# Presentation: 716415

## Interannual Variability of the Rainy Season in the Tropics of South America

- Brant Liebmann
- NOAA-CIRES Climate Diagnostics Center
- Boulder, Colorado, USA
- José A. Marengo
- Centro de Previsão de Tempo e Estudos Climáticos
- Cachoeira Paulista, Brasil
-  Anji Seth
- Suzana Camargo
- International Research Institute for Climate Prediction
- The Earth Institute at Columbia University
- Palisades, New York

## Themes:

- The variability of the initiation (and of the end) of the rainy season is important for determining calendar season rainfall totals (when the initiation or end occurs during the season of interest).

- b)  When there is a relationship between seasonal rainfall anomalies and sea surface temperature (SST), it is almost always through the relationship between SST and the variation of the starting or ending date.

## Rainfall: 1976-1999

- We wish to thank the following agencies:
- Agência Nacional de Energia Elétrica (Brasil)
- UADASC - AASANA - Santa Cruz (Bolivia)
- Ministerio del Ambiente y los Recursos Naturales (Venezuela)
- National Climatic Data Center (USA)
- FUNCEME, SIMEPAR, DAEE (Brasil)
- C.T.M. Salto Grande (Uruguay - Argentina)
- Administración Provincial de Agua (Argentina)
- Servicio Meteorológico Nacional (Argentina, Paraguay, Uruguay)
- Meteorologische Dienst Suriname
- METEO-France (French Guiana)
- UTE (Uruguay)

## Slide 4

## September-November

- climatology

- December-February
- climatology

## March-May

- climatology

- June-August
- climatology

## Linear Correlation

- Niño 3.4 =
- SST in the
- region
- 5N - 5S
- 170W -120W

- (simultaneous)

## A(day) = Anomalous accumulation

*A(day)* = Anomalous accumulation

*R(n)* = daily rainfall

*R* = annual average daily precipitation

- The rainy season is considered to be when the slope of the curve is positive (*R(n) **>** R*).

## Average date of initiation: 30 December

- East Central Amazon

## East Central Amazon

- composite about initiation and end

- day relative to initiation or end

- rainfall (mm/day)

## 1982-83

- (6 March onset)

## Month of initiation of rainy season

- (Climatology)

- october

- november

- december

- january

- february

- march

- april

## Slide 13

## Slide 14

## Month of end of rainy season

- (from climatology)

- november

- october

- september

- august

- july

- june

- march

- april

- may

## Correlations:

- Initiation versus DJF total: -0.86

- Rate versus DJF total: 0.77

- Rate versus initiation date: -0.56

- .....in the east central Amazon Basin, the average date of onset is 30 December

- Standard Deviation: 28 days

- Therefore, usually
- the rainy season begins during DJF.

- Therefore, the variability of the day of initiation
- Is important in determining the DJF total.

- The rate of precipitation is defined as the total
- between the initiation and the end of
- February divided by the number of days.

## DJF total rainfall versus SST

- Start versus SST

- Rate (start to end of February) versus SST

## Average ending date = 13 june

- AMJ rain versus SST

- Rate (1 april to end of rainy season) versus SST

- End date versus SST

- 2.5N-2.5S
- 60W-55W

## Average ending date = 20 June 

- 2S-4N
- 56W-49W

- Average ending date = 20 June

- May-July rain versus SST

- Rate (1 May to end) versus SST

- Ending date versus SST

## Summary:

- Variability of the start (and end) of the rainy
-  	season is important in determining the quantity of rain during the calendar season (when the start or end occurs during the season of interest).

- b)  When the variability of total rainfall during the rainy season is related to SST, this relationship usually is due to the relationship between SST and the variation of the start (or end) of the rainy season.

## General Circulation Model

- European Centre / Hamburg Model (ECHAM-3)
- (Used operationally at IRI until January 2002)
- 1 member (of 8)
- Run 50 years with observed SST (we use 1976-1997)
- Precipitation fields only
- Horizontal resolution: T42
- Vertical resolution: 19 Levels
- Tiedke mass flux convection scheme
- Modified SiB land surface scheme

## Same, except ECHAM-3

- December-February Climatology
- 1976/77 – 1996/97

## March-May Climatology

- Same, except ECHAM-3

## June-August Climatology

- Same, except ECHAM-3

## September-November Climatology

- Same, except ECHAM-3

## GCM uses 360 day year

- oct

- nov

- dec

- jan

- feb

- mar

- apr

## Note different

- scales

## ~ during wet season:

-                                 OBS               GCM
- % dry days                 7.0                 8.4
- rainy day average     10.1                 9.0

## Dec-Feb 

- Dec-Feb
- Obs. rain vs. Nino 3.4 SST

- Same, except ECHAM-3

## Dec-Feb rain vs. nino 3.4 SST

- Regression for 1 std SST anomaly

- observed

- ECHAM-3

## Note different scales

## Slide 32

## Slide 33

## Slide 34

##                               Grid point at 56W, 1.4S

-                                         Observations           GCM
-         Average onset=       30 December         13 December
-         Std of onset=           28 days                   48 days

- Using ensembles may reduce standard deviation of
- onset date

## Slide 36

## Slide 37

## We argue that the influence of SST on seasonal total

- precipitation is through its influence on the timing of the
- rainy season.

- Conclusions

- The same relationship may hold in a GCM, but if timing
- is biased or spread is erroneous, it will appear that the
- GCM does not properly simulate calendar season
- interannual variability.

- Perhaps bias and spread will be improved through
- utilization of ensembles.

- Statistical techniques (e.g., adjusting for bias) may
- improve interannual simulations.