Relationships between Northern Gulf of California Sea Surface Temperatures and Arizona Summer Rainfall

ABSTRACT

Perhaps the most regular and predictable weather pattern in North America is the North American (NA) or Mexican monsoon. Occurring in summer, it delivers about 35% and 45% of Arizona's and New Mexico's annual precipitation, respectively, and about 60% of northern Mexico's. While recent studies have linked strong NA monsoons to summer drought in the U.S. mid-west, the sequence of events which produce the NA monsoon remain unclear.

This empirical study builds on the findings of many other studies which implicate the Gulf of California (henceforth the gulf) as the dominant moisture source for the monsoon. It examines five monsoon seasons in detail, and quantitatively relates gulf sea surface temperatures (SST) to the timing, amount and regional extent of monsoon rainfall. The study is based on multi-channel satellite SST (MCSST) data at 18 km spatial and weekly temporal resolution, along with satellite SSM/I pentad (5 day) precipitation data having a spatial resolution of 0.25 x 0.25 degrees. Four coastal ocean regions (three in the gulf) were evaluated for SSTs, and four monsoon regions northwest of each ocean region (consistent with known moisture transport) were evaluated for rainfall amount. The main results were: 1) Monsoon rainfall did not occur prior to the onset of gulf SSTs exceeding 26 C. 2) The incremental advance of SSTs > 26 C up the mainland coast of Mexico appears necessary for the northward advance of the monsoon. 3) In the southern gulf region, an SST parameter was lag-correlated with rainfall amount in adjacent land regions (r=0.75) occurring during a 1-15 day period after an SST increase. 4) For the period June through August, 69% of the rainfall in the Arizona/New Mexico region occurred after SSTs in the northern gulf exceeded 29.5 oC, 79% occurred after these SSTs exceeded 28.5 oC, and 91% after SSTs exceeded 27.5 oC. 5) While the timing of when SSTs became 29.5 oC was similar south of the N. gulf, this timing was significantly delayed in the N. gulf. This warming delay in the N. gulf coincided with a relative delay in the primary monsoon rainfall period for Arizona-New Mexico. 6) The evolution of N. gulf SSTs for these five moderate-to-weak monsoon seasons was compared with the SST evolution for six strong monsoon years regarding the Arizona/New Mexico region. Five of the six anomalously wet monsoon years exhibited significantly higher SSTs ( > 29.4 oC) over a 1-2 week period in early July, suggesting that strong monsoon years in the U.S. may be predicated by this condition.

Recent oceanographic studies indicate the heat content of the upper layer (e.g. 70 m) in the Gulf of California during spring/summer is primarily due to horizontal advection from the south, allowing SSTs to be higher than solar insolation alone would produce. These findings, in combination with ours, suggest that regular seasonal changes in the strength of gulf currents may play a role in determining monsoon activity, especially in the Arizona-New Mexico region.