Heavy rainfall/flood (HRF) cyclones contribute close to two-thirds of the total rainfall PT in both parts of Malaysia [peninsular Malaysia (M) and west Borneo (B)]. Judging by the rainfall variance produced by these cyclones and its correlation (~0.9) with the interannual PT variation, this variation is caused primarily by HRF cyclones through two factors: 1) their westward propagation properties and 2) their rain-producing efficiency. The former is regulated by the change of the cyclonic shear flow around the near-equator trough, while the latter is determined by the change of the convergence of water vapor flux toward tropical Southeast Asia. During November–December of cold (warm) ENSO phases, the westward propagation of the cyclone's parent cold surge vortices (CSVs) from the Philippine (P) vicinity (Borneo) to peninsular Malaysia CSVPMs (CSVBMs) and intensified (weakened) convergence of water vapor flux toward tropical South/Southeast Asia act to enhance (reduce) the rain-producing efficiency of HRFPM (HRFBM) cyclones. During winter cold (warm) phases, the deepening (filling) of the near-equator trough crossing west Borneo allows some CSVs formed/trapped in Borneo CSVBBs to develop into HRFBB (HRFBBM) cyclones (to propagate westward to peninsular Malaysia). The rain-producing efficiency of HRFBB and HRFBBM cyclones is also increased (reduced) by the intensified (weakened) convergence of water flux toward tropical South/Southeast Asia. Interannual variations of both PT(M) and PT(B) caused by the impacts of the circulation pattern changes on occurrences of HRFPM and HRFBB/HRFBBM cyclones, respectively, and their rain-producing efficiency may pose a new challenge to simulate the weather–climate relationship in climate modeling.
It is revealed from the seasonal rainfall variation along the coasts of the South China Sea that a rainfall maximum occurs in central Vietnam during October–November (Chen et al. 2012), in peninsular Malaysia during November–December, and over west Borneo during December–February (DJF) (Chen et al. 2013). This seasonal progression of the rainfall maximum around this sea (Fig. 1) is caused by different rain-producing system activities in response to the seasonal variation of the large-scale circulation in the Southeast Asia–western tropical Pacific region. For central Vietnam, the major rain-producing disturbances are cold surge vortices (CSVs) formed in the vicinity of the Philippines and heavy rainfall/flood (HRF) cyclones/events that develop from some of these CSVs. The major rainfall in peninsular Malaysia is produced by the CSVs from both the Philippine vicinity and Borneo and from HRF cyclones that evolve from these disturbances. Differing from the previous two regions, the major rainfall in Borneo is produced by domestically formed CSVs and in situ HRF cyclones that develop from these CSVs. About two-thirds of the rainfall maximum in central Vietnam is produced by HRF cyclones, which constitute only 20% of the total rain-producing disturbances. In Malaysia, close to 60% of the rainfall maximum in peninsular Malaysia and west Borneo is produced by HRF cyclones, whose population is less than one-third of the rain-producing weather disturbances in both regions.
North of the tropical South Pacific anticyclone, the near-equator trough migrates equatorward, following this anticyclone, during the boreal cold season (Fig. 2). This trough is located between 5° and 10°N during October–November and the monsoon northeasterlies north of this trough propagate CSVs from the Philippine vicinity to central Vietnam. In November–December, this trough line migrates farther equatorward to around 5°N. Because of the southward migration of this trough, the monsoon northeasterlies north of this trough enable CSVs to propagate from the Philippine vicinity and Borneo to peninsular Malaysia. Eventually, the near-equator trough moves closer to the equator and anchors there during winter (DJF) and traps CSVs and HRF cyclones/events in Borneo. The genesis regions of major rain-producing disturbances and their propagation properties are regulated by the seasonal variation of monsoon flows in East/Southeast Asia. The activity of the rain-producing disturbances and the rainfall produced by these disturbances downstream over central Vietnam, peninsular Malaysia, and west Borneo likely undergo an interannual variation caused by the interannual variation of the cold-season monsoon circulation over this region.
Examining the relationship between Malaysian rainfall during winter (DJF) and the El Niño–Southern Oscillation (ENSO) activity, Tangang and Juneng (2004) suggested that an anomalous cyclonic circulation over the South China Sea, northern Borneo, and the Philippine Sea may modulate the winter Malaysian rainfall. Analyzing the interannual variation of the late fall/early winter rainfall in central Vietnam from a weather–climate perspective, Chen et al. (2012) explored how the interannual variation of the large-scale circulation over the Southeast Asia–western tropical Pacific region affected the activity and rain-producing efficiency of rain-producing disturbances. The cyclonic flow around the near-equator trough modulated by the cross-Pacific shortwave train in response to the western tropical Pacific (WTP; 8°–12°N, 128°–132°E) sea surface temperature (SST) anomalies was revealed from this analysis (Chen 2002). This modulation acted to deepen the trough west of 120°E, but filled it east of this longitude during the cold ENSO phase [i.e., SST (WTP) anomalies ≥0.5°C]. The opposite situation occurs during the warm ENSO phase [SST (WTP) anomalies ≤−0.5°C]. Chen et al. (2012) found the deepened (filled) trough west of 120°E during the cold (warm) ENSO phase facilitates (hinders) the westward propagation of CSVs from the vicinity of the Philippines and enhances (reduces) the rain-producing efficiency of HRF events in central Vietnam.
To illustrate the interannual variation of late fall rainfall in central Vietnam, Chen et al. (2012) showed the latitude–time distribution of October–November rainfall along the coast from southern China, across Indochina, to peninsular Malaysia. The interannual variation of the late fall rainfall in central Vietnam is coincident with SST (WTP) anomalies, but out of phase with SST (Niño-3.4; 5°S–5°N, 170°–120°W) anomalies. Surprisingly, the rainfall in peninsular Malaysia also exhibits an interannual variation coincident with the late fall central Vietnam rainfall, although the maximum rainfall season of peninsular Malaysia is a month behind central Vietnam. In light of Tangang and Juneng's (2004) suggestion for the interannual variation mechanism of the Malaysian winter rainfall and Chen et al.'s (2012) demonstration for the interannual variation of central Vietnam rainfall by an anomalous cyclonic circulation, it seems likely a similar mechanism could work for the interannual variation of Malaysian winter rainfall. This suggested mechanism will be substantiated in terms of a new perspective through the impact of the interannual variation of the Southeast Asia–western tropical Pacific circulation on the activity and rain-producing efficiency of the rain-producing weather systems.
The major effort of the present study is to explore the characteristics and cause for the interannual variation of the Malaysian winter rainfall. The entire study is outlined as the following. Data used in our analysis and identification, formation, synoptic condition, rainfall, and characteristics of rain-producing disturbances over Malaysia during November–February from 1979 to 2009 have been compiled and analyzed by a companion study (Chen et al. 2013). To make the present study more self-contained, a brief description of data and identification of rain-producing distribution are provided in section 2. The major effort of this study focuses the analyses on the interannual Malaysian rainfall variations in winter through the activity of the rain-producing disturbances and rainfall amount produced by these disturbances in section 3. The impacts of the interannual variation of the low-level circulation on the rain-producing disturbance activity, rainfall amount, and hydrological conditions are presented in section 4. Concluding remarks are provided in section 5.
2. Data and identification of rain-producing disturbances
The data sources for rainfall used in the present study include land measurements by the World Meteorological Organization (WMO) surface stations (from 1979 to present), the daily gridded precipitation compiled and analyzed by the Asian Precipitation–Highly-Resolved Observational Data Integration Towards Evaluation of Water Resources [APHRODITE, generated by Yatagai et al. (2009) with a resolution of 0.25° longitude × 0.25° latitude from 1951 to 2007], and four different analyses of the global precipitation/precipitation proxy generated with measurements of atmospheric radiative properties by satellites. The precipitation/precipitation proxy datasets used in this study include 1) the Goddard Precipitation Index [GPI; Susskind et al. (1997), daily with a resolution of 1° × 1° from 1985 to 1995]; 2) the Global Precipitation Climatology Project [GPCP; Huffman et al. (1997, 2001), daily with a resolution of 1° × 1° from 1996 to 2006]; 3) the Microwave Sounding Unit [MSU; Spencer (1993), daily with a resolution of 2.5° × 2.5° from 1979 to 1996]; and 4) the Tropical Rainfall Measurement Mission [TRMM; Simpson et al. (1996), every 3 h with a resolution of 0.25° × 0.25° from 1998 to present]. The APHRODITE rainfall data are comparable to the TRMM rainfall over peninsular Malaysia, but not over Borneo. For the latter region, a 0.25° × 0.25° gridded rainfall dataset is generated by merging the WMO station rainfall measurements and other satellite precipitation/precipitation proxy data with a 16-point Bessel interpolation scheme (Jenne 1975). Using the same interpolation scheme over the oceans, different sources of precipitation/precipitation proxy data are interpolated onto the 0.25° × 0.25° grid, also.
The streamline charts used to identify rain-producing disturbances are generated with three sets of reanalysis data: 1) National Centers for Environmental Prediction (NCEP) Global Forecast System [GFS; Kanamitsu et al.(1991) and Yang et al. (2006), every 6 h with a resolution of 0.5° × 0.5° from 2004 to present], 2) European Centre for Medium Range Weather Forecast (ECMWF) Interim Re-Analysis [ERA-Interim; Dee et al. (2011), every 6 h with a resolution of 1.5° × 1.5° from 1979 to 2011], and 3) Goddard Earth Observing System Data Assimilation System, version 5 [GEOS5; Rienecker et al. (2008), every 6 h with a resolution of 0.667° longitude × 0.5° latitude from 1979 to present]. The identification of a rain-producing weather disturbance is validated by three daily surface analysis maps: 1) the NCEP tropical strip surface analysis and observations archived in the Service Record Retention System (SRRS) and analysis and forecast charts archived at the National Oceanic and Atmospheric Administration (NOAA) Operational Model Archive Distribution System (NOMADS; Rutledge et al. 2006), 2) Japan Meteorological Agency synoptic charts (JMA 2008), and 3) Thai Meteorological Department weather maps (TMD 2008).
Rain produced by four types of rain-producing disturbances is analyzed: 1) easterly wave, 2) tropical cyclone, 3) CSV, and 4) HRF event. According to Chen et al. (2013), the last two types of disturbances combined contribute 90% of the total rainfall over peninsular Malaysia during November–December and 87% of the total rainfall over west Borneo during December–February. The interannual variations of rainfall over these two parts of Malaysia are essentially attributed to the rain produced by these two types of weather disturbances. The CSV is a closed vortex formed by the interaction of an easterly wave with the cold surge flow in the Philippine vicinity and Borneo. The verification of HRF events that occurred in Malaysia are provided by two centers: 1) the Dartmouth Flood Observatory (DFO 2010) and 2) the International Emerging Disaster Dataset from the Center for Research on the Epidemiology of Disaster (CRED 2009). A backtracking verification of every identified CSV and HRF cyclone/event to its parent CSV was also conducted.
3. Interannual variation of Malaysian winter rainfall
Because the major rainfall season in central Vietnam occurs during October–November, a rainfall latitude–time diagram during this period along the east coast of Indochina was prepared by Chen et al. (2013) to show the interannual rainfall variation in central Vietnam. Surprisingly, the interannual variation of the October–November rainfall over peninsular Malaysia is coincident with that in central Vietnam. Additionally, the interannual rainfall variation in central Vietnam also follows the intensity variation of the near-equator trough in tropical Southeast Asia, opposite the ΔSST(Niño-3.4) anomalies. In fact, the maximum rainfall season in peninsular Malaysia occurs in November–December, while that over west Borneo occurs in December–February. This leads to the question of whether rainfall in these two parts of Malaysia varies interannually opposite the ΔSST(Niño-3.4) [ΔSST(WTP)] index, as does the central Vietnam rainfall, modulated by the intensity variation of the near-equator trough.
The latitude–time diagrams of rainfall and zonal wind at 850 hPa, u(850 hPa) during November–December along the east coasts of Indochina and peninsular Malaysia and during December–February along the west coasts of Palawan and Borneo are shown in the left and right columns, of Fig. 3, respectively. The interannual variation of the November–December rainfall maximum in peninsular Malaysia (Fig. 3b) appears opposite the interannual variation of ΔSST(Niño-3.4) anomalies (Fig. 3a), but it is coincident with ΔSST(WTP) and the intensities of subtropical easterlies centered at 17.5°N and equatorial westerlies centered at 2.5°N (Fig. 3c). The intensification (weakening) of north–south wind shear is reflected by the increase (decrease) of the peninsular Malaysian rainfall maximum, when the near-equator trough is deepened (filled). The December–February rainfall along the west coast of Palawan and Borneo exhibits maxima near 2.5°N (Fig. 3e), slightly south of the peninsular Malaysia maxima. The maximum easterlies (westerlies) of the near-equator trough at 12°N (south of the equator) (Fig. 3f) undergo an interannual variation opposite the interannual ΔSST(Niño-3.4) variation (Fig. 3d). Similar to the variation of the peninsular Malaysian rainfall, the west Borneo rainfall increases (decreases) when the tropical cyclonic shear flow is intensified (weakened).
In view of the interannual variation of rainfall maxima over peninsular Malaysia and west Borneo, several issues should be explored and clarified:
The maximum rainfall season over west Borneo has a 1-month phase lag behind peninsular Malaysia. The cause of the interannual rainfall variation in these two regions should be examined independently.
The major rainfall in both regions is produced by HRF cyclones (<60%). Are interannual rainfall variations in these two regions primarily caused by HRF events?
Populations of CSVs in different regions are larger than those of HRF cyclones in these regions, but Issue 2 indicates the rainfall produced by HRF cyclones should be much larger than that by CSVs. Can CSVs also significantly contribute to interannual rainfall variations in both parts of Malaysia?
How does the interannual variation of the tropical South Pacific anticyclone affect the near-equator trough in tropical Southeast Asia and the western tropical Pacific, and, in turn, modulate the rainfall variation over both parts of Malaysia?
a. Peninsular Malaysia
Populations of major rain-producing disturbances, CSVs and HRF cyclones/events, in peninsular Malaysia (M) are shown in the left column of Fig. 4, with a summary of this figure presented in the top half of Table 1. Populations of these disturbances are larger (smaller) during November–December of a cold (warm) ENSO phase. Note, here cold or warm is defined by ΔSST(Niño-3.4), thus opposite ΔSST(WTP) over the western tropical Pacific. Chen et al. (2013) observed these disturbances are originally formed in the vicinity of the Philippines (P) and Borneo (B). The simple cold/warm population stratification of rain-producing disturbances shown in the left column is further divided into those originated from the two regions—HRFPM cyclones and CSVPM1 do not appear during warm November–December, while HRFBM + HRFBBM cyclones and CSVBM dominate during this extreme ENSO phase. This cold/warm population contrast of rain-producing disturbances is likely created by the circulation change associated with the near-equator trough. As shown in Fig. 3c, the strengthening of subtropical easterlies during a cold November–December facilitates the southwestward propagation of CSVPM and HRFPM cyclones. In contrast, the weakened equatorial westerlies during the warm November–December enable CSVBM and parent CSVB of HRFBM cyclones to propagate westward. This inference will be further substantiated later, when the composite anomalous circulations are presented.
The rainfall histograms corresponding to the rain-producing disturbance population (Fig. 4) are shown in Fig. 5, with a summary presented in the bottom half of Table 1. From Table 1, an interesting contrast between the occurrence frequency of rain-producing disturbances and the precipitation produced by these disturbances emerges. The populations of rain-producing weather systems identified as HRF cyclones and CSVs, are 25% and 75%, respectively. In contrast, the HRF cyclones produce 57% of the total rainfall over the northeast Malay Peninsula, while CSVs generate only about 32%. Clearly, the major rainfall in peninsular Malaysia is produced by HRF cyclones. The correlation coefficient between the time series of total rainfall PT in this region (Fig. 5a) and PHRF rainfall produced by HRF cyclones (Fig. 5b) is about 0.86, and variances of PHRF (~105 mm) and PT (~122 mm) are very close to each other. These two factors indicate the interannual variation of the peninsular Malaysian rainfall is primarily caused by HRF cyclones/events.
The coherence between interannual variations of PT and PHRF may be attributed to two factors: 1) the rain-producing efficiency of HRF cyclones and 2) the large-scale environment favorable for maintaining the high rain-producing efficiency of HRF cyclones. The rain-producing efficiency should increase when the near-equator trough and the low-level convergence around this trough intensify. Note that rain is maintained by the convergence of water vapor flux. The higher rain-producing efficiency of HRF cyclones should be attributed to the intensified convergence of the large-scale environmental flow. This rain-producing efficiency of HRF cyclones and the impact of environmental flow on this efficiency will be addressed later.
The population ratio of rain-producing weather systems (HRF cyclones + CSVs) between cold and warm November–December is about 1.6, but their rainfall ratio is about 2.1 (Table 1). Because the intensification of the subtropical easterlies north of the near-equator trough during a cold November–December facilitates the southwestward propagation of HRFPM cyclones and CSVPM, rainfall in peninsular Malaysia during this climate condition is produced primarily by these two types of rain-producing weather systems. On the contrary, rain is produced by HRFBM cyclones, and CSVBMs are produced more often than the HRFPM cyclone and CSVPMs during a warm November–December. It may not be unique, but it is certainly unusual that the rainfall maxima in peninsular Malaysia during different climate regimes are produced by rain-producing weather systems originating from two different regions in response to opposite climate flow patterns.
b. West Borneo
The seasonal evolution of the East/Southeast Asian circulation leads to an equatorward migration of the near-equator trough, which can be shown by the contrast of the latitude–time diagrams for u(850 hPa) at 105°E (Fig. 3c) and u(850 hPa) at 112.5°E (Fig. 3f). As illustrated by the companion study (Chen et al. 2013) about winter rainfall in Malaysia, this trough deepens and anchors near the equator across Borneo during deep winter. The deepening of this trough extends vertically to the upper troposphere (Fig. 11 of Chen et al. 2013). During the cold season, the East Asian cold surge flow intrudes equatorward underneath the western Pacific subtropical anticyclone and interacts with Borneo's orography to form CSVBs. Both CSVBs and HRFB cyclones may be trapped in Borneo by some special synoptic conditions (Fig. 8 of Chen et al. 2013) developed primarily by the South China Sea–type cold surge flows. In fact, the seasonal variation of the East/Southeast Asian circulation affects not only the propagation of rain-producing weather systems, but also their occurrence frequency, rainfall amount, and rain-producing efficiency.
Following Figs. 4 and 5, the population of major rain-producing weather systems over west Borneo is shown in Figs. 6a–c, while rainfall produced by these weather systems are displayed in Figs. 6d–f. A summary of this figure is shown in the top half of Table 2. The total averaged population of rain-producing disturbances is 4.4 in Borneo, slightly larger than for peninsular Malaysia (~4.0). The major contributor to the population of rain-producing disturbances in west Borneo is the CSVB, about 3.2, (~73% of total disturbances), but HRFB cyclones average only 1.1 (~25% of total disturbances). Figures 3c and 4c show the interannual variation of CSVMs coincides with the interannual variation of intensity of the near-equator troughs. This coincidence also appears in the interannual variations of Borneo's rain-producing disturbances (Fig. 6a) and the zonal wind around the near-equator trough (Fig. 3f). The population ratio between HRFB events and CSVBs is about 1:3, smaller than the ratio between HRFM cyclones and CSVMs (about 1:2). Note that the correlation coefficient between total disturbances and CSVBs is about 0.83. It may be implied by such a high correlation that the interannual population variation of the total rain-producing disturbances is largely determined by CSVBs. On the other hand, the HRFB cyclone population (Fig. 6b) does not exhibit a high correlation in the annual population variation of total disturbances.
The rainfall contribution from HRFBB + HRFBBM cyclones and CSVBB in Fig. 6 and the bottom half of Table 2 are 58% and 28% of total rainfall, respectively. Regardless of the small population ratio between these two types of rain-producing disturbances, the rainfall ratio between and is about 2:1. The correlation coefficient between PT and in west Borneo is 0.88, and variances for PT and are 95 and 76 mm month−1, respectively. Obviously, the interannual variation of rainfall in west Borneo is primarily caused by , despite the fact also exhibits a relative, significant correlation coefficient of 0.72.
Similar to the relationship between PT and in peninsular Malaysia (Fig. 5), the coherency between interannual variation of PT and can also be attributed to two factors: 1) the rain-producing efficiency of HRFBB + HRFBBM cyclones and 2) the large-scale environment for maintaining this efficiency. The ratio of between these two extreme ENSO phases is ≥5:2. This rain-producing efficiency (Table 2, bottom half) will be addressed in the next section. The value of is larger during the cold ENSO phase than the warm phase in response to the change in the large-scaled circulation pattern caused by the tropical Pacific SST anomalies. Differing from , is produced by domestically formed and developed CSVBs in Borneo (Chen et al. 2013). As a result of the seasonal variation of the cyclonic shear flow around the near-equator trough shown in Figs. 2c and 3f, it would be of interest to determine the effect of circulation changes during the two extreme ENSO phases on the propagation properties and rain-producing efficiencies of major rain-producing disturbances.
c. Rain-producing efficiency of HRF events
The water vapor flux can be split into divergent and rotational component components:
where , with g, po, q, and V representing gravity, surface pressure, specific humidity, and velocity vector, respectively, and and , with and representing the streamfunction and potential function of the water vapor flux. The major water vapor flux is depicted by , but precipitation is primarily maintained by the convergence of water vapor flux,
illustrated by the spatial distribution of , , and P (Chen 1985).
The water vapor transports associated with three types of HRF cyclones/events are depicted by the (, P) charts of individual HRFPM, HRFBM, and HRFBB cyclones/events in Figs. 7a–c, respectively. The rainfall maintenance for these cyclones/events is illustrated by the convergence of water vapor flux (, , P) in Figs. 7d–f, respectively. The hydrological relationship between (, , ) and P for individual HRF cyclone/events shown in Fig. 7 is “common” to its corresponding group. The HRFBB and HRFBBM cyclones are developed in a similar manner, except the former is trapped in Borneo by a type of cold surge flow in the South China Sea (Chen et al. 2013). It is redundant to illustrate the hydrological conditions of both events, so we shall focus on the HRFBB cyclone to save space. As revealed from the fields, the center of the HRF cyclone at peninsular Malaysia and west Borneo is juxtaposed with a much larger East Asian continental anticyclone. These two asymmetric circulation elements are separated by tropical trade easterlies. The spatial relationship between and centers has an important dynamical implication. Figures 7a,d and 7b,e show the convergent centers of and are located to the west of the cyclonic (low) center for both HRFPM and HRFBM cyclones, because both propagate westward. On the other hand, the comparison between Figs. 7c and 7f reveals the cyclonic (negative) center and the convergent (positive) (, ) center are almost spatially coincident for the HRFBB case. As demonstrated by the vorticity budget analysis of HRFBB cyclones by Chen et al. (2013), both (850 hPa) and positive (850 hPa) centers coincide, and this positive (850 hPa) center is primarily maintained by the vorticity stretching process.
The rain-producing efficiency of rain-producing weather systems over both sides of Malaysia during cold and warm ENSO phases may be inferred from a rainfall histogram shown in Figs. 5 and 6b. These HRF cyclones primarily contribute (approximately two-thirds) to both the rainfall and its interannual variation in both parts of Malaysia . To obtain a more quantitative estimate for the rain-producing efficiency of an individual HRF cyclone, two assessment procedures were developed by Chen et al. (2012) for this efficiency of the central Vietnam HRF event: 1) the rainfall measurements of WMO stations over a land area produced by a specific type of weather disturbance and 2) the rainfall/rainfall proxy measured by satellites over the area encircling the rainfall center with P ≥ 3 mm day−1 produced by a specific type of weather system.
Figures 5 and 6 show close to three-fifths of the total precipitation over both parts of Malaysia is produced by HRF cyclones. Therefore, we will focus on the rain-producing efficiency change between the cold and warm ENSO phases. The area-averaged rainfall (over the yellow areas shown in Fig. 8c) contributed by HRF events PHRF and their corresponding convergence of water vapor flux over peninsular Malaysia during November–December and Borneo during December–February are shown in Figs. 8a and 8b, respectively, and are also tabulated in Table 3. The difference between PHRF and over both regions is about 9% of PHRF and may be considered as convergence of uncondensed water vapor, evaporation, and computational error. On the other hand, differences of PHRF and between the cold and warm phases are slightly over 100% of these two variables averaged over the entire analysis period over both parts of Malaysia. In fact, increases of PHRF and during the cold ENSO phase are generally over twice their decrease during the warm ENSO phase. Apparently, the rain-producing efficiency of HRF cyclones during the former climate condition is larger than the latter.
The area-averaged rainfall for an individual HRF event is also computed over the region surrounding the rainfall center with P ≥ 3 mm day−1 encircled by the red line in Fig. 7. Histograms of PHRF (darker color) and (lighter color, averaged over the same area of P) are shown in Figs. 9a and 9b for both groups of HRFPM + HRFBM + HRFBBM and HRFBB + HRFBBM events, respectively. Numerical values for these hydrological variables are tabulated in Table 4. The difference between PHRF and for individual HRF events over both parts of Malaysia is about 9%, which may consist of convergence of uncondensed water vapor flux, evaporation, and slight computation error. The difference in rain-producing efficiencies of individual HRF cyclones in both parts of Malaysia is over 50% of the averaged values of both PHRF and . Compared to these long-term average values, the rain-producing efficiency of HRF cyclones is about 50% higher during the cold ENSO phase than the warm ENSO phase. The interannual variation of rainfall in both parts of Malaysia is not only affected by the population of rain-producing weather disturbances, but also by their rain-producing efficiency, particularly HRF cyclones. However, a concern arises here. How is the hydrological condition of the environment affected by the large-scale circulation change? Does the rain-producing efficiency of the HRF cyclones increase or decrease through the convergence of the environmental water vapor flux?
4. Impact of the low-level circulation change on the rain-producing disturbance activity
As shown in Figs. 3b and 3c, the interannual rainfall variation in peninsular Malaysia during November–December coincides with the intensity variation of the near-equator trough but is opposite the interannual variation of the SST(Niño-3.4) index. During December–February, the near-equator trough migrates closer to the equator and deepens across Borneo. The interannual rainfall variation in west Borneo also follows the intensity variation of this trough (Figs. 3e,f). Because the tropical cyclonic shear flow around this trough is part of the Asian–Australian monsoon system, the interannual variation of this trough should be part of this monsoon's interannual variation. In addition, the impact of this monsoon's interannual variation on the rain-producing disturbance activity and rain-producing efficiency should be examined.
a. Peninsular Malaysia
1) Interannual variation of zonal flow
The 850-hPa zonal wind during November–December in Asia and the western North Pacific is characterized by a meridional juxtaposition of the equator westerly flow, tropical/subtropical trade easterlies, and the midlatitude westerlies (Fig. 10a). The first two zonal flows are separated by the near-equator trough, while the final two zonal flows are divided by the subtropical high. The East Asian subtropical high juxtaposed with the midlatitude trough along Japan forms a cold surge-like flow across the eastern seaboard of northeast Asia. Figure 3c infers the response of the low-level circulation to the negative (positive) tropical Pacific SST anomalies over the NCEP Niño-3.4 region is to deepen (fill) the near-equator trough over South/Southeast Asia. To substantiate this inference, the V(850 hPa) streamline charts during cold and warm November–December are shown in Figs. 10b and 10c, respectively. During cold November–December, the East Asian high is intensified and the western North Pacific trough is deepened. Thus, the northeast Asian cold surge-like flow is strengthened. At low latitudes, the near-equator trough in South/Southeast Asia deepens, although this trough in the western tropical Pacific is filled. Consequently, the northeasterlies north of this trough and the equatorial westerlies south of this trough intensify. Thus, the near-equator trough in South/Southeast Asia deepens during the cold November–December. The opposite condition occurs during warm November–December. The deepening and filling of the near-equator trough revealed from the latitude–time diagram for u(850 hPa) at 105°E in Fig. 3c follow the alternation between the cold (Fig. 10b) and warm (Fig. 10c) anomalous circulations in response to the tropical SST anomalies. The cross-Pacific shortwave train along the North Pacific rim identified by Chen (2002) emanates from this anomalous circulation cell in tropical Southeast Asia.
2) Trajectories of CSVs
Depicted by the 850-hPa streamfunction (850 hPa), the north–south juxtaposition of the subtropical anticyclone and the near-equator trough exhibits strong gradients of the streamfunction, maintaining the well-organized tropical/subtropical easterlies. As confirmed by trajectories2 of all related tropical disturbances, including CSVPMs (red and green lines) formed in the Philippine vicinity and CSVBMs (blue, purple, and light blue lines) formed over Borneo (Figs. 10d, A1a), these easterlies facilitate the westward propagation of these tropical disturbances. A more illustrative depiction of the anomalous circulation during the cold and warm November–December is presented with (850 hPa) [(850 hPa) − (850 hPa), where the first term represents the November–December average and the overbar denotes the long-term average during November–December]. This anomalous circulation possesses a four-leaf structure [a deformation pattern depicted by Hess (1979)] with a saddle point of these (850 hPa) cells located near New Guinea. The composite (850 hPa) structures during cold and warm November–December exhibit an opposite spatial phase. During cold November–December, the near-equator trough deepens over South/Southeast Asia and facilitates the formation of CSVPs and their exclusively westward propagations (Figs. 10e, A1b) as indicated by Figs. 4d and 4f. On the contrary, the near-equator trough fills over South/Southeast Asia during the warm November–December, and mostly CSVBMs are formed and propagated westward during this climate condition (Figs. 10f, A1c). The circulation pattern change functions as a regulator to determine the formation and propagation property of CSVs and HRF cyclones that reach peninsular Malaysia.
3) Hydrological condition
During November–December, the three tropical rainfall centers are located in the tropical Indian Ocean/Maritime Continent, tropical South America, and equatorial Africa (e.g., Chen 1985). These rainfall centers are maintained by the convergence of water vapor flux toward them (e.g., Chen 1985). The maintenance of the first rainfall center is illustrated by in Fig. 10g. It is of interest to note the cold-season rainfall maxima appear in the upwind side of central Vietnam, peninsular Malaysia, the Philippines, and Sumatra. The maintenance mechanism of the central Vietnam rainfall center is illustrated by Chen et al. (2012), while the maintenance mechanism of peninsular Malaysia is part of the companion study (Chen et al. 2013). The maintenance of the final two rainfall centers is under investigation and will be reported in the future.
The large-scale circulation changes presented in Figs. 10b,c and 10e,f should be accompanied by the corresponding changes of divergent circulation. These interannual variations of divergent circulation also lead to changes in the divergent water vapor flux, as portrayed by during cold (warm) November–December in Fig. 10h (Fig. 10i). During a cold November–December, the convergent center of water vapor flux over the Asian monsoon region is strengthened to maintain the rainfall increase along the North Pacific and South Pacific convergence zones, and the upwind rainfall centers in tropical Southeast Asia. Obviously, the convergent center of water vapor flux over the South China Sea and the Philippine Sea forms a hydrological environment favorable to enhance rain production. On the contrary, the hydrological condition opposite the cold November–December appears during warm November–December (Fig. 10i). Comparing the rain-producing weather system activity between these two extreme ENSO conditions, one can easily find that the former hydrological environment stimulates more rain-producing weather systems for HRFPM cyclones and CSVPMs (Fig. 10e), but the latter suppresses these weather systems (Fig. 10f) and allows HRFBM + HRFBBM cyclones and CSVBMs to occur in peninsular Malaysia.
The quantitative measurements for the impact of large-scale environment on the rain-producing efficiency may be achieved by superimposing histograms for on PT histograms (Fig. 11a) and histograms for on PHRF histograms (Fig. 8a). Note that these hydrological variations are averaged over the yellow area in peninsular Malaysia (Fig. 8c). As shown in Fig. 11a, PT is slightly larger than its corresponding . The difference is about 9% of PT between PT and (Table 5). This difference may be attributed to the convergence of uncondensed water vapor flux, evaporation, and some computational error over peninsular Malaysia. In other words, PT is well maintained by . The correlation coefficient between (Fig. 8a) and (Fig. 11a) is about 0.9. Statistical amplitudes of and measured in terms of their variances are 113 and 93 mm, respectively. It is implied by these statistics that and are not only well correlated but are also comparable in their amplitudes. Because = , the interannual variation of large-scale divergent circulation can modulate through . Embedded in this circulation, the rain-production efficiency of the HRF event cyclone is obviously affected by the large-scale divergent circulation.
b. West Borneo
1) Interannual variation of zonal wind and trajectories of CSVBs
The north–south juxtaposition in December–February of the equatorial westerlies in the Southern Hemisphere, the tropical trade easterlies in the Northern Hemisphere, and the midlatitude westerlies (Fig. 12a) are similar to those occurring in November–December. The latitude–time diagram for u(850 hPa) at 112.5°E in Fig. 3f reveals the near-equator trough migrates farther southward, compared to November–December, and enables itself to almost align with the equator. The East Asian anticyclone is centered at the south China coast, and the ridge line of the subtropic high is located close to 20°N slightly south of the November–December subtropical high (Fig. 10a). In addition to its southward migration to the equator, the near-equator trough also becomes deeper (Chen et al. 2013, their Fig. 11f).
During the cold December–February (winter), the composite anomalous circulation (Fig. 12b) exhibits a four-leaf structure centered at the western tip of New Guinea, similar to that during the cold November–December (Fig. 10b). The near-equator trough west of New Guinea is intensified/deepened, so the tropical trade easterlies around 20°N and the equatorial westerlies are intensified, as shown in Fig. 3f. During the warm winter, the composite anomalous circulation opposite the cold winter (Fig. 12c) appears and weakens/fills the near-equator trough.
Chen et al. (2013, their Fig. 9c) show some CSVBs and HRFB cyclones during December propagate westward to peninsular Malaysia. In contrast, all CSVBs and HRFB cyclones during January–February become nonpropagating, rain-producing disturbances trapped in Borneo. The synoptic condition of the cold surge flow in the South China Sea (Compo et al. 1999) prevents the westward propagation of these disturbances (Chen et al. 2013). These nonpropagating CSVBs (blue dots) and HRFB cyclones (red crosses) are superimposed on (850 hPa) and cold and warm (850 hPa) anomaly fields in Figs. 12d–f (expanded in Figs. A1d–f), respectively. Although some HRF cyclones propagate westward from west Borneo (Figs. 12f, A1f), the greater majority of HRF events are trapped in Borneo (Figs. 12e, A1e). The interannual rainfall variation in west Borneo is attributed to PHRF. The ratio of PHRF between the cold and warm winters is ≥5:2. It is of interest to discover the interannual rainfall variation is essentially regulated by the propagation property of HRF cyclones in Borneo in response to the interannual variation of the large-scale circulation pattern in South/Southeast Asia.
2) Hydrological conditions
As shown by in Fig. 12g, the hydrological conditions during the winter (December–February) over the Asian monsoon–western Pacific region seem similar to those for November–December. However, there are some major changes within the convergent center for water vapor flux. The November–December rainfall center over peninsular Malaysia (Fig. 10g) is weakened, but the December–February rainfall center over west Borneo is intensified. The rainfall also appears over northern Australia, after the monsoon onset in late December. Following this seasonal evolution, the rainfall over the so-called warm pool in the western tropical Pacific is also enhanced.
Coupled with the interannual variation of the low-level flow field depicted by the composite (850 hPa) field shown in Figs. 12e and 12f, the corresponding composites (cold) and (warm) are shown in Figs. 12h and 12i, respectively. The rainfall increase (decrease) over west Borneo is maintained by the intensified convergent (divergent) water vapor flux toward (out of) the tropical southwest Asia–western tropical Pacific region. The interannual variation of west Borneo rainfall during December–February (Fig. 3f) is a result of the response of the water vapor flux to the large-scale divergent circulation over this region.
The area average for over west Borneo (the yellow area in Fig. 8c) superimposed on the corresponding is shown in Fig. 11b. The difference is about 9% of between and (Table 5). As pointed out by the peninsular Malaysia rainfall changes, this difference is likely convergence of uncondensed water vapor flux, evaporation, and some computational error. Total rainfall is well maintained by . Variances for (Fig. 11b) and (Fig. 8a) are 80 and 74 mm, respectively, which are very close to each other. The correlation coefficient between these two hydrologic conditions is about 0.9. These statistics indicate interannual variations for are primarily caused by . Note that = and are determined by the large-scale divergent circulation, and a HRFB cyclone is embedded in this divergent circulation, affecting the rain-producing efficiency of that cyclone.
5. Concluding remarks
Chen et al. (2013) observed the population ratio between HRF cyclones/events and CSVs, two major rain-producing disturbances, is about 1:3 in peninsular Malaysia during November–December and about 1:3 in west Borneo during December–February. In contrast, the ratio of rainfall produced by HRF cyclones and CSV is ≥2:1 in the former region and about 2:1 in the latter region. The major rainfall over these two parts of Malaysia is generated by HRF cyclones. The total rainfall for both parts of Malaysia, denoted as PM and PB, undergoes an interannual variation, opposite NOAA Niño-3.4 SST anomalies but coincident with SST(WTP) anomalies. Analyzed by the present study, this rainfall variation for Malaysia is characterized by the following two features: 1) interannual variations of and are highly coherent with those for PM and PB, respectively, with a correlation coefficient close to 0.9, and 2) variances for and are close to 90% of those for PM and PB, respectively. Apparently, interannual rainfall variations for both parts of Malaysia are primarily attributed by HRF events.
The boreal cold-season lower-tropospheric circulation in tropical Southeast Asia is characterized by the near-equator trough and the associated cyclonic shear flow (Fig. 2). This tropical trough migrates equatorward during the cold season from the location between 5° and 10°N in October–November (the rainfall maximum in central Vietnam, Fig. 2a), around 5°N in November–December (the rainfall maximum in peninsular Malaysia, Fig. 2b), and between 5°N and the equator in December–February (the rainfall maximum in west Borneo, Fig. 2c). The monsoon northeasterlies flow toward peninsular Malaysia in the second period, although the near-equator trough crosses the most northern part of Borneo. The cyclonic shear flow north of this trough facilitates the westward propagation of rain-producing disturbances from the vicinity of the Philippines to peninsular Malaysia in November–December. Since the near-equator trough crosses west Borneo later in December–February, rain-producing disturbances are trapped in Borneo by the tropical cyclonic shear flow. In summary, the major part of PM is produced by both HRFPM cyclones and CSVPMs from the Philippines and HRFBM + CSVBBM cyclones and CSVBMs from Borneo, and the major part of PB is generated by domestically formed/developed HRFBM + HRFBBM cyclones and CSVBBs. Thus, the interannual rainfall variations for both parts of Malaysia are caused by the circulation changes of the cyclonic shear flow around the near-equator trough in response to the tropical Pacific SST anomalies. The impact of these circulation changes is reflected by 1) the propagation properties of rain-producing disturbances and 2) the rain-producing efficiency of these disturbances.
The peninsular Malaysian rainfall PM maximum during November–December is mostly produced by HRFPM cyclones and CSVPMs propagating from both the Philippines vicinity and Borneo. During the cold ENSO phase, the near-equator trough deepens, and the tropical trade easterlies north of this trough intensify. This circulation change facilitates the southwestward propagation of both HRF and non-HRF CSVs from the Philippines to reach peninsular Malaysia; PM is exclusively contributed by and during this ENSO phase. In contrast, the near-equator trough is filled and the tropical trade easterlies north of this trough weaken. The circulation change under this climate condition prevents major rain-producing disturbances from the Philippines vicinity but enables them to propagate from Borneo to reach peninsular Malaysia; PM is exclusively produced by HRFBM events and CSVBMs. Different from PM, the west Borneo rainfall PB maximum during winter is generated by the domestically formed/developed and HRFBM events. During the cold ENSO phase, the near-equator trough crosses west Borneo and deepens. Thus, these rain-producing disturbances are trapped in Borneo. In the opposite extreme ENSO condition, some of the rain-producing disturbances may propagate westward from Borneo. The response of the large-scale divergent circulation to the central and eastern tropical Pacific SST anomalies intensifies (weakens) the convergence of water vapor flow toward tropical Southeast Asia during the cold (warm) ENSO phase. Thus, the rain-producing efficiency of HRF events is enhanced (reduced) during the cold (warm) ENSO phases. Therefore, HRFPM cyclones developed from CSVPMs become more effective in rain production than those developed from CSVBMs. Similarly, HRFBB cyclones developed from trapped CSVBBs are also more effective rain producers than HRFBBM cyclones.
The circulation pattern change of the cyclonic shear flow around the near-equator trough in tropical South/Southeast Asia functions as a filter to regulate the propagation property of HRF cyclones and CSVs in both parts of Malaysia. Additionally, the corresponding change of global divergent circulation also intensifies (weakens) the convergence of water vapor flux toward tropical Southeast Asia to enhance (reduce) the rain-producing efficiency of HRF events during the cold (warm) ENSO phase. Conventional climate modeling usually focuses simply on the change of rainfall climatology caused by the climate change in response to the tropical Pacific SST anomalies associated with the El Niño–Southern Oscillation. However, the tropics–midlatitude interaction to generate CSVs and develop it into HRF cyclones/events is a major factor to produce the regional rainfall climatology. Can this tropics–midlatitude interaction modulated by the circulation change in South/Southeast Asia be properly simulated by the climate model in response to the ENSO activity? This task will pose a new challenge to climate modeling.
It was pointed out earlier that the formation mechanism and propagation properties of major rain-producing disturbances in central Vietnam (Chen et al. 2012) and both parts of Malaysia (Chen et al. 2013) are basically regulated by the cyclonic shear flow associated with the near-equator trough. The interannual rainfall variations in these regions are caused by the interannual variation of the activity and rain-producing efficiency of HRF events affected by the circulation change of this cyclonic shear flow in response to the tropical Pacific SST anomalies. The HRF events often bring costly disasters to the Southeast Asian countries around the South China Sea. Forecasts for these HRF events become one of the most important operational missions to the weather services in these regions. According to the occurrence statistics of HRF events in both parts of Malaysia identified by the DFO over the period 1979–2009, close to 90% of the HRF events in both parts of Malaysia follow the occurrence of these events in central Vietnam (usually occurring 1–2 months after). Can the occurrence of HRF events in the latter region be used as the precursor to the occurrence of HRF events in the former region and facilitate forecasts for these events? Property damage and life loss caused by the HRF events in Malaysia make it extremely worthwhile to explore this possibility.
This study was partially sponsored by the Cheney Research Fund, the NSF Grant ATM-0836220, the NSC Grant NSC100-2111-M-008-014, and the Grant-in-Aid for Scientific Research (23240122) from the Japan Society for the Promotion of Science. Comments offered by three reviewers were of great help to improve the presentation of this paper.
The Zoomed-In Versions of Figs. 10d–f and Figs. 12d–f
Trajectories of HRF and non-HRF CSVs shown in Figs. 10d–f and 12d–f are too crowded. For the convenience of the reader, the zoomed-in versions of these panels are shown in Figs. A1a–c and A1d–f, respectively.
Symbols used hereafter are presented by the following conventions. CSVAM is a CSV formed in region A and propagated to peninsular Malaysia, while CSVBB is a CSV formed in Borneo and then trapped there. HRFAM is an HRF cyclone/event evolved from its parent CSVAM in region A and then propagated to peninsular Malaysia. HRFBBM is an HRF cyclone/event evolved from its parent CSVB in Borneo, then trapped/developed into a HRF cyclone/event there, but it is eventually propagated to peninsular Malaysia and is still maintained as a HRFBBM cyclone/event.