SYNOPTIC CONTROLS ON PRECIPITATION IN THE VALLEY OF OAXACA, MEXICO

The valley of Oaxaca is an important agricultural centre in the highlands of southern Mexico. Precipitation is a major control on rain- fed agriculture. Daily, monthly, and annual precipitation in the valley of Oaxaca are linked synoptically to distinct meso- and large-scale patterns of atmospheric pressure and temperature. During the summer rainy season, daily precipitation in the valley coincides with lower atmospheric pressures over Mexico. Anomalous lows increase convergence and draw maritime air landward. From June through to September, monthly average 850 hPa level temperatures in the Mexico–USA border region are consistently higher during the valley's wettest years compared with dry years. This link with atmospheric temperatures in the border region suggests that precipitation in the valley of Oaxaca is affected by variations in the Mexico monsoon, a tongue of maritime air that is drawn up the Mexican west coast and into the south-west USA during summer. Precipitation in the valley of Oaxaca also varies according to the frequency and position of tropical storms and hurricanes in the Gulf of Mexico and Pacific Ocean. Significant correlations between precipitation in the valley and storm frequency occur during August and September. Finally, El Niño–Southern Oscillation (ENSO) warm events are associated with dry conditions in the valley owing to a southward displacement of the Inter Tropical Convergence Zone (ITCZ) during the low phase of the Southern Oscillation and diminished storm frequency in the Gulf of Mexico.