Lin, J. W.-B., J. D. Neelin, and N. Zeng, 2000: "Maintenance of tropical intraseasonal variability: Impact of evaporation-wind feedback and mid-latitude storms," J. Atmos. Sci., Vol. 57, No. 17, pp. 2793-2823. © Copyright 2000 by the American Meteorological Society (AMS). Permission to place a copy of this work on this server has been provided by the AMS. The AMS does not guarantee that the copy provided here is an accurate copy of the published work.
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.