South America

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The South American
Monsoon System (SAMS)
Vernon Kousky & Viviane Silva
1 June 2011
Outline
• Key topographic features of South America
• Characteristic features of SAMS
• Life Cycle of the SAMS wet season
– Evolution (precipitation and atmospheric circulation)
– Onset, mature and demise phases
• SAMS variability
–
–
–
–
–
El Niño/ Southern Oscillation (ENSO) influences
Atlantic SST dipole pattern
Madden-Julian Oscillation (MJO) impacts
South Atlantic Convergence Zone
Diurnal Cycle
Key Topographic Features
Amazon Basin
Andes Mountains
BP
Brazilian High
Plains (Planalto)
Parana Basin
SAMS: Characteristic Features
The annual cycle of precipitation
Distinct wet and dry seasons
occur between the equator and
25ºS.
Many areas within that region
receive more than 50% of their
total annual precipitation during
the austral summer (Dec-Feb),
and less than 5% of their total
annual precipitation during
austral winter (Jun-Aug).
Percent of annual mean (1975-2003) precipitation.
SAMS: Characteristic Features
A
During the wet season an upper-tropospheric anticyclone (red A)
dominates the circulation over tropical and subtropical South America,
while cyclonic flow (troughs – dashed black lines) dominates the uppertropospheric circulation over low latitudes of the eastern South Pacific and
central South Atlantic
H
L
H
Prominent low-level features include:
1) surface high pressure systems and anticyclonic circulation over the
subtropical oceans (Pacific and Atlantic),
2) a surface low-pressure system (Chaco Low) centered over northern
Argentina, and
3) a low-level northwesterly flow (low-level jet) extending from the
southwestern Amazon to Paraguay and northern Argentina.
Throughout the region one
notes a reversal of
circulation features
between the lower
troposphere and the upper
troposphere, which is
typical in the global Tropics.
A
C
A
A
Life Cycle of the SAMS Wet Season
The annual cycle of upper-tropospheric
circulation features over South America is
intimately linked to the seasonally varying
horizontal temperature gradients, which
arise from differential heating between
land and water.
Life Cycle of the SAMS Wet Season
During summer,
temperatures over the
continent become warmer
than the neighboring
oceanic regions. This
results in a direct thermal
circulation with low-level
(upper-level) convergence
(divergence), midtropospheric rising motion
and wet conditions over the
continent, and low-level
(upper-level) divergence
(convergence), midtropospheric sinking motion
and dry conditions occur
over the neighboring
oceanic areas.
Red (blue)
indicates
divergence
(convergence)
South America
Blue indicates
cold cloud top
temperatures
Life Cycle of the SAMS Wet Season
During winter, temperatures
over the continent and
nearby oceanic regions are
more uniform, which gives
rise to a more zonally
symmetric uppertropospheric circulation
pattern over the region and
little or no evidence of any
east-west direct thermal
circulation
Red (blue)
indicates
divergence
(convergence)
South America
Blue indicates
cold cloud top
temperatures
Meridional Circulation: January and July
10-20S
SH Summer:
upper-level
divergence,
low-level
convergence,
rising motion
SH Winter:
upper-level
convergence,
low-level
divergence, and
subsidence
South
America
Life Cycle of the SAMS Wet Season
Green and
blue colors
indicate cold
cloud top
temperatures,
which in the
Tropics are
associated
with deep
convection.
Life Cycle of the SAMS Wet Season
The development of the South American
warm season Monsoon System, during
Sep-Nov, is characterized by a rapid
southward shift of the region of intense
convection from northwestern South
America to the southern Amazon Basin
and Brazilian highlands (altiplano) (see
next slide).
Blue colors
indicate cold
cloud top
temperatures,
which in the
Tropics are
associated
with deep
convection.
Life Cycle of the SAMS Wet Season
Lower-Atmospheric temperatures reach their annual maximum over the southern
Amazon and altiplano region in early September, just prior to the onset of the
rains.
Life Cycle of the SAMS Wet Season
• Transient synoptic systems at higher latitudes
play an important role in modulating the
southward shift in convection.
• Cold fronts that enter northern Argentina and
southern Brazil are frequently accompanied by
enhanced deep convection over the western
and southern Amazon and an increase in the
southward flux of moisture from lower latitudes.
A
• Cold fronts are also important in the formation
of the South Atlantic Convergence Zone
(SACZ), which becomes established in austral
spring over Southeast Brazil and the
neighboring western Atlantic.
South Atlantic Convergence Zone
(SACZ)
Green and
blue colors
indicate cold
cloud top
temperatures,
which in the
Tropics are
associated
with deep
convection.
• During spring an upper-tropospheric anticyclone (Bolivian High) becomes established
near 15ºS, 65ºW, as the monsoon system develops mature-phase characteristics.
• Upper-level troughs and dry conditions are found over oceanic areas to the east and
west of the Bolivian High.
• The deep convection over central Brazil and the Bolivian High reach their peak
intensities during December-February.
• These features shift northward and weaken during March-May, as the summer
monsoon weakens and a transition to drier conditions occurs over subtropical South
America.
Brazil Wet Season Onset & Demise
ONSET
DEMISE
SAMS Variability
• Interannual Variabiliy - El Niño and La
Niña impacts, Atlantic SST dipole pattern
• Intraseasonal Variability
– Madden-Julian Oscillation (MJO)
– South Atlantic Convergence Zone (& cold
fronts)
• Diurnal Cycle
Precipitation Anomaly Patterns during
El Niño Episodes over Brazil
The pattern of anomalous
precipitation over Brazil for the
water year (July-June) for El-Nino
episodes.
Composite shows wetter than
average to the south and drier than
average to the north, with
considerable event-to-event
variability in the intensity and extent
of the anomalies.
From Silva et al., 2007
Precipitation Anomaly Patterns during
La Niña Episodes over Brazil
The pattern of anomalous
precipitation over Brazil for the
water year (July-June) for La Nina
episodes.
Composite shows wetter than
average to the north and drier than
average to the south, with
considerable event-to-event
variability in the intensity and extent
of the anomalies, especially over
southern Brazil.
From Silva et al., 2007
Atlantic Dipole
Atlantic Dipole Pattern
Suppressed
convection
Enhanced
convection
Low-level westerly wind
anomalies are consistent
with positive SST anomalies
along the equator and a
southward shift in the ITCZ,
resulting in heavy rainfall in
Northeast Brazil.
MJO Impacts
• The MJO modulates summer rainfall over
northern and northeastern South America
with a period of 30-60 days.
• Very important for Northeast Brazil (semiarid region), which experiences a short (34 month) wet season.
200-hPa Velocity Potential
Anomalies (5°S-5°N)
Positive anomalies (brown shading)
indicate unfavorable conditions for
precipitation
Negative anomalies (green shading)
indicate favorable conditions for
precipitation
• From mid-March to early May 2009, eastward
propagating velocity potential anomalies
indicated moderate-to-strong MJO activity.
• The active phase of the MJO was in the South
American sector (phase 8) during the end of
March and again in the beginning of May,
contributing to excessive rainfall and flooding
over northeastern Brazil during those periods.
Time
Longitude
Rainfall over northeastern South America (MAM)
BP
- Rainfall totals over northeast Brazil
during 11 March-6 June 2009 were
above-average (surpluses of 210-600
mm). Note the increased rainfall at the
end of March and again at the
beginning of May, associated with the
convectively active phase of the MJO.
SACZ Variability: The South
American Dipole Pattern
The OLR anomaly patterns often show a dipole of positive and negative
values along the east coast of Brazil.
SACZ Variability: The South
American Dipole Pattern
The dipole pattern is found over eastern South America in
the region of greatest OLR variability, which extends from
southern Brazil northeastward to Northeast Brazil.
SACZ Variability: The South
American Dipole Pattern
The mid-tropospheric vertical
motion also displays a dipole
pattern, with upward vertical motion
(negative omega) in the region of
negative OLR anomalies
(anomalously strong convection
and rainfall) and sinking motion to
the south.
SW
NE
SACZ Variability: The South
American Dipole Pattern
• Cold fronts play an important role in
organizing convective activity in the region
of the SACZ.
• Sometimes this leads to extreme events
(exceptionally heavy rainfall and flooding
over a period of several days).
Diurnal Cycle of
Precipitation Based on
CMORPH
Vernon E. Kousky, John E. Janowiak
and
Robert Joyce
Climate Prediction Center, NOAA
CMORPH (CPC Morphing technique)
Uses IR data along with passive microwave
data to create global rainfall analyses (60N60S) at high spatial and temporal resolution.
CMORPH uses IR only as a transport vehicle, i.e. IR
data are NOT used to make estimates of rainfall when
passive microwave data are not available.
The underlying assumption is that the error in using IR to
transport precipitation features is less than the error in
using IR to estimate precipitation.
Specifics
• Spatial Grid: 0.0728o lat/lon (8 km at equator)
• Temporal Resolution: 30 minutes
• Domain: Global (60o N - 60o S)
• Period of record: Dec. 2002 – present
For more information about CMORPH:
http://www.cpc.ncep.noaa.gov/products/janowiak/cmorph.html
Methodology
• The seasonal mean precipitation rates, from CMORPH,
are computed for each time interval (1-h or 3-h).
• The mean daily precipitation rates are computed by
summing the 1-h rates for the 24-h period.
• In the absence of any diurnal variability, the same amount
of rainfall would be expected during each time interval
(e.g., 100/24 = 4.2% for 1-h intervals and 100/8 = 12.5%
for 3-h intervals).
• Brown (green) colors are used to depict times when the
observed % is less (more) than the expected values
(precipitation uniformly distributed throughout the 24-h
period).
DJF - South America (1mm mask)
The amplitude of the
diurnal cycle is large
over northeastern
South America, the
Andes Mts. And
southeastern Brazil.
(Compare top left
and bottom right
panels, or toggle
back and forth
between this slide
and the following
slide.)
DJF - South America (1mm mask)
The amplitude of the
diurnal cycle is large
over northeastern
South America, the
Andes Mts. And
southeastern Brazil.
(Compare top left
and bottom right
panels, or toggle
back and forth
between this slide
and the previous
slide.)
South America: DJF (1 mm/d mask)
Westward moving
organized
features are
evident over
Tropical South
America, and
eastward moving
features are
evident in the
vicinity of the
Andes Mts. (15S30S).
Time (LST) of Max. Precipitation:
South America – DJF 2002-03
15-18 LST
21-24 LST
08-11 LST
03-06 LST
02-05 LST
09-12 LST
20-23 LST
15-18 LST
Time-Longitude Diurnal Cycle EQ DJF
The mean diurnal
cycle is repeated four
times. A westward
moving feature
(dashed black line) is
evident between 50W
and 60W.
Time
Longitude
East Coast
MAM Mean Diurnal Cycle – EQ-5N
Convective
rainfall
systems start
along the east
coast on day1
and
propagate
westward,
reaching the
western
Amazon
Basin on
day3.
Day1
Day2
Day3
Day4
West Coast
East Coast
Seasonal
variations of the
Diurnal Cycle of
Precipitation
over the Amazon
Basin
An afternoon peak is observed
throughout the year. A secondary
nocturnal peak is evident during
Jan-May.
topography
A strong diurnal cycle is evident over Southeast Brazil and the
western Atlantic Ocean.
Seasonally varying diurnal cycle
of preciitation (area averaged
2x2 degrees), 2S.
55W
52W
49W
46W
Westward propagating lines near the Equator occur primarily
during February-May.
Summary
• Characteristic features of SAMS (precipitation,
atmospheric circulation)
• Life Cycle of the SAMS wet season
• SAMS variability
–
–
–
–
–
El Niño/ Southern Oscillation (ENSO) influences
Atlantic SST dipole pattern
Madden-Julian Oscillation (MJO) impacts
South Atlantic Convergence Zone
Diurnal Cycle
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