| The lowland of Piura, in northwestern Peru, is very strongly impacted by
El Niño. Its climate is arid but can experience very heavy rainfall
associated with the high nearby sea surface temperature (SST) during El Niño
events. Rainfall, however, tends to occur in discrete, intense events and
an understanding of the physical conditions favoring a particular day with
heavy rainfall over others is of both scientific and practical interest.
In this work, we consider the rainy periods of December 1997 to April 1998 and March to April
2002, corresponding to very strong and weak to moderate El Niño conditions, respectively,
and search for systematic differences in the atmospheric circulation that may account
for the day-to-day variability of rainfall in Piura. Composites of vertical
profiles of winds measured by wind profiling radars in Piura, as well as composites of
NCEP/NCAR Reanalysis wind fields, suggest that rainy days are associated with an enhanced
onshore westerly low-level flow, which may help the triggering of convection by
orographic lifting over the western slope of the Andes.
Synoptic control was evident in the rainfall record for 1997-1998 but was not as clear in
that of 2002. However, in both periods of study the low-level flow over Piura, which we found
to be important for the triggering of rainfall, was modulated by tropical synoptic scale
disturbances. The structures of the
composited wind differences suggest that they may be related to equatorially trapped
tropospheric waves, particularly Kelvin and n=1 Rossby waves. |