5. Conclusions

Interrelationships between SSTs in the Gulf of California and rainfall amounts in adjacent land regions were evaluated for six monsoon seasons (June-August), with one season (1999) addressing only SSTs in the northern gulf due to persistent cloud cover further south. This was made possible by using high temporal and spatial resolution satellite data. These results lend considerable support to the hypothesis that Gulf of California SSTs play an important role in determining the timing, rainfall amount and northwestward extent of the Mexican or North American (NA) monsoon. The major findings from this study are as follows:

  • A 20 year mean seasonal cycle, from March through August, of SSTs and OLR was evaluated, indicating a developing warm pool off southern Mexico and Central America in spring, when solar insolation is high. Relatively warm (e.g. 29°C) water advances far up the Mexican west coast into the Gulf of California during July, coinciding with a similar extreme northward advance in deep convection (i.e. OLR < 240 W/m-2). A five season intensive observation period revealed that convection did not occur over northwestern Mexico until corresponding gulf SSTs exceeded 26°C, implicating the advance of warm water up the coast as a precondition for monsoon onset.

  • For the pre- and southern gulf regions, an SST parameter was lag-correlated (r = 0.75) with rainfall amount occurring in adjacent land regions to the east or northeast about 0-15 days after the SST increase.

  • For the period monsoon onset through August, 69% of the rainfall in the Arizona/New Mexico region occurred after SSTs in the northern Gulf of California exceeded 29.5 °C, and about 80% occurred after these SSTs exceeded 28.5°C. Relatively heavy rains generally occurred within 0 to 7 days following N. gulf SSTs reaching or exceeding 29°C.

  • A relative climatological delay in the monsoon onset for Arizona, as observed by Higgins et al. (1999) via 26 years of daily precipitation data, appears to result from a delay in the warming of northern gulf waters relative to gulf waters further south.

  • For the strong monsoon year of 1999, the timing of GOES daily N. gulf SSTs exceeding 29°C (adjusted to MCSST values) coincided with the timing of the of the first heavy monsoon rains in Arizona and the Las Vegas flood two days later. With one apparent exception, relatively heavy rains occurred within 0 to 7 days following N. gulf SSTs reaching or exceeding 29°C.

  • The evolution of N. gulf SSTs for the five moderate-to-weak monsoon seasons was compared with the SST evolution for six strong monsoon years for Arizona. Five of the six anomalously wet monsoon years exhibited significantly higher SSTs ( >29°C) over a 1-2 week period in early July, suggesting that strong monsoon years in Arizona may often be predicated by this condition.

  • Anomalously wet June-August periods in Arizona do not correspond to anomalously wet periods in New Mexico, based on 1950 to present. The wettest Arizona seasons, about one standard deviation wetter than normal, were strongly related to summer drought in the mid-west, being about 0.8 standard deviations drier than normal. This was not true for the wettest New Mexico years, but these years exhibited dry conditions in the interior northwest, with standard deviations being about 0.6 to 0.8 drier than normal. 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.

    In conclusion, the factors governing the timing, intensity and extent of the NA monsoon may be oceanographic in nature as much as they are atmospheric, and greater "cross-fertilization" between the fields of oceanography and the atmospheric sciences may provide the key to optimizing progress toward a mechanistic understanding of the NA monsoon.

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