Jwb. Lin et al., Maintenance of tropical intraseasonal variability: Impact of evaporation-wind feedback and midlatitude storms, J ATMOS SCI, 57(17), 2000, pp. 2793-2823
An intraseasonal tropical oscillation with a period of 20-80 days is simula
ted in the Neelin-Zeng Quasi-Equilibrium Tropical Circulation Model. This m
odel is an intermediate-level atmospheric model that includes primitive equ
ation nonlinearity, radiative-convective feedbacks, a simple land model wit
h soil moisture, and a Betts-Miller-type moist convective adjustment parame
terization. Vertical temperature and moisture structures in the model are b
ased on quasi-equilibrium profiles taken from deep convective regions. The
tropical intraseasonal variability is reasonably broadband. The eastward pr
opagating 20-80-day variability is dominated by zonal wavenumber 1, shows f
eatures similar to an irregular Madden-Julian oscillation (MJO), and exhibi
ts amplitude and phase speeds that vary both seasonally and between events.
At higher wavenumbers, the model has a distinction between the low-frequen
cy MJO-like band and the moist Kelvin wave band, similar to that found in o
bservations. In the model, it is conjectured that this arises by interactio
n of the wavenumber-1 moist Kelvin wave with the zonally asymmetric basic s
tate.
Experiments using climatological sea surface temperature forcing are conduc
ted using this model to examine the effects of evaporation-wind feedback an
d extratropical excitation on the maintenance of intraseasonal variability,
with particular attention paid 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 it is not
by itself sufficient to sustain this oscillation for the most realistic pa
rameters. Excitation by extratropical variability is a major source of ener
gy for the intraseasonal variability in this model. When midlatitude storms
are suppressed, tropical intraseasonal variability is nearly eliminated. H
owever, the eastward propagating intraseasonal signal appears most clearly
when midlatitude excitation is aided by the evaporation-wind feedback.