The ASEAN region has a tropical climate that is characterised by relatively uniform warm and humid weather all year round. The rainfall patterns however, vary greatly across different parts of the ASEAN region. Based on the rainfall patterns, we can further classify the tropical climate into two types; (1) Tropical Rainforest climate that has high and uniform rainfall through the year and (2) Tropical Monsoon or Savanna climate that has distinct dry and wet seasons. The former is usually the case for the near-equatorial regions such as Malaysia, Brunei, Singapore and some parts of Indonesian Archipelago including Sumatra and Kalimantan, whereas the latter scenario takes place mainly over the Northern ASEAN region including Thailand, Myanmar, Cambodia, Lao PDR, Vietnam and the Philippines. Due to the elevated geographic features, temperate climate can also be found in the northern parts of Myanmar, Lao PDR and Vietnam.Our 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 from December to March, and the Southwest Monsoon 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. Countries such as the Philippines, Vietnam and Myanmar are particularly at risk of being affected by Typhoons during this time of the year. 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 common 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 raged out of control, consequently wreaking havoc on the environment and economy of affected countries.

The El Niño phenomenon is a non-regular occurrence in the tropical pacific region where warmer waters develop over the Eastern Tropical Pacific Ocean along the coast of South America. In South East Asia, this brings drier weather and increases the risk of forest fires and smoke haze. The La Niña phenomenon is the reverse of the El Niño where cooler waters develop over the Eastern Tropical Pacific Ocean along the coast of South America. In South East Asia, higher than normal rainfall tends to occur during a La Niña episode which may result in an increased occurrence of floods.

The correlation between El Niño/La Niña and its associated weather impacts on South East Asia differ from one place to another and for different seasons.

The image above shows the precipitation anomalies averaged over the El Niño and La Niña years. For instance, the impact of El Niño is typically stronger over the southern and eastern part of South East Asia during the months of Jun – Oct.

A seasonal forecast provides forecast information about the general weather conditions at seasonal timescales up to several months ahead. Long range predictions rely on predicting components of the atmosphere and the oceans which show variations at large spatial and temporal scales. Among these key components is the sea-surface temperature – an important indicator in the El Niño Southern Oscillation (ENSO) cycle.

To account for the chaotic nature of the atmosphere, seasonal forecast adopts a probabilistic approach and the forecast weather condition is presented as an average over an extended period of time, usually with reference to climatology.

ASMC carries out monthly seasonal forecast predictions based on various seasonal model forecasts from the leading meteorological centres, supplemented by individual assessments from respective countries in South East Asia. The skilfulness of seasonal forecast relies greatly on the performance of seasonal models, which differ from one place to another and for different seasons of the year.

For example, one way to assess the seasonal model skills is to examine the ROC score. From the figures above, we can see that ROC score map differs greatly from one season to another. During the period from Dec to Feb, a high ROC score region is observed mainly over the Philippines whereas during the period from May to Jul, a high ROC score region is observed over the near equatorial region such as Peninsular Malaysia and Sumatra.