Onset and End of the Rainy Season in the Brazilian Amazon Basin

Onset and end of the rainy season in the Amazon Basin are examined for the period 1979–96. The onset and end dates are determined by averaging daily rainfall data from many stations, and then constructing 5-day averages (pentads). Onset (end) is defined as the pentad in which rainfall exceeds (falls below) a given threshold, provided that average rainfall was well below (above) the threshold for several pentads preceding onset (end), and well above (below) the threshold for several pentads after onset (end). For the criteria chosen, the climatological onset progresses toward the southeast, arriving in mid-October, and then toward the mouth of the Amazon, arriving near the end of the year. The end dates are earliest in the southeast and progress toward the north, but withdrawal is slower than onset. The onset dates, however, are quite sensitive to changes in the threshold. If the threshold is doubled, for example, the sense of onset is reversed, with onset occurring toward the northwest. Changes in threshold do not change the direction of the progression of the end of the rainy season. The central Amazon shows the largest variation in the date of onset. In several years, onset in the southeast occurs before that in the central Amazon, but onset near the mouth is always latest. There is an unexpectedly low relationship between the length of the rainy season and total accumulation. Likewise, there is little relationship between the onset (and end) date and the total accumulation. Composites of outgoing longwave radiation and the low-level wind field show that in the central Amazon, onset is associated with an anomalous anticyclone and enhanced trade winds in the Atlantic. Near the mouth of the Amazon, however, onset is associated with large-scale northerly anomalies, and the zonal component of the trade winds is reduced. There is an apparent association between sea surface temperature anomalies in the tropical Atlantic and Pacific and the pentads of onset and end of the rainy season in the northern and central Amazon, and near its mouth. The sense is that a warm Pacific and cold Atlantic result in a delayed onset and early withdrawal. Although the strong El Niño of 1982/83 and La Niña 1988/89 were examples of a delayed and early onset, respectively, the relationships it still holds these years are not considered.