Tropical monsoon climate

Topic: World Climate Classification: Koeppen's Scheme · NCERT: Class 9, Ch 3 "Atmosphere and Climate"; Class 11, Ch 11 "World Climate and Climate Change"

Meaning

Tropical monsoon climate (Koeppen's Am) is a hot, wet climate where every month averages above 18°C, heavy rain falls in summer from monsoon winds, and there is a short dry winter in which one or more months get under 60 mm of rain [2][3]. It matters because it is India's wettest climate type. Its India locations, the west coast south of Goa and parts of north-east India, are a common map question.

Explanation

What puts a climate in the Am type

• Group A test: the coldest month averages 18°C or more. No month is cool, so there is no real winter.

• In Am, the monthly mean temperature is above 18°C in every month [3].

• Rain test (the 60 mm line): one or more months get under 60 mm of rain [2][3].

• So Am has a dry season, but it is short.

• Forest test: the total rain is still heavy enough for forest to survive.

• The yearly total is often higher than in Af, the equatorial rainforest climate [4].

• Koeppen's letter: m = monsoon [2]. In India the type is written Amw, which means monsoon type with a short dry winter.

How the monsoon brings the rain

• Summer: wet season
• The land heats faster than the sea, so low pressure forms over the land.

• Moist winds from the sea are pulled in. In India these are the south-west monsoon winds from the Indian Ocean.

• Mountains push the rain out

• The winds hit the Western Ghats and are forced to rise.

• Rising air cools. Its water vapour condenses, and very heavy rain falls on the side facing the wind (the windward side).

• Winter: dry season

• The winds now blow from land to sea.
• Land air carries little moisture, so the season is dry.

Temperature pattern

• Hottest month comes before the rains: the hottest month comes just before the rains begin [2].
• Once the clouds and rain arrive, they block sunlight and cool the land.

• In India, May is usually hotter than July.

• Small yearly range: the annual range of temperature (warmest month minus coldest month) is smaller than in savanna (Aw) [3].

• It is still a little larger than in Af, where the range is often under 3°C.

Its place in the Group A sequence

• Moving away from the equator: Af → Am → Aw → B (dry).
• The dry season gets longer.
• Total rain falls.

• Forest thins into grassland.

• Am sits in between:

• Unlike Af, it has a dry season.
• Unlike Aw, its dry season is short, and its rain is heavy enough for forest.

Where it occurs

• World:
• The Indian subcontinent.
• North-eastern South America.
• Northern Australia.

• Most of the Am area lies in southern and south-eastern Asia [4].

• India (Amw):

• The west coast south of Goa, which is the Konkan–Malabar coast: coastal Karnataka and Kerala.
• Parts of north-east India.

• NCERT's India Climate chapter puts Amw on the "west coast of India south of Goa".

• Neighbouring types in India:

• Aw covers most of the Peninsular plateau, which lies in the rain-shadow (the dry side of a mountain, away from the wind) of the Western Ghats.
• As covers the Coromandel coast of Tamil Nadu.

Don't confuse with

• Af (tropical wet / equatorial): in Af the driest month still gets 60 mm or more, so there is no dry season. Am has one or more months under 60 mm [2]. Even so, Am's yearly total is often higher than Af's [4].
• Aw (tropical wet and dry / savanna): Aw has more than two months under 60 mm. That gives a long dry season, unreliable rain and grassland [2]. Am's dry season is short, and forest survives.
• As (dry summer): in As the dry season comes in summer, while in Aw/Amw it comes in winter [2]. In India, As is the Coromandel coast, which gets its main rain in winter from the north-east monsoon. Amw is the west coast.
• Monsoon climate vs equatorial rain-maker: Am's rain comes from seasonal monsoon winds. Af's rain comes from ITCZ convection (afternoon thundershowers in the low-pressure belt where trade winds meet and rise) all year.

Prelims Hooks

• Am rule: every month averages above 18°C, and one or more months get under 60 mm, but total rain is very heavy [2][3].
• In Am, the hottest month comes just before the monsoon rains [2]. In India this is May, not June or July.
• India map trap: Amw = west coast south of Goa (coastal Karnataka, Kerala) plus parts of north-east India. As = Coromandel coast (Tamil Nadu). Aw = most of the Peninsular plateau.
• "Equatorial means wettest" is a trap: Am's yearly rain is often more than Af's [4].
• Am's annual temperature range is smaller than Aw's [3].
• World Am regions: Indian subcontinent, north-eastern South America, northern Australia. Most of the Am area is in southern and south-eastern Asia [4].

Mains Points

• Relief divides India's climates (GS-I): the Western Ghats split Amw on the windward coast from Aw on the plateau in their rain-shadow.
• Moist south-west monsoon winds rise over the Ghats and drop very heavy rain on the coast.
• Air that crosses the Ghats has lost much of its moisture, so the Deccan gets less rain.

• Effect: Kerala and coastal Karnataka grow plantation crops and keep evergreen forests. The Deccan relies on rain-fed crops and faces repeated drought.

• Am as a link in the ITCZ–monsoon sequence (GS-I): Group A types follow the seasonal north–south shift of the ITCZ.

• Where the ITCZ stays all year, rain falls every month (Af).
• Where it arrives only in summer, there is a short wet season and a long drought (Aw).

• Where seasonal monsoon winds strengthen it, a short dry season comes with very heavy rain (Am). Use this to explain the zones as a sequence, not a list.

• Climate change risk to tropical forests (GS-III): cutting forests in Af regions reduces moisture recycling (trees giving water back to the air). This can lengthen dry spells.

• Over time this can push Af towards Am or Aw conditions.
• Link this to biodiversity loss and the loss of carbon sinks.

Related concepts

• Tropical humid climates (A)
• Tropical wet climate (Af)
• Tropical wet and dry climate (Aw)

Read more

• Group A: tropical humid climates (Af, Am, Aw)