Some observed characteristics of frontal systems in the Amazon Basin

The precipitation and the subsequent decline in temperature at Manaus (3 ° S, 60 ° W) associated with strong cold frontal passages occur 1–3 days after those effects are observed at Sao Paulo (23 ° S, 46 ° S). The average rainfall of 40.4 mm day−1 on the day of frontal passage over the Amazon Basin is substantially higher than that on the preceding and succeeding days. The decline in maximum temperature of about 8 °C in 48 h is much greater than the decline in minimum temperature (3 °C) observed at Manaus. The development of an inverted trough in the lower troposphere over the continent in the subtropics strengthens the southerly winds and makes them reach the Amazon Basin. The extratropical synoptic-scale trough at 925 hPa level, associated with cold front over the Amazon Basin in the South Atlantic moves from 40 ° W to 25 ° W in 2 days. The vertical westward tilt of the anomalous low is about 12 ° longitude from 925 hPa to 200 hPa. The rainfall has no correlation with either the temperature or the decline in temperature associated with fronts in the Amazon Basin. During cold frontal events, especially in winter cases, the transport of moisture from the Amazon Basin to the south is either weak or absent. The differences between winter and summer cases are large in terms of precipitation, temperature, wind anomalies and eastward and northward movements of the synoptic system.

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