ISSN:
1436-5065
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geography
,
Physics
Notes:
Summary 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,θ 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−1 to 79 ms−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° 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 Phillippines 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 15–20 days from late September to early December. The first peak is well known and is associated with typhoon 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 with the findings of Hartmann et al. (1992). For some years of the investigation period (1985, 1986, and 1988), however, our results showed that westward propagating convective disturbances that fail to reach tropical depression 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 found at all. Therefore, it appears that further work needs to be done with regard to the relationship between convective systems and their accompanying relationships on time scales ranging between 10 and 25 days.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01030207
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