Extreme orographic precipitation events over the Central Andes of Argentina and Chile

Winter precipitation events in the Central Andes of Argentina and Chile (35ºS-30ºS) have a great impact in the regional climatic system. The amount of accumulated snow by the end of the season is the main agent controlling the hydrologic cycle in the region. However, these events can have a negative impact when they are of great intensity. Every year, the road traffic trade between Argentina and Chile is interrupted due to adverse meteorological conditions causing huge economic losses. To characterize the orographic precipitation episodes of highest intensity in the Central Andes for 1967-1976 period, we obtained the synoptic scale circulation patterns linked to them, and examined the local climatologic aspects associated with the strong orographic influence in the region.

High mountain daily precipitation series are used to classify extreme events, considering their intensity and regional extension. We propose two subcategories: 1) intense events with precipitations occurring in 100% of the available stations and areal daily mean higher than 50 mm/day; and 2) moderate events with areal daily mean precipitations higher than 35 mm/day and lower or equal than 50 mm/day. These two thresholds represent less than 5% of the daily precipitation distribution in the mountain range. Fifteen episodes were recorded, 4 of which are intense and 11 moderate. The most frequent duration of each event was 4 days and they were in general evenly distributed along the analyzed period of 10 years. Although the considered period is not very long and with a low-frequency variability included, it could be said that this mostly uniform temporal distribution and the presence of an extreme event in years of intense drought (extreme case in 1969), suggest a frequency of an intense event per year and an extreme event every two years.

Using the reanalysis data, we saw that the circulation associated with these extreme events has a particular pattern with strong negative anomalies in geopotential heights located off the Pacific Southwest coast. Besides, an anomalous westerly flow at mid-level impinges on the high Central Andes and favors intense orographic lifting on windward slope and downward motion immediately at lee side. Together with this mechanic/orographic influence, the presence of dynamic factors like an upper divergent airflow in jet stream approaching the mountain range and an anomalous northwest wind in low levels, promote the ascending mechanisms and humidity transport that generate intense precipitations over the mountain range. A more regional analysis of the daily precipitation series in high mountain and surrounding lowlands, showed that precipitations are not recorded in Argentinean stations lee side of the Andes during the first 2 days of each event (including the one of highest intensity), and some stations reported the occurrence of “Zonda” wind (warm and dry downslope wind) during these days in most of the events. This reveals the strong influence of orography over precipitations, inhibiting them leeward due to the descending character of the airflow. Rain shadows are recorded in low-lying stations on lee side mostly in the last days of each event. Moreover, when calculating the reduction rates between daily mean precipitations on the lowland region of Argentina and the high mountains, there is a sharply reduction of 90% at lee side, except in the southernmost region where the mean altitude of the mountain range is lower, hence this reduction rate was lower.