Airborne pollen transport over the Andes: statistical study of trajectories

CLAUDIO F. PÉREZ, M. ELIZABETH CASTAÑEDA, M. I. GASSMANN AND M. M. BIANCHI

Santiago de Chile. Chile

Conferencia; 4th EGU Alexander von Humboldt International Conference. The Andes: Challenge for Geosciences; 2008

European Geosciences Union y Fac. de Cs. Fís. y Matemáticas, Universidad de Chile

Comprehension of atmospheric pollen transport episodes requires proper knowledge of the role of interacting mechanisms from different time and space scales. Examples of these mechanisms are: turbulent mixing, dry and wet deposition, synoptical weather systems, location, strength and phenology of vegetation sources. Airborne pollen is of great importance due to its interest for many fields of science, from palaeoecology to allergology. Studies of pollen transport in South America are particularly scarce. Recent airborne pollen records in Southern Patagonia, suggest that pollen transport takes place from the west to the east slope of the Andes but, the main atmospheric characteristics responsible of this phenomenon are not studied yet. The aim of this paper is to assess potential source areas and to describe the involved synoptic mechanisms which drive airborne pollen transport above the Andes. Methodology relies on the analysis of backward trajectories of air masses calculated for particular days where airborne pollen of Weinmannia trichosperma Cav. was detected at Bariloche aerobiological station (41.143S, 71.375W) located east of the Andes. This pollen type was selected due to the geographical distribution of their source plants located mainly on the west slope of the Andes. Eighty-eight backward trajectories were calculated with the HYSPLIT 4.5 regional model developed by the National Oceanic and Atmospheric Administration (http://www.arl.noaa.gov/ready.html) for events recorded between October and March for the period 2002 -2006. Correspondence between trajectories and the position of potential sources were checked throw GIS maps provided by the Laboratorio de teledetección -SIG-INTA-EEA Bariloche. Mode T, Principal Component Analysis (PCA) with Varimax rotation was used to identify the main spatial structure of geopotential height anomalies leading the calculated trajectories. Meteorological data came from daily NCEP reanalysis provided by the NOAA-CIRES Climate Diagnostic Center, stored at their web site (http://www.cdc.noaa.gov/). Results show two trajectory patterns. The first one is mainly from the west, passing over the Chilean region of W. trichosperma distribution, while the second one come first from the west to later return to Bariloche area with an E-NE direction. This pattern is not directly related with potential source areas. Analysis of the first PC scores (31.17% of the total variance) show a pattern of negative anomalies at high and mid latitudes and positive ones extending towards the tropics; the more negative at higher latitudes while the more positive show the position of semi-permanent anticyclones. Composite fields of mean geopotential heights at 1000, 850 and 700 Hpa constructed with significant PC loadings, show the characteristic field of strong westerlies over Patagonia, which support the quasizonal trajectories displayed in 70% of the selected cases. The Second PC scores (26.12% of the total variance) show the presence of an anticyclone centred at 40S, 68W. This pattern supports those cases where trajectories showed E-NE components before Weinmannia pollen were finally collected.