Summertime precipitation extremes and the influence of atmospheric flows on the western slopes of the southern Andes of Perú

• Published in: International journal of climatology 2022-12, Vol.42 (16), p.9909-9930
• Main Authors: Villalobos‐Puma, Elver, Flores‐Rojas, Jose Luis, Martinez‐Castro, Daniel, Morales, Annareli, Lavado‐Casimiro, Waldo, Mosquera‐Vásquez, Kobi, Silva, Yamina
• Format: Article
• Language: English
• Subjects: Andes; Anomalies; Atmospheric circulation; Atmospheric circulation changes; Atmospheric circulation patterns; Atmospheric flows; Blocking effects; Convective available potential energy; Dynamic height; Extreme weather; Geopotential; Geopotential height; large scale; Moisture; Moisture effects; moisture flows; Potential energy; Precipitable water; Precipitation; precipitation extremes; precipitation gradients; Principal components analysis; Rainfall; Slopes; Troposphere; Vertical velocities; Water vapor; Water vapour
• ISSN: 0899-8418; 1097-0088
• DOI: 10.1002/joc.7871

Although climatologically dry, the western slopes of the southern Andes of Peru (WSA) can experience precipitation extremes (PEs) during the summer (December–February) resulting in great economic and human losses. Generally, WSA has a positive upslope gradient in precipitation, meaning more rain falls at higher elevations, but observations have shown this gradient can become negative with higher rainfall near the coastal cities. In this study we analyse 2000–2019 regional atmospheric patterns associated with different upslope precipitation gradients and PEs in WSA using principal component analysis methods and surface station observations. Results show important changes in the atmospheric circulation patterns during the occurrence of PE events. A prevailing pattern of negative southerly wind anomalies and regional warming of the southeastern Pacific Ocean leads to significant increases in moisture along the coast of WSA. Eastern moisture flows associated with the presence of the Bolivian High are observed at upper levels of the atmosphere and transport water vapour from the Amazon to the western side of the Andes. Additionally, there is a blocking effect aloft in response to an intense gradient of geopotential height that attenuates the easterly circulations. These large‐scale mechanisms act to concentrate high precipitable water amounts and high levels of convective available potential energy in the troposphere which favours the vertical velocities essential to trigger PEs. These results increase our knowledge of the large‐scale characteristics of PEs to help with forecasting these impactful events and protecting the more than 1.8 million people living in WSA. Schematic representation of the atmospheric mechanisms controlling to the PE events during the summer (December–February) on WSA region. For the PE events it generates a high concentration of the integrated water vapour in the vertical column (IWV) on WSA region and its coast. This well‐defined hot‐spot region of IWV depends on the following settings: There is a weakening of the south winds at levels close to the sea surface and regional warming of the sea (SST, sea surface temperature). At levels of 800–875 hPa the northwesterly moisture flow (NWMF) intensifies as a water vapour corridor along the Peruvian coast. At levels of 500–100 hPa it generates an atmospheric blocking effect (BE), and at levels of 650–200 hPa the easterly moisture flow (EMF) moisture transport from the Amazon and this flow ac