Chap. 5
Conceptual models of synoptic
Tropical disturbances in summer
Definition of tropical disturbances :
• A discrete tropical weather system of apparently organized
convection generally 200 to 600 km in diameter, originating
in the tropics or subtropics, having a nonfrontal migratory
character, and maintening its identity for 24 hours or more.
• It may or may not be associated with a detectable
perturbation of the wind field.
• Disturbances associated with perturbations in the wind
field and progressing through the tropics from east to
west are also known as ‘easterly waves’.
Chap. 5
Conceptual models of synoptic
Tropical disturbances in summer
An example of evolution of tropical
disturbances over Atlantic in summer:
Chap. 5
Conceptual models of synoptic
Tropical disturbances in summer
5.1 Easterly waves
(West Africa, Caribbean Sea, Pacific)
5.2 Troughs of Upper Tropical Troposphere
(TUTT)
5.3 Monsoon Depression
5.4 Mid-tropospheric cyclones
5.5 Tropical storms
5.6 Hurricanes
sommaire général
5.1 African Easterly waves
• Located at the southern flank (12°N) of AEJ (15°N) from June
to October
• Initiate by barotropic instability (inflexion point of PV on the
isentrope surface 315°K)
• Growth by release of latent heat
• Speed phase = -8 m/s
• Period = 3 to 5 days
• Scale lenght = about 2500 km
• Bad weather in general on the western side of the trough
5.1 African Easterly waves
15°N
10°N
5°N
Streamline and wind at 700 hPa : we observe 2 easterly waves at
the southern flank of AEJ
Troughs of easterly waves are visibles through the sharp
shift of the wind
5.1 African Easterly waves
On NIMBUS software, look after the streamline at 850 hPa
(closed circulation) or at 700 hPa (sinusoide circulation)
but easterly waves are not visible in upper troposphere.
5.1 African Easterly waves
T= Trough R= Ridge
Composite structure : rain, clouds on the western side of the trough
: divergence at 200 hPa and convergence at
850 hPa on the western side of the trough
5.1 Carribean Easterly waves
• Origin : African easterlies wave which cross Atlantic
• Occurrence/ frequency over Carribean : from may to november
once every 3 to 4 days, either about 60 over the season
• Speed phase = -5 to -8 m/s
• Scale lenght = about 3000 km
• Bad weather, generally, on the eastern side of the trough
5.1 Carribean Easterly waves
• Maximum signal of the wave = between 500 and 700 hPa
• Vertical tilt of the trough : below 700 hPa, westward tilt with altitude
(not shown on the above figure)
above 700 hPa, eastward tilt with altitude
5.1 Pacific Easterly waves
• Eastern Pacific (80°E to 170°E) :
- scale lenght = 3000 to 3500 km
- period = 4 to 6 days
- phase speed = -5 to -7 m/s
- in surface, we observe a fluctuation of wind as large as 4 m/s
- in upper troposphere, lighter fluctuation of wind
• Western Pacific (170°E to 120°E) :
- scale lenght = 3500 to 4000 km
- period = 6 days
- phase speed = -9 m/s
- in low troposphere (800 hPa), first peak of fluctuation
of wind as large as 3 to 4 m/s and strong convergence
- in upper troposphere (175 hPa), second peak of fluctuation
of wind as large as 2 to 3 m/s and strong divergence
- in mid- troposphere (500 to 300 hPa) : no fluctuation of wind
• The vertical tilt of the trough:
It varies from eastern Pacific (eastward vertical tilt)
to western Pacific (westward vertical tilt); it probably depends
of the environment, either the vertical shear wind which is
the opposite between Eastern and Western Pacific
Chap 5.2
Chap. 5
Differents types of
tropical disturbances in summer
5.1 Easterly waves
(West Africa, Caribbean Sea, Pacific)
5.2 Troughs of Upper Tropical Troposphere
(TUTT)
5.3 Monsoon Depression
5.4 Mid-tropospheric cyclones
5.5 Tropical storms
5.6 Hurricanes
sommaire général
5.2 Trough of Upper Tropical Troposphere (TUTT)
200 hPa streamline in august :
• In summer, over N. Atlantic, an upper through quasi-zonal, lying
WSW/ENE, is visible on monthly maps (not necessarily daily !)
• Hypothesis of initiation
This upper trough is a typical example of the interactions between
tropics and mid-latitudes :
‣ we observe low ratio of water vapour along the TUTT linked to
(?) the successive Rossby Wave Breakings (RWB) which occur in
summer over mid-oceans (called ‘surf zone’). RWB result in
stratospheric mid-latitude intrusion (strong PV and light
humidity) into the tropical troposphere
‣ along the axis of TUTT, as the atmosphere is very dry, we observe
a very strong radiative cooling (-6°K/day) which implies a strong
synoptic-scale subsidence (- 2 cm/s) along the axis (cf. thermo
equation in tropics)
5.2 TUTT ‘cells’ or TUTT ‘lows’
Definition of a TUTT ‘cell’:
Cold vortex observed in upper troposphere along the climatological
TUTT disturbing the tropical atmosphere, sometimes, down to the
surface (should the occasion arise, a subtropical disturbance occur
and rarely tropical storm).
hPa
Anomaly of VV,
in microbars/s
• Vertical velocity :
in vortex, subsidence
max. at 300 hPa
whence sky clear (or
isolated cumulus) and
radiational cooling
• Cold core max.
at 300 hPa,
• Cyclonic vorticity
max. at 200 hPa
• Deep convection :
located upstream the
vortex in the warm and
divergent upper tropo,
either at SE of the vortex
in northen hemisphere
(resp. NE in southern
hemisphere)
5.2 TUTT ‘cells’ or TUTT ‘lows’
Other features of a TUTT ‘cell’:
• Move
: Stationnary or westward move (in both
hemisphere) from 0 to 6° of longitude per day
• Life span : 2 to 31 days !!
• MSLP : the fall of MSLP depends on the interaction between
the upper tropo (intrusion of high PV) and the
low tropo (line of convergence coupled with high tp’w ..)
• Future study :
a TUTT event seems to be linked with an intrusion of
stratospheric air into tropical troposphere (linked with the
Rossby wave Breaking ?)
So, high values of PV (>2 PVU) on 350°K should be a good
proxy to detect ‘TUTT cell’ ?
5.2 TUTT ‘cells’ or TUTT ‘lows’
Relation between ‘TUTT cell’ and tropical cyclone: *
⇨ The ‘TUTT cell’ vanish the tropical cyclone when
the vertical shear (induced by the TUTT ‘cell’) is
greater than 12m/s between the surface and 200 hPa .
⇨ Under this threshold vertical shear, a tropical
cyclone could be enhanced when it is located on the
eastern side on the TUTT ‘cell’, that is to say in the
Upper warm and ascent outflow
Chap 5.3
Chap. 5
Differents types of
tropical disturbances in summer
5.1 Easterly waves
(West Africa, Caribbean Sea, Pacific)
5.2 Troughs of Upper Tropical Troposphere
(TUTT)
5.3 Monsoon Depression
5.4 Mid-tropospheric cyclones
5.5 Tropical storms
5.6 Hurricanes
sommaire général
5.3 Indian monsoon : monsoon dépression
200 hPa : Streamline and isotach
‣ upper tropo :
H
light anticyclonic
circulation and light
signal of divergence
500 hPa: Streamline and isotach
‣ mid-tropo :
moderate signal
of convergence
C
Shaded area=wind > 40 kt
850 hPa: Streamline and isotach
‣ low tropo :
Maximum signal of
wind and convergence
from 600 to 800 hPa
C
5.3 Indian monsoon :
Monsoon depression
Main Key features :
• Synoptic-scale : about 2000 km of diameter
‣ 80 % in tha Bay of Bengale,
‣ 10% in Arabian Sea
‣ 10 % over land (Bangladesh)
• MSLP ~ as low as 990 hPa
• Location of initiation
• 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, between 500-200 hPa, hot core
‣ 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)
5.3 Indian monsoon :
Monsoon depression
Location of rain and vertical velocity :
• Deep convection and vertical velocity max. in the SW quadrant
• Rain : in the SW quadrant between 100 mm to 300 mm per day
Chap. 5
Differents types of
tropical disturbances in summer
5.1 Easterly waves
(West Africa, Caribbean Sea, Pacific)
5.2 Troughs of Upper Tropical Troposphere
(TUTT)
5.3 Monsoon Depression
5.4 Mid-tropospheric cyclones
5.5 Tropical storms
5.6 Hurricanes
sommaire général
5.4 Indian monsoon
Mid-tropospheric cyclone
925 hPa
600 hPa
Streamline and isotach at (left) 925 hPa, (right) 600 hPa
• 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
5.4 Indian monsoon
Mid-tropospheric cyclone
Main Key features :
• 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. 5
Differents types of
tropical disturbances in summer
5.1 Easterly waves
(West Africa, Caribbean Sea, Pacific)
5.2 Troughs of Upper Tropical Troposphere
(TUTT)
5.3 Monsoon Depression
5.4 Mid-tropospheric cyclones
5.5 Tropical storms
5.6 Hurricanes
sommaire général
Chap. 5
Differents types of
tropical disturbances in summer
5.1 Easterly waves
(West Africa, Caribbean Sea, Pacific)
5.2 Troughs of Upper Tropical Troposphere
(TUTT)
5.3 Monsoon Depression
5.4 Mid-tropospheric cyclones
5.5 Tropical storms
5.6 Hurricanes
sommaire général
5.2 Rossby Wave Breaking (RWB)
Event of RWB from 27/05 06TU to 29/05/98 18TU
Source : ERA15 CEP : PVU on the surface 350°K
high PV
RWB
high PV
RWB
high PV
high PV
Cutt-off of
high PV
Cutt-off of
high PV
high PV
high PV
5.2
Rossby Wave Breaking
Event of RWB : zoom on 27/05/98 18TU
Source : ERA15 CEP : PVU on the surface 350°K
Cut-off of
high PV :
Stratospheric air
Cut-off of
low PV :
tropospheric air
• Deep convection occur upstream the RWB, northeastward
the ‘TUTT cell’, in the upper warm and divergent flow
5.2 Example of a RWB over the Southern Atlantic
04/02/04 : Geopotential and wind on the surface 2 PVU :
• Deep convection occur upstream the RWB, northeastward
the ‘TUTT cell’, in the upper warm and divergent flow