A Climatology of Waves in the Equatorial Region

Abstract

Propagating anomalies of moisture and moist deep convection in the Tropics are organized into a variety of large-scale modes. These include (but are not limited to) the so-called intraseasonal oscillations, convectively coupled waves similar to those predicted by shallow water theory on the equatorial beta plane, and tropicaldepression-type disturbances. Along with the annual and diurnal cycles, these modes act and interact to control much of the variance of tropical convection. Analyses of 10 yr of outgoing longwave radiation (OLR) and precipitable water (PW) data are carried out to develop comparative climatologies of these wavelike modes. The analysis relaxes the commonly used cross-equatorial symmetry constraints, which allows study of the portions of the wavelike processes that are asymmetric across the equator. Mean background states are found for OLR and for PW as functions of day of the year. Examination of anomalies together with the background reveals much about how the waves are affected by their environments. Zonal wavenumber-frequency spectral analyses are performed on these anomalies. Following the spectral analyses, the OLR and the PW data are then filtered for specific regions of the wavenumber-frequency domain. Results show how variance generated by propagating modes is distributed in time and space, approximately illustrating the relative contributions of the wave modes to regional OLR and PW variability.

The skeleton of tropical intraseasonal oscillations

“…For the standard parameter values used here, the oscillation period corresponding to Eq. 8 is 45 days, in agreement with observations of the MJO (24)(25)(26). Notice that this formula is independent of the wavenumber k; i.e., this model recovers the peculiar dispersion relation dω/dk ≈ 0 from the observational record (24)(25)(26).…”

“…2 shows that eastward-propagating waves, like the MJO (24)(25)(26), have the peculiar dispersion relation dω/dk ≈ 0. Moreover, this dispersion relation is robust over a wide range of parameter values, and the oscillation periods spanned by these reasonable parameter values are in the range of 30-60 days, which is the observed range of the MJO's oscillation period (24)(25)(26). The westward-propagating waves, however, which are plotted with positive ω and negative k, have variable ω, and their oscillation periods are seasonal, not intraseasonal, for k = 1 and 2.…”

Analysis of vertically propagating convectively coupled equatorial waves using observations and a non‐hydrostatic Boussinesq model on the equatorial beta‐plane

“…Liebmann and Hendon, 1990;Wheeler and Kiladis, 1999;Roundy and Frank, 2004;Yang et al, 2007aYang et al, , 2007bYang et al, , 2007c, substantially different values of N and l also yield reasonable dispersion solutions. Comparison of this dispersion relation with the shallow-water model of Matsuno (1966) reveals that the shallow-water model equivalent depth…”

“…We chose the imaginary part for convenience of the associated zonal phasing, but the conclusions are the same regardless of this choice. We select the geographical points from the region that shows the greatest OLR variance in the wavenumber-frequency band of the MRG wave (Wheeler et al, 2000;Roundy and Frank, 2004), from 160 • E to 170 • W. The time series from each of those points serve as predictors in regression models at each grid point across a broad geographical domain to diagnose the associated structures. One grid of regression models is calculated for each base point time series.…”

Convectively coupled Kelvin and easterly waves in a regional climate simulation of the tropics

“…For example, easterly waves are typically most active during boreal summer when convection is generally located well north of the equator (Serra et al 2008;Magnusdottir and Wang 2008). Meanwhile, Kelvin waves are typically most active during boreal spring when convection is generally closest to the equator (Roundy and Frank 2004). This section begins with an assessment of the time-mean distribution of rainfall simulated by the WRF channel model, followed by an evaluation of the simulated space-time spectral characteristics of rainfall.…”