Climate of North America 101

• What are the major controls on North American climate?

• What is the dominant flow pattern across North America in winter?

• How does this pattern influence North American climate?

• What are the main spatial patterns of temperature and precipitation across the US and what controls them?

• How are the major climate regions in the US related to precipitation regimes?

• How are these related to hydrologic variability?

Factors Influencing the Climatic Regions

• Latitude and the solar radiation received

• Location of global high and low pressure centers

• Heat exchange from ocean currents

• Continentality; distribution of land and sea

• Topography; distribution of mountain barriers

• Pattern of prevailing winds

Controls on North American Climate

North America lies between ~20N and 80N. Moving from south to north, climate is influenced by:

• Poleward arm of the Hadley cell associated subtropical high pressure

• Convergence of subtropical and polar air at the polar front

• These features shift seasonally, and influence climatehttp://www.physicalgeography.net/fundamentals/7v.html

Controls on North American Climate

Wind belts and jet streams associated with patterns of circulation shift seasonally as well. The polar front and polar jet stream are two of the most important controls on midlatitude North American climate.

Controls on North American Climate

Distinctive, large-scale patterns of surface pressure and winds result from solar radiation, global circulation, and thermal properties of oceans and land.

• In winter, low pressure in the region of the Aleutian Islands and Iceland (relatively warm oceans), and high pressure over North American (Great Basin High) are key features.

• In summer, the oceanic high pressure (relatively cool surface) in both the Pacific and Atlantic Oceans affect west coast and southeastern US Climate

Controls on North American Climate

Ocean currents, warm and cold, especially on the west coast can moderate temperatures.

• Oceans as a moisture source; Pacific and Gulf of Mexico

• Continentality – the influence of land mass and distance from the ocean results in the diurnal (from night to day) range of temperatures, as well as the difference in temperature between summer and winter.

http://photojournal.jpl.nasa.gov/jpegMod/PIA03377_modest.jpg

Controls on North American Climate

Topography: mountain ranges influence air flow, precipitation patterns, and local climate because of elevation.

Circulation across North American

• Dominant flow pattern is called the Pacific/North American (PNA) pattern

• Flow pattern is guided by semi-permanent upper-tropospheric pressure centers, and the topography of the Rocky Mountains

700 mb pressure patterns

Winter 700 mb jet stream variability controls precipitation across the USA

Pacific/North American (PNA) pattern

Variations of the Pacific/North American pattern influence the jet stream and the winter storm track, and well as the incidence of cold outbreaks in the eastern US.

US Temperature Patterns

• Maximum and minimums show influence of seasonality

• In the eastern half of the US, pattern reflect latitude

• In the western US, patterns are complicated by topography of mountain ranges and high plateaus

• Elevation plays an important role

Scale is not the same in these 2 figures

US Precipitation Patterns

• More complex due to topography, moisture source regions, circulation patterns

• Marine west coast climates restricted by coastal ranges, with rainshadow east of mountain ranges (e.g., east of Sierras, Cascades, Rocky Mountains

North American Climate Regions based on Precipitation Regimes

• seasonal precipitation

• snowmelt vs rainfall dominated watersheds

Regional climates are reflected in the temporal patterns of runoff

http://www.epa.gov/owow/watershed/wacademy/wam2003/3e-hydrology.pdf

annual hydrograph for winter storm-driven regime

http://www.fws.gov/southwest/sjrip/pdf/DOCHGHf_Chapter_2.pdf

There is variability in the amount and timing of runoff, and well as variations of annual runoff due to natural climate variability at a variety of time scales….

Lees Ferry Streamflow, Annual Flow, Reconstructed and Observed

… as well as changed projected due to warming from anthropogenic climate change

Observed and projected conditions for the Colorado River Basin above Lees Ferry, using 11 models and 2 scenarios downscaled to the Colorado River basin (from Christensen and Lettenmaier 2007)

SUMMARY POINTS:

• A variety of factors influence North American climate, including the distribution of solar radiation, atmospheric circulation, land masses and oceans, and topography.

• The Pacific/North American (PNA) pattern is the dominant flow pattern across North America, and is most important in winter.

• Patterns of temperature are largely related to latitude in the eastern US, and to latitude and elevation in the western US

• Patterns of precipitation are more complex, especially in the western US.

• Precipitation regimes (characteristics that include relative amount, seasonality, moisture source) can be used to define North American climate regions.

• Precipitation regimes, along with elevational effects of temperature and snowpack, influence annual hydrographs

• Natural and anthropogenic climate variability (as well as other human activities) translate to hydrologic variability.