Relation between the onset and end of the South American summer monsoon and rainfall in subtropical South America

摘要

ABSTRACT: This paper explores the relation between the inter-annual variability of the austral South American monsoon onset and end dates and the inter-annual variability of rainfall in September and May in subtropical South America. Outgoing longwave radiation (OLR) was used to define the onset and end dates of the convective season. Precipitation records were taken from national or state agencies of the region. Other surface and tropospheric variables were taken from NCEP/NCAR reanalysis. A 5 d canonical correlation analysis between rainfall in subtropical South America east of the Andes and the tropical convection of the South American monsoon estimated by the OLR shows that these variables are related during the austral autumn and spring. In both seasons, the first modes are very similar and indicate that rainfall in southern Brazil is negatively correlated with convection over the eastern part of the tropical continent. Because a delay (an early advent) of the monsoon end increases (reduces) the period with convection over the tropics, while the contrary occurs with the onset, rainfall composites between months corresponding to cases with extreme dates at the onset or the end of the tropical convective season show an important rainfall signal in southern Brazil during May and September. During May the early (delayed) end date is associated with enhanced (decreased) rainfall in most of the subtropical region. In September, the early (delayed) onset is associated with decreased (enhanced) rainfall only in southern Brazil, while in Argentina and Uruguay there is an opposite signal, non symmetric with respect to the May case. In May, the mean low-level flow of moisture from the tropical continent over eastern subtropical South America is considerably greater in the cases with an advanced monsoon end than in those with a delayed end, while in September, the composite differences between years with an early and a delayed onset show enhanced moisture advection from the tropical continent over most of Argentina and Uruguay, and a reduction over southern Brazil and Paraguay.