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Evolution of the North American Monsoon

sat-image-tropical-easterly-wave-mx-thbSatellite image showing a tropical easterly wave offshore of Mexico and strong surge from Gulf of California toward the southwestern US and further north.
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The North American Monsoon (NAM) is a large-scale feature having a strong impact on summer rainfall patterns and amounts over North America. For example, it supplies about 60%-80%, 45% and 35% of the annual precipitation for northwestern Mexico, New Mexico (NM) and Arizona (AZ), respectively. Also, anomalously wet NAMs in Arizona are strongly anti-correlated with anomalously dry summers in the mid-west. Although regional climate models have succeeded in reproducing some features of the NAM, its onset, strength and regional extent are not well predicted, and a physical understanding of key processes governing its life-cycle remain elusive.

Here we propose a partial mechanistic understanding of the NAM incorporating local- and planetary-scale processes that quantitatively relates the Gulf of California (GC) sea surface temperatures (SSTs) to the timing, amount and extent of NAM rainfall. The proposed hypothesis is supported with satellite observations of SST, sea surface height (SSH) and rainfall amount; temperature and humidity profiles from ship soundings launched over the GC; climatologies of SST, outgoing longwave radiation (OLR) and 500 hPa geopotential height reanalysis.

A physical understanding of the NAM is needed to guide improvements in regional and global scale modeling of the NAM and its remote impacts on the summer circulation and precipitation patterns over North America. This understanding is also needed to predict the NAM’s response to global warming. 

bams-2012Summer precipitation bias (% difference; model-observation) for regional models indicates that summer rainfall is generally most under-predicted in the NAM region. (shown by several studies).



Many studies implicate the GC as the dominant moisture source for the monsoon. Our empirical studies of SSTs and NAM rainfall (Erfani and Mitchell, 2014Mitchell et al. 2002) show that:

1) Monsoon rainfall did not occur prior to the onset of GC 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 GC 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 GC reached 29.5°C during a similar time frame, but such warming was delayed in the northern GC. This warming delay coincided with a rainfall delay for AZNM relative to regions farther south. 

The figures show the relationship between GC SSTs and NAM rainfall.  Further details on our methods and results can be found in our Journal of Geophysical Research (JGR) paper: Erfani and Mitchell (2014), and our Journal of Climate (JC) paper: Mitchell et al. (2002).

aznm-cumulative-rainfallAZNM region cumulative normalized rainfall for periods having N. GC SSTs ≤ indicated SST. Time is implicit with increasing SSTs. aznm-mean-rainfallMean rainfall rates for the AZNM region for N. GC SST intervals of 0.5°C, based on five June–August seasons (Mitchell et al. 2002).



Since  Mitchell et al. (2002) was published, we have analyzed 3 other monsoon seasons  at higher temporal resolution regarding SST and AZ rainfall amounts, resulting  in similar findings. Above is the 2012 analysis. These findings indicate relatively heavy rainfall begins after the mean N. GC SST reaches ~ 29.5°C. On  the other hand, relatively heavy convective rainfall occurred in the Great  Basin and Arizona during the first days of July in 2013 when the N. GC SST was well below this threshold. Thus, our local scale  mechanism may describe the main factors governing the Arizona monsoon onset during most years but not all years (Erfani and Mitchell, 2014).


The time evolution of SSTs in 2 different regions of the GC for August 2003.  Note that the Las Vegas flood occurs a few days after the warmest northern GC SST.

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