Monday, June 20, 2011

The June Climate Transition

June is typically an important transition month over southwestern North America.  It is during this month that an upper-level anticyclone develops and builds westward from over the Gulf Coast to northern Mexico, enabling relatively dry, large-scale westerly and southwesterly flow to be replaced by comparatively moist easterly and southeasterly flow over much Mexico.  This can be seen in the 10-year climatology of 500-mb height below.

Mean pentad (5-day averages) 500-mb height for 1979-88.
Source: Adams and Comrie (1997)
This circulation change is characteristic of the so-called North American Monsoon, and it leads to a pronounced warm-season peak in precipitation across much of Mexico and portions of the American SouthWest (especially southeast Arizona and New Mexico).

Seven-day mean 500-mb analyses for the first part of June show evidence of this large-scale circulation change this year.  In particular, note below the development and intensification of locally high 500-mb heights over northern Mexico, especially during the past week.

People often talk about the monsoon here in Utah, but I've never considered Utah to be part of the monsoon region.  Instead, we are in what I like to call the monsoon "surge" region, meaning that we are north of the primary monsoon region, but can get surges of monsoon moisture and precipitation into our area.

Anyone care to bet what sort of monsoon "surge" season we will have this year?

1 comment:

  1. I'll take a crack at it!

    The storm track has stayed active across our area a little longer than usual... evidenced by below normal heights over southwestern Canada. So even though the climatological high over northern Mexico seems like it's off to a decent start, I'll take Intermountain trough passages to remain above normal for a bit still.

    That would slow/curtail the ability of the high to expand northwestward towards our region, and limit the monsoonal surges.

    (...sure to be lamenting this post in a few months)