This study investigates major characteristics of summer (June-August) climate variability in Taiwan during the period of 1950-2000 and associated regulating processes imposed by large-scale background variations, including the western North Pacific summer monsoon (WNPSM), the Pacific interdecadal climate change (PICC) related to abrupt climate change in the late 1970s, and the western North Pacific (WNP) tropical cyclone (TC) activity.In Taiwan, summer rainfall and temperature anomalies tend to be opposite in sign: anomalous cold summer is often wet, and vice versa. Based upon characteristics of temperature and rainfall anomalies in Taiwan, low-level circulation pattern overlying Taiwan, and the phase of PICC, we categorize Taiwan's summer climate variability into six major types. The major regulatory mechanism for four of these types is anomalous vertical motion over an east-west elongated low-level circulation anomaly across Taiwan, a feature associated with the WNPSM variability. For the other two types, the primary regulating mechanism is anomalous moisture transport by a southwest-northeast oriented low-level circulation anomaly over the WNP, which is a salient feature of the PICC.Influence of the WNP TC activity on Taiwan's summer climate is examined in terms of TC rainfall (rainfall in Taiwan induced by TC passages) variability. For the types of summer climate variability with a low-level anomalous high over the WNP, the circulation enhances vertical wind shear to suppress TC genesis and the subsequent TC activity over the WNP, leading to decreased TC rainfall in Taiwan. On the other hand, increased TC rainfall does not necessarily occur in Taiwan for summer climate variability types with a low-level anomalous low over the WNP. A favorable condition for TC rainfall in Taiwan is to have more TCs forming in a region southeast of Taiwan, which are then steered by the WNP anomalous low to recurve northward toward Taiwan.
The perturbation in the National Centers for Environmental Prediction (NCEP) Regional Spectral Model (RSM) is defined by the difference between the RSM and its outer model, such as the NCEP global model, on the same given coordinate surfaces. This perturbation is dominated by the difference between terrains of the RSM and the global model if model terrain exists over the domain. After being transformed into spectral space, this difference in terrains is characterized by two distinct groups of coefficients i.e., long-wave and short-wave groups. The long-wave group is considerably larger in amplitude than the short-wave group, and considered to be the cause of error in horizontal diffusion of perturbations on sigma surfaces. A higher order of diffusion and diffusion without long waves on sigma surfaces with perturbations are formulated to investigate the possibility of using diffusion on sigma surfaces.In a typical case, erroneous circulation occurred when fourth-order diffusion on a sigma surface was used; however, reasonable circulation was simulated with second-order diffusion on the pressure surface. Nevertheless, diffusion on a pressure surface is a non-economical method in spectral modeling. The results of sixth-and eighth-order horizontal diffusions of perturbations and no-diffusion on the group of long waves for all orders (second, fourth, sixth, and eighth) on sigma surfaces did not show erroneTerr. Atmos. Ocean. Sci., Vol. 18, No. 1, March 2007 68 ous circulations. Thus, diffusions greater than and equal to sixth-order diffusions and no-diffusion on a long-wave group can be implemented into NCEP RSM as an economical computation as it leads to an absence of erroneous circulation comparable to diffusion on pressure surfaces.
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