UC Santa Barbara

Understanding the spatiotemporal variability of rainfall over mountainous regions such as the Andes is crucial for the maintenance of water resources and ecosystems. Rainfall variability and change in the Central and Southern Peruvian Andes (CSPA) and their relationship with large-scale atmospheric dynamics is not fully understood. This study examines observed intraseasonal-to-interannual variability and long-term rainfall changes in CSPA.

Chapter 1 describes the motivation, research questions, hypotheses and objectives of this study.

Chapter 2 explains observed regional rainfall features over CSPA during 1965-2010. A regionalization of stations was performed using principal component and clustering analyses of forty-seven daily gauged time-series. Four major homogeneous regions are identified: Amazon, Central Pacific, Southern Pacific and Titicaca. The total and extreme rainfall indices proposed by the Expert Team on Climate Change Detection and Indices (ETCCDI) were calculated for each station. Rainfall indices and geographic features are similar among stations of the same region. Furthermore, this chapter investigates the effect of El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) on rainfall indices. ENSO and PDO influences on rainfall are regionally dependent. Most stations in CSPA exhibit positive (negative) anomaly in total and extreme rainfall indices (consecutive dry days) during La Niña (El Niño) years of 1965-2010. Nevertheless, these rainfall patterns are decade-dependent. Positive (negative) PDO is associated with positive (negative) anomalies of annual total rainfall in Titicaca basin, and with positive (negative) anomalies of consecutive dry days in Amazon, Southern Pacific and Titicaca basins.

Chapter 3 provides a comprehensive analysis of the signal, statistical significance and spatial pattern of rainfall trends from 1965 to 2010 in CSPA. Trends were examined with Mann-Kendall test and Sen’s slope estimator applied to yearly rainfall indices following the hydrologic calendar. Significant regional patterns of changes in rainfall extremes were investigated and compared with previous studies. The drying signal within the southern Peruvian Andes is more complex than pointed out in previous studies. Here, statistically significant trends observed in about 30% of stations in each CSPA region is described. The annual total rainfall has decreased in the Amazon basin, despite the increase in the number of rainy days and some extreme rainfall indices. Decrease in one-day and five-day yearly maximum rainfall is observed in Central Pacific, along with an increase in the number of wet days. Positive trends in indices related to the intensity of very strong daily rainfall are evident in Southern Pacific. Titicaca basin shows an increase in the intensity of rainfall extremes. The ENSO-PDO low-frequency conditions seem to influence the complex and mostly non-statistically significant long-term trends in CSPA.

Chapter 4 examines the Madden-Julian Oscillation (MJO) and ENSO combined influence on rainfall during November-March 1979-2010 in CSPA. Positive standardized rainfall anomalies and a higher frequency of extreme rainfall events occurs during active MJO than during inactive MJO phases. This pattern is enhanced (suppressed) during La Niña (El Niño). MJO-ENSO modulation of rainfall varies according to the location of the stations. During ENSO-neutral conditions, enhanced rainfall in the upper Amazon and Central Pacific basins is evident during MJO phases 4 and 5, whereas MJO phase 2 (phases 6, 7 and 8) enhances rainfall in Southern Pacific (Titicaca) basin. During La Niña, enhanced rainfall occurs during MJO phases 5, 6 and 8 in the Amazon and Central Pacific basins, whereas MJO phases 1, 3 and 6 (phases 2, 4, 7 and 8) enhance rainfall in Southern Pacific (Titicaca) basin. During El Niño, suppressed rainfall occurs during MJO phases 1, 2, 5 and 8 in the Amazon basin, during MJO phase 4 in the Central Pacific basin, during MJO phase 8 in the Southern Pacific basin, and during MJO phases 1, 3 and 5 in the Titicaca basin. Certain MJO-ENSO conditions related with positive and negative anomalies of rainfall show different spatially-coherent statistically significant moisture flux anomalies in each CSPA region.

Chapter 5 presents the final conclusions of this study and proposes some recommendations for future research.