A mechanistic understanding of the evolution of the North American monsoon

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Description

The North American Monsoon (NAM) is a large-scale synoptic feature having a strong impact on summer rainfall patterns and amounts over North America. For example, anomalously wet NAMs in Arizona are strongly anti-correlated with anomalously dry summers in the Midwest. 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 evolution remains elusive. A correct physical understanding of the NAM is thus likely to improve the prediction of summer precipitation over North America in regional and global-scale models.

Here we propose a partial mechanistic understanding of the NAM incorporating local- and planetary-scale processes. The proposed hypothesis is supported with satellite observations of sea-surface temperature (SST), sea-surface height (SSH), and rainfall amount; temperature and humidity profiles from soundings launched over the Gulf of California (GC); climatologies of SST, outgoing longwave radiation (OLR), and 500 hPa streamline reanalysis; and regional-scale modeling of the NAM region.

On the local scale, these measurements and modeling demonstrate that relatively heavy summer precipitation in Arizona generally begins within several days after northern GC SSTs exceed 29°C. The mechanism for this relates to the marine boundary layer (MBL) over the northern GC. For SSTs < 29°C, GC air is capped by an inversion ~ 50–200 meters above the surface, restricting GC moisture to this MBL. The inversion weakens with increasing SST and generally disappears once SSTs exceed 29°C, allowing MBL moisture to mix with free tropospheric air. This results in a deep, moist layer that can be advected inland to produce thunderstorms.

On the synoptic scale, climatologies of NAM region SST, OLR, and NCEP/NCAR 500 hPa streamline reanalysis support the hypothesis that relatively warm SSTs (≥ 27.5°C) are generally required for widespread deep convection to initiate in the NAM region, and that the poleward evolution of the monsoon anticyclone during June–July is driven by the associated descending air north of the convective region. As warm Pacific SSTs propagate northwards up the Mexican coastline, deep convection follows this northward advance, advancing the position of the anticyclone. This evolution brings mid-level tropical moisture into the NAM region. This study may provide a basis for more productive NAM research.