On precipitation anomalies over Central Andes during ENSO episodes

On precipitation anomalies over central Andes during ENSO episodes

Precipitation over the central Andes in South America exhibits a marked interannual variability with a tendency toward dryness (wetness) during ENSO (non-ENSO) episodes.

Such relationship is supported by important simultaneous correlations between the NINO-4 Outgoing Longwave Radiation (OLR) index with the precipitation over a specific region in the area; e.g. the Peru-Bolivian Altiplano (PBA). A similar conclusion may be obtained from Empirical Orthogonal Functions (EOF) analysis of the OLR over the Pacific Ocean and the South American continent.

These findings are highly suggestive of the existence of an atmospheric teleconnection between the Southern Oscillation (SO) and the precipitation regime over the PBA ; but instead of trying to find statistical significance of the correlation, we rather try to identify the physical mechanisms responsable for their existence.

A precipitation dipole is found between a region in Northern Peru and Southern Ecuador (NPSE) and another in the PBA zone. During ENSO (non-ENSO) years, positive (negative) anomalies are present over the NPSE (PBA) region.

The precipitation in the area is produced by atmospheric perturbations associated with the SPCZ, that enter into South America from the Pacific Ocean in central Chile latitudes. They crossed the Andes in the form of mid-tropospheric anticyclonic circulations and in some cases they move along the eastern flank of the Andes. During the ENSO episodes, the perturbations are so energetic that can reach northern Peru latitudes, leaving without precipitation the PBA region. On the contrary, during non-ENSO episodes they are only able to reach southern Peru latitudes with negative rainfall anomalies in the north.

As it has been stated before, the atmospheric conditions related to this study are the ones associated with the SPCZ. Whenever it is closer (farther) to the South American continent as during ENSO (non-ENSO) years, the perturbations are able to reach northern (only southern) Peru.

Finally, with a strong physical support for the teleconnection, we may use this knowledge to predict the rainfall regime over the PBA region with some months in advance. To do that, we can predict the future NINO 4 OLR index based on the forecasted SST routinely produced by several institutes in the world wit some monts in advance. Now having an estimation of the OLR index, we can get an idea of the future rainfall regime over the PBA region based on the stated teleconnection. The performance of the prediccion scheme is presented.