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monsoons: a brief introduction. Not quite an ordinary day in monsoon land!. A. C. D. B. what is a monsoon. Monsoon is a climatological feature covering roughly half the tropics (1/4 of the global surface)

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a brief introduction

what is a monsoon
what is a monsoon
  • Monsoon is a climatological feature covering roughly half the tropics (1/4 of the global surface)
  • Strictly, a system where the winds and precipitation reverses (summer rain, winter dry)
  • Sufficiently reproducible to host the most successful agricultural system (5000 years of success)
  • Host 65% of the world’s population
  • Small changes in year-to-year climate can be catastrophic

a description of the

south asian



Halley (of comet fame) was the first to describe monsoon

and attribute differential heating between

land and ocean as the cause


mean august and january precipitation

from MSU satellite (mm/day)


Monsoon regions are the

largest region where precipitation exceeds evaporation.

This excess water must come from somewhere …


…….which defines the scale of the monsoon.

The monsoon is intrinsically inter-hemispheric with the

winter hemisphere being the source of moisture


monsoon of the americas

  • There are three major
  • monsoon systems:
  • Asian-Australian
  • African
  • Americas
  • Each has its own
  • characteristics and
  • differences but each is
  • characterized by summer
  • rains and seasonal wind-
  • reversals


Surface pressure



Annual cycle of radiative temperature

“Cold” areas denote

Cloudiness and precipit-

ation. “Warm” areas

show clear regions.

Note that there are

large gradients both

east-west and north-

south indicating that

the monsoon is driven

by heating gradients

in both directions.


Climatological precipitation over South-Asia/Indian Ocean

  • Three maxima (Bay of Bengal, east coast India, and south of equator)
  • Equatorial maximum indicates importance of intraseasonal oscillations

variability of the

south asian monsoon

time scales of monsoon variability
Time Scales of Monsoon Variability
  • Interannual variability: Variations on the annual cycle of the monsoon producing anomalously wet or dry years. Generally influenced by sea-surface temperature variations associated with ENSO or inherent variability in the Indian Ocean.
  • Monsoon Weather: Weather events such as monsoon lows and depressions, tropical cyclones (hurricanes) and etc. Produce short-lived local flooding (or drought), erosion, high winds and etc.
  • Intraseasonal Variability: “Envelopes”: or clusters of weather events leading to 20-40 day droughts or flood periods. Most difficult to forecast but perhaps the most important of all time scales for economic and agricultural sectors.

El Nino influences Indian Ocean

Changes SST, sea surface slope and regions of maximum precipitation

However, other factors are also important …..


Indian Ocean Zonal Mode:

Inherent mode of Indian Ocean invoked by outside

perturbations including ENSO

Time series of E-W

SST gradient




Sea-level variations associated with IOZM:

Positive phase: warm SST WIO, low SSH EIO

Negative phase: warm SST EIO, high SSH EIO


Intraseasonal variability of the monsoon:

Active and break periods of the Summer South Asian Monsoon

The summer monsoon is divided into distinct periods of wet and dry.

When it is dry on the plains, it is wet in the foothills of the Himalaya, just as

occurred this year. The result was Nepal, Assam and Bangladesh floods, the latter due to

strong Brahmaputra discharge.


Intraseasonal Modes: Impact on Rainfall

  • Intraseasonal variability imposes a distinct form to precipitation.
  • Histograms of precipitation, shown for 9 years show distinct wet periods with lulls in between.
  • Lower diagram shows the distinct spatial character

Precipitation histograms in

central India


Forecasting monsoon variability

on 20-30 day time scales:

Taking advantage of a new

understanding of monsoon dynamics

Peter J. Webster

Georgia Institute of Technology


Differences between active and break periods of the monsoon

Note distinct bands of precipitation excess or deficit. During breaks, there is a 20-40 day drought across plains of India and excess rain over southern, northern and north-eastern regions


Latitude-time section of OLR along 90E: 1995

Active phases of the monsoon commence near the equator and

propagate northward (and southward) across South Asia.






active phase

physical ingredients for a monsoon
Physical Ingredients for a Monsoon
  • Land-Sea differences: land and water have different heating capacities & water can store more heat because it is a fluid and can mix heat down for future release
  • Planetary rotation: introduces swirl and much stronger winds
  • Moisture: water vapor, collected over oceans through evaporation, condenses over land and marginal seas releasing vast quantities of heat
  • Orography: acts as elevated heat source which intensifies flow & also ducts flow.

Impacts of Elevated Heat Sources

Note the dominating effect of the Himalayas


Changes in Tropospheric Temperature in Monsoon Region

Temperature changes much larger over Himalayas than elsewhere.


Impact of East African Highlands

Arguably, without the Himalayas & the East African

Highlands, South Asia would be a desert like North Africa


Impact of Rotation:

increases in intensity and scale


Impact of anomalous monsoon seasons

Strong and weak monsoons invoke different

upwelling effects creating SST gradients. Then …….


Explanation of why there is an IOZM with

approximately biennial period

Atmosphere responds

to SST anomaly associated

with strong/weak


Ocean responds to

anomalous winds producing

ocean dynamic modes

Anomalous SST produces

a monsoon of opposite

anomaly which wipes out



predictability of the

south asian monsoon