What controls the seasonality of What controls the seasonality of the Amazon rainfall and its the Amazon rainfall and its

interannual variations?interannual variations?How strong it interacts with land surface?How strong it interacts with land surface?

Rong Fu and Wenhong Li

Earth & Atmospheric Sciences, Georgia Tech.

Amazon climate and hydrology workshop, Duke, May 9-10, 2005

Length of dry season: Sombroek 2001Length of dry season: Sombroek 2001

Malhi 2004Malhi 2004

Ecosystem and agriculture of Ecosystem and agriculture of the Amazônia depend more on the Amazônia depend more on the length and rainfall of the the length and rainfall of the dry season than on wet season dry season than on wet season rainfall. Yet, most of the rainfall. Yet, most of the meteorological studies have meteorological studies have been focus only on the wet been focus only on the wet season rainfall.season rainfall.

Regions with dry season rainfall < Regions with dry season rainfall < 100mm could potentially be converted 100mm could potentially be converted to Savanna given enough of to Savanna given enough of deforestation - Steinberg 2001deforestation - Steinberg 2001

Outline:Outline:

Processes that controls the wet season onset– How do land surface, moisture transport and extratropical

influence wet season onset?

Mechanisms that control the interannual variation of the wet season onset

– - Why is the relationship between Amazon rainfall and SST anomalies so complex?

The influence of Amazonian rainfall on tropical and North Atlantic atmospheric circulation (NAO, ITCZ)

Key uncertainties and future challenges

Seasonal Cycle:Seasonal Cycle:

Kousky 1979; Kousky 1979; Horel et al. 1989; Marengo et al. 2000; Marengo et al. 2000

• What causes rapid increase What causes rapid increase of rainfall across broad of rainfall across broad latitudes during austral latitudes during austral spring (non-ITCZ like)?spring (non-ITCZ like)?

• Why is the northward Why is the northward withdraw of the rainy area withdraw of the rainy area (ITCZ-like) more gradual (ITCZ-like) more gradual during austral fall?during austral fall?

Change of seasonal cycles is a main source of Change of seasonal cycles is a main source of

interannual variation of annual rainfall:interannual variation of annual rainfall: Liebmann and Marengo 2001

Marengo et al. 2001

JulJul DecDec

What cause such strong variations What cause such strong variations of wet season onset?of wet season onset?

What control the changes of rainfall and wet season onset in What control the changes of rainfall and wet season onset in the western Amazon?the western Amazon?

Ronchail et al. 2002Ronchail et al. 2002

El Nino relatedEl Nino related El Nino+cold El Nino+cold SATLSATL

Oceanic influences cannot be clearly detected in the western Oceanic influences cannot be clearly detected in the western Amazon.Amazon.

Key issues-1:Key issues-1: What is the primary forcing of the wet season

onset? – Evaluated heating is most efficient to drive overturn

of the large-scale circulation! Surface fluxes over Altiplano: Schwerdtfeger 1961; Gutman &

Schwerdtfeger 1965 Amazonian convection drives the wet season circulation: e.g., Silva Dias et

al. 1983, Kleeman 1989; Gandu & Geisler 1991; Lanters & Cook 1995; Convection increases prior to the onset of monsoon circulation: Horel et al

1989• Is Amazon convection a forcing or a result of seasonal

reversal of the large-scale circulation? • What causes increase of convection over Amazon during

the transition from dry to wet season?

Key issues-2:Key issues-2: What are the relative roles of land surface flux

and moisture transport in determine the seasonality of the rainfall, esp. the wet season onset?

– Local Recycling: e.g., Salati et al. 1979, Shuttleworth 1988, ≥50%– Transport from Atlantic: e.g., Gibbs 1979; Marengo 1992; Rao et al. 1996;

“Mechanisms that explain the various precipitation maxima .. are all apparently linked to either large-scale features … or to other local and mesoscale forcings. They do not appear to depend, to a first approximation, on type of underlying vegetation. Yet, there is a wealth of observational evidence showing that evapotranspiration accounts for more than 40% of the precipitation.”

— Salati and Nobre 1991

• Is land surface important to the wet season onset?

Key issues-3:Key issues-3: What cause the sudden increase of rainfall across broad latitudes

(~20˚) in western and southern Amazon? What is the role of cold fronts incursion?

– Strong cold surges account for about 50% of the total summertime precipitation south of 25S, about 30% over the western Amazon basin. (Kousky 1979; Kousky and Ferreira 1981; Marengo et al. 1997; Garreaud and Wallace 1998; Garreaud 1999; Garreaud 2000a; Vera and Vigliarolo 2000; Vera et al. 2002)

Could cold fronts Could cold fronts trigger the wet season trigger the wet season onset given adequate onset given adequate large-scale large-scale thermodynamic thermodynamic conditions over the conditions over the Amazon?Amazon?

Convective Clouds associated with cold front incursions (Garreaud Convective Clouds associated with cold front incursions (Garreaud & Wallace 1998)& Wallace 1998)

cold

cold

SummerSummer WinterWinter

Data Sets:Data Sets: Data: 15-year (1979-1993) pentad

– ECMWF reanalysis data (4-times a day, 2.5 lat x 2.5 lon, 17 pressure levels)

– Rain gauge data (daily): National Water and Electric Energy Agency of Brazil (ANEEL).

– GPCP precipitation data (daily, 2.5 lat x 2.5 lon)

– TRMM daily rainrate– Radiosonde: 7 years– ABRACOS flux tower data, 1992 and

1993 Domain (5-15S 45-75W): Onset Sept. 10

to Oct. 1. Define the wet season onset:

– The pentad before which rain rate is less than 6.1 mm day–1 during 6 out of 8 preceding pentads and after which rain rate is greater than 6.1 mm day–1 during 6 out of 8 subsequent pentads (Marengo et al. 2001; Liebmann & Marengo 2001)

gauge

GPCP

Marengo et al. 2001Marengo et al. 2001

ERA15

Li & Fu 2004Li & Fu 2004

initiating developing Onset

Kinetic energy conversion function

V-index

850mb Equivalent potential temperature change with time

mm

/day

K/d

aym

/s

pentad

10-6(m

2 s-3)

ECMWF

15-yr composite results

gauge

GPCP

onset

1000hPa 45W 15S

200hPa

45W 5S Buoyancy,Buoyancy,

Li and Fu 2004Li and Fu 2004

Rainfall begins to Rainfall begins to increase before the increase before the transition of the transition of the circulation. Thus, it circulation. Thus, it may provides elevated may provides elevated heating and initiates heating and initiates circulation transition.circulation transition.

How is transition initiated?

e/t e/T*(T/t+Lc/Cp* q/ t)

T/t

Lc/Cp* q/ te/t

What causes increase of air buoyancy near surface?

How does it influence convective instability?

CAPECINE

initiating pentad

K/d

aykJ

/kg

maturing

hei

ght

hei

ght

temperaturetemperature

Positively buoyant,CAPE

Negatively buoyant,CINE• Increase of air humidity dominates the increase of air

buoyancy near surface• Increase air buoyancy rapidly reduce the convecitve

inhibition energy (CINE) prior to the occurrence of moisture convergence.

LFCLFC

Sensible heat

Latent Heat

Net Radiation

Downward Solar at surface

Onset

LH

: W/m

2W

/m2

SH

: W/m

2

Latent flux increase leads to higher surface air buoyancyLatent flux increase leads to higher surface air buoyancy

Increase of air buoyancy

1000hPa 75W 15S 45W 15S

100hPa

45W 5S

Increase of surface air buoyancy peaks prior to the moisture convergence during the initial phase of the transition.

Moisture convergence

Change of surface buoyancy

Surface latent flux

OnsetOnset

Land surface latent flux initiates the transition.

InitialInitial developdevelop

What is the primary drive of the transition?

Wind (u,w) and Relative humidity(shaded)

10oS

onset maturing

initiating developing

onset maturing

Pre

ssur

e(hP

a)

Longitude

Init. Develop.

onset mature

Humidity begins Humidity begins to increase prior to increase prior to the increase of to the increase of moisture moisture transport.transport.

Aug - Dec.Aug - Dec.

Jan-Mar.Jan-Mar. Aug.Aug. Sept - Mar.Sept - Mar.

wetwetdrydrydrydrywetwet

Fu et al. 1999, J. ClimateFu et al. 1999, J. Climate

In contrary to Asian monsoon, circulation transition to the wet In contrary to Asian monsoon, circulation transition to the wet season is NOT driven by land-ocean temperature reversal in the season is NOT driven by land-ocean temperature reversal in the upper troposphere. It is driven by convection in Amazon.upper troposphere. It is driven by convection in Amazon.

Observations: 7 years radiosonde dataObservations: 7 years radiosonde data

How is transition initiated and accelerated?How is transition initiated and accelerated?

Elevated Elevated heatingheating

Upper troposphere Upper troposphere high formshigh forms

What causes rapid onset?

Synoptic episodes induce increase of rainfall across broad latitude during the transition from dry to wet season.

TRMM daily rainrate, averaged over 5 days

Aug. 1- Nov. 12,

1998-2001

What is the role of cold front incursions?What is the role of cold front incursions?

Cold front index: Cold front index: SLPSLP among among the top 10% for SON season, the top 10% for SON season, SLP≥1018 hPa, SLP≥1018 hPa, TTsfcsfc>8˚C>8˚C

(similar to Garreaud 2000)(similar to Garreaud 2000)

JunJun DecDec

rainfallrainfall

Rainfall associated with cold air incursionRainfall associated with cold air incursion

Li and Fu, 2005,Li and Fu, 2005,Composite for 15 transition seasons (1979-93)Composite for 15 transition seasons (1979-93)

• Rainfall increases in in western Amazon 2 days after the cold front passes 25˚S and move northward into Amazon.

The spatial pattern is very similar to that of rainy area during the wet season onset.

1 day before cold events1 day before cold events 1 day before cold events1 day before cold events

1 day after cold events1 day after cold events1 day after cold events1 day after cold events

Under what condition can cold fronts trigger wet season onset?Under what condition can cold fronts trigger wet season onset?C

ompo

site

of

all c

old

even

ts

Com

posi

te o

f al

l col

d ev

ents

pr

ior

to th

e w

et s

easo

n o

nse

tspr

ior

to th

e w

et s

easo

n o

nse

ts

Com

posite of all cold events th

at C

omposite of all cold even

ts that

triggered wet season

onsets.

triggered wet season

onsets.

Readiness of the large-scale thermodynamic condition is central.Readiness of the large-scale thermodynamic condition is central.

WithdrawWithdraw

Lack of extratropical influence over northern Amazon perhaps contribute to gradual northward withdraw of rainy area associated with the wet season demise.

Processes that control wet season onset:Processes that control wet season onset:

Increase of land surface latent flux initiates the rainfall increases.

Increases of rainfall initiates moisture transport.

Positive feedback between moisture transport and rainfall accelerates the circulation transition.

Cold front incursions trigger the wet season onset when the atmosphere becomes sufficiently unstable.

Year Onset pentad

Calendar date Annual mean rain (mm/day)

79 49 Aug 29-Sep 2 8.280 57 Oct 8-12 6.981 59 Oct 18-22 7.582 68 Dec 2-6 6.083 60 Oct 23-27 6.484 73 Dec 26-31 4.985 61 Oct 28-Nov 1 6.086 73 Dec 26-31 5.787 64 Nov 12-16 5.488 63 Nov 7-11 5.689 63 Nov 7-11 6.490 60 Oct 23-27 5.991 62 Nov 2-6 6.092 66 Nov 22-26 4.593 60 Oct 23-27 6.0

Early onset

Late onset

Late onset

Normal onset

Fu & Li 2004

Interannual variations of the wet season onsetsInterannual variations of the wet season onsets - Can we explain them? - Can we explain them?

Late onset

1979-early

1986-late

1984-late

1990-norm

Early onset: more unstable (lower CINE, higher CAPE) in dry season Later onset: more stable (higher CINE, lower CAPE) in dry season

Fu&Li, 2004, LBA special issue

Influence of thermodynamic condition:Influence of thermodynamic condition:

Normal

Early

late

late

Early Onset: Higher air buoyancy and humidity in dry and transition seasonEarly Onset: Higher air buoyancy and humidity in dry and transition seasonLate Onset: lower air buoyancy and humidity in dry and transition seasonLate Onset: lower air buoyancy and humidity in dry and transition season

Influence of land surface fluxes:

1979-early

1986-late

1984-late

1990-norm

Early onset: lower Bowen ratio in dry seasonLater onset: higher Bowen ratio

1984-late 1990-norm

The differences in atmospheric circulation:

Differences during transition are not as clear as those of land surface.

Abnormally dry/wet land surface during dry season delays initiation of the transition, can strongly delay/accelerate wet season onsets (e.g., 1984 and 86, 1979).

1979-early1986-late

Influence of cold air incursion:Influence of cold air incursion:

a. CINE (kJ/kg)

b. CAPE (kJ/kg)

1983: normal

1979: early

1979

1982: late,

1984

Atmospheric instability cannot explain late onset in 1982.

1982: air is more unstable than “normal” 1983, but with delayed wet season onset.

Sept. Dec Oct.

cold events

Lack of cold air incursion as a trigger appears to delay Lack of cold air incursion as a trigger appears to delay wet season onset, even though the atmospheric wet season onset, even though the atmospheric thermodynamic condition was “ready”.thermodynamic condition was “ready”.

Late onsetLate onset normal onsetnormal onset

What might influence the cold air incursion?What might influence the cold air incursion?

Stronger subtropical jets in 1982 may suppress cold front incursions. ENSO and S. Atlantic SSTs can influence the subtropical jet (e.g.,

Horel and Wallace 1981, Grimm et al. 2000).

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

19821982ClimClim

Can we explain interannual Can we explain interannual variations of the wet season onsets?variations of the wet season onsets?

Year Onset pentad

Calendar date Annual mean rain (mm/day)

79 49 Aug 29-Sep 2 8.280 57 Oct 8-12 6.981 59 Oct 18-22 7.582 68 Dec 2-6 6.083 60 Oct 23-27 6.484 73 Dec 26-31 4.985 61 Oct 28-Nov 1 6.086 73 Dec 26-31 5.787 64 Nov 12-16 5.488 63 Nov 7-11 5.689 63 Nov 7-11 6.490 60 Oct 23-27 5.991 62 Nov 2-6 6.092 66 Nov 22-26 4.593 60 Oct 23-27 6.0

Early onset: Wetter land surface & stronger cold air incursion

Late onsets: drier pre-seasonal land surface

Li and Fu 2005

Late onset: weaker cold air incursions may be due to El Niño

Land surfaceLand surface(soil moisture & (soil moisture &

vegetationvegetation

Extratropical Extratropical Influence (cold Influence (cold fronts fronts incursions)incursions)

Pacific and Pacific and AtlanticAtlantic

Influences (cross-Influences (cross-equatorial flow)equatorial flow)

Why is interannual change of rainfall/wet Why is interannual change of rainfall/wet season onset so complex?season onset so complex?

• These factors can These factors can either work for or either work for or against each other to against each other to influence the wet influence the wet season onset.season onset.

• Importance of land Importance of land surface cannot be surface cannot be represented by represented by correlation and its correlation and its fraction in wet fraction in wet season rainfall.season rainfall.

soil moisture/ soil moisture/ Vegetation Vegetation

Memory and Memory and feedbackfeedback

Key uncertainties in understanding rainfall Key uncertainties in understanding rainfall seasonality and its climate changes:seasonality and its climate changes:

Soil moisture/vegetation memory and Soil moisture/vegetation memory and feedback: feedback:

• How would soil moisture& How would soil moisture& vegetation memory & feedbacks vegetation memory & feedbacks affect subsequently dry and affect subsequently dry and transition season?transition season?

• To what extent can soil/moisture To what extent can soil/moisture feedback mitigate or amplify the feedback mitigate or amplify the externally forced rainfall externally forced rainfall variability?variability?

Interference between various Interference between various external forcings:external forcings:

• ENSO+NAO, ENSO+SALT on ENSO+NAO, ENSO+SALT on cold air incursioncold air incursion

Human influences:biomass burning, land use

ENSOENSO

AtlanticAtlantic Cold airCold air

OnsetOnset

Biomass Biomass burningburningLand useLand use

MoistureMoisturetransporttransport

Implications to climate changes:Implications to climate changes:

Reduce forest would delay wet season onset and prolong dry season, esp. in the areas where dry season is already 3-5 months, “savannization” appears to be highly probable.

Changes in latitudinal SST gradient in South Pacific and Atlantic: subtropical jets, extratropical cold air incursions

Nobre 2004Nobre 2004

Remote Influence of Amazon RainfallRemote Influence of Amazon Rainfall

- - The influence of Amazon rainfall on NAOThe influence of Amazon rainfall on NAO

Amazon rainfall can amplify NAO during boreal winters.– Data: QuikSCAT

and TRMM for winters of 1999-2004.

- The influence of Amazon rainfall on Atlantic ITCZ- The influence of Amazon rainfall on Atlantic ITCZ::

Day -3

Day -2

Day -1

Day 0

Day 1

Day 2

Day 3

Composite of Rainfall and Ocean Surface Wind Anomalies

April 2000-2003

Eastward propagation Kelvin wavesPhase speed: 10-12 m/s

Wang & Fu 2005, Data: TRMM & QSCAT

West Phase

East Phase

Future ChallengesFuture Challenges

• In current GCMs, In current GCMs, dry season rainfall is dry season rainfall is too low in current too low in current climate to sustain climate to sustain rainforest.rainforest.

Duration of rain ~ 30 Duration of rain ~ 30 mm/mon:mm/mon:

Obs. 3 months Obs. 3 months Models: > 5 monthsModels: > 5 months

Future ChallengesFuture Challenges

Rainfall predictions for Rainfall predictions for the 21st. century in the 21st. century in the 11 models in the 11 models in CMIP-IPCC AR4:CMIP-IPCC AR4:

• 4 models: increase 4 models: increase rainfall significantlyrainfall significantly

• 5 models: no 5 models: no significant rainfall significant rainfall changechange

• 2 models: decrease of 2 models: decrease of rainfall significantly. rainfall significantly. HadCM3: stronger HadCM3: stronger interannual changes.interannual changes.

How can we reduce the uncertainty in understanding and predicting long-term rainfall variabilities in past and future?

Surface heat fluxes comparison

0

20

40

60

80

100

120

140

160

180

19 22 25 28 31 34 37 40 46 49 52 55

pentad

LH_obs

LH_ERASH_obs

SH_ERA

Apr May Jun Jul Sep

W m

-2

1993 1992ECMEF: LH

Obs:LH

ECMEF:SHObs:SH

Comparison between ECMWF and ABRACOS in situ observations

V index of the MonsoonV index of the Monsoon

V index: (65-75W, 5S-5N)Lu and Chan (J. Climate 1999): A unified monsoon index for South China.

convectionconvection

CINECINE

CINECINE

850 mb850 mb

400 mb400 mb

eses

Seasonal Cycle Influenced by Pacific SSTsSeasonal Cycle Influenced by Pacific SSTs

Seasonal Cycle Influenced by Pacific SSTsSeasonal Cycle Influenced by Pacific SSTs