily time series of these momentum fluxes from high-resolution radiosonde data. Daily time series are produced for sounding sites operated by the US Department of Energy (DOE) and from the recent Dynamics of the MaddenJulian Oscillation (DYNAMO) field campaign. Our momentum flux estimates are found to be robust to different data sources and processing and in quantitative agreement with estimates from prior studies. Testing the sensitivity to vertical resolution, our estimated momentum fluxes are found to be most sensitive to vertical resolution greater than 1 km, largely due to overestimation of the vertical wavelength. Climatological analysis is performed over a selected 11-year span of data from DOE Atmospheric Radiation Measurement (ARM) radiosonde sites. Analyses of this 11-year span of data reveal the expected seasonal cycle of momentum flux maxima in boreal winter and minima in boreal summer, and variability associated with the quasi-biennial oscillation of maxima during easterly phase and minima during westerly phase. Comparison between periods with active convection that is either strongly or weakly associated with the MaddenJulian Oscillation (MJO) suggests that the MJO provides a nontrivial increase in the lowermost stratospheric momentum fluxes.

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“…After the first discovery of observational evidence of Kelvin wave and westward-moving mixed Rossbygravity (WMRG) wave in the stratosphere (Yanai and Maruyama 1966;Wallace and Kousky 1968), there have been many subsequent observational studies of equatorial waves in the troposphere and stratosphere, particularly Kelvin and WMRG waves (e.g., Zangvil andYanai 1980, 1981;Dunkerton 1993;Takayabu 1994;Dunkerton and Baldwin 1995;Wheeler and Kiladis 1999;Wheeler et al 2000;Straub and Kiladis 2003;Yang et al 2003;Randel and Wu 2005;Tindall et al 2006;Yang et al 2007 a,b,c;Alexander et al 2008;Suzuki and Shiotani 2008;Kiladis et al 2009;Lott et al 2009;Alexander and Ortland 2010;Yang et al 2011). However, there is relatively less observational knowledge of the tropospheric origin of the stratospheric waves, their zonal and vertical propagation, and how the propagation is influenced by ambient flows in the stratosphere.…”

Section: Introductionmentioning

confidence: 99%

The Influence of the QBO on the Propagation of Equatorial Waves into the Stratosphere

Yang

Hoskins

Gray

2012

Journal of the Atmospheric Sciences

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The variation of stratospheric equatorial wave characteristics with the phase of the quasi-biennial oscillation (QBO) is investigated using ECMWF Re-Analysis and NOAA outgoing longwave radiation (OLR) data. The impact of the QBO phases on the upward propagation of equatorial waves is found to be consistent and significant. In the easterly phase, there is larger Kelvin wave amplitude but smaller westward-moving mixed Rossby-gravity (WMRG) and n 5 1 Rossby (R1) wave amplitude due to reduced propagation from the upper troposphere into the lower stratosphere, compared with the westerly phase. Differences in the wave amplitude exist in a deeper layer in summer than in winter, consistent with the seasonality of ambient zonal winds. There is a strong evidence of Kelvin wave amplitude peaking just below the descending westerly phase, suggesting that Kelvin waves act to bring the westerly phase downward. However, the corresponding evidence for WMRG and R1 waves is less clear.In the lower stratosphere there is zonal variation in equatorial waves. This reflects the zonal asymmetry of wave amplitudes in the upper troposphere, the source for the lower-stratospheric waves. In easterly winters the upper-tropospheric WMRG and R1 waves over the eastern Pacific region appear to be somewhat stronger compared to climatology, perhaps because of the accumulation of waves that are unable to propagate upward into the lower stratosphere. Vertical propagation features of these waves are generally consistent with theory and suggest a mixture of Doppler shifting by ambient flows and filtering. Some lower-stratosphere equatorial waves have a connection with preceding tropical convection, especially for Kelvin and R1 waves in winter.

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Equatorial waves in the lower stratosphere. II: Annual and interannual variability

Cited by 30 publications

References 46 publications

Wave fluxes of equatorial Kelvin waves and QBO zonal wind forcing derived from SABER and ECMWF temperature space-time spectra

Wave fluxes of equatorial Kelvin waves and QBO zonal wind forcing derived from SABER and ECMWF temperature space-time spectra

Intraseasonal to interannual variability of Kelvin wave momentum fluxes as derived from high-resolution radiosonde data

The Influence of the QBO on the Propagation of Equatorial Waves into the Stratosphere

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