What controls the seasonality of the Amazon rainfall and

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					  What controls the seasonality of the
  Amazon rainfall and its interannual
     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
                                       Ecosystem and agriculture of
                                       the Amazônia depend more on
                                       the length and rainfall of the
                                       dry season than on wet season
                                       rainfall. Yet, most of the
                                       meteorological studies have
                                       been focus only on the wet
 Length of dry season: Sombroek 2001
                                       season rainfall.

Regions with dry season rainfall <
100mm could potentially be converted
to Savanna given enough of
deforestation - Steinberg 2001         Malhi 2004

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

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

Kousky 1979; Horel et al. 1989; Marengo et al. 2000
            Change of seasonal cycles is a main source of
              interannual variation of annual rainfall:
                                             Liebmann and Marengo 2001

     Marengo et al. 2001

                   Jul     Dec

What cause such strong variations
of wet season onset?
What control the changes of rainfall and wet season onset in
the western Amazon?

 Oceanic influences cannot be clearly detected in the western

              El Nino related                    El Nino+cold

Ronchail et al. 2002
                           Key issues-1:
   What is the primary forcing of the wet season
    – 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
• 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:
   What are the relative roles of land surface flux
    and moisture transport in determine the
    seasonality of the rainfall, esp. the wet season
     –   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:
      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 25S, 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                                      Summer                            Winter
trigger the wet season
onset given adequate
conditions over the
                                   Convective Clouds associated with cold front incursions (Garreaud
                                   & Wallace 1998)
                                       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                 Marengo et al. 2001
       and Electric Energy Agency of Brazil
    – GPCP precipitation data (daily, 2.5 lat x
       2.5 lon)
    – TRMM daily rainrate
    – Radiosonde: 7 years
    – ABRACOS flux tower data, 1992 and
 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)

                                               Li & Fu 2004        gauge

                                                          15-yr composite results
    200hPa                                      initiating developing Onset

                                                                               850mb Equivalent potential
                                                                               temperature change with time

                                                                                  Kinetic energy

                                                                                  conversion function

                           45W 5S                     V-index
1000hPa          45W 15S                     ECMWF


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

     Li and Fu 2004                                                    pentad
How is transition initiated?
           What causes increase of air buoyancy near surface?
                 How does it influence convective instability?

                                                            Lc/Cp* q/ t

                                      e/t  e/T*(T/t+Lc/Cp* q/ t)




                                           initiating   pentad   maturing

  •     Increase of air humidity dominates the increase of air                             CINE
        buoyancy near surface
  •     Increase air buoyancy rapidly reduce the convecitve
        inhibition energy (CINE) prior to the occurrence of
        moisture convergence.                                                        temperature
Latent flux increase leads to higher surface air buoyancy

                                       Increase of air buoyancy

                                                                  SH: W/m2
    LH: W/m2

                                        Sensible heat

                  Latent Heat

                                                    Solar at

                            Net Radiation           surface

          Land surface latent flux initiates the transition.


                        45W 5S               Change of surface buoyancy

75W 15S   1000hPa   45W 15S

                                                         Moisture convergence
   Increase of surface air
   buoyancy peaks prior
   to the moisture
   convergence during                                     Surface latent flux

   the initial phase of the
                                   Initial    develop Onset
What is the primary drive of the
                 Wind (u,w) and Relative humidity(shaded)
                          initiating                            developing


                          onset                        maturing

Humidity begins
to increase prior          onset                                     maturing

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

                         wet        dry
                                                     dry           wet

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

Observations: 7 years radiosonde data                       Fu et al. 1999, J. Climate
How is transition initiated and accelerated?

   Upper troposphere
   high forms

What causes rapid onset?
         Synoptic episodes induce increase of rainfall
          across broad latitude during the transition from
          dry to wet season.

Aug. 1- Nov. 12,

            TRMM daily rainrate, averaged over 5 days
    What is the role of cold front incursions?

Cold front index: SLP among   Jun              Dec
the top 10% for SON season,
SLP≥1018 hPa, Tsfc>8˚C
(similar to Garreaud 2000)
          Rainfall associated with cold air incursion
                                                • Rainfall increases in in
                                                  western Amazon 2 days
                                                  after the cold front
                                                  passes 25˚S and move
                                                  northward into
                                                 The spatial pattern is
                                                  very similar to that of
                                                  rainy area during the
                                                  wet season onset.

Composite for 15 transition seasons (1979-93)
                                                          Li and Fu, 2005,
Under what condition can cold fronts trigger wet season onset?
                                 1 day before cold events   1 day before cold events

                                                                                       triggered wet season onsets.
                                                                                       Composite of all cold events that
prior to the wet season onsets
Composite of all cold events

                                 1 day after cold events     1 day after cold events

Readiness of the large-scale thermodynamic condition is central.

         Lack of extratropical
          influence over
          northern Amazon
          perhaps contribute to
          gradual northward
          withdraw of rainy
          area associated with
          the wet season
    Processes that control wet season onset:
   Increase of land surface latent flux initiates the rainfall

   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.
          Interannual variations of the wet season onsets
                     - Can we explain them?
                     Year   Onset    Calendar date   Annual
                            pentad                   mean rain
       Early onset    79      49      Aug 29-Sep 2      8.2
                      80      57       Oct 8-12         6.9
                      81      59       Oct 18-22        7.5
       Late onset     82      68        Dec 2-6         6.0
                      83      60       Oct 23-27        6.4
       Late onset     84      73       Dec 26-31        4.9
                      85      61      Oct 28-Nov 1      6.0
        Late onset    86      73       Dec 26-31        5.7
                      87      64       Nov 12-16        5.4
                      88      63       Nov 7-11         5.6
                      89      63       Nov 7-11         6.4
      Normal onset    90      60       Oct 23-27        5.9
                      91      62        Nov 2-6         6.0
                      92      66       Nov 22-26        4.5
Fu & Li 2004          93      60       Oct 23-27        6.0
        Influence of thermodynamic condition:





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




 Influence of land surface fluxes:





Early onset: lower Bowen ratio in dry season
Later onset: higher Bowen ratio
The differences in atmospheric circulation:

                                   Differences during
                                    transition are not as
                                    clear as those of land

                                   Abnormally dry/wet
                                    land surface during dry
       1984-late   1990-norm        season delays initiation
       1986-late   1979-early       of the transition, can
                                    delay/accelerate wet
                                    season onsets (e.g.,
                                    1984 and 86, 1979).
         Influence of cold air incursion:

   1982: air is more    a. CINE (kJ/kg)   1982: late,
                                           1983: normal
    unstable than
                                           1979: early
    ―normal‖ 1983, but
    with delayed wet
    season onset.
                         b. CAPE (kJ/kg)                  1979

   Atmospheric
    instability cannot                     1984

    explain late onset
    in 1982.
     Late onset         cold events             normal onset

  Sept.                    Dec        Oct.

Lack of cold air incursion as a trigger appears to delay
wet season onset, even though the atmospheric
thermodynamic condition was ―ready‖.
What might influence the cold air incursion?

        Clim                               1982

                                 QuickTime™ and a
                             TIFF (LZW) decomp resso r
                          are need ed to see this picture.

   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).
    Can we explain interannual
variations of the wet season onsets?
                              Year   Onset    Calendar date   Annual
                                     pentad                   mean rain
Early onset: Wetter land
surface & stronger cold air    79      49     Aug 29-Sep 2       8.2
incursion                      80      57      Oct 8-12          6.9
                               81      59      Oct 18-22         7.5
Late onset: weaker cold air
                               82      68       Dec 2-6          6.0
incursions may be due to El
                               83      60      Oct 23-27         6.4
                               84      73      Dec 26-31         4.9
Late onsets: drier pre-        85      61     Oct 28-Nov 1       6.0
seasonal land surface          86      73      Dec 26-31         5.7
                               87      64      Nov 12-16         5.4
                               88      63      Nov 7-11          5.6
                               89      63      Nov 7-11          6.4
                               90      60      Oct 23-27         5.9
                               91      62       Nov 2-6          6.0
                               92      66      Nov 22-26         4.5
Li and Fu 2005                 93      60      Oct 23-27         6.0
Why is interannual change of rainfall/wet
        season onset so complex?

                                      • These factors can
                                        either work for or
   Pacific and      Extratropical       against each other to
     Atlantic       Influence (cold     influence the wet
Influences (cross- fronts               season onset.
 equatorial flow) incursions)
           Land surface               • Importance of land
         (soil moisture &               surface cannot be
            vegetation                  represented by
                                        correlation and its
                                        fraction in wet
                                        season rainfall.
     Key uncertainties in understanding rainfall
        seasonality and its climate changes:
                                    Soil moisture/vegetation memory and
Biomass    soil moisture/              feedback:
burning     Vegetation              • How would soil moisture&
Land use   Memory and                  vegetation memory & feedbacks
              feedback                 affect subsequently dry and
                                       transition season?
                                    • To what extent can soil/moisture
                                       feedback mitigate or amplify the
ENSO                                   externally forced rainfall
            Moisture        Onset      variability?
            transport               Interference between various
                                       external forcings:
                                    • ENSO+NAO, ENSO+SALT on
                                       cold air incursion
Atlantic     Cold air               Human influences:
                                       biomass burning, land use
           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
                                      Nobre 2004
     Remote Influence of Amazon Rainfall
        - The influence of Amazon rainfall on NAO

   Amazon rainfall
    can amplify NAO
    during boreal
    – Data: QuikSCAT and
      TRMM for winters of
- The influence of Amazon rainfall on Atlantic ITCZ:
      Composite of Rainfall and
                                                    Day 0
    Ocean Surface Wind Anomalies
             April 2000-2003
                                       West Phase
   Eastward propagation Kelvin waves
        Phase speed: 10-12 m/s
                       Day -3                       Day 1

                       Day -2                       Day 2

                       Day -1                       Day 3

                                                 East Phase

                                       Wang & Fu 2005, Data: TRMM & QSCAT
Future Challenges

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

             Duration of rain ~ 30
                Obs. 3 months
                Models: > 5 months
                   Future Challenges
                                          Rainfall predictions for
                                            the 21st. century in
                                            the 11 models in
                                            CMIP-IPCC AR4:
                                             • 4 models: increase
                                               rainfall significantly
                                             • 5 models: no
                                               significant rainfall
                                             • 2 models: decrease of
How can we reduce the uncertainty in           rainfall significantly.
understanding and predicting long-             HadCM3: stronger
term rainfall variabilities in past and        interannual changes.
              Comparison between ECMWF and
               ABRACOS in situ observations
                                      Surface heat fluxes comparison

        180         LH_obs
        160         SH_obs

        120                                                 Obs:LH

W m-2

                                                ECMEF: LH                   1992

        40                                                    Obs:SH

              19   22     25    28    31   34     37   40       46     49   52     55
                   Apr          May        Jun pentad Jul                   Sep
V index of the Monsoon

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


400 mb


850 mb
Seasonal Cycle Influenced by Pacific SSTs
Seasonal Cycle Influenced by Pacific SSTs