Observed precipitation in the Paraná-Plata hydrological basin: long-term trends, extreme conditions and ENSO teleconnections

BOULANGER J.P.,LELOUP J., PENALBA O., M. RUSTICUCCI, F. LAFON, W. VARGAS

CLIMATE DYNAMICS

SPRINGER

Lugar: Berlin; Año: 2005 vol. 24 p. 393 - 413

0930-7575

The Parana´ -Plata basin is the second largesthydrological basin in South America and is of greatimportance for the countries of the region (Argentina,Bolivia, Brazil, Paraguay and Uruguay). The presentstudy focuses on the long-term trends in basin-scaleprecipitation with special emphasis on the role of distributionchanges in extreme large-scale precipitationevents and on the characteristics and evolution of ENSOteleconnections over the last 50 years. First, we defined aParana´ -Plata basin total precipitation index (PTPI) asthe precipitations spatially averaged over the hydrologicalbasin. On interannual time scales, such an index ismainly representative of anomalous monsoon precipitationsin the northern part of the basin and large convectiveprecipitation anomalies in the center of the basin(Paraguay-southern Brazil-Uruguay-northern Argentina)typical of the canonical ENSO teleconnectionpattern. Our major findings clearly highlight a positivetrend in yearly averaged PTPI mainly from the late1960s to the early 1980s with a strong dependence frommonth-to-month. The largest precipitation increase isobserved from November to May in southern Brazil andArgentina. A close examination of PTPI distributionsduring the two halves of the period 1950?2001 showsthat the changes in the mean state from 1950?1975 to1976?2001 result from significant changes in each calendarmonth mean state and in the tails of the PTPIanomaly distributions in May with lesser and weakerlarge-scale dry events and stronger large-scale wetevents. Further studies will be needed to assess whetherthe observed trend in large-scale extreme precipitationconditions can be related to natural climate variabilityor anthropogenic activities and whether it is associatedto changes in local/regional extreme events. The strongerwet conditions in different months seem to be associatedto changes in ENSO characteristics (amplitude, propagation,spatial structure, ...) since the 1982?1983 ElNin? o. Indeed, spatial ENSO teleconnections (stronger inNovember and April?May) have greatly evolved from1950?1975 to 1976?2001. Moreover, we demonstratethat there is a strong modulation and displacement ofthe teleconnection patterns from one event to another,impeding the definition of robust statistical relationshipbetween ENSO and precipitation in the Parana´ -Platabasin (except maybe over a very limited area near thecommon border between Paraguay, Argentina andBrazil). Finally, the non-antisymmetrical patterns ofprecipitation between El Nin? o and La Nin? a conditionsand the non-linear relationship between precipitationand either Nin? o3.4 or Nin? o1+2 sea surface temperatureindices show that linear statistical forecast systems areactually of very limited use for impact predictions onsociety on a local or regional scale.