A wavenumber-frequency spectral analysis of intraseasonal variability in the standard BMRC atmosphere general circulation model
A.G. Marshall, O. Alves, H.H. Hendon, and M.C. Wheeler
2006: BMRC Research Report, Australian Bureau of Meteorology,
No. 122, 28pp.
Abstract
Wavenumber-frequency spectral analyses were performed over all longitudes
in the +/-15 degree latitude band using satellite-observed outgoing
long-wave radiation (OLR) and NCEP-NCAR Re-analysis 10m (surface) zonal
wind data. We removed estimated background spectra from the absolute spectra
to reveal the Madden-Julian Oscillation (MJO) spectral peak. Using these
observed relative spectra as our reference, we investigated the ability of
several versions of the standard BMRC unified atmosphere model (BAM) to
simulate the MJO. Atmosphere only integrations as well as coupled model
eight-month hindcasts were investigated. Neither the coupled nor the
uncoupled models showed a peak at intraseasonal scales characteristic of the
MJO. Coupling had no marked effect on the variability at MJO scales. The
only exception was coupled model eight-month hindcasts that were initialised
with true atmospheric initial conditions from the NCEP/NCAR Re-analysis,
which showed a peak of similar magnitude to the observed MJO in eastward
propagating variability at wavenumber 1 and period 100 days.
While observed OLR and surface zonal wind anomalies propagate toward the
east, the models' anomalies mainly propagated toward the west. Furthermore,
several anomalous westerly wind bursts (WWB) during the development of the
1997 El Nino were stationary in time, being a local response to increases
in SST rather than MJO events. While all models showed the correct
low-frequency response to SST increases across the Pacific basin, they
did not correctly capture the internal dynamics of the atmosphere required
to generate the correct intraseasonal variability, such as the MJO.