Chap. 6 Interactions between the tropics and midlatitudes

1
Chap. 6Interactions between the tropics and
midlatitudes
Intro Interactions between the tropics and
midlatitudes occurred the most
frequently during the winter season
6.1 Cold front or pseudo-cold front 6.2
Winter subtropical cyclone or winter
mid-tropospheric cyclone 6.3 Waves in the winter
easterlies 6.4 Shear lines 6.5 Cold surge
6.6 Dust storm 6.7 Influence of the
extratropics on tropical convection over
N.Pacific
general content
2
6.1 Westerly troughs Key features
30N
Move of the Cold front

W.
E.
Upper westerly trough
Move of the pseudo-cold front
equator
Schematic airflow about a retarded cold front in
the tropics. Broken stremalines for upper flow
solid lines for lower levels. Source daprès
Met Office College

• Synoptic Features
• In winter, the extension of the upper trough,
  which is a mid-latitude feature, into the tropics
  occurs when the cold front advances equatorwards
  and then becomes retarded in its southward
  (northen hemisphere) movement. Then, the cold
  front is often represented as a pseudo
  cold-front
• The upper through, however, continues to dig
  south and moves ahead of the retarted portion of
  the front which soon loses its identity as a
  surface feature

3
6.1 Westerly troughs Key features

• Weather associated
• ? In the southward flow ahead of the cold front,
  trades winds vanish and convection by diurnal
  evolution enhances day after day
• ? Pre-frontale convergence with moist and warm
  air ahead of the front (high instability) which
  may induce heavy rains (100mm)
• ? Strenghtening of northeasterlies behind the
  cold front with drop of temperature
• Frequency 3 to 5 times per month over
  Carribean,
• Mascareign, Polynesian islands
• Tracking the convection
• ? Z, theta on 1.5 PVU since vertical velocity
  
• are enhanced ahead of a stratospheric
  intrusion
• ? divergence at 200 hPa and convergence at
  925 hPa
• Further possible scenario
• If the upper through extends equatorward far
  enough to
• interact with the ITCZ, then marked
  intensification of convection

Sommaire chap.6
4
6.1 Westerly troughs Carribean Islands

Source images du satellite GOES E. Météo-France
Source Analyse Arpège Météo-France
Sommaire chap.6
5
6.1 Westerly troughs Reunion Islands
example1 of 11/04/2001
image satellite du 11/04/2001. Source
Météo-France

Satellite image of 11/04/2001

• Cold front affecting Mascareign Islands and
  taking place in a trough beetween two ridges of
  the Mascareign high
• This cold front has generated heavy rains at
  Reunion , from 60 mm to 300 mm over Volcano!

Sommaire chap.6
6
6.1 Westerly troughs Reunion Islands
example2 of 09/04/2003
Z sur la surface 2 PVU image vapeur deau du
09/04/03. Source Météo-France

Z on the 2 PVU Water Vapor image of 09/04/03
? intrusion of stratospheric air with
subsidence behind and ascending motion
enhanced ahead
Sommaire chap.6
7
6.1 Westerly troughs Reunion Islands
example2 of 09/04/2003
vitesse de vent seuillée à 40 kt sur la 2 PVU
(CEP) image vapeur deau du 09/04/03 du
satellite INSAT. Source Météo-France.

Speed Wind gt 40 kt on the 2 PVU (CEP) Water
Vapor image of 09/04/03
? Convection enhanced at the left entry (we are
in the Southern hemisphere ) of the STJ
Sommaire chap.6
8
6.1 Westerly troughs French Polynesian

Les dépressions d origine polaire le 31 mai
2000. Source Météo-France
Marquises
A
Tahiti
A
Rikitea
Tubuai
D
140W

• Cold front generally not active over Polynesian
  Islands, except for Gambier and Austral Islands,
  since cold front go generally polewards,
  eastwards of 140W
• Over Tahiti, and nortwards regions, the weather
  associated is only
• a line of Stratocumulus, but ahead of the cold
  front a convergence
• line very unstable may develop

Sommaire chap.6
9
6.1 Westerly troughs New Caledonia

• Cold front
• occur in winter ,
• from mid-may to
• mid-september
• Cold front
• may induce
• westerlies up to
• 60 km/h and
• heavy rains
• (100 mm/day)

Source Météo-France
Sommaire chap.6
10
Chap. 6Interactions between the tropics and
midlatitudes
Intro Interactions between the tropics and
midlatitudes occurred the most
frequently during the winter season
6.1 Cold front or pseudo-cold front 6.2
Winter subtropical cyclone or winter
mid-tropospheric cyclone 6.3 Waves in the winter
easterlies 6.4 Shear lines 6.5 Cold surge
6.6 Dust storm 6.7 Influence of the
extratropics on tropical convection over
N.Pacific
general content
11
6.2 Winter subtropical cyclone or winter
mid-tropospheric cyclone

Initiation In winter, as a rule, cold front
move northeastward out of tropics region before
any major deepening takes place. However, if
the upper through in the westerlies, located
behind the cold front, has a considerable
meridional extension, then a major
deepening can take place in low
latitudes. If high pressure build across the
trough to its north, a cut-off is created and
the vigorous system has a slow moving. It is
commonly referred to as a subtropical
cyclone.
To sum-up, the winter subtropical cyclone
originate from cut-off lows in the upper-level
subtropical westerlies but the heart of the
system is in mid-troposphere (400-600 hPa)
12
6.2 Winter subtropical cyclone or winter
mid-tropospheric cyclone
Divergence représentée par , convergence par
Source daprès Ramage, 1971

Main features Closed circulation between 700
and 300 hPa (mid-tropospheric cyclone). Light
signature at surface and 200 hPa at its early
stage. Cold core (but subtropical cyclone is
not marked as a front cold) Occurrence
(northern hemisphere) from november to
january Location ? in the Atlantic
generally found btw 15N/35N and 30W/60W
(SW of the Azores) but may exist everywhere, only
1 or 2 per year ? more common in Pacific
btw 15 and 35N and 175E and 140W (next
to Hawaii), called as Kona cyclone
13
6.2 Winter subtropical cyclone or winter
mid-tropospheric cyclone
- convergence divergence

Source daprès Ramage, 1971
Main features Maximum of wind, between 400
and 600 hPa, on the eastern side (about 500 km
from the center) of the cyclone Max. of
convergence 600 to 500 hPa Divergence at 300
hPa and at 800 hPa (trade winds inversion not
vanished !) Maximum of rain on the eastern
side (about 500 km from the center) of the
cyclone. Sky clear at the center. In surface,
light wind and slack pressure gradient but it may
exist a great variability with the time
14
6.2 Winter subtropical cyclone or winter
mid-tropospheric cyclone
tropopause

Source daprès Ramage, 1971
Life span several days to several weeks !
High PV (i.e. dry air) and low tropopause at
center of cyclone A great variability of
structure with the time Usually, subtropical
cylone remain cold core, but sometimes, the
release of latent heat is filling litlle by
little the cold pool and subtropical cyclone
become a warm core and it can develop an eye,
as a hurricane. Should the occasion arise, the
MSPL fall and the surface wind can become violent
!!
Sommaire chap.6
15
6.2 Winter subtropical cyclone Carribean
Islands example of 23/05/04
23/05/04 at 00UTC

Source Météo-France
Z VVgt0 on 1.5 PVU Wind gt 40 kt on 1.5
PVU Water vapor
16
6.2 Winter subtropical cyclone Carribean
Islands example of 23/05/04

Source Météo-France
23/05/04 at 18UTC
Cutoff
VVgt0 at Righ entry of STJ and upstream the cutoff
Cutoff
Z VVgt0 on 1.5 PVU Wind gt 40 kt on 1.5 PVU
Water vapor
17
6.2 Winter subtropical cyclone Carribean
Islands example of 23/05/04

Source Météo-France
23/05/04 at 18UTC
Cutoff
VV gt0 Right entry of STJ and upstream the cutoff
Cutoff
Z VVgt0 on 1.5 PVU Wind gt 40 kt
18
6.2 Winter subtropical cyclone Carribean
Islands example of 23/05/04

Source Météo-France
24/05/04 at 00 UTC
30N
25N
1008 hPa
20N
Location of the convection in subtropical
cyclone
right entry of STJ
upstream the cutoff
Pmer IR image
Sommaire chap.6
19
Chap. 6Interactions between the tropics and
midlatitudes
Intro Interactions between the tropics and
midlatitudes occurred the most
frequently during the winter season
6.1 Cold front or pseudo-cold front 6.2
Winter subtropical cyclone or winter
mid-tropospheric cyclone 6.3 Waves in the winter
easterlies 6.4 Shear lines 6.5 Cold surge
6.6 Dust storm 6.7 Influence of the
extratropics on tropical convection over
N.Pacific
general content
20
6.3 Waves in the winter easterlies
North Pole
30N
Upper westerly trough

W.

E.
trough in easterlies
equator
equator
Schematic airflow about a retarded cold front in
the tropics. Broken stremalines for upper flow
solid lines for lower levels Source Daprès Met
Office College

• Occasionally a trough in the upper westerlies
  forms and induces
• a surface trough in the trade wind easterlies,
  which moves
• eastward against the surface flow
• The trough of the easterly waves induce
  relative vorticity and so
• it may enhance convection

21
6.3 Waves in the winter easterlies
illustration in Guyane
D
A
Westerlies trough


A
Trough in easterlies
Streamline 700 Hpa
Source Météo-France
22
6.3 Waves in the winter easterlies
illustration in Guyane
D
A
Westerlies trough


A
Trough in easterlies
Source Météo-France
Infra-Red Image
Heavy rains 200 to 250 mm in 12h. on Guyana
coast !!!
Sommaire chap.6
23
Chap. 6Interactions between the tropics and
midlatitudes
Intro Interactions between the tropics and
midlatitudes occurred the most
frequently during the winter season
6.1 Cold front or pseudo-cold front 6.2
Winter subtropical cyclone or winter
mid-tropospheric cyclone 6.3 Waves in the winter
easterlies 6.4 Shear lines 6.5 Cold surge
6.6 Dust storm 6.7 Influence of the
extratropics on tropical convection over
N.Pacific
general content
24
6.4 Shear lines the origins

25N
20N
Source Atkinson, 71 daprès Palmer et al.,55
Origins of the shear line When a cold front
move into the tropics, the cold air behind it is
much modified by passage over the warm water. In
addition, warming by sudsidence takes place in
the cold air aloft and so the front ceases to
exist as a density discontinuity at about 20
of latitude, surface air temperature and dew
point difference across the front are small or
zero.
25
6.4 Shear lines the origins
a
b

c
d
Source daprès Met Office College
Origins of the shear line (following
) Conditions favouring the transformation of a
cold front into a shear line rather than for
simple frontolysis to take place, occur when the
cold front is followed by an intensifying high
which moves or builds equatorwards behind the
front as represented at above figure
26
6.4 Shear lines definition

25N
20N
Source Atkinson, 71 daprès Palmer et al.,55

• Definition
• However, instead of decaying as a synoptic
  feature, the
• front may retain its identity weather wise, by
  virtue of cyclonic shear induced by the
  continuance os strong winds in what was the cold
  air.
• A shear line is defined as a line or narrow zone
  where
• there is an abrupt vector change in horizontal
  wind component parallel to the line. In other
  words, shear lines
• are defined as a line of maximal horizontal shear.

27
6.4 Shear lines main features

Main features

• Lifespan may persist several days, sometimes
  even longer
• Period and location all tropical regions in
  winter months
• move generally, slowly equatorward and
  penetrating deep into
• the tropics. Occasionally, the
  equatorwards portion may
• merge with the ITCZ and if the ITCZ
  is in the other
• hemisphere, the shear line may even
  cross the equator.
• Weather associated
• narrow band of convective clouds of varying
  development.
• Cloud tops are usually not excessively high
  (10000 to 15000 ft)
• although isolated cumulonimbus may exist.
• The weather starts to deteriorates in the
  lights winds just ahead
• the shear line, while along its axis there may
  be low cloud and
• poor visibility associated with the showers

28
6.4 Shear lines tracking

Streamline in low tropo (unbroken line) isotach
in kt (dashed line) Source
Atkinson, 1971, daprès Palmer et al., 1955

• Tracking ? through streamline, speed wind,
  vorticity in low
• troposphere or/and in surface
• ? satelitte cloud imagery makes
  identification and
• tracking easy
• Foreraster be careful ! The activity can wax
  and wane and so
• there should be no hurry to drop a shear line
  from the analysis on
• the strenght of one image without strong
  supporting evidence
• that the high is weakening or mowing away.

Sommaire chap.6
29
Chap. 6Interactions between the tropics and
midlatitudes
Intro Interactions between the tropics and
midlatitudes occurred the most
frequently during the winter season
6.1 Cold front or pseudo-cold front 6.2
Winter subtropical cyclone or winter
mid-tropospheric cyclone 6.3 Waves in the winter
easterlies 6.4 Shear lines 6.5 Cold surge
6.6 Dust storm 6.7 Influence of the
extratropics on tropical convection over
N.Pacific
general content
30
6.5 Cold surges Definition

Schematic diagram of a left) synoptic gravity
wave right) cold front. Source Li et Ding, 1989
and from Monsoons over China, chap.2, p.165.

• Definition
• During winter seasons, disturbances of
  midlatitude origin
• may penetrate deep into the tropics.
  Characteristics of cold surges
• is the build-up of a cold air reservoir in the
  midlatitudes, the
• establishement of a steep meridional pressure
  gradient in low
• troposphere, and a subsequent abrupt cold-air
  outbreak
• (southward of 40N over East Asia).

31
6.5 Cold surges The winter Asian cold surge

Schematic diagram of a left) synoptic gravity
wave right) cold front. Source Li et Ding, 1989
and from Monsoons over China, chap.2, p.165.

• Stage 1 Initiation of a cold surge over the
  East Asia
• ? Simultaneous cyclogenesis on the Eastern Sea
  China
• (called extratropical cyclone) and
  anticyclogenesis over Siberia
• and North China.
• ? Following an increase of pressure force and
  the cold
• airstream in low troposphere burst out of the
  anticyclone initiating
• a cold surge
• ? In addition, it must be outlined thr role
  played by the
• Himalaya which provide a suitable setting for the
  generation of a
• vast pool of cold air and its outpout eastward
  of 90E.

32
6.5 Cold surges The winter Asian cold surge

Schematic diagram of a left) synoptic gravity
wave right) cold front. Source Li et Ding, 1989
and from Monsoons over China, chap.2, p.165.
Stage 2 Southward propagation of the cold
surge divided itself into 2
distinct phases 1) the fisrt phase (left
figure) is a southward propagation of a synoptic
gravity wave coupled with an increase of
pressure, with a speed velocity of 40 m/s. This
is not associated with a change in the synoptic
weather. 2) the second phase (right figure) is
caracterised by a drop of the dew point which is
in phase with the passage of the surface cold
front (speed velocity of about 10 m/s). Heaviest
rainfall over Malaysia-Indonesia area.
33
6.5 Cold surges The winter Asian cold surge

Streamline (solid line) and isotherm (dashed
line) at 900 hPa on the
a) 10 dec. b) 11
dec. C) 12dec.
Source Johnson et
Zimmerman, 1986

• The increase of northeasterlies
• may be associated with both phases
• (gravity wave and cold front ) with
• about
• ? 20/30 kt over the North China Sea
• ? 30/40 kt over the South China Sea
• The maximum of wind speed is
• observed at 900/950 hPa, just
• above the boundary layer

Sommaire chap.6
34
Chap. 6Interactions between the tropics and
midlatitudes
Intro Interactions between the tropics and
midlatitudes occurred the most
frequently during the winter season
6.1 Cold front or pseudo-cold front 6.2
Winter subtropical cyclone or winter
mid-tropospheric cyclone 6.3 Waves in the winter
easterlies 6.4 Shear lines 6.5 Cold surge
6.6 Dust storm 6.7 Influence of the
extratropics on tropical convection over
N.Pacific
general content
35
6.6 Dust storm in sudan the 27/12/2004

The airborne dust has a cooling effect on the
observed temperature (30c cooler than
surrounding desert )
Egypt
Sudan
Nile River
Source images from MODIS (spectroradiometer of
1 km per pixel) left image natural color
right image temperature
36
6.6 Dust storm

Definition Strong and sustained winds, capable
of raising large amounts of dry opaque particles,
causing a serious reduction in visibility (lt 1000
m) ? in Sudan Area (Sudan and Lybia), they
are called Haboobs ? in Middle-East area
(Iraq, Saudi Arabia), they are called
Shamal
37
6.6 Dust storm
3 kinds of synoptic situations in Sudan Area

1) instability type associated with
thunderstorm activity, for example ahead a squall
line. ? between May and August
? short-lived and localized, with a
width of 10-80km 2) pressure gradient type
related with to the steepening of the large-scale
meridional pressure gradient ? between May and
August during the advancing
summer monsoon or early monsoon period. ?
widespread and blow for long periods 3)
pressure gradient type, associated with cold
front and strong southward pressure gradient ?
between February and May with the passage of
Mediterranean depression ?
widespread and blow for long periods (24-36 h)
and 2-3 times a month Statistics over
Central Sudan 20 dust storms per year,
more than 70 of which are of
types 1) and 2)
between May and August Over middle-East Area
Dust Storms occur only with the third above
synoptic situation depicted

38
6.6 Dust storm over the Red Sea the 30/06/2003
Thick plumes of tan-colored dust are blowing
across the Red-sea

This dust storm is related to strong post-frontal
north-westerly winds and has persisted 24-36 h
The dust storms also severely affect the Darfur
region in Sudan and MSG, with its capacity to
observe dust storm in the whole area at a
frequency of 15 mn, plays an essential role in
helping United nations to coordinate their aid.
Source images from MODIS natural color
39
Chap. 6Interactions between the tropics and
midlatitudes
Intro Interactions between the tropics and
midlatitudes occurred the most
frequently during the winter season
6.1 Cold front or pseudo-cold front 6.2
Winter subtropical cyclone or winter
mid-tropospheric cyclone 6.3 Waves in the winter
easterlies 6.4 Shear lines 6.5 Cold surge
6.6 Dust storm of the Sudan 6.7 Influence of
the extratropics on tropical convection
over N.Pacific
general content
40
6.7 Influence of the extratropics on
tropical convection (over North Pacific in
winter)

Schematic showing the sequence of events
describing tropical- extratropical interaction
processes over the Pacific Ocean as simulated by
an AGCM . Source Slingo, 98.
1) Cold surge in low troposphere at 110E
initiated by the increasing Of the Siberian high
associated with the passage of deepening mid-latit
ude weather system 2) This cold surge enhances
convection over the maritime continent
intensifying the local Hadley circulation 3)
which in turns interacts with the extratropics by
accelerating the Subtropical Westerly Jet
over Eastern Asia
Sommaire chap.6
41
6.7 Influence of the extratropics on
tropical convection (over North Pacific in
winter)

Source Slingo, 98.
Reminder ? Upper easterlies generally observed
between 15N/15S vanish Rossby wave
propagation ? On the contrary, Upper
westerlies observed at 30 of
latitude and over Equatorial East Pacific are
considered as a Rossby wave duct or waveguide
4) The subsequent eastwards extension of the
Subtropical Westerly Jet considered as a
Rossby canal duct lead to an amplification of a
Rossby wavetrain 5) The westerly canal duct
observed over Equatorial East Pacific allow the
intrusion of an upper tropospheric through in
deep tropics over the East Pacific some 3-4 days
later resulting in a flaring of convection
Sommaire chap.6
42
6.7 Influence of the extratropics on
tropical convection (over North Pacific in
winter)

Source Slingo, 98.
Schematic showing the sequence of events
describing tropical- extratropical interaction
processes over the Pacific Ocean as simulated by
an AGCM (Slingo, 98)
6) The final link in the chain is the excitation
of equatorial easterly waves by the enhanced
convection in the East Pacific. These easterly
waves, propagating across the Pacific with a
speed phase of 7m/s, may contribute to convective
activity in the West Pacific and can, if the
phasing is correct, subsequently interact with
later cold surge event. Meehl et al. (96)
describe such an interaction btw easterly wave
and cold surge can contribute to extreme
weather event in that region, such as
tropical cyclone and associated westerly wind
burst
Sommaire chap.6
43
References (1)

• Atkinson, G. D. 1971 Forecaster guide to
  tropical meteorology. Rapport technique 240,U.S.
  Air Weather Service.
• Chang, C. P. , Millard, J. E.,Chen, G.T. J.,
  1983 Gravitationnal character of cold surges
  during winter MONEX. Mon. Wea. Rev., Vol.111,
  p.293-307
• Ding Yihui, 1994 Monsoons over China. Kluwer
  Academic Publishers, 419 p.
• Johnson, R. H. and J. R. Zimmerman, 1986
  Modification of the boundary layer over the South
  China Sea during a winter MONEX cold surge event.
  Mon. Wea. Rev., Vol.114, p.2004-2005
• Lau, K.-M., and H. Lim, 1984 On the dynamics
  of equatorial forcing of climate
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• J. of the Atm. Sci., Vol.41, p.161-176.
• Li, C. and Ding, Y. H., 1989 A diagnostic
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• - Malardel S. , 2005 Fondamentaux de
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  Editions,
• 708 p.
• - Matthews, A. J. and G. N. Kiladis, 2000 A
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• Meteorological Office College, FitzRoy Road,
  Exeter, Devon Courses Note Introduction to
  tropical Meteorology

44
References (2)

• - Ramage, C. S., 1971 Monsoon Meteorology.
  Academic Press, New York and London, 296 p.
• Ropelewski C. F. et Halpert M. S., 1987
  Global and Regional scale précipitations and
  temperature patterns associated with El
  Nino/Southern Oscillation. Mon. Wea. Rev., Vol.
  115, p. 1606-1626
• Ropelewski C. F. et Halpert M. S., 1989
  Précipitations patterns associated with the high
  index of the Southern Oscillation. J. Clim,
  Vol.2, p.268-284
• Slingo, J. M., 1998 Extratropical forcing of
  tropical convection in a northern winter
  simulation with the UGAMP GCM. Quarterly Journal
  of the Royal Met. Soc., Vol.124, p.27-51