NORTH PACIFIC AND GULF OF CALIFORNIA SEA SURFACE TEMPERATURES OF THE PAST 15,000 YEARS PREDICT CHANGING PRECIPITATION PATTERNS IN THE SOUTHERN DESERTS OF THE U.S

Winter precipitation in the southern deserts of the U.S. is mainly derived from the North Pacific, whereas the summer monsoonal rains of Arizona and adjacent areas are mainly caused by pulses of moisture traveling northward up the Gulf of California. The intensity and geographic distribution of these precipitation patterns varies from year to year due to changing sea surface temperature (SST) associated with the El Niño-Southern Oscillation (ENSO) and to latitudinal shifts in the Intertropical Convergence Zone (ITCZ). During past glacial intervals, it is widely acknowledged that a more southerly position of the winter jet stream brought increased precipitation to the deserts, resulting in extensive lake systems, whereas it has been suggested that the Arizona Monsoon was reduced or absent.

During the current interglacial, variability in precipitation patterns in the deserts of North America has been considerable, affecting desert flora, fauna, and land forms. North Pacific and Gulf of California SST records of the past 15,000 years can be used to explain variation in both winter and summer precipitation patterns in this region. Between 15,000 and 11,000 cal yr B.P., summer insolation was relatively high and SSTs along the California coast and in the Gulf of California were warmer than present, promoting increased summer precipitation in the desert. Cooler SST during the Younger Dryas (ca. 12,900 to 11,600 cal yr B.P.) likely coincided with cooler, drier conditions in the deserts. Between 11,000 and 8,000 cal yr B.P., SSTs declined, first in the Gulf of California and later off the coast of California, resulting in a decrease in monsoonal rains. At ca. 6,000 cal yr B.P., SST warmed in the Gulf of California, probably due to a northerly shift in the ITCZ, leading to an intensification of monsoonal precipitation. Winter rainfall began increasing throughout much of the West at ca. 5,000 cal yr B.P., with the rate of change increasing at about 3,500 cal yr B.P., causing an end to widespread aridity which had characterized most of the West during the middle part of the Holocene. The last 3,000 years have been marked by generally warmer fall and winter SSTs off California, and increased expression of ENSO cycles. Winter precipitation, although varying with ENSO cycles, has increased throughout most of the West during this period, coinciding with increased winter insolation.