1. Chap. 3 Regional climates in tropics
3.2 Ocean circulations
3.3 Structure of the InterTropical
Convergence Zone (ITCZ)
3.4 Monsoon circulations and
associated jets
sommaire général

2. 3.4 Monsoon circulations and associated jets
Which interests ?
• Affect a large area of tropics, from West Africa to Western Pacific passing by Indian Ocean, either 50% of the tropics.
• Connections with global circulations
(ex : modulate intensity of STJ, TEJ …)
• Various synoptic disturbances occur as :
- monsoon depression (Bay of Bengale,
Arabian Sea)
- mid-tropospheric cyclones (Arabian sea)
- shear lines (Southern China)
- westward wind burst (WWB) over
Western Pacific
- cold surges (South China Sea or North Indian Ocean during the winter monsoon)
sommaire monsoon

3. 3.4 Monsoon circulations JJA DJF ‘Mausim’ means ‘season‘ in Arabic
Source : from Webster, 87, p.5, Fig. 1.1
DJF
‘Mausim’ means ‘season‘ in Arabic
‘Monsoon flow’ = transequatorial trades wind flow shifted by the
Coriolis force
‘Monsoon region’= the prevailing wind direction shifts by
at least 120° between july and january.
With this definition, the three greatest summer monsoon are:
‣ from june to september over West African, India/China
‣ from december to march over Indonesia and North Australia
sommaire monsoon

4. 3.4 Monsoon circulations Mechanism of monsoon
JJA
Strenghtened
by a contrast
continent-ocean
Differential
heating
between
SH and NH
Source : Webster,
87, p.28, figure 1.9
Breeze
circulation …
DJF
… at large-scale, which implies the
effect of the Coriolis force on the
circulation
Source : Webster,
87, p.26, figure 1.8
sommaire monsoon

5. 3.4 Monsoon circulations 3.4.1 Indian Monsoon
3.4.2 West African Monsoon
sommaire général

6. 3.4.1 Indian Monsoon : The pre-onset
heat low
Hatched region for
RR>1000 mm
Source : Rao, 81
Pre-onset :
① Early june, as the heat low over the Tibetan Plateau is deepening, the monsoon flow at 850 hPa over the Arabian Sea increase within a few days, from 10 kts (about 3rd of june) to 40 kts (about 7th of june) : this jet is called ‘Somali Jet’
② By barotropic instability, on the northern flank of the Somali Jet,
a vortex, manifested as trough or depression, is developping over SE of Arabian Sea about 10th to 20th of June. Following an increase of fluxes and convergence and two weeks later onset occur on the coast.
③ End of June, this vortex moves NW to die over Arabian Peninsula.

7. 3.4.1 Indian monsoon Tropical Easterly Jet (TEJ)
Flow and isotach in july at 200 hPa but the peak occur at 100 hPa Source : d’après Koteswaram (1958)
JJA ‣ 1rst maximum at 100 hPa/15°N from SE Asia to
Eastern Africa passing from South Indian (60/70 kt)
‣ 2nd max. at 8°N/100 hPa over Western Africa (35 kt)
DJF ‣ located in a latitud band 10°S-equator and at 100 hPa from
Central Pacific (20 kt at 180°) passing from Indonesia (30 kt)
and equatorial Indian Ocean (15 kt) ending over
Central Africa (20kt at 20°E)
Carte de flux :chap 3.1 H
Origin of high geopotential and TEJ?
a consequence of
heating and release of
latent heat, especially
in Indian monsoon
hPa layer thickness in gmp
Source : Osman et Hastenrath, 1969

8. 3.4.1 Indian monsoon Tropical Easterly Jet (TEJ)
and associated precipitation
TEJ
Mean july precipitation (in inches) and position of the TEJ .
1 inch=2.54 cm. Source : d’après Koteswaram, 1958
Maximum of rain and upper divergence at the right
entry and left exit
Forecasters : Survey right entry and left exit of the TEJ
(it’s the opposite when the TEJ is located in
the southern hemisphere in DJF)

9. 3.4.1 Indian Monsoon : Key feature from may to october
Hatched region for
RR>1000 mm
Source : Rao, 81
Hatched region for RR>1000 mm
Mid-tropospheric cyclone
Monsoon depression
Map of climatological rain
Chap 3.4.2

10. 3.4.1 Indian monsoon : monsoon dépression
200 hPa : Streamline and isotach
‣ upper tropo :
light anticyclonic
circulation and light signal of divergence H
500 hPa: Streamline and isotach
‣ mid-tropo :
moderate signal
of convergence
Shaded area=wind > 40 kt
Source : d’après Krisnamurti, 79 C
850 hPa: Streamline and isotach
‣ low tropo :
Maximum signal of wind and convergence from 600 to 800 hPa C

11. 3.4.1 Indian monsoon : Monsoon depression
Main Key feature :
Synoptic-scale : about 2000 km of diameter
Location of initiation ‣ 80 % in tha Bay of Bengale,
‣ 10% in Arabian Sea
‣ 10 % over land (Bangladesh)
MSLP ~ as low as 990 hPa
Livespan : from 3 to 5 days
Frequency : twice a month over Bay of Bengale
Move : westward or northwestward at 2 or 3 m/s in direction of
heat low at least as far as Central India before decaying
Closed circulation between surface-300 hPa
max. intensity (wind, convergence) from 600 to 800 hPa
Temperature ‣ cold core between surface-600 hPa
‣ above, hot core between hPa
‣ But, some of them no cold core observed
Evolution : no risk of development of tropical cyclone because of the strong vertical shear (TEJ in upper tropo and SW monsoon flow in low tropo)
Origin hypothesis : baroclinic instability (eastward vertical tilt
coupled with westward vertical shear )
+ CISK (convergence linked to the surface trough)

12. 3.4.1 Indian monsoon : Monsoon depression
Location of rain and vertical velocity :
Sources : d’après Daggupaty et Sikka, 77.
• Deep convection and vertical velocity max. in the SW quadrant
• Rain : in the SW quadrant between 100 mm to 300 mm per day

13. 3.4.1 Indian Monsoon : Key feature from may to october
Hatched region for
RR>1000 mm
Source : Rao, 81
Hatched region for RR>1000 mm
Mid-tropospheric cyclone
Monsoon depression
Map of climatological rain
Chap 3.4.2

14. 3.4.1 Indian monsoon Mid-tropospheric cyclone
925 hPa
600 hPa
Streamline and isotach at (left) 925 hPa, (right) 600 hPa
Source : Atkinson, 1971, d’après Miller et Keshavamurthy, 1968
• Closed circulation between hPa
Maximum of intensity (wind, convergence) in
mid-troposphere from 500 to 600 hPa
At low and upper troposphere : absence or light signature
in wind (manifested as a trough in the streamline) whence
risk of ‘messing- up’ for forecasters

15. 3.4.1 Indian monsoon Mid-tropospheric cyclone
Main Key feature :
Location of initiation : Mid-tropospheric cyclone occur in
NE of Arabian Sea, South Vietnam, South China Sea from
Period : from may to october
Synoptic scale : ~ 3000 km
Livespan : from 3 to 7 days, even 10 days !
Frequency : less than monsoon depression
Move : stationnary or westward
Temperature ‣ cold core between surface-600 hPa
‣ above between hPa, hot core
Heavy rains : in western quadrant (location of ascending motion) up to 200 mm per day
Origin hypothesis ‣ for initiation, barotropic instability of the monsoon flow at 700 hPa
‣ for growth, release of latent heat

16. 3.4.1 Indian Monsoon : Key feature from may to october
Hatched region for
RR>1000 mm
Source : Rao, 81
Hatched region for RR>1000 mm
Mid-tropospheric cyclone
Monsoon depression
Map of climatological rain
Chap 3.4.2

17. 3.4.1 Indian monsoon : climatological rain
Precipitation associated with
winter monsoon (left) and summer monsoon (right)
[shaded in blue if total RR> 1000 mm ].Source : Atlas Bordas, p.87
Nov. to april
May to oct.
Summer, 3 max of rain :
- max. over Ghates mountains
(mid- tropospheric cyclone,
pre-onset vortex), drier downwind side the mountain
- max. over Bangladesh situated
along the trajectory of monsoon depression
-max. over southern China, from may
to mid-june, linked to monsoon surge (when low level SW
jet > 25 kts entre 900 et 700 hPa) coupled with a vortex at surface. Over this period, they produce rain between 500
and 1000 mm
Winter :
Dry except Sri Lanka and
the far SE India Peninsula

18. 3.4.1 Indian Monsoon : Key feature from may to october
Hatched region for
RR>1000 mm
Source : Rao, 81
Hatched region for RR>1000 mm
Mid-tropospheric cyclone
Monsoon depression
Map of climatological rain
Chap 3.4.2

19. 3.4 Monsoon circulations 3.4.1 Indian Monsoon
3.4.2 West African Monsoon
sommaire général

20. 3.4.2 West African monsoon The West African Monsoon can be seen :
Vapor image, Météosat, 17/06/97
The West African Monsoon can be seen :
‣ either on daily timescales (MCS shown as )
‣ or on monthly to annual timescales
(ITCZ shown as ).
The ITCZ exists only on these climatological
timescales, and not on daily timescales.

21. 3.4.2 West African monsoon : Key features in july/august
Source :
Thorncroft, 2001
AEJ : African Easterly Jet at 15°N, maximum of 20/30 kt
at 600 hPa
Saharan Air Layer (SAL) occur sometimes between 1500-
6000 m over Sahara (more information with )
Heat low (at surface) at 25°N over Sahara
ITCZ located at 10/12°N in july/august over western
Africa
Cold tongue (fall of 1 to 2°C of SST) in Gulf of Guinea

22. 3.4.2 West African monsoon : African Easterly Jet (AEJ)
Zonal wind (m/s)at 600 hPa. Source : Thorncroft, 2001
20°N
15°N
15°N
10°N
Equ.
40°W
0°W
40°E
> 6 m/s
Location ‣ over Africa from early july to september about at 15°N
‣ located between 600 and 700 hPa
Strenght (monthly mean, ERA 40 source) :
‣ 18 kt over Eastern Africa (50°E),
‣ decrease over Central Africa (15kts btw 20/30°E)
‣ is maximum over Western Africa (22kt btw 0°W/40°W)
Origin : Thermal wind . The horizontal gradient temperature
directed northward btw SFC-700 hPa produce easterlies
Consequence of AEJ : initiation of easterly wave at its
southern flank (12°N) by barotropic instability
and at its northern flank (17 to 25°N) for not-well understood reasons

23. 3.4.2 West African monsoon : Key features in july/august
Source :
Thorncroft, 2001
AEJ : African Easterly Jet at 15°N, maximum of 20/30 kt
at 600 hPa
Saharan Air Layer (SAL) occur sometimes between 1500-
6000 m over Sahara (more information with )
Heat low (at surface) at 25°N over Sahara
ITCZ located at 10/12°N in july/august over western
Africa
Cold tongue (fall of 1 to 2°C of SST) in Gulf of Guinea

24. 3.4.2 West African monsoon Seasonal variation of ITCZ : pre-onset, onset, retreat
Source :Sultan et al., 2003
« Onset »
« retreat »
« Pre-onset »
Rain per day (in mm) : t1 t0
‣ Early May, rains shift from equator to 5°N : it’s called the
pre-onset monsoon phase
‣ End of June, rains shift from 5°N to 10°N : it’s called the onset monsoon phase
‣ ITCZ retreat is linear from end of August to October : end of the
summer monsoon

25. 3.4.2 West African monsoon: The onset simulated by meso-NH, 2D
pre-onset : may and early june
STJ
TEJ
AEJ
harmattan
monsoon
FIT
ITCZ
Pre-onset :
ITCZ : 5°N
AEJ : 10°N
SubTropical Jet : 30°N
Monsoon flow : 5/10kt
Onset of monsoon :
1. Fall of SST (26 to
24°C) and increase
of pressure in Gulf
of Guinea.
2. Meanwhile, Tsurf
increase over Sahara
and the ‘heat low’ is
deepening
⇨ horizontal pressure
and temperature
gradients increase
btw Sahara and Gulf
of Guinea
3. Monsoon flow
increase (10/20 kt)
4. ITCZ move
northwards from
5°N to 10°N within a
few days (mean of
24th June). AEJ move
to 15°N, STJ to 40°N.
10°S Eq °N °N °N °N
10°S
40°N
zonal wind (06/2000)
onset :end of june
10°S Eq °N °N °N °N
FIT
monsoon
TEJ
ITCZ
STJ
AEJ
Zonal wind (07/2000)
Sources : Peyrillé, 2004

26. 3.4.2 West African monsoon: conceptual model
Conceptual model in july/august .
Sources : from Fontaine, 1989 and Germain, 1968
: link to squall line
FIT (Front InterTropical) = discontinuity in the flow pattern (NE northward the FIT, SW southwards the FIT) and the FIT is visible through a strong horizontal gradient of θ’w or deep point at surface.
The FIT is less vertically tilted in august than in february

27. 3.4.2 West African monsoon : An international Project : AMMA
Vapor image, 17/06/97
Vapor image, Météosat, 17/06/97
Why an African Monsoon Multidisciplinary Analysis (AMMA) Project ?
To improve our knowledge and understanding of the West African Monsoon and its variability with the emphasis :
‣ on daily timescales (MCS shown as )
‣ to interannual timescales (ITCZ shown as )
Sommaire général

28. 3.4.2 West African Monsoon : squall line (SL)
Schematic diagram of the squall line.
Move of the squall line over Africa E W
Hundreds km large
Up to 1000 km long
Source : Lafore, 2004.
Definition :
‣ Mescoscale Convective System organized in line which can extent
to 1000 km long and hundreds km karge.
‣ Lifetime : 12 to 36 h >> life of a CB (max of 1 hour).
‣ Can produce cyclonic vorticity, at 3 km height, after tens hours of
life and originate tropical storm over mid-Atlantic several days after.
‣ Violent phenomenon : wind, lightning, heavy rain during 10 to 30 mn
‣ Finally, amount of rain by SQ are light over Sahelian area (as large
as tens of mm because a large part is evaporated by dry middle
tropo air) but are moderate more southwards (as large as 100 mm)

29. 3.4.2 West african monsoon : squall line
5 favorables conditions :
⒈Warm moist air in low-level (SFC-700 hPa) visible
through high CAPE or
⒉Dry air in mid-tropo (~ 600 hPa) organize convection :
the evaporation process originates strong downdrafts and
cold pool in low-levels.
But, in some cases (light CAPE, no relief, over ocean), the
midle-tropo dry air can kill convection.
In another words, the midle-tropo dry air play a role of
energetic barrier crossed by thermodynamics (CAPE) or
external conditions (relief, inertial-gravity wave)
⒊Strong vertical shear (>10 m/s between SFC-3km)
increase lifetime and intensity of the squall line and delays
the formation of stratiform region.
Remark : the only component of low-level shear that
contributes is the component perpendicular to squall Line
orientation.
⒋Favorable surface conditions :
‣ occurred more over land than ocean
‣ occurred more over mountain than plain
⒌Favorable synoptic conditions : trough of easterly wave
‣ activity of squall line enhanced
‣ faster squall line
retour coupe ZCIT
sommaire général

30. 3.4.2 West African monsoon squall line : simulation model
Impact of the vertical shear on the intensity
Simulation of squall lines in weak and strong vertical
shear, from hour 2:00 to 3:40
Weak shear
Strong shear
Cross-sectional views show reflectivity (weak in blue ; strong
in red), wind vectors and cloud outline (blank line).
Source : COMET®
retour coupe ZCIT
sommaire général

31. 3.4.2 West African monsoon squall line : simulation model
Impact of the vertical shear on the intensity
Simulation from hour 1:00 to 6:00 without Coriolis
With weak vertical shear
With strong vertical shear
horizontal views show reflectivity (weak in blue ; strong
in red), wind vectors and cloud outline (blank line)
Source : COMET®
retour coupe ZCIT
sommaire général

32. 3.4.2 West african monsoon squall line : simulation model
Impact of Coriolis on the shape
Simulation from hour 1:00 to 6:00 with strong vertical shear
With Coriolis force
Without Coriolis force
horizontal views show reflectivity (weak in blue to strong in red), wind vectors and cloud outline (blank line). Source : COMET®
‣ Coriolis force enhances the cyclonic vortex after
several hours and originates tropical storm over
mid-Atlantic several days after
retour coupe ZCIT
sommaire général

33. 3.4.2 West african monsoon squall line
More about the squall line on our web-site :
: chap 4.1
retour coupe ZCIT
sommaire général

34. 3.4.2 West Africa monsoon
Conceptual model of barotropic instability (C. Thorncroft and Pytharoulis, JAS 99, vol.127) :
Source : Pytharoulis et Thorncroft, 99
Barotropic instability
AEJ
>0 +
Equator
‣ In the northern (resp. southern) hemisphere, the barotropic
instability area is situated at the northern (respec. southern)
flank of the maximum of Potentiel Vorticity (i.e., where
meridian gradient PV is reverse ). In surface, the meridian
gradient temperature is directed poleward.
‣ If we observe a collocated jet, the atmosphere can convert
Zonal Kinetic Energy into Eddy kinetic Energy and initiate
waves.
‣ Over Western Africa, southward the AEJ, easterly wave are
initiated by this process
5.1 African easterly wave

35. References for mechanism of monsoon and Indian monsoon (1)
Atkinson, G. D : Forecaster guide to tropical meteorology. Rapport technique 240,U.S. Air Weather Service.
Atlas Bordas historique et géographique, Editeur Hözel à Vienne
Daggaputy S. M., Sikka, 1977 : ‘On the vorticity budget and vertical distribution associated with the life cycle of a monsoon depression’, Journal of Atm. Sci., vol.34, n°5, p
Johnson, R. H. and R. A. Houze, Jr., Precipitating clouds systems of the Asian monsoon, in Monsoon Meteorology, C.-P. Chang and T. N. Krisnamurti, eds., Oxford University Press, p , 1987
Koteswaram, P., 1958 : ‘The easterly jet stream in the tropics’, Tellus, 10, p.43-57
Krisnamurti, T. N., 1979 : Tropical Meteorology. Compendium of meteorology, vol.2, part 4, editor A. Wiin-Nielsen, WMO N°364, World Meteorologic Organization, Geneva, 428 p.
Miller, B. R. et R. N. Keshavamurthy, 1968 : Structure of an Arabian Sea summer monsoon system. Monographies météorologiques de l’Expédition internationales dans l’Océan Indien, N°1, East-West Centre Press, Honolulu, Hawaï
Osman, O. E., Hastenrath, S., 1969 : ‘On the synoptic climatology of summer rainfall over Central Sudan’. Archiv. Meteor. Geophys. Bioklim., Ser. B., 17, p
- Rao, Y. P., The climate of the Indian subcontinent. In Climates of southern and western Asia. Vol.9. World Survey of climatology, ed. H. E. Landsberg (Volume editors K. Takahashi and H. Arakawa) Elsevier, Amsterdam.

36. References for mechanism of monsoon and Indian monsoon (2)
- Ramage, C. S. , 1971 : Monsoon meteorology. Academic Press, New York and London, 296 p.
Webster, Peter, J., 1987 : The Elementary Monsoon. In Fein and Stephens (ed.). Monsoons. J. Wiley, p. 3-32

37. References for African monsoon (1)
Carlson, T. N., Lee, J. D., 1978 : Tropical Meteorology. Pennsylvania State University, Independent Study by Correspondence, University Park, Pennsylvania, 387 p.
COMET : ‘The source of this material is the Cooperative Program for Operational Meteorology, Education, and Training (COMET®) website at of the University Corporation for Atmospheric Research (UCAR) pursuant to a Cooperative Agreement with National Oceanic and Atmospheric Administration. © University Corporation for Atmospheric Research. All Rights Reserved’.
- Fontaine, B., 1989 : Les moussons pluvieuses dans l’espace africano-asiatique : Afrique Occidentale et Inde. Thèse d’Etat, Univ. Dijon, France, 2 vol., 687 p.
Germain, H., 1968 : ‘Météorologie dynamique et climatologie; application au régime des pluies au Sénégal’ ASECNA, Direction de l’Exploitation Météorologique, Dakar, Senegal, 15 p.
- Lafore, J. Ph., 2004 : Orages en Fanfare. Atmosphérique n°21, disponible sur
rubrique institutionnel /publication. Illustration de F. Poulain.
Peyrille, P. et al., 2004 : ‘An idealized approach of the West African Monsoon’. AMS Conference on Hurricanes and Tropical Meteorology. Vol.26,[np],
Piriou C., J P. Lafore, Tomasini M. : Climatologie des MCS sur l’Afrique de l’Ouest. Note technique CNRM en cours.
Pytharoulis, L., Thorncroft, C., 1999 : “The low-level structure of African easterly waves in 1995”. Month. Wea. Rev., Boston, MA. Vol.127, n°10, p

38. References for African monsoon (2)
Roca, R., Lafore, J.-P., Piriou, C. et Redelsperger, J.-L., 2005 : ‘Extratropical dry air intrusion into the west African monsoon mid-troposphere : an important factor for the convective activity over Sahel’. J. Atsmos. Sci., vol.62, n°2, p
- Reed, R.J., D. C. Norquist et E. E. Recker, 1977 : The structure and properties of African wave disturbances as observed during phase III og GATE. Monthly Weather Review, 105, p
- Sultan, B., S. Janicot, Diedhiou, A. , 2003: The West African Monsoon Dynamics. Part I: ‘Documentation of Intraseasonal variability’. Journal of Climate, Vol.16, n°21, p
- Sultan, B. and S. Janicot, 2003: The West African Monsoon Dynamics. Part II: ‘ The ‘Preonset’ and ‘Onset’ of the Summer Monsoon’. Journal of Climate, Vol.16, n°21, p
- Thorncroft et al., 2001 : ‘The JET2000 experiment : large-scale overview of the 2001 season’. Proceedings on the 25th AMS Conference on Hurricanes and Tropical Meteorology. [np]. AMS Conference on Hurricanes and Tropical Meteorology , Vol.25
- Webster et al., 1998 : ‘Monsoons : processes, predictability, and the prospects for prediction’, Journal of Geophysical Research, Washington, DC. Vol. 103, n°C7 (TOGA, special issue), June 29, p