Toward Understanding the North American Monsoon and Forecasting Summer Energy Demand in the Desert Southwest

Cities in the desert southwest have recently faced potential ‘brown out' conditions during summer due to excessive power demand. Summer energy demand could be better anticipated if advanced knowledge of the likelihood of an anomalously dry (warmer) or wet (cooler) monsoon season were known. Arizona (AZ) and New Mexico (NM) receive on average 35% and 45% of their annual precipitation, respectively, from the North American monsoon. Based on the last 20 years, the correlation between July-August mean temperature and precipitation is r=-0.59 for AZ and r=-0.65 for NM, consistent with the relative impact of the monsoon in each state. The monsoon has its greatest impact in S.E. AZ and S.W. NM, where the mean temperature- precipitation correlation is -0.71 and -0.75, respectively, for this period. These correlations would be higher if only maximum temperatures were used (clouds increase nighttime temperatures). Moreover, just knowing a likely temperature range is valuable.

A number of N.A. monsoon studies will be reviewed from the angle of synoptic influences, sea surface temperature (SST) forcing, diabatic heating, gulf surges and climate diagnostics, with the emphasis on understanding the monsoon's evolution. These aspects will be brought to bear on findings from our recent study, which provides a means of estimating the timing and strength of the N.A. monsoon just prior to onset in AZ, using SSTs in the Gulf of California (GOC). Collectively these studies support a new conceptual model of the monsoon, an important component of which is the ocean circulation in the GOC.

Some findings from our six season study, based on satellite rainfall (SSM/I), sea surface temperature (MCSST and GOES-10), and other data fields at high spatial and temporal resolution, include: (1) SSTs in the GOC must be at least 26 deg. C before significant rainfall begins in northern Mexico or the U.S. southwest during late spring/early summer. (2) An SST parameter was correlated with rainfall amount over N. Mexico (r=0.75) occurring during a 0-15 day lag period after an SST increase. (3) For the period June through August, 75% of the rainfall in the Arizona/New Mexico region (AZNM) occurred after N. GOC SSTs exceeded 29 C, with relatively heavy rains beginning 0-7 days after this. (4) The dates that SSTs reached 29.5 C were fairly uniform in the southern and central GOC, but were delayed in the N. GOC. This warming delay coincided with a rainfall delay for AZNM relative to regions further south. (5) Based on the 17 years of available SST data, 14 of those years exhibited the following behavior: When N. gulf SSTs during the first half of July were relatively high, rainfall during June-August in Arizona was relatively high. Otherwise, June-August Arizona rainfall was normal or below normal. This agrees with the work of Higgins in that early monsoons tend to be wetter.