The South American Monsoon System and the 1970s climate transition

Abstract

ABSTRACT:The South American Monsoon System (SAMS) is characterised by intense convective activity and precipitation during austral summer. This study investigates changes in the onset, demise and duration of SAMS during 1948SAMS during -2008. The results show a significant change in these characteristics in the early 1970s. Onset becomes steadily earlier from 1948 to early 1970s and has occurred earlier than 23-27 October after 1972-1973. Demise dates have remained later than 21-25 April after the mid-to-late 1970s. SAMS duration shows a statistical changepoint in the summer of 1971-1972 such that the mean duration was ∼170 days (1948-1972) and 195 days (1972-1982). Vertically integrated moisture flux is used to diagnose changes in mean state and reveal statistically significant increases over South America after 1971-1972.

Multidecadal variability of moisture and heat budgets of the South American monsoon system

“…Previous papers have all documented an increase in the moisture flux over TSAM after the mid-1970s (Curtis and Hastenrath 1999;Marengo 2004;Carvalho et al 2011).…”

“…Still, regarding multidecadal SAMS variability, Carvalho et al (2011) found an earlier onset and a delayed demise (i.e., an overall increase in duration) of the SAMS rainy season from the beginning of the 1970s. The same authors also argued that a rise in the vertically integrated moisture flux over TSAM favored the observed precipitation increase over the southern Amazon andCentral Brazil after 1971-1972. Analyzing rainfall over the Amazon for the 1929-1998 period, Marengo (2004) discovered shifts in rainfall regimes in both northern and southern sub-basins in 1946-1947and 1976-1977, consistent with PDO climatic shifts.…”

“…One aspect of South American climatic variability which has been extensively investigated and which is related to the SAMS is the relationship between the low-frequency variability modes, mostly manifested in the sea surface temperature (SST) field, with rainfall variation over continental South America (Zhou and Lau 2001;Nogués-Paegle and Mo 2002;Grimm 2003Grimm , 2004Marengo 2004;Andreoli and Kayano 2005;Grimm et al 2007;Kayano and Andreoli 2007;Kayano et al 2009;Grimm and Zilli 2009;Carvalho et al 2011). One of these modes, the Pacific Decadal Oscillation (PDO), is part of the interdecadal variability recorded in the Pacific, with anomaly patterns of the SST, sea level pressure (SLP), and wind stress fields similar to those of the El Niño-Southern Oscillation (ENSO) (Mantua et al 1997;Zhang et al 1997;Garreaud and Battisti 1999;Mestas-Nuñez and Enfield 2001).…”

Severe Convection Features in the Amazon Basin: A TRMM-Based 15-Year Evaluation

“…The austral winter corresponds to the dry season in most of the region, although it represents a rainy maximum for the far northwest of the basin. Well-defined wet and dry seasons are associated with the so-called South American Monsoon System, hereafter SAMS (Carvalho et al, 2011). Within the SAMS, the South Atlantic Convergence Zone (SACZ) represents a dominant feature of the wet season and is seen as a wide region of rainfall extending from the northwest to the southeast of the Basin and reaching the southern Atlantic.…”

La Niña events before and after 1979 and their impact in southeastern South America during austral summer: role of the Indian Ocean

“…Studies have investigated regional anomalies of atmospheric circulation , the effects of low-level circulation (Nogués- Paegle & Mo 1997, Salio et al 2002, Liebmann et al 2004, Silva et al 2009), the effects of soil conditions in regions close to SESA (Grimm 2003, Grimm et al 2007, Grimm & Zilli 2009, changes in the onset and duration of the monsoon season during long-term periods (Carvalho et al 2011), and the influence of SST anomalies in oceans other than the Pacific.…”