The South American Monsoon System: Recent Evolution and Current Status

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The South American Monsoon System Recent
Evolution and Current Status
Update prepared by Climate Prediction Center /
NCEP 21 December 2007
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Outline

• Recent Evolution and Current Conditions
• NCEP/GFS Model Forecasts
• Summary
• Climatology

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Rainfall Patterns Last 90 Days
During the last 90 days rainfall has been below
normal over most of Brazil, with rainfall
deficits between 175-250 mm in portions of
central and southeastern Brazil. Deficits of up
to 175 mm are found in northern Argentina.
Above-normal rainfall was observed over Bolivia,
Paraguay and portions of southern Brazil.
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Rainfall Patterns Last 30 Days
During the last 30 days rainfall has been near
normal over northern Brazil, and below normal
over central, southeastern and southern Brazil.
Above-normal rainfall was observed over Bolivia
and northern Paraguay, and below-normal rainfall
was observed over Uruguay and the northern half
of Argentina.
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Rainfall Patterns Last 7 Days
During the last 7 days rainfall has been slightly
above normal over the northwestern Amazon and
eastern Brazil, while below-normal rainfall was
observed over the southern Amazon, central and
southern Brazil, and Uruguay.
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Atmospheric Circulation Last 7 days

• During the last 7 days a strong 200-hPa
  anticyclonic circulation was centered over
  Bolivia. A well defined trough was located at low
  latitudes over the western Atlantic.
• Anomalous low-level southeasterly flow extended
  from the Atlantic ocean towards southern Brazil,
  Paraguay and Bolivia (top-right panel). This
  anomalous flow represents a weaker-than-normal
  flow of moisture from the Amazon Basin towards
  southern and southeastern Brazil.

A
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Recent Evolution Rainfall

• The SAMS wet season was delayed in many areas.
• Since October, rainfall has increased over
  western Brazil.
• Rainfall deficits continue over the Brazilian
  Plateau (BP), with deficits between 120-170 mm
  during the last 3 months.

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NCEP/GFS Model Forecasts Bias-Corrected
Precipitation
Week 1 (22-28 December)
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NCEP/GFS Model Forecasts Bias-Corrected
Precipitation
Week 2 (29 Dec 07 4 Jan 08)
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Summary

• Rainfall has been slightly above normal in
  western Brazil and much below normal in central
  and southeastern Brazil during the last 90 days.
• The rainy season was slow to start over central
  and southeastern Brazil, where rainfall deficits
  of 100-200 mm (locally up to 250 mm) have been
  observed during the last 90 days.
• During the last 7 days rainfall has been slightly
  above normal over the northwestern Amazon and
  eastern Brazil and below normal over southern
  Amazon, central and southern Brazil.
• NCEP/GFS MODEL
• For week 1 (22-28 Dec) the model forecast is for
  below-normal rainfall over most of Brazil, with
  the exception of the Amazon region.
• Below-normal rainfall is forecast over central
  and southeastern Brazil during the entire period
  (22 Dec 2007- 4 Jan 2008).

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Information About the NCEP/GFS Model Forecasts

• The forecast uses four operational NCEP/GFS
  forecast runs from 00Z, 06Z, 12Z, and 18Z initial
  conditions.
• 2. PDF (Probability Density Function) correction
  is made for each forecast
• - For each individual forecast and each grid
  point, cumulative PDFs for both forecasts and
  observations (CMORPH) are defined using values of
  the past 35 days within 15X15 boxes.
• - GFS forecasts are replaced by observations at
  the same percentile level of the cumulative PDFs.
• 3. Forecast anomalies are taken as the
  differences of the bias-corrected ensemble mean
  from the 2003-2006 CMORPH average.
• 4. Week-1 forecast is at 1-day lead and week-2
  forecast is at 8-day lead.

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ClimatologyRainy Season Dates
ONSET
DEMISE
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Precipitation Climatology