The South American Monsoon System Summary

July 2011-June 2012

Update prepared byClimate Prediction Center / NCEP

June 2012

For more information, visit:

http://www.cpc.ncep.noaa.gov/products/Global_Monsoons/American_Monsoons

Summary• The rainy season onset phase (SON) featured large rainfall deficits

exceeding 270 mm over the western Amazon basin, and near-average rainfall over the monsoon core region (central Brazil). La Nina conditions were present in the tropical Pacific.

• During the mature phase of the monsoon (DJF) rainfall was below average over most of Brazil and the northern half of Argentina, with rainfall deficits greater than 210 mm in many areas. La Nina conditions began to weaken, as negative SST anomalies decreased in magnitude in the extreme eastern equatorial Pacific.

• During the decay phase of the rainy season (MAM), below-average rainfall was observed throughout most of Brazil, with deficits greater than 270 mm observed over the eastern Amazon basin and northern Northeast Brazil, in part associated with the dry phase of the MJO (see slide 12). SSTs were near average in central equatorial Pacific, and above average in the extreme eastern equatorial Pacific.

• In general, the pattern of rainfall departures for the water year (July 2011-June 2012) is much drier over most of Brazil when compared with previous La Nina episodes.

Outline

• Onset phase

• Mature phase

• Decay phase

• Long-Term Departures from Average

• Summary

Precipitation Climatology

ONSET DATESONSET DATES

Precipitation Departures: Onset Phase (Sep-Nov 2011)

During September-November 2011 rainfall was below average over most of western, central-western and southern Brazil, with rainfall deficits exceeding 270 mm in parts of the Amazon basin. Much above-average rainfall was observed over Colombia and portions of Peru. Central and Northeast Brazil received near to above-average rainfall.

Tropical Pacific and Atlantic SST Anomalies (Sep-Nov 2011)

During September–November 2011, equatorial SSTs were below average over most of the Pacific Ocean. This pattern is consistent with La Niña conditions. In the Atlantic, equatorial SSTs were slightly above average from northeastern South America to the West African coast. Negative SST anomalies dominated the subtropical South Atlantic Ocean.

Precipitation Departures: Mature Phase (Dec 2011-Feb 2012)

During December 2011-February 2012 rainfall was below average over most of Brazil and northern Argentina (red oval, with the exception of northern Paraguay), with rainfall deficits greater than 210 mm in many areas. Much above-average rainfall was observed over northwestern South America.

Tropical Pacific and Atlantic SST Anomalies (Dec11-Feb12)

During December 2011–February 2012, negative SST anomalies persisted in the central equatorial Pacific and weakened in the extreme eastern equatorial Pacific. This pattern is consistent with NOAA’s definition of La Niña. Over the Atlantic equatorial, SSTs were slightly below average from northern South America to the African coast.

Precipitation Climatology

END DATESEND DATES

Precipitation Departures: Decay Phase (Mar-May 2012)

During March-May 2012 rainfall was below average throughout most of Brazil, with deficits greater than 270 mm observed over the eastern Amazon basin and northern Northeast Brazil. Above-average rainfall was observed over central Colombia, northern Peru and portions of the western Amazon basin in Brazil.

Tropical Pacific and Atlantic SST Anomalies (Mar-May 2012)

During March-May 2012, equatorial SSTs were above average in the Pacific Ocean near South American coast and near average in the central Pacific, as La Niña conditions weakened. In the Atlantic, SSTs were slightly below average in many areas in the Tropics and Subtropics.

200-hPa Velocity Potential Anomalies (5°S-5°N)

The loop above represents the climatological MJO propagation. As you can see the MJO has some influence over portions of Southeastern Brazil (near the location of the South Atlantic Convergence Zone – SACZ) and Northeastern Brazil.

The 200-hPa velocity potential anomaly field shows that the dry phase of the MJO was centered over the longitudes of Brazil during mid-February to the end of March, which probably contributed to the severe dry conditions observed over Northeastern Brazil during that period.

Time

Longitude

Outgoing Longwave Radiation (OLR) Matthews, 2000

MJO propagation – Wet & Dry Phases

Precipitation Departures: July 2011- June 2012

During 1 July 2011- 17 June 2012 rainfall was below average throughout most of Brazil, and near average over the remainder of South America, except for above average over central Colombia, Ecuador and northern Peru.

Precipitation Anomaly Patterns during La Niña Episodes over Brazil

From Silva et al., 2007

2011-2012

The pattern of anomalous precipitation over Brazil for the water year (July 2011-June 2012, figure above) shows much drier conditions over most of Brazil when compared with previous La Nina episodes.

Summary• The rainy season onset phase (SON) featured large rainfall deficits

exceeding 270 mm over the western Amazon basin, and near-average rainfall over the monsoon core region (central Brazil). La Nina conditions were present in the tropical Pacific.

• During the mature phase of the monsoon (DJF) rainfall was below average over most of Brazil and the northern half of Argentina, with rainfall deficits greater than 210 mm in many areas. La Nina conditions began to weaken, as negative SST anomalies decreased in magnitude in the extreme eastern equatorial Pacific.

• During the decay phase of the rainy season (MAM), below-average rainfall was observed throughout most of Brazil, with deficits greater than 270 mm observed over the eastern Amazon basin and northern Northeast Brazil, in part associated with the dry phase of the MJO (see slide 12). SSTs were near average in central equatorial Pacific, and above average in the extreme eastern equatorial Pacific.

• In general, the pattern of rainfall departures for the water year (July 2011-June 2012) is much drier over most of Brazil when compared with previous La Nina episodes.