Low-frequency variability is an important mode of rainfall variation on the Altiplano and in the southern lowlands. We remind that a 25% rainfall increase occurred at the beginning of the seventies.
High-frequency rainfall variability associated to ENSO varies in this heterogeneous space: - During El Nino year, significant positive anomalies on the northern coast of Peru and summer (JFMA) negative anomalies in the highlands (-30%), already known, are confirmed; but the important inter-event variability is documented. - On the Altiplano, unexpectedly, the driest years are as well non-ENSO years than ENSO years (warm or cold). So dryness is not only associated with El Nino. - These results about the Altiplano are somehow persistent in time as they are the same for the 1885-1998 La Paz-San Calixto rainfall serie. - In the southern lowlands, negative significant anomalies only occur during La Nina events in late summer (JFMA) and in winter (MJJA), after the 1970 rainfall rupture.
The first mode of temperature variability in Peru an Bolivia is associated with ENSO with an opposition between the coastal Peru and the Andes in phase with the Pacific sea surface temperature (SST), and the southern lowlands that are out of phase. On the Altiplano, during El Nino, the above normal temperature are significantly due to day temperature (+ 1 to 2°) and not to night temperature. This is consistent with the lack of cloudiness and the negative rainfall anomaly.
The specific comportment of the rainfall and temperature in the lowlands, the specific position of this region, in between those whose ENSO signal is strong, gives way to understand the variability of its circulation. So we analyze the relationship between the low-level meridian circulation in the lowlands and the climatic anomalies in this domain, using the long-time series of reanalisis of the National Center for Environmental Prevision (NCEP).
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