EVOLUTION OF THE NORTH AMERICAN SUMMER ANTICYCLONE

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• EVOLUTION OF THE NORTH AMERICAN SUMMER
  ANTICYCLONE
• David L. Mitchell1, Beth Hall1, Dorothea Ivanova1
  and Miguel F. Lav?n2
• Desert Research Institute, Reno, Nevada, USA
• 2. Centro de Investigaci?n Cient?fica y de
  Educaci?n Superior de Ensenada (CICESE),
• Ensenada, Baja California, Mexico

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Annual Rainfall Contributions of the North
American Monsoon NW Mexico 60-80 Arizona
35 New Mexico 45
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Mean rainfall rates for the AZNM region for N. GC
SST intervals of 0.5oC based on five June-August
seasons.
AZNM region cumulative normalized rainfall for
periods having N. GC SSTs indicated SST. Time
is implicit with increasing SSTs.
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Modeled surface circulation of the GC by Beier
(1997 J. Phys. Oceanography). Islands in red
sea surface elevation in yellow.
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Dynamic height and geostrophic current velocities
(blue arrows) for 15 May 2003. Black line is 50
cm contour. From NOAA/AOML website, developed by
Joaquin Trinanes.
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Dynamic height and geostrophic current velocities
(blue arrows) for 25 May 2003. Black line is 50
cm contour. From NOAA/AOML website, developed by
Joaquin Trinanes.
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Evolution of the 26oC and 28.5oC isotherms for
the warm pool, based on SST climatology
(1983-2000). Images are 5-day means centered on
June 3rd and June 28th. Courtesy of Miguel Lav?n
at CICESE.
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Poleward Migration of the North American
Anticyclone Working Hypothesis

• As warm SSTs ( gt 26oC) move up the Mexican coast,
  deep convection is triggered
• as shown previously and in earlier work.
• 2. The poleward advance of deep convection over
  the Sierra Madre Occidental is
• accompanied by a branch of descending air aloft
  to the north of the deep convection.
• This may position the center of the anticyclone.
• 3. Given (2), the observed SST-rainfall relation
  implies the anticyclones position
• depends on the poleward extent of the warm SSTs.
• The mid-level monsoon moisture supply is largely
  governed by this anticyclone,
• with moist tropical air often to the south and
  west of the anticyclone center. The summer
• circulation over North America is dominated by
  this anticyclone.

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Evolution of NA Monsoon Onset From Higgins et
al. 1999, J. Climate The timing of onset
coincides with the arrival of SSTs gt 26oC
north of Cabo Corrientes, as shown in Mitchell et
al. 2002 (J. Climate).
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Evolution of the 27.5oC isotherm and the 500 mb
anticyclone
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Evolution of the 27.5oC isotherm, the 500 mb
anticyclone center, and the 250-260 W/m2 OLR
gradient (transition between regions of rising
and descending air).
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MM5 simulations of the anticyclone at 600 mb for
July, using 3 convection schemes (a)
Betts- Miller-Janjic, (b) Kain-Fritsch and (c)
Grell. 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. Simulations based on Reynolds-Smith
SST data (optimum interpolation method), which
under- estimate the GC SSTs by 2-6oC 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.
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Reynolds-Smith climatological SSTs (1974-1993)
for July with OLR fields (numbers and solid
curves). Dark orange 29oC, with 1oC change per
color change. Northern GC is about 24oC.
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Mean 500 mb circulation showing anticyclone
center (in yellow) and mixing ratios. Based on
MM5 simulation for 10 July 10 August, where GC
SST 29.5oC. Taken from Stensrud (1995 J.
Climate).
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SUMMARY

• Previous work based on satellite observations and
  modeling reveal the existence of a
• threshold SST of about 29.5oC for the northern GC
  that triggers widespread and relatively
• heavy rainfall in Arizona. Moreover, the
  incremental advance of SSTs gt 26oC up the
• Mexican coast appears necessary for the northward
  advance of the monsoon.
• The extremely warm water in the GC appears
  primarily due to advection of warm
• pool water into the GC during May-July, resulting
  in a dramatic reconfiguration of the
• warm pool 28.5oC isotherm (based also on earlier
  studies from UABC and CICESE).
• More research is needed to confirm this.
• 3. Satellite SST and OLR data, and reanalysis
  data, suggest that the poleward
• migration of the North American 500 mb
  anticyclone during spring/summer may
• depend on the poleward advance SSTs gt 26oC up the
  Mexican coast. This corroborates
• well with MM5 modeling results, that show the
  mean July position of the anticyclone
• is much farther south when unrealistically cool
  GC SSTs are used. This anticyclone
• dominates the summer mid-level circulation over
  North America.