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
and attribute differential heating between
land and ocean as the cause
from MSU satellite (mm/day)
largest region where precipitation exceeds evaporation.
This excess water must come from somewhere …
The monsoon is intrinsically inter-hemispheric with the
winter hemisphere being the source of moisture
“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.
south asian monsoon
Changes SST, sea surface slope and regions of maximum precipitation
However, other factors are also important …..
Inherent mode of Indian Ocean invoked by outside
perturbations including ENSO
Time series of E-W
Positive phase: warm SST WIO, low SSH EIO
Negative phase: warm SST EIO, high SSH EIO
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.
Precipitation histograms in
on 20-30 day time scales:
Taking advantage of a new
understanding of monsoon dynamics
Peter J. Webster
Georgia Institute of Technology
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
Active phases of the monsoon commence near the equator and
propagate northward (and southward) across South Asia.
Note the dominating effect of the Himalayas
Temperature changes much larger over Himalayas than elsewhere.
Arguably, without the Himalayas & the East African
Highlands, South Asia would be a desert like North Africa
increases in intensity and scale
Strong and weak monsoons invoke different
upwelling effects creating SST gradients. Then …….
approximately biennial period
to SST anomaly associated
Ocean responds to
anomalous winds producing
ocean dynamic modes
Anomalous SST produces
a monsoon of opposite
anomaly which wipes out
south asian monsoon