Johnny Lin, J. David Neelin, and Ning Zeng
J. Atmos. Sci., 2000.
Paper (PDF 1.4MB).
© Copyright 2000 by the American Meteorological Society.
Abstract
An intraseasonal tropical oscillation with period between 20-80 days
is simulated in the Neelin-Zeng Quasi-Equilibrium Tropical Circulation
Model. This model is an intermediate-level atmospheric model that
includes primitive equation non-linearity, radiative-convective
feedbacks, a simple land model with soil moisture, and a Betts-Miller
type moist convective adjustment parameterization. Vertical temperature
and moisture structures in the model are based on quasi-equilibrium
profiles taken from deep convective regions. The tropical intraseasonal
variability is reasonably broadband. The eastward propagating 20-80 day
variability is dominated by zonal wavenumber 1, shows features similar to
an irregular Madden-Julian oscillation, and exhibits amplitude and phase
speeds that vary both seasonally and between events.
At higher wavenumbers, the model has a distinction between the
low-frequency MJO-like band and the moist Kelvin wave band, similar
to that found in observations. In the model, it is conjectured
that this arises by interaction of the wavenumber 1 moist Kelvin
wave with the zonally asymmetric basic state.
Experiments using climatological sea surface temperature forcing are conducted using this model to examine the effects of evaporation-wind feedback and extratropical excitation on the maintenance of intraseasonal variability, with particular attention to the low wavenumber mode in the 20-80 day band. These experiments indicate that evaporation-wind feedback partially organizes this intraseasonal variability by reducing damping, but is not by itself sufficient to sustain this oscillation for the most realistic parameters. Excitation by extratropical variability is a major source of energy for the intraseasonal variability in this model. When mid-latitude storms are suppressed, tropical intraseasonal variability is nearly eliminated. However, the eastward propagating intraseasonal signal appears most clearly when mid-latitude excitation is aided by the evaporation-wind feedback.
Citation Lin, J. W., J. D. Neelin, and N. Zeng, 2000: Maintenance of tropical intraseasonal variability: impact of evaporation-wind feedback and midlatitude storms. J. Atmos. Sci., 57, 2793-2823.