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Tsing-Chang Chen and Ming-Cheng Yen

Abstract

It has been observed that the low-level monsoon circulation, especially the Somali jet, exhibits a 40–50 day oscillation and obtains its maximum intensity in this oscillation when the migrating transient monsoon trough approaches ∼20°N. The data generated by the FGGE III-b analyses of the European Centre for Medium Range Weather Forecasts for the northern summer were used to explore the air mass source of this oscillation and to explain energetically and synoptically the intensification and decay of the low-level monsoon circulation in association with the 40–50 day oscillation.

A synoptic analysis of the divergent wind fields suggests that the convergence induced by the intertropical convergence zone and the deepening of the monsoon trough over northern India supplies the air mass to the 40–50 day oscillation. The energetics analysis shows that the 40–50 day oscillation of the low-level monsoon circulation is essentially described by the rotational mode. The 40–50 day oscillation of this flow field is maintained both by the generation of the 40–50 day rotational kinetic energy, i.e., the work done by the cross contour rotational mode [G(kRν)], and by the interaction between the 40–50 day divergent and rotational modes with the former process of primary importance. It is inferred from the dominant role of [G(kRν)] that the intensity of the Somali jet is highly related to the gradients of height field over the southwest fringe of the monsoon trough over northern India. This trough is deepened when the transient migrating monsoon trough reaches ∼20°N. It is clear that the concerned gradients of height field and the monsoon trough over northern India must develop synchronously. This synoptic relationship between them explains why the Somali jet intensifies or decays with the same pace as this monsoon trough.

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Ming-Cheng Yen and Tsing-Chang Chen

Abstract

Active wild fires over continental landmasses occur in the warm, dry summer. In contrast, in a study by Yen and Chen it was observed that the fire occurrence in Taiwan exhibits an annual variation with a peak in the cool, dry winter. Analysis of the fire-disaster reports released recently by the National Fire Administration of Taiwan shows that fire-related house damage, property loss, and human casualties, overall, exhibit annual variations in concert with the annual variation in the fire-occurrence frequency. The fire-disaster statistics support Yen and Chen’s observation of the annual variation of the fire-occurrence frequency on a subtropical maritime island.

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Ming-Cheng Yen and Tsing-Chang Chen

Abstract

Twelve years (1985–96) of monthly house fire reports for 22 districts in Taiwan, a maritime subtropical island of east Asia, were analyzed to characterize its fire nature. The major effort focused on the identification of temporal variation signals and their possible links with meteorological variables. Two significant modes of house fires were identified: annual and diurnal. As revealed from the power spectral analyses of fire time series in every fire district, a pronounced annual cycle peak emerges, with a peak phase in December and a minimum phase in June. In contrast to the warm and dry summer fire season of three continental landmasses (i.e., the U.S. West, the Northwest Territories of Canada, and the large wildland of Australia), an active fire season appears during the cool, dry winter in Taiwan. The fires on this island are highly correlated with several hydrometeorological variables; a decrease (increase) in rainfall in the dry (wet) cool (warm) environment with strong (weak) winds facilitates (hinders) fire occurrence. Under the modulation of the annual variation, two distinct fire regimes are identified in the diurnal variation of fire occurrence over the entire year: midnight–early morning and late morning–night. A sharp increase in fire occurrence occurs in the midmorning after a phase of constant fire occurrence frequency in the first regime and a gradual reduction over the nighttime hours in the second regime. Although fire occurrence is significantly suppressed by rainfall during the warm wet summer, an inverse relationship between fire occurrence and relative humidity for both annual and diurnal variations in Taiwan suggests that relative humidity plays a crucial role in fire occurrence.

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Tsing-Chang Chen and Ming-Cheng Yen

Abstract

An inverse annual variation is observed between surface pressure on the highest mountain, which has an elevation of approximately 4000 m, and in the lowlands of Taiwan (a subtropical island in east Asia). This inverse annual variation in surface pressure of high and low elevation in low latitudes reflects, essentially, a vertical phase reversal of the tropical circulation, which is illustrated with the annual variation in the vertical structure of tropical geopotential height.

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Tsing-Chang Chen and Ming-Cheng Yen

Abstract

Previous studies have shown diagnostically and statistically that the interannual variation of the Indian monsoon is closely correlated with the tropical Pacific sea surface temperature (SST). It seems likely that the interannual variation of the Indian monsoon results from the response of this monsoon system to the interannual variations of the Pacific SST. This hypothesis has not been substantiated in the past. In order to test it, Version 1 of the National Center for Atmospheric Research Community Climate Model (CCM) was used to perform two parallel climate simulations: a control run using the 12 calendar month climatological SST and a run using real-time Pacific SST. The SST data used in this study are derived from the Comprehensive 0cean-Atrnosphere Data set. Significant interannual variations of the Indian monsoon circulation are generated by the real-time Pacific SST experiment, but not the clmatological SST control experiment in real-time Pacific SST simulation weakened during the 1982 and 1987 summers and intensified in the 1984 and 1988 summers. The interannual variation of the model monsoon circulation resembles the observed in many ways. According to the linear theory of Matsuno and Gill, summertime stationary eddies are generated by steady tropical forcing. Because the Indian monsoon is a part of summertime stationary eddies, interannual variation of steady tropical heating induced by interannual Pacific SST anomalies results in interannual variations of summertime stationary eddies and the associated monsooon. Various diagnostic analyses are thus engaged to illustrate this explanation of the monsoon interannual variation.

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Tsing-Chang Chen and Ming-Cheng Yen

Abstract

An effort is made to explore the interannual variation of stationary eddies for the past 13 summers (1979–1991). Equatorial λ − t diagrams of sea surface temperature (SST), outgoing longwave radiation (OLR), the potential function of OLR (χOLR), and velocity potential (χ) derived from National Meteorological Center (NMC) analyses reveal the existence of a coherent and pronounced interannual variation, particularly over the Pacific. Performing regular empirical orthogonal function (EOF) analyses of eddy streamfunction and velocity potential departures from their multiple-summer averages at 200 mb [ψE(200 mb)] and χE(200 mb)] and correlating the distributions between these two variables, it is possible to identify two interannual variation modes of summertime stationary eddies. The horizontal structure of these two interannual variation modes, which differs from the Pacific-North American (PNA) teleconnection pattern, resembles the Matsuno-Gill type of tropically trapped modes. It is inferred from the coherent interannual variation of the tropical Pacific SST and ψE(200 mb) anomalies that a dynamic coupling exists between interannual variations of the Pacific SST and summertime stationary eddies. The coherent eigencoefficient time series of χOLR, the χE(200 mb) and ψE(200 mb) anomalies, the EOF analysis of the ψE(200 mb) budget, and numerical simulations of the response of summertime stationary eddies to the interannual variation of tropical forcing all reveal that the aforementioned dynamic coupling is accomplished through the interannual variation of the global-scale divergent circulation.

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Tsing-Chang Chen and Ming-Cheng Yen

Abstract

Previous studies examining the seasonal contrast and spatial structure of the interdecadal variation in the Southern Hemisphere (SH) atmospheric circulation and the relationship between this interdecadal variation and cyclone activity have been extended using the SH data generated by the Australian Bureau of Meteorology for 1972–92. The major findings of this study are the following:

  1. In addition to the deepening of the circumpolar trough and the increase of midlatitude sea level pressure (SLP) in winter, a zonal wavenumber 3 pattern appears in the high middle latitudes. During summer, the interdecadal change is dominated by three latitudinal zones of SLP anomalies with negative values in low and high latitudes and positive values in midlatitudes.

  2. A zonal wavenumber 3 pattern that develops during the winter persists into early spring, while the three latitudinal SLP anomaly zones that emerge in late spring and early summer continue on through the summer.

  3. An enhancement of cyclone activity is associated with the deepening of the SH circumpolar low centers.

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Tsing-Chang Chen, Ming-Cheng Yen, and Masato Murakami

Abstract

In this study, we examine the water vapor transport over the entire Asian monsoon region using the FGGE III-b data of the European Centre for Medium Range Weather Forecasts (ECMWF) and the Geophysical Fluid Dynamics Laboratory (GFDL). Effort is made to explore the possible effect of the 30–50 day oscillation on water vapor transport. We apply the empirical orthogonal function (EOF) analysis to the potential and stream-function of the vertically integrated water vapor transport over a region (25°S–60°N, 40°E–130°W). It is found that the water vapor transport over the Indian and East Asian monsoon regions are related to each other through their interactions with the eastward propagating 30–50 day oscillation. Several occurrences follow the 30–50 day oscillation of the Indian monsoon westerlies in a coherent manner: The east-west shifting of the water vapor convergence center over the Asian monsoon regions; the north-south movement of the North Pacific convergence zone (NPCZ); and the merging of the water vapor flux transported by the Indian monsoon westerlies and the Pacific anticyclone trade winds over the South China Sea. The development of the East Asian monsoon is characterized by the north-south movement of water vapor fronts, such as the Mei-Yü and Baiu fronts, associated with the NPCZ. This study reveals that this north-south movement is caused by the eastward propagation of the 30–50 day mode.

We also apply the EOF analysis to the global potential function of the water vapor transport between 45°S and 45°N over the entire FGGE year. It is suggested that the phase relationship between the annual cycle and the 30–50 day mode of the potential function of the water vapor transport can be used to explain the development of the large-scale environment for the Indian monsoon over its entire life cycle. It is also inferred from the time lag between onsets of the Indian and East Asian monsoon that the two monsoons may be induced by the same mechanism, namely, the interaction between the annual cycle and the 30–50 day low-frequency mode.

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Tsing-Chang Chen, Ming-Cheng Yen, and Siegfried Schubert

A substantial amount of precipitation in the midlatitudes occurs in association with extratropical cyclones. Using the data generated by version 1 of the Goddard Earth Observing System (GEOS-1) Data Assimilation System for 1985–89, hydrologic processes and the water vapor budget over the United States were analyzed to illustrate the maintenance of precipitable water and precipitation associated with extratropical cyclones. The area-mean divergence of water vapor flux covering the Great Plains and the eastern region of the United States (80°–105°W, 30°–50°N) was adopted as a hydrologic index. The cyclones over this region that have values of this index smaller than minus one standard deviation over a season were selected for analysis. On average, 15 cases were selected for each season. The composite results show a developing baroclinic wave coupled with a low-level cyclone in which the low-level convergent (divergent) center and the upper-level divergent (convergent) center ahead of the trough (ridge) are linked by an upward (downward) branch of the divergent circulation, consistent with the classic cyclone model. Thus, water vapor converges (diverges) through the low-level divergent circulation of the cyclone wave to maintain precipitation (evaporation) centers ahead of the trough (ridge). It is estimated that the amount of water vapor accumulating in the Great Plains and the eastern United States throughout winter (November–March) could be converged by typical cyclones within a month. During summer (May–September), it would take only about half a month for typical cyclones to converge water vapor toward this region sufficient to account for the summer season runoff by streamflow over this region.

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Tsing-Chang Chen, Ming-Cheng Yen, and Shu-Ping Weng

Abstract

The summer monsoons in East and Southeast Asia are characterized, respectively, by the Mei-yu (in eastern China)–Baiu (in Japan) front (MBF) and by the monsoon trough stretching from northern Indochina to the Philippine Sea. These two major monsoon elements are separated by the North Pacific anticyclone. As indicated by the 850-mb zonal wind and cumulus convection over some key areas, a distinct opposite-phase intraseasonal variation exists between the two monsoon elements. Two approaches are adopted to explore the cause of this opposite-phase variation (which reflects the coupling between the two monsoon components): 1) the correlation coefficient patterns between the 850-mb zonal-wind monsoon index and the 850-mb streamfunction field and 2) the composite 850-mb streamline charts and the 120°E zonal-wind cross sections. It is shown that the opposite-phase variation between the two monsoon elements is caused by the anomalous circulation associated with the northward-migrating 30–60-day monsoon trough/ridge from the equator to 20°N and with the westward-propagating 12–24-day monsoon low–high along the latitude of ∼15°–20°N. Results obtained in this study are used to address two often discussed phenomena of the East Asian monsoon: 1) the rapid northward shift of the MBF across the Yangtze River basin during the Mei-yu onset is related to the north–south meridional oscillation of the MBF, and 2) the three longitudinally oriented location zones of extremely heavy rain events in eastern China are formed by the alternation of deep cumulus convection zones associated with the intraseasonal monsoon vortices (centered in the northern part of the South China Sea) between extreme monsoon conditions.

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