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In NATURAL HISTORY OF THE COLORADO PLATEAU AND GREAT BASIN, edited by Kimball T. Harper, Larry L. St. Clair, Kaye H. Thorne, and Wilford M. Hess, pp. 27-53. University Press of Colorado, Niwot, Colorado (1994).

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3

Modern and Pleistocene Climatic Patterns in the West Kenneth Lee Petenen OVERVIEW

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C/im£l/e ill tile westenl United States is cOIII/>/ex, {(lid gClleral,ll!tlems are lin/,ed to 1lI11clliarger Il/o/Jalwemher systems l!1ul (0 a deep geologic l1islOr),. A /lcrs/)eclive 011 rile I'mt sllollld hel/> llS face OIlT cllrrcnt concerns ((bOlet Illtl1Wll··illduccd global change, m .I11c11 Ll11iew 1I'ill hetter ellahl" II.I to (>re/J(lre for tile fUllIre. A lirief look at full glacial di­ milles suggests clwt 1111St cllanges have /1al weather system are far more dYllamic (illd complex tItan collld IJ(ltie hern iHwgined a few gencmliolls ago (Earlll S)'s­ tem Sciences COlllmilfee 1988). The klllJlcit of tile fint experimental satellite ill 1960 showed im(l,~es of the carth net'cr be[clre scen lIlld provided a global perspective tlwt was 1III/Iconl of only (l few years em lier. The launch of tile fint 110/(11' orbiting weat!ler sate/­ tiles ill I Y66 inilia/ed (I new era in 1l'eatilcr forecasting £Iwt we now cake for granred. NIlIlleriCllI simil/a/ion of almospheric processes began to /Jecome I)mclical in lite 19605 1I'il/1 tlte 11£II'enr of Itig/l-speed com/llllcrs, lIitllOllg/l tlw I>e,~illltillg of nllmericalll1()delillg 11/1/1 hegllll (IS early as 1948. All [/lese at/I'wlCe; 11(ll'e Iwilled II> IlcHer 1I11dersllllld ollr gill/lalweather, which 1111151 he treated to some dcgrcc ill tilis ella/'ler to /JTOvide a basic hoc/drop for il more dellliled disCII.IS;OIl of tile weatlwr and climate in tile western United Stifles W!c(lt/rer is defined lIS it Slate or wllt/ition of the lItllw.li!ilcre at any /JaniClliar time or

. A 1»)'Oad. iJlte,~mtcd tl'eilliler /Iictrlre is 1{)I()11'11 (15 (l 1t'eatlter system. Meteor%­

Iwl/ooJlS, shiJiS, {llltl cOln/lHlers hi o/!sertJC alld forecllSt the dny~w­

ill COlloml, i.~ really /I sUltis/iUlI WIISlrilCt; (/ dwnl!e in climate

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K. L. Petersen

call be inferred only from a change in swtistics, which hy ac]ml[/()ll are [;alllerea (WeI' long periods of time (LandsiJerg 1987). Climate can he vieu'ed as the cOln/1osite of all weather system.s and events, and it fluctuates on all time scales: lllOn thly, yearly, dec­ centenllially, mille llll ially , and longer. To characterize J)ast c/imme or even to J)roject the futllre course of OIlT climate, cli­ matologists must take into consideration the wide cOIll/llex of natural inflllences that climate; variations in the earth's orbit and tilt, iJreakup and eluHering of cOllti­ nelHal plates and land masses, s/)(lTculic conceHtratiollS of too/ulnic dust in t/IC allIlO­ sphere, great t l arimiol1S in area of .mow and glaciers, changes in the concentratio/l and compmition of gases in the atmosJ)ilere, and the iHi/)((ct of ll1tmans. To assess thest' influences, climawlogists ttlm to long-term instrumental records, old manuscripts, tree rings, fossil pollen, plant and animal remains, isotopes, ice from Greenland and the Antarctic, mud fTom tile bottom of Lakes alld the ocean, and the world's ji.IS/cst COlll/lli!­ ers using extremely coHlJ)lex programs to simulate atmo-lJ)heric and oce(lnic circulation. GENERAL ATMOSPHERIC CIRCULATION

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One cannot describe the climate in the Great Basin and the Colorado Plateau without first examining general concepts of atmospheric circulation. Most simply, the energy imbalance hetween lower and higher latitudes of the cart h dril'e., the general circulation system like a giant engine attempting to energy equilibrium (for example, Oliver and Fairbridge 1987). Surplus energy in several forms is transported from the equatorial regions poleward to the higher hll i­ tudes, where outgoing radiation exceeds incoming solar mutation. This energy transport creates the general patterns of wind for the glohe. A rotating earth pro­ duces a slight but persistent right-hund turn on air parcels moving polewmd. This coriolis deflection, as it is known, results in a band of strong, westerly wind currents in the upper atmosphere of the middle latitudes that surrounds the pole. This band lies at the boundary of the cooler, polar air masses on its poleward side and the W:lrmer, tropical air masses on its equatorward side. If the temperature gradient is relatively small between the lower and higher latitudes, these wind currents travel around the globe in a generally btitudinal fashion, referred to as wnal flow. I Iow­ ever, if this temperature gradient steepens, the circllbtion flow forms a sinuolls pat­ tern moving around the hemisphere with a stronger north-south component, still at the boundary of the polar and tropical air masses. This second configuration that tends to move air in either a north or south direction is referred to as meridional flow. Whatever the configuration, the wind band, called the circumpolar vortex, significantly affects the weather and climate of the earth '5 surface, parricubrly in the middle latitudes, where the boundary between the polar and tropical air masses is normally located. The circumpolar vortex tends to persist (say with four descend­ ing wave troughs), but a change in the annual energy halance of the earth, for whatever reason, can force a stepwise change in the configuration of the circmnpo­ lar vortex (such as a contraction in size and a reconfiguration to three descending wave troughs), creating a c1imat ic fluctuation. At high altitude within the circumpolar vortex is a narrow hand or stream of fast­ moving air called the jet stream (Balling et aL 1987). Variation in wind velocities

MOt/em lind Pleistocene Climatic Patterns in tlte \'\/esf

29

the year is one of the interesting aspects of the jet stream - and of all the upper-air flow. According to the concept of thermal wind, the greater the contrast in thermal energy acr"ss a region, the greater the resulting wind velocity. Therefore, tiJejet slteam, as well as most lipper-air flow, rl'aches its m,lxil11UIIl velocities during the fall and early winter and early spring, when the latitudinal thermal gradient is at a maxilllulIL Conversely, during the summer se,lson of either hemisphere, the veloc­ ity of the lipper-air flow decreases as the latitudinal thermal gradient diminishes to a minimulH. The undulations ohsNved in the lipper-air flow also reach the amplitude (crossing the largest amount of latitude) during the winter. During the sutlllner the high-amplitude wave pattern becomes greatlv reduced as a result of a reduced latitudinal thenn,ll The type of climatic condition found at the surface is associated with the posi­ tiOll of the circumpolar vortex (Oliver and Fairhritlge 1(87). In the vicinity of H wave t rough (counterclockwise circubtion of the westerlies), where air flows fmm highn [0 lower latitudes, temperatures cool as polar air masses, transported south­ ward hy prevailing northwesterly nnw, sprend over the region. Beneath the wave (clockwise circulation of [he westerlies), in cOlltrast, warmer air is advected northward and temperatures \\'mm. Higher than IHlnn:11 pressures are found in the region dominated hy an upper-leVl'I ridge, and I{)\ver pressures are found at the Sut­ r:ICl' in ! he vicinity ()f the upper-level tn . The mean position of these wave features also affects vertical lllotions in the atlllllSI,hne critical to precipitation (Oliver ,1I1d F:lirhridge 1987). Just ahead (do\\'nstream) of the upper-level ridge, air flows in a converging pattern, favoring suhsidence of air parcels toward the surface that inhihits precipitation and fosters conditions. The suhsidence pat!efll alsll favors slower than normal wind speeds. Ahead of the upper-level trough, dh'ergence occurs in the upper atmosphere. This condition favors surface CUl1ver/!ence and convection, with stronger wind speeds at the surface, increasing the potential fm precipitation and prevailing wet conditions. Increases in the amplitude (north-south dimensions) of the long waves accentuate these vertical motions in the atmosphere and reinforce the wet-dry contrast. The surf;lce weather data will reflect these ('at terns only if these long waves remain rela­ tively stationary long enough so that they might be evident on daily or weekly weather maps. the Northern llemisphere winter, all the circulation regimes migrate southward from their normal summertime positions. Conversely, these circulation features migrate northward with the advancement of summer into the Northern Hemisphere (Balling et al. 1987). One of the significant deviations from the gener­ alized pattern of seasonal migration of circumpolar vortex is the development of regions of lower pressure over land durin/! slimmer within the suhtf0pical high-pres­ sure heir. These low-pressure cells, called thermal lows, are predominantly surface manifestations, while aloft the controlling circulation pattern is still one of descending dry air with no precipitation. Fronts (contrasting atmospheric conditions that lllark the contact between two air masses) arc one of the primary mechanisms for generation of precipita­ tion and km(lmndum, Report !lO. 50. Nal;oll'.! Wl'atiwr Serviu', Nat ional (kcanic' and i;;trat ion, Silver SPI ings, 1\·11 \ K. T. 19R6. Historical (·Il\·il'