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 false color image was taken by GOES-8 during the summer
of 1994, and shows an active monsoon over northern Mexico and New
Mexico. This moisture surge may have been triggered by the
passage of the easterly wave shown off the western coast of Mexico,
which are believed to initiate pulses of moisture up the Gulf of
California axis, known as gulf surges (Stensrud et al. 1997). In this
photo, it is apparent that monsoon moisture is propagating across the
central U.S. where it is becoming entrained into a frontal system in the
upper mid-west. This infusion of tropical moisture appears to have
intensified this system, at least relative to the frontal system further
west, judging from cloud reflectance. This type of injection of monsoon
moisture into frontal systems near the U.S. - Canadian border appears to
be common immediately after northbound monsoon moisture penetrates into
the mid-latitudes (i.e. the U.S.), especially during strong monsoon
years. To witness animations of this phenomena from July 2-9 1999, using
total precipitable water and lifted index from GOES Sounder, see the
website
of our collaborator Dr. Robert Rabin at either of the following
addresses:
http://zonda.ssec.wisc.edu/~rabin/monsoon/monsoon.html
or
http://www.nssl.noaa.gov/~rabin/monsoon/monsoon.html
This page is based on the following paper:
Mitchell, D.L., D. Ivanova, R. Rabin, K. Redmond, T.J. Brown, and T. Hooker, 2000: Sea surface temperatures and the North American monsoon: mechanistic implications. Submitted to the Journal of Climate.
This web-site describes this paper, building 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 six 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. This design applies to the first 5 seasons studied. Findings from these first 5 seasons were further tested using daily SSTs in the N. gulf only, and relating them to the timing of rainfall in Arizona and the flood in Las Vegas, Nevada, for the strong monsoon year of 1999. In addition, the SST evolution in the N. gulf was evaluated for 1983-1999, and anomalously wet June-August seasons in (1) Arizona and (2) New Mexico were contrasted with precipitation anomalies in other parts of the United States.
The main results of this study are:
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 correlated with rainfall amount in adjacent land regions (r = 0.75) occurring during a lag period after an SST increase. 4) For the period June through August, 69% of the rainfall in the Arizona/New Mexico region (AZNM) occurred after SSTs in the northern gulf exceeded 29.5°C, 79% occurred after these SSTs exceeded 28.5°C, and 91% after SSTs exceeded 27.5°C. 5) While the timing of when SSTs became 29.5°C 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 AZNM. 6) 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 first heavy monsoon rains in Arizona and the Las Vegas flood two days later. In general, relatively heavy rains occurred within 0 to 7 days following N. gulf SSTs reaching or exceeding 29°C. 7) 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 regarding 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. 8) 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.
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 Arizona.
For more information, contact Dr. David Mitchell; E-mail: mitch@dri.edu.