# Presentation: 164081

## Observed Trends inSouth American Precipitation

- Brant Liebmann			Vicente Barros
- Carolina S. Vera			Julián Báez
- Leila M.V. Carvalho                       Anji Seth
- Inés Camilloni
- Marty P. Hoerling
- Dave Allured
- Mario Bidegain
- José A. Marengo

## If a flood is defined as monthly flow at Corrientes more than 2 standard deviations above its mean, there were almost 6 times as many floods in the 20 years from 1980-1999 as there were in the 60 years from 1920-1979.

- If a flood is defined as monthly flow at Corrientes more than 2 standard deviations above its mean, there were almost 6 times as many floods in the 20 years from 1980-1999 as there were in the 60 years from 1920-1979.

## Previous work:

- Barros, Castaneda, Doyle (2000): Increase in precipitation over most of Argentina from 1956-1991.
- Castaneda and Barros (1994): Humid Pampa (Argentina) increase in rain, mainly after 1960.
- Robertson and Mechoso (1998): Decadal variability and non-linear upward trend in southeast South America rivers.
- Marengo (2004): Northern Amazon Basin decrease related to El Nino?

## Data 

- We wish to thank the following agencies for providing the data used in this study:
*Agência Nacional de Águas (Brasil)*
*Agência Nacional Energia Elétrica (Brasil)*
*U.T.E. Uruguay*
*C.T.M. Salto Grande*
*Servicio Meteorologico Nacional (Argentina, Paraguay, Uruguay)*
*FUNCEME (Ceará, Brasil)*
*IAPAR - SIMEPAR (Paraná, Brasil)*
*DAEE (São Paulo, Brasil)*
*Minesterio del Ambiente y los Recursos Naturales (Venezuela)*
*Meteorogische Dienst Suriname*
*METEO-France*
*ASANA (Bolivia)*

## Observed Precipitation Climatology

- mm

## Observed Precipitation Trend

- mm/season

## Statistical Relevance of Observed Trends

## Observations versus AMIP runs

- January – March 1976-1999

- mm/season

**Notes:**
Xgsetting trend.color

## Southern Brazil

- 1976-1999 January – March standardized percent trend
- Stations with at least 22 yrs data

## Average onset date = 25 October

- Average end date = 9 April

- 				      _76-81    94-99_
- Rainy day average =           14.8      17.5
- Percent dry days =              57.2      46.0

- Trend in rainy day rain rate
- = 0.078 mm/day/season

- (There is no apparent trend in synoptic activity)

- Southern Brazil

## Simultaneous Correlation (SST versus Southern Brazil Rain)

- JFM SST trend (1976 – 1999)

## January – March rainfall – Southern Brazil

- Slope:
- 1976 - 1999 = 7.20
- 1948 - 1975 = 1.11
- 1948 - 1999 = 1.53

## Slope (m3/s/yr):

- 1976-1999 = 49.2
- 1948-1975 = 20.2
- 1948-1999 = 17.2

- Precipitation has increased by 35% (along trend line)
- River flow has increased by 161%
- (amplification noted by Berbery and Barros (2002))

## SST in southwest Atlantic

- 1976 - 1999 = 0.045 deg/yr
- 1948 - 1975 = 0.022
- 1948 - 1999 = 0.022

- Slope:
- 1976 - 1999 = 7.20 mm/yr
- 1948 - 1975 = 1.11
- 1948 - 1999 = 1.53

- Jan-March rainfall

- Season rainfall (mm/yr)

- Jan-Mar SST (C)

## Running 24-year correlations

- Using 2-sided
*t*-test

- Southern Brazil versus southwest Atlantic
- Trend removed from each segment

- Stations with
- 50 years of data

## Slide 16

## Slide 17

## Running 24-year trends

- Stations with at least 50 years data
- (1 degree grid)

**Notes:**
Southern brazil is 50 yr 1 deg 47w-53w, 23-29s
SACZ is 50 yr Custom 2

## all stations

- Stations with 50 year records

- R = 0.998

- R = 0.92

- R = 0.91 (detrended)

**Notes:**
Centered at 25S, 50W, good data goes back to 1948

## An observed positive precipitation trend during the rainy season in Southern Brazil is most prominent from 1976-1999, but is evident earlier.

- The trend is characterized by an increase in the number of rainy days per season and an increase in the amount per rainy day.

- The precipitation trend is related to one in SST, although perhaps not causally.

- The SST trend seems to result from a decrease in the strength of the South Atlantic high.

- The negative trend near SACZ is not ‘significant’, but
- seems to be indicative of decadal scale variability (near
- the coast). Data in Amazon are of insufficient length to
- draw conclusions.

## http://www.cdc.noaa.gov/people/

- brant.liebmann/data/s_america_rain.html