Two possible mechanisms for interannual modulation of precipitation anomalies in the Southwest U.S. are examined. The association between El Ni¿o/Southern Oscillation (ENSO) fluctuations and New Mexico precipitation anomalies is found to vary with season: positive Pacific Ocean surface temperature anomalies are associated with positive winter-spring precipitation anomalies in New Mexico but negative summer anomalies. We propose that the spring snow extent across the west-central U.S. acts to enhance or suppress the subsequent summer monsoon circulation affecting the American Southwest, such that excessive snow leads to deficient summer rain, and deficient snow leads to abundant rain. This mechanism is consistent with the observed seasonal variation in the ENSO-precipitation relationship.¿ 1997 American Geophysical Union