The weather in Malaysia is characterised by two monsoon regimes, namely, the Southwest Monsoon from late May to September, and the Northeast Monsoon from November to March. The Northeast Monsoon brings heavy rainfall, particularly to the east coast states of Peninsular Malaysia and western Sarawak, whereas the Southwest Monsoon normally signifies relatively drier weather. The transition period in between the monsoons is known as the intermonsoon period.
What is Monsoon ?
The word "monsoon" is derived from the Arabic word "mausim" which means season. Ancient traders plying in the Indian Ocean and adjoining Arabian Sea used it to describe a system of alternating winds which blow persistently from the northeast during the northern winter and from the opposite direction, the southwest, during the northern summer.
What causes Monsoon ?
Monsoon is caused by land-sea temperature differences due to heating by the sun's radiation. In winter, the continental landmass cools rapidly resulting in extremely low temperatures over central Asia. As temperature drops, atmospheric pressure rises and an intense high pressure system (anticyclone) develops over Siberia. Cold air flows out of Siberia as northwesterlies and turns into northeasterlies on reaching the coastal waters of China before heading towards Southeast Asia.
From time to time, strong outbursts of cold air (termed as monsoon surges) interact with low pressure atmospheric systems and cyclonic vortices are formed near the equator resulting in strong winds and high seas in the South China Sea and heavy rainfall to east coast states of Peninsular Malaysia as well as the west coast of Sarawak in East Malaysia.
In summer, intense solar heating leads to scorching temperatures over the Asian landmass. As hot air expands and rises upwards, a semi-permanent low-pressure area develops. Moist southeasterlies originating from the southern Indian Ocean and the Indonesian-Australian region transforms into southwesterlies on crossing the equator and flow across Southeast Asia before converging towards Indochina, China and Northwest Pacific.