The 1985/86 Intraseasonal Oscillation and the Role of the Extratropics

An intraseasonal oscillation that occurred in the 1985/86 northern winter is documented in this study. The tropical convection of this event is dominated by the mixture of a standing oscillation over the maritime continent and an eastward moving feature from the Indian Ocean into the central Pacific. The time evolution of the upper tropospheric circulation patterns, instead of propagating eastward along the equator as suggested in the existing composites of the intraseasonal oscillation, is characterized by a series of wave patterns in the Northern Hemisphere and does not complete the cycle around the globe. The familiar moist Kelvin wave explanation for the intraseasonal oscillation receives little support from diagnosis of this event using zonal wind, height field, streamfunction, and potential vorticity. Only in the lower troposphere near the date line is the convincing evidence for its existence found. A scenario for the intraseasonal oscillation, which is suggested by the analysis, includes the initiation of the event through organization of tropical convection in the Indian Ocean by a subtropical Rossby wave train. This wave train also triggers a modal meridional dipole response in the west Pacific. The eastern Asia and western Pacific portion of this wave pattern is further reinforced by downstream propagation from the Indian Ocean convection region. The wave train creates the conditions in which synoptic cold surge events can occur over China. The propagation of these surges into the Indonesian region leads to markedly increased convection there. This process may be aided by the conditions created by a tongue of high potential vorticity that is advected equatorward and westward towards the Indonesian region by the flow associated with the dipole. The Indonesian convection gives rise to a North Pacific wave pattern and increased upper tropospheric, equatorial westerlies in the eastern Pacific. Aspects of this scenario are supported with previous theoretical studies and new numerical model experiments. It describes a mixture of eastward propagation and the flaring of stationary features of tropical convection. However, it does not describe an oscillation. It is possible that equatorial Kelvin waves of very small magnitude do play a role in making such an oscillation possible and that the variable magnitude and period of the oscillation depend on the match of the extratropical structures with the Kelvin wave.