Holocene changes in the of the North American Monsoon and its Pacific moisture sources

The North American monsoon (NAM) is constrained by a climatology (atmospheric pressure systems, sea surface temperatures {SST}) that has evolved substantially during the Holocene (last 11,500 years), driven largely by changes in Northern Hemisphere insolation. During the late Holocene (past 4000 cal yr), the geographic expression of NAM within the southwestern US has been limited to a core region east of 113°W in eastern Arizona and New Mexico. Prior to 7000 cal yr BP, however, paleoproxy and modeling studies suggest that NAM covered a much broader region, encompassing most of the southwest and extending northward into the northern Great Basin and central Rockies. Proxy SST records (biogenic sediment, diatoms, and silicoflagellates) from the Gulf of California (GoC) and the adjacent Pacific suggest that surges of tropical moisture up the axis of the Gulf that presently contribute monsoon precipitation to Arizona and eastern New Mexico were absent during the most of the early Holocene ( 11,000 to 7000 cal yr BP). During the early Holocene, the North Pacific high-pressure system was further north, SSTs in the California Current System (CCS) off Baja and southern California were warmer, and GoC SSTs were cooler. Under these conditions, summer frontal storms and tropical cyclones could have taken more northerly trajectories, delivering substantial summer precipitation into an expanded area of the southwestern US. Holocene proxy precipitation records (lakes, vegetation/pollen, and caves) from the southwestern US and northwestern Mexico are reviewed in the context of this hypothesis of changing Holocene NAM climatologies.