Synoptic Patterns Associated With Wet Season Onset In The Tropical High Andes Of Southern Peru And Bolivia

In the outer tropical Andes where a clear distinction between the wet and dry season exist, precipitation is one of the most relevant factors in determining glacier mass balance. This study examines daily precipitation observations from 1979 to 2017 in southern Peru and Bolivia to identifies the wet season timing and examine its interannual variability and tendencies. The ERA-Interim Reanalysis (0.75° Lat/Lon - 6 hours) provides insights into atmospheric circulations related to the wet season timing. We identify spatiotemporal variations in the wet season timing mostly associated with the distance to the equator and to the Amazon basin, in which onset dates exhibits a pronounced variability. Significant trends showing a delay of the wet season onset in 0.4 to 0.8 days/year were found in the southwestern subregions, closely related with the occurrence of early/late wet season onset cases. Main features related to the wet season onset are a low-level northwesterly flow east of the Andes, an anticyclonic circulation in mid-troposphere, and northwesterly winds in the upper troposphere. Changes in the position and strength of these circulations are observed during early vs late wet season onset cases. This result has implications for improving seasonal precipitation predictions from tropical high Andes.