ORBITAL HISTORY AND SEASONALITY OF REGIONAL PRECIPITATION

The Arizona monsoon, a major source of precipitation in the Southwest, shares many features with the monsoons of other continents. Computer modeling and fossil data indicate maximum extent of the African and As ian monsoons 9000 years ago. Fossil data indicate increased summer pre cipitation 9000 years ago, synchronous with the maxima of the African and Asian monsoons and, paradoxically, with the early-Holocene xerothe rmic of the Pacific Northwest. Climate model runs for 6000, 9000, 11,5 00, 13,000, and 18,000 years ago indicate increased summer precipitati on 9000 years ago and a reciprocal relationship between precipitation in the Northwest and Southwest, but they relegate insolation to a role secondary to the North American ice sheet in regulating climate, and suggest a non-monsoon source for much of the summer precipitation in t he Southwest prior to 9000 years ago.