Ch 18 Global Climate Scales of global climate Recall that climate is loosely defined as "average weather" However, you need to be careful of what geographic area you are talking about Need to define climate on different spatial scales: microclimate - near the ground over your front yard mesoclimate - over a field or few fields (a few square kilometers) macroclimate - scale of a state or country global climate - over entire earth We will focus on large-scale climate Indeed, climate extremes exist on the planet, so what controls climate? 1
Climate Controls Intensity of solar radiation and its variation with latitude Distribution of land and water Ocean currents Prevailing winds Positions of semi-permanent high and low pressure systems Mountain barriers Altitude Distribution of global temperatures Temps have been reduced to sea level Latitudinal change N.H. - difference between land and ocean U.S. west coast (upwelling and cold current) v.s. U.S. east coast (gulf stream) Influence of gulf stream in north Atlantic highest temps are in the subtropical N.H. desert regions West coast of S. America is cool while the east coast is warm, due to the ocean currents Much less variability in the zonal direction in the S. H. Coldest mean temps are over Antarctica, yet the figure to the right suggests it is over the arctic...? 2
Extreme climates Highest temperature ever recorded in North America -- 57 C (134 F) at Greenland Ranch in Death Valley, CA average max temp in July in Death Valley is 116 F!! Hottest urban area is Yuma AZ high temps average 108 F in July Hottest place in the world - Dallol Ethiopia - located south of the Red Sea average annual temperature between 1960-1996 was 94 F! One of the coldest spots in U.S. - International Fall, MN Average temperature in January is 3 F Longest cold streak - Langdon, ND - winter of 1936 temp remained below 0 F for 41 consecutive days Coldest temperature reading in US - Rogers Pass, Montana -70 F on 20 Jan, 1954 Coldest areas of the US: Yukon and the Northwest Territories of Canada Coldest areas in N.H.: Siberia and Greenland Coldest temperature ever recorded anywhere in the world: -129 F at Vostok, Antarctica Global precipitation distribution is tied to the general circulation... Tropics are wet abundant moisture and ITCZ Subtropics are influenced by subtropical highs Mid latitudes wet due to polar front dry on lee-side of mountains wet on upslope side of mountains Polar regions tend to be dry little moisture polar high Distribution of global precipitation A vertical cross section along a line running north to south illustrates the main global regions of rising and sinking air and how each region influences precipitation. 3
Distribution of Global Precipitation Climate Classification - The Koppen System Widely used system that classifies climatic regimes according to: annual and monthly averages of temperature annual and monthly averages of precipitation Devised by Waldimir Koppen (1846-1940), but modified and refined. identified five primary climate types: Tropical moist climates: all months have an average temperature above 18 C (64 F) - no real winter season dry climates: deficient precipitation most of the year. Potential evaporation and transpiration exceed precipitation. moist mid-latitude climates with mild winters: warm-to-hot summers with mild winters. The average temperature of the coldest month is below 18 C (64 F) and above -3 C (27 F) moist mid-latitude climates with severe winters: warm summers and cold winters. The average temperature of the warmest month exceeds 10 C (50 F) and the coldest monthly average drops below -3 C (27 F) Polar climates: extremely cold winters and summers. The average temperature of the warmest month is below 10 C (50 F). There is no real summer season. 4
Climate Classification The Koppen system has been modified, most notably by C. Warren Thornthwaite (1899-1963) Thornthwaite emphasized the importance of precipitation (P) and evaporation (E) by devising the P/E ratio... The P/E ratio is monthly average precipitation divided by monthly average evaporation. Five major humidity provinces and their characteristic vegetations: rain forest, forest, grassland, steppe, and desert. Potential evapotranspiration (PE) the amount of moisture that would be lost from the soil and vegetation if the moisture were available. Worldwide distribution of climatic regions (after Köppen). 5
Worldwide distribution of climatic regions (after Köppen). Tropical Moist Climates General Characteristics - year-round warm temperatures (all months have mean temperatures above 18 C [64 F]) and abundant rainfall (typical annual average exceeds 150 cm [59"]) Extent - 15-25 north and south of the equator Major types: 1. Tropical wet (Af) small seasonal temperature variations (less than 3 C) solar radiation is constant year round the diurnal temperature variation is larger than the seasonal variation! moist, convective showers most every afternoon example at Iquitos, Peru 2. Tropical monsoon (Am) similar to Af, with the exception that monthly average precipitation falls below 6 cm for a month or two - not enough to affect the rain forest, however. 3. Tropical wet and dry (Aw) located just pole ward of the tropical wet regions have a distinct dry season jungle areas transforms into savanna grasslands obtain same amount of precip as Af, rain rates fluctuate greatly during the year and from year to year example at Timbo, Guinea (11 N) 6
Tropical wet (Af) Tropical rain forest near Iquitos, Peru. Temperature and precipitation data for Iquitos, Peru, latitude 4 S. Tropical wet and dry (Aw) Baobob and acacia trees illustrate typical trees of the East African grassland savanna, a region with a tropical wet-and-dry climate (Aw). Climatic data for Timbo, Guinea, latitude 11 N. 7
Dry Climates General Characteristics - deficient precipitation most of the year; potential evaporation and transpiration exceed precipitation Occupies 26% of total land area - more than any other climate type evaporation > precipitation Extent - the subtropical deserts extend from roughly 20-30 latitude in large continental regions of the middle latitudes, often surrounded by mountains. Major types: - 1. arid (BW) - the "true desert" further subdivided into BWh (hot) and BWk (cold),12% of worlds land mass created by subsidence associated with semi-permanent highs or subsidence on lee side of mountain ranges BW has highest temps in the world, winters can be somewhat moderate - min temps in the 20s example of Phoenix AZ 2. semi-arid (BS) further subdivided into BSh (hot) and BSk (cold), also called Steppe climate short bunch grass, low bushes, sagebrush similar to BW, but receives more rainfall, found on edges of BW regions example of Denver, CO Hot arid BWh Creosote bushes and cacti are typical of the vegetation found in the arid southwestern American deserts (BWh). Climatic data for Phoenix, Arizona, latitude 33.5 N. A station with an arid climate (BWh). 8
Semi-arid (BS) Cumulus clouds forming over the steppe grasslands of western North America, a region with a semi-arid climate (BS). Climatic data for Denver, Colorado, latitude 40 N. A station with a semi-arid climate (BSk). Moist Subtropical Mid-latitude Climates General Characteristics - humid with mild winters (average temperature of the coldest month below 18 C (64 F) and above -3 C 27 F)); have distinct summers and winters and ample precipitation Extent - on the eastern and western regions of most continents, from about 25-40 latitude Major types: 1. humid subtropical (Cfa) largely found along east coasts of continent, hot, muggy summers -- usually on western side of subtropical highs mild winters, some snow, frost, ice, though heavy snows are rare afternoon convection is common in summer, tropical storms example from mobile, Alabama 2. marine (Cfb) extend to 40-60 latitude on the western sides of continents prevailing westerlies moderate the climate near the coast, compared to the same latitude further inland relatively mild winters, cool summers; lots of low clouds, fog, drizzle, especially during fall, winter, and spring; little snow, except at higher altitudes in the mountains example for Port Hardy, Vancouver, Island 9
Humid subtropical climate (Cfa) Marine climate (Cfb) Climatic data for Mobile, Alabama, latitude 30 N Climatic data for Port Hardy, Canada, latitude 51 N. Moist Subtropical Mid-latitude Climates 3. dry-summer subtropical (also referred to as Mediterranean) (Cs) similar to marine (Cfb), but with a more pronounced dry season found to the south of marine regions, where high pressure areas are more dominant (Oregon, California) mild-hot, dry summer mild, wet winters- subtropical high moves south allowing midlatitude cyclones to affect the region further subdivide Mediterranean into: coastal Mediterranean (Csb) moderated by coastal upwelling mild, summertime highs in the 70's, lows in the 50's-60's example at San Francisco, CA interior Mediterranean (Csa) much warmer, summertime highs in the 90's example at Sacramento, CA 10
Comparison of a coastal Mediterranean climate, Csb (San Francisco, at left), with an interior Mediterranean climate, Csa (Sacramento, at right) Moist Continental General Characteristics - warm to cool summers and cold winters. Winters are severe with snow storms, blustery winds, bitter cold - climate is controlled by large continent controlled by large land masses - found only in northern hemisphere found between about 40-70 latitude warm-cool summers, cold winters average temperature of coldest month must dip below -3 C (27 F)) - Koppen found that this temperature value marks the southern extent of persistent snow cover during the winter Extent - north of moist subtropical mid-latitude climates Major types: - 1. humid continental with hot (Dfa) summers or cool summers (Dfb) observed between 40-60 latitude evenly distributed precip during the year - 20-40 inches per year Dfa - hot, humid summers - summertime temps in the 90's - warm, humid evenings Dfa - growing season is about 5-6 months example for Des Moines, IA Dfb - cooler, located north of Dfa region growing season is about 3-5 months slightly larger temperature range than Dfa example for Winnipeg, Canada 11
Comparison of a humid continental hot summer climate, Dfa (Des Moines, at left), with a humid continental cool summer climate, Dfb (Winnipeg, at right). The leaves of deciduous trees burst into brilliant color during autumn over the countryside of Adirondack Park, a region with a humid continental climate. Moist Continental 2. sub polar (Dfc) much of Canada, Alaska, Norway, Siberia source regions of cp, ca air masses severe winters, short, cool summers 1-3 months with mean temperature exceeding 10 C (50 F) small amounts of precip, though evaporation rate is also low hence, there is adequate moisture to support the boreal forests of conifers and birches example for Fairbanks, Alaska Coniferous forests (taiga) such as this occur where winter temperatures are low and precipitation is abundant. 12
Polar General Characteristics - year round low temperature (average temperature of the warmest month is below 10 C (50 F)) Extent - northern coast areas of North America and Eurasia; Greenland and Antarctica Major types: - 1. polar tundra (ET) ground is permanently frozen to depths of hundreds of meters - called permafrost meager annual precip very cool summers - low sun angle extremely cold winters example for Barrow, Alaska 2. polar ice caps (EF) average temperature for each month is below freezing interior ice sheets of Greenland and Antarctica very meager precip - no moisture example for Eismitte, Greenland Climatic data for Eismitte, Greenland, latitude 71 N, a polar ice cap climate (EF). Tundra vegetation in Alaska. Climatic data for Barrow, Alaska, latitude 71 N. A station with a polar tundra climate (ET). 13
Highlands Polar climates can be observed at lower latitudes, you just need to increase your elevation, as one would ascending a mountain chain 14
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