Monsoon

Changing winds

 Monsoon

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.

 

Characteristics of Monsoon

The northeast monsoon is the major rainy season in the country. Monsoon weather systems which develop in conjunction with cold air outbreaks from Siberia produce heavy rains which often cause severe floods along the east coast states of Kelantan, Trengganu, Pahang and East Johore in Peninsular Malaysia, and in the state of Sarawak in East Malaysia.

 

Monsoon floods

Following the southward march of the monsoons, typical expected monthly rainfall maximum are:

Kelantan, Trengganu

600 mm (November)

Pahang and East Johore

600 mm (December)

Sarawak

400-700 mm (January)

The southwest monsoon is comparatively drier throughout the country except for the state of Sabah in East Malaysia. During this season, most states experience monthly rainfall minimum (typically 100 - 150 mm). This is attributed to relatively stable atmospheric conditions in the equatorial region. In particular, the dry condition in Peninsular Malaysia is accentuated by the rain shadow effect of the Sumatran mountain range. Sabah is relatively wetter (exceeding 200 mm) due to the tail effect of typhoons which frequently traverse the Philippine islands in their journey across the South China Sea and beyond.

During the intermonsoon periods, winds are light and variable. Morning skies are often clear and this favours thunderstorm development in the afternoon. In the west coast states of Peninsular Malaysia, thunderstorms contribute to a mean monthly rainfall maximum in each of the two transition periods.

Weather Advisories and Warnings

The Malaysian Meteorological Service monitors closely the changing weather situation and issues weather statements, outlooks, advisories and warnings when the need arises. The different categories of monsoon advisories and warnings are :

Heavy Rainfall Advisories

issued when there is a possibility of heavy rainfall occurrence within 24 to 48 hours.

Heavy Rainfall Warnings

issued when latest information received indicate that heavy rainfall is expected.

Strong Wind and Rough Seas Advisories

issued when there is a possibility of strong wind and rough seas within the Malaysian waters within 24 to 48 hours.

Strong Wind and Rough Seas Warnings

issued when latest information received indicate that strong wind and rough seas are expected.

These severe weather advisories and warnings are published in the newspapers and broadcasted over radio and television.

 

Be prepared for the wet Monsoon season

Before the Monsoon season ....

Listen regularly to the radio and television for warnings of strong wind and rough seas

Ensure that sufficient non-perishable food and water supplies are on hand.

Clear loose and clogged drains and rain gutters.

During the Monsoon ....

Stay on firm ground. Keep away from fast flowing drains, rivers and flooded areas.

Do not drive into flooded areas. Avoid weakened bridges and washed out roads.

Closely follow the instructions of your local disaster preparedness council .

Prepare to evacuate upon instruction if you live along the coastline, near a river or in low-lying areas.

Make sure your vehicles are parked on higher ground, and other property are securely protected from loss and damage by flood waters.

Avoid going out to sea in small vessels and fishing boats .

Tropical Cyclones

Monsoon

Haze

Thunderstorms

Squall-Lines

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