Variability of the Southern Andes rain shadow

Abstract

The southern Andes mountain range plays a determinant role in separating humid Chilean climate from dry Argentinian climate. However, there is substantial event-to-event variability in rain shadow strength, such that some precipitation events are able to produce significant spillover precipitation over the Argentinian side. This study investigates the mechanisms modulating the rain shadow strength across the Southern Andes. Rain gauge measurements from Chile and Argentina were used to classify events according to their rain shadow strength, as strong rain shadow (SRS) or weak rain shadow (WRS). ERA5 reanalysis was used to analyze synoptic conditions during these events. Differences between events show that the 0 °C isotherm is higher during SRS events. SRS cases exhibit a larger windward deceleration of cross-barrier wind than WRS events, indicating a blocking signature. Strong mountain wave activity increases precipitation upwind of the Andes and suppresses precipitation on the lee for SRS events, while WRS events exhibit weaker wave activity. Zonal moisture flux offshore is larger for SRS events than for WRS. Analysis of the drying ratio suggests that more water vapor is precipitated out or deflected by the mountains and does not reach the leeside in the SRS events compared to WRS events. The following mechanisms controlling the rain shadow strength are suggested: (1) A higher 0 °C causes more liquid precipitation to fall upstream and over the windward slope of the Andes for SRS events, while WRS cases with a lower 0 °C level allow ice particles and snow at the top of the mountain to be advected to the leeside, decreasing the rain shadow, (2) the low-level impinging flow is blocked by the Andes during SRS events, forcing the flow to rise upstream, increasing precipitation upwind and on the windward slope of the Andes, increasing the rain shadow, (3) leeside descent for SRS events suppresses precipitation on the leeside, while weaker wave activity for WRS cases allows for spillover precipitation decreasing the rain shadow, (4) SRS events that are also characterized by strong orographic enhancement could be produced by the landfall of atmospheric rivers to the area, increasing precipitation upstream and decreasing the rain shadow, as a result of the northerly orientation of the flow in contrast with the more zonal orientation of WRS cases.