2019 Klimtrop MONSOONS-converted

Tropical Climatology
MONSOONS
Akhmad Faqih, PhD.
Department of Geophysics and
Meteorology
Introduction
• Monsoon  arabic word “mausim”
• The main characteristics of the monsoon regions:
• The prevailing wind direction shifts by at least 120° between January and July;
• The average frequency of prevailing directions in January and July exceeds
40%
• The mean resultant winds in at least one of the months exceeds 3 ms-1, and
less than one cyclone anticyclone alteration occurs on average every two
years in any one month in a 5° latitude-longitude rectangle (Ramage 1971).
• Monsoon areas are important in terms of their geographical coverage
and demographic that cover more than 50% world’s population.
3 mechanisms cause the Monsoon
(Webster 1987):
• The differential seasonal heating of the oceans and continents;
• The importance of seasonal temperature and pressure changes
• Seasonal contrasts in land surface temperatures produce atmospheric pressure
changes which produce reversals of the pressure gradient force, the basic driving
force of the winds  there are major wind reversals.
• Moisture processes in the atmosphere;
• A moist warm air rises during summertime heated land surface  condenses
&releasing energy  latent heat of condensation.
• Add to the strength of the monsoon
• The earth’s rotation.
• Earth’s rotation  the Coriolis force  causes wind to change direction as they
cross the equator  Monsoon current moves in curved paths.
The annual monsoon
cycle
• In transitional months
• ITCZ in the equator  maximum surface heating
• The northern hemisphere tropical-subtropical
latitudes are beginning to warm up
• Vertical motion is present, but weak
• The northern hemisphere Hadley cell and
offshore winds still predominates
• May to June
• the heating of northern tropical land masses and
vertical motions intensifies
• Onshore wind is predominant
• ITCZ has moved well north of the equator
• June to July
• Sensible heat input at the surface, the vertical
motion and atmospheric moisture over the
northern hemisphere tropical land masses are
close to maximum
• Pressure gradient force and the monsoon reach
maximum intensity
September
Surface heating has decreased with maximum
insolation positioned close to the position in
April  the structure is very similar to that
ofApril
A stopping of the northern hemisphere
monsoon wet season and the onset of dry
season.
December
The southern hemisphere wet season is
developing , and the ITCZ has moved south of
the equator
Monsoons variability
• There is significant variability in the onset, duration and magnitude of the
monsoons
• There are two mechanisms responsible for monsoon interannual variability (Sukla 1987):
1.
Internal Dynamics
A variety of aperiodic variations in the atmospheric circulation  e.g. travelling disturbances, thermal
andorographic forcing, non-linear associations between different scale of atmospheric motion and tropicalextratropical interactions constitute some of the internal dynamic controls on the monsoon (Shukla 1987)
2.
Boundary Forcing
Change in surface conditions  the extension of snow cover, surface hydrological effects and sea surface
temperature
Change in boundary conditions affect geographical distribution of heat and moisture source and sinks in the
atmosphere
• Both mechanisms interact to produce variations in the monsoos
• Various monsoon systems experience intra-seasonal variations in the form of active
and break phases  Rainfall and no rainfall conditions between one to the next
period
Regional Monsoon System
• Traditionally, only three main monsoon system have been
recognized: African, Asian and Australian Monsoon.
• Due to more data available, more monsoon systems are recognized:
•
•
•
•
•
}
The Indian Monsoon
Asian Monsoon
The East Asian Monsoon
The Australian Monsoon
The African Monsoon
Eastern North Pacific Monsoon
The Indian Monsoon
• Is made up of a number of components:
1.
2.
3.
4.
5.
6.
7.
The monsoon through the northern India
The Mascarene anticyclonic system
The low level cross-equatorial jet
The Tibetan high pressure system
The tropical easterly jet
Monsoon cloudiness
Rainfall (Krishnamurti and Bhalme 1976)
Indian Monsoon: Onsets and Withdrawals
Indian Monsoon: Processes
The East Asian Monsoon
• It is not a simply an eastward extension of the Indian Monsoon, but a
separate component of the large Asian monsoon system.
• Facts to this conclusion include:
• the frequent opposite behavior of monsoon activity over East Asia and India,
• the existence of heat source and sink regions over the South China Sea and
Australian region,
• intra-seasonal changes in the origins of the air masses involved in the
respective monsoons, and
• contrast in onset and cessation times of the two monsoons
• Unique characteristic: the East Asian monsoon has a very strong cold
winter signature , which is not possessed by any other monsoon
systems.
The East Asian Monsoon: Onsets
East Asian Summer Monsoon
• Can be divided into eight major components:
1.
2.
3.
4.
5.
6.
7.
8.
The Australian High
The cross-equatorial jet at about 110°E
The monsoon through
zones of convection
Tropical easterly jet, part of the upper level north-easterly return flow
The western Pacific high
The Mei-Yu Front
Mid-latitude disturbance
East Asian Winter Monsoon
• The associated high pressure centre reaches a great intensity at
latitudes of about 40°-60° over Mongolia and middle Siberia.
• Air moves out from the anticyclonic centre over those regions in a
southerly direction over Korea, China, Japan, Indo-China and the
western Pacific.
• Over the South China Sea, at about 15-20°, the northerly airflow
converges with the north-easterlies from the Pacific
The Australian Monsoon
A southern
hemisphere
extension of the
larger Asian winter
monsoon system
The African Monsoon
• The circulations differ from the Indian and East Asian systems in their
magnitude, thickness of flow and geographical coverage.
• There are basic differences in the structure and physical properties of
the monsoon system between West and East Africa
• In West Africa, a large continental area north of the equator contrast
with the oceanic region of the South Atlantic Ocean
Eastern North Pacific Monsoon
• Analysis of climatological data for the Pacific Ocean coast of Mexico
(Douglas et al 1993) and over the eastern north pacific (Wang 1994)
 monsoon system exist in the region.
• This is due to:
• Seasonal rainfall distribution along the Pacific coast of Mexico are similar to
those of monsoon Asia
• A distinct ocean summer rainfall season exists, as reflected by low outgoing
longwave radiation amounts in July
• Temperatures peak prior to the onset of the rains
• An annual reversal of the surface wind system from easterly in January to
westerly in July occurs (Douglas et al 1993, Wang & Murakami 1994).
(American Monsoons)
A.
The definition of Monsoon Indices
Indian Monsoon (IM) and Western North
Pacific Monsoon (WNPM)
Reference:
Wang, B. and Z. Fan, 1999: Choice of South Asian summer monsoon indices. Bull. Amer. Meteor. Soc., 80, 629-638.
Wang, B., R. Wu, K.-M. Lau, 2001: Interannual variability of Asian summer monsoon: Contrast between the Indian and
western North Pacific-East Asian monsoons. J. Climate, 14, 4073-4090.
Daily Indian Monsoon Index
Daily Western North Pacific Monsoon Index
B. Webster and Yang Monsoon Index (WYM)
Reference:
Webster, P. J. and S. Yang, 1992: Monsoon and ENSO: Selectively interactive systems. Quart. J. Roy. Meteor. Soc., 118, 877-926.
C. Australian Monsoon (AUSM)
Reference:
Kajikawa, Y., B. Wang and J. Yang, 2010: A multi-time scale Australian monsoon index, Int. J. Climatol, doi: 10.1002/joc.1955.
Daily Australian Monsoon Index
Daily Australian Monsoon Index
Cor/Reg of P recip/V850
on Eost—Asion Monsoon Index for JUN (1979—200s)
40N
20N
E§
20S
40S
60E
120E
180
120¥/
60¥/
Cor/Reg o”f Precip/V850
on South—Asian2 Monsoon Index for SEP (1979—20D6)
20N
0.37
EQ
—0.37
40S
'
0
IPB
i
60E
,»
120E
Li and Zsng [2002): DNS (2.5N-20N, 7DE-110E)
Universit
180
120¥/
60¥
0
Summary
• The monsoon are characterized by seasonal reversal of the prevailing
wind and seasonal contrast in regimes of cloudiness, precipitation
and temperature.
• Differential seasonal heating of the oceans and continents, moisture
processes in the atmosphere and the earth’s rotation are the main
factors which explain the existence of the monsoon.
Terima Kasih