COMPARISON OF HOLOCENE EASTERN PACIFIC AND GULF OF MEXICO SEA SURFACE TEMPERATURE RECORDS WITH TERRESTRIAL RECORDS OF HOLOCENE PRECIPITATION IN THE WESTERN UNITED STATES

Modern precipitation over most of the western US is strongly seasonal and derived from different marine sources. As a result of ENSO processes, winter (November to March) precipitation maxima in California, Oregon, and Washington as well as most of Nevada are associated with Pacific frontal storms. During El Niño and/or positive Pacific Decadal Oscillation (PDO) conditions, rainfall is enhanced in the southern and decreased in the northern parts of this region. Opposite regional trends are associated with La Niña and/or negative PDO conditions. Monsoonal (July-September) precipitation in Arizona is derived from the Gulf of California (GoC), whereas the Gulf of Mexico (GoM) is the primary source for monsoonal precipitation in much of New Mexico. Summertime sea surface temperatures (SSTs) in the GoC and GoM respectively exert a strong control on the amount of monsoonal precipitation delivered to Arizona and New Mexico.

Marine and terrestrial proxy evidence suggest that these modern regional precipitation patterns were established between ~8000 and 4000 cal yr BP in response to changing SSTs. For example, prior to ~8000 cal yr BP, spring-summer SSTs off California were 2-4°C warmer and the North Pacific High (NPH) was located further to the north, allowing westerlies to deliver spring-summer precipitation to the southwest. Conversely, cooler summer SSTs (<26°C) in the GoC prior to ~6000 cal yr BP suppressed the northward flow of tropical moisture up the axis of the GoC, diminishing monsoonal precipitation in Arizona. After ~8000 cal yr BP spring-summer SSTs cooled off California and associated high pressure blocked the eastward flow of Pacific moisture into the southwest. Conversely, warming of GoC waters after ~6000 cal yr BP promoted the flow of tropical moisture up the axis of the GoC into Arizona. In the GoM SSTs appear to have warmed until ~5600 cal yr BP, promoting monsoonal precipitation in eastern parts of the southwestern US. However, a southward shift of the Intertropical Convergence Zone (ITCZ) between ~5600 and 4700 cal yr BP lead to cooler GoM SSTs and possibly caused decrease in monsoonal precipitation in some eastern areas of the southwestern US. Enhanced ENSO strength and variability after ~4000 cal yr BP led to increased winter precipitation from Pacific frontal storms throughout much of the western US.