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

 Brant Liebmann

NOAA-CIRES Climate Diagnostics CenterBoulder, Colorado, USA

 José A. Marengo

Centro de Previsão de Tempo e Estudos ClimáticosCachoeira Paulista, Brasil

  Anji Seth

Suzana CamargoInternational Research Institute for Climate Prediction

The Earth Institute at Columbia UniversityPalisades, New York

Themes:

a) 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 SurinameMETEO-France (French Guiana)UTE (Uruguay)

September-Novemberclimatology

December-Februaryclimatology

March-Mayclimatology

June-Augustclimatology

Niño 3.4 =SST in the

region5N - 5S

170W -120W

Linear Correlation(simultaneous)

1

( ) ( ( ) )day

n

A day R n R

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 Amazoncomposite about initiation and end

day relative to initiation or end

rain

fall

(mm

/day

)

1982-83

(6 March onset)

Month of initiation of rainy season(Climatology)

october

november

december

january

february

march

april

Month of end of rainy season(from climatology)

november

october

september

august

july

june

march

april

may

Correlations:Initiation versus DJF total: -0.86Rate versus DJF total: 0.77Rate 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 initiationIs important in determining the DJF total.

The rate of precipitation is defined as the totalbetween the initiation and the end of

February divided by the number of days.

DJF total rainfall versus SST

Start versus SSTRate (start to end of February) versus SST

AMJ rain versus SST

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

Average ending date = 13 june

2.5N-2.5S60W-55W

2S-4N56W-49W

Average ending date = 20 June

May-July rain versus SST

Rate (1 May to end) versus SSTEnding date versus SST

Summary:

a) 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 ModelEuropean 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: T42Vertical resolution: 19 Levels

Tiedke mass flux convection schemeModified SiB land surface scheme

Same, except ECHAM-3December-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

oct

nov

dec

jan

feb

mar

apr

GCM uses 360 day year

Note differentscales

~ during wet season: OBS GCM% dry days 7.0 8.4rainy day average 10.1 9.0

Dec-Feb Obs. rain vs. Nino 3.4 SST Same, except ECHAM-3

Dec-Feb rain vs. nino 3.4 SSTRegression for 1 std SST anomaly

observed ECHAM-3

Note different scales

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 ofonset date

•We argue that the influence of SST on seasonal totalprecipitation is through its influence on the timing of therainy season.

Conclusions

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

•Perhaps bias and spread will be improved throughutilization of ensembles.

•Statistical techniques (e.g., adjusting for bias) mayimprove interannual simulations.