Chap. 3 Regional climates in tropics

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
2
3.3 The InterTropical 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).

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3.3 The InterTropical Convergence Zone
Définition (2)
Source Météo-France
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3.3 The ITCZ Hypothesis 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 (gt
  28C).
• On continent ? ITCZ is collocated with the zone
  of tpw maxi in low
• troposphère ( proxy of warm and
  moist air)

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3.3 The ITCZ Hypothesis 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 (f0) but
off-equator (hundreds of km southward or
northward).
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3.3 ITCZ Analysis and forecasting

• The ITCZ is visible through monthly mean
  patterns
• precipitations, Outgoing Longwave Radiation
  (OLR), tpw 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 ?w at 850 hPa (gt 21C, over
  Atlantic and Pacific)
• ? Vertical velocitis maximum at 600-700
  hPa
• ? Divergence at high troposphère 200
  hPa

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3.3 ITCZ and OLR
Source Données NOAA
OLR lt 240 W/m2 over tropics (red) deep
convection
OLR lt 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 2N (Atl.)
  and 5N (E. Pacific)
• in july between 8N
  (Atl.) and 10N (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 10S
  (january) and
• 15-20N(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)

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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
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
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3.3 La ZCIT Formation Hypothesis 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 (f0),
  the airflow is
• divergent
• ? at a few degrees northward and southward the
  equator, as f
• increases fastly, the airflow is convergent.

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3.3 La ZCIT Formation Hypothesis 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 an acceleration of
  the trade winds and so divergence-subsidence.
• ? At around 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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3.3 La ZCIT Formation Hypothesis 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

1. Dynamical Factor in the boundary layer

3. Thermodynamical factor Over océan ?
Between 2S and 2N, 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 5N, 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 tpw is located in the
summer hemisphere but
the continental ITCZ doesnt have
latitudinal preference as over ocean

chap 3.4 moussons
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3.3 ITCZ formation Dynamical factor above the
boundary layer (1)

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

(1)
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3.3 ITCZ formation Dynamical factor above the
boundary layer (2)
(3)
cotanf
Remind on cotan f
South Pole f-?/2
North Pole f?/2
equator
f

• Following the equation (3), we deduce that
• When an air parcel moves equatorward (vgt0 dans
  HS, et vlt0 dans HN), the flow become divergent
  and descends down.
• On the contrary, when an air parcel moves
  poleward
• (vlt0 dans HS, et vgt0 dans HN), the flow become
  convergent and ascends up.

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3.3 ITCZ formation Dynamical 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
25N
High pressure
z
5N
5N
4 km
Dynamical valley effect of the Equator
divergence and subsidence
2 km
5S
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)
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3.3 La ZCIT Formation Hypothesis 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 an acceleration of
  the trade winds and so divergence-subsidence.
• ? At around 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
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3.3 ITCZ formation Dynamical 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)
?
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3.3 ITCZ formation Dynamical factor in the
boundary layer (2)

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

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3.3 ITCZ formation Dynamical 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
25N
Dt
Ekman Regime
z
1 km
5N
Equator
2S
Mascareigns high pressure
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3.3 ITCZ formation Dynamical factor in the
boundary layer (4)

• Explanations of the physical porcesses of the
  previous figure
• Between 2S et 5N 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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3.3 ITCZ formation Dynamical factor in the
boundary layer (5)

• Explanations of the physical porcesses of the
  previous figure
• Between 2S et 5N Advective regime
• Vers 5N the transition regime towards the
  Ekman regime
• The fast increase of the Coriolis force around
  5N 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 5N is located
in the transition zone between the Advective
regime (2S-5N) and the Ekman regime (nothward
5N) In other words, ITCZ is located at that
latitude (5N) where the period ? of atmospheric
weather systems, like easterly waves, equals the
period of the Coriolis parameter f 2?/ßy 6
days. For more informations, see Asnani book,
p.1060 and Fig. 11.4(27).
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3.3 Formation de la ZCIT Dynamical factor in the
boundary layer (6) TheEkman 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 a0 angle of inflow between observed
wind and geostrophic wind at the bottom of
Ekman layer ?g 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 (f0). ? 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 ZCIT Formation Hypothesis 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

1. Dynamical Factor in the boundary layer

3. Thermodynamical factor Over océan ?
Between 2S and 2N, 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 5N, 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 tpw is located in the
summer hemisphere but
the continental ITCZ doesnt have
latitudinal preference as over ocean

sommaire chap.3
chap 3.4 moussons
35
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 t in the
  northern (southern) hemisphere. The magnitude of
  E is proportional to the strenght of t.
• Following this rule, at the equator, E is
  directed away
• from the equator producing divergence and
  upwelling
• along the equator.

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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.

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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

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3.3 ITCZ formation role of the ocean-atmosphere
coupling (4)
Link between the Ekman convergence and the
downwelling zone
Convergence dEkman 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
  t.
• Since the southeasterlies decrease while they
  approach the ITCZ,
• the Ekman transport decrease too
• ? we observe a strong convergence of Ekman
  towards 4N
• ? producing downwelling and fast increasing of SST

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3.3 ITCZ formation role of the ocean-atmosphere
coupling (5)
Strong correlation between maxi. of SST and maxi.
of précipitation
10N
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 (gt28C)
• Under annual mean, the ITCZ ( ) is located
  between 5N-10N over Central Pacific Eastern
  Pacific - Atlantic

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chap 3.4 moussons
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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, 30N to
  30S. Mon. Wea. Rev., 107, 896-910
• - Dorman, C., E., Bourke, R.,R., H., 1981
  Precipitation over the Atlantic Ocean, 30N to
  30S. Mon. Wea. Rev., 109, 554-563