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

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3.3 The InterTropical Convergence Zone : Définition (1)

• The Satellite pictures show that the ITCZ is formed by a zonal band of deep convection generally narrow (102 km).

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• 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.

Source : Météo-France

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• The MCS move generally westward in the mean flow. Animation of satellite picture (InfraRed canal) : click on

MCS linked to the MJO

• 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 The InterTropical Convergence Zone : Définition (2)


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 :

1. 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 océanOn 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 continentOn continent ‣ ITCZ is collocated with the zone of tp’w maxi in low troposphère (= proxy of warm and moist air)

3.3 The ITCZ Hypothesis formation (1)

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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 pumpingEkman pumping and moisture availability (thermodynamical factor).

The Ekman pumpingEkman pumping produces ascending motion which aremaximum 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).

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3.3 The ITCZ Hypothesis formation (2)


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 : 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 climatologyFor climatology (for monthly location of ITCZ)(for monthly location of ITCZ) ‣ OLR <240 W/m2

‣ SST >=28°C

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3.3 ITCZ and OLR

OLR < 240 W/m2 over tropics (red)= deep convection

OLR < 240 W/m2 over tropics (red)= deep convection

Chap 3.4

Source : Données NOAA

• Seasonal move of the ITCZ :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 :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 :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)

3.3 ZCIT Seasonal move

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3.3 ITCZ Seasonal move

Precipitations (mm/s) in january : mean 68-96 (Analysis of NCEP)

Indonesian monsoon

SouthConvergencePacific Zone (SCPZ)

Amazonianconvection area

Eastern African monsoon and Madagasikara monsoon


Precipitations (mm/s) in february : mean 68-96 (Analysis of NCEP)


Precipitations (mm/s) in march : mean 68-96 (Analysis of NCEP)


Precipitations (mm/s) in april : mean 68-96 (Analysis of NCEP)


Precipitations (mm/s) in may : mean 68-96 (Analysis of NCEP)


Precipitations (mm/s) in june : mean 68-96 (Analysis of NCEP)


Precipitations (mm/s) in july : mean 68-96 (Analysis of NCEP)

Indian and SE Asian monsoon Central America convection area

Western Africanmonsoon


Precipitations (mm/s) in august : mean 68-96 (Analysis of NCEP)


Precipitations (mm/s) in september : mean 68-96 (Analysis of NCEP)


Precipitations (mm/s) in october : mean 68-96 (Analysis of NCEP)


Precipitations (mm/s) in november : mean 68-96 (Analysis of NCEP)

Back-up ITCZ january

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Precipitations (mm/s) in december : mean 68-96 (Analysis of NCEP)

3.3 La ZCIT Formation Hypothesis : more informations

Introduction :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).

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1.1. Dynamical Factor above the boundary layer 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 equatorat equator, without the Coriolis force (f=0), the airflow is

divergent ‣ at a few degrees northward and southward the equatorat a few degrees northward and southward the equator, as f

increases fastly, the airflow is convergent.

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2.2. Dynamical Factor in the boundary layerDynamical Factor in the boundary layer

Under synoptic conditions of monsoon flow in the PBL :

‣ at the equatorat the equator, the lack of the Coriolis force in the PBL is balanced by the increase of the advection which produces an acceleration of the trade winds and so divergence-subsidence.

‣ AtAt around 5° of latitudearound 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)

1.1. Dynamical Factor above the boundary layerDynamical Factor above the boundary layer



3. Thermodynamical factor : Thermodynamical factor :

Over océanOver océan ‣ Between Between 2°S and 2°N2°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°NAt 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 continentOver continent the maximum of tp’w is located in the summer ‣ hemisphere but the continental ITCZ doesn’t have

latitudinal preference as over oceansommaire chap.3



chap 3.4: moussons



adv

yf

fvVdiv hh

1)sin2(ln

1⇨

hhrr Vdivf

dt

fd )(

)(

• The equation of conservation of absolute vorticity (above PBL):

3.3 ITCZ formation Dynamical factor above the boundary layer (1)

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(1)

hhVdivfy

fv

x

fu

t

f

dt

df .

Eulerian evolution of f equal to 0

= 0

• Under the hypothesis of ζr =0 (hypothesis realistic around the equator) :

(2)

cotan

)sin(ln

a

vd

a

vVdiv hh

⇨ (3)

cotana

vVdiv hh

Remind on cotan 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.

equator

South Pole

φ=-Π/2

North Pole

φ=+Π/2

cotanφ

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φ

(3)


Illustration over the Eastern Pacific in january with :Illustration over the Eastern Pacific in january with :- subsidence at the equator - subsidence at the equator - ascendance at a few degrees northward or southward the - ascendance at a few degrees northward or southward the

equator equator 25°N

High pressure

5°N

Eq.

5°S

High pressure

4 km

z

5°N

Dynamical valley effect of theEquator =divergence andsubsidence

2 km

Conclusion : convergence and ascent motions have preferentiallocations off -equator but to develop deep convection, the lowerlayers must be also favorable (for instance : convergence in the the boundary layer + SST maximum)

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Surface followed by

an air parcel


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Under synoptic conditions of monsoon flow in the PBL :

‣ at the equatorat the equator, the lack of the Coriolis force in the PBL is balanced by the increase of the advection which produces an acceleration of the trade winds and so divergence-subsidence.

‣ AtAt around 5° of latitudearound 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)


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fcph FFFAt

V

hhhh VVz

wwVVA

..

• To explain the ITCZ formation at about 5° of latitude, start to write the equation of the horizontal movement in the boundary layer (PBL):

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

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0

fcp FFFA

(1)

(2)⇨

3.3 ITCZ formation Dynamical factor in the boundary layer (1)

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0

fcp FFF

1. ‘Ekman regime’‘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)

0

fp FFA (4)

2. ‘Advective regime’‘Advective regime’ in the equatorial zonein 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.

• Define the different regimes of the tropical PBL for atmospheric phenomenon longer than 5 days :


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°NDt

Equator

Ekman Regime

2°S

Mascareigns high pressure

EkmanRegime

pF

cF

fF

hV

•

5°N

Advective Regime

•pF

fF

A

hV

z

~ 1 km

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pF

cF

fF

hV

•A


Explanations of the physical porcesses of the previous figure :

1.1. Between 2°S et 5°N : ‘Advective regime’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.

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Explanations of the physical porcesses of the previous figure:

1.1. Between 2°S et 5°N : ‘Advective regime’Between 2°S et 5°N : ‘Advective regime’

2.2. Vers 5°N : the transition regime towards the ‘Ekman 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’).

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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).

Link between convergence, absolute vorticity and Link between convergence, absolute vorticity and vertical upward ascents (called Ekman pumping)vertical upward ascents (called Ekman pumping)

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gH f

Kw .2sin.

2 0wH: vertical velocity at the top of the Ekman layer = Ekman pumpingK: coeffecient of eddy viscosityα0 : angle of inflow between observed wind and geostrophic wind at the bottom of Ekman layerζg: 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.

• Equation of the vertical velocity at the top of the PBL, called Equation of the vertical velocity at the top of the PBL, called ‘ ‘Ekman pumping’ : Ekman pumping’ :

3.3 Formation de la ZCIT Dynamical factor in the boundary layer (6) :

The‘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

3. Thermodynamical factor : Thermodynamical factor :

Over océanOver océan ‣ Between Between 2°S and 2°N2°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°NAt 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 continentOver continent the maximum of tp’w is located in the summer ‣ hemisphere but the continental ITCZ doesn’t have

latitudinal preference as over oceansommaire chap.3



chap 3.4: moussons



• 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.

E

E

3.3 ITCZ formation role of the ocean-atmosphere coupling (1)

The origin of the equatorial upwelling is the Ekman divergence :

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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.


Link between upwelling and ‘cold’ tongues of SST:

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Source : Météo-France (F.Beucher)

Monthly mean of Sea surface température Source : RéAnalyse NCEP 1981-2002

• 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

Strong correlation between upwelling (mini of SST) and mini. of precipitations :


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Sources : Dorman et Bourke (79,81), Dorman (82), Baumgartnet et Reichel (75)

• 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


Convergence d’Ekman et zone de downwelling :Link between the Ekman convergence and the downwelling zone :

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Source : Météo-France(F.Beucher)

• 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

10°N


Strong correlation between maxi. of SST and maxi. of précipitation :

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Sources : Dorman et Bourke (79,81), Dorman (82), Baumgartnet et Reichel (75)

references

-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

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Chap. 3 Regional climates in tropics