Abstract
Abstract. Wind measurements from a meteor radar on Ascension Island (8° S, 14° W) and simultaneous temperature measurements from the Aura MLS instrument are used to characterise ultra-fast Kelvin waves (UFKW) of zonal wavenumber 1 (E1) in the mesosphere and lower thermosphere (MLT) in the years 2005–2010. These observations are compared with some predictions of the Kyushu-general circulation model. Good agreement is found between observations of the UFKW in the winds and temperatures, and also with the properties of the waves in the Kyushu-GCM. UFKW are found at periods between 2.5–4.5 days with amplitudes of up to 40 m s−1 in the zonal winds and 6 K in the temperatures. The average vertical wavelength is found to be 44 km. Amplitudes vary with latitude in a Gaussian manner with the maxima centred over the equator. Dissipation of the waves results in monthly-mean eastward accelerations of 0.2–0.9 m s−1 day−1 at heights around 95 km, with 5-day mean peak values of 4 m s−1 day−1. Largest wave amplitudes and variances are observed over Indonesia and central Africa and may be a result of very strong moist convective heating over those regions. Rainfall data from TRMM are used as a proxy for latent-heat release in an investigation of the excitation of these waves. No strong correlation is found between the occurrence of large-amplitude mesospheric UFKW events and either the magnitude of the equatorial rainfall or the amplitudes of E1 signatures in the rainfall time series, indicating that either other sources or the propagation environment are more important in determining the amplitude of UFKW in the MLT. A strong semiannual variation in wave amplitudes is observed. Intraseasonal oscillations (ISOs) with periods 25–60 days are evident in the zonal background winds, zonal-mean temperature, UFKW amplitudes, UFKW accelerations and the rainfall rate. This suggests that UFKW play a role in carrying the signature of tropospheric ISOs to the MLT region.
Highlights
Kelvin Waves are equatorially-trapped planetary waves that travel eastwards with respect to the background winds
The fact that the period of oscillation is about 3–4 days, combined with the larger amplitudes in the zonal component, is a strong indication that these winds are the signature of ultra-fast Kelvin waves
We investigated the connection between tropospheric heating and ultra-fast Kelvin waves (UFKW) observed in the mesosphere and lower thermosphere (MLT) using TRMM precipitation data
Summary
Kelvin Waves are equatorially-trapped planetary waves that travel eastwards with respect to the background winds They propagate upwards away from their sources in the troposphere where they are thought to be excited by the latent heat release associated with tropospheric convection (Holton, 1973; Salby and Garcia, 1987). The “slow” Kelvin waves have periods in the range 15–20 days and vertical wavelengths of around 10 km. They were first observed in the lower stratosphere in tropical radiosonde measurements by Wallace and Kousky (1968). The “fast” Kelvin waves were first observed by Hirota (1978) in rocketsonde data They have periods in the range 6–10 days and vertical wavelengths of around 20 km. The “Ultra-Fast Kelvin Waves”, hereafter referred to as UFKW, were first observed by Salby et al (1984) in Published by Copernicus Publications on behalf of the European Geosciences Union
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