Elsevier

Science of The Total Environment

Volume 544, 15 February 2016, Pages 908-918
Science of The Total Environment

Equatorial atmospheric Kelvin waves during El Niño episodes and their effect on stratospheric QBO

https://doi.org/10.1016/j.scitotenv.2015.12.009Get rights and content

Highlights

  • Enhanced atmospheric Kelvin Wave amplitudes observed during El Nino of 2010.

  • The waves are probably produced by processes generating El Nino.

  • The enhanced wave activity resulted in a fast descending westerly of the QBO.

  • This effect is observed only due to westward zonal winds in lower stratosphere.

  • Wave–mean flow interactions determine if El Nino can affect QBO.

Abstract

Equatorial atmospheric Kelvin waves are investigated during a positive El Niño Southern Oscillation (ENSO) episode using temperature data retrieved from GPS Radio Occultation (RO) observations of FORMOSAT-3/COSMIC during the period from August 2006 to December 2013. Enhanced Kelvin wave amplitudes are observed during the El Niño episode of 2009–2010 and it is also observed that these amplitudes correlate with the Niño 3.4 index and also with outgoing longwave radiation and trade wind index. This study indicates that the enhanced equatorial atmospheric Kelvin wave amplitudes might be produced by geophysical processes that were involved in the onset and development of the El Niño episode. Further, easterly winds above the tropopause during this period favored the vertically upward propagation of these waves that induced a fast descending westerly regime by the end of 2010, where the zero-wind line is observed to take only 5 months to descend from 10 to 50 hPa. The current study presents observational evidence of enhanced Kelvin wave amplitudes during El Niño that has affected the stratospheric quasi-biennial oscillation (QBO) through wave–mean flow interactions. Earlier El Niño episodes of 1987 and 1998 are also qualitatively investigated, using reanalysis data. It is found that there might have been an enhancement in the equatorial Kelvin wave amplitudes during almost all El Niño episodes, however, an effect of a fast descending westerly is observed in the QBO only when the ambient zonal winds in the lower stratosphere favor the upward propagation of the Kelvin waves and consequently they interact with the mean flow. This study indicates that the El Niño and QBO are not linearly related and wave mean flow interactions play a very important role in connecting these two geophysical phenomena.

Introduction

Atmospheric, planetary scale Kelvin waves are eastward propagating waves that play a very important role in the equatorial stratospheric dynamics, especially the stratospheric quasi-biennial oscillation (QBO) (Baldwin et al., 2001, Dunkerton, 1997, Randel and Wu, 2005). They were first suggested by Matsuno (1966) and later by Holton and Lindzen (1968) as a special solution to linearized equations for an equatorial β-plane and were first discovered in radiosonde observations by Wallace and Kousky (1968). Since then, these waves have been investigated in detail for their effects on the equatorial atmospheric dynamics, particularly in reversing and bringing down the westerlies of the stratospheric QBO (Baldwin et al., 2001, Ern and Preusse, 2009a, Ern and Preusse, 2009b, Pan et al., 2011, Venkat Ratnam et al., 2006). Their contribution towards QBO wind reversal is found to be approximately 30–50% and 20–35% of the expected total wave forcing (Dunkerton, 1997, Ern and Preusse, 2009a). Kelvin waves have been investigated using observations of temperatures, winds and outgoing longwave radiation (OLR) by radiosondes, radars, satellite borne instruments, re-analysis and also in model data (Lott et al., 2014, Mote et al., 2002, Salby et al., 1984, Sridharan et al., 2006, Suzuki et al., 2010, Tsuda et al., 1994, Wheeler and Kiladis, 1999). The latest addition has been temperatures obtained by the GPS Radio Occultation (RO) technique (Randel and Wu, 2005, Tsuda et al., 2006, Venkat Ratnam et al., 2006). More recently, FORMOSAT-3/COSMIC mission is providing a large number of temperature profiles that can be used for the investigation of these waves (Alexander et al., 2008, Das and Pan, 2013, Pan et al., 2011).

Daily fluctuations in convection in the tropical troposphere produce heating and excite a spectrum of Kelvin waves with vertical wavelength corresponding to twice the effective depth of heating (Salby and Garcia, 1987). These high frequency disturbances radiate vertically upward away from the source and become free modes. There are also waves excited due to slow heating that evolve over seasonal time scales and do not radiate vertically upward. These are convectively coupled modes and confined mostly to the troposphere (Garcia and Salby, 1987). Simulations reproduced the distribution and mean magnitude of convective heating reasonably well; however, there were large differences between the variability of the heating in the model and that inferred from observations (Ricciardulli and Garcia, 2000). Both convectively coupled Kelvin waves (Kiladis et al., 2009) as well as free modes (Alexander et al., 2008, Das and Pan, 2013, Pan et al., 2011, Venkat Ratnam et al., 2006) have been investigated using ground and satellite observations. Their properties are reasonably well investigated and found to vary over a wide range of wave numbers, periods and vertical wavelengths. Das and Pan (2013) summarize these characteristics from literature in their Table 1. Based on period, Kelvin waves are classified as slow (10–20 days), fast (5–10 days) and ultra-fast (2–5 days) Kelvin waves. Vertical wavelengths of slow waves are smaller and hence are dissipated at lower altitudes modifying the stratospheric dynamics. Vertical wavelengths of ultra-fast Kelvin waves are very large and they can propagate up to mesosphere and lower thermosphere (Forbes et al., 2009). Signatures of ultra-fast Kelvin waves have also been observed in peak electron densities and heights in the ionosphere (Takahashi et al., 2007). Model studies show that wind dynamo modulation and direct vertical wave propagation are the plausible mechanisms for these waves to reach the upper atmospheric altitudes (Chang et al., 2010). These studies show the effects of Kelvin waves in ionospheric and thermospheric dynamics in particular and the importance of waves in coupling from lower to upper atmosphere in general (Forbes, 2000).

Changes in atmospheric convections and associated latent heat release can affect the excitation of Kelvin waves and thus it is envisaged that during active periods of El Niño Southern Oscillation (ENSO), which is an ocean atmosphere coupled phenomena, Kelvin wave activity and thereby the stratospheric QBO might be influenced. However, mixed results have been reported so far on effects of ENSO on QBO. Earlier, Maruyama and Tsuneoko (1988) observed a rapid descent of stratospheric westerly regime in 1987 and attributed it to the El Niño episode of 1986–87. They postulated that intense Kelvin wave activity could have contributed to the descent but were not successful in establishing the same, probably due to lack of sufficient data. Wheeler and Kiladis (1999), on the other hand, did not observe any significant differences in wave spectra of OLR data spanning 18 years (1979–1996) that can be linked to the phase of ENSO. Recent studies are, however, showing increasing evidences that Kelvin waves have a greater role to play between ENSO and stratospheric QBO (Maury et al., 2013, Taguchi, 2010, Yang and Hoskins, 2013). Investigations of stratospheric winds using radiosonde observations at equatorial stations during 1953–2008 showed unprecedented evidence of clear variations of stratospheric QBO with ENSO with the former exhibiting weaker amplitude and faster phase propagation for El Niño conditions and was attributed to enhanced equatorial wave activity due to changes in convection (Taguchi, 2010). Simulation studies also show that ENSO can have an influence on QBO by significantly affecting the stratospheric equatorial Kelvin waves (Maury et al., 2013). Yang and Hoskins (2013) analyzed ECMWF (European Centre For Medium-Range Weather Forecasts) re-analysis data and NOAA OLR and found that the phase of ENSO has a substantially positive impact on Kelvin waves. All these studies emphasised stratospheric dynamics based on ENSO phase, but few studies have so far investigated Kelvin waves in relation to ENSO using observational data in the stratosphere. In the present study we investigate the equatorial stratospheric Kelvin waves during an El Niño episode and thereby on QBO using temperature data retrieved from state-of-the-art GPS RO observations by FORMOSAT-3/COSMIC from August 2006 to December 2013. This dataset spans more than seven years covering three QBO cycles and reliable data extends up to 50 km (Das and Pan, 2014). These measurements present high global coverage, vertical, spatial and temporal resolutions, long-term stability, self-calibration and capability to operate in all weather conditions.

The paper is organized as follows. Section 2 describes the data utilised and the analysis technique. Section 3 presents the characteristics of Kelvin waves observed during the study period with emphasis on the El Niño episode of 2010. Section 4 describes the possible sources of the waves during El Niño and discussion of effects on QBO by Kelvin waves follows in Section 5 followed by summary and concluding remarks in Section 6.

Section snippets

Temperature data from FORMOSAT-3/COSMIC

The FORMOSAT-3/COSMIC mission comprises of a constellation of six micro satellites that were launched into a circular, 72° inclination orbit at an altitude of 512 km on 15 April 2006. The mission goal was to deploy the six satellites into six orbit planes at 800 km with a 30° separation for evenly distributed global coverage, which was successfully achieved for five of the satellites. Rising of FM-3 was stopped at ~ 750 km owing to solar panel related problems. It is the first constellation of

Kelvin wave properties from FORMOSAT-3/COSMIC

Free atmospheric Kelvin waves excited by latent heat release in convective clouds radiate vertically outward; however, their upward propagation depends on the ambient zonal winds in the lower stratosphere which in turn are affected by these upward propagating Kelvin waves. These eastward moving planetary scale waves (and eastward propagating gravity waves) are allowed to propagate upward only through a regime of easterly wind and on encountering westerly winds equal to their phase speeds at a

Kelvin wave generation during El Niño

Equatorial trade winds in central and western Pacific Ocean play an important role in El Niño evolution (Boulanger et al., 2004, Menkes et al., 2014, Vecchi and Harrison, 2000). High frequency westerly wind variability termed as westerly wind events (WWE) are believed to trigger eastward currents that shift the warm ocean pool and also generate downwelling oceanic Kelvin waves that deepen the thermocline in the central and eastern Pacific Ocean. These processes along with location of the

ENSO and stratospheric QBO

Fig. 1 shows easterlies at and above the tropopause during El Niño episode from mid-2009 to mid-2010 that favor the vertically upward propagation of Kelvin waves. The high amplitude Kelvin waves carrying greater momentum in total than usual thus travel through this easterly regime and when the waves encounter the westerly shear, interact with the mean flow and deposit the energy generating the westerlies. With time Kelvin waves encounter the westerly shear at lower altitudes resulting in a

Summary and conclusions

Temperature data retrieved from GPS RO observations of FORMOSAT-3/COSMIC is investigated to understand equatorial atmospheric Kelvin waves during El Niño episodes. Using space–time spectral analysis technique, spectral amplitudes of Kelvin waves are derived at wavenumbers 1 and 2 and at periods ranging from 8 to 24 days. During the El Niño episode of 2009–2010, enhanced amplitudes of slow waves at wavenumbers 1 and 2 are observed. An increase in vertical wavelength of some of these waves is also

Acknowledgements

Authors thank Prof. William Ward for very fruitful discussions and Mr. S. S. Yang and Ms. Ling-Yun Cheng for their help and support. Authors acknowledge the UCAR/COSMIC program team for providing free access to temperature data. ECMWF ERA-Interim data used in this study have been obtained from the ECMWF data server; NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, for the interpolated OLR data from their web site at http://www.esrl.noaa.gov/psd/; and NOAA/NWS/CPC for TWI index from their website //www.cpc.ncep.noaa.gov/data/indices

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