PREDICTING THE ONSET OF THE NORTH AMERICAN MONSOON ANDPROGRESS TOWARD A MECHANISTIC UNDERSTANDING

David L. Mitchell1, Dorothea Ivanova1, Beth Hall1, Miguel F. Lavn2, and Affonso S. Mascarenhas, Jr.3

1. Desert Research Institute, Reno, Nevada, USA2. Centro de Investigacin Cientfica y de Educacin Superior de Ensenada (CICESE),

Ensenada, Baja California, Mexico3. Instituto de Investigaciones Oceanologicas, Universidad Autonoma de Baja California,

Ensenada, Baja California, Mexico

Annual RainfallContributions of theNorth American Monsoon:

NW Mexico: 60-80%Arizona: 35%New Mexico: 45%

AZNM region cumulative normalized rainfallfor periods having N. GC SSTs ≤ indicatedSST. Time is implicit with increasing SSTs.

Mean rainfall rates for the AZNM regionfor N. GC SST intervals of 0.5oC based onfive June-August seasons.

7 July 2002at 00 UTC,Total Precip.Water (TPW)

9 July 2002at 00 UTC,TPW

11 July 2002at 00 UTC,TPW

2.4oC increase in N. GC SST over 2 days,reaching 29.8oC.

11 July 2002at 00 UTC.24 hour rainfallaccumulation

15 July 2002at 00 UTC.24 hour rainfallaccumulation

N. GC 26oC; 28oC otherwise N. GC 29oC; 30oC otherwise N. GC. 30oC; 30oC otherwise

Sensitivity of CAPE (Convective Available Potential Energy) to GCsea surface temperatures. Simulation time = 54 h; 11 pm LST.

Yellow ≈ 1300 J/kg (CAPE)Orange ≈ 2100 J/kgRed ≈ 2900 J/kgBrown ≈ 3700 J/kg

Modeled surface circulation of the GC by Beier (1997; J. Phys. Oceanography).Islands in red; sea surface elevation in yellow.

Sea surface height images courtesy of Dr. Ted Strub, Cooperative Institute of Oceanography, Oregon State University

SSTs on16 May 1999Red ≈ 29-30oC,Light orange ≈ 25oC March-April

9-year climatology

May-Juneclimatology

July-Augustclimatology

Sea surface heights

Sea surface heightsSea surface heights

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.

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.

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 Lavn at CICESE.

Sea height anomaly (SHA) index based on anomalies in 3 coastal and 1 open ocean region.Coastal: GC entrance, between the “Cape” and GC entrance, and southern MexicoOpen Ocean: Off southern Mexico (the “dome”)1 rating point = | 3 cm sea surface height |Negative coastal ratings: SHA Index = |coastal rating – 1.5| + off-shore ratingPositive coastal ratings: SHA Index = off-shore rating – coastal rating

Based on 10 seasons ofTOPEX/POSEIDONsatellite altimeter data.

Preliminary analysis relating the May SHA Index to the onset of the 29.5oC N. GC SST.

Statewide AZ June-August rainfall amounts, 1983-2002, vs. N. GC 29.5oConset date, using 18 or 14 km weekly SSTs from JPL or NOAA. Zero and negative values are prior to July 1st. Horizontal line = median rainfall value.

Deviation from normal for Jul-Sep precipitation, based on the 9 wettest AZ seasons for theperiod 1950-99, relative to the long-term mean 1950-95. Color legend gives std. deviations.

Poleward Migration of the North American Anticyclone:Working Hypothesis

1. As warm SSTs ( > 26oC) move up the Mexican coast, deep convection is triggeredas 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. The observed SST-rainfall relation implies the anticyclone’s position depends on the poleward extent of the warm SSTs.

4. 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 summercirculation over North America is dominated by this anticyclone.

Evolution of NA Monsoon Onset

From Higgins et al. 1999, J. Climate

Note timing of onset coincides withthe arrival of SSTs > 26oC northof Cabo Corrientes, as shown inMitchell et al. 2002 (J. Climate).

Evolution of the 27.5oC isotherm and the 500 mb anticyclone

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).

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 theanticyclone below US-Mexico border is consistentwith hypothesis that its position depends on the latitude of the warmest coastal SSTs.

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

Mean 500 mb circulation showing anticyclone center (in yellow) and mixingratios. Based on MM5 simulation for 10 July – 10 August, where GC SST =29.5oC. Taken from Stensrud (1995; J. Climate).

SUMMARY1. Previous work based on satellite observations and modeling reveal the existence of athreshold SST of about 29.5oC for the northern GC that triggers widespread and relativelyheavy rainfall in Arizona. Moreover, the incremental advance of SSTs > 26oC up the Mexican coast appears necessary for the northward advance of the monsoon.

2. 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. The onset of relatively heavy rainfall in AZ may be forecast 1-2 months in advance,based on a relationship linking sea surface topography with the arrival of N. GC SSTs 29.5oC. The timing of this onset indicates whether AZ ( the Midwest) is likely to be average-to-wet (average-to-dry) or whether AZ is likely to be average-to-dry during theperiod June-August. A more rigorous analysis is needed to confirm these results.

4. 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 > 26oC up the Mexican coast. This corroborates well with MM5 modeling results, that show the mean July position of the anticycloneis much farther south when unrealistically cool GC SSTs are used. This anticyclonedominates the summer mid-level circulation over North America.