Chap. 3 Regional climates in tropics 3.1 Regional climates 3.2 Ocean circulations 3.3 Structure of...

Chap. 3 Regional climates in tropics

3.1 Regional climates

3.2 Ocean circulations

3.3 Structure of the InterTropical Convergence Zone (ITCZ)

3.4 Monsoon circulations and associated jets

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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)

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3.4 Monsoon circulations

• ‘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

JJA

DJF

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Source : from Webster, 87, p.5, Fig. 1.1

3.4 Monsoon circulationsMechanism of monsoon

JJA

DJF

Strenghtenedby a contrastcontinent-ocean

Differentialheatingbetween SH and NH

Breeze circulation …

… at large-scale, which implies theeffect of the Coriolis force on the circulation

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Source : Webster,87, p.26, figure 1.8

Source : Webster,87, p.28, figure 1.9


3.4.1 Indian Monsoon

3.4.2 West African Monsoon

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3.4.1 Indian Monsoon :The pre-onset

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.

Hatched region for RR>1000 mm Source : Rao, 81

heat low

H

H

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)

200-850 hPa layer thickness in gmp Source : Osman et Hastenrath, 1969

Origin of high geopotential and TEJ?

a consequence of heating and release of latent heat, especially in Indian monsoon

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

Tropical Easterly Jet (TEJ) and associated precipitation

• 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)

Mean july precipitation (in inches) and position of the TEJ .1 inch=2.54 cm. Source : d’après Koteswaram, 1958

3.4.1 Indian monsoon Tropical Easterly Jet (TEJ)

TEJ

3.4.1 Indian Monsoon :Key feature from may to october

Hatched region for RR>1000 mm

• Mid-tropospheric cyclone

• Monsoon depression • Map of climatological rain

Chap 3.4.2


3.4.1 Indian monsoon : monsoon dépression

‣ mid-tropo :moderate signalof convergence

Shaded area=wind > 40 ktSource : d’après Krisnamurti, 79

‣ low tropo :Maximum signal of wind and convergence from 600 to 800 hPa

‣ upper tropo : light anticyclonic circulation and light

signal of divergence

200 hPa : Streamline and isotach

500 hPa: Streamline and isotach

850 hPa: Streamline and isotach

H

C

C

3.4.1 Indian monsoon : Monsoon depression

• 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 500-200 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)

Main Key feature :

3.4.1 Indian monsoon : Monsoon depression

• Deep convection and vertical velocity max. in the SW quadrant

• Rain : in the SW quadrant between 100 mm to 300 mm per day

Location of rain and vertical velocity :

Sources : d’après Daggupaty et Sikka, 77.





Chap 3.4.2


3.4.1 Indian monsoon Mid-tropospheric cyclone

Streamline and isotach at (left) 925 hPa, (right) 600 hPaSource : Atkinson, 1971, d’après Miller et Keshavamurthy, 1968

• Closed circulation between 700-300 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

925 hPa 600 hPa

Main Key feature :

3.4.1 Indian monsoon Mid-tropospheric cyclone

• 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 500-200 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





Chap 3.4.2


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

Winter :Dry except Sri Lanka andthe far SE India Peninsula

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

Nov. to april May to oct.





Chap 3.4.2



3.4.1 Indian Monsoon

3.4.2 West African Monsoon

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3.4.2 West African monsoon

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.

Vapor image, Météosat, 17/06/97

3.4.2 West African monsoon :Key features in july/august

• 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

Source :Thorncroft, 2001

3.4.2 West African monsoon :African Easterly Jet (AEJ)

• 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

10°N

20°N

Equ.

15°N

Zonal wind (m/s)at 600 hPa. Source : Thorncroft, 2001

15°N

> 6 m/s

40°W 0°W 40°E

3.4.2 West African monsoon :Key features in july/august

• 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

Source :Thorncroft, 2001

3.4.2 West African monsoonSeasonal variation of ITCZ : pre-onset, onset, retreat

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

« Onset » « retreat »« Pre-onset »

Rain per day (in mm) :

Source :Sultan et al., 2003

3.4.2 West African monsoon:The onset simulated by meso-NH, 2D

10°S Eq 10°N 20°N 30°N 40°N

Zonal wind (07/2000)

onset :end of june

zonal wind (06/2000) 10°S Eq 10°N 20°N 30°N 40°N

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.

pre-onset : may and early june

Pre-onset :ITCZ : 5°NAEJ : 10°NSubTropical Jet : 30°N Monsoon flow : 5/10ktSTJTEJ

AEJ

harmattanmonsoon

FIT

ITCZ

10°S 40°N

FIT

monsoon

TEJITCZ

STJ

AEJ

Sources : Peyrillé, 2004

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

3.4.2 West African monsoon :An international Project : AMMA

• 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 )

Vapor image, 17/06/97

http://amma.mediasfrance.org/france/

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Vapor image, Météosat, 17/06/97

3.4.2 West African Monsoon : squall line (SL)

Hundreds km large

Up to 1000 km long

Move of the squall line over Africa

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)

W E

Schematic diagram of the squall line.

Source : Lafore, 2004.

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

e

retour coupe ZCIT sommaire général

3.4.2 West African monsoon squall line : simulation model

Simulation of squall lines in weak and strong verticalshear, from hour 2:00 to 3:40

Cross-sectional views show reflectivity (weak in blue ; strongin red), wind vectors and cloud outline (blank line).Source : COMET®

Impact of the vertical shear on the intensity


Weak shear

Strong shear

3.4.2 West African monsoon squall line : simulation model

horizontal views show reflectivity (weak in blue ; strongin red), wind vectors and cloud outline (blank line)Source : COMET®

Impact of the vertical shear on the intensity

With weak vertical shear With strong vertical shear

Simulation from hour 1:00 to 6:00 without Coriolis


3.4.2 West african monsoon squall line : simulation model

Impact of Coriolis on the shape

horizontal views show reflectivity (weak in blue to strong in red), wind vectors and cloud outline (blank line). Source : COMET®

With Coriolis force Without Coriolis force

Simulation from hour 1:00 to 6:00 with strong vertical shear

‣ Coriolis force enhances the cyclonic vortex after several hours and originates tropical storm over mid-Atlantic several days after


3.4.2 West african monsoon squall line

More about the squall line on our web-site :http://intraenm.enm.meteo.fr/pages/enm/dep_et_serv/departements/ufr/ufr_index.htm : chap 4.1


Conceptual model of barotropic instabilityConceptual model of barotropic instability

(C. Thorncroft and Pytharoulis, JAS 99, vol.127)(C. Thorncroft and Pytharoulis, JAS 99, vol.127) : :

3.4.2 West Africa monsoon

‣ 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

Equator

Barotropic instability

>0+

AEJ

5.1 African easterly wave

Source : Pytharoulis et Thorncroft, 99

References for mechanism of monsoon and

Indian monsoon (1)

- Atkinson, G. D. 1971 : Forecaster guide to tropical meteorology. Rapport technique 240,U.S. Air Weather Service.

- Atlas Bordas historique et géographique, 1985. 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. 773-792

- 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.298-353, 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.297-324

- Rao, Y. P., 1981. 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.

- 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

References for mechanism of monsoon and

Indian monsoon (2)

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 http://meted.ucar.edu/ of the University Corporation for Atmospheric Research (UCAR) pursuant to a Cooperative Agreement with National Oceanic and Atmospheric Administration. ©1997-2005 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 http://intramet.meteo.fr, 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.2266-2280.

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.390-407

- 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.317-333.

- 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.3389-3406

- 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.3407-3427

- 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. 14451-14510

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