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

This paper investigates tropical-extratropical interactions over the northw estern Pacific Ocean that involve tropical cyclones and subtropical jet str eaks. Another aspect of this study is to examine the relation between 6-25 day convective variability and tropical cyclones. This investigation is con ducted for the fall and early winter season, with a focus on the months, Oc tober through December (OND). In addition to outgoing longwave radiation (O LR) data, we use 10 years (1985-1994) of WCRP/TOGA archive II analyses prod uced by ECMWF to compute equivalent temperature, theta(e), precipitable wat er, W, and kinematic and kinetic energy transfer variables. These variables are composited for two classes of tropical cyclones, recurving cyclones (R Cs) and non-recurving cyclones (NCRs), in order to examine the influence of tropical cyclones and baroclinic processes on changes in the jet streak in tensity. We found that RCs interacted with extratropical regions during all composit e days. A strong baroclinic zone developed throughout the troposphere on th e north side of the composite cyclone as it propagated poleward. Between th e day of recurvature, DR, and the day after recurvature, DR+1, the main ban d of convection shifted from the RC to a frontal band within the baroclinic zone indicating a transformation of the tropical cyclone into an extratrop ical one. An eastward propagating jet streak at 200 hPa, located north of t he RC and in the vicinity of the baroclinic zone, increased its speed from 57 ms(-1) to 79 ms(-1) on DR+1. Although we could not measure the role of b aroclinic 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 als o 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 subtr opical 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 develope d 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 15-20 days from late September to early December. The first peak is well known and is associated with typhoo n activity. In several of the investigated autumn seasons (1987, 1989, 1992 , and 1993), the second peak was clearly related to the recurrence interval of northwestern Pacific tropical cyclones. This result is in accordance wi th the findings of Hartmann et al. (1992). For some years of the investigat ion period (1985, 1986, and 1988), however, our results showed that westwar d propagating convective disturbances that fail to reach tropical depressio n strength also contribute to the power in the 15-25 day band, whereas in a few years (1990 and 1991), no OLR peak between 15 and 20 days could be fou nd at all. Therefore, it appears that further work needs to be done with re gard to the relationship between convective systems and their accompanying relationships on time scales ranging between 10 and 25 days.