Abstract
Abstract. Equatorial Kelvin and mixed Rossby–gravity (MRG) waves in the tropical tropopause layer and stratosphere represented in recent reanalyses for the period of 1981–2010 are compared in terms of spectral characteristics, spatial structures, long-term variations, and their forcing of the quasi-biennial oscillation (QBO). For both wave types, the spectral distributions are broadly similar among most of the reanalyses, while the peak amplitudes exhibit considerable spread. The longitudinal distributions and spatial patterns of wave perturbations show reasonable agreement between the reanalyses. A few exceptions to the similarity of the spectral shapes and spatial structures among them are also noted. While the interannual variations of wave activity appear to be coherent for both the Kelvin and MRG waves, there is substantial variability in long-term trends among the reanalyses. Most of the reanalyses which assimilate satellite data exhibit large increasing trends in wave variance (∼15 %–50 % increase in 30 years at 100–10 hPa), whereas one reanalysis (Japanese 55-year Reanalysis assimilating conventional observations only; JRA-55C) produced without satellite data does not. Several discontinuities are found around 1998 in the time series of the Kelvin and MRG wave variances, which manifest in different ways depending on the reanalysis, and are indicative of impacts of the transition of satellite measurements during that year. The equatorial wave forcing of the QBO, estimated by the Eliassen–Palm (EP) flux divergence, occurs in similar phase-speed ranges in the lower stratosphere among the reanalyses. However, the EP flux and its divergence are found to be dependent on the zonal-mean winds represented in reanalyses, exhibiting different magnitudes, altitudes, and phase-speed ranges of the Kelvin wave forcing between the reanalyses, especially at 20–10 hPa. In addition, at around 20 hPa, a wave signal which appears only in easterly mean winds with westward phase speeds is found and discussed.
Highlights
Stratospheric equatorial waves are known to be generated in response to the heat sources associated with tropical convection and to play an important role in the tropics (Salby and Garcia, 1987; Garcia and Salby, 1987)
The reanalyses commonly exhibit a gradual shift of the Kelvin wave spectrum to larger equivalent depths (h) with respect to the altitude: the majority of the spectral power appears at h < 60 m at 100 hPa, at h ∼ 60 m at 70 hPa, and at h = 60–240 m above 70 hPa
The equatorial Kelvin and mixed Rossby–gravity (MRG) waves in the tropical tropopause layer (TTL) and stratosphere represented in six reanalyses (ERA-I, MERRA, MERRA-2, CFSR, JRA-55, and JRA-55C) are compared for the period of 1981–2010
Summary
Stratospheric equatorial waves are known to be generated in response to the heat sources associated with tropical convection and to play an important role in the tropics (Salby and Garcia, 1987; Garcia and Salby, 1987). In contrast to analyses, which are made using operationally changing, state-of-the-art versions of the prediction model and assimilation system, reanalyses are derived using fixed versions of the data assimilation system and firstguess models for the whole period of the product This helps to generate a temporally homogeneous product, which benefits studies of long-term changes in meteorological variables. Examples include the introduction of radiance/temperature profiles derived from the Television Infrared Observation Satellite (TIROS) Operational Vertical Sounder (TOVS) suite (Smith et al, 1979) around 1978 and those from the Advanced TOVS (ATOVS) suite around 1998 While these measurements have improved the quality of reanalyses, it has been reported that the transition from the TOVS to ATOVS suites induced temporal discontinuities in assimilated variables such as upperstratospheric global-mean temperature (Onogi et al, 2007; Simmons et al, 2014) and equatorial stratospheric mean wind and temperature (Kawatani et al, 2016).
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