Recent changes in the atmospheric circulation patterns during the dry-to-wet transition season in south tropical South America (1979-2020): Impacts on precipitation and fire season

Abstract We analyze the characteristics of atmospheric variations over tropical South America using pattern recognition framework weather typing or circulation patterns (CPs). During 1979-2020, nine CPs are defined in region, a k-means algorithm based on daily unfiltered 850 hPa winds 0035°N-30°S, 90°W-30°W. primarily interpreted as stages annual cycle low-level circulation. identified three “winter” (CP7, CP8 and CP9), “summer” (CP3, CP4 CP5) “transitional” (CP1, CP2 CP6). Significant long-term changes detected during dry-to-wet transition season (July-October) south (STSA). One wintertime (CP9) increases from 20% 1980s to 35% last decade while decreases 13% 7%. CP9 is characterized by enhancement American Low-Level Jet increasing subsidence STSA. southerly cold-air incursions anomalous convective activity The years high (low) frequency (CP2) associated with delayed Monsoon onset dry conditions Consistently, higher intensifies fire STSA (1999-2020). Over Brazilian states Maranhão, Tocantins, Goiás São Paulo, seasonal explains around 35%-44% interannual counts.