Onset and End of the Rainy Season in South America in Observations and the ECHAM 4.5 Atmospheric General Circulation Model

LIEBMANN, BRANT; CAMARGO, SUZANA; SETH, ANJI; MARENGO, J.; CARVALHO, LEILA; ALLURED, DAVE; FU, RONG; VERA, CAROLINA

JOURNAL OF CLIMATE

American Meteorological Society

Lugar: Boston, Ma, USA; Año: 2007 vol. 20 p. 2037 - 2050

0894-8755

Rainfall in South America as simulated by a 24-ensemble member of the ECHAM 4.5 atmosphericgeneral circulation model is compared and contrasted with observations (in areas in which data are available)for the period 1976–2001. Emphasis is placed on determining the onset and end of the rainy season,from which its length and rain rate are determined.It is shown that over large parts of the domain the onset and ending dates are well simulated by the model,with biases of less than 10 days. There is a tendency for model onset to occur early and ending to occur late,resulting in a simulated rainy season that is on average too long in many areas. The model wet season rainrate also tends to be larger than observed.To estimate the relative importance of errors in wet season length and rain rate in determining biases inthe annual total, adjusted totals are computed by substituting both the observed climatological wet seasonlength and rate for those of the model. Problems in the rain rate generally are more important thanproblems in the length.The wet season length and rain rate also contribute substantially to interannual variations in the annualtotal. These quantities are almost independent, and it is argued that they are each associated with differentmechanisms.The observed onset dates almost always lie within the range of onset of the ensemble members, even inthe areas with a large model onset bias. In some areas, though, the model does not perform well. In southernBrazil the model ensemble average onset always occurs in summer, whereas the observations show thatwinter is often the wettest period. Individual members, however, do occasionally show a winter rainfallpeak. In southern Northeast Brazil the model has a more distinct rainy season than is observed. In thenorthwest Amazon the model annual cycle is shifted relative to that observed, resulting in a model bias.No interannual relationship between model and observed onset dates is expected unless onset in themodel and observations has a mutual relationship with SST anomalies. In part of the near-equatorialAmazon, there does exist an interannual relationship between onset dates. Previous studies have shown thatin this area there is a relationship between SST anomalies and variations in seasonal total rainfall.