P48 Mechanisms governing the onset, extent and strength of the North American Monsoon

Erfani, Ehsan, David L. Mitchell, Desert Research Institute, Dorothea Ivanova, Embry-Riddle Aeronautical University

The North American Monsoon (NAM) is a large-scale feature having a strong impact on summer rainfall patterns and amounts over North America. 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. 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 ship soundings launched over the Gulf of California (GC); climatologies of SST, outgoing longwave radiation (OLR) and 500 hPa geopotential height reanalysis; regional scale modeling of the NAM region (WRF).

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 a strong inversion ~ 50-200 m 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 large scale, climatologies of NAM region SST, OLR and NCEP/NCAR 500 hPa geopotential height reanalysis from 1983 to 2010 support the hypothesis that relatively warm GC 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. A set of carefully designed simulations is used to investigate the dependence of NAM convection and the 500 hPa anticyclone on SSTs along the Mexican coastline and in the GC.

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.