On extreme precipitation in northern Peru during ENSO episodes

On extreme precipitation in northern Peru during ENSO episodes

Torrential rainfall and severe flodding along the normally arid coasts of northern Peru and southern Ecuador usually cause considerably hardship to the inhabitants of those regions during ENSO episodes. The primary goal of the present paper is to explore the physical mechanisms responsable for such flooding.

Using NCEP-NCAR reanalysis as well as local surface information, we obtained results that suggest that abnormally high coastal ocean temperatures during those episodes may aid the outbreak of convection. However the latitudinal extent and timing of the rainfall was quite different from that of sea surface temperature. These differences suggest that regional and large scale meteorological conditions play the dominant role in determining the severity of the flooding in the region.

The atmospheric reanalysis let us investigate these large scale features associated with the flooding. Several cases in 1983 and 1998 were analyzed, but in the later year, the routine analysis issued by the Center of Weather Forecasting in Brazil were used. Our results suggest that prior to the flooding episodes, important processes take place over the South American continent. First, atmospheric perturbations from the Pacific Ocean enter into the continent at central Chile latitudes with significant mid-tropospheric cold advection and anticyclogenesis tendencies producing weak or moderate precipitation over the eastern flank of the Andes, first starting in southern Peru and then extending northward. Eventually they are able to reach northern Peru where due to the relatively low Andes elevations, they can move westward where the high coastal sea surface temperatures let them amplify.

We have also compared the rainfall spatial distrbution for different ENSO episodes, mainly the corresponding to 1982-83 and 1997-98 over the peruvian region and found that although some differences are present, an important feature related to the existence of common zones with intense precipitation amounts on most of the events can be noticed. That is the case in northern Peru near the Peru-Ecuador border. However, there are other regions with precipitation negative anomalies which take place over relatively slightly different locations on different ENSO episodes, although they occur over the same general area.

All these results suggest that geographic conditions related to topography are of primary importance in determining the location of areas with extreme rainfall impacts.