Convectively Coupled Equatorial Waves: Analysis
of Clouds and Temperature in the Wavenumber-Frequency Domain
Matthew Wheeler and George N. Kiladis
1999: J. Atmos. Sci., 56, 374-399.
Abstract
A wavenumber-frequency spectrum analysis is performed for all longitudes in the
domain 15degS to 15degN using a long (~18 years) twice-daily record of
satellite-observed outgoing longwave radiation (OLR), a good proxy for deep
tropical convection. The broad nature of the spectrum is red in both zonal
wavenumber and frequency. By removing an estimated background spectrum, numerous
statistically significant spectral peaks are isolated. Some of the peaks
correspond quite well to the dispersion relations of the equatorially-trapped
wave modes of shallow-water theory with implied equivalent depths in the range
of 12 to 50 m. Cross-spectrum analysis with the satellite-based microwave
sounding unit (MSU) deep-layer temperature data shows that these spectral peaks
in the OLR are "coupled" with this dynamical field. The equivalent depths of the
convectively-coupled waves are shallower than those typical of equatorial waves
uncoupled with convection. Such a small equivalent depth is thought to be a
result of the interaction between convection and the dynamics. The
convectively-coupled equatorial waves identified correspond to the Kelvin, n=1
equatorial Rossby (ER), mixed Rossby-gravity (MRG), n=0 eastward inertio-gravity
(EIG), n=1 westward inertio-gravity (WIG), and n=2 WIG waves. Additionally, the
Madden-Julian Oscillation (MJO) and tropical depression-type (TD-type)
disturbances are present in the OLR spectra. These latter two features are
unlike the convectively-coupled equatorial waves due to their location away from
the equatorial wave dispersion curves in the wavenumber-frequency domain.
Extraction of the different convectively-coupled disturbances in the
time-longitude domain is performed by filtering the OLR dataset for very
specific zonal wavenumbers and frequencies. The geographical distribution of the
variance of these filtered data gives further evidence that some of the spectral
peaks correspond to particular equatorial wave modes. The results have
implications for the cumulus parameterization problem, for the excitation of
equatorial waves in the lower stratosphere, and for extended-range forecasting
in the Tropics.
This figure is a presentation of the wavenumber-frequency power spectral peaks
of the "convectively-coupled equatorial waves". The power spectra were
calculated from a long (~18 years) twice-daily dataset of outgoing
longwave radiation (OLR) between the latitudes of 15°S and 15°N. The power s
pectra have then been divided by
a red-noise background spectrum to give the contours as shown above. (a)
is for the antisymmetric component (w.r.t. the equator), and (b) is
for the symmetric component. Contour interval of this power ratio is 0.1,
and the shading begins at a value of 1.1, for which the spectral
signatures are statistically significantly above the background at the 95%
level (based on 500 d.o.f.). Superimposed are the dispersion curves of the
respectively even and odd meridional-numbered equatorial waves as in Matsuno (1966) fo
r the three equ
ivalent
depths of h=12, 25, and 50m.