The El Niño Southern Oscillation influences precipitation levels in northwest Argentina. During an El Niño year, the westerly flow is strengthened while moisture content from the east is reduced, resulting in a drier rainy season. In contrast, during a La Niña year, there is enhanced easterly moisture transport, resulting in a more intense rainy season. Nonetheless, this trend is highly variable both spatially and temporally. Temperatures in northwest Argentina vary by altitude. In the temperate valleys which includes the cities of Salta and Jujuy, they have a temperate climate, with mild temperatures in the summer (a mean of in Salta, in Jujuy in January) and extremely dry and cool winters with regular frosts (a mean of in Salta and in Jujuy in July). The diurnal range in these cities is fairly large, particularly in the winter. In the Quebrada de Humahuaca valley in Jujuy province, north of Jujuy city, the diurnal range is large with a thermal amplitude between . A major reason for the large thermal amplitude is that during the day, there is intense radiation from the sun, causing the land to heat up while during the night, there is less radiation, causing the land to cool and temperatures to fall down. Mean temperatures in the warmest month in the Quebrada de Humahuaca valley range from in Humahuaca at the higher altitudes to in Volcan at the lower altitudes; in the coldest month, the mean temperature ranges from . The mean annual temperatures in the Quebrada de Humahuaca valley ranges from , depending on altitude. In the Calchaquí Valleys in Salta province, the climate is similar to the valleys in La Rioja province and Catamarca province, by being temperate and arid with large thermal amplitudes, long summers, and a long frost free period which varies depending on altitude. Mean temperatures in the warmest month in the Calchaqui valleys range from at the higher altitudes to at lower altitudes; in the coldest month, the mean temperature ranges from . The mean annual temperatures in the Calchaqui valleys ranges from in La Poma at the higher altitudes to in San Carlos which lies at a lower altitude. In both the Quebrada de Humahuaca and Calchaqui valleys, winters are cold with frosts that can occur between March and September. In the valleys in the south in La Rioja province and Catamarca province along with the southwest parts of Santiago del Estero Province which is part of the arid Chaco ecoregion, the climate is mild year long in terms of temperature.