Spring Heavy Rain Events in Taiwan during Warm Episodes and the Associated Large-Scale Conditions

VOLUME 131 MONTHLY WEATHER REVIEW JULY 2003 Spring Heavy Rain Events in Taiwan during Warm Episodes and the Associated Large-Scale Conditions GEORGE TAI-JEN CHEN, ZHIHONG JIANG,* AND MING-CHIN WU Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan, China (Manuscript received 4 April 2002, in final form 6 November 2002) ABSTRACT Daily rainfall data at 15 stations of the Taiwan Central Weather Bureau (CWB) and the gridded dataset of the National Centers for Environmental Prediction National Center for Atmospheric Research (NCEP NCAR) reanalysis during the period of February April 1951 2000 were used to reveal the characteristics of temporal and spatial variations of spring rainfall over Taiwan in relation to the sea surface temperature (SST) over the Niño-3 (5 S 5 N, 90 150 W) area. Extremely heavy rain events during a warm episode were selected to study the characteristics of the associated large-scale circulations. Results showed that the spring rainfall in Taiwan was positively correlated to the Niño-3 SST not only for the overall rainfall events but also for the heavy rain events. Extremely heavy rain events occurred significantly more frequently during warm episodes as compared to those occurring during cold and normal episodes. A varimax-rotated empirical orthogonal function (REOF) analysis revealed the existence of two spatial modes, with one over northern Taiwan and the other over southern Taiwan. It was found that the intrusion of the midlatitude frontal system into the eastern China coastal area coupled with the mean state of the Pacific East Asian teleconnection pattern was primarily responsible for the extremely heavy rain events during springtime warm episodes. 1. Introduction Taiwan is an island located in the subtropical latitudes (22 26 N) to the southeast of China and is regulated by the East Asian monsoon with prevailing northeasterlies in winter and southwesterlies in summer. The seasonal rainfall was largely controlled by the monsoonal flows and modified by the local topography (Yen and Chen 2000). The onset of the summer southwesterly monsoon generally occurred in the middle of May as the mei-yu front established its quasi-stationary position over southern China, Taiwan, and the western North Pacific (Tao and Chen 1987; Chen 1988, 1994; Chang and Chen 1995). Climatologically, the mei-yu front affected Taiwan for about one month from mid-may to mid-june and produced a seasonal rainfall peak in the mei-yu season around the island (Chen and Wu 1978; Wang et al. 1984). Before the mei-yu season, monthly rainfall showed that the late winter and spring seasons (February April) are relatively dry particularly over * On leave from Department of Environmental Science, Nanjing Institute of Meteorology, Nanjing, China. Corresponding author address: Dr. George Tai-Jen Chen, Dept. of Atmospheric Sciences, National Taiwan University, 61, Ln. 144, Sec. 4, Keelung Rd., Taipei, Taiwan 10772, China. E-mail: george@georgez.as.ntu.edu.tw southwestern Taiwan to the lee side of the high Central Mountain Range under the prevailing northeast monsoon flows (Fig. 1). This was also observed by Yen and Chen (2000). Although February is still in the winter season and exhibits its circulation characteristics, the precipitation distribution over Taiwan is very much like that for the spring season. Therefore, the rainfall in February April is generally called spring rainfall in this study. Pronounced interannual variation of spring rainfall is an important meteorological issue because it is closely related to the availability of water resources in the dry season. Within this interannual variation mode, drought and heavy rain events are not uncommon phenomena locally or islandwide. The spatial variation of the spring rainfall, perhaps primarily due to the topographical effects, is also an interesting topic to be explored in addition to the interannual variation. Therefore, the temporal and spatial variations of the spring rainfall will be studied in this paper. The summer monsoon rainfall over East Asia was observed to be closely related to ENSO (e.g., Huang and Wu 1989; Liu and Ding 1992; Tanaka 1997) and to the western Pacific warm pool SST (Huang and Sun 1992). It was also found that the East Asia winter monsoon tends to be weaker and warmer (Ropelewski and Halpert 1987; Halpert and Ropelewski 1992; Tomita and Yasunari 1996; Zhang et al. 1996; Wang et al. 2000) and springtime tends to be wetter over East Asia (Kang 2003 American Meteorological Society 1173

1174 MONTHLY WEATHER REVIEW VOLUME 131 2. Data and analyses FIG. 1. Locations of the surface rainfall stations used and topography (m) over Taiwan. and Jeong 1996; Tao and Zhang 1998; Wang et al. 2000) during the mature phase of ENSO. Wang et al. (2000) found that ENSO exerted a significant impact on the spring rainfall along the East Asian polar front from southern China via the east China Sea to the Kuroshio extension region. The relationship of this spring rainfall to the ENSO event was manifested in the so-called Pacific East Asian teleconnection pattern (PEA) in the surface vorticity wave pattern and the surface streamfunction anomaly field that started from the central Pacific and extended poleward and westward to East Asia. During the mature phase of ENSO, a pair of anomalous surface anticyclones formed over the western Pacific with one center located over the Philippine Sea well within the Tropics. Although this pattern became weaker, it still persisted through the ensuing spring. It will be interesting to learn if this anomalous low-level high pressure pattern modulates the spring rainfall in Taiwan. Although a close relationship between the spring rainfall amount over Taiwan and ENSO was documented (Wu and Chen 1996), the relationship between the spring heavy rain events and ENSO has not been studied to our knowledge. What are the characteristics of the largescale circulations associated with the heavy rain events if they occurred during ENSO? What are the roles of PEA pattern and the midlatitude transient synoptic disturbances during the heavy rain events? These are some of the interesting questions related to the spring rainfall over Taiwan to be answered and they will be considered in this study. Daily rainfall data at 15 stations of the Taiwan Central Weather Bureau (CWB) (Fig. 1) observed in February April 1951 2000 were used to reveal the characteristics of temporal and spatial variations of spring rainfall. The dataset used to analyze the large-scale environmental conditions is the National Centers for Environmental Prediction National Center for Atmospheric Research (NCEP NCAR) global atmospheric reanalysis dataset covering a 50-yr period from February to April 1951 2000. A detailed description of the data assimilation system that produces this dataset was given by Kalnay et al. (1996). The data assimilation system was based on the global forecast model that was implemented operationally at NCEP in January 1995. The daily data at standard pressure levels in 2.5 latitude longitude grids were used in this study. The monthly sea surface temperature (SST) in 2 latitude 2 longitude grids used is the NCEP NCAR reanalysis dataset for 1951 2000, which was specified using the reconstructed monthly mean SST by Reynolds and Smith (1994). As reflected in the mean sea surface temperature anomaly (SSTA) over the central and eastern equatorial Pacific, either the El Niño or the La Niña event usually reached its mature phase in the cold season of November March (e.g., Ropelewski and Halpert 1987; Wang et al. 2000). This SSTA was suggested to be responsible for the cooling over the tropical western Pacific as well as the generation of the PEA pattern. Therefore, the SSTA in the cold season over Niño-3 (5 S 5 N, 90 150 W) was used to identify the warm, cold, and normal episodes. One standard deviation of SSTA (0.7 C) appeared to be a reasonable choice for differentiating warm versus cold episodes as compared to the season episodes issued by the National Oceanographic and Atmospheric Administration s Climate Prediction Center (NOAA/CPC; information available online at http:// www.cpc.noaa.gov). In this study, warm, cold, and normal episodes are thus defined by Niño-3 SSTA 0.7 C, SSTA 0.7 C, and 0.7 C SSTA 0.7 C, respectively. It turns out that the event years of warm episodes include 1957/58, 1965/66, 1968/69, 1972/73, 1982/83, 1986/87, 1987/88, 1991/92, 1994/95, and 1997/98 during the period 1951 2000. The cold-episode event years include 1950/51, 1954/55, 1955/56, 1970/ 71, 1973/74, 1975/76, 1984/85, 1988/89, 1995/96, 1998/99, and 1999/2000. And the other years belong to the normal episodes. To examine the existence of teleconnection between the spring rainfall and the SST, the correlation coefficient between SST over Niño-3 and rainfall amount, rainfall days, heavy rainfall amount, and heavy rainfall days were analyzed at each station. In this regard, the ratios of each yearly spring rainfall to the long-term spring mean rainfall during the warm, cold, and normal episodes at each station were also analyzed. A heavy rainfall day in this study was defined as having a daily

JULY 2003 CHEN ET AL. 1175 FIG. 2. Time series of the ratio of the averaged spring rainfall (Feb Apr) over 15 stations to the long-term (1951 2000) averaged spring rainfall over the same stations in Taiwan. Std dev of the ratio is 0.423. rainfall amount greater than the long-term mean by 1.5 standard deviations at each station. To better reveal the spatial variation, a varimax-rotated empirical orthogonal function (REOF) analysis was performed (Richman 1986). A rotated EOF mode refers to a linear transformation of the initial EOF modes utilizing the varimax method. The varimax methods maximizes the variance of the squared correlation coefficients between the time series of each REOF mode and each of the original time series. This causes the loadings of the REOF modes to be widely distributed, with a few large loadings and many close to zero. Hence, REOF analysis reveals simple patterns and increases the discrimination among the loadings and makes them easier to interpret. The remarkable usefulness of the REOF analysis to reveal the spatiotemporal variability of the anomaly pattern has been well documented by various researchers (e.g., Horel 1981; Richman 1986; Kawamura 1994). Finally, the large-scale environmental conditions were composited for each mandatory levels to reveal the general characteristics of flow patterns associated with the extremely heavy rain events during the warm episode over northern and southern Taiwan. 3. Temporal and spatial variations of spring rainfall A time series of the ratio between the averaged spring rainfall for 15 stations to the long-term mean spring rainfall for the same stations was constructed to illustrate the interannual variability of the spring rainfall over Taiwan (Fig. 2). Obviously large interannual variations existed in the spring rainfall over Taiwan. It is clear that the rainfall deviated remarkably from the longterm mean for a majority of the years. The interannual variability showed an obvious change in the mid-1970s with a greater variation and more wetness for the period 1978 2000 as compared to the period 1951 77. This phenomenon might be related to the interdecadal variations of East Asian climate and the tropical Pacific SSTs as discussed by Chang et al. (2000a,b) and will be illustrated in a companion paper (Jiang et al. 2003). The unusually wet years of 1983, 1988, 1992, and 1998 correlated well with the warm episodes identified in this study. This relationship appeared to be more pronounced in the period after 1978, particularly the first (1983) and second (1998) wettest springs corresponded nicely with the two strong ENSO events of 1982/83 and 1997/98 in the twentieth century. The dry years of 1955, 1971, 1976, and 1999 also appeared to correspond with cold episodes, although to a lesser degree as compared to that between wet years and warm episodes. It is quite clear that the spring rainfall over Taiwan was much more abundant during warm episodes (El Niño) as compared to that during cold (La Niña) and normal episodes. Since the spring rainfall appeared to vary coherently with the SST over the Niño-3 region, the question of how this relationship varied spatially deserves further exploration. The spatial distribution of various correlation coefficients between spring rainfall and SST is presented in Fig. 3. Positive correlation existed between the rainfall amount and SST over all stations (Fig. 3a) with a mean correlation coefficient of 0.36. The correlation coefficients over northern, central, and southwestern Taiwan were much higher than the mean value with a 99% significance level (i.e., 0.35). The correlation between the rainfall days and SST (Fig. 3b) shows a similar pattern although with a somewhat lower value. Again, the correlation coefficients over northern, central, and southwestern Taiwan were much higher than the mean value of 0.2. Similar features existed in the correlation coefficients between heavy rainfall amount and SST and heavy rainfall days and SST, as presented in Figs. 3c and 3d, respectively. It is interesting to note that the correlation coefficients were greater than 0.5 at some stations over north, central, and southwestern Taiwan. Apparently, the heavy rain events over these areas are highly correlated with the SST over the Niño-3 region. To explore the question of how the spring rainfall varied spatially during different SST episodes, the distribution of the ratio of spring rainfall to the long-term spring mean rainfall is presented in Fig. 4. It is clear that the spring rainfall during the warm episode was much greater than the long-term mean at all the stations, particularly over northern, central, and southwestern Taiwan. Less rainfall was observed during the cold and normal episodes at most of the stations except for a few stations over eastern and southern Taiwan. This result is consistent with the correlation coefficient analyses presented in Fig. 3, which shows that spring rainfall was positively correlated to the SST and exhibited spatial variations. In other words, more rainfall and more heavy rain events during the warm episode would be expected, particularly over northern, central, and southwestern Taiwan. In view of the high correlation over these areas, the question arises regarding the existence of the spatial variation of the spring daily rainfall. Therefore, the

1176 MONTHLY WEATHER REVIEW VOLUME 131 FIG. 3. Correlation coefficient between SST over Niño-3 (50 S 5 N, 90 150 W) and (a) rainfall amount, (b) rainfall days, (c) heavy rainfall amount, and (d) heavy rainfall days over Taiwan in the spring seasons (Feb Apr) of 1951 2000. Numbers indicate the correlation coefficients in 0.01 units and Rave is the averaged value. REOF technique was employed to further study the spatial mode. The REOF eignen mode illustrates the simple structure of the spatial variation. The spatial distributions of the first and the second REOF modes are presented in Fig. 5. A region of high loadings in mode 1 of REOF was found over central and southwestern Taiwan (designated as southern Taiwan to be differentiated from northern Taiwan). On the other hand, a region of high values in mode 2 of REOF was found over northern Taiwan. Clearly, the spring rainfall over Taiwan possessed two distinct spatial modes with one over northern Taiwan and the other over southern Taiwan. These two areas were also expected to have more heavy rain events during the warm episode as indicated in Fig. 3d. As would be expected, these two principal components were orthogonal to each other and the heavy rain events

JULY 2003 CHEN ET AL. 1177 FIG. 4. The ratio of spring rainfall to the long-term spring mean rainfall (1951 2000) during (a) warm, (b) cold, and (c) normal SST episodes. Numbers indicate percentage (%). should occur at the peaks in the time series. Therefore, the characteristics of large-scale environmental conditions associated with heavy rain events that occurred during warm episodes for these two spatial modes are worth exploring and this will be discussed in a later section. 4. Spring heavy rain events and SST As indicated in Fig. 3d, the spring heavy rainfall days (defined as daily rainfall greater than long-term daily mean by 1.5 standard deviation) were positively correlated to the SST over the Niño-3 area. Also, as illus-

1178 MONTHLY WEATHER REVIEW VOLUME 131 FIG. 5. Factor loadings (units in 10 2 ) of the (a) first and (b) second REOF modes of daily rainfall anomalies in the spring seasons of 1951 2000. PVE indicates the percentage variance explained by the REOF modes. Southern Taiwan (S) and northern Taiwan (N) are shaded. trated by REOF analyses in Fig. 5 the spring rainfall possessed one spatial mode over northern Taiwan and the other mode over southern Taiwan. Therefore, it is interesting to know specifically how the heavy rain events did over these two areas associate with the SST over Niño-3. This is presented in Fig. 6 for the top 50 and top 25 daily rainfall events for both northern and southern Taiwan cases. The daily rainfall was computed by averaging all the stations in each area as presented in Fig. 5 and was obtained by using a 1 2 1 running mean. It is clear that extreme heavy rain events occurred much more frequently during the warm episodes at a 95% significance level as compared to those that occurred during either cold or normal episodes for both northern and southern Taiwan cases. Both areas could expect about two events to occur annually during the warm episode among the top 50 daily rainfall events. About one event would be expected to occur annually among the top 25 daily rainfall events. During the warm episode, there were eight cases among the top 50 daily rainfall events that occurred solely over northern Taiwan and another nine cases solely over southern Taiwan (Table 1). To alleviate the seasonal march of the mean state in the composite, five out of eight cases over northern Taiwan in February and five out of nine cases over southern Taiwan in April were selected (Table 2) to reveal the characteristics of the associated large-scale environmental conditions. The cases selected had a daily rainfall anomaly close to or greater than three standard deviations and can be considered as extremely heavy rain events in spring. The averaged date of the five cases over northern Taiwan was 11 February and it was 13 FIG. 6. Annual expectation of the number of event occurrences (y axis) for the (a) top 50 and (b) top 25 daily rainfall events in northern and southern Taiwan during the warm, cold, and normal SST episodes in 1951 2000. Dashed lines indicate the 95% significance level.

JULY 2003 CHEN ET AL. 1179 TABLE 1. Number of events among the top 50 daily rainfall events occurring over northern and southern Taiwan in warm episodes in Feb Apr of 1951 2000. Region Month Northern Taiwan Feb Mar Apr Southern Taiwan Feb Mar Apr No. 5 1 2 2 2 5 April for southern Taiwan case. Thus, the heavy rain event selected for the northern Taiwan case occurred in the early spring, whereas for the southern Taiwan case it occurred in the late spring. In view of the fact that the spring rainfall for both the northern and southern Taiwan cases was positively correlated to the Niño-3 SST as discussed in the previous section, question arises regarding how did the basin-wide Pacific SST correlate to the Niño-3 SST in spring, particularly during the heavy rain events. Figure 7a presents the distribution of correlation coefficients between SST and Niño-3 SST in spring. A basin-wide pattern existed with a negative correlation area located in a SW NE direction over the central Pacific and a positive area extending from the South China Sea northeastward to the western North Pacific including the East China Sea. Thus, the spring rainfall over Taiwan was not only correlated positively to the Niño-3 SST but also to the SST over the South China Sea and the western Pacific including the East China Sea. A similar pattern was found in the correlation between the Niño-3.4 SST (5 S 5 N, 120 170 W) and the spring rainfall over the area from southern China and the northern South China Sea via the East China Sea to the Kuroshio extension by Wang et al. (2000). Thus, the interannual variation of spring rainfall over Taiwan is not a local phenomenon but is part of the regional climate that correlated positively to the SST. The SST anomaly distributions were very similar to each other for the heavy rain events over northern and southern Taiwan (Figs. 7b and 7c) and were also similar to the distribution of the correlation coefficient presented in Fig. 7a. Positive anomalies existed over the area from the South China Sea northeastward to the western North Pacific including the East China Sea. Therefore, positive correlation between the SST over this area and the Niño-3 SST would also be expected for the heavy rain events over Taiwan during the warm episode. 5. Conditions associated with heavy rain events a. Synoptic situations Five cases of extremely heavy rain events over northern Taiwan in February were selected (Table 2); the 1951 2000 long-term mean on the event day in February will serve as the climatological mean state. Similarly, the long-term mean on the event day in April will serve as the mean state for the extremely heavy rain events over southern Taiwan. Five cases of extremely heavy rain events over northern Taiwan and another five cases over southern Taiwan, as presented in Table 2, were averaged with respect to the event day to obtain the composite field. Synoptic situations at different mandatory levels will be discussed using the composite maps and the anomaly fields of the composite from the climatological mean state. Figure 8 presents the geopotential height and wind fields at 1000 hpa. It would be expected that the difference in climatological mean state for the northern and southern Taiwan cases was primarily a reflection of the seasonal difference between the early and late spring. For example, the intensities of the Mongolian anticyclone and the accompanied northeast monsoon over Taiwan and the South China Sea and the trough to the east of Japan for the northern Taiwan case were much stronger as compared to that for the southern Taiwan case. The synoptic patterns at the surface for the northern Taiwan case were quite similar to those occurring during the winter monsoon cold surges (e.g., Boyle and Chen 1987). During the heavy rain events, a frontal trough was located from east of Japan southwestward, passing through the Bashi Channel to the northern South China Sea. Anomalous anticyclones developed over the North Pacific and extended southwestward to the Philippine Sea such that the strong southwesterly anomalous flows developed to the southeast of this frontal trough particularly over the northern South China Sea and Taiwan area. Strong southwesterly flow was an important ingredient during the extremely heavy rain event for supplying the moisture from the TABLE 2. The five extremely heavy rain events occurring in warm episodes over northern Taiwan in Feb and southern Taiwan in Apr. The daily precipitation is the area average over four and five stations in the northern and southern Taiwan cases, respectively. Normalized anomaly is the daily rainfall anomaly divided by the spring (Feb Apr) standard deviation. Region Northern Taiwan Southern Taiwan Date Events 23 Feb 1998 20 Feb 1983 6 Feb 1983 6 Feb 1958 2 Feb 1983 Precipitation (mm) 43.6 35.6 32.4 29.4 29.3 Normalized anomaly 4.76 3.75 3.35 2.99 2.97 Date 9 Apr 1973 14 Apr 1987 20 Apr 1988 1 Apr 1995 22 Apr 1992 Precipitation (mm) Normalized anomaly 45.4 6.10 44.7 5.99 29.4 3.78 29.2 3.74 27.6 3.52

1180 MONTHLY WEATHER REVIEW VOLUME 131 FIG. 7. (a) Correlation coefficients between the SST and Niño-3 SST in the spring season analyzed at 0.2 intervals. Heavy solid line indicates zero correlation. Areas with a 95% significance level are shaded. SSTAs ( C) on the event month of the five extremely heavy rain events occurring over (b) northern Taiwan and (c) southern Taiwan. Positive areas of SSTA are shaded. tropical area. It was suggested by Wang et al. (2000) that an anomalous lower-tropospheric anticyclone located in the western North Pacific is the key system that bridges the warm events in the eastern Pacific and the weak East Asian winter monsoons. A similar situation occurred in this case as the anomalous southwesterly flows over Taiwan and the South China Sea developed and thus the northeasterly flows over the same areas were replaced by the southerly flows or relatively weak northeasterly flows. This seems to suggest that the PEA during the mature phase of ENSO as proposed by Wang et al. (2000) also existed for the spring extremely heavy rain event during the warm episode. Another important feature was the movement of the frontal trough from

JULY 2003 CHEN ET AL. 1181 FIG. 8. The 1000-hPa geopotential heights (gpm) and wind vectors (m s 1 ) of long-term mean, composite, and anomalies for the five extremely heavy rain events occurring over (a) northern Taiwan and (b) southern Taiwan. Areas with negative anomalies of geopotential heights are shaded. Heavy dashed line indicates frontal trough line. the northwest into the area. The negative geopotential height anomaly associated with this midlatitude synoptic-scale transient system was located over the eastern China coastal area. This negative anomaly coupled with the positive anomaly over the Philippine Sea was the key feature in generating strong anomalous southwesterly flows favorable for producing extremely heavy rain. The synoptic situation at 850 hpa is presented in Fig. 9. Unlike that at 1000 hpa, the climatological mean state over Taiwan and its vicinity for the northern Taiwan case was quite different from that for the southern Taiwan case. Over this area, an anticyclonic circulation system was observed for the former case, whereas for the latter case strong southwesterly flows prevailed. A frontal trough extended from Japan southwestward passing through the East China Sea to southern China for

1182 MONTHLY WEATHER REVIEW VOLUME 131 FIG. 9. As in Fig. 8 except for 850 hpa. both cases. The trough position shifted northwestward from its position at 1000 hpa indicating a typical baroclinic midlatitude system. Again, strong southwesterly flows prevailed to the southeast of this trough particularly over the northern South China Sea and Taiwan and were important for producing extremely heavy rain events. The distribution of geopotential height anomalies over the Philippine Sea and the eastern China coastal area and the accompanied strong anomalous southwesterly flows were very similar to those observed at 1000 hpa. At 500 hpa, the climatological mean state was quite similar for both cases except that the westerlies over Taiwan and its vicinity were stronger for the northern Taiwan case than for the southern one (Fig. 10). However, the synoptic situation during the heavy rain event was quite different. For the northern Taiwan case, the trough was located to the northwest of Taiwan such that strong southwesterlies prevailed over Taiwan and its vicinity. On the other hand, the trough was located to the immediate east of Taiwan and the Bashi Channel and only relatively weak westerlies were observed over

JULY 2003 CHEN ET AL. 1183 FIG. 10. As in Fig. 8 except for 500 hpa. the same area for the southern Taiwan case. The pattern of geopotential height anomalies was similar to that at 850 hpa with a positive anomaly over the Philippine Sea and a negative anomaly over the eastern China coastal area. An exception for the southern Taiwan case was that an area of negative anomaly occurred over southwestern China such that anomalous cyclonic flows prevailed over Taiwan and its vicinity. In general, the mean state at 200 hpa was quite similar for both cases except the circulation intensity was different (Fig. 11). A westerly jet streak was located over East Asia with a much stronger intensity for the northern Taiwan case as compared to that for the southern one. However, Taiwan was located on the anticyclonic side of the entrance region of the jet streak for both cases. Therefore, stronger upward motion would be expected from the stronger upper-level jet streak forcing for the northern Taiwan case (Uccellini and Johnson 1979). The jet streak intensity during the heavy rain event was stronger than the climatological mean state for both cas-

1184 MONTHLY WEATHER REVIEW VOLUME 131 FIG. 11. As in Fig. 8 except for 200 hpa with 40, 50, 60, and 70 m s 1 isotaches (shaded) in the long-term mean and composite maps. es, indicating that the upper-level jet streak forcing was an important factor during extremely heavy rain events by providing a lifting mechanism. Obviously, the stronger upward motion associated with the upper-level jet streak would be expected for the northern Taiwan case as compared to that for the southern one. The pattern of the geopotential height anomaly observed in the lower and midtroposphere was still discernible at 200 hpa. A positive anomaly was observed over the Philippine Sea and a negative anomaly again occurred over the eastern China coastal area. From the previous discussion on synoptic situations, it is clear that the midlatitude transient baroclinic system was an important factor for the extremely heavy rain events in both cases. The question arises regarding as to how this transient system affected the Taiwan area during the extremely heavy rain events in both cases. To illustrate the characteristics of this transient system, pressure surface time cross sections for the northern Taiwan case (25 N, 120 E) and the southern Taiwan case (22.5 N, 120 E) for the period from 3 days before to 3 days after the event day (day 0) are presented in Fig.

JULY 2003 CHEN ET AL. 1185 12. As indicated by the cyclonic wind shear and baroclinicity, the weak northeasterly wind strengthened at the surface and southwesterly wind veered at 850 and 700 hpa during the passage of the frontal trough. The frontal trough with relatively strong baroclinicity in the lower troposphere below 700 hpa passed northern Taiwan during the extremely heavy rain event day for the northern Taiwan case, whereas for the southern Taiwan case, the frontal trough with relatively weak baroclinicity in the lower troposphere passed southern Taiwan right after the extremely heavy rain event day. In other words, extremely heavy rain occurred during the frontal passage for the northern Taiwan case and occurred before the passage of a frontal system for the southern Taiwan case. The different intensities of the frontal systems for both cases were perhaps primarily a reflection of the seasonal difference between early spring (February) for the northern Taiwan case and late spring (April) for the southern Taiwan case. Also, the atmosphere was potentially stable for the former case and potentially unstable for the latter case as indicated in Fig. 13. b. Moisture flux and vertical motion As discussed previously, the anomalous southwesterly flows over the northern South China Sea and Taiwan to the southeast of the frontal trough were important for the extremely heavy rain by providing moisture from the tropical area. It is interesting to examine the distribution of the moisture flux and the flux convergence in the lower troposphere during the extremely heavy rain event. The strong southwesterly moisture flux occurred to the southeast of the 850-hPa frontal trough, particularly over Taiwan and the northern South China Sea for both cases (Fig. 14), as would be expected. This strong southwesterly moisture flux converged to the southeast of the frontal trough to form a band of maximum flux convergence. The maximum band extended from southern China and the northern South China Sea east-northeastward, passing through the Taiwan area to the western North Pacific. Therefore, the existence of the lower-tropropheric frontal system and the accompanied moisture flux convergence appeared to be instrumental in producing the extremely heavy rain event. The moisture flux convergences during the extremely heavy rain events for the northern and southern Taiwan cases are presented in Fig. 15a. For the northern Taiwan case, the flux convergence increased dramatically from 2 days before to 1 day before the event day and maintained a high value on the event day. It then decreased rapidly after the event day in agreement with the frontal passage as indicated in Fig. 12a. For the southern Taiwan case, the flux convergence increased gradually from 2 days before to 1 day before, and then increased dramatically to reach a maximum on the event day. Again, it decreased rapidly after the event day in agreement with the frontal passage as indicated in Fig. 12b. In FIG. 12. Pressure surface time cross section from 3 days before ( 3) to 3 days after ( 3) the event day (0) for temperature anomalies ( C) and winds at (a) 25 N, 120 E for the five extremely heavy rain events over northern Taiwan and (b) 22.5 N, 120 E for the five extremely heavy rain events over southern Taiwan. Pennants represent 25ms 1, full wind barb 5ms 1, and half-wind barb 2.5 m s 1. Heavy dashed lines indicate the frontal position. agreement with the variation of the moisture flux convergence, the rainfall amount increased dramatically from 1 day before to reach a maximum on the event day and then decreased rapidly after the event day (Fig. 15b). The stronger moisture flux convergence on the event day together with the potentially unstable atmosphere as indicated in Fig. 13b were consistent with the greater rainfall amount observed for the southern Taiwan case. Figure 16 presents the vertical motion at 700 hpa during the extremely heavy rain event. It is clear that a band of maximum upward motion was located to the south of the 850-hPa frontal trough. The maximum band extended from southern China eastward, passing through Taiwan to the western North Pacific. The area of maximum upward motion was approximately collocated with the area of maximum moisture flux convergence as presented in Fig. 14. Therefore, the existence of the lower-tropospheric frontal system is instrumental in the heavy rain event by providing maximum moisture flux convergence and maximum upward motion. The temporal variations of vertical motion for the extremely heavy rain event are presented in Fig. 17 for the northern and southern Taiwan cases. Upward motion

1186 MONTHLY WEATHER REVIEW VOLUME 131 FIG. 13. As in Fig. 12 except for equivalent potential temperature (K). increased dramatically from 2 days to 1 day before the event day and reached a maximum value of 12 10 2 Pa s 1 right over the lower-tropospheric front on the event day for the northern Taiwan cases. Upward motion associated with the across-front thermally direct secondary circulation appeared to be an important lifting mechanism for extremely heavy rain events. For the southern Taiwan case, upward motion prevailed before the event day and increased more rapidly from 1 day before to reach a maximum value of 8 10 2 Pa s 1 at 700 hpa on the event day ahead of the lowertropospheric front. Again, upward motion associated with the across-front thermally direct secondary circulation was an important lifting mechanism for extremely heavy rain events. The maximum upward motion on the event day was much greater for the northern Taiwan case. This was consistent with a stronger upper-level jet at 200 hpa (Fig. 11) and a stronger lower-tropospheric front (Fig. 12). The temporal variation of the rainfall amount as presented in Fig. 15b is not only consistent with the temporal variation of moisture flux convergence presented in Fig. 15a but also is consistent with the upward motion presented in Fig. 17. Although the maximum upward motion on the event day was weaker for the southern Taiwan case, the greater rainfall amount was consistent with the stronger moisture flux convergence and potentially unstable atmosphere. FIG. 14. Moisture flux (qv arrows; ms 1 gkg 1 ) and moisture flux convergence ( qv; 10 5 s 1 gkg 1 ) in the 1000 850-hPa layer for the extremely heavy rain events occurred over (a) northern and (b) southern Taiwan. Flux convergence analyzed at 2 10 5 s 1 g kg 1 intervals. Heavy dashed line indicates the 850-hPa frontal trough. 6. Summary and conclusions Before the mei-yu rainy season of mid-may to mid- June, Taiwan experiences a relatively dry spring from February to April particularly over the central and southwestern regions. The spring rainfall, however, exhibits a large interannual variation such that the occurrence of drought and heavy rain events is not uncommon. Therefore, better understanding of the temporal and spatial variations of spring rainfall becomes one of the important meteorological issues that is crucial for the management of water resources. In this study, rainfall data at 15 stations from the Central Weather Bureau (CWB) of Taiwan and gridded data of meteorological parameters at different mandatory levels and the sea surface temperature from the National Centers for Environmental Prediction National Center for Atmospheric Research were used to reveal the characteristics of spring rainfall in relation to the SST over the Niño-3 (5 S 5 N, 90 150 W) area and to the large-scale circulations. From the analyses of spatial distributions of correlations between various spring rainfall parameters and the Niño-3 SST, it is clear that the spring rainfall was

JULY 2003 CHEN ET AL. 1187 FIG. 15. Time series from 3 days before ( 3) to 3 days after ( 3) the event day (0) for (a) moisture flux convergence ( qv; 10 5 s 1 gkg 1 ) in the 1000 850-hPa layer and (b) daily rainfall amount for the five extremely heavy rain events over northern Taiwan (solid line) and for the five extremely heavy rain events over southern Taiwan (dashed line). positively correlated to the Niño-3 SST, particularly over northern Taiwan and southern Taiwan (including central Taiwan). This is true not only for the overall rainfall events but also for the heavy rain events. Particularly interesting is that the heavy rain events occurred significantly more frequently during the warm episode (ENSO) as compared to those occurring during the cold and normal episodes. It was also found that the spring rainfall exhibited two distinct spatial modes, by using a varimax-rotated empirical orthogonal function analysis (REOF), with one over southern Taiwan and the other over northern Taiwan. To investigate the largescale environmental conditions associated with extremely heavy rain events for the two distinct spatial modes, five cases that occurred solely over northern Taiwan and another five cases that occurred solely over southern Taiwan were selected. Composite and anomaly fields reveal that large-scale features were quite similar to each other with only minor differences. A Pacific East Asian teleconnection mechanism (PEA) during the mature phase of ENSO, as proposed by Wang et al. (2000), appeared to exist for the springtime extreme rain event during warm episodes. The anomalously strong southwesterly flows in the lower troposphere over Taiwan and the northern South China Sea were essential for producing heavy rain events by providing abundant FIG. 16. Vertical p velocity (10 2 Pa s 1 ) on the event day at 700 hpa for the five extremely heavy rain events occurring over (a) northern and (b) southern Taiwan. Negative values (upward motions) are shaded. Heavy dashed line indicates the 850-hPa frontal trough. moisture from the tropical area. These anomalous flows were associated with the positive geopotential height anomaly over the Philippine Sea and the negative anomaly over the eastern China coastal area in the troposphere. The midlatitude transient baroclinic system was important in this regard as the lower-tropospheric frontal trough moved into the area to produce the negative geopotential height anomaly. Composite fields also showed that the band of maximum moisture flux convergence tended to collocate with the band of maximum upward motion to the south of the 850-hPa frontal trough. The moisture flux convergence and upward motion were the key factors for generating heavy rain. Therefore, it is clear that the intrusion of a midlatitude frontal system into the eastern China coastal area coupled with the mean state of the PEA teleconnection pattern were primarily responsible for the spring extremely heavy rain events during warm episode. For extremely heavy rain events over northern Taiwan, the frontal system as well as the upper-level jet streak were stronger and thus upward motion on the event day was much stronger as compared to that observed for the southern Taiwan case. This difference is perhaps primarily due to the seasonal variation between the northern Taiwan events occurring

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