Synoptic Systems and Weather

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Synoptic Systems and Weather

• H. Annamalai
• IPRC, Univeristy of Hawaii

Contributions from Ding and Sikka, Chapter 4,
Monsoon Book (Bin Wang)
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Synoptic Systems
and Weather

• Quasi-stationary Monsoon Trough
• Monsoon lows and depressions
• Tropical cyclones
• Floods and droughts
• Mid-tropospheric cyclones
• Cold waves and cold surges
• Local severe storms and
• mesoscale convective complex events

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Quasi-stationary monsoon trough

• Observed features (Blanford 1884)
• Basic dynamics
• rainfall maximum- south of the troughs
• normal position (participants)
• Impact on monsoon lows, depressions
• (Elliot 1886)

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Normal position of the monsoon trough
analogous to the equatorial trough seasonal
displacement of the TCZ
west-east moisture gradient
shear vorticity Ekman pumping
Heat low dry weather Pakistan
Moist low
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favorable for successive genesis of
lows/depressions
Active Phase
Break Phase
foot hills of the Himalayas lead to floods in
the rivers
in both cases, some regions over India face
flood situations
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trough south of the normal position
trough foot hills of the Himalayas
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Participants.

• The maximum rainfall observed lies to the
  south of the mean axis of the trough. Why?
• MONTBLEX Monsoon Trough Boundary Layer
  Experiment

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Monsoon lows and depressions

• Role of the basic flow
• Genesis
• Westward movement
• Rainfall distribution (southwest quadrant)
• Vertical structure (participants)
• Decadal frequency
• Response to global warming
• Monsoon onset vortex (participants)

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30 20S 10S EQ 10N 20N
30N 40N
PV gradient Precipitation Climatology
Charney and Stern (1962) Dickinson and
Molinari (2000)
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Mean Monsoon and Synoptic Systems
Role of the basic flow
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Number of lows and depressions
Monsoon lows and depressions Number of
low-pressure systems (lows, depressions, and
cyclonic storms) which formed over 4º lat/long
blocks over the south Asian region in the summer
monsoon season during 1888-1983
Main features Maximum numbers of low-pressure
systems form over the northern Bay of
Bengal. Climatologically, 7 depressions and 1.5
cyclonic storms form over the Bangladesh-India-Pak
istan region. Mean 13 Standard deviation
2.2
most preferred genesis region
mid-tropospheric cyclones
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Rainfall variance
Hartmann and Nichelson, 1989
850hPa vorticity Variance Lau and Lau (1990)
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Tracks of Synoptic Systems (1993 Boreal summer)
850hPa Vorticity
Strong Indian and West Pacific monsoon
Tracks of Synoptic Systems (1994 - Boreal summer)
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closey packed Isobars
westward progression of monsoon depression
westward progression of a monsoon depression
vorticity budget divergence term - Maximum
rainfall SW quadrant - Rossby waves east-west
divergent circulation
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Rainfall associated with an intense depression
in August 2006.
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rivers lie along the path of the monsoon lows
and depressions
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1998 1999 2000


1999 2000 2001 2002

2003 2004
2005 2006



Figure 4 Spatial distribution of observed
seasonal (JJAS) rainfall anomalies over the
Indian sub-continent during 1998-2006.
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Growth/decay rate
Lysis density
Growth/decay rate
Feature density
Genesis density
Annamalai et al. (1999, Mon. Wea. Rev.,)
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Decadal variations - Synoptic systems
Monsoon lows and depressions
The decadal frequency of cyclonic disturbances
(low and depressions) and cyclonic storms over
the north Indian Ocean during the period 1899-1999
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Number of monsoon depressions over BOB
Stowasser and Annamalai (2007)
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Feature density of monsoon depressions
MRI
MRI
ERA
ERA
MPI
GFDL
MRI
MPI
GFDL_CM2.1
In models, depressions are concentrated over the
ocean The mean monsoon trough is located
southwards in the models
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Response to Global Warming
Stowasser and Annamalai (2007)
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Participants

• Vertical structure of depressions?
• Onset vortex in the Arabian Sea
• Bay Bengal Monsoon Experiment (BOBMEX) SST
  variations in the north Bay of Bengal CAPE -
  Depressions genesis

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onset vortex intensified into a severe cyclone
in June of 2007
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Synoptic systems East Asia

• Cyclones/typhoons
• Genesis
• Land fall
• Decadal frequency
• Global warming (participants)
• Vortices off Tibet

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warmest SST
Eastward extension of the monsoon trough
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Synoptic systems East Asian Monsoon
The ITCZ monsoon trough
Mean 850-hPa streamline fields for July (a),
August (b), September (c), and October (d),
averaged for 1979-1999. Shaded area is with weak
vertical wind shear, with V200 - V850 gt 7.5 m
s-1.
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Synoptic systems Active / break phases
The east Asian monsoon region

• The ITCZ monsoon trough
• With many typhoons
• 2-3 times more than (b)
• With an active ITCZ near 20ºN, extending
  eastward to 160ºE
• With few typhoons
• With an inactive ITCZ near 10ºN, ending at about
  30ºE

Mean streamline at 850 hPa in summer
(July-September)
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Tropical cyclones Landfall
Very significant impacts Socioeconomic aspects
and human lives in China, Japan,
Korean Peninsular, Vietnam, and Philippines, etc.
Statistics in China Annual Frequency 7
Interannual Variation Max 12
(1971) Min 3 (1998) Decadal Variability
Max 7.6 (1980s) Min 6.1
(1950s) IDV of persistent duration
Min 4 (1950s) Max 6
(1960s, 1970s, 1980s) Spatial Dist. (442 in
recent 50 yrs) 95 in coastal south-east China
Max 33 in Guangdong Sec 19 in
Taiwan Damage
Increases during mid-1980smid-1990s alleviates
afterwards.
Track frequency of tropical cyclones in the
north-west Pacific Track frequency Tropical
cyclones occur and move over a very broad
area 91 between 5ºN and 22.5ºN over the
western North Pacific Most active place is in
(110º-145ºE, 10º-30ºN) Max to the east of
Philippines and the south-west of Guam
Statistics in Japan Mainly the western
part 30 (1949-2000)
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Tropical cyclones Genesis - ENSO
Major original regions To the east of the
Philippines and to the south-west of Guam
Central part of the Caroline Islands Central
part of the SCS
Intensification into typhoons 5º-10º lon
east of the Philippines and SCS Characteristics
of cyclogenesis 80 originate in or just
poleward of ITCZ or Monsoon trough Related to
large-scale north-westward wave Vary
seasonally with annual migration of ITCZ
Enhance of flow on either side of monsoon trough
increases low-level relative vorticity makes
favorable environmental condition Clustering
phenomenon (3-5 in 1-3 weeks) in an active or
extended ITCZ over WNP with ITCZ holding its
easternmost position Impacts of ENSO
Formation location, frequency and movements
Strongly depends on the intensity of ENSO Strong
evens Significant Moderate Not El Nino
yrs La Nina yrs
SE Enhanced (31) Suppressed (2)
NW Suppressed (7) Enhanced (28)
Life span Longer (7 day) Shorter
Tracks (lt15ºN-gt35ºN) 2.5 times relation
SE
NW
Locations of initial tropical storm formation
and SST anomalies in peak season (Jul-Sep) during
the (a) six strongest El Nino and (b) six
strongest La Nina years.
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Tropical cyclones Western North Pacific
The western North Pacific basin Most frequently
occur from a global point of view 33 of total
global 80 tropical cyclones (with mean wind
maximum gt 17.3 m s-1) each year Typhoon season
is during Jun (1.76)Nov (2.58), with peaks in
Aug (5.82), Sep (2.58) and Oct (3.98), 69 of
total annual number. The Indian Ocean Mainly
occur during premonsoon and postmonsoon seasons,
with monsoon depressions dominating during the
monsoon season.
Statistics in WNP Annual Frequency 28.3
(1951-1979) 26.7 (1971-2000) 27.6 (1951-2000)
Interannual Variation Maximum 40
(1967) Minimum 14 (1998) Decadal
Variation 1950s 26 (below normal) 1960s 31.1
(above) 1970s 27.8 (nearly) 1980s 27.5
(nearly) 1990s 25.8 (much below an apparent
decease especially 22.2 during 1995-2001)
Interdecadal Variability 29.03
(1961-1990) 27.03 (1971-2000)
Annual number of tropical cyclones in the western
north Pacific basin
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Floods and droughts South Asia
The south Asian monsoon region Great interannual
variability Severe droughts 1877, 1899, 1902,
1905, 1918, 1920, 1965, 1972, 1979, 1987, 2002
of which phenomenal droughts 1877, 1899, 1918.
Floods 1882, 1917, 1961 Factors Independent
short-medium, low-frequency modes (boundary
forcing or dynamical instability) Floods Caused
by very high rainfall spells associated with
monsoon disturbances including Tropical
cyclones, depressions, low-pressure areas, MTCs,
and western disturbances Individual
flood events occur in all the major rivers of the
subcontinent Designated by excessively
wet monsoon seasons over India (excess rainfall
gt10 of normal)
Droughts Natural disaster known to India
from ancient Simultaneous over vast parts of
south Asia especially in NW India and W-Central
India Opposite behavior NE India and
Bangladesh Categories Mild (1.0-1.24 SD)
Moderate (1.25-1.49) Severe (1.5-1.99)
Phenomenal (gt2.0 SD) Relationship with El
Nino warming low SOI Associated except 1920
and 1979 Coincidence warm phase monsoon
onset Complex, important aspect of monsoon
IAV El Nino warming -gt No floods
Departure of monsoon rainfall from norm (Jun-Sep)
over India
Long-term (LT) characteristics Quite stable
Smallest IAV of all major regional monsoon
systems No strong quasiperiodity but perhaps
quasibiennial oscillation Only 3 consecutive
droughts gives strength to LT stability?
Decreasing NE India increasing NW India Weak
inter-relationship between India other
countries.
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Floods and droughts East Asian region
Leader of floods Prolonged, persistent and
excessively heavy rainfall during the Meiyu and
Baiu seasons Special large-scale circulation
conditions for bloods Large negative pressure
anomaly zone stretching from central Asia to
western north America Positive anomaly areas
over Seas of Japan and Okhotsk and over central
Siberia Unusually active low-pressure
disturbances in the middle latitudes over east
Asia and Pacific Ocean High pressure ridges,
blocking highs (maintain stability and force
westerlies to flow into lower latitudes) Frequent
passage of short-wave troughs Westward
extensions of the subtropical high over the
coastal areas south of Yangtze River Droughts Se
vere weather and climate disaster or extreme
events that may persistently affect extensive
areas Time 1955, 1957, 1960, 1962, 1965, 1972,
1978, 1980, 1983, 1986, 1987, 1989, 1991,1993,
1997, 1999, 2000, 2001
Most vulnerable regions Huaihe, Yellow and
Haihe valleys (north China)
Threatens Agriculture, water supply, trans.
Remarkable features Prolonged duration or long
persistence Extensive affected areas Great
severity and degree of impact Simultaneous
occurrence of high temperature
Causes Stationary Rossby wave train of EU
mode ENSO events Global warming
Interannual fluctuation in Meiyu rainfall along
the Yangtze River basin
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Synoptic systems Low-level vortex off Tibet
The east Asian monsoon region
Low-level vortex

• The low-level vortices produced locally over the
  Tibetan Plateau (TP) and its sloping periphery
  are unique weather systems of a subsynoptic scale
  in the EASM.
• These topographically generated or influenced
  circulation systems are among the most important
  rain-bearing systems in east Asia during spring
  and summer.
• The Plateau Vortices are obviously a weather
  system comprised of small-scale, shallow-depth,
  weak intensity and short life cyclones
• Geographical origins
• South-west (SW) vortices South-eastern part of
  the Plateau
• Plateau vortices Main body of the Plateau
• North-west vortices Northern part of the Plateau
• The genesis and development, especially for SW
  vortices, are closely related to activity of
  monsoonal airflows
• Existence of a vigorous southerly airflow of
  eastern slope of the TP to the Sichuan Basin
  (SB)
• Dynamically, it produces differential frictional
  effects forming low-level cyclonic circulation
• Thermally, it transport abundant warm, moist air
  into eastern slope of TP and SB, providing major
  source of precipitation and release of latent
  heat
• Necessary triggering mechanism
• Low-pressure troughs passing over the TP

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Synoptic systems Blocking E. Asia
The blocking situation at mid and high latitudes
in Eurasia

• Blocking high in Eurasia in spring delays ISM
• Blocking at Oral Mountains, Baikal Lake, and
  Okhotsk Sea
• Forces the planetary frontal zone in east Asia
  further to the south
• Affects YR basin due to a splitting of westerlies
• Leads to prolonged heavy rainfall over the YR and
  HR basins
• Suppresses the seasonal northward advance of the
  subtropical high

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Cold waves and cold surges
Definition of a cold wave (AMS glossary) As used
in U.S. National Weather Service, a rapid fall in
temperature within 24 hours to temperatures
requiring substantially increased protection to
agriculture, industry, commerce, and social
activities. Cold wave Winter monsoon affecting
mid- high latitudes Cold surge Winter monsoon
invading into low latitudes Prevailing weather
process in winter east Asia Outbreaks of cold
air marked by invasion of extreme dry and cold
air over Mongolia High winds, abrupt temperature
drop, severe frost, freezing rain, heavy
snowfall, even sandstorms Severity of cold
waves Nationwide outbreak of cold air regional
outbreak of cold air severely cold air cold air
of moderate intensity For 1951-1975, annual mean
20 cold air outbreaks includes 6.3, 4.2, and 8.8,
respectively. Greatly depend upon Coldness of
air masses, pattern of synoptic systems and
circulation, regional or local terrain
features East Asian winter monsoon
(EAWM) Activity closely related to outbreaks of
cold air or cold waves Blows out of huge
Siberian High, sweeps away the east Asian
continental region, turns into cold surge at
lower latitudes, can further flow into southern
hemisphere to enhance the summer ITCZ there Key
component of Asian-Australian monsoon system
reflecting significant interhemispheric
interaction Part of winter local Hadley cell in
east Asia and WNP, Abruptly building up in
NovDec and withdrawing in AprMay Active (cold
air or cold waves) -break cycles modulated mainly
by 10-20-day low-frequency oscillation Three
stages Onset, southward propagation, interaction
between low and mid-latitudes Significant
variability on multi-time scales QBO, IDV
Strong in 1950s-1970s, weak since mid-1980s (ass.
Global warming)
Siberian High Affects winter temperature
in China with important southward invasion (SI)
Since 1980s, intensity decreased (significant
warming in north China), but extent of SI
enhanced (slight warming or even cooling in south
China) Stages Formation, intensification,
s-ward excursion, transformation, dissi.
Early tracks (1) NE track (2) N track (3) NW
track I (4) NW II (5) W track Tracks in
China (a) SE-ward from west of Lake Baikal,
passing middle Yellow River valley, northern
China, lower Yangtze River valley, and
eventually extending to (a1) the sea south of
Japan or (a2) down to southern China (b) E-ward
from Mongolia, crossing NE China, moving over Sea
of Japan
4
3
2
1
5
b
a
a1
a2
Major tracks of Siberian Highs related to
outbreaks of cold air, with relative frequency
(isopleths, )
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Mid-troposheric cyclones
Streamlines at 500hPa
Three cyclones (i) NE Arabian Sea, (ii) Bay of
Bengal and (iii) Plains of Indo-China
Vertical Structure
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Summary

• Monsoon trough quasi-stationary
  important feature
• Synoptic systems (lows, depressions,
  cyclones)
• Land-fall river leads to floods
• Response to Global warming suggest for
  an increase

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Local severe storms and mesoscale convective
complex events
Seasons in EASM region Premonsoon and
sometimes monsoon seasons Type Isolated
squall lines, hailstorms, thunderstorms,
tornados important components of convective
rainstorms and persistent heavy rainfall events
Conditions compared to heavy rainfall
(HR) Temperature inversion or isothermal layer
near surface Above 700 hPa, T lower than HR (8ºC
at 400 hPa) Lapse rate (lt7 km) is 1-3ºC km-1,
greater than HR Layer of potential instability
deeper and LFC higher Significant differences in
moisture content and transport
Hailstorms Spring and early summer in
China Eastern Tibetan Plateau, Western Sichuan,
Eastern and central Gansu, Most of Yunnan
Guizhou, Eastern Inner Mongolia, northern north
China, eastern north-east China, middle, lower
valleys of Yangtze R. Factors Topographic
effects (TP), interaction between cold and warm
moist air with cold fronts, extratropical
cyclones and wind shear lines Mesoscale
convective systems (MCSs) Meso-a-scale (200-2000
km), elliptic shape smooth edge Frequently
occur in south-west China with seasonal
belts High possibility in active area for
south-west vortices
Cloud top blackbody temperature of a MCS
generated in a south-west vortex for the period
from 1 to 2 Aug with grid length of 20 km
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Supplementary slides
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4.2 Synoptic systems
The south Asian monsoon region
(i) The regional monsoon trough (MT) its
latitudinal oscillations (ii) Low-pressure
systems in MT (iii) Indian WC offshore
troughs (iv) Monsoon depressions in MT (v)
Mid-tropospheric cyclones (vi) Minor cyclonic
circulations (vii) North-south troughs (viii)
Western disturbances (ix) Low-frequency MJO
viii
ii
iv
i
v
vii
iii
ix

• These systems are influenced by the interaction
  of the south Asian monsoon with
• Extratropical circulation systems in the south
  Indian Ocean which pulsate the LLJ
• Extratropical systems of the northern hemisphere
  like western disturbances and associated troughs
• Remnants of typhoon/tropical cyclones traveling
  westward and rejuvenating into monsoon lows
• Northward ISOs and eastward MJO, and the
  Himalayan rim
• Westward moving 10-20-day oscillation between
  10º-15ºN from the SCS toward India

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Floods and droughts East Asian region
The east Asian monsoon region Major Factor
Timing of onset and intensity of the summer
monsoon have significance on droughts floods
Droughts in the Yellow River basin and north
China are prone to occur during weak and late
onset of monsoon, while the Yangtze and Huaihe
River basins play an opposite role. The book has
typos in the descript for floods in river
basins! Intensity of SCS SM has negative
correlation with south China precipitation in
summer and positive in postrainy season
Sstrong, Nnormal, Wweak, Eearly onset, Llate
onset, Ddrought, Fflood (1961-1998)
Severe floods in China since 1931 Time 1931,
1935,1954, 1958, 1963, 1975, 1991, 1998, 1999,
2003 Regions Southern China (1-2 times every
3 years) Northern part of Hunan and Jiangxi
(1/2-3yrs) Middle and lower valleys of Yangtze
River and coastal area of eastern China
(1/2-3yrs) Huaihe River valley (1/2-3yrs)
Yellow and Haihe River valleys (1/3yrs)
Songhuajiang and Liaohe River valleys
Consistent with seasonal March of EASM and
geographical distribution of heavy rainfall
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Active/break phases synoptic systems

• Western disturbances and active-break cycle
• Influence of the Western disturbances
• (i) Intensify a lower troposphere low over
  western India
• (ii) Enhance rainfall in a pre-existing low over
  NW India
• (iii) Cause recurvature of depressions lows
  (75º-78ºE)
• (iv) Shift the monsoon trough to the foothills
  break
• Comparisons between active and break phases
• A typical sea level pressure pattern in which
  there are five weather disturbances during the
  active phase of the monsoon
• Typical sea level pressure pattern over south
  Asia during a break monsoon situation with the
  monsoon trough lying across the foothills of the
  Himalayas and a north-south trough along the east
  coast of India