Perhaps the most regular and predictable weather pattern in North America is the North American (NA) or Mexican monsoon. While recent studies have linked strong NA monsoons to drought in the U.S. mid-west (and vice-versa) and drier winters in southern California (and vice-versa), the sequence of events which produce the NA monsoon remain unclear.
Recent oceanographic studies document the role of a coastal warm current in raising sea surface temperatures (SST) in the Gulf of California in spring and early summer, allowing SSTs to climb beyond that achievable from solar insolation/sensible heating alone. If gulf SSTs are important in determining monsoon rainfall, this warm current could be an important mechanistic component. Thus far, there has been little work linking gulf SSTs to NA monsoon rainfall. This latter aspect is the contribution of this study.
The period of interest here is from the first onset of monsoon rainfall to just after the highest SSTs are reached (in August), referred to here as the onset period. The data used in this three year empirical study consisted of 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 were defined (three in the Gulf of California) within which SSTs were evaluated, and four monsoon regions northwest of each ocean region were evaluated for rainfall amount.
The onset of monsoon rainfall did not occur prior to the onset of gulf SSTs exceeding 26 C. SSTs were often relatively constant for 2 to 5 weeks, and then rapidly increased by up to 2 C. For SSTs > 26 C, weekly SST "jumps" > 0.4 C were related to an increase in rainfall amount to the west or northwest (to be consistent with prevailing wind directions), occurring about 5 to 15 days later, for 91% of the SST jumps. SST increases in or just below the southern 1/3 of the gulf were related to rainfall amount in adjacent regions over this lag period with a correlation coefficient of 0.78, suggesting gulf SST jumps may be an important precondition for monsoon rainfall. Moreover, some evidence suggests this convection may propagate northwards, possibly related to "gulf surges" described elsewhere.
It was also found that the Arizona-New Mexico region, from late May through August, received 2/3 of its rainfall after SSTs in the northern 1/3 of the gulf climbed past 29.5 C. Most of this rainfall could be attributed to SST jumps in the N. gulf, occurring 5-15 days prior. These results support the findings from an earlier monsoon modeling study by Stensrud.
While these results do not conclusively prove a cause-and-effect relationship between SST increases and monsoon rainfall, they do lend considerable support to the hypothesis that an observed warm current, by affecting Gulf of California SSTs, may play a significant role in determining the timing and amount of NA monsoon rainfall