SEASONAL ATMOSPHERIC CIRCULATION WESTERN NORTH AMERICA

Sheppard 2002

REGIONAL CLIMATE

• The Pacific High moves northward and prevents storms from entering the continent<
• Arizona Monsoon: circulation around the Bermuda High brings moisture northward
• Arizona Monsoon: Plateau Thermal Low brings moist air into Southwest from the Tropical Pacific as surges of moist air

Tonopah Low

1983 flood

CLIMATIC REGIONS: Equivalent Potential Temperature Boundaries and wind-flow patterns follow major topographic features Pyke, 1972; Mitchell, 1976

SEASONAL ATMOSPHERIC CIRCULATION DURING THE Insolation Maximum 12 to 9,000 ya (Northern Hemisphere ): Increased summer temperatures increased stability of east Pacific High, and increased evaporation, producing an "EARLY HOLOCENE XEROTHERMIC" in the Pacific Northwest. Hebda & Matthews (1984), Clague & Mathewes (1989), Barnosky (1984), (Davis et al., 1984), (Davis et al., 1985) Greater summer contrast (cooler ocean, warmer land), and enhanced circulation around the Bermuda High increased monsoonal precipitation, producing higher lake levels and an "EARLY HOLOCENE PLUVIAL" Oviatt (1988), Spaulding and Graumlich (1986), Davis and Sellers (1987, 1989)

CLIMATE MODELS:

Energy Balance Models (simple, fast, global or latitudinal)

Radiative-Convective Models (vertical distribution gasses)

General Circulation Models (horizontal distribution, mountains)
Resolution low: 4.5^o Lat x 7.5 ^o Long, scale limited by complexity of climatic system and cost of computer time.

Mid-Holocene 8-3K, Orbital Forcing Diffenbaugh & Sloan 2004. Amer. Meteorol. Soc. Northwest Drier!

• Complexity: # of atmosphere layers, topography, computation vs. parameterization
• Sensitivity: same input, 3-fold differences in output (precip.) (Chess et al., 1989)
• Stability: wild fluctuations due to feedbacks
• Resolution: 4.5^o x 7.5 ^o or less
• Stability: linkages to oceanic, vegetation, other models

Past: Insolation, Aerosols, Ice Distribution, Sea Surface Temp., CO₂

Past: Temperature, Precipitation, Wind direction where input parameters not gathered.

CLIMAP 1976

Cooperative Long-range Investigation, Mapping and Prediction


• reconstructed climate for the the Last Glacial Maximum (LGM)
  based on certain organism abundances and ¹⁸O/¹⁶O
  annual: greatest cooling N. Atlantic, S. Indian/Pacific Oceans
• Latitudinal temperature gradient was stronger than today
  • zonal flow may have been enhanced
  • highs and lows may have been shifted equatorward
• Lowering of sea surface temperature was greatest in summer on the Atlantic coast, but greatest in winter along the Pacific coast south of 40^o
  
  
• Climatic change in tropics was minimal: 0 - 2 ^oC - WRONG!
  • current thinking: 3 - 6 ^oC colder in tropics, 12-16 ^oC at poles
    
  • snow lines 1000 m lower in tropics (Rind & Peteet, 1985)
    Pattern of lowering! Mark et al. (205)
    
  • tropical glaciers ¹⁸O/¹⁶O indicate 8 - 12 ^oC colder (Thompson et al., 1995)
    
  • vegetation zones lower in Brasil (Clapperton, 1993)
    
  • ocean coral Sr/Ca ratios indicate 6 ^oC colder (Stiute et al., 1995)
    
  • heat transport by oceans (8.2-4.6 ^oC) colder (Webb et al 1997)
    
  
  

COHMAP 1988

Annual, Seasonal Temperature, Precipitation, Soil Moisture, Atm. Pressure, Vegetation
• including CO₂ reduces temperature 8.2-4.6 ^oC
  
• sea ice (reduced surface currents) increases temperature 8.2 - 4.2. ^oC (Webb et al 97)
  
• albedo difference Turdra vs. Boreal forest = 4 ^oC (Foley et al., 1994)
  

COHMAP "Input Paramaters" IVA COST

COHMAP Results The depressed polar jetstream see (Licciardi, 2001)