Gulf of California Sea Surface Temperatures and the North American Monsoon: Mechanistic Implications from Observations

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. Midwest, the sequence of events that produce the NA monsoon remain unclear. This empirical study builds on the findings of many other studies that implicate the Gulf of California [(GOC) or simply the gulf] as the dominant moisture source for the monsoon. It examines six monsoon seasons in detail, and quantitatively relates GOC sea surface temperatures (SST) to the timing, amount, and regional extent of monsoon rainfall. This six season study is based on satellite measurements of rainfall [using the Special Sensor Microwave Imager (SSM/I)] and GOC SST at high spatial and temporal resolution. Key findings include the following. 1) Monsoon rainfall did not occur prior to the onset of GOC SSTs exceeding 26°C, and the incremental advance of SSTs > 26°C up the mainland coast of Mexico appears necessary for the northward advance of the monsoon. 2) For the period June?August, 75% of the rainfall in the Arizona?New Mexico region (AZNM) occurred after northern GOC SSTs exceeded 29°C, with relatively heavy rains typically beginning 0?7 days after this exceedance. 3) For a given year, SSTs in the southern and central GOC reached 29.5°C during a similar time frame, but such warming was delayed in the northern GOC. This warming delay coincided with a rainfall delay for AZNM relative to regions farther south. 4) Based on the 17 yr of available SST data, 14 of those years exhibited the following behavior: When northern gulf SSTs were relatively high for some period during the first half of July, rainfall during June?August in Arizona was relatively high. Otherwise, June?August Arizona rainfall was normal or below normal. 5) Anomalously wet July?September periods in Arizona do not correspond to anomalously wet periods in New Mexico, based on data from 1950 to the present. The wettest Arizona seasons, about 1.1 standard deviations wetter than normal, were strongly related to summer drought in the Midwest, being about 0.8 standard deviations drier than normal. This was not true for the wettest New Mexico years (Midwest rainfall was near normal), but these years exhibited dry conditions in the interior Northwest, with standard deviations being about 0.6?0.9 drier than normal. Collectively, this research suggests that the cause of these two wet monsoon modes may be related to SSTs in the northern gulf, which appear to affect Arizona more than New Mexico rainfall.