# Presentation: 914962

## The South American Monsoon System: Recent Evolution and Current Status

**Update prepared by**
**Climate Prediction Center / NCEP**
**28 November 2008**

**For more information, visit:****  **_[http://www.cpc.ncep.noaa.gov/products/Global_Monsoons/American_Monsoons](http://www.cpc.ncep.noaa.gov/products/Global_Monsoons/American_Monsoons)_

## Outline

** ****Highlights**
** ****Recent Evolution and Current Conditions**
** ****NCEP/GFS Model Forecasts**
** ****Climatology**

## Highlights

** ****The heavy rains in Santa Catarina, Brazil during the past week were associated with an unusually intense high pressure system over the western South Atlantic that created a very strong onshore pressure gradient. Those conditions favored upward motion along the Sierra do Mar, which was enhanced by the 500-hPa trough.**** ****See slides 7 to 17.**

**Highlights**

## Rainfall Anomaly Patterns:

**Rainfall Anomaly Patterns:**

_**NOTE:**_** ****Due to a lack of station reports, the rainfall deficits over northern Colombia and southern Venezuela are not real (red circle).**

**Last 90 Days**

**Last 30 Days**

**Last 7 Days**

**Long-term rainfall deficits over the southern Amazon, central and southeastern Brazil  decreased during the last 30 days. During the last 7 days rainfall was near or above normal over most of the monsoon region (Central and Southeastern Brazil), and below normal over southwestern Brazil, Bolivia, Paraguay and northern Argentina. Much above normal rainfall was observed over coastal Santa Catarina. Long-term (90 & 30 days) rainfall deficits are evident over Uruguay and eastern Argentina. **

## Recent Evolution: Rainfall

** ****90-day rainfall totals are below normal in the southern Amazon (deficits between 120-170 mm), and in central and southeastern Brazil (deficits ****between 100-150 mm****).**

## Atmospheric Circulation Recent 7 days

**During 19-25 November anomalous 200-hPa  cyclonic circulation was centered near the coast of Southern Brazil, with anomalous upward motion (negative omega) observed from the southern Amazon basin to southeastern Brazil. These features are consistent with an enhanced South Atlantic Convergence Zone (SACZ) and wet conditions observed in that region (see slide 4). **

**A**

**C**

## Circulation Features over South America Accompanying the Heavy Rains over Santa Catarina, Brazil

- H

- H

- L

**On 16 November, a 500-hPa trough was located over the western Atlantic and a surface cold front was located along the Southeast Brazil coast. High pressure and sinking motion (positive omega, upper left panels) dominated northern Argentina, Uruguay and extreme southern Brazil.**

## H

- L

**On 17 November, the 500-hPa trough was located over the Atlantic and a surface cold front was slowly moving northeast along the Southeast Brazil coast. High pressure and sinking motion covered northern Argentina, Uruguay and southern Brazil.**

## H

- L

**On 18 November, another short wave 500-hPa trough moved northward and reinforced the mean trough over the Atlantic. The surface cold front was nearly stationary along the Southeast Brazil coast. A strong high pressure system and sinking motion moved northward behind the 500-hPa trough.**

## H

**On 19 November, the second short wave 500-hPa trough moved northeastward over the Atlantic. The surface cold front remained nearly stationary along the Southeast Brazil coast. A strong high pressure system and sinking motion moved northward behind the 500-hPa trough. This surface high pressure system created a strong onshore pressure gradient along the coast of southern Brazil.**

## H

**On 20 November, the second short wave 500-hPa trough split with a portion moving eastward over the Atlantic and a portion remaining along the coast of southern Brazil. The surface cold front moved northeastward along the Brazil coast. A strong high pressure system dominated the western Atlantic. A strong onshore pressure gradient and rising motion occurred along the coast of southern Brazil.**

## H

**On 21 November, a 500-hPa short wave trough remained along the coast of southern Brazil. The surface cold front moved slowly northeastward along the Brazil coast. A strong high pressure system dominated the western and southern Atlantic. A strong onshore pressure gradient and rising motion occurred along the coast of southern Brazil.**

## H

**On 22 November, a closed 500-hPa low formed along the coast of southern Brazil. The surface cold front weakened along the NE Brazil coast. A strong high pressure system continued to dominate the western and southern Atlantic. A strong onshore pressure gradient and rising motion continued along the coast of southern Brazil.**

- L

## H

**On 23 November, a weak closed 500-hPa low remained along the coast of southern Brazil. A strong high pressure system continued to dominate the western and southern Atlantic. A strong onshore pressure gradient and rising motion continued along the coast of southern Brazil.**

- L

## The heavy rains in Santa Catarina, Brazil were associated with an unusually intense high pressure system over the western South Atlantic that created a very strong onshore pressure gradient. Those conditions favored upward motion along the Sierra do Mar, which was enhanced by the 500-hPa trough. 

## 925-hPa Temperature 

**During the last 7 days (19-25 November 2008), near-normal temperatures were observed over most of Brazil, with the exception of southern Brazil where below-normal temperatures were observed.**

**Recent 90 Days**

**Recent 30 Days**

**Recent 7 Days**

**A**

## NCEP/GFS Model Forecasts  Bias-Corrected Precipitation

**Week 1**

**Forecasts from 28 November 2008**

**Week 2**

_**Note:**_** Bias correction based on last 30-day forecast error. **

## For week 1 (28 Nov – 4 Dec2008), above-normal rainfall is predicted for the northeastern third of Brazil. Below-normal rainfall is predicted for the western Amazon basin, portions of southeastern and southern Brazil, Paraguay, Ecuador and Peru.
For week 2 (5-11 Dec 2008) above-normal rainfall is predicted for most of the Amazon basin. Below-normal precipitation is forecast for most of eastern and southern Brazil.

**For week 2 (5-11 Dec 2008) above-normal rainfall is predicted for most of the Amazon basin. Below-normal precipitation is forecast for most of eastern and southern Brazil.**

**NCEP/GFS MODEL FORECASTS**

_**NOTE**_**:**** See forecast verification in the next slide.**

## Forecast Verification

**Forecast from  22 November  2008 **
**Valid 22-28 Nov 2008**

**Observed**
** ****21-27 Nov 2008**

## ClimatologyRainy Season Dates

**ONSET**

**DEMISE**

## Precipitation Climatology