Tropical cyclones, 6-25 day oscillations, and tropical-extratropical interaction over the northwestern Pacific

This paper investigates tropical-extratropical interactions over the northwestern Pacific Ocean that involve tropical cyclones and subtropical jet streaks. Another aspect of this study is to examine the relation between 6-25 day convective variability and tropical cyclones. This investigation is conducted for the fall and early winter season, with a focus on the months, October through December (OND). In addition to outgoing longwave radiation (OLR) data, we use 10 years (1985-1994) of WCRP/TOGA archive II analyses produced by ECMWF to compute equivalent temperature, theta sub(e) , precipitable water, W, and kinematic and kinetic energy transfer variables. These variables are composited for two classes of tropical cyclones, recurving cyclones (RCs) and non-recurving cyclones (NCRs), in order to examine the influence of tropical cyclones and baroclinic processes on changes in the jet streak intensity. We found that RCs interacted with extratropical regions during all composite days. A strong baroclinic zone developed throughout the troposphere on the north side of the composite cyclone as it propagated poleward. Between the day of recurvature, DR, and the day after recurvature, DR+1, the main band of convection shifted from the RC to a frontal band within the baroclinic zone indicating a transformation of the tropical cyclone into an extratropical one. An eastward propagating jet streak at 200 hPa, located north of the RC and in the vicinity of the baroclinic zone, increased its speed from 57 ms super(-) super(1) to 79 ms super(-) super(1) on DR+1. Although we could not measure the role of baroclinic processes in this regard, we were able to infer that upper-level outflow from the RC did supply momentum and energy to the jet streak. Whereas we expected tropical-extratropical interactions for the RCs, we also found evidence that NRCs that stay south of 20 degrees N throughout their lifetime and that dissipate over Indo-China have an influence on the subtropical jet by their upper-level outflow, especially in the late OND season. The tropical (i.e., momentum) forcing did appear to cause increases in the speed of the jet after the composited storm crossed the Philippines on the fourth day of its life cycle, D4. Concurrently, a baroclinic zone developed along the coast of southern China by about D4, but it was confined to the lower troposphere. Finally, our spectral analysis investigations for the northwestern Pacific showed significant peaks at 6-10 days and 1