Introduction
Regional Weather Conditions
El Niño
Fire Danger Rating System
Introduction
The climate of the ASEAN region is mainly tropical with hot and humid conditions all year round and a lot of rainfall. The climate is influenced by maritime wind systems which originate in the South China Sea and the Indian Ocean. Two main monsoon seasons predominate in the region - the Northeast Monsoon occurs from December to March and the Southwest Monsoon occurs from June to September. The seasons are separated by two relatively shorter Inter-Monsoon periods.
The Northeast Monsoon is characterised by a dry season in the northern ASEAN region (Cambodia, Lao PDR, Myanmar, northern Philippines, northern Thailand, and Vietnam) and a rainy season in the southern ASEAN region (Brunei Darussalam, Indonesia, Malaysia, southern Philippines, Singapore, southern Thailand). The converse applies for the Southwest Monsoon i.e. a wet season prevails in the northern ASEAN region and a dry season in the southern ASEAN region. During the Inter-Monsoon season, diurnal-type weather conditions characterised by afternoon and evening showers with light variable winds along the tropical belt predominate across the ASEAN region.
During the traditional dry season, fires from land clearing and traditional slash-and-burn activities are prevalent in the region. The situation is exacerbated when the dry season is enhanced due to disruptions in the normal monsoon cycle. These disruptions are caused by a combination of factors such as the El Niño. In 1992, 1995, 1997 and 2006, prolonged dry periods brought on by the El Niño created a catastrophe when the fires got out of control, consequently wreaking havoc on the environment and economy of affected countries.
Regional Weather Conditions
Weather parameters such as rainfall and wind speed and direction are important indicators in monitoring the development and transport of regional smoke haze from large scale fires. The weather parameters are also used to derive the fire danger rating for assessing the fire risk in the region.
The ASMC produces a monthly review and outlook of the weather and smoke haze situation in the region, including the El Niño and rainfall outlook. It also updates the daily rainfall charts of selected locations in the region on a weekly basis. The daily rainfall reports and monthly rainfall outlook are collated from inputs provided by the respective ASEAN National Meteorological Services.
El Niño
El Niño is an ocean-atmosphere climate phenomenon that is linked to the periodic warming of waters across the central and eastern tropical Pacific Ocean. This warming is part of the natural climate system, known as the El Niño/Southern Oscillation (ENSO) cycle. The El Niño refers to the warm phase of the ENSO cycle. On average, an El Niño event occurs every 2 to 7 years and typically lasts about 9 to 18 months.
Under normal conditions, the following oceanic and atmospheric conditions prevail across the tropical belt:
• Area of warm water and active convection over Indonesia-Australia and western tropical Pacific Ocean
• Cold waters on the western coast of South America
• Easterly trade winds in the tropics
During an El Niño event, the above oceanic and atmospheric conditions are weakened or reversed:
• Area of warm water over Indonesia-Australia and western tropical Pacific Ocean cools and the warmest water and convection is displaced eastward to the central and eastern Pacific Ocean
• Warming of cold waters on the western South American coast
• Weakening of tropical easterly trade winds
The impact of El Niño is a disruption in normal weather patterns across some parts of the world:
• Dry conditions over Southeast Asia, Australia, western Pacific and eastern Africa
• Wet conditions over central Pacific, western tropical South America
• Warmer winters in some regions of east Asia and North America
El Niño is typically strongest during November to April when the tropical Pacific Ocean sea surface temperatures are normally warmest. Hence the above impacts are usually strongest during this period.
The cold phase of the ENSO cycle is known as the La Niña, which is essentially the opposite of El Niño. La Niña is characterized by cooling of waters in the central and eastern tropical Pacific Ocean and stronger than usual trade winds. It occurs almost as often as El Niño and also affects the normal weather patterns in some parts of the world, such as higher than normal rainfall in Southeast Asia.
The correlation between El Niño/La Niña and its associated weather impacts vary in different parts of Southeast Asia, but it is known to be quite high in Indonesia.
Fire Danger Rating System
The smoke haze danger period in the ASEAN region typically coincides with the dry season each year, when forest and plantation areas are highly vulnerable to ignition and development of fires. Increased solar radiation and limited rainfall reduces soil moisture, and kindling temperatures over wide areas allow either a natural or man-made spark to ignite a fire.
To assess the potential for a fire on any given day, a Fire Dating Rating System (FDRS) is used. The system uses daily weather information (temperature, relative humidity, rainfall, wind speed) and data on vegetation as a potential fuel to predict the risk of fires starting and spreading. The FDRS products can be used as a guide to policy-makers in developing actions to protect life, property and the environment.
The Malaysian Meteorological Department (MMD) has been producing a FDRS for Southeast Asia on a daily basis since September 2003. The Southeast FDRS was adopted from the Canadian FDRS developed by the Canadian Forest Service. Daily maps of the Southeast FDRS are available at the MMD website.
