VERA
ET
AL.
Toward
a
Unified
View
of the American
Monsoon
Systems
C.
VERA,a
W.
HIGGINS,b
J.
AMADOR,c
T.
AMBRIZZI,d
R.
GARREAUD,e
D.
GOCHIS,f
D.
GUTZLER,g
D.
LETTENMAIER,h
J.
MARENGO,'
C.
R.
MECHOSO,j
J.
NOGUES-PAEGLE,k
P.
L.
SILVA
DIAS,d
AND
C.
ZHANG!
CIMAIUniversity
of
Buenos Aires,
Buenos Aires,
Argentina
"Climate
Prediction
Center,
NOAAINWSINCEP,
Camp Springs,
Maryland
University
of
Costa Rica,
San
Jose,
Costa
Rica
d
University
of
Sao
Paulo,
Sao
Paulo,
Brazil
'Universidad
de
Chile,
Santiago,
Chile
'National
Center
for
Atmospheric
Research/RAP,
Boulder,
Colorado
9
University
of
New
Mexico,
Albuquerque,
New Mexico
h
University
of
Washington,
Seattle,
Washington
iCPTECIINPE,
Cachoeira
Paulista,
Brazil
I
University
of
California,
Los
Angeles,
Los
Angeles,
California
k
University
of
Utah,
Salt
Lake
City,
Utah
'RSMAS,
University
of
Miami,
Miami,
Florida
(Manuscript
received
31
August
2004,
in final
form
14
February
2005)
ABSTRACT
An
important
goal
of
the Climate
Variability
and
Predictability
(CLIVAR)
research
on
the
American
monsoon
systems
is
to
determine the
sources
and
limits
of
predictability
of
warm
season
precipitation,
with
emphasis
on
weekly
to
interannual
time
scales.
This
paper
reviews
recent
progress
in
the
understanding
of
the
American
monsoon
systems and
identifies
some
of
the
future
challenges
that
remain
to
improve
warm
season
climate
prediction.
Much
of
the
recent
progress
is
derived
from
complementary
international
pro-
grains
in
North
and
South
America,
namely,
the
North
American
Monsoon
Experiment (NAME)
and
the
Monsoon
Experiment
South
America (MESA),
with
the
following
common
objectives:
1)
to
understand
the
key
components
of
the
American
monsoon
systems
and
their
variability,
2)
to
determine the
role
of these
systems
in
the global
water
cycle,
3)
to
improve
observational
datasets,
and
4)
to
improve simulation
and
monthly-to-seasonal
prediction
of
the
monsoons
and
regional water resources.
Among
the
recent
obser-
vational
advances
highlighted
in
this
paper
are
new
insights
into
moisture
transport
processes,
description
of
the
structure
and
variability
of
the
South
American
low-level
jet,
and
resolution
of
the
diurnal
cycle
of
precipitation
in
the
core
monsoon
regions.
NAME
and
MESA
are
also
driving
major
efforts
in
model
development
and
hydrologic
applications.
Incorporated
into
the postfield
phases
of
these
projects
are
assessments of
atmosphere-land
surface
interactions
and
model-based
climate
predictability
experiments.
As
CLIVAR
research
on
American
monsoon
systems
evolves,
a
unified
view
of
the
climatic
processes
modulating
continental
warm
season
precipitation
is
beginning
to
emerge.
1.
Introduction
Monsoon
circulation
systems, which
develop
over
low-latitude
continental
regions
in
response
to
seasonal
changes
in
the
thermal
contrast
between
the continent
and
adjacent
oceanic
regions,
are
a
major
component
of
Corresiponding
auihor
address:
Dr.
Carolina Vera,
CIMA,
Pab.
II,
2"i
piso,
Ciudad
Universitaria,
(1428)
Buenos
Aires,
Argen-
tina.
E-mail:
carolinaCa)cima.fcen.uba.ar
continental
warm
season
precipitation
regimes.
Both
North
and South
America
are
characterized
by
such
systems
[hereafter
referred
to
as
the
North
American
Monsoon
System
(NAMS)
and
the
South
American
Monsoon
System
(SAMS), respectively].
The
NAMS
and
SAMS
provide
a
useful
framework for
describing
and
diagnosing
warm
season
climate
controls,
and
the
nature
and
causes of
year-to-year
variability.
A
number
of
studies
during the
past
decade
have
revealed
the
major
elements
of
these
systems,
including
their
con-
text
within
the
annual
cycle,
and
some
aspects
of
their
variability.
CO
2006
American
Meteorological
Society
4977
15
OcioluriR
2006
JOURNAL
OF
CLIMATE-SPECIAL
SECTION
Due
in
large
part
to
the
success
of
the
World
Climate
Research
Programme/Climate
Variability
and
Predict-
ability/Variability
of
the
American
Monsoon
Systems
(WCRP/CLIVAR/VAMOS)
program,
a
unifying
view
of
the
NAMS
and
SAMS
is
beginning
to
emerge.
In
particular,
CLIVAR/VAMOS
has
implemented
complementary
international
programs
in
North
and
South
America,
namely
the
North
American
Monsoon
Experiment
(NAME)
and
the
Monsoon
Experiment
South
America
(MESA).
These
programs
have
com-
mon
objectives:
1)
to
understand
the
key
components
of
the
American
monsoon
systems
and
their
variability,
2)
to
determine
the
role
of
these
systems
in
the
global
water
cycle,
3)
to
improve
observational
datasets,
and
4)
to
improve
simulation
and
monthly-to-seasonal
pre-
diction
of
the monsoon
and
of
regional
water
resources.
Several
recent
publications
summarize
the
accom-
plishments
of
the
scientific
community
toward
achiev-
ing
a
better
understanding
of
the
monsoon
systems
of
the Americas.
These
include
a
review
of
the
South
American
Monsoon
System
(Noguds-Paegle
et
al.
2002)
and
a
review
of
the
North
American
Monsoon
System
(Higgins
et
al.
2003).
The
intention
of
this
paper
is
to
discuss
recent
advances
in
our
understanding
of
the
NAMS
and
SAMS
in an
integrated
framework,
highlighting
recent
papers
that
illustrate
progress
made
during
the
initial
phase
of
CLIVAR.
More
complete
historical
reference
lists
are
found
in
the
overview
pa-
pers
cited
above.
The
basic
features
of
the
NAMS
and
SAMS
and
their
variability
within
the context
of
the
land
surface-atmo-
sphere-ocean
annual
cycle
are
discussed
in
sections
2
and
3,
respectively.
The
role
of
land
surface
memory
in
the variability
and
predictability
of
the
NAMS
and
SAMS
is
considered
in
section
4.
Hydrologic
character-
istics
of
the
monsoon
systems
are
described
in
section
5.
Outstanding
science
questions
associated
with
gaps
in
our
understanding
are
reviewed
in
section
6.
2.
Basic
features
Both
the
NAMS
and
the
SAMS
exhibit
many
of
the
features
of
their
Asian
counterpart,
including
large-
scale
land-sea
temperature
contrast,
a
large-scale
ther-
mally
direct
circulation
with
a
continental
rising
branch
and
an
oceanic
sinking
branch,
land-atmosphere
inter-
actions
associated
with
elevated
terrain
and
land
sur-
face
conditions,
surface
low
pressure
and
an
upper
level
anticyclone,
intense
low-level
inflow
of
moisture
to the
continent,
and
associated
seasonal
changes
in
precipi-
tation
(both
increases
and
decreases).
Moreover,
the
poleward
extension
of
the
summer
convection
in
the
tropical
Americas
seems
to
be
associated
with
similar
mechanisms
(such
as
the ventilation
processes)
to
those
acting
in
the
Asian
monsoon
system
(Chou
and
Neelin
2003).
Both
the
NAMS
and
the
SAMS
receive
more
than
50%
of
total
annual
precipitation
during
the
respective
summer
monsoons,
though
the
SAMS
precipitation
amounts
are
considerably
greater
(Figueroa
and
Nobre
1990;
Higgins
et
al.
1997).
The
SAMS
exhibits
some-
what
distinct
characteristics
compared
to
the
other
monsoon
systems,
given
that
most
of
South
America
is
situated
in
the
Tropics,
and
seasonal
temperature
dif-
ferences
are
less
pronounced
than
in
subtropical
mon-
soon
regimes.
A
clear
annual
cycle
characterizes
the
convection
over
the
tropical
Americas
that
exhibits
a
seasonal
regularity
and
degree
of
symmetry
with
respect
to
the
equator
(Horel
et
al.
1989).
The
NAMS
and
SAMS
can
be
interpreted
as
the
two
extremes
of
the
same
cycle
(Fig.
1)
and
their
corresponding
life
cycle
can
be
de-
scribed
using
terms
that
have
been
traditionally
re-
served
for
the
Asian
summer
monsoon
system,
namely,
onset,
mature,
and
decay
phases.
Seasonal
evolution
of
the
convection
During
May-June
(the
onset
phase
of
the
North
American
monsoon),
heavy
rains
spread
northward
along
the
western
slopes
of
the
Sierra
Madre
Occiden-
tal
(SMO;
Fig.
2;
Douglas
et
al.
1993;
Stensrud
et
al.
1995;
Adams
and
Comrie
1997).
Precipitation
increases
over
northwestern
Mexico
coincide
with
increased
ver-
tical
transport
of
moisture
by
convection
(Douglas
et
al.
1993)
and
southerly
winds
flowing
along
the
Gulf
of
California.
During
this
season,
weather
in
the
NAMS
region
changes
abruptly
from
relatively
hot,
dry
condi-
tions
to
cool,
rainy
ones
(Mock
1996;
Adams
and Com-
rie
1997).
Increases
in
precipitation
over
the
southwestern
United
States
occur
abruptly
around
the
beginning
of
July
(e.g.,
Mock
1996;
Higgins
et
al.
1997)
and
coincide
with
the
development
of
a
pronounced
anticyclone
at
the
jet
stream
level
(e.g.,
Okabe
1995),
the
develop-
ment
of
a
thermally
induced
trough
in
the
desert
South-
west
(Rowson
and
Colucci
1992),
northward
displace-
ments
of
the
Pacific
and
Bermuda
highs
(Carleton
1987),
the
formation
of
southerly
low-level
jets
over
the
Gulf
of
California
(Douglas
1995),
and
the
formation
of
the
Arizona
monsoon
boundary
(Adang
and
Gall
1989).
The
onset
of
the
summer
monsoon
rains
over
southwestern
North
America
has
been
linked
to
an in-
crease
of
rainfall
along
the
East
Coast
of
the
United
States
(e.g.,
Higgins
et
al.
1997)
and
to
a
decrease
of
rainfall
over
the
Great
Plains
of
the
United
States
(e.g.,
Douglas
et
al.
1993;
Mock
1996;
Higgins
et
al.
1997).
4978
VOLUME
19
VERA
ET
AL.
Mean
monthly
precipitation
averaged over
each
spatial
domain
300
G
275
_
250
2
225
E
c 200
175
•
150
a.
125
S100
o
75
S50
m
25
0
1
2
3
4
5
6
7
8
9
10
11
12
Months
NA
-S
-
SAMS
-
Sahel
-
PNW
-
India
Fie;.
1.
Mean
annual
cycle
of
precipitation
over
several
major
monsoon
areas
[NAMS
(20'-37-N,
248°-257°E);
SAMS
40'-60'W);
India (6°-37°N,
68°-98°E);
Sahel
(10°-20'N,
15'-
15'W)].
For comparison,
one
nonmonsoon
region
with
a
large
annual
cycle
is
also
shown
[Pacific
Northwest
(PNW:
42°-50'N,
112°-124°W)].
Moisture
transport
onto
the
North
American continent
is
accomplished
by
boundary
layer
flow
from
the
south
(Gulf
of
California),
and
in
the
middle
troposphere
via
southeasterly
flow
from
the
Gulf of
Mexico
(Schmitz
and
Mullen
1996;
Fig.
3a).
During
July-August-early
September,
the
NAMS
is
fully
developed.
The heaviest
precipitation
is
west
of
the
SMO
and
near
the
Bay of
Campeche
(Fig.
4b).
The
northern
edge
of
the
monsoon
extends into
Arizona
and
New
Mexico
(e.g.,
Douglas
et
al.
1993),
but the
rainfall
is
much
lighter
and more
directly
influenced
by
midlatitude
effects.
Precipitation
also
occurs
in
Central
America
and
northwestern
South
America,
with
a
rela-
tive
maximum
of
precipitation
in
southeast
(SE) sub-
tropical South
America
(Fig.
4a).
The
diabatic
heating
released
by
the
NAMS
in
combination
with
the
oro-
graphic
forcing
induces
descent
over
the
eastern
North
Pacific
while
it
promotes
the
development
of
the
North
Atlantic subtropical
high
(Rodwell
and
Hoskins
2001).
Surges of
maritime
tropical
air
move
northward
along
the
Gulf
of
California
and are linked
to
bursts
and
breaks
of the
monsoon
rains over
the deserts
of
Arizona
and
California
(Stensrud
et
al.
1995).
Gulf
surges
are
triggered
by
a
variety
of
synoptic-scale and
mesoscale
disturbances,
including
tropical
easterly
waves,
tropical
cyclones,
mesoscale convective
systems,
and
upper-level
inverted troughs
(e.g.,
Stensrud
et
al.
1995;
Higgins
et
al.
2004).
Equatorward
of
the
tropic
of
Cancer
the monsoon exhibits
a
double-peak
structure
in
precipitation
and
diurnal
temperature
range.
From
south-central
Mexico
into
Central America,
this
mid-
summer
dry
spell
or
"canicula"
is
sufficiently
regular
as
to
appear
in
climatological averages
(Magana
et
al.
1999).
Recent
evidence
indicates
that
the
trade
winds,
evaporation,
and
precipitation
patterns
over
the
warm
pool region
to
the
southwest
of Mexico
modulate
sea
surface
temperatures
(SSTs)
in
a
manner
consistent
with
the
double-peak structure
in
precipitation
(Ma-
gafia
et
al.
1999).
During
late
September-October
the
decay
phase
of
the
NAMS
occurs.
The
ridge
over the
western United
States
weakens
as
the
monsoon
high
retreats
southward
and
precipitation
diminishes.
Simultaneously
the
mid-
latitude
westerly regime
shifts
equatorward,
and
pre-
cipitation
events
are
more
frequently
associated
with
synoptic-scale
frontal
systems
rather
than
with localized
convective
instability.
By
September,
the
convection migrates
from
Central
America
into
South America,
and
the
onset
of
the
wet
season
over South
America
starts
first
in
the
equatorial
Amazon
and
then
spreads
quickly
to
the
east
and
southeast
(Fig.
2b).
The onset
across
the Amazon
basin
lasts
about
one
month
(e.g.,
Kousky
1988:
Horel
et
al.
1989;
Marengo
et
al.
2001;
Liebmann
and
Marengo
2001)
and
is
followed
by
abundant
rainfall.
The
onset
(demise)
of
the Amazon
rainy season
is
preceded
by
an
increase
in
the
frequency
of
northerly
(southerly)
cross-
equatorial
flow
over South
America
(Marengo
et
al.
2001;
Wang and
Fu
2002).
Changes
in
the
moistening
of
the
planetary
boundary
layer
and
changes
in
the
tem-
4979
1.5
Oct"oiw,R
2006
JOURNAL
OF
CLIMATE-SPECIAL
SECTION
a)
b)
W
- E
t,,
I
1 - '•/,
4-./
-.
"
96W 86Wl 86W 7iW 7iW UýW 6tW
5iW
as
44
405
45W
30W
FiG.
2.
Mean
calendar
date
of
onset
for
(a)
the
North
American
monsoon
(from
Higgins
et
al.
1999),
and
(b)
the South
American
Monsoon
(courtesy
of
V.
Kousky).
perature
at
the top
of
the
boundary
layer
seem
to
ex-
plain
the
seasonal
changes
of
convection
in
tropical
South
America
(Fu
et
al.
1999;
Marengo
et
al.
2001;
Liebmann
and
Marengo
2001).
The
onset
of
the
wet
season
in
central
and
southeast-
ern
Brazil
typically
occurs
between
the
end
of
Septem-
ber
and early
October
and
may
sometimes
occupy
only
a
single
5-day
period
(Sugahara
1991).
Intraseasonal
oscillations
may
promote
rapid
onset
over
central
Bra-
SW
- NE
FIG.
3.
(a)
Schematic
vertical
(longitude-pressure)
cross
section
through
the
NAMS
at
27.5°N.
Topography
data
were
used
to
establish
the
horizontal
scale
and
NCEP-National
Center
for
At-
mospheric
Research
(NCAR)
reanalysis
wind
and
divergence
fields
were
used to
establish
the
vertical
circulations
(from
Hig-
gins
and
NAME
Science
Working
Group
2003).
(b)
Schematic
vertical
section
across
South
America
displaying
the
major
large-
scale
elements
affecting
the
SAMS
(from
CLIVAR
Web
site
on-
line
at http://www.clivar.org).
zil
(Vera
and
Nobre
1999).
By
late
November,
deep
convection
covers
most
of
central
South
America
from
the
equator
to
20'S,
but
is
absent
over
the eastern
Amazon
basin
and
northeast
Brazil.
Throughout
this
period,
deep
convection
associated
with
the
intertropi-
cal
convergence
zone
(ITCZ)
is
confined
to the
central
Atlantic
between
50
and
8'N
(Zhou
and
Lau
1998).
During
late
November
through
late
February
(the
mature
phase
of
the
SAMS),
the
main
convective
ac-
tivity
is
centered
over
central
Brazil
and
linked
with
a
southeastward
band
of
cloudiness
and
precipitation
ex-
tending
from
southern
Amazonia
toward
southeastern
Brazil
and
the surrounding
Atlantic
Ocean
(Fig.
4b).
That
convection
band,
known
as
the South
Atlantic
convergence
zone
(SACZ),
is
a
distinctive
feature
of
the
SAMS
(Kodama
1992).
Also,
the
heavy
rainfall
zone
extends
over the
Altiplano
Plateau
and
the
south-
ernmost
Brazilian
highland.
The
upper-level
circulation
during
the
South
Ameri-
can
summer
includes
well-defined
regional
circulation
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ET
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20N
1ON
10S
20S
30S
40S
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90W
80W
70W
JJA
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40S
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120W 110W 100W
9OW 80W 70W 66W
50W 40W
30W
Fi(i.
4.
Climatological
mean
accumulated
precipitation
(from
Xie
and
Arkin
1997)
and
vertically
averaged
climatological
mean
moisture
fluxes
(from
NCEP-NCAR
reanalysis)
for
(a)
DJF
and
(h)
Junc-August
(JJA).
Contour
interval
is
150
mm.
Vector
units
are
kg
(m
s)
'.
Orography
higher than
1000
m
is
contoured
in
black.
features,
including
a
large
anticyclonic
circulation
(the
"Bolivian
High")
centered
near
15'S, 65°W
and
an
up-
per-level
trough
near
the
coast
of
northeast
Brazil (not
shown).
At
low
levels,
the
"Chaco
Low"
is
the
most
conspicuous
feature
downslope
of
the
Andes and
can
be
considered
together
with
the
Bolivian
High
as
the
regional
response
of
the
tropospheric
circulation
to the
strong
convective
heating
over
the
Amazon-central
Brazil.
The Andes
effect
reinforces
the
strength
of
the
Chaco
Low
through
the
barrier
role
of
the
mountains
(Gandu
and
Silva
Dias
1998,
and
references
therein).
A
continental-scale
gyre
transports
moisture
westward
from
the
tropical
Atlantic
Ocean
to
the
Amazon
basin,
and
then
southward
toward
the
extratropics
of
South
America
(Fig.
4a).
The
diabatic
heating released
in
the
SAMS
region
seems to
promote
that
gyre,
and
the
maintenance
of
the
South
Atlantic
subtropical
high
during
austral
summer
(Rodwell
and
Hoskins
2001).
A
regional
intensification
of
this
gyre
circulation
to the
east
of
the Andes Mountains
is
due to
the
South
Ameri-
can
low-level
jet
(SALLJ),
with
strongest
winds
in
Bo-
livia
near
Santa
Cruz
(18'S,
63°W).
The SALLJ
trans-
ports
considerable
moisture between
the
Amazon
and
the
La
Plata
basins
and
is
present
throughout
the
year
(e.g.,
Berbery
and
Barros,
2002);
it can
be
explained
using
simple,
adiabatic
models
in
which
orography
pro-
vides
dynamical,
rather
than
thermodynamical
modifi-
cation
of
the
zonally
averaged
circulation
(Byerle
and
Paegle
2002;
Campetella
and
Vera
2002).
During
the
warm season,
thermodynamic
processes associated
with
precipitation
either
over
the
SACZ
region or
southeast-
ern
South
America
(SESA)
modulate
the
low-level
flow
in
tropical
regions
(Berbery
and
Collini
2000).
Between
March
and
May,
the
SAMS
decay
phase
begins,
as
regions
of
heavy
precipitation
over the
south-
ern
Amazon
and
central
Brazil
decrease
and
gradually
migrate
northwestward
toward
the
equator
and
as
the
rainy
season
along
the
eastern
coast
of
northeast
(NE)
Brazil
gets
under
way
and
continues
from
April
through
June (Rao
and
Hada
1990).
Throughout
the
decay
phase
of
the
SAMS,
deep
convection
associated
with
the
Atlantic
ITCZ
is
relatively
weak.
3.
Climate variability
a.
Diurnal
and
mesoscale
variability
1)
DIURNAL
CYCLE
The
timing
of
daily
maximum
convection
in
the
Americas
is
location
specific
and
closely
related
to
to-
pographic
features
such
as
mountain
ranges
and
coast
lines.
In
the
NAMS
there
are
large-scale
shifts
in
the
regions
of
deep
convection
during
the
day
from
over
land
during
the
afternoon
and
evening
(especially along
the
western
slopes
of
the
SMO)
to
offshore
locations
during
the
morning
hours
(Figs.
5a,b;
e.g.,
Higgins
et
al.
2003).
In general,
onshore
flow
begins
during
the
morn-
ing
transporting
moist air from
the Gulf
of
California
inland
and
up
the
slopes
of
the
SMO.
Enhanced
by
inland
mountain
valley
circulations,
convection
initiates
near
midday
along
the
western
slopes
and
high
ridges
of
the
SMO
(Gochis et
al.
2003;
Fig.
6).
During
the
eve-
ning
and
nighttime
upslope
flow
reverses
and
generates
a
westward-propagating
zone
of
convergence
that
mi-
grates downslope
across
the
coastal plains
and
out
over
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