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  • View in gallery

    Latitude–time diagrams of monthly-mean rainfalls at WMO stations (red dots in the top panel) along (a) the east coast of Peninsular Malaysia and (b) the northwest coast of Borneo. The color scale of rainfall is shown in the bottom right of (b), while the rainfall contour interval is shown in the top right of (b). The period of major rainfall season in each region is indicated by vertical red lines.

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    (a),(b) Monthly-mean V(850 hPa) streamline charts superimposed with the TRMM precipitation, and trajectory (thin red line) CSVP, which moved from the Philippine Sea to Peninsular Malaysia and became HRFPM events: HRFPM events are marked by golden crosses, while CSVs in Philippine Sea are denoted by small black crosses. (c)–(f) Monthly-mean V(850 hPa) streamline charts superimposed with the TRMM precipitation, and trajectories of CSVB (thin, blue line) moving from west Borneo to Peninsular Malaysia: HRFBM and HRFBBM events in Peninsular Malaysia are marked by red and green crosses, while HRFBB events in Borneo are marked by purple crosses. The shear line of the near-equator trough is marked by a red dashed line. The isotach center is marked by a blue arrow (northeasterlies) or a red arrow (southwesterlies). The scale of precipitation is shown in the top right corners of (a) and (c). Small black crosses are locations of parent CSVs of HRF events in each panel.

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    Time periods covered by different rain/rain proxy datasets. Details of each dataset are described in Table 1.

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    (a) Monthly-mean occurrence frequency of tropical cyclone (TC), easterly wave (EW), CSVBM, CSVPM, (HRFBM + HRFBBM), and HRFPM in Peninsular Malaysia are shown in the left column. Precipitation amounts produced by these disturbances individually are shown in the right column. The bottom panel of the right column shows the monthly-accumulated precipitation contributed by all disturbances. (b) Monthly occurrence frequency of TCs, EWs, (CSVBB + CSVBBM), CSVPB, and (HRFBB + HRFBBM) in Borneo are shown in the left column. Precipitation amounts contributed by these disturbances individually are shown in the right column. The accumulated monthly precipitation by all disturbances is shown in the bottom panel of the right column. Precipitation shown in (a) and (b) is averaged over the yellow areas of Peninsular Malaysia and Borneo, respectively, in the bottom middle panel.

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    (a) Histograms of the rainfall contributions from easterly wave, tropical cyclones, cold surge vortices, and HRF events in (a) Peninsular Malaysia during November–December and (b) west Borneo during December–February.

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    The synoptic evolution of an HRFPM event (15 Dec 2005) from a CSVP (5 Dec 2005) depicted by the V(925 hPa) streamline charts superimposed with TRMM precipitation: (a),(c), and (e). The upper-level synoptic conditions corresponding to the 925-hPa flow patterns are presented by the V(500 hPa) streamline chart superimposed with isotachs. The shearline of the near-equator trough at 925 hPa is depicted by a red-dashed line. Scales of precipitation and isotachs are shown above the upper right corner of (a) and (b), respectively.

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    As in Fig. 6, but for an HRFBM event (29 Dec 2008).

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    As in Fig. 6, but for an HRFBB event (1 Feb 2009).

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    (a) Direction and speed of the cold surge flow related to the formation of three different types of CSVs at two locations (large red dot at Dongsha Island and large blue dot in the Philippine Sea) shown in the bottom panel: the HRF and non-HRF CSVs are marked by (•) and (), respectively, and ()PM, ()BM + ()BBM, and ()BB are denoted by blue, green, and red, respectively, in a small table in the left bottom corner of the top panel. All selected cold surge flows are shown by small blue dots (SCS types) and red dots (PHS types). The large blue and red dots are averaged locations of the SCS and PHS groups, respectively. The blue and red oblongs are one standard deviation of locations used to determine cold surges. (b) Magnitude changes of surface pressure (Δps) and temperature (ΔTs) over a period of 24 h by cold surge flows at locations shown in (a). The cold surge flows of SCS (PHS) types are shown in the left (right) panels: Δps(ΔTs) is shown in the top (bottom) panel. Averaged values of Δps and ΔTs for cold surges related to three different groups of (CSVs, HRF events) are tabulated in the table at the bottom of (b). (c) Formation date of every CSV.

  • View in gallery

    Basic characteristics of non-HRF CSVs, HRF CSVs, and HRF cyclones/events: (top) maximum speed, (middle) size, and (bottom) rainfall. Enhancements of these characteristics for the three groups of rain-producing disturbances are shown in three columns. (a) Enhancement from HRF CSVs (non-HRF CSVs) in the vicinity of the Philippines to HRFPM events (non-HRF CSVPMs) over Peninsular Malaysia. (b) As in (a), but from CSVBs in west Borneo to HRFBM events (non-HRF CSVBMs) over Peninsular Malaysia. (c) As in (a), but from CSVBs in west Borneo to in situ HRFBB events (non-HRF CSVBBs). Life cycles for the three CSV groups to become HRF events are shown at the bottom of every column.

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    (a) The January–February V(850 hPa) streamline chart superimposed with vertical motion, (b) the January–February V(925 hPa) streamline chart superimposed with vorticity, (c) the difference of V(925 hPa) and vorticity between January–February and November–December. Also shown are the latitude–height cross section along 115°E for (d) vertical motion during January–February, (e) vorticity during January–February, and (f) differences of streamfunction and vorticity between January–February and November–December. Scales of (850 hPa) in (a), (925 hPa) in (b) and (925 hPa) in (c) are shown at the top right of each panel.

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    The V(850 hPa) streamline charts superimposed with the vorticity budget of an HRFBM event (0000 UTC 24 Dec 2008), which moved from west Borneo to Peninsular Malaysia: the vorticity tendency caused by (a) horizontal advection of relative vorticity, (b) meridional advection of planetary vorticity, and (c) sum of (a) and (b); the vorticity tendency caused by (d) vortex stretching of relative vorticity, (e) vortex stretching of planetary vorticity, and (f) sum of (d) and (e); and (g) vorticity, (h) vorticity tendency, and (i) the sum of (c) and (f). Scales of vorticity tendency and vorticity are shown at the top and bottom of the left column.

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    As in Fig. 12, but for an HRFBB event (0000 UTC 30 Jan 2009).

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    Formation dates of CSVs in west Borneo. Non-HRF CSV is denoted by (•), while HRF CSV is marked by (); CSVBM and (HRFBM + HRFBBM) events are green, and CSVBBs and HRFBB events are red.

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The Winter Rainfall of Malaysia

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  • 1 Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa
  • 2 Department of Atmospheric Sciences, National Central University, Chung-Li, Taiwan
  • 3 Department of Geography, Tokyo Metropolitan University, Tokyo, and Research Institute for Global Change, JAMSTEC, Yokosuka, Japan
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Abstract

Malaysia is geographically separated into Peninsular Malaysia and west Borneo. The rainfall maximum in the former region occurs during November–December, whereas that in the latter region occurs during December–February. This difference of maximum rainfall period indicates that the formation mechanism is different for the rainfall centers in these two parts of Malaysia. Since rainfall is primarily produced by severe weather systems, the formation of a climatological rainfall center is explored through synoptic activity and the rainfall amount of this center is estimated through contributions by rain-producing disturbances. The major cause of the rainfall maximum of Peninsular Malaysia is cold surge vortices (CSVs) and heavy rainfall/flood (HRF) events propagating from the Philippine area and Borneo. In contrast, the major cause of the rainfall maximum of Borneo is these rain-producing disturbances trapped in Borneo. Disturbances of the former group are formed by the cold surge flows of the Philippine Sea type, whereas disturbances of the latter group are formed by cold surge flows of the South China Sea (SCS) type. The population of HRF events is about one-fourth of the rain-producing disturbances in both Peninsular Malaysia and Borneo, but they produce less than ~60% rainfall for these two regions. It is revealed from the synoptic and dynamic analyses that the major Borneo rain-producing disturbances propagate westward before December by strong tropical easterlies, but they are trapped after December by strong northeasterlies of the SCS-type cold surge flow.

Corresponding author address: Tsing-Chang “Mike” Chen, Atmospheric Science Program, Department of Geological and Atmospheric Sciences, 3010 Agronomy Hall, Iowa State University, Ames, IA 50011. E-mail: tmchen@iastate.edu

Abstract

Malaysia is geographically separated into Peninsular Malaysia and west Borneo. The rainfall maximum in the former region occurs during November–December, whereas that in the latter region occurs during December–February. This difference of maximum rainfall period indicates that the formation mechanism is different for the rainfall centers in these two parts of Malaysia. Since rainfall is primarily produced by severe weather systems, the formation of a climatological rainfall center is explored through synoptic activity and the rainfall amount of this center is estimated through contributions by rain-producing disturbances. The major cause of the rainfall maximum of Peninsular Malaysia is cold surge vortices (CSVs) and heavy rainfall/flood (HRF) events propagating from the Philippine area and Borneo. In contrast, the major cause of the rainfall maximum of Borneo is these rain-producing disturbances trapped in Borneo. Disturbances of the former group are formed by the cold surge flows of the Philippine Sea type, whereas disturbances of the latter group are formed by cold surge flows of the South China Sea (SCS) type. The population of HRF events is about one-fourth of the rain-producing disturbances in both Peninsular Malaysia and Borneo, but they produce less than ~60% rainfall for these two regions. It is revealed from the synoptic and dynamic analyses that the major Borneo rain-producing disturbances propagate westward before December by strong tropical easterlies, but they are trapped after December by strong northeasterlies of the SCS-type cold surge flow.

Corresponding author address: Tsing-Chang “Mike” Chen, Atmospheric Science Program, Department of Geological and Atmospheric Sciences, 3010 Agronomy Hall, Iowa State University, Ames, IA 50011. E-mail: tmchen@iastate.edu
Keywords: Asia

1. Introduction

Annual variations of rainfall measured by the World Meteorological Organization (WMO) stations along the east coast of Peninsular Malaysia and the west coast of Borneo are illustrated by the latitude–time diagrams shown in Fig. 1; major rainfall occurs during the cold season (e.g., Oki and Musiake 1994). The rainfall maximum in the former region appears during November–December but in the latter region during December–February, as indicated by vertical red lines in Fig. 1. The rainfall maximum occurs in the west about one month earlier than it occurs in the latter region. Because rainfall is produced primarily by severe weather, it is suggested by this one-month difference in the onset of maximum rainfall across the tropical South China Sea that there should be a difference in the activity of the rain-producing weather systems between these two regions. Cheang (1977) observed that winter rainfall in Peninsular Malaysia is produced by an easterly vortex. After the Winter Monsoon Experiment (WMONEX; Greenfield and Krishnamurti 1979), some studies (e.g., Houze et al. 1981; Geotis and Houze 1985) showed that onshore rainfall over west Borneo is generated in the late afternoon by the sea breeze and the offshore rainfall at night by the interaction of the land breeze with the low-level northeasterly monsoon flow. Additionally, the monsoon northeasterly flow toward coastal mountain ranges in Vietnam, Malaysia, and the Pacific side of the Philippines also produced rainfall because of the orographic effect (e.g., Johnson 2005). Regardless of efforts made by these studies, Johnson and Houze (1987) pointed out that flooding may occur along Peninsular Malaysia and west Borneo during strong cold surge events, but this was not a major focus of WMONEX.

Fig. 1.
Fig. 1.

Latitude–time diagrams of monthly-mean rainfalls at WMO stations (red dots in the top panel) along (a) the east coast of Peninsular Malaysia and (b) the northwest coast of Borneo. The color scale of rainfall is shown in the bottom right of (b), while the rainfall contour interval is shown in the top right of (b). The period of major rainfall season in each region is indicated by vertical red lines.

Citation: Journal of Climate 26, 3; 10.1175/JCLI-D-12-00174.1

The monthly-mean Tropical Rainfall Measuring Mission (TRMM; Simpson et al. 1996) rainfalls, superimposed with monthly-mean 925-hPa streamlines for the cold season of November–February, are shown in Figs. 2a–f. The seasonal variation of the low-level circulation in tropical Southeast Asia is characterized by the southward migration of the near-equatorial trough, indicated by the red dashed lines, north of the equator (e.g., Sumi and Murakami 1981). Accompanying the migration of this trough is the seasonal variation of rainfall in Peninsular Malaysia and west Borneo. During November–December, the northeasterly flow (marked by a blue arrow) north of the near-equatorial trough heads toward Peninsular Malaysia, so heavy rainfall occurs both onshore along the east coast of Peninsular Malaysia and offshore over the adjacent South China Sea. During December–February, the southward migration of the near-equatorial trough slows down and almost anchors at west Borneo. At this stage, this trough line is located across the northern part of this island, but rainfall is trapped along the trough line over west Borneo.

Fig. 2.
Fig. 2.

(a),(b) Monthly-mean V(850 hPa) streamline charts superimposed with the TRMM precipitation, and trajectory (thin red line) CSVP, which moved from the Philippine Sea to Peninsular Malaysia and became HRFPM events: HRFPM events are marked by golden crosses, while CSVs in Philippine Sea are denoted by small black crosses. (c)–(f) Monthly-mean V(850 hPa) streamline charts superimposed with the TRMM precipitation, and trajectories of CSVB (thin, blue line) moving from west Borneo to Peninsular Malaysia: HRFBM and HRFBBM events in Peninsular Malaysia are marked by red and green crosses, while HRFBB events in Borneo are marked by purple crosses. The shear line of the near-equator trough is marked by a red dashed line. The isotach center is marked by a blue arrow (northeasterlies) or a red arrow (southwesterlies). The scale of precipitation is shown in the top right corners of (a) and (c). Small black crosses are locations of parent CSVs of HRF events in each panel.

Citation: Journal of Climate 26, 3; 10.1175/JCLI-D-12-00174.1

The following concerns arise from the rainfall climatology of Malaysia presented in Figs. 1 and 2:

  1. Regardless of studies made by the post-WMONEX research to understand the convective activity over west Borneo, the orography–monsoon northwesterly interaction, and the cold surge vortex formation [over the tropical South China Sea as reviewed by Johnson and Houze (1987) and Johnson and Chang (2007)], no quantitative estimate of rainfall over Malaysia by rain-producing weather systems has been attempted. Rainfall proxies, generated with radiative properties of the atmosphere measured by satellites in the past decades and the spaceborne measurements of TRMM rainfall available since 1998, have made this effort possible.
  2. Chen et al. (2012a) observed that cold surge vortices, formed in the vicinity of the Philippines, can propagate westward to produce rainfall in central Vietnam. Since cold surge vortices can form over west Borneo (Johnson 2005; Johnson and Chang 2007), can these vortices not only produce rainfall over Borneo, but also propagate westward to Peninsular Malaysia and generate rainfall there?
  3. During November–December, rainfall is produced in the downwind side of the monsoon northeasterlies over Peninsular Malaysia, but why is rainfall produced and trapped over west Borneo along the near-equatorial trough during December–February?

To answer these concerns, several tasks are pursued in the present study: 1) to identify rain-producing weather systems and heavy rainfall events, 2) to estimate the rainfall amount produced by these weather systems, and 3) to understand their synoptic activity. The identification of rain-producing weather systems and their genesis regions over Southeast Asia was made possible by two types of synoptic maps: 1) streamline charts (superimposed with rainfall/rainfall proxy) with high-resolution reanalyses and 2) daily surface analysis maps issued by operational centers. The Dartmouth Flood Observatory (DFO; http://floodobservatory.colorado.edu/) and International Emergency Disaster Database (EM-DAT; http://www.emdat.be/) identified the heavy rainfall/flood events. The rainfall distribution and amounts contributed by different rain-producing weather systems are analyzed and estimated with several high-resolution global/regional (Asian) rainfall and rainfall proxy datasets for different time periods after 1979, compiled and archived in the past three decades by different operational centers and a Japanese research project, Asian Precipitation–Highly-Resolved Observational Data Integration toward Evaluation of Water Resources (APHRODITE).

Because rainfall is a sensitive indicator of climate change, rainfall distribution and amount usually have been the major concerns of climate research. Contrary to this conventional perspective, the rainfall center over a region may be formed by rain-producing weather disturbances propagating from remote regions. It is important to understand how the synoptic activity of rain-producing weather disturbances in an upstream region contributes to the formation of a rainfall center downstream. This offers us a different approach in search of the cause of climate change. Thus, the synoptic activity of rain-producing weather systems related to the development of the rainfall centers in Malaysia is explored.

This study is organized in the following manner. Details of the data used for this study and identification of rain-producing weather systems are presented in section 2. The formation of the rainfall centers over Peninsular Malaysia and west Borneo by contributions from different rain-producing weather systems is analyzed in section 3. The synoptic development and analysis of the rain-producing weather systems from the vicinity of the Philippines and Borneo are provided in section 4. How the rain-producing systems are trapped in Borneo during December–February is examined in terms of a vorticity dynamics perspective in section 5. Concluding remarks are offered in section 6.

2. Data and identification of rain-producing disturbances

a. Data

Covering the past three decades (1979–2009), the analysis in this study was performed with four different data sources—rainfall, reanalysis, daily surface analysis charts, and heavy rainfall/flood (HRF) events identified over Malaysia. Data used should be compiled uniformly over a long analysis period. This requirement is met by the three latter data sources and HRF events currently available, but not by rainfall. Rainfall measurements by the network of WMO stations are only available over land. Over the ocean, rainfall is compiled with rainfall proxies generated indirectly with measured radiative properties or direct spaceborne measurements by satellites. Some merging processes of different rainfall data sources were applied. The details of these processes are explained below.

Malaysia is separated by the South China Sea into two parts—Peninsular Malaysia and west Borneo (Sarawak and Sabah). Located on the south side of the South China Sea, Borneo is divided into two political entities—west Borneo and Kalimanta (part of Indonesia). Over land, rainfall measurements by WMO surface stations are incorporated into the daily-gridded precipitation data compiled/analyzed by the APHRODITE project (Yatagai et al. 2009) with a spatial resolution of 0.25° lon × 0.25° lat for the period 1951–2007. A validation for TRMM (Simpson et al. 1996) rainfall shows that APHRODITE rainfall is comparable over the Malaysia Peninsula, but not over Borneo, except along its coast. A merging process of the WMO station rainfall is applied over Borneo. The 16-point Bessel interpolation scheme (Jenne 1975) is applied to blend station rainfall with the TRMM rainfall and other rainfall proxies into a 0.25° lon × 0.25° lat mesh over the periods covered by these rainfall data, shown in Fig. 3. If there are no station rainfall data available within a circle of 0.25° radius of a grid point, the interpolated TRMM rainfall or rainfall proxy is used. This newly merged rainfall dataset over Borneo is validated against surface analysis charts issued by three operational centers shown in Table 1. The corresponding 850- and 925-hPa streamline charts generated by reanalyses superimposed with Geostationary Meteorological Satellite (GMS)/Multifunctional Transport Satellites (MTSAT) equivalent Black Body Temperature, TBB, or infrared, IR. data determine if these rainfall data match the synoptic condition well; if not, then an adjustment of rainfall based on the TBB or IR distribution is made. Streamline charts, vorticity, streamfunction, water vapor flux, and the vorticity budget analysis used by the present study are prepared with a horizontal grid of 0.5° lon × 0.5° lat. Only the National Centers for Environmental Prediction (NCEP) Global Forecasting System (GFS) initial analyses meet this resolution, but they are only available after June 2004. The Goddard Earth Observing System Data Assimilation System version 5 (GEOS-5) reanalyses are interpolated by the 16-point Bessel interpolation scheme to reach the GFS resolution. Additionally, the GEOS-5 reanalyses over any mountain area are filled by the interpolated interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data.

Fig. 3.
Fig. 3.

Time periods covered by different rain/rain proxy datasets. Details of each dataset are described in Table 1.

Citation: Journal of Climate 26, 3; 10.1175/JCLI-D-12-00174.1

Table 1.

Details of datasets used in this study. Abbreviations used herein include MMD—Malaysian Meteorological Department; TMD—Thai Meteorological Department; MCGAI—Meteorological, Climatological and Geophysical Agency, Indonesia; GPI—Goddard Precipitation Index; and SRRS—Service Records Retention System; OI SST—Optimum Interpolation SST.

Table 1.

Most rain-producing weather systems reaching Malaysia during November–December are not generated in situ; instead, they are generated over the surrounding sea/ocean. Thus, rainfall and rainfall proxies measured over the ocean by spaceborne observations are used. These data are supplied by the following sources: 1) the Tropical Rainfall Measurement Mission (Simpson et al. 1996), 2) the Goddard Precipitation Index (GPI; Susskind et al. 1997), 3) the Global Precipitation Climatology Project (GPCP; Huffman et al. 1997; Huffman et al. 2001), and 4) the Microwave Sounding Unit (MSU; Spencer 1993). The details and periods covered by these data sources are provided in Table 1 and Fig. 3, respectively. Data not measured over the same time period and compiled/archived with the same spatial–temporal resolution are interpolated with the 16-point Bessel interpolation into a uniform rainfall dataset. This interpolation approach is also utilized to merge the rainfall data between land and ocean.

The validation of rainfall pattern, synoptic conditions of rain-producing weather systems, and the circulation structure over Malaysia are depicted with high-resolution reanalyses. Three high-resolution analyses meet this requirement: 1) the initial analysis of the NCEP GFS (Kanamitsu et al. 1991; Yang et al. 2006), 2) ERA-Interim data (Dee et al. 2011), and 3) GEOS-5 (Rienecker et al. 2008). The details of these three high-resolution reanalyses are listed in Table 1. Daily surface analysis maps are issued by three operation centers: 1) Tropical strip surface analysis and observations archived in the Service Records Retention System (SRRS) analysis and forecast charts archived at the National Oceanic and Atmospheric Administration (NOAA) Operational Model Archive Distribution System (NOMADS; http://nomads.ncdc.noaa.gov/; Rutledge et al. 2006), 2) Japan Meteorological Agency (JMA) synoptic charts (data are available on a CD; see http://www.jmbsc.or.jp/english/index-e.html), and 3) Thai Meteorological Department (TMD) weather maps (http://www.tmd.go.th/en/). Finally, the heavy rainfall/flood events that occurred in Malaysia were identified by two sources: 1) The Dartmouth Flood Observatory (http://floodobservatory.colorado.edu/) and 2) the International Emerging Disaster Database (http://www.emdat.be/) compiled by the Center for Research on the Epidemiology of Disaster (CRED). Both data sources are used to verify each other.

b. Identification of rain-producing disturbances

Analyzing the heavy rainfall/flood events in central Vietnam, Chen et al. (2012a,b) identified four types of rain-producing disturbances: easterly wave (EW), tropical cyclone (TC), cold surge vortex (CSV), and HRF event. An easterly wave is an open cyclonic perturbation with rainfall occurring ahead of its trough line, which can be recognized with the 850-hPa streamline chart east of the Philippines and Borneo. Tropical cyclones and their tracks are operationally issued/archived by the Joint Typhoon Warning Center (JTWC; http://www.usno.navy.mil/JTWC/). HRF events are identified/archived by the DFO and EM-DAT. The CSVs of concern to the current study are those that bring rain to Peninsular Malaysia and Borneo and evolve into HRF events.

The detection approach of CSVs and the parent CSVs of HRF events performed by Chen et al. (2012b) is applied in the present study to identify these disturbances and to explore how they contribute to the rainfall centers in Malaysia. To make this study more self-contained, this detection approach is concisely outlined below.

1) Synoptic identification

Two identification approaches are adopted; they are described below.

(i) Forward tracking approach

The 925-hPa streamline charts superimposed with precipitation are used to identify a closed vortex formed by the interaction of an easterly wave with the northeastern Asian cold surge flow in the vicinity of the Philippines and Borneo. The identified CSV is verified/validated against the NCEP 6-h tropical strip surface analysis and observation SRRS charts and TMD and JMA surface analysis maps. Only those that can either reach Peninsular Malaysia and Borneo or evolve into HRF events are included in the estimate of rainfall contribution.

(ii) Backtracking approach

A backtracking approach is also adopted to trace the CSVs and HRF events from Peninsular Malaysia and Borneo backward to their formation locations in the vicinity of the Philippines and Borneo. This backtracking approach is also a way to validate the CSVs and parent CSVs of HRF events identified by the forward tracking approach.

2) Statistics of three CSV basic characteristics (maximum speed, size, and rainfall)

It will be shown in section 4c that the three basic characteristics of HRF and non-HRF CSVs become distinctive as soon as they move across 120°E into the South China Sea. When a CSV reaches Peninsular Malaysia and Borneo, enhancements of the three CSV basic characteristics are ≥30%. For an HRF cyclone/event, enhancements of these three basic characteristics from the parent CSV are ≥150%. Such distinguishable enhancements are useful means to separate HRF or non-HRF CSVs.

3. Formation of rainfall center

a. Contribution of rain-producing disturbances

Rainfall occurs along the coast of the Peninsular Malaysia in early winter and along the near-equator trough across west Borneo in winter. Because rain is produced by severe weather systems, the relationship of the rain-producing weather systems over these two parts of Malaysia with their respective environmental flows may be different. To understand this relationship, the formation of the two Malaysia rainfall centers was examined from a synoptic perspective in a climatological context. Several hundred rain-producing weather disturbances [including EWs, TCs, CSVs, and HRF cyclones/events] were identified during November–February of 1979–2009. To save space in presenting tracks of these disturbances, we focus only on those developing into HRF cyclones/events—major contributors to the rainfall centers for both parts of Malaysia. Superimposed on the long-term monthly-mean 850-hPa streamline charts are tracks of these weather systems and rainfall during November–February shown in Figs. 2a–f. In addition to the population statistics of these disturbances (Fig. 4), the nature of these tracks is divided into two different types that are illustrated below.

Fig. 4.
Fig. 4.

(a) Monthly-mean occurrence frequency of tropical cyclone (TC), easterly wave (EW), CSVBM, CSVPM, (HRFBM + HRFBBM), and HRFPM in Peninsular Malaysia are shown in the left column. Precipitation amounts produced by these disturbances individually are shown in the right column. The bottom panel of the right column shows the monthly-accumulated precipitation contributed by all disturbances. (b) Monthly occurrence frequency of TCs, EWs, (CSVBB + CSVBBM), CSVPB, and (HRFBB + HRFBBM) in Borneo are shown in the left column. Precipitation amounts contributed by these disturbances individually are shown in the right column. The accumulated monthly precipitation by all disturbances is shown in the bottom panel of the right column. Precipitation shown in (a) and (b) is averaged over the yellow areas of Peninsular Malaysia and Borneo, respectively, in the bottom middle panel.

Citation: Journal of Climate 26, 3; 10.1175/JCLI-D-12-00174.1

1) Early winter rainfall center in Peninsular Malaysia

Based on the formation and track of CSVs related to all identified HRF cyclones/events, which contribute to the Peninsular Malaysia rainfall center during November–December, two different groups of disturbances are identified.

  • Group I (westward-propagating CSVs through the southern Philippines): The interaction of the easterly wave with the East Asian cold surge flow may form a CSV in the vicinity of the Philippines. Propagated by the easterlies north of the near-equator trough across the tropical South China Sea, some CSVs may intensify to become HRF cyclones over the South China Sea and eventually evolve into HRF events (1, thick golden crosses) in Peninsular Malaysia; seven HRFPM events formed in November (Fig. 2a) and eight in December (Fig. 2b).
  • Group II (CSVs formed in Borneo): CSVs may form in Borneo by the interaction of the East Asian cold surge flow with the mountains between west Borneo and Kalimantan. Some of these CSVs may develop into three types of HRF events: 1) those propagating westward across the South China Sea by the tropical easterlies north of the near-equator trough to become Peninsular Malaysia HRF events (, bold red crosses), 2) those evolving into Borneo HRF events, but eventually propagating westward to become Peninsular Malaysia HRF events (HRFBBM, bold green crosses), and 3) those trapped in Borneo and developing in situ into Borneo HRF events (HRFBB, bold purple crosses). The populations of these HRF events during November–February are shown in the bottom right corner of Figs. 2c–f.

2) Winter rainfall center in west Borneo

In December, the near-equator trough (Fig. 2d) is located across northern Borneo, so some easterly waves can propagate through this part of Borneo (not shown). In fact, most CSVs are formed in Borneo by the interaction of the East Asian cold surge flow with Borneo’s orography: 18 out of 40 Borneo CSVs generated by this process became HRF events (specifically, 6 Borneo CSVs moved across the South China Sea to become HRFBM events in Peninsular Malaysia, and 12 Borneo CSVs evolved into in situ Borneo HRFBB cyclones/events). Eventually, four propagated across the South China Sea to join the other six HRFBM cyclones/events. During the next two months (January–February), Borneo CSVs are trapped over west Borneo because the near-equator trough is located across the central part of Borneo. Some CSVs evolved into in situ HRFBB events—nine in January and five in February. As indicated by Figs. 2e,f and 4b, almost all Borneo CSVs are trapped over northwestern Borneo. Apparently, the equatorward migration of the near-equator trough has a dynamical effect on the propagation properties of Borneo CSVs and HRF events.

The formation mechanisms of the rainfall centers in Peninsular Malaysia and west Borneo are distinctively different. The former is primarily established by the westward-propagating CSVs and HRF cyclones in November–December from both the southern Philippines and northwestern Borneo, whereas the latter is established by the in situ CSVs and HRF cyclone/events during December–February. It becomes clear the seasonal movement of the large-scale circulation over tropical Southeast Asia significantly affects the propagation properties of rain-producing weather disturbances. This impact results in the transition from the major rainy season of Peninsular Malaysia in early winter to that of west Borneo in late winter.

b. Statistics

The quantitative rainfall contributions to the two rainfall centers on both sides of Malaysia are presented by the area-averaged rainfall over the yellow areas in the geography shown in the central bottom panel of Fig. 4. The population of rain-producing weather systems over the two rainfall centers is shown in the left columns of Figs. 4a and 4b, while the averaged monthly-mean rainfalls contributed by these disturbances are shown in the right columns of these two figures.

1) Peninsular Malaysia (November–December)

The population of EWs is larger than that of TCs, but the amounts of rainfall produced by them are similar. Observed by Cheang (1977), the equatorial vortex can be intensified in the presence of the East Asian cold surge flow. Later, Yokoi and Matsumoto (2008) showed the necessary condition for the development of HRF events is the presence of tropical depression–like vortices. CSVs can form over the Philippines vicinity (CSVP) and Borneo (CSVB). The CSVB population is larger than the CSVP population, but the rainfall produced by the former group (15%) is somewhat smaller than the latter group (18%). Both regions contribute the same population of HRF events to Peninsular Malaysia, but the rainfall contributions by these two groups of HRF events are different: the former group produces 25%, whereas the latter generates 32%. According to the ratio between rainfall contribution and disturbance population, both CSVPM and HRFPM are more efficient in rain production than their corresponding counterparts from Borneo. Is this difference in the rain-producing efficiency caused by the longer track from the Philippines vicinity to pick up more moisture across the South China Sea? This question may need consideration for future research. The accumulation of rainfall contributions from different synoptic disturbances is shown in the bottom panel of the right column in Fig. 4a. Interesting features of the Peninsular Malaysia rainfall include the following:

  1. The Peninsular Malaysia rainfall center is created by rain-producing disturbances from both the vicinity of the Philippines and Borneo.
  2. The rain-producing efficiency of the former group is larger than the latter group.
  3. The major rain-producing disturbances are CSVs and HRF events, but the latter group is the most important.

2) Borneo (December–February)

During October–November, the CSVBB and CSVPB populations are about 52 individually (the left column of Fig. 4b), but the CSVPB produces 32% of Borneo rainfall against 14% of the CSVBB group. About one-third of Borneo’s rainfall during October–November is produced by foreign CSVs, although two-thirds of Borneo’s rainfall is produced in situ by domestic disturbances, particularly HRFBM and HRFBBM events. In contrast, during December–February the foreign group produces only 5% of Borneo’s rainfall against 87% of the domestic group. The rainfall regime of Borneo during December–February is recognized as a domestic (in situ) one. The most important concern raised by these rainfall statistics over Borneo is: Why are the rain-producing disturbances trapped over this island? Is it caused by lower-level convergence along the near-equator trough? This question will be answered by the synoptic and dynamic analyses.

3) Summary

The statistics of population and rain amount contributed by different rain-producing weather systems are summarized in Fig. 5 with the following salient features:

  1. The ratio of rainfall per month between Peninsular Malaysia and west Borneo is close to . The rainfall-producing efficiency in Peninsular Malaysia region is higher than west Borneo.
  2. The ratios of rainfall produced by the Philippines disturbances (CSVP + HRFP event) and the Borneo disturbances (CSVB + HRFB) are ~ in Peninsular Malaysia and <⅛ in west Borneo. The rainfall in the Peninsular Malaysia region is mainly produced by foreign disturbances, whereas the west Borneo region is primarily produced by domestic disturbances.
Fig. 5.
Fig. 5.

(a) Histograms of the rainfall contributions from easterly wave, tropical cyclones, cold surge vortices, and HRF events in (a) Peninsular Malaysia during November–December and (b) west Borneo during December–February.

Citation: Journal of Climate 26, 3; 10.1175/JCLI-D-12-00174.1

It was shown by Chen et al. (2012a) that two-thirds of rainfall in central Vietnam during October–November (peak rainfall season) is produced by HRF events, but the population of HRF events compared to other rain-producing weather systems, including CSVs and TCs, is about (HRF event)/(HRF event + CSV + TC) ~31/(31 + 102 + 25) ~20% (⅕). Populations of all HRF events in Peninsular Malaysia and west Borneo compared to the total population of rain-producing weather systems are >20% [all HRF events / (all HRF events + CSVP() + CSVB() + TC)]. As shown in Fig. 4, the major population of rain-producing weather systems is formed by CSVs, but the major rainfall (<60%) over these two parts of Malaysia is produced by HRF events, just like those HRF events occurring in central Vietnam during October–November.

4. Synoptic development and basic characteristics of rain-producing weather systems

Synoptically, the HRF event that occurs in central Vietnam evolves from a parent CSV formed in the vicinity of the Philippines by the interaction of the easterly wave with an East Asian cold surge flow (Chen et al. 2012a). As shown in Fig. 2, the propagation tracks of rain-producing weather systems reaching Peninsular Malaysia and west Borneo are more complicated than those reaching central Vietnam. Thus, there is a need to synoptically understand the formation of HRF events in both parts of Malaysia from their parent CSVs. In view of the complicated propagation tracks and patterns, we are concerned with several salient synoptic features:

  1. Under what synoptic conditions are CSVPs and CSVBs, and other disturbances, able to propagate westward, across the South China Sea, to form HRF events in Peninsular Malaysia during November–December? Why should a greater majority of CSVBs and HRFB events be trapped in Borneo during December–February?
  2. Based on the flow paths, Compo et al. (1999) classified the East Asian cold surges into two types—the Philippine Sea (PHS) and South China Sea (SCS) types. How do these two types of East Asian cold surge flows affect formations and tracks of rain-producing weather systems related to the two parts of Malaysia?
  3. The HRF event evolves from its parent CSV. How do we distinguish the HRF and non-HRF CSVs?

As shown in Fig. 4, the total number of rain-producing weather systems identified in both parts of Malaysia is over 200. To answer these three questions in the following three subsections, respectively, we focus primarily on the major rain-producing weather systems selected by the following criterion: the ratio P(X)/N(X) ≥ 100%, where P and N are the percentages of rainfall and population contributions of disturbances (X) to both parts of Malaysia (Table 2). These disturbances are HRF events and their parent CSVs.

Table 2.

Contribution (%) of rainfall (P) and population (N) by major rain-producing weather systems to both parts (Peninsular Malaysia and Borneo) of Malaysia during November–December and December–February, respectively.

Table 2.

a. Synoptic analysis

1) HRFPM event developed from CSVP

The well-developed/organized East Asian cold surge flow appeared in the Yellow Sea on 5 December 2005 (Fig. 6a). The downstream part of this cold surge covered the western North Pacific, east of Taiwan: the PHS type. At 500 hPa (Fig. 6b), an easterly disturbance (a short, red-dotted line) propagated toward the Philippine Archipelago and interacted with this PHS-type cold surge flow to form a CSVP, located within an active near-equator trough (a long, red-dashed line in Fig. 6a). It is revealed from the contrast between Figs. 6a and 6b that this East Asian cold surge flow was coupled with a northeast Asian trough that moved eastward, while the CSVP was centered at the southwest periphery of the eastern Pacific subtropical anticyclone. Several days later on 9 December, the northeast Asian trough propagated farther eastward, but the western part of this trough started to deepen and intensify the surface high associated with the northeast Asian cold surge (Fig. 6c). In the tropics, northeasterlies of the PHS-type cold surge and the tropical easterly flow of the Pacific subtropical anticyclone propagated the CSVP southwestward across the Celebes Sea. This CSVP developed into an HRF cyclone across the South China Sea (Fig. 6c) and reached 500 hPa (Fig. 6d) on 9 December. On its way to Peninsular Malaysia, a new Yellow Sea cold surge, adjacent to the remnant cold surge, appeared to interact with this HRF cyclone (not shown). On 15 December, this interaction led this cyclone to develop into an HRF event over Peninsular Malaysia (Fig. 6e). The synoptic evolution of the CSVP into an HRFPM event, presented in Figs. 6a,c,e, is verified by surface analysis maps from NCEP SRRS, JMA, and TMD (not shown).

Fig. 6.
Fig. 6.

The synoptic evolution of an HRFPM event (15 Dec 2005) from a CSVP (5 Dec 2005) depicted by the V(925 hPa) streamline charts superimposed with TRMM precipitation: (a),(c), and (e). The upper-level synoptic conditions corresponding to the 925-hPa flow patterns are presented by the V(500 hPa) streamline chart superimposed with isotachs. The shearline of the near-equator trough at 925 hPa is depicted by a red-dashed line. Scales of precipitation and isotachs are shown above the upper right corner of (a) and (b), respectively.

Citation: Journal of Climate 26, 3; 10.1175/JCLI-D-12-00174.1

2) HRFBM event developed from CSVB

Blocked by the subtropical easterlies of a remnant cold surge in the northern part of the central Pacific, the cold surge flow across the Yellow Sea was channeled into the SCS to form a SCS-type cold surge on 22 December 2008 (Fig. 7a). A vortex formed in west Borneo by the interaction of this cold surge flow with the Borneo orography named the Borneo cold surge vortex, CSVB. As indicated by rainfall and the cyclonic vortex, this CSVB was located at the near-equator trough (a red-dashed line). The Yellow Sea cold surge flow was coupled with a midtropospheric short-wave trough along the eastern seaboard of northeast Asia (Fig. 7b). Located to the south of this trough was a subtropical anticyclone juxtaposed with the tropical Southeast Asian cyclonic shear flow. Two and one-half days later on 24 December, this CSVB evolved into an HRF cyclone (Fig. 7c), located south of the western Pacific subtropical anticyclone (Fig. 7d). Because the remnant from the northeast Asia cold surge flow moved eastward with the upper-air short-wave trough, a new northeast Asian cold surge already appeared over northern China. Later, the remnant cold surge flow in the Pacific became easterly and joined the upper easterlies of the subtropical anticyclone to transport the CSVB westward out of Borneo. On 25 December, a new northern China cold surge flow reached the Yellow Sea and strengthened the midlatitude–tropics interaction with the HRF cyclone, eventually reaching Peninsular Malaysia on 29 December and forming an HRFBM event there (Fig. 7e). The westward propagation of the CSVB out of Borneo by the upper-level easterly flow of the subtropical anticyclone is still well reflected by the 700-hPa streamline chart in Fig. 7f.

Fig. 7.
Fig. 7.

As in Fig. 6, but for an HRFBM event (29 Dec 2008).

Citation: Journal of Climate 26, 3; 10.1175/JCLI-D-12-00174.1

3) HRFBB event developed from CSVB

On 26 January 2009, a well-developed and organized near-equator trough (red-dashed line) emerged along the equator (Fig. 8a). A CSVB formed in west Borneo by the interaction of the SCS-type cold surge flow and the Borneo orography. At this stage, the near-equator trough is not well discernable at 700 hPa (Fig. 8b). This SCS cold surge flow was coupled with a short-wave trough in the northern SCS and also linked to a Korean cold surge coupled with the short-wave trough along Japan. Several days later on 30 January, the Korean cold surge flow moved eastward with the upper-level short-wave trough (Fig. 8d). A new short wave propagated with its trough located along the coast of East Asia and coupled with a new cold surge connected to the already existing SCS-type cold surge flow west of the HRFB cyclone, evolving from the west Borneo CSVB (Fig. 8c). On 1 February, this HRFB cyclone developed into a west Borneo HRFBB event (Fig. 8e). Regardless of the eastward movement of the northeast Asian cold surge flow with the upper-air short-wave trough (Fig. 8f), the near-equator trough and the SCS-type cold surge flow across the tropical SCS are more or less stationary over the entire development period of the HRFBB event, since the date its parent CSVB formed.

Fig. 8.
Fig. 8.

As in Fig. 6, but for an HRFBB event (1 Feb 2009).

Citation: Journal of Climate 26, 3; 10.1175/JCLI-D-12-00174.1

b. Formation of CSV and its track leading to formation of HRF event

It was shown in Fig. 4 the Peninsular Malaysia rainfall is produced by 39 non-HRF CSVP, 15 HRFPM, 57 non-HRF CSVB, and 15 HRFBM + HRFBBM events, and the west Borneo rainfall is generated by 88 non-HRF CSVB and 26 HRFBB events. Can CSVP and CSVB be formed by the PHS- and SCS-type cold surge flows, respectively, as shown by sample cases in Figs. 68? To answer this question, let us define these two types of cold surges by the following two criteria:

  1. Both types of cold surge flows at Dongsha Island (20.72°N, 116.7°E) in the northern South China Sea and a location (18.28°N, 130.8°E, ) in the Philippine Sea should exhibit a surface pressure increase 1 hPa and a surface temperature drop 1°C, within 24 h. These two locations are shown at the bottom of Fig. 9a.
  2. The SCS- and PHS-type cold surges are determined by the following wind directions: The direction of Vs at Dongsha Island, , is within the range 225° ≥ ≥ 180°, while the direction of V(925 hPa) at the Philippine Sea location, , is within the range 270° ≥ ≥ 225°.
Formations of the non-HRF and HRF CSVP and CSVB related to the two (PHP and SCS) types of cold surges are illustrated by three basic characteristics (direction/speed, magnitudes of Δps and ΔTs, and preferred season) in Fig. 9:
  1. Direction and speed of cold surge related to the CSV formation (Fig. 9a): Directions () of most cold surge flows related to the formations of HRFPM (blue asterisk) and non-HRF (blue dot) CSVPMs are located within the range of 270° ≥ ≥ 225°, and those related to HRFBB (red asterisk) and non-HRF (red dot) CSVBB are contained within the range of 225° ≥ ≥ 180°. Speeds of cold surge flows at the two designed locations are measured by the radius of their circles. We also identified another group of CSVBM that propagate westward, across the South China Sea, to Peninsular Malaysia, but either are maintained as non-HRF CSVBM or form HRFBM events. Directions of the cold surges related to this group are colored green, and located primarily within the range of 270° ≥ ≥ 225°.
  2. Magnitudes of and (Fig. 9b): Both and for each season in both locations are plotted with symbols corresponding to those used in Fig. 9a. Formations of all related CSVP and CSVB, whether or not they lead to the development of HRF events, are related to cold surges.
  3. Preferred season of the CSV formation (Fig. 9c): The HRF and non-HRF CSVs formed by the two types of cold surge flows in the vicinity of the Philippines and Borneo are marked with color symbols corresponding to those in Fig. 9a. Most CSVPM and CSVBM are formed during November–December by the PHS and SCS types of cold surge flows, respectively. On the other hand, most CSVBBs are generated in Borneo during December–February by the cold surge flows of the latter type. The seasonal stratification of CSV formation by these two types of cold surge flows reveals an important implication. The northeast Asian cold surge activity, in response to the seasonal variation of the winter atmospheric circulation over the northeast Asia–northwest Pacific region, may affect the seasonal variation of the HRF event activity in Malaysia.
Fig. 9.
Fig. 9.

(a) Direction and speed of the cold surge flow related to the formation of three different types of CSVs at two locations (large red dot at Dongsha Island and large blue dot in the Philippine Sea) shown in the bottom panel: the HRF and non-HRF CSVs are marked by (•) and (), respectively, and ()PM, ()BM + ()BBM, and ()BB are denoted by blue, green, and red, respectively, in a small table in the left bottom corner of the top panel. All selected cold surge flows are shown by small blue dots (SCS types) and red dots (PHS types). The large blue and red dots are averaged locations of the SCS and PHS groups, respectively. The blue and red oblongs are one standard deviation of locations used to determine cold surges. (b) Magnitude changes of surface pressure (Δps) and temperature (ΔTs) over a period of 24 h by cold surge flows at locations shown in (a). The cold surge flows of SCS (PHS) types are shown in the left (right) panels: Δps(ΔTs) is shown in the top (bottom) panel. Averaged values of Δps and ΔTs for cold surges related to three different groups of (CSVs, HRF events) are tabulated in the table at the bottom of (b). (c) Formation date of every CSV.

Citation: Journal of Climate 26, 3; 10.1175/JCLI-D-12-00174.1

c. Basic characteristics of CSV and HRF cyclone/event

It was shown in Fig. 4 the population of HRF events is much smaller than the population for CSVs in both Peninsular Malaysia and west Borneo, but major rainfall is produced by the HRF events. Because HRF events develop from CSVs, it is important to distinguish them by quantifying some of their basic characteristics during their life cycles. These basic characteristics include maximum speed, size (east–west dimension), and rainfall around centers of CSV and HRF cyclone/event. Measurements of these three basic characteristics are performed with the following definitions:

  1. Maximum speed: The maximum isotachs at 850 hPa, |V(850 hPa)|, around the CSV and HRF cyclone is defined as their maximum speeds. The CSV and HRF cyclone are defined with |V(850 hPa)|max 8 m s−1 and 12 m s−1, respectively.
  2. Size (east–west dimension): the east–west distance between the maximum isotachs in the eastern and western periphery of a CSV or HRF cyclone. This is validated by the rainfall distribution, if the isotach distribution against the streamline chart is unclear.
  3. Rainfall: The area average of rainfall 3 mm day−1 around a CSV or HRF cyclone.

It was shown in Fig. 2 that the track properties of the CSV and HRF events that produce rainfall in Peninsular Malaysia and Borneo are different. The rainfall of the former region is produced by CSVs and HRF cyclones from two different regions—the vicinity of the Philippines and Borneo. The rainfall in the latter region is primarily generated by CSV and HRF cyclone events formed in Borneo. The three basic characteristics of CSV and HRF cyclone/events originally propagating from the vicinity of the Philippines and Borneo to the Peninsular Malaysia are presented in Figs. 10a and 10b, respectively, and those CSVB that become trapped and evolve into HRFBB are shown in Fig. 10c. For CSVP and CSVB, their three basic characteristics do not show significant changes between their original and final locations in Peninsular Malaysia. The three basic characteristics of the CSVB trapped in Borneo behave somewhat similar as the CSVP and westward-moving CSVB. In contrast, these three basic characteristics are notably enhanced, particularly size and rainfall, between non-HRF CSVs and HRF events. To concisely illustrate this enhancement, let us summarize the characteristic contrasts between the HRF and CSV cyclone/events in the Malaysia Peninsula and Borneo in Table 3. The basic characteristics of parent CSVP and CSVB do not exhibit significant differences (~30%) over their life time between original locations and Peninsular Malaysia, or stay within Borneo, but magnitudes of these three characteristics of HRF cyclone/events are much larger (≥150%) than those for CSVs. It is of interest to see in Fig. 10—regardless of the lifetime shown in the bottom panels of Figs. 10a and 10b, and the ordinate in each panel of Fig. 10c or the travel distance of parent CSVs—that all three characteristics of CSVs or HRFP cyclones/events behave similarly.

Fig. 10.
Fig. 10.

Basic characteristics of non-HRF CSVs, HRF CSVs, and HRF cyclones/events: (top) maximum speed, (middle) size, and (bottom) rainfall. Enhancements of these characteristics for the three groups of rain-producing disturbances are shown in three columns. (a) Enhancement from HRF CSVs (non-HRF CSVs) in the vicinity of the Philippines to HRFPM events (non-HRF CSVPMs) over Peninsular Malaysia. (b) As in (a), but from CSVBs in west Borneo to HRFBM events (non-HRF CSVBMs) over Peninsular Malaysia. (c) As in (a), but from CSVBs in west Borneo to in situ HRFBB events (non-HRF CSVBBs). Life cycles for the three CSV groups to become HRF events are shown at the bottom of every column.

Citation: Journal of Climate 26, 3; 10.1175/JCLI-D-12-00174.1

Table 3.

The enhancement ratio (%) of three basic characteristics (maximum speed, size, and rainfall) from three types parent CSVs to their HRF events and to their mature CSVs in Peninsular Malaysia and Borneo.

Table 3.

5. Propagation and nonpropagation mechanisms of Borneo CSVs and HRF cyclones/events

The winter atmospheric circulation over East Asia and the western North Pacific region is characterized by a vertically westward tilting trough located in the eastern seaboard of northeast Asia, separated from the subtropical anticyclone in the western Pacific by the Japan jet stream (not shown). South of this midlatitude trough, a lower-tropospheric anticyclone centered at Taiwan is meridionally juxtaposed with the near-equator trough (Fig. 11a). A cold surge–like northwesterly flow is sandwiched by this continental anticyclone and the northwest Pacific trough (Fig. 11b). The CSVB and HRFB disturbances either propagate westward to Peninsular Malaysia during late fall–early winter or become nonpropagating (trapped) during late winter. One may wonder whether the seasonal evolution of the large-scale circulation in East and Southeast Asia may affect the propagation properties of these disturbances.

Fig. 11.
Fig. 11.

(a) The January–February V(850 hPa) streamline chart superimposed with vertical motion, (b) the January–February V(925 hPa) streamline chart superimposed with vorticity, (c) the difference of V(925 hPa) and vorticity between January–February and November–December. Also shown are the latitude–height cross section along 115°E for (d) vertical motion during January–February, (e) vorticity during January–February, and (f) differences of streamfunction and vorticity between January–February and November–December. Scales of (850 hPa) in (a), (925 hPa) in (b) and (925 hPa) in (c) are shown at the top right of each panel.

Citation: Journal of Climate 26, 3; 10.1175/JCLI-D-12-00174.1

During January–February, the near-equator trough anchors across Borneo (Figs. 11a,b). Along this trough, the upward motion (Fig. 11d) and cyclonic vorticity (Fig. 11e) penetrating vertically beyond 400 hPa are juxtaposed with anticyclonic vorticity and a downward motion in the subtropics of both hemispheres. This circulation structure reflects the deepening of the near-equator trough and the intensification of the subtropical high in both hemispheres from late fall–early winter to late winter. As revealed from the latitude–height cross section of the corresponding change in streamfunction and vorticity at 115°E [ (115°E); Fig. 11f], this circulation change reaches beyond 400 hPa. At the surface, the Asian cold surge flow can penetrate equatorward underneath the western Pacific subtropical anticyclone to reach the near-equator trough (Fig. 11b). The meridional stratification of the environmental vorticity (Figs. 11e,f) may facilitate CSVB to become trapped in Borneo.

It was inferred from the synoptic analysis that a CSVB propagates westward when the tropical easterlies penetrate across the South China Sea. In contrast, a CSVB is trapped in Borneo when the SCS-type cold surge flow is strong. This inference will be substantiated dynamically by the vorticity () budget analysis:
e1
eq1
Note that V, υ, f, and are the velocity vector, meridional wind, Coriolis parameter (=2), and , respectively. This analysis is performed with developments of the HRFBM and HRFBB events presented in Figs. 7 and 8, respectively.

a. Westward-propagating CSVB/HRFBM event

The 850-hPa streamline charts on 24 December 2008 are superimposed with the four dynamical processes shown in Fig. 12. The comparison between Figs. 7c and 12a indicates that the upper-level tropical easterlies between Taiwan and northern Borneo penetrated down to 850 hPa. At this time, the CSVB was centered at the western tip of Borneo. The well-organized tropical easterlies enabled the (850 hPa) tendency (advection of relative vorticity) to exhibit a positive (negative) value west (east) of this vortex center. The strong northeasterlies (weak southeasterlies) northwest (southeast) of this center also make the (850 hPa) tendency (meridional advection of planetary vorticity) positive (negative) west (east) of this center (Fig. 12b). Combining these two dynamic processes results in a positive–negative juxtaposition of tendency in such a way to propagate this vortex westward (Fig. 12c).

Fig. 12.
Fig. 12.

The V(850 hPa) streamline charts superimposed with the vorticity budget of an HRFBM event (0000 UTC 24 Dec 2008), which moved from west Borneo to Peninsular Malaysia: the vorticity tendency caused by (a) horizontal advection of relative vorticity, (b) meridional advection of planetary vorticity, and (c) sum of (a) and (b); the vorticity tendency caused by (d) vortex stretching of relative vorticity, (e) vortex stretching of planetary vorticity, and (f) sum of (d) and (e); and (g) vorticity, (h) vorticity tendency, and (i) the sum of (c) and (f). Scales of vorticity tendency and vorticity are shown at the top and bottom of the left column.

Citation: Journal of Climate 26, 3; 10.1175/JCLI-D-12-00174.1

Regardless of the tropical easterly flow across the South China Sea and north Borneo, the well-developed/organized vertical motion over this CSVB generates a strong positive (850 hPa) tendency (stretching of relative vorticity) center juxtaposed with a minor negative center on both sides (Fig. 12d). Because f = 0 at the equator, the (850 hPa) tendency (stretching of planetary vortex) is insignificant over this CSVB (Fig. 12e), but it is significant over the SCS north of this vortex. Thus, a positive (850 hPa) [=(+)(850 hPa)] center over the CSVB and its northern vicinity are juxtaposed with a minor negative center on both sides (Fig. 12f).

A pattern closely resembling (850 hPa) (Fig. 12h) across the CSVB center emerges from the summation of (850 hPa) and (850 hPa) tendencies (Fig. 12i); a west (positive)–east (negative) juxtaposition of (850 hPa) spatially in quadrature with this CSVB vortex. It is revealed from this spatial quadrature relationship between centers of (850 hPa) and (850 hPa) that the analyzed CSVB is propagated westward by the dynamic process (+). As inferred synoptically, the westward penetration of tropical easterlies into the South China Sea after the CSVB formation on 22 December 2008 is the most crucial factor enabling the CSVB to propagate westward.

b. Nonpropagating CSVB/HRFBB event

Four days later after formation on 23 January 2009 (Fig. 8a), the CSVB evolved into an HRFB cyclone. The SCS northeasterly flow at this stage was still maintained by the tropical easterlies that originated from the remnant cold surge flow (Fig. 13). These northeasterlies generated the north–south stratified (850 hPa) tendency (Fig. 13a) and the dominant (850 hPa) tendency (Fig. 13b). The (850 hPa) tendency (Fig. 13c), ( + )(850 hPa), exhibits an east–west juxtaposition of the positive value southwest of this vortex center over southwestern Borneo and the tropical South China Sea, with the negative value east of this vortex center covering the region from northern Borneo to the Celebes Islands of Indonesia.

Fig. 13.
Fig. 13.

As in Fig. 12, but for an HRFBB event (0000 UTC 30 Jan 2009).

Citation: Journal of Climate 26, 3; 10.1175/JCLI-D-12-00174.1

Because of the well-organized vertical motion over the CSVB, both (850 hPa) (Fig. 13d) and (850 hPa) tendencies (Fig. 13e) over this vortex resemble those of the propagating CSVB shown in Fig. 12. These two dynamic processes combined, the (850 hPa) tendency, exhibit a negative (west)–positive (center)–negative (east) train across Borneo where the positive (850 hPa) center coincides with the positive (850 hPa) center. The combination of the (850 hPa) and (850 hPa) tendencies (Fig. 13i) across Borneo shows a spatial pattern resembling that for (850 hPa) tendency (Fig. 13h); a positive center of (+)(850 hPa) juxtaposed with a minor negative center on each side of this positive center. Apparently, both centers of (850 hPa) and (850 hPa) are spatially in phase. This dynamic relationship indicates that the two dynamic processes ( and ) work together to maintain/intensify the CSVB. The northeasterly flow of this SCS-type cold surge plays a crucial role to keep the CSVB and HRFB trapped in Borneo.

c. Summary

According to Figs. 2 and 4, 57 CSVBM and 15 (HRFBM + HRFBBM) cyclones/events occur during November–December and 88 CSVBB and 26 HRFBB cyclones/events appear during December–February. The spatial relationship between centers of (850 hPa) and (850 hPa) was analyzed so far only for a propagating HRFBM event (Fig. 12) and a nonpropagating HRFBB event (Fig. 13). One may question whether the spatial relationship between centers of (850 hPa) and (850 hPa) revealed from these two cases are common to others in these two groups. This concern is clarified by the spatial relationship between (850 hPa) and (850 hPa) centers for all disturbances in these two groups. The in-quadrature spatial relationship between (850 hPa) and (850 hPa) centers of CSVBM and HRFBM cyclone are marked by green dots and asterisks, respectively, in the upper-left corner of Fig. 14. The spatially in-phase relationship between (850 hPa) and (850 hPa) centers of CSVBB and HRFBB events are denoted with red dots and asterisks, respectively, in the lower half of Fig. 14. The separation of propagation properties between these two groups of disturbances and the seasonal preference of occurrence are very distinctive. Note that the latter aspect is surprisingly similar to Fig. 9c. Apparently, the propagation properties of CSVB and HRFB events are determined by the cold surge flow pattern (i.e., synoptic condition) related to the development of these Borneo disturbances. The westward-propagating CSVBM and HRFBM events are primarily formed by the PHS-type cold surge flow with well-organized strong easterlies across Borneo at 850–700 hPa. In contrast, the nonpropagating (trapped) CSVBB and HRFBB events are formed by the SCS-type cold surge flow with a strong meridional flow across the South China Sea.

Fig. 14.
Fig. 14.

Formation dates of CSVs in west Borneo. Non-HRF CSV is denoted by (•), while HRF CSV is marked by (); CSVBM and (HRFBM + HRFBBM) events are green, and CSVBBs and HRFBB events are red.

Citation: Journal of Climate 26, 3; 10.1175/JCLI-D-12-00174.1

6. Concluding remarks

Malaysia geographically consists of Peninsular Malaysia and west Borneo, separated by the South China Sea. The maximum rainfall in the former region begins about a month ahead of the latter region. It was shown in Fig. 2 the near-equator trough during January–February migrates equatorward and anchors across Borneo. Diminishing in Peninsular Malaysia after December, rainfall is trapped in west Borneo during deep winter. This seasonal difference in the rainfall maxima between these two regions may be caused by different rain-producing mechanisms. Because rainfall is produced by rain-producing weather systems [easterly waves (EWs), tropical cyclones (TCs), cold surge vortices (CSVs), and heavy rainfall/flood (HRF) events] over tropical southeast Asia, an effort was made to quantitatively estimate over both parts of Malaysia the rainfall contributed by these weather systems during the maximum rainfall seasons:

  1. During November–December the major rainfall in Peninsular Malaysia is primarily contributed by CSVs and HRF events developed from their parent CSVs from the vicinity of the Philippines and Borneo. The population (N) ratios of major rain-producing weather systems from these two regions are
    eq2
    The rainfall (P) contributions by CSVs and HRFs are
    eq3
    eq4
    The total rainfall produced by these two groups is 90% of the Peninsular Malaysia rainfall; the most important rainfall is produced by HRF events and the rain-producing efficiency is higher by [CSVPM, HRFPM event] than [CSVBM, (HRFBM + HRFBBM) event].
  2. During December–February, the population ratio of major rain-producing weather systems in west Borneo is N(HRFBBM + HRFBB)/N(CSVBB) = 26/88. The rainfall contribution by CSVBB and HRFB events are P[CSVBB(29%)] + P[(HRFBBM + HRFBB)(58%)] = 87% of the total rainfall of west Borneo. Undoubtedly, HRFBB and HRFBBM events are major rain-producing weather systems in this region.

The major weather systems to produce the rainfall centers in Peninsular Malaysia and west Borneo are CSV and HRF events. The rainfall centers in Peninsular Malaysia are formed by the interaction of an easterly wave with the East Asian cold surge, while the rainfall centers in west Borneo are developed from the former. In additional to the formation of these rain-producing weather systems, their propagation properties are also dependent on the cold surge flow patterns classified by Compo et al. (1999) into two types—the Philippine Sea (PHS) and South China Sea (SCS) types. Both CSVP and CSVB during November–December are primarily formed by the PHS-type cold surge flows, but CSVBB are almost exclusively formed by the SCS-type cold surge flows during December–February. The three basic characteristics of CSV and HRF cyclones/events (maximum speed, size, and rainfall) are enhanced about 30% and over 150%, respectively, from their parent CSVs. The enhancement rates of these basic characteristics can be applied to separate CSVs and HRF events. Regardless of traveling distances from the formation regions of parent CSVs to reach their destination regions, the life cycles of CSVs and their evolution into HRFs may reach seven days.

It was synoptically observed that CSVBMs, and HRFBM and HRFBBM events are formed by the PHS-type cold surge flow and propagated westward by the well-organized tropical easterly penetrating across the tropical SCS. In contrast, CSVBBs and HRFBB events are trapped in west Borneo by the well-organized strong SCS-type cold surge flow across the SCS. The vorticity budget analysis at 850 hPa was performed to illustrate dynamically how the flow pattern affects the propagation properties of these two groups of rainfall-producing weather systems. A spatially quadratic relationship between centers of (850 hPa) and (850 hPa) appears in the first group, while a coincidence of these two centers occurs in the second group.

The rainfall center in Peninsular Malaysia during November–December is formed by the rain-producing weather systems propagating from two remote genesis regions—the vicinity of the Philippines and west Borneo. In contrast, the Borneo rainfall center during December–February is formed mostly by the in situ rain-producing weather systems. This finding provides a new perspective about the formation mechanisms of rainfall centers in both parts of Malaysia, namely how the midlatitude–tropics interaction affects the formation and propagation properties of the rain-producing weather systems. The HRF events often bring disaster to Malaysia, so a better understanding about the formation mechanisms of the Malaysian rainfall centers would be useful to improve the prediction of HRF events. Some studies (e.g., Tangang and Juneng 2004) were devoted to exploring the interannual rainfall variation in Malaysia from the perspective of rainfall climatology affected by the El Niño–Southern Oscillation (ENSO) activity. Because the role played by the CSVs and HRF events in the formation of Malaysian rainfall centers, the effort to explore the cause of this interannual rainfall variation may be made through the ENSO impact on the synoptic activity and rain-producing efficiency of these rain-producing weather systems in Southeast Asia. This effort is made by a companion study that will be reported in the near future.

Acknowledgments

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 Japanese Ministry of Education, Culture, Sports, Science and Technology. We would also like to thank two reviewers for offering constructive comments to improve this paper.

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1

CSV(HRF)AB is a symbol used to indicate a CSV vortex formed in region A and transported to become a CSV (HRF) vortex (cyclone) in region B. Subscripts P, M, and B represent the Philippines, Peninsular Malaysia, and Borneo, respectively. When this symbol () is attached with double BB subscripts, ()BB, it means the () formed in Borneo is trapped there. If this symbol () is attached with triple subscripts, ()BBM, it denotes the ()BB vortex/cyclone transported to Peninsular Malaysia.

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