4. Annual Cycle of Southeast Asia-maritime Continent Rainfall and the Asymmetric Monsoon Transition

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  Annual Cycle of Southeast Asia—Maritime Continent Rainfall and the AsymmetricMonsoon Transition C.-P. C HANG AND  Z HUO  W ANG Department of Meteorology, Naval Postgraduate School, Monterey, California J OHN  M C B RIDE Bureau of Meteorology Research Centre, Melbourne, Australia C HING -H WANG  L IU Department of Atmospheric Sciences, Chinese Culture University, Taipei, Taiwan (Manuscript received 21 October 2003, in final form 9 June 2004)ABSTRACTIn general, the Bay of Bengal, Indochina Peninsula, and Philippines are in the Asian summer monsoonregime while the Maritime Continent experiences a wet monsoon during boreal winter and a dry seasonduring boreal summer. However, the complex distribution of land, sea, and terrain results in significant localvariations of the annual cycle. This work uses historical station rainfall data to classify the annual cycles of rainfall over land areas, the TRMM rainfall measurements to identify the monsoon regimes of the fourseasons in all of Southeast Asia, and the QuikSCAT winds to study the causes of the variations.The annual cycle is dominated largely by interactions between the complex terrain and a simple annualreversal of the surface monsoonal winds throughout all monsoon regions from the Indian Ocean to theSouth China Sea and the equatorial western Pacific. The semiannual cycle is comparable in magnitude tothe annual cycle over parts of the equatorial landmasses, but only a very small region reflects the twice-yearly crossing of the sun. Most of the semiannual cycle appears to be due to the influence of both thesummer and the winter monsoon in the western part of the Maritime Continent where the annual cyclemaximum occurs in fall. Analysis of the TRMM data reveals a structure whereby the boreal summer andwinter monsoon rainfall regimes intertwine across the equator and both are strongly affected by thewind–terrain interaction. In particular, the boreal winter regime extends far northward along the easternflanks of the major island groups and landmasses.A hypothesis is presented to explain the asymmetric seasonal march in which the maximum convectionfollows a gradual southeastward progression path from the Asian summer monsoon to the Asian wintermonsoon but experiences a sudden transition in the reverse. The hypothesis is based on the redistributionof mass between land and ocean areas during spring and fall that results from different land–ocean thermalmemories. This mass redistribution between the two transition seasons produces sea level patterns leadingto asymmetric wind–terrain interactions throughout the region, and a low-level divergence asymmetry in theregion that promotes the southward march of maximum convection during boreal fall but opposes thenorthward march during boreal spring. 1. Introduction The region of Southeast Asian landmasses, which in-cludes Indochina, the Malay Peninsula, and the Mari-time Continent, is situated between the Asian (Indian)summer monsoon and the Asian winter (Australiansummer) monsoon in both space and time. This areaforms the “land bridge” along which maximum convec-tion marches gradually from the Asian summer mon-soon to the Asian winter monsoon during boreal fall(e.g., Lau and Chan 1983; Meehl 1987; Yasunari 1991;Matsumoto 1992; Matsumoto and Murakami 2002;Hung et al. 2004). The seasonal march is not symmetric.During boreal spring the convection tends to stay nearthe equator as if it were blocked from moving north-ward. It stays until the reversed meridional thermal gra-dient is established when it jumps over the northernequatorial belt to mark the onset of the Asian summermonsoon.Indochina is often considered as part of the Asian Corresponding author address:  Dr. C.-P. Chang, Department of Meteorology, Naval Postgraduate School, Code MR/Cp,Monterey, CA 93943.E-mail: cpchang@nps.edu15 J ANUARY  2005 CHANG ET AL.  287 © 2005 American Meteorological Society  summer monsoon region while rainfall over most loca-tions in the Maritime Continent tends to reach maxi-mum during the boreal winter. This wet season is oftenrelated to the Australian summer monsoon (e.g.,McBride 1987) due to the proximity of the two regions.However, significant geographical variations of the sea-sonal march have been recognized since Braak (1921 – 29, see Ramage 1971). A main reason for these varia-tions is the complex terrain due to islands of differentsizes interspersed among the surrounding seas. Thevariations motivated Wyrtki (1956) to divide the searegion into 12 climate subregions that were presentedin Ramage (1971, Fig. 5.9). Other studies on the Indo-nesian rainfall climatology include Schmidt and Fergu-son (1951), Sukanto (1969), and McBride (1998). Morerecently, Hamada et al. (2002) documented the sea-sonal variations of rainfall at 46 Indonesian stationsduring a 30-yr period (1961 – 90) and classified the sta-tions objectively into five climatological types depend-ing on the phase and relative amplitudes of the annualand semiannual cycles of rainfall. Another approachwas used by Aldrian and Susanto (2003) and Aldrian etal. (2003), who used spatial correlation among stationrainfalls to divide the Maritime Continent into threesubregions. All these studies showed complex geo-graphical variations of the annual cycle of rainfall in theregion.The large variation in local rainfall over small dis-tances indicates that high-resolution wind data are re-quired to analyze the wind – terrain interaction of theMaritime Continent. The lack of historical high-resolution and consistent wind observations made suchan analysis difficult. Furthermore, since the rainfalldata used in the previous studies are all from land sta-tion reports and often concentrate only in Indonesia, agap exists in our knowledge of the distribution of theannual cycle of rainfall in the inner and surroundingseas of the Maritime Continent as well as regions northof Indonesia.Recently available satellite-based observations makeit possible to alleviate both problems. In this study,we analyze the annual cycle of rainfall by using twotypes of satellite data together with historical stationrainfall datasets. The Tropical Rainfall MeasuringMission (TRMM) precipitation radar data, availablesince late 1997, are used to enhance the analysis of the geographical variations of the annual cycle of rain-fall in a domain encompassing most of Southeast Asiaand the Maritime Continent. The National Aeronauticsand Space Administration ’ s Quick Scatterometer(QuikSCAT) winds, available since July 1999 at the seasurface, are used to help examination the relationshipbetween the monsoon – terrain interaction and the geo-graphic variations. The result of the analysis will beused to address the cause of the asymmetric seasonalmarch of maximum convection during the transitionalseasons. 2. Data Two datasets of monthly station rainfall are used inthis study. The first is an extension of the Indonesianrainfall dataset prepared by Kirono et al. (1999) andused in Haylock and McBride (2001). This dataset,hereinafter referred to as the INDO dataset, containsrainfall during 1950 – 97 at 63 Indonesian stations. Thesecond is the Association of Southeast Asian Nations(ASEAN) Climatic Atlas Project (ACAP) data, fur-nished by the Malaysian Meteorological Service. Thisdataset covers 935 rainfall stations in all the membernations of ASEAN during the data collection phase.The beginning dates of the stations vary, with the ear-liest date in each country as follows: Singapore, 1875;Malaysia, 1876; Indonesia, 1879; Philippines, 1902; andThailand, 1911. The majority of the stations start from1951 or earlier, and almost all start from 1958 or earlier.All stations end in 1975. The two sets have overlappingobservations over Indonesia between 1950 and 1975.The TRMM microwave precipitation data (Simpsonet al. 1996) became available November 1997. In thisstudy, we use the data of January 1998 – December 2002(data for 7 – 24 August 2001 are missing). The srcinaldata at resolution of 4 km  4 km are smoothed into agrid of 0.5 °    0.5 °  using the two-step filter of Leise(1982).The QuikSCAT scatterometer winds (Liu 2002)cover the period January 1999 – December 2002. Theseare scatterometer winds at the sea surface at 25 km  25 km resolution. On a typical day the available datacovers 75% of the equator and increased percentage of area away from the equator. These data are used toproduce monthly mean winds at the sea surface. 3. Analysis based on station rainfall reports a. Monthly mean rainfalls As background, the large influence of the annualcycle can be seen clearly in Fig. 1, which includesmonthly mean rainfall from both the INDO and ACAPdata for January, April, July, and October (Chang et al.2004). For this figure all station rainfall data havebeen interpolated to a 0.5 °  0.5 °  grid using the meth-odology of Cressman (1959). This figure should be in-terpreted with reference to the data locations or sta-tions in Fig. 2. The stations north of 10 ° N experience awet season during July and October and those south of 5 ° S have their wet season during January and April.Rainfall amounts are high throughout the region withindividual monthly totals on the order of 300 to 500mm. Most of the region experiences a distinct dry sea-son at some time of year, with the exception being partsof Borneo and New Guinea, which have high rainfallyear-round. (Place names are shown in Fig. 2.) Theseasonal shift is such that in January the center of massof the heavy rainfall is south of the equator while in 288  JOURNAL OF CLIMATE V OLUME  18  July the rainfall is northward of 10 ° N. This is associatedwith the seasonal migration of the intertropical conver-gence zone (ITCZ) in the region, as has been docu-mented by Johnson and Houze (1987), Waliser andGautier (1993), McBride et al. (1995), and Qian andLee (2000).The other major feature in Fig. 1 is the existence of strong rainfall gradients existing at all times of the year;for example, notice the east – west gradients of rainfallacross the Philippines in January and across the MalayPeninsula in April. As will be discussed below, thesepatterns result largely from the interaction between thehigh topography in the region and the moisture-bearinglow-level monsoon flow, whereby rainfall is enhancedwhen the flow is lifted on the upstream side of a moun-tainous island or peninsula. Conversely the flow on thedownstream side experiences a rainfall minimum asso-ciated with a lee or rain-shadow effect. b. Annual cycles The annual cycle and semiannual cycle modes are thefirst two harmonics of the climatologically averaged an-nual rainfall variation at each location. Because onlymonthly mean station rainfall data are available, eachtime series has only 12 data points. Figure 2 shows theannual cycle mode at land stations, with the amplituderepresented by the length of each arrow and the phaseshown as a 12-month clock with a northward arrowindicating maximum rainfall in January. The arrow ro-tates clockwise with eastward, southward, and west-ward arrows indicating April, July, and October, re-spectively. This figure is examined along with the meanQuikSCAT winds in January and July shown in Fig. 3,which also includes the distribution of topography.North of the Maritime Continent, data are availablein two Southeast Asian regions, Indochina (Thailandstations) and the Philippines (Fig. 2). In Indochina, theeffect of the Asian summer monsoon is clearly indi-cated with most Thailand stations showing maximumrainfall around June. Over the Philippines, the Asiansummer monsoon rainfall is defined at most stations inthe south and west where the rainfall maximum occursaround July. These are stations on the windward sideduring the southwest monsoon (Fig. 3). On the otherhand, most northern and eastern stations show maxi-mum rainfall in late summer or early fall. There are twofactors influencing this. A major factor for this varia-tion is that during the peak of the southwest monsoonin July these stations are on the leeward side of the high F IG . 1. Monthly mean rainfall and topography for (a) Jan, (b) Apr, (c) Jul, and (d) Oct (from Chang et al. 2004).15 J ANUARY  2005 CHANG ET AL.  289  topography on the islands. Because of the shelteringeffect of the topography, they do not experience a wetmonsoon at this time of year. However, during Septem-ber through December when the southwest monsoonbegins its retreat, northeasterly flow occurs and thesesame stations are now on the windward side. At thattime of year (fall – summer) there is still upward motionassociated with the retreating ITCZ; so there is a latewet season associated with terrain-lifted rainfall. Thesecond factor is that the number of typhoons peaks inSeptember; and tropical cyclones are considered to bemajor contributors to rainfall in the northeastern Phil-ippines (Coronas 1912; Mas ó  1914; Flores and Balagot1969). The relative contributions of these two factorsare not known without further study.The southern Philippine island of Mindanao is con-sidered part of the Maritime Continent, based on Ra-mage ’ s (1968) definition. Here, the seasonal cycle overmost of the region is characterized by the summer mon-soon rainfall. The exception is in its northeastern cor-ner where maximum rainfall occurs around Novemberand December, again as a result of the prevailing north-easterly onshore winds (Fig. 3) during the northernwinter monsoon.In Fig. 2 the distribution of the annual cycle over thePhilippines appears as a counterclockwise pattern inthe phase diagram, associated with a progression froma November – December maximum on the southeasternside, an August – September maximum on the northeast,and a July maximum along the western coastline. Incontrast, over the western Maritime Continent fromIndochina to the Malay Peninsula, phase of the annualcycle moves in a clockwise manner reflecting the sea-sonal march of the deep convection that follows the sun(Lau and Chan 1983). Thus, the annual cycle maximumoccurs mostly during northern fall in the Malay Penin-sula and northern Sumatra and changes to around De-cember in southern Sumatra. Over the rest of Indonesiathe boreal winter maximum in the annual cycle occursin most places, especially in southern Borneo, Java, andother islands near 10 ° S.As pointed out by previous investigators (e.g., Ra-mage 1971; Hamada et al. 2002), the majority of thelocations south of the equator have monsoonal rainfall F IG . 2. The annual cycle mode at rainfall stations. The phase of the cycle is shown as a 12-month clock with anorthward arrow indicating maximum rainfall in Jan. The arrows rotate clockwise with eastward, southward, andwestward arrows indicating Apr, Jul, and Oct, respectively. The length of the arrow defines the amplitude of thecycle. This figure also serves to show the locations of the observation stations for the monthly rainfall datasets usedin this study. 290  JOURNAL OF CLIMATE V OLUME  18
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