Intraseasonal variability of organized convective systems in the Central Andes: Relationship to Regional Dynamical Features

The Andes extend from the west coast of Colombia (10N) to the southern tip of Chile (53S). In southern Peru and Bolivia, the Central Andes is split into separate eastern and western cordilleras, with a high plateau (≥ 3000 m), the Altiplano, between them. Because 90% of the Earth's tropical mountain glaciers are located in the Central Andes, our study focuses on this region, defining its zonal extent as 7S-21S and the meridional extent as the terrain 1000 m and greater. Although intense convection occurs during the wet season in the Altiplano, it is not included in the lists of regions with frequent or the most intense convection. The scarcity of in-situ observations with sufficient density and temporal resolution to resolve individual storms or even mesoscale-organized cloud systems and documented biases in microwave-based rainfall products in poorly gauged mountainous regions have impeded the development of an extensive literature on convection and convective systems in this region. With the tropical glaciers receding at unprecedented rates, leaving seasonal precipitation as an increasingly important input to the water balance in alpine valley ecosystems and streams, understanding the nature and characteristics of the seasonal precipitation becomes increasingly important for the rural economies in this region. Previous work in analyzing precipitation in the Central Andes has emphasized interannual variability with respect to ENSO, this is the first study to focus on shorter scale variability with respect to organized convection. The present study took advantage of the University of Utah's Precipitation Features database compiled from 14 years of TRMM observations (1998-2012), supplemented by field observations of rainfall and streamflow, historical gauge data, and long-term WRF-simulations, to analyze the intraseasonal variability of precipitating systems and their relationship regional dynamical features such as the Bolivian High. Through time series and wavelet analysis, we found an important 8-10 day cycle related to but lagging convective surges in the Amazon basin and enhanced upper-level cyclonic flow around the Bolivian High. The majority of the organized convection in the region tended to be weak (< 5 mm/hr rain rates) and shallow (< 12 km). The timing of response (i.e., formation and distribution of organized convection) due to changes in moisture transport around the Bolivian High was similar in the wetter eastern and drier western cordilleras of the Central Andes. The response to upper level moisture transport was modulated by local soil moisture and elevation slope and aspect, with higher elevation, eastern facing peaks having a stronger response than western-facing and lower elevation areas. Streamflow data support the hypothesis that the majority of the light rainfall infiltrates the shallow sub-surface, rather than contributing to surface channel runoff, helping to sustain the high altitude peatlands in the Andean valleys.