Climatological onset dates of summer monsoon over Myanmar

INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 31: 382 393 (11) Published online 1 February in Wiley Online Library (wileyonlinelibrary.com) DOI:.2/joc.76 Climatological onset dates of summer monsoon over Myanmar Ohnmar Htway a *andjunmatsumoto b,c a Department of Natural Environmental Studies, The University of Tokyo, Kashiwa, Chiba, 277-863, Japan b Department of Geography, Tokyo Metropolitan University, Hachioji, Tokyo, Japan c Institute of Observational Research for Global Change (IORGC), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan ABSTRACT: Asian summer monsoon is the predominant climate system in Myanmar and nearly 9% of total rainfalls are from summer monsoon. The timing of monsoon onset is of importance to the agricultural sector and water replenishment. However, there is a lack of systematic method to detect the climatological monsoon onset dates of Myanmar. In this study, the climatology of the summer monsoon onset over Myanmar is defined using the mean pentad precipitation data of 29 stations from 1968 to. As a result, the climatological monsoon onset dates over Myanmar are on 18th May (the middle date of pentad number 28) in the southern and central Myanmar and on 28th May (the middle date of pentad number 3) in the northern Myanmar. These climatological onset dates in two areas of Myanmar are confirmed by examining seven meteorological parameters of the observed station data and ERA4 re-analysis data during the period between 21st April and 9th June. Copyright Royal Meteorological Society KEY WORDS Myanmar; climatological onset; summer monsoon; meteorological parameters; observed station data Received 6 October 7; Accepted 21 November 9 1. Introduction Myanmar is located in the northwestern part of the Indochina Peninsula, between latitudes 9 32 N and 28 31 N and longitudes 92 E and 1 11 E. The most important feature in the climate of Myanmar is the alternation of seasons known as the monsoon. According to the Asia-Pacific monsoon division by Wang and LinHo (2), Myanmar is located between Indian summer monsoon and a broad transitional zone of the Indochina Peninsula. Zhang et al. (2) also pointed out that the Indochina Peninsula is a unique region which lies within the South and East Asia, and the monsoon system over it shows a transitional feature of South Asian Monsoon and East Asian Monsoon. Meteorologists in Myanmar consider the seasons of Myanmar based on the five periods: the pre-monsoon season from mid-april to mid-may, the monsoon season from mid-may to mid-october, the postmonsoon season from mid-october to end-november, the dry and cool season from end-november to mid- March, and the hot season from mid-march to mid-april (Lwin, ). Mid-April to mid-may is a period of continuous and rapid rise of maximum temperature in the whole country. Generally, the summer monsoon advances into the country during the middle of May with copious rain along the coastal areas. The study of local monsoon * Correspondence to: Ohnmar Htway, Department of Natural Environmental Studies, The University of Tokyo, Room 24, Environmental Bldg., Kashiwa Campus, -1-, Kashiwanoha, Kashiwa, Chiba, 277-863, Japan. E-mail: ohnmarhtway@nenv.k.u-tokyo.ac.jp system is essential as Myanmar is one of the world s great rice-growing countries and agricultural sector relies heavily on the summer monsoon rain. Of particular importance is to study the timing of monsoon onset. Though there are few studies of climate characteristics and classification of Myanmar using station data, for example, Eguchi (1996) and Roy and Kaur (), no one emphasizes to depict the image of long-term monsoon onset climatology of Myanmar except Matsumoto (1997) who investigated the mean onset and withdrawal of summer rainy season over the Indochina Peninsula using -day averaged rainfall data from 197 to 1987. Although other studies (Tao and Chen, 1987; Tanaka, 1992; Murakami and Matsumoto, 1994; Lau and Yang, 1997; Wang and LinHo, 2) also mentioned the onset maps of Asia including Myanmar, not only global dataset but also specific local station data should be examined if the local monsoon onset climatology of Myanmar calls for interpretation. Matsumoto (1997) described that a simple definition of onset and withdrawal of summer rainy season for the local level is needed so as to compare the largescale situation. The present definition which is used in the Department of Meteorology and Hydrology (DMH), Myanmar, is that the date of onset of the summer monsoon at various places in Myanmar is defined as the date of the first day of three consecutive rainy days with daily rainfall amount of 2.4 mm or more (Aung and Thoung, 198). Based on this definition, the climatological onset dates have been defined by Lwin (2) using frequency distribution Copyright Royal Meteorological Society

MYANMAR SUMMER MONSOON ONSET CLIMATOLOGY 383 of long-term onset dates for four different regions of Myanmar that summer monsoon advances in the southern Myanmar, which exists between latitudes 9 N and N, on th May, in the deltaic area between N and 17 N on th May, in the eastern, central and the western coast of Myanmar between 17 N to 23. N on th May and in the northern Myanmar of 23. N to28 N on 1st June. The weakness of this definition is that it uses the daily rainfall amount of each station and thus needs to omit any rainfall prior to th April as it is not monsoon rain. Some parts of Myanmar can get rain from thunderstorm formation due to insolation before monsoon onset and rainfall amounts of some stations are sometimes above 2.4 mm and rainy days are also more than 3 days though the rainfall duration is very short. For that reason, more objective definition should be considered to define the climatological onset dates of summer monsoon over Myanmar. DMH also traditionally determined four different regions of Myanmar based on the map of onset date isochrones over India and Myanmar which was modified in 1967 (Moe, 2). The rain gauge stations of Myanmar in 1967 were not many and those four regions should also be reconsidered according to the present rainfall stations. Thus the purpose of this study is to define the climatological onset dates of summer monsoon objectively using long-term precipitation data of Myanmar as monsoon rains are vital for agriculture and replenishment of water resources. Subsequently, the climatological structure of various parameters, not only the observed station data but also re-analysis data, from late April to early June will also be depicted and analysed to confirm the climatological monsoon onset dates and areas of Myanmar based on precipitation data. 2. Datasets Twenty-nine stations of Myanmar with less than 3-year missing data from 1968 to are used in this study. The 33-year mean pentad precipitation data are utilized to define the climatological summer monsoon onset dates of Myanmar. The 33-year mean pentad precipitation, maximum and minimum temperatures, relative humidity, and cloudiness data of 29 stations and the mean pentad sea level pressure (SLP) of 24 stations (Among 29 stations, five stations have no barometer to measure the SLP.) between the periods of 21st April and 9th June are examined to confirm the period and place of the significant changes of these meteorological parameters. The original daily observed station data are obtained from DMH, Myanmar. The locations of 29 stations are shown in Figure 1 and listed in Table I. Mean pentad data of 8-hPa horizontal wind are derived from daily 8-hPa wind of European Centre for Medium-Range Weather Forecasts (ECMWF) 4 years Re-analysis data (ERA-4) with a spatial resolution of 2. 2. from 1968 to. The -min gridded global relief data from the National Geophysical Data Center (NGDC) of the National Oceanic and Atmospheric 28 N 26 24 22 18 16 14 12 N C 92E Bay of Bengal B Andaman Sea A 94 96 98 2E m 4 3 Figure 1. Topography (shading) and distribution of 29 stations (dots) in Myanmar. White dots represent stations without barometer. A to C denote the stations used for calculating the pressure gradient in Figure 9. 9 32 N to 23. N is known as southern and central Myanmar and 23. N to28 31 N is called northern Mynamar. Administration Satellite and information Service is used for the stations map of Myanmar and model topography from the Climate Diagnostics Center (CDC) is used to mask 8-hPa wind field with its corresponding topographical feature. 3. Climatological summer monsoon onset dates of Myanmar (1968 ) In previous studies, various meteorological parameters are used to define the onset of the Asian summer monsoon such as wind speed and direction (Matsumoto, 1992), precipitation amount (Matsumoto, 1997; Wang and LinHo, 2), outgoing longwave radiations (Murakami and Matsumoto, 1994) and cloud amount (Tanaka, 1992). Although there is no infinite way to decide which meteorological parameter is best to define the summer monsoon onset, rainfall can be considered as an important meteorological parameter operationally, and rainfall variation displays the variability of the entire monsoon circulation system. Zhang et al. (2) constructed the domainaveraged rainfall index of the central Indochina Peninsula to define the onset of the summer monsoon over the Indochina Peninsula using -day running mean rainfall data. Compared with other countries of Southeast Asia, Myanmar does not lie completely within the tropics. According to the theoretical climatic zones, northern Copyright Royal Meteorological Society Int. J. Climatol. 31: 382 393 (11)

384 O. HTWAY AND J. MATSUMOTO Table I. Station list of Myanmar. Number Station Latitude ( N) Longitude ( E) 1 Putao 27.33 97.42 2 Hkamti 26. 9.7 3 Myitkyina.37 97.4 4 Homalin 24.87 94.92 Bhamo 24.27 97. 6 Kalewa 23. 94.3 7 Lashio 22.93 97.7 8 Monywa 22. 9.13 9 Mandalay 21.98 96. Kengtung 21.3 99.62 11 Taunggyi.78 97. 12 Meiktila.88 9.83 13 Minbu.17 94.88 14 Sittwe. 92.9 Kyaukpyu 19.42 93. 16 Pyinmana 19.72 96.22 17 Loikaw 19.68 97.22 18 Toungoo 18.92 96.47 19 Pyay 18.8 9.22 Thandwe 18.47 94.3 21 Pathein 16.77 94.77 22 Yangon 16.87 96.18 23 Bago 17.33 96. 24 Hpa-an 16.83 97.67 Mawlamyine 16. 97.62 26 Ye. 97.8 27 Dawei 14.7 98.18 28 Myeik 12.43 98.6 29 Kawthoung 9.97 98. Myanmar exists in the Temperate Zone, above 23. N (above the Tropic of Cancer) to 28 31 N and the other part of Myanmar which locates within the latitudes of 9 32 N and 23. N is known as southern and central Myanmar. Hence two domain-averaged rainfall indexes and the annual mean pentad precipitations (Pm = Annual precipitation/73) of these two areas over Myanmar are calculated using mean pentad precipitation data of 29 stations from 1968 to. Afterwards, the following definition which is proposed by Matsumoto (1997) for the Indochina Peninsula is used to redefine the climatological summer monsoon onset dates of two areas over Myanmar. The first pentad when the mean pentad precipitation exceeds annual mean pentad precipitation (Pm) in at least three consecutive pentads after lowering it in more than three consecutive pentads. The middle date of thus defined pentad is regarded as onset date. Figure 2 displays the pentad mean climatology of the rainfall index for southern and central Myanmar and northern Myanmar from 1968 to. Based on the above definition, the climatological onset dates of summer monsoon in Myanmar are defined as 18th May (the middle date of pentad number 28, hereafter P28) for the southern and central Myanmar and as 28th May (the middle date of P3) for the northern Myanmar. The wet season over southern and central Myanmar is recognized from mid-may to early October. Regarding the northern Myanmar, the wet season is from late May to early October. The beginning of a wet season is characterized by the increase of pentad rainfall more than annual mean pentad precipitation. 4. Analysis of the observed station data and re-analysis wind data So far, the climatological monsoon onset dates of two areas over Myanmar are defined based on only rainfall data and theoretical climatic zones. Thus, other meteorological parameters should be examined to verify the time and place of onset over Myanmar. Consequently, 33-year averaged pentad of six observed station data and re-analysis wind data between the periods of 21st April and 9th June are analysed. Climatology of rainfall Index for Southern & Central Myanmar (1968-) Climatology of rainfall Index for Northern Myanmar (1968-) 1 4 7 13 16 19 22 28 31 34 37 4 43 46 49 2 8 61 64 67 7 73 1 4 7 13 16 19 22 28 31 34 37 4 43 46 49 2 8 61 64 67 7 73 3 Rainfall (mm) P28 Rainfall (mm) P3 Jan1- Jan16- Jan31-Feb4 Feb-19 Mar2-6 Mar17-21 Apr1- Apr16- May1- May16- May31-Jun4 Jun-19 Jun3-Jul4 Jul-19 Jul3-Aug3 Aug14-18 Aug29-Sep2 Sep13-17 Sep28-Oct2 Oct13-17 Oct28-Nov1 Nov12-16 Nov27-Dec1 Dec12-16 Dec27-31 Pentad Jan1- Jan16- Jan31-Feb4 Feb-19 Mar2-6 Mar17-21 Apr1- Apr16- May1- May16- May31-Jun4 Jun-19 Jun3-Jul4 Jul-19 Jul3-Aug3 Aug14-18 Aug29-Sep2 Sep13-17 Sep28-Oct2 Oct13-17 Oct28-Nov1 Nov12-16 Nov27-Dec1 Dec12-16 Dec27-31 Pentad Southern & Central Myanmar (1968-) Pm Northern Myanmar (1968-) Pm Figure 2. The climatological onset dates of summer monsoon in Myanmar based on the precipitation data. The horizontal dashed line denotes the annual mean pentad precipitation (Pm) defined in the text. Copyright Royal Meteorological Society Int. J. Climatol. 31: 382 393 (11)

MYANMAR SUMMER MONSOON ONSET CLIMATOLOGY 38 Figure 3. Difference between the two consecutive pentads of maximum temperature (C) from P24 to P32 in Myanmar (1968 ). (a) P24 (Apr26 3) P23 (Apr21 ), (b) P (May1 ) P24 (Apr26 3), (c) P26 (May6 ) P (May1 ), (d) P27 (May11 ) P26 (May6 ), (e) P28 (May16 ) P27 (May11 ), (f) P29 (May21 ) P28 (May16 ), (g) P3 (May26 3) P29 (May21 ), (h) P31 (May31 Jun4) P3 (May26 3) and (i) P32 (June 9) P31 (May31 June4). This figure is available in colour online at wileyonlinelibrary.com/journal/joc 4.1. Analysis of the observed station data To know the period and place of the significant changes of the observed station data, the differences between two consecutive pentads of 33-year averaged maximum and minimum temperature, relative humidity, cloudiness and rainfall of each station during the periods from 21st April to 9th June are calculated and displayed by ArcMap (geographical information system). Figure 3 shows the difference between the two consecutive pentads of 33-year averaged maximum temperature for each station from 21st April to 9th June. When the maps of these maximum temperature are examined, the area from southern part of Myanmar up to about 23. N, called the southern and central Myanmar, is covered with the minimum difference of (. C to 2. C) between P27 and P28 (Figure 3e). At that time, the maximum difference of (1. C to. C) can be seen in the northern Myanmar. But the minimum difference of (. C to 1. C) over this area shows between P29 and P3 (Figure 3g). Figure 4 exhibits the difference between the two consecutive pentads of 33-year mean minimum temperature for each station from 21st April to 9th June. The minimum difference of ( C to. C) can be seen over the southern and central Myanmar between P27 and P28 (Figure 4e), but that area is not exactly the same as the former area of the maximum temperature. The minimum temperature difference of the northern part of Myanmar does not change significantly compare to the southern and central part of Myanmar during the study periods. Figure indicates relative humidity. The southern and central Myanmar is covered with the maximum difference of (4.% to 8.%) during P28 minus P27 (Figure e). The minimum difference of (% to 4.%) covers in the northern Myanmar at that pentad. But the maximum difference of (4.% to 6.%) over this area is described in P3 minus P29 (Figure g). Figure 6 depicts the cloudiness difference. Similar to the relative humidity, maximum difference (. 1. Okta) of cloudiness in the former area occurs in P28 minus P27 (Figure 6e), and in the latter area in P3 minus P29 (Figure 6g). Figure 7 displays the rainfall difference. Although the maximum difference (8 mm) of precipitation can be seen in P28 minus P27, it is difficult to distinguish the different area of these changes like the above data. It is supposed that the whole country can get rain more or less during these periods and the difference areas are not significant compared with other parameters. When the analyses of the above five parameters are summarized, two step conspicuous changes of these data, except rainfall and minimum temperature, can be seen over Myanmar. The first step change occurs in the southern and central Myanmar from 9 32 N to 23. N during P28 minus P27 and the second step change takes place in the northern Myanmar from 23. N to28 31 N in P3 minus P29. Copyright Royal Meteorological Society Int. J. Climatol. 31: 382 393 (11)

386 O. HTWAY AND J. MATSUMOTO Figure 4. Same as Fig. 3, but for difference between the two consecutive pentads of minimum temperature (C) from P24 to P32 in Myanmar (1968 ). (a) P24 (Apr26 3) P23 (Apr21 ), (b) P (May1 ) P24 (Apr26 3), (c) P26 (May6 ) P (May1 ), (d) P27 (May11 ) P26 (May6 ), (e) P28 (May16 ) P27 (May11 ), (f) P29 (May21 ) P28 (May16 ), (g) P3 (May26 3) P29 (May21 ), (h) P31 (May31 Jun4) P3 (May26 3) and (i) P32 (Jun 9) P31 (May31 Jun4). This figure is available in colour online at wileyonlinelibrary.com/journal/joc Figure. Same as Fig. 3, but for difference between the two consecutive pentads of relative humidity (%) from P24 to P32 in Myanmar (1968 ). (a) P24 (Apr26 3) P23 (Apr21 ), (b) P (May1 ) P24 (Apr26 3), (c) P26 (May6 ) P (May1 ), (d) P27 (May11 ) P26 (May6 ), (e) P28 (May16 ) P27 (May11 ), (f) P29 (May21 ) P28 (May16 ), (g) P3 (May26 3) P29 (May21 ), (h) P31 (May31 Jun4) P3 (May26 3) and (i) P32 (Jun 9) P31 (May31 Jun4). This figure is available in colour online at wileyonlinelibrary.com/journal/joc Copyright Royal Meteorological Society Int. J. Climatol. 31: 382 393 (11)

MYANMAR SUMMER MONSOON ONSET CLIMATOLOGY 387 Figure 6. Same as Fig. 3, but for difference between the two consecutive pentads of cloudiness (Okta) from P24 to P32 in Myanmar (1968 ). (a) P24 (Apr26 3) P23 (Apr21 ), (b) P (May1 ) P24 (Apr26 3), (c) P26 (May6 ) P (May1 ), (d) P27 (May11 ) P26 (May6 ), (e) P28 (May16 ) P27 (May11 ), (f) P29 (May21 ) P28 (May16 ), (g) P3 (May26 3) P29 (May21 ), (h) P31 (May31 June4) P3 (May26 3) and (i) P32 (Jun 9) P31 (May31 Jun4). This figure is available in colour online at wileyonlinelibrary.com/journal/joc Figure 7. Same as Fig. 3, but for difference between the two consecutive pentads of rainfall (mm) from P24 to P32 in Myanmar (1968 ). (a) P24 (Apr26 3) P23 (Apr21 ), (b) P (May1 ) P24 (Apr26 3), (c) P26 (May6 ) P (May1 ), (d) P27 (May11 ) P26 (May6 ), (e) P28 (May16 ) P27 (May11 ), (f) P29 (May21 ) P28 (May16 ), (g) P3 (May26 3) P29 (May21 ), (h) P31 (May31 Jun4) P3 (May26 3) and (i) P32 (Jun 9) P31 (May31 Jun4). This figure is available in colour online at wileyonlinelibrary.com/journal/joc Copyright Royal Meteorological Society Int. J. Climatol. 31: 382 393 (11)

6 6 388 O. HTWAY AND J. MATSUMOTO (a) P23 (b) P24 (c) P (d) P26 (e) P27... 8. 8. 8. 11 8.. 8. 8.. 8.. 8. 8.. 7. 8 8.. 7 7. 8 8. 9 9 9 9 9 (f) P28 (g) P29 (h) P3 (i) P31 (j) P32 7. 8 8. 6. 6. 7 8... 6 7 7. 8 6. 8.. 4. 1. 6. 7 7. 8 7. 8 8. 4. 4.. 6. 7 6 8 8. 4. 4. 6. 7 7.. 6 9 9 9 9 9 Figure 8. Mean pentad sea level pressure (hpa) from P23 to P32 over Myanmar (1968 ). The interval of the isobar line is. hpa. (a) P23 (Apr21 ), (b) P24 (Apr26 3), (c) P (May1 ), (d) P26 (May6 ), (e) P27 (May11 ), (f) P28 (May16 ), (g) P29 (May21 ), (h) P3 (May26 3), (i) P31 (May31-Jun4) and (j) P32 (Jun 9). Regarding the mean pentad SLP; without calculating the difference between two consecutive pentads of mean pentad SLP, original 33-year mean pentad values of 24 stations are displayed for each pentad from 21st April to 9th June (Figure 8). There are only three to five isobar lines with.-hpa interval along the coast of Myanmar from P23 to P27 (Figure 8a e). But the isobar lines start to compact and eight isobar lines can be seen along the coast of Myanmar from P28 (Figure 8f), and it shows the increase of the velocity of southwesterly wind, which carries water vapour from the Bay of Bengal and the Andaman Sea to Myanmar. To further investigate this, time series of SLP differences of three stations which exist along the coastal area are drawn to examine the notable changes of SLP (Figure 9). The locations of these three stations are shown as A, B and C in Figure 1. Both of the increase of pressure gradient for the southern part of the coast (SLP difference of the southernmost station A and the middle station B of the coast) and the northern part of the coast (SLP difference of the middle station B and the northernmost station C of the coast) shows the significant increase in the same pentad (P28). In the inland area of the northern Myanmar, SLP is almost uniform and the changes are not significant. Thus, SLP difference can represent only the southern and central Myanmar area and the period of the significant changes occurs there at the same time in P28 like changes in other parameters in the observed station data. 4.2. Analysis of re-analysis wind data Concerning wind data, the climatological mean pentad horizontal wind fields at 8 hpa and its pentad differences are displayed and analysed to know the changes of wind direction and speed over Myanmar. When 8-hPa pentad horizontal wind patterns are examined (Figure ), the subtropical westerly from the northern Indian subcontinent prevails over the northern and central Myanmar area during P23 to P. The strength of the southwest component of the equatorial westerly starts to increase in the southern part of the Bay of Bengal from P and it reaches to the southern part of Myanmar coast in P27. The distinct increase of the southwesterly wind can be seen over the southern and central Myanmar in P28. But the decrease of the subtropical westerly cannot be seen clearly in these maps although the southwesterly wind prevails over the whole country in P3. To clarify this, the difference between the two consecutive pentads of 8 hpa horizontal wind is analysed (Figure11). There is no significant change of 8-hPa wind over Myanmar from P23 minus P22 to P27 minus P26 but strong westerly over the southern Myanmar and southerly over the central Myanmar are Copyright Royal Meteorological Society Int. J. Climatol. 31: 382 393 (11)

MYANMAR SUMMER MONSOON ONSET CLIMATOLOGY 389 8. 6. Pressure gradient (hpa) 4. 2.. P28 P28 1 3 7 9 11 13 17 19 21 23 27 29 31 33 3 37 39 41 43 4 47 49 1 3 7 9 61 63 6 67 69 71 73-2. -4. Pentad Number Figure 9. Time series of pressure gradients along the coast of Myanmar. Solid line represents the pressure gradient between A and B which locations are shown in Figure 1, and dashed line for between B and C. shown in P28 minus P27. Strong southwesterly can also be seen over the northern Myanmar in P3 minus P29. Thus three grid areas, (.. N, 9.. E), (. 22. N, 92. 97. E) and (. -. N, 9. 9. N), are selected to verify the significant period over Myanmar. In the time series of climatological 8-hPa horizontal wind velocity for the area averaged of these grid areas, the significant increase of wind velocity for each grid area occurs in the same P28 for the southern and central Myanmar and in P3 for the northern Myanmar (Figure 12) as in other parameters in the observed station data. To sum up the significant results of the observed station data and re-analysis wind data, it can be seen clearly that the significant increase of relative humidity and cloudiness, the significant decrease of maximum temperature at the southern and central Myanmar during P28 minus P27 and at the northern Myanmar during P3 minus P29 (Figure 13). As for the SLP field, eight isobar lines with.-hpa interval along the coast of Myanmar can be seen from P28 (Figure 8f) and the abrupt increase of SLP difference also occurs during P28 (Figure 9) along the coastal area which can represent the southern and central Myanmar area. The significant increase of wind velocity for three grid areas which can represent the southern Myanmar, central Myanmar and northern Myanmar, occurs in the same pentad P28 for the southern and central Myanmar and in P3 for the northern Myanmar as observed in other parameters. Thus, the climatological monsoon onset dates of two areas over Myanmar can be confirmed as P28 for the southern and central Myanmar and as P3 for the northern Myanmar although those are defined by rainfall only and theoretical climatic zones, the other five meteorological fields also exhibit simultaneous changes over the same area during the onset.. Discussion In this study, the climatological summer monsoon onset dates over two areas of Myanmar are defined by mean pentad precipitation data. The monsoon onset definition which is used in DMH is based on the daily rainfall amount and it has to omit rainfall prior to th April subjectively. The criterion of -day averaged rainfall can avoid this problem as no rain can persist more than days before monsoon onset according to the analysis of longterm precipitation data. Previous study (Lwin, 2) also says there are four pulsations to accomplish the establishment of monsoon over Myanmar. However, these four regions are decided according to the 1967 onset date isochrones over India and Myanmar, not based on the present station data. This study uses 29 well-distributed stations with more than 3-years data period and results from the analysis of the seven meteorological parameters during the periods between 21st April and 9th June of 1968 to also show that there are two consequential stages to advance the summer monsoon over Myanmar. The significant increase of relative humidity and cloudiness, the significant decrease of maximum temperature at the southern and central Myanmar during P28 minus P27 and at the northern Myanmar during P3 minus P29 (Figure 13) can be seen in the analysis of the observed station data. When the climatological seasonal variations graphs of both areas (Figure 14) over Myanmar are examined, mean pentad maximum temperature begins to decrease from the onset pentad over the whole country. Mean pentad relative humidity and cloudiness also increase obviously after the onset. Concerning SLP and 8-hPa wind analyses, the figures of SLP and 8- hpa wind patterns (Figures 8 ) show that increase of isobar lines and pressure gradient at SLP occurs concurrently with the increase of wind velocity at 8 hpa. Copyright Royal Meteorological Society Int. J. Climatol. 31: 382 393 (11)

39 O. HTWAY AND J. MATSUMOTO Figure. Same as Fig. 8, but for 8 hpa mean pentad horizontal wind (m/s) from P23 to P32 (1968 ). Reference arrow is m/s and an elevation of above metres is shaded. (a) P23 (Apr21 ), (b) P24 (Apr26 3), (c) P (May1 ), (d) P26 (May6 ), (e) P27 (May11 ), (f) P28 (May16 ), (g) P29 (May21 ), (h) P3 (May26 3), (i) P31 (May31-Jun4) and (j) P32 (Jun 9). The difference between the two consecutive pentads of 8-hPa horizontal wind analysis also reveals the changes of low-level circulation associated with the local monsoon onset. All these changes are considered to occur associated with the sudden increase of cloud cover and convective activity after the monsoon onset. The monsoon onset dates for individual year can be defined according to the above definition as the further work to understand more about the mechanism of monsoon onset over Myanmar. Moreover, Zhang et al. (2) have identified a close relationship between the interannual variations of the monsoon onset over the central Indochina Peninsula and El Nino/La Nina phenomena that years with warm SST in the western Pacific and cold SST in the centraleastern Pacific in the early spring have early onsets and years with the reverse conditions of SST have late onsets. But Roy and Kaur () described that there has no one-to-one relationship between El Nino and Myanmar s monsoon rainfall. Nevertheless, Myanmar is located in the northwestern part of the Indochina Peninsula and thus the relationships between the monsoon Copyright Royal Meteorological Society Int. J. Climatol. 31: 382 393 (11)

MYANMAR SUMMER MONSOON ONSET CLIMATOLOGY 391 4N (a) P23 - P22 (b) P24 - P23 3N N N EQ 4N (c) P - P24 (d) P26 - P 3N N N EQ 4N (e) P27 - P26 (f) P28 - P27 3N N N EQ 4N (g) P29 - P28 (h) P3 - P29 3N N N EQ 4N (i) P31 - P3 (j) P32 - P31 3N N N EQ 6E 8E E 1E 6E 8E E 1E 3 Figure 11. Same as Fig. 3, but for difference between the two consecutive pentads of 8 hpa mean pentad horizontal wind (m/s) from P23 to P32 (1968 ). Reference arrow is 3 m/s and an elevation of above metres is shaded. (a) P23 (Apr21 ) P22 (Apr16 ), (b) P24 (Apr26 3) P23 (Apr21 ), (c) P (May1 ) P24 (Apr26 3), (d) P26 (May6 ) P (May1 ), (e) P27 (May11 ) P26 (May6 ), (f) P28 (May16 ) P27 (May11 ), (g) P29 (May21 ) P28 (May16 ), (h) P3 (May26 3) P29 (May21 ) and (i) P31 (May31 Jun4) P3 (May26 3) (j) P32 (Jun 9) P31 (May31 Jun4). onset of Myanmar and SST should be examined in order to reveal the El Nino associated with the monsoon onset of Myanmar or not. Besides, many studies (He et al., 1987; Li and Yanai, 1996; Wu and Zhang, 1998) have been shown that the formation of Asian summer monsoon is as consequences of not only the atmospheric response to the changes in the contrast of thermal heating between land and ocean but also the elevated heating of the Tibetan Plateau. Thus, the Tibetan Plateau, located more than 4 m above Copyright Royal Meteorological Society Int. J. Climatol. 31: 382 393 (11)

392 O. HTWAY AND J. MATSUMOTO 4N 3N N N c EQ 6E 7E 8E 9E E 1E 1E 13E 8 hpa V-Wind (m/s) b (b) Time series of the climatological 8hPa V-wind (m/s) in b (.-22.N, 92.-97.E) 1968-. 4. 4. 3. 3. 2. 2. 1. 1... 23 24 26 27 28 29 3 31 32 Pentad Number a 8 hpa U-Wind (m/s) 8 hpa V-Wind (m/s) (a) Time series of the climatological 8 hpa U-wind (m/s) in a (.-.N, 9.-.E) 1968-9. 8. 7. 6.. 4. 3. 2. 1.. 23 24 26 27 28 29 3 31 32 Pentad Number (c) Time series of the climatological 8hPa V-wind (m/s) in c (.-.N, 9.-9.E) 1968-4. 4. 3. 3. 2. 2. 1. 1. 23 24 26 27 28 29 3 31 32 Pentad Number Figure 12. The time series of climatological 8 hpa wind speed. (a) Zonal wind speed in southern Myanmar (..N, 9..E), (b) Meridional wind speed in central Myanmar (. 22.N, 92. 97.E) and (c) Meridional wind speed in Northern Myanmar (..N, 9. 9.E). Arrow in each figure indicates the largest increase of wind speed during the pentad. Figure 13. Significant changes of the observed station data in Myanmar. This figure is available in colour online at wileyonlinelibrary.com/journal/joc the sea level with a length of km and width of 6 km in the west and 1, km in the east, should also be taken into account in the future work of interannual variations of monsoon onset over Myanmar. It would be interesting to study the variability of monsoon onset of two different areas in Myanmar whether the land-sea thermal contrast is the only mechanism to trigger off the monsoon because of the different and complex topography of these areas. 6. Summary In this study, the objective definition to determine the climatological monsoon onset over Myanmar is proposed by using mean pentad precipitation data. The climatological summer monsoon onset occurs with two steps in Myanmar. The first is the monsoon onset over the southern and central Myanmar on May 18 (the middle date of P28). It is followed by the onset of the summer monsoon over the northern Myanmar by 28 May (the middle date of P3). The time and place of monsoon onset stage are also confirmed by analysing the observed station data and re-analysis wind data. The general conditions of meteorological parameters during monsoon onset period of Myanmar are as follows: maximum temperature decrease, relative humidity and cloud amount increase, SLP gradient along the coast of Myanmar abruptly increase, low-level equatorial Copyright Royal Meteorological Society Int. J. Climatol. 31: 382 393 (11)

MYANMAR SUMMER MONSOON ONSET CLIMATOLOGY 393 (a) Rainfall(mm)&Tmax (C) 4 3 3 Climatological seasonal variations of four meteorological parameters for Southern & Central Myanmar (1968-) 1 4 7 13 16 19 22 28 31 34 37 4 43 46 49 2 8 61 64 67 7 73 P 28 8 6 4 RH(%) & Cld*(Okta) (b) Rainfall(mm)&Tmax(C) 3 3 Climatological seasonal variations of four meteorological parameters for Northern Myanmar (1968-) 1 4 7 13 16 19 22 28 31 34 37 4 43 46 49 2 8 61 64 67 7 73 P 3 8 6 4 RH(%) & Cld* (Okta) Jan1- Jan16- Jan31-Feb4 Feb-19 Mar2-6 Mar17-21 Apr1- Apr16- May1- May16- May31-Jun4 Jun-19 Jun3-Jul4 Jul-19 Jul3-Aug3 Aug14-18 Aug29-Sep2 Sep13-17 Sep28-Oct2 Oct13-17 Oct28-Nov1 Nov12-16 Nov27-Dec1 Dec12-16 Dec27-31 Jan1- Jan16- Jan31-Feb4 Feb-19 Mar2-6 Mar17-21 Apr1- Apr16- May1- May16- May31-Jun4 Jun-19 Jun3-Jul4 Jul-19 Jul3-Aug3 Aug14-18 Aug29-Sep2 Sep13-17 Sep28-Oct2 Oct13-17 Oct28-Nov1 Nov12-16 Nov27-Dec1 Dec12-16 Dec27-31 Pentad Pentad Southern & Central Myanmar (Rainfall) Northern Myanmar (Rainfall) Pm Tmax RH Cld* Pm Tmax RH Cld* Figure 14. Climatological seasonal variations of pentad mean rainfall, maximum temperature (Tmax), relative humidity (RH) and cloudiness (Cld) for (a) southern & central Myanmar and (b) northern Myanmar averaged for the period (1968 ). southwesterly wind from the Bay of Bengal prevails in the whole country. Acknowledgements We wish to express our sincere appreciation and gratitude to the Department of Meteorology and Hydrology (DMH), Myanmar, for providing the observed station data, the European Centre for Medium-Range Weather Forecasts (ECMWF) for ERA-4 re-analysis data, the Climate Diagnostics Center (CDC) and the National Geophysical Data Center (NGDC) for the model topography and the -min gridded global relief data. The Grid Analysis and Display System(GrADS), distributed from the Center for Ocean-Land-Atmosphere Studies (COLA), and the Generic Mapping Tools (GMT), distributed from the School of Ocean and Earth Science and Technology (SOEST) at the University of Hawaii, were utilized for the drawing part of the Figures. Special thanks are indebted to Dr Yasushi Agata, Assistant Prof. of the Department of the Natural Environmental Studies (NENV), and Mr Yoshitaka Masuda (NENV) who supported making figures using ArcMap. Thanks are also dedicated to the members of the Climate Colloquium in the Department of Earth and Planetary Science, the University of Tokyo, in particular, to Dr Satoru Yokoi and Mr Tomoshige Inoue, for their helpful discussions. Part of this study is financially supported by the Core Research for Evolutional Science and Technology (CREST) of Japan Science and Technology Agency (JST). 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