The Atmospheric System 27 the vertical circulation described by the Ferrell N and S latitudes also produces relatively cell(Fig.2.6)and a horizontal pattern of high an area the w-pre sure commonly observed fro moves the expected easterly t by the Co dir ing west on on a plan The locations of the ITCZ and of each cir- by both land-ocean heating contrasts and the culation cell shifts seasonally because the zone locations of large mountain ran such as of maximum solar radiation input varies the Rockies and the Himalavas.These moun. from summer to winter due to Earth's 23.5 tain barriers force the Northern Hemisphere tilt with respect to the plane of its orbit around westerlies vertically upward and to the north. the sun.The seasonal changes in the location Downwind of the mountains,air descends of these cells contribute to the seasonality of and moves to the south,forming a trough,much climate. like the standing waves in the rapids of a The uneven distribution of land and oceans fast-moving river that are governed by the eanthat modes the general latitadial location of rocks in the riverbed.Temperatures the gen are comparatively low in the troughs,due to the and the igh polar air,and are com in the air is over the an tha over the examp results ir creates hie over the Atlantic southerly of the and Pacific Oceans (the Bermuda and Pacific eastern north am Although planetary highs,respectively)and over the southern waves have preferred locations they are no oceans (Fig.2.7).At 60N.where air is rising static.Changes in their location or in the there are seminermanent low. sure zones number of waves alter regional patterns of over iceland and the aleutian islands (the ice. climate.These step changes in the circulation landic and Aleutian lows.respectively).These pattern are referred to as climate modes. lows are actually time averages of mid-latitude Planetary waves and the distribution of storm tracks rather than stable features of the major high-and low-pressure centers explain circulation.In the Southern Hemisphere,there manv details of horizontal motion in the atmo is little land at 60S;therefore,there is a trough sphere and therefore the patterns of ecosystem centers su ow-pre movemen is, e we He 60 Eig.2.7du emis olis for )(i 7 subtro ssure centers at 30N and S cause inward toward low-pressure centers in a quator on the terclockwise direction in the Northern Hemi west coasts of continents.creating dry medi sphere and in a clockwise direction in the terranean climates at 30 N and s(Fig 2 7)Or Southern Hemisphere.Air in the low-pressure the east coasts of continents subtropical highs centers rises to balance the subsiding air in cause warm moist equatorial air to move north- high-pressure centers.The long-term average of ward at 30N and S,creating a moist subtropi- these vertical and horizontal motions produces cal climate (Fig.2.7)
The Atmospheric System 27 early sailors as the doldrums. Subsiding air at 30° N and S latitudes also produces relatively light winds, an area known as the horse latitudes. The surface air that moves poleward from 30° to 60° N and S is deflected toward the east by the Coriolis forces, forming the prevailing westerlies, or surface winds that blow from the west. The locations of the ITCZ and of each circulation cell shifts seasonally because the zone of maximum solar radiation input varies from summer to winter due to Earth’s 23.5° tilt with respect to the plane of its orbit around the sun. The seasonal changes in the location of these cells contribute to the seasonality of climate. The uneven distribution of land and oceans on Earth’s surface creates an uneven pattern of heating that modifies the general latitudinal trends in climate. At 30° N and S, air descends more strongly over cool oceans than over the relatively warm land because the air is cooler and more dense over the ocean than over the land. The greater subsidence over the oceans creates high-pressure zones over the Atlantic and Pacific Oceans (the Bermuda and Pacific highs, respectively) and over the southern oceans (Fig. 2.7). At 60° N, where air is rising, there are semipermanent low-pressure zones over Iceland and the Aleutian Islands (the Icelandic and Aleutian lows, respectively). These lows are actually time averages of mid-latitude storm tracks rather than stable features of the circulation. In the Southern Hemisphere, there is little land at 60° S; therefore, there is a trough of low pressure instead of distinct centers. Air that subsides in high-pressure centers spirals outward in a clockwise direction in the Northern Hemisphere and in a counterclockwise direction in the Southern Hemisphere (Fig. 2.7) due to an interaction between Coriolis forces and the pressure gradient force produced by the subsiding air. Winds spiral inward toward low-pressure centers in a counterclockwise direction in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere. Air in the low-pressure centers rises to balance the subsiding air in high-pressure centers.The long-term average of these vertical and horizontal motions produces the vertical circulation described by the Ferrell cell (Fig. 2.6) and a horizontal pattern of highand low-pressure centers commonly observed on weather charts (Fig. 2.7). These deviations from the expected easterly or westerly direction of prevailing winds are organized on a planetary scale and are known as planetary waves. These waves are influenced by both land–ocean heating contrasts and the locations of large mountain ranges, such as the Rockies and the Himalayas. These mountain barriers force the Northern Hemisphere westerlies vertically upward and to the north. Downwind of the mountains, air descends and moves to the south, forming a trough, much like the standing waves in the rapids of a fast-moving river that are governed by the location of rocks in the riverbed. Temperatures are comparatively low in the troughs, due to the southward movement of polar air, and are comparatively high in the ridges. The trough over eastern North America downwind of the Rocky Mountains (Fig. 2.7), for example, results in relatively cool temperatures and a more southerly location of the arctic treeline in eastern North America. Although planetary waves have preferred locations, they are not static. Changes in their location or in the number of waves alter regional patterns of climate. These step changes in the circulation pattern are referred to as climate modes. Planetary waves and the distribution of major high- and low-pressure centers explain many details of horizontal motion in the atmosphere and therefore the patterns of ecosystem distribution. The locations of major high- and low-pressure centers, for example, explain the movement of mild moist air to the west coasts of continents at 60° N and S, where the temperate rainforests of the world occur (the northwestern United States and southwestern Chile, for example) (Fig. 2.7). The subtropical high-pressure centers at 30° N and S cause cool polar air to move toward the equator on the west coasts of continents, creating dry mediterranean climates at 30° N and S (Fig. 2.7). On the east coasts of continents subtropical highs cause warm moist equatorial air to move northward at 30° N and S, creating a moist subtropical climate (Fig. 2.7)
2.Earth's Climate System 90 180 90 January H H H so July 18 H H 60 Longitude 180 by C.Ahrens1998,by permission of Brooks/Cole,an imprint of the Wadsworth Group,a division of Thomson Learning:Ahrens 1998.) The Oceans wher Ocean Structure is less maintainraitherstabielayesihaldoahGtocCal Oceans maintain rather stable lavers with ix.The t limited vertical mixing between them.The sun er.which interacts the atmos
28 2. Earth’s Climate System ITCZ L L L H H H H H H H L H L H 90 90 90 90 90 90 180 180 0 0 60 60 60 60 0 0 30 30 30 30 Latitude Longitude Icelandic low Bermuda high Pacific high Siberian high Aleutian low January A 90 90 90 90 180 90 180 0 0 60 60 60 60 0 0 30 30 30 30 Latitude Longitude 90 ITCZ L H H H H H Bermuda high Pacific high H Icelandic low L Siberian low L Thermal low L July B Figure 2.7. Average surface wind-flow patterns and the distribution of sea level pressure for January (A) and July (B). (Redrawn from Essentials of Meteorology: An Invitation to the Atmosphere, 2nd edition, by C. Ahrens © 1998, by permission of Brooks/Cole, an imprint of the Wadsworth Group, a division of Thomson Learning; Ahrens 1998.) The Oceans Ocean Structure Oceans maintain rather stable layers with limited vertical mixing between them. The sun heats the ocean from the top, whereas the atmosphere is heated from the bottom. Because warm water is less dense than cold water, oceans maintain rather stable layers that do not readily mix. The uppermost warm layer of surface water, which interacts directly with the atmos-
The Oceans 29 phere,extends to depths of 75to200m,depend- brates)and are the locations of many of the world's major fisheries. primary prod apte diff Ocean Circulation er major dete ed by Ocean cri paysloeinErth' that density of both t ocean water air warm n water can sink.if it is salty enough fer from the equator to the oles.with the There are relatively sharp gradients in tem perature (thermocline)and salinity (haloeline) through the atmosphere.The ocean is the dom between warm surface waters of the ocean and inant heat transporter in the tropics.and the cooler more saline waters at intermediate atmosphere plays the stronger role at mid- depths(200 to 1000m)(Fig.2.8).These two ver- latitudes.The surface currents of the oceans tical gradients cause the surface waters to be are driven by surface winds and therefore less dense than deep water,creating a stable show global patterns(Fig.2.9)that are gener vertical stratification.The deep layer there- ally similar to those of the prevailing surface fore mixes with the surface waters slowly over winds (Fig.2.7).The ocean currents are hundreds to of years These deeper however,deflected 20 to 40 relative to the wind emen n by Cor ivity anc the edges of cause n contin tio the winds th at driv g equato idly to the inds.unt the nents,where they split and flov warm tropical water to higher latitudes.On their way poleward currents are defected by Temperature (C Coriolis forces.Once the water reaches the high 0 5 10 1 latitudes,some returns in surface currents toward the tropics along the eastern edges of ocean basins (Fig.2.9),and some continues 25 poleward. Deep ocean waters show a circulatior pattern quite different from the wind-driver rculation. e winte air co ls the surta 750 the asine ch on)in ters,also increasing their density.The high 1000 32 33 6 density of these cold saline waters causes then Salinity(ppt) to sink.This downwellins to form the North FIGURE 2.8.Typical vertical profiles of ocean tem- Atlantic deep water off of Greenland,and the Antarctic bottom water off of Antarctica drives and ones the global thermohaline circulation in the middle and deep ocean that ultimately transfers with depth.These transition zones usually coincide water between the major ocean basins (Fig. approximately. 2.10).The descent of cold dense water at high
The Oceans 29 phere, extends to depths of 75 to 200 m, depending on the depth of wind-driven mixing. Most primary production, detrital production, and decomposition take place in the surface waters (see Chapter 10). Another major difference between atmospheric and oceanic circulation is that density of ocean waters is determined by both temperature and salinity, so, unlike warm air, warm water can sink, if it is salty enough. There are relatively sharp gradients in temperature (thermocline) and salinity (halocline) between warm surface waters of the ocean and cooler more saline waters at intermediate depths (200 to 1000 m) (Fig. 2.8).These two vertical gradients cause the surface waters to be less dense than deep water, creating a stable vertical stratification. The deep layer therefore mixes with the surface waters slowly over hundreds to thousands of years. These deeper layers nonetheless play critical roles in element cycling, productivity, and climate because they are long-term sinks for carbon and the sources of nutrients that drive ocean production (see Chapters 10 and 15). Upwelling areas, where deep waters move rapidly to the surface, support high levels of primary and secondary productivity (marine invertebrates and vertebrates) and are the locations of many of the world’s major fisheries. Ocean Circulation Ocean circulation plays a critical role in Earth’s climate system. On average, ocean circulation accounts for 40% of the latitudinal heat transfer from the equator to the poles, with the remaining 60% of heat transfer occurring through the atmosphere. The ocean is the dominant heat transporter in the tropics, and the atmosphere plays the stronger role at midlatitudes. The surface currents of the oceans are driven by surface winds and therefore show global patterns (Fig. 2.9) that are generally similar to those of the prevailing surface winds (Fig. 2.7). The ocean currents are, however, deflected 20 to 40° relative to the wind direction by Coriolis forces. This deflection and the edges of continents cause ocean currents to be more circular (termed gyres) than the winds that drive them. In equatorial regions, currents flow east to west, driven by the easterly trade winds, until they reach the continents, where they split and flow poleward along the western boundaries of the oceans, carrying warm tropical water to higher latitudes. On their way poleward, currents are deflected by Coriolis forces. Once the water reaches the high latitudes, some returns in surface currents toward the tropics along the eastern edges of ocean basins (Fig. 2.9), and some continues poleward. Deep ocean waters show a circulation pattern quite different from the wind-driven surface circulation. In the polar regions, especially in the winter off southern Greenland and off Antarctica, cold air cools the surface waters, increasing their density. Formation of sea ice, which excludes salt from ice crystals (brine rejection), increases the salinity of surface waters, also increasing their density. The high density of these cold saline waters causes them to sink. This downwelling to form the North Atlantic deep water off of Greenland, and the Antarctic bottom water off of Antarctica drives the global thermohaline circulation in the middle and deep ocean that ultimately transfers water between the major ocean basins (Fig. 2.10). The descent of cold dense water at high 0 250 500 750 1000 32 33 34 35 36 Salinity (ppt) Depth (m) Temperature (oC) 0 5 10 15 Surface water Intermediate water Temperature S alinity T / H Figure 2.8. Typical vertical profiles of ocean temperature and salinity. The thermocline (T) and halocline (H) are the zones where temperature and salinity, respectively, decline most strongly with depth. These transition zones usually coincide approximately
2.Earth's Climate System 18 Longitude 90 % 2.9.Major surface ocean currents Warm currents are shown by solid arrows and cold currents by ow awn from of Brooks /Cole y:An at of th oup. a division Thomson Learning:Ahrens 1998.) latitudes is balanced by the upwelling of deep trols latitudinal heat transport,changes in its water on the eastern margins of ocean basins strength have significant effects on climate.In at lower latitudes,where along-shore surface addition,it transfers carbon to depth,where it currents are deflected offshore by Coriolis remains for centuries (see Chapter 15). forces and easterly trade Oceans,with their high heat capacity,heat up movement p water oward down much than does land ave a m influence on the equator.Because thermo circulation con- climate of adjacent land. 2.10.Circulation patterns of de mong the major ocean basins
30 2. Earth’s Climate System latitudes is balanced by the upwelling of deep water on the eastern margins of ocean basins at lower latitudes, where along-shore surface currents are deflected offshore by Coriolis forces and easterly trade winds. Net poleward movement of warm surface waters balances the movement of cold deep water toward the equator. Because thermohaline circulation controls latitudinal heat transport, changes in its strength have significant effects on climate. In addition, it transfers carbon to depth, where it remains for centuries (see Chapter 15). Oceans, with their high heat capacity, heat up and cool down much less rapidly than does land and thus have a moderating influence on the climate of adjacent land. Wintertime temperaGulf Stream Bengue al C. N. Pacific Drift Kuroshio C. Humboldt C. North Atlantic Drift West Wind Drift 90 90 90 90 180 180 0 0 60 60 60 60 0 0 30 30 30 30 Longitude 90 90 Latitude Figure 2.9. Major surface ocean currents. Warm currents are shown by solid arrows and cold currents by dashed arrows. (Redrawn from Essentials of Meteorology: An Invitation to the Atmosphere, 2nd edition, by C. Ahrens © 1998, by permission of Brooks/Cole, an imprint of the Wadsworth Group, a division of Thomson Learning; Ahrens 1998.) Warm shallow current Cold salty deep current Figure 2.10. Circulation patterns of deep and surface waters among the major ocean basins
Landform Effects on Climate 31 India enhances vertical motion,increasing the han at similar lati proportion of water vapor that is converted to mer A onal change win sea of the tream) 2.9 patter and on th e the rd t rom th tsca of a few kilometers,the differ California current for example,which runs heating between land and ocear n produces land north to south along the west coast of the and sea bre es.During the day. ong heating United States.keeps summer temperatures in over land causes air to rise.drawing in cool air northern California lower than the Is east from the ocean.The rising of air over the land coast at similar latitudes windward coastal sit increases the height at which a given pressure uations are typically more strongly influenced occurs,causing this upper air to move from land by prevailing onshore winds and thus have toward the ocean.The resulting increase in the more moderate temperatures than do coastal mass of atmosphere over the ocean augments situations that are downwind of predominantly the surface pressure,which causes surface ai continental air.New York City,on the eastern to flow from the ocean toward the land.Th edge of Nort Ame ica,therefore experie res nland n occu es on sia the of the the he cea the surfac ems that occur to .The ne different parts of the globe reextre s and increase precipitation on land oceans or large lakes Landform Effects on Climate Mountain ranges affect local atmospheric cir culation and climate through several types of The spatial distribution of land,water,and orographic effects-that is.effects due to pres mountains modify the general latitudinal trends ence of mountains.As winds carry air up the in climate.The greater heat capacity of water windward sides of mountains.the air cools compared to land influences atmospheric cir- and water vapor condenses and precipitates culation at local to continental scale The sea. Therefore,the windward side tends to be cold sonal reversal o winds (monsoon)in and wet.when the air moves down the le exampl is driven largely side of the mo con shad 20 e in nter,the la nd is the 11 nd India to 2.7 's m summer.however.the land heats relative to the rom Colorado 0 m)to llinoi ocean.The heatins over land forces the air to (1000m rise.in turn drawing in moist surface air from Deserts or de pes)are ofter the ocean.Condensation of water vapor in the found immediately downwind of the majo rising moist air produces large amounts of pre mountain ranges of the world. cipitation.Northward migration of the trade systems can also influence climate by channel winds in summer enhances onshore flow of air. ing winds through valleys.The Santa Anna and the mountainous topography of northern winds of southern California occur when high
Landform Effects on Climate 31 tures in Great Britain and western Europe, for example, are much milder than at similar latitudes on the east coast of North America due to the warm North Atlantic drift (the poleward extension of the Gulf Stream) (Fig. 2.9). Conversely, cold upwelling currents, or currents moving toward the equator from the poles, cool adjacent land masses in summer. The cold California current, for example, which runs north to south along the west coast of the United States, keeps summer temperatures in northern California lower than the U.S. east coast at similar latitudes. Windward coastal situations are typically more strongly influenced by prevailing onshore winds and thus have more moderate temperatures than do coastal situations that are downwind of predominantly continental air. New York City, on the eastern edge of North America, therefore experiences relatively mild winters compared to inland cities like Minneapolis, but its winter temperatures are lower than those of cities on the western edge of the continent. These temperature differences play critical roles in determining the kinds of ecosystems that occur over different parts of the globe. Landform Effects on Climate The spatial distribution of land, water, and mountains modify the general latitudinal trends in climate. The greater heat capacity of water compared to land influences atmospheric circulation at local to continental scales. The seasonal reversal of winds (monsoon) in eastern Asia, for example, is driven largely by the differential temperature response of the land and the adjacent seas. During the Northern Hemisphere winter, the land is colder than the ocean, giving rise to cold dense air that flows southward across India to the ocean (Fig. 2.7). In summer, however, the land heats relative to the ocean. The heating over land forces the air to rise, in turn drawing in moist surface air from the ocean. Condensation of water vapor in the rising moist air produces large amounts of precipitation. Northward migration of the trade winds in summer enhances onshore flow of air, and the mountainous topography of northern India enhances vertical motion, increasing the proportion of water vapor that is converted to precipitation. Together, these seasonal changes in winds give rise to predictable seasonal patterns of temperature and precipitation that strongly influence the structure and functioning of ecosystems. At scales of a few kilometers, the differential heating between land and ocean produces land and sea breezes. During the day, strong heating over land causes air to rise, drawing in cool air from the ocean. The rising of air over the land increases the height at which a given pressure occurs, causing this upper air to move from land toward the ocean. The resulting increase in the mass of atmosphere over the ocean augments the surface pressure, which causes surface air to flow from the ocean toward the land. The resulting circulation cell is identical in principle to that which occurs in the Hadley cell (Fig. 2.6) or Asian monsoon (Fig. 2.7). At night, when the ocean is warmer than the land, air rises over the ocean, and the surface breeze blows from the land to the ocean, causing the circulation cell to reverse. The net effect of sea breezes is to reduce temperature extremes and increase precipitation on land near oceans or large lakes. Mountain ranges affect local atmospheric circulation and climate through several types of orographic effects—that is, effects due to presence of mountains. As winds carry air up the windward sides of mountains, the air cools, and water vapor condenses and precipitates. Therefore, the windward side tends to be cold and wet. When the air moves down the leeward side of the mountain, it contains little moisture, creating a rain shadow, or a zone of low precipitation downwind of the mountains. The rain shadow of the Rocky Mountains extends 1500 km to the east, resulting in a strong west-to-east gradient in annual precipitation from Colorado (300 mm) to Illinois (1000 mm) (see Fig. 14.1) (Burke et al. 1989). Deserts or desert grasslands (steppes) are often found immediately downwind of the major mountain ranges of the world. Mountain systems can also influence climate by channeling winds through valleys. The Santa Anna winds of southern California occur when high