Maintenance of tropical intraseasonal variability: Impact of evaporation-wind feedback and midlatitude storms

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.