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Chap. 3 Regional climates in tropics

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. The ITCZ is defined by a zone ( ) where may develop

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Chap. 3 Regional climates in tropics

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  1. Chap. 3Regional 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

  2. The ITCZ is defined by a zone ( ) where may develop • some Meso-scale Convectif Systems (~ 1000 km long) separated by • area of sky clear of the same scale magnitude or lightly greater. 3.3 TheInterTropical Convergence Zone : Définition (1) Source : Météo-France • The Satellite pictures show that the ITCZ is formed by a zonal band of deep convection generally narrow (102 km). sommaire chap.3

  3. MCS linked to the MJO • The MCS move generally westward in the mean flow. • Animation of satellite picture (InfraRed canal) : click on • Over Indian Ocean, the MCS can reach 2500 km long under the • influence of low frequency oscillations as the Madden Julian • Oscillation (MJO). 3.3 TheInterTropical Convergence Zone : Définition (2) Source : Météo-France sommaire chap.3

  4. 3.3 The ITCZHypothesis formation (1) Source : Météo-France • Intro : Physical mechanisms regulating the formation of latitudinal • preference of the ITCZ have been a subject of numerical • observational, theoretical and numerical modeling investigations • ITCZ formation dépends essentially of two main factors : • Thermodynamical factor : • The warm and moist air which feed the convection is supplied by • the trades winds which have sailed thousands of km over warm seas • On océan‣ The earliest researches (Bjerkness, 69) have related the • spatial distribution of SST to the spatial structure of • tropical convection which underlines the role of the • ocean-atmosphere coupling under tropics. • ‣ The mean location of the MCS is collocated with the • zone os SST maximum (>= 28°C). • On continent‣ ITCZ is collocated with the zone of tp’w maxi in low • troposphère (= proxy of warm and moist air) sommaire chap.3

  5. 3.3 The ITCZHypothesis formation (2) Source : Météo-France 2. Dynamical factor : SST forcing alone cannot explain all observed features of the ITCZ. For instance, many observational studies showed that the highest SST is not collocated with the ITCZ (Lietzke, 2001, Journal of Climate). Charney (1971) put forward an explanation for the ITCZ in terms of two competiting processes, namely Ekman pumping and moisture availability (thermodynamical factor). The Ekman pumping produces ascending motion which are maximum at the top of the boundary layer. The Ekman Pumping is proportional to the Coriolis parameter (f) and thus increases poleward. This explains, partly, that the ITCZ in never located along the equator (f=0) but off-equator (hundreds of km southward or northward). sommaire chap.3

  6. 3.3 ITCZ Analysis and forecasting • The ITCZ is visible through monthly mean patterns : • precipitations, Outgoing Longwave Radiation (OLR), tp’w etc. • The weather may be fine within the area of ITCZ during several days if the large scale conditions are unfavorable for the convection: • ex 1 : negative phase of MJO which produce large scale • subsidence • ex 2 : dry intrusion in middle or high troposphere which suppress deep convection over oceans. • Good proxies of the ITCZ : • For analysis and forecasting : • ‣ Convergence at 850/925 hPa • ‣ High θ’w at 850 hPa (> 21°C, over Atlantic and Pacific) • ‣ Vertical velocitis maximum at 600-700 hPa • ‣ Divergence at high troposphère : 200 hPa • For climatology(for monthly location of ITCZ) • ‣ OLR <240 W/m2 • ‣ SST >=28°C sommaire chap.3

  7. 3.3 ITCZ and OLR Source : Données NOAA OLR < 240 W/m2 over tropics (red)= deep convection OLR < 240 W/m2 over tropics (red)= deep convection Chap 3.4

  8. 3.3 ZCIT Seasonal move • Seasonal move of the ITCZ : • The position of the ITCZ follow the apparent movement of the sun with a mean lag of 6 to 8 weeks. Because of the high thermal inertia of the oceans, the lag reaches 10-12 weeks over the Atlantic and Eastern Pacific. • Eastern Pacific and Atlantic : • ‣ The ITCZ is located throughout the year in the Northern • hemisphere : in january between 2°N (Atl.) and 5°N (E. Pacific) • : in july between 8°N (Atl.) and 10°N (E. Pacific) • ‣ The ITCZ is a narrow band of deep convection (300- • 500 km of large) with annual precipitation of 2-3 meters. • Western Pacific and Eastern Indian Océan : • ‣ The ITCZ fluctuates between 10°S (january) and • 15-20°N(july) • ‣ the ITCZ is a lot larger (2000 à 3000 km of large) and the annual precipitations are the heaviest of the earth (3-4 m. by year) sommaire chap.3

  9. Amazonian convection area Indonesian monsoon South Convergence Pacific Zone (SCPZ) Eastern African monsoon and Madagasikara monsoon 3.3 ITCZ Seasonal move Precipitations (mm/s) in january : mean 68-96 (Analysis of NCEP)

  10. 3.3 ITCZ Seasonal move Precipitations (mm/s) in february : mean 68-96 (Analysis of NCEP)

  11. 3.3 ITCZ Seasonal move Precipitations (mm/s) in march : mean 68-96 (Analysis of NCEP)

  12. 3.3 ITCZ Seasonal move Precipitations (mm/s) in april : mean 68-96 (Analysis of NCEP)

  13. 3.3 ITCZ Seasonal move Precipitations (mm/s) in may : mean 68-96 (Analysis of NCEP)

  14. 3.3 ITCZ Seasonal move Precipitations (mm/s) in june : mean 68-96 (Analysis of NCEP)

  15. 3.3 ITCZ Seasonal move Precipitations (mm/s) in july : mean 68-96 (Analysis of NCEP)

  16. Western African monsoon Central America convection area Indian and SE Asian monsoon 3.3 ITCZ Seasonal move Precipitations (mm/s) in august : mean 68-96 (Analysis of NCEP)

  17. 3.3 ITCZ Seasonal move Precipitations (mm/s) in september : mean 68-96 (Analysis of NCEP)

  18. 3.3 ITCZ Seasonal move Precipitations (mm/s) in october : mean 68-96 (Analysis of NCEP)

  19. 3.3 ITCZ Seasonal move Precipitations (mm/s) in november : mean 68-96 (Analysis of NCEP)

  20. 3.3 ITCZ Seasonal move Precipitations (mm/s) in december : mean 68-96 (Analysis of NCEP) Back-up ITCZ january chap 3.4: moussons

  21. 3.3 La ZCITFormationHypothesis : more informations Introduction : It is interesting to understand why the ITCZ is nearly never located along equator but off-equator at hundreds km northward or southward equator (depends on areas and seasons). • Dynamical Factor above the boundary layer • The equation of conservation of the absolute vorticity applied above • the atmospheric boundary layer (PBL) give a link between the • Coriolis parameter (f) and the divergence. This equation indicates that : • ‣ at equator, without the Coriolis force (f=0), the airflow is • divergent • ‣ at a few degrees northward and southward the equator, as f • increases fastly, the airflow is convergent. sommaire chap.3

  22. 3.3 La ZCITFormationHypothesis : more informations Introduction : It is interesting to understand why the ITCZ is nearly never located along equator but off-equator at hundreds km northward or southward equator (depends on areas and seasons). • Dynamical Factor above the boundary layer • Dynamical Factor in the boundary layer • Under synoptic conditions of monsoon flow in the PBL : • ‣ at the equator, the lack of the Coriolis force in the PBL is balanced by the increase of the advection which produces anacceleration of the trade winds and so divergence-subsidence. • ‣ Ataround 5° of latitude (in the summer hemisphere), as the Coriolis force becomes again suddenly significant, the advection decreases suddenly which produces deceleration and so convergence-ascendance (Ekman pumping) sommaire chap.3

  23. 3.3 La ZCITFormationHypothesis : more informations Introduction : It is interesting to understand why the ITCZ is nearly never located along equator but off-equator at hundreds km northward or southward equator (depends on areas and seasons). • Dynamical Factor above the boundary layer • Dynamical Factor in the boundary layer 3. Thermodynamical factor : Over océan ‣ Between 2°S and 2°N, cold tong of SST linked with the equatorial upwelling. The fluxes of sensible and latent heat are reduced whence the absence of deep convection ‣ At about 5°N, the SST maximum is linked with the downwelling. The fluxes of sensible and latent heat are maximum and enhance deep convection. Over continent ‣ the maximum of tp’w is located in the summer hemisphere but the continental ITCZ doesn’t have latitudinal preference as over ocean chap 3.4: moussons sommaire chap.3

  24. Under the hypothesis of ζr =0 (hypothesis realistic around the equator) : (2) Eulerian evolution of f equal to 0 = 0 ⇨ (3) ⇨ 3.3 ITCZ formationDynamical factor above the boundary layer (1) • The equation of conservation of absolute vorticity (above PBL): (1) sommaire chap.3

  25. 3.3 ITCZ formationDynamical factor above the boundary layer (2) (3) cotanφ Remind on cotan φ : South Pole φ=-Π/2 North Pole φ=+Π/2 equator φ • Following the equation (3), we deduce that : • When an air parcel moves equatorward (v>0 dans HS, et v<0 dans HN), the flow become divergent and descends down. • On the contrary, when an air parcel moves poleward • (v<0 dans HS, et v>0 dans HN), the flow become convergent and ascends up. sommaire chap.3

  26. 3.3 ITCZ formationDynamical factor above the boundary layer (3) Illustration over the Eastern Pacific in january with : - subsidence at the equator - ascendance at a few degrees northward or southward the equator 25°N High pressure z 5°N 5°N 4 km Dynamical valley effect of the Equator = divergence and subsidence 2 km 5°S Surface followed by an air parcel Eq. High pressure Conclusion : convergence and ascent motions have preferential locations off -equator but to develop deep convection, the lower layers must be also favorable (for instance : convergence in the the boundary layer + SST maximum) sommaire chap.3

  27. 3.3 La ZCITFormationHypothesis : more informations Introduction : It is interesting to understand why the ITCZ is nearly never located along equator but off-equator at hundreds km northward or southward equator (depends on areas and seasons). • Dynamical Factor above the boundary layer • Dynamical Factor in the boundary layer • Under synoptic conditions of monsoon flow in the PBL : • ‣ at the equator, the lack of the Coriolis force in the PBL is balanced by the increase of the advection which produces anacceleration of the trade winds and so divergence-subsidence. • ‣ Ataround 5° of latitude (in the summer hemisphere), as the Coriolis force becomes again suddenly significant, the advection decreases suddenly which produces deceleration and so convergence-ascendance (Ekman pumping) chap 3.4: moussons sommaire chap.3

  28. Magnitude Scale : • W~ 10-3 U ⇨ the vertical advection is not significant respect to the horizontal advection • ∂ Vh/ ∂ t ~ 0 ⇨ the eulerian acceleration of the horizontal wind • is not significant 3.3 ITCZ formationDynamical factor in the boundary layer (1) • To explain the ITCZ formation at about 5° of latitude, start to write the equation of the horizontal movement in the boundary layer (PBL): (1) (2) ⇨ sommaire chap.3

  29. 1. ‘Ekman regime’ southward of 2°S and northward of 5°N : Through scale analysis of synoptic-scale in the tropical PBL, the equation (2) show a balance of the forces between the pressure force, the Coriolis force and the friction forces. The advection A is constant and not significant. (3) 2. ‘Advective regime’in the equatorial zone, between 2°S and 5°N : Because of the lack of the Coriolis force, the advection A induced by the mean flow Vh increase and permit the balance of the forces in the equatorial PBL. Within this latitud band, since the modulus of the advection |A| increases, the mean flow, that is to say, the trade winds accelerate. (4) 3.3 ITCZ formationDynamical factor in the boundary layer (2) • Define the different regimes of the tropical PBL for • atmospheric phenomenon longer than 5 days : sommaire chap.3

  30. • Advective Regime • Ekman Regime • 3.3 ITCZ formationDynamical factor in the boundary layer (3) Illustration over the Indian Ocean in july with a heat low (Dt) situated over Pakista and subtropical highs over the Southern Ocean. Explanation of the physical processes in the next slide : 25°N Dt Ekman Regime z ~ 1 km 5°N Equator 2°S Mascareigns high pressure sommaire chap.3

  31. 3.3 ITCZ formationDynamical factor in the boundary layer (4) • Explanations of the physical porcesses of the previous figure : • Between 2°S et 5°N : ‘Advective regime’ • Because of the lack of the Coriolis force in the equatorial PBL, the advection term increases and reaches the same order of magnitude that the pressure forces or friction forces. • The fast increase of the advective flow induces the acceleration of the mean flow (shown on the figure through the elongation of the blank arrows). • Lastly, the acceleration of the mean flow in the PBL produces divergence and vertical subsidence. sommaire chap.3

  32. 3.3 ITCZ formationDynamical factor in the boundary layer (5) • Explanations of the physical porcesses of the previous figure: • Between 2°S et 5°N : ‘Advective regime’ • Vers 5°N : the transition regime towards the ‘Ekman regime’ • The fast increase of the Coriolis force around 5°N is compensated by the fast decrease of the horizontal advection. • The decrease of the term of advection causes the deceleration of the mean flow (shown on the figure through the shrinking of the blank arrows and yellow arrow). • Lastly, the deceleration of the mean flow in the PBL produces convergence and maximum vertical upward ascents at the top of the PBL (called ‘Ekman pumping’). To sum-up, the convergence zone at 5°N is located in the transition zone between the Advective regime (2°S-5°N) and the Ekman regime (nothward 5°N) = In other words, ITCZ is located at that latitude (5°N) where the period ω of atmospheric weather systems, like easterly waves, equals the period of the Coriolis parameter f = 2Π/βy ~ 6 days. For more informations, see Asnani book, p.1060 and Fig. 11.4(27). sommaire chap.3

  33. 3.3 Formation de la ZCITDynamical factor in the boundary layer (6) :The‘Ekman pumping’ Link between convergence, absolute vorticity and vertical upward ascents (called Ekman pumping) • Reminder : • Both, convection and friction forces in the boundary layer • generates convergent low-level fields • - The equation of absolute vorticity explains why inflow produces • cyclonic spin-up in proportion to the existing environmental • vorticity field • Equation of the vertical velocity at the top of the PBL, called • ‘Ekman pumping’ : wH: vertical velocity at the top of the Ekman layer = Ekman pumping K: coeffecient of eddy viscosity α0 : angle of inflow between observed wind and geostrophic wind at the bottom of Ekman layer ζg: geostrophic vorticity f: Coriolis parameter ⇨ Vertical velocity at the top of Ekman layer, wH, is proportionnal to the geostrophic vorticity and f. Consequently, the Ekman pumping is null at the equateur (f=0). ⇨ We can also add that vertical velocity, w, increase with height inside the boundary layer (not explained with this equation) and is maximum (wH) at the top of the Ekman layer. sommaire chap.3

  34. 3.3 La ZCITFormationHypothesis : more informations Introduction : It is interesting to understand why the ITCZ is nearly never located along equator but off-equator at hundreds km northward or southward equator (depends on areas and seasons). • Dynamical Factor above the boundary layer • Dynamical Factor in the boundary layer 3. Thermodynamical factor : Over océan ‣ Between 2°S and 2°N, cold tong of SST linked with the equatorial upwelling. The fluxes of sensible and latent heat are reduced whence the absence of deep convection ‣ At about 5°N, the SST maximum is linked with the downwelling. The fluxes of sensible and latent heat are maximum and enhance deep convection. Over continent ‣ the maximum of tp’w is located in the summer hemisphere but the continental ITCZ doesn’t have latitudinal preference as over ocean sommaire chap.3 chap 3.4: moussons

  35. E E 3.3 ITCZ formation role of the ocean-atmosphere coupling (1) The origin of the equatorial upwelling is the Ekman divergence : Source : Météo-France (F.Beucher) • The oceanic Ekman mass transport, E, is directed at right angles to the right (left) of τ in the northern (southern) hemisphere. The magnitude of E is proportional to the strenght of τ. • Following this rule, at the equator, E is directed away • from the equator producing divergence and upwelling • along the equator. sommaire chap.3

  36. 3.3 ITCZ formation role of the ocean-atmosphere coupling (2) Link between upwelling and ‘cold’ tongues of SST: Monthly mean of Sea surface température Source : RéAnalyse NCEP 1981-2002 Source : Météo-France (F.Beucher) • The equatorial upwelling and • the coastal upwelling are pronounced • in the sectors of Eastern Pacific and Eastern Atlantic, • which explains that cold tongues of SST occur • in these areas. sommaire chap.3

  37. 3.3 ITCZ formation role of the ocean-atmosphere coupling (3) Strong correlation between upwelling (mini of SST) and mini. of precipitations : Sources : Dorman et Bourke (79,81), Dorman (82), Baumgartnet et Reichel (75) • As atmosphere-ocean coupling plays an important role in tropics (latent heat and sensible fluxes are linked with the SST) shallow convection (St/Sc or shallow Cu) and rare rain ( ) occur in upwelling areas : along the equator + E. Pacific + E. Atlantic sommaire chap.3

  38. 3.3 ITCZ formation role of the ocean-atmosphere coupling (4) Link between the Ekman convergence and the downwelling zone : Convergence d’Ekman et zone de downwelling : Source : Météo-France (F.Beucher) • We remind that the Ekman transport E is proportional to the intensity of the wind stress τ. • Since the southeasterlies decrease while they approach the ITCZ, • the Ekman transport decrease too : • ⇨ we observe a strong convergence of Ekman towards 4°N • ⇨ producing downwelling and fast increasing of SST sommaire chap.3

  39. 3.3 ITCZ formation role of the ocean-atmosphere coupling (5) Strong correlation between maxi. of SST and maxi. of précipitation : 10°N Sources : Dorman et Bourke (79,81), Dorman (82), Baumgartnet et Reichel (75) • As the ocean-atmosphere coupling plays an important role under tropics (flux of latent heat and sensible heat are linked to SST), we observe heavy rains over areas of SST maximum (>28°C) • Under annual mean, the ITCZ ( ) is located between 5°N-10°N over Central Pacific – Eastern Pacific - Atlantic sommaire chap.3 chap 3.4: moussons

  40. Baumgartner, A., Reichel, E., 1975 : The World water balance. Elsevier, Amsterdam, Oxford, New York, 179 pp. • Beucher, 2005 : Schéma conceptuel de la Zone de Convergence Intertropicale sur le Pacifique Est en juillet-Août pendant une année normale. Atmosphérique n° 26, avril 2005, disponible sur • http://intramet.meteo.fr, rubrique institutionnel /publication. Illustration de F. Poulain. • - Dorman, C. E. , 1982 :4Indian Ocean Rainfall’. Tropical Ocean-Atmosphere Newsletter,10,4. • - Dorman, C., E., Bourke, R.,R., H., 1979 :’Precipitation over the Pacific Ocean’, 30°N to 30°S. Mon. Wea. Rev., 107, 896-910 • - Dorman, C., E., Bourke, R.,R., H., 1981 :’Precipitation over the Atlantic Ocean’, 30°N to 30°S. Mon. Wea. Rev., 109, 554-563 references

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