where does the warm moist air mass over the andes mountains in argentina come from
The north–south orientation of the Andes creates a barrier for humid air masses originating from the Pacific Ocean.  Most of the moisture is dropped on the Chilean side, causing abundant precipitation and cloudiness while on the Argentine side, the air warms adiabatically, causing it to become drier as it descends.
Climate of Argentina
Owing to the higher altitudes of the Andes north of 40S, they completely block the normal westerly flow, preventing low pressure systems containing moisture from the Pacific Ocean from coming in. Thus, much of Argentina north of 40S is dominated by wind circulation patterns from the South Atlantic High. South of 40S, the Andes are lower in altitude, allowing much of Patagonia to be dominated by westerly winds and air masses from the Pacific Ocean. However, the north–south orientation of the Andes creates a barrier for humid air masses originating from the Pacific Ocean. This is because they force these air masses upwards, cooling adiabactically. Most of the moisture is dropped on the Chilean side, causing abundant precipitation and cloudiness while on the Argentine side, the air warms adiabatically, causing it to become drier as it descends. Thus, an extensive rain–shadow is present in much of Patagonia, causing it to receive very little precipitation. The Sierras Pampeanas influences 
wiki:2869677
Climate of Argentina [SEP] Geographic factors
The most important geographical factors that influence the climate of Argentina are latitude, elevation, and distance from the sea. With Argentina extending from 22S to 55S, there are differences in the amount of incoming solar radiation and the amount of daylight received in each season, which affects temperature. Thus, temperatures decrease from north to south due to the differences in latitudes. Although the centre and the eastern parts of the country are mostly flat, the west is mountainous. Both the Andes and Sierras Pampeanas affect the climate of Argentina, leading to differences in temperature, pressure, and spatial distribution of precipitation depending on the topography and altitude.