The remote impacts of a Western Pacific tropical cyclone

Abstract

The short term teleconnections arising from an individual tropical cyclone in tne western Pacific (typhoon Seth, October 1994) were examined using an operational global data assimilation system and numerical weather prediction model. During the data assimilation, the model's initial conditions were modified using a tropical cyclone bogusing procedure that either maintained or eliminated the individual storm. These different initial conditions were used in six extended-range forecasts of about 3.5 weeks duration. Three of these forecasts simulated the atmosphere with tne tropical cyclone and three without the storm. The ensemble average differences between the forecasts with the storm and those without it were used to infer the global teleconnection response to the tropical cyclone. This response was dominated by a strong and persistent Rossby wave train that extended from east Asia across the North Pacific into North America. This wave train was initiated when an anticyclonic circulation formed near Japan as the tropical cyclone approached the east Asian jet. The anticyclone formation was primarily the result of two factors: (1) vortex stretching; and (2) absolute vorticity advection as divergent outflow from the tropical cyclone crossed the large absolute vorticity gradient of the east Asian jet. The wave response was quasi-stationary. However, the basic wave train (i.e., the teleconnection pattern) developed within a week due to a relatively rapid eastward propagation of wave energy across the North Pacific and North America. In regions of strong jet flow, this propagation tended to parallel the flow while in regions of weaker flow, the propagation had stronger poleward or equatorward components. The wave train intensified well after the tropical cyclone and the initial wave formation process had dissipated.