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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.

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Presentation on theme: "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."— Presentation transcript:

1 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 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 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 sommaire monsoon Source : from Webster, 87, p.5, Fig. 1.1

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

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 Pre-onset : 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 3 rd of june) to 40 kts (about 7 th of june) : this jet is called Somali Jet 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 10 th to 20 th of June. Following an increase of fluxes and convergence and two weeks later onset occur on the coast. End of June, this vortex moves NW to die over Arabian Peninsula. Hatched region for RR>1000 mm Source : Rao, 81 heat low

7 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 : daprès 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 1 rst maximum at 100 hPa/15°N from SE Asia to Eastern Africa passing from South Indian (60/70 kt) 2 nd 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

8 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 (its 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 : daprès Koteswaram, 1958 3.4.1 Indian monsoon Tropical Easterly Jet (TEJ) TEJ

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

10 3.4.1 Indian monsoon : monsoon dépression mid-tropo : moderate signal of convergence Shaded area=wind > 40 kt Source : daprès 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

11 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 :

12 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 : daprès Daggupaty et Sikka, 77.

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

14 3.4.1 Indian monsoon Mid-tropospheric cyclone Streamline and isotach at (left) 925 hPa, (right) 600 hPa Source : Atkinson, 1971, daprès Miller et Keshavamurthy, 1968 Closed circulation between 700-300 hPa Maximum of intensity (wind, convergence) in mid-troposphere from 500 to 600 hPa A t 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 hPa600 hPa

15 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

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

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 Winter : Dry except Sri Lanka and the 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 aprilMay to oct.

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

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 : 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

21 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

22 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

23 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

24 3.4.2 West African monsoon Seasonal variation of ITCZ : pre-onset, onset, retreat t1t1 t0t0 Early May, rains shift from equator to 5°N : its called the pre-onset monsoon phase End of June, rains shift from 5°N to 10°N : its 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

25 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, T surf 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 24 th June). AEJ move to 15°N, STJ to 40°N. pre-onset : may and early june Pre-onset : ITCZ : 5°N AEJ : 10°N SubTropical Jet : 30°N Monsoon flow : 5/10kt STJ TEJ AEJ harmattan monsoon FIT ITCZ 10°S 40°N FIT monsoon TEJ ITCZ STJ AEJ 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 θw 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 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/ Sommaire général Vapor image, Météosat, 17/06/97

28 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.

29 3.4.2 West african monsoon : squall line 5 favorables conditions : Warm moist air in low-level (SFC-700 hPa) visible through high CAPE or Dry 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) Strong 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. Favorable surface conditions : occurred more over land than ocean occurred more over mountain than plain Favorable 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 Simulation of squall lines in weak and strong vertical shear, from hour 2:00 to 3:40 Cross-sectional views show reflectivity (weak in blue ; strong in red), wind vectors and cloud outline (blank line). Source : COMET® Impact of the vertical shear on the intensity retour coupe ZCIT sommaire général Weak shear Strong shear

31 3.4.2 West African monsoon squall line : simulation model horizontal views show reflectivity (weak in blue ; strong in 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 retour coupe ZCIT sommaire général

32 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 retour coupe ZCIT sommaire général

33 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/departeme nts/ufr/ufr_index.htmhttp://intraenm.enm.meteo.fr/pages/enm/dep_et_serv/departeme nts/ufr/ufr_index.htm : chap 4.1 retour coupe ZCIT sommaire général

34 Conceptual model of barotropic instability (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

35 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 lExpédition internationales dans lOcéan 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.

36 - 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)

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 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.http://meted.ucar.edu/ - Fontaine, B., 1989 : Les moussons pluvieuses dans lespace africano- asiatique : Afrique Occidentale et Inde. Thèse dEtat, Univ. Dijon, France, 2 vol., 687 p. - Germain, H., 1968 : Météorologie dynamique et climatologie; application au régime des pluies au Sénégal ASECNA, Direction de lExploitation Météorologique, Dakar, Senegal, 15 p. - Lafore, J. Ph., 2004 : Orages en Fanfare. Atmosphérique n°21, disponible sur http://intramet.meteo.frhttp://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 lAfrique de lOuest. 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.

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.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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