Large-scale climatologies of North American monsoon

On the large scale, climatologies of North American monsoon (NAM) region, sea surface temperature (SST), outgoing longwave radiation (OLR) and NCEP/NCAR 500 hPa geopotential height reanalysis from 1983 to 2010 support the hypothesis that relatively warm GC (Gulf of California) SSTs (≥ 27.5°C) are generally required for widespread deep convection to initiate in the NAM region, and that the poleward evolution of the NAM anticyclone during June-July is driven by the associated descending air north or north east of the convective region. Indeed, the Rossby wave response to the monsoon latent heating forms a region of adiabatic descent and the anticyclone develops east of monsoon system.  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.  More details are provided in our Journal of Geophysical Research (JGR) paperErfani and Mitchell (2014)and our Atmospheric System Research (ASR) meeting poster: Erfani et al. (2013).

OLR = outgoing longwave radiation | 

The firgure below shows time evolution of the 27.5°C SST isotherm (red curves), the anticyclone center at 500 hPa geopotential height (blue ellipses), and the 240–250 W m−2 outgoing longwave radiation (OLR) gradient (shades of golden) from 18 May to 13 July for the 1983–2010 climatology.  The OLR approximates the NAM boundary; a transition between regions of rising air (wet) and descending air (dry) (Erfani and Mitchell, 2014).

1893 2010 sst olr climatology

The images below show time evolution of a zonal vertical cross section of the 1983–2010 specific humidity (q) (shaded) and horizontal wind vector climatology at 25°N latitude from 18 May to 8 July every 10 days. Specific humidity shading interval is 0.5 g kg−1 (Erfani and Mitchell, 2014).

1983 2010 time evolution zonal cross section

Below is an animation of SST climatology from 1983 to 2000 provided from NASA/JPL data, courtesy of Miguel Lavin, CICESE. Abrupt poleward advance of warmest water is observed from 10 to 20 June.  Northern GC SST is warmer than 29°C from 20 to 25 July. (click to play and click to stop)

Below is an animation of 500 hPa streamline climatology from 1971 to 2000 provided from NCEP/NCAR reanalysis data. Rapid poleward advance of center of 500 hPa high is observed from 8 to 20 June.  Center of 500 hPa high moves from New Mexico to 4-corners region from 22 to 25 July. (click to play and click to stop)

The three images below represent the July mean surface-600 mb pressure integrated divergence (s-1) and surface-600 mb pressure integrated streamline pattern.  Taken from Gochis et al. 2002, Sensitivity of the Modeled North American Monsoon Regional Climate to Convective Parameterization, Mon. Wea. Rev., Vol. 130, 1282-1298.

MM5 simulations of the anticyclone at 600 mb for July, using 3 convection schemes: (a) Betts- Miller-Janjic, (b) Kain-Fritsch and (c) Grell.

Simulations are based on Reynolds-Smith SST data (optimum interpolation method), which under-estimate the GC SSTs by 2-6°C during July, especially in the northern GC.  The position of the anticyclone below US-Mexico border is consistent with hypothesis that its position depends on the latitude of the warmest coastal SSTs.

 july-mean-surface-a july-mean-surface-b 



Reynolds-Smith climatological SSTs (1974-1993) for July with OLR fields (numbers and solid curves).  Dark orange = 29°C, with 1°C change per color change.  Northern GC is about 24°C.



Below is a simulation of the NAM for July when Gulf of California SSTs were fixed at 29.5°C.  The position of the 500 hPa anticyclone is now consistent with climatology, possibly due to realistic GC SSTs.  Taken from Stensrud, D. J., R. L. Gall, S. L. Mullen and K. W. Howard, 1995: Model climatology of the Mexican monsoon. J. Climate, 8, 1775-1794.