Print Version (PDF Format)1. Review of Regional Weather Conditions in July 2007
1.1 Moderate Southwest Monsoon conditions prevailed in the region during July 2007 with the strengthening of the southwesterly winds in the region north of the equator. The inter-tropical convergence rain belt was mainly confined between latitudes 5°N and 15°N, leaving areas south of 5°N with reduced rainfall. In early July 2007 the first tropical storm to form within the ASEAN region this year, Tropical Storm “Toraji (03W)”, developed in the South China Sea just southeast of Hainan Island. The storm made landfall on the island before tracking across the Gulf of Tonkin and making its final landfall on northern Vietnam on 5 July 2007. An intrusion of moist easterly winds from the equatorial Pacific Ocean in the later part of the month led to the development of extensive rain clouds over large parts of the southern ASEAN region.
1.2 Drier weather conditions in the southern ASEAN region of southern Thailand, Malaysia, Singapore, Brunei Darussalam, Indonesia and the southern Philippines continued into the start of July 2007. This marked the onset of the traditional dry season, bringing reduced rainfall to most of the southern ASEAN region. In the later part of the month, some parts of the southern ASEAN region were affected by spells of unseasonal wet weather that brought heavy rain and floods in some areas. In the northern ASEAN region, the rainy season continued to prevail but some areas experienced drier than normal weather conditions.
1.3 Most of the southern ASEAN region experienced above normal rainfall in July 2007 except for Java, Bali, Nusa Tenggara and Sulawesi which received less than 50% of normal rainfall. On the other hand, the northern parts of the northern ASEAN region recorded below normal rainfall. The regional rainfall pattern in July 2007 is shown in Fig 1.
Fig 1. Percentage of Normal Rainfall for July 2007
2. Review of Land/ Forest Fires and Smoke Haze Situation
2.1 Drier weather conditions at the start of the dry season in the southern ASEAN region resulted in a significant increase in hotspot activities in Sumatra in early July 2007. Active hotspot clusters with moderate to dense smoke haze were detected mainly in North Sumatra and Riau provinces. The prevailing southwesterly winds transported some of the smoke haze to the northern states of Peninsular Malaysia. Air quality in the affected areas deteriorated to between moderate and unhealthy levels for several days. In Borneo, isolated hotspot activities with localised smoke haze were observed in various parts of Sarawak and Kalimantan. As weather conditions became wetter in the later part of the month, the increased rainfall helped to subdue the hotspot activities in Sumatra and Borneo. By the end of the month, the return of drier weather saw the emergence of active hotspot activities in southern Sumatra. Satellite pictures depicting the hotspot activities in Sumatra and Borneo in July 2007 are shown in Figs 2 to 7.
2.2 Hotspot activities in the northern ASEAN region remained subdued during the month as the rainy season continued to prevail. During intervals of drier weather, isolated hotspot activities were detected sporadically mainly in Vietnam.
2.3 The hotspot charts for July 2007 for:-
a) Myanmar, Thailand, Lao PDR and Vietnam;
b) Sumatra, Borneo and Peninsular Malaysia; and
c) Java, Sulawesi and the Philippines;
are shown in Figs 8 to 10, respectively.
Fig 2. NOAA-18 Satellite Picture of Sumatra on 2 July 2007 showing scattered hotpot activities in central parts of Sumatra.
Fig 3. NOAA-12 Satellite Picture of Sumatra on 3 July 2007 showing the spread of smoke haze from hotspot clusters in North Sumatra and Riau provinces across the Malacca Straits.
Fig 4. NOAA-18 Satellite Picture of Sumatra on 4 July 2007 showing smoke haze from hotspot clusters in Riau province being transported by southwesterly winds toward the northern states of Peninsular Malaysia.
Fig 5. NOAA-12 Satellite Picture of Borneo on 5 July 2007 showing isolated hotspot activities with prominent smoke plumes near coastal areas of Sarawak and in West Kalimantan.
Fig 6. NOAA-18 Satellite Picture of Sumatra on 10 July 2007 showing isolated hotspot activities in central Sumatra.
Fig 7. NOAA-18 Satellite Picture of Borneo on 16 July 2007 showing localised hotspot activities in West and Central Kalimantan
Fig 8. Hotspot Counts in Myanmar/Thailand/Lao PDR/Cambodia/Vietnam for July 2007
Fig 9. Hotspot Counts in Sumatra/Borneo/P. Malaysia for July 2007
Fig 10. Counts in Java/Sulawesi/Philippines for July 2007
3.1 After a pause in the cooling trend suggestive of La Niņa development in the early part of July 2007, cooling of the eastern equatorial Pacific and strengthening of the easterly trade winds resumed in the later part of the month. As of end July 2007, the equatorial sea surface temperatures (SSTs) are slightly below average in the east-central Pacific and below average in the far eastern Pacific (Fig 11). and the trade winds are stronger than average across much of the equatorial Pacific. However, recent fluctuations in the cooling trend and strength of the trade winds have slightly increased the uncertainty of a La Niņa development in 2007.
3.2 The majority of dynamical and statistical model forecasts continued to indicate cooler than average SSTs in the eastern and central equatorial Pacific in the coming months (Fig 12). Overall, the model forecasts are approximately split between La Niņa conditions and neutral conditions during the rest of 2007. The development of the El Niņo is unlikely.
Fig 11. Sea Surface Temperature Anomalies in Tropical Pacific
(source US NOAA)
Fig 12. Forecast of the SST Anomalies (source US NOAA)
4.1 The Southwest Monsoon conditions are expected to prevail over the next two months. With the current rainy season in the northern ASEAN region expected to continue into October 2007, increased rainfall can be expected in Myanmar, northern Thailand, Lao PDR, Cambodia, Vietnam and the northern Philippines during this period. On the other hand, reduced rainfall can be expected in most parts of the southern ASEAN region during the current dry season which should last till end September or early October 2007. Thereafter increased shower activities can be expected in the region when the Southwest Monsoon makes way for the Inter-Monsoon in October 2007.
4.2 Hotspot activities in the northern ASEAN region are expected to remain mostly subdued during the rainy season. However isolated hotspot activities could develop occasionally during intervals of drier weather. In the southern ASEAN region, an increase in hotspot activities can be expected in Sumatra and Kalimantan during periods of dry weather.
4.3 With neutral to La Niņa conditions being forecast for the rest of the year, rainfall is expected to be mostly normal or slightly above normal in large parts of the ASEAN region over the next few months. However, Indonesia is expecting slightly below normal to below normal rainfall except eastern parts. The expected rainfall for the region is as shown in Fig 13. With slightly below rainfall being forecast for the traditionally fire-prone areas in Sumatra and Kalimantan, escalated levels of hotspot activities could lead to transboundary smoke haze could occur during extended periods of drier weather. Hence vigilance should be stepped up for an increase in hotspot activities in these areas during the dry season.
