Climate

The distribution of temperate ecosystems in western North and South America is related primarily to the seasonal patterns of precipitation along latitudinal, longitudinal, and altitudinal gradients (Lawford, 1996; Aceituno et al., 1993; Veblen and Alaback, 1996). Both

*In this chapter, we use cal ka (or kiloannum) to mean 1000s of calendar years before present and 14C ka to mean 1000s of radiocarbon years before present. The radiocarbon ages have been converted to calendar years based on data that accompany the program of Stuiver and Reimer (1993).

western North and South America are affected by conditions in the Pacific Ocean and the interaction between the tropics and extratropics. Key elements of the climate patterns include the strength and position of the Intertropical Convergence Zone (ITCZ); the subtropical high-pressure systems and associated trade wind belts; and in the midlatitudes, upper atmospheric features (i.e., trough and ridge patterns and the locations of polar and subtropical jet streams, and of regions of large-scale subsidence or uplift). These circulation systems show latitudinal shifts related to the seasonal changes in the temperature gradient between the equator and poles. On average, this gradient is steeper in the Southern Hemisphere than in the Northern Hemisphere, primarily because of the extremely cold Antarctic continent, while the greater land/ocean temperature contrast in the Northern Hemisphere leads to greater amplitude of the waves of the westerlies in that hemisphere. As a result, circulation patterns are asymmetric between the two hemispheres, as are their seasonal variations.

During the northern winter/ southern summer (December-January-February [DJF]), the northern mid-latitude westerlies are displaced equatorward and are accelerated relative to the northern summer ( June-July-August [JJA]) (Fig. 1A), while at the surface, the northern subtropical high-pressure systems (STHs) are reduced in size while the Aleutian and Icelandic low-pressure systems are expanded (Figs. 1B and 1C) (Kousky and Ropelewski, 1997). Northwestern North America is dominated by large-scale rising motions at the 500-mbar level (negative values in Fig. 1D), while the region from northern México and the southwestern United States across the cordillera to the Great Plains is dominated by subsidence. Precipitation reaches its maximum during the year along the northwestern coast of North America (Fig. 1E). In the tropics, the ITCZ and the band of precipitation along it are shifted southward (Figs. 1C and 1E).

In the Southern Hemisphere during DJF, the westerlies are slower relative to those during the southern winter (JJA) and are shifted poleward (Fig. 1A), while at the surface, the southern STHs are expanded (Figs. 1B and 1C). Onshore flow from the South Atlantic Ocean into South America is at its seasonal maximum (Fig. 1C), and most of the continent is dominated by large-scale rising motions (Fig. 1D), convergence at the surface, and divergence aloft (Figs. 1A and 1C); as a consequence, precipitation is relatively high over the continent, except for its farthest southern tip. Together, these surface and upper level winds, and the related precipitation patterns, show the wet phase of the "southern monsoon."

During the northern summer/southern winter

A 200-hPa Wind 1979-9Ë Sea Level Pressure C 925-hPa Stream Lines D 500-hPa VVEL 1979-95 E Precipitation

A 200-hPa Wind 1979-9Ë Sea Level Pressure C 925-hPa Stream Lines D 500-hPa VVEL 1979-95 E Precipitation

FIGURE 1 Features of the present climate of the Western Hemisphere: (A) upper level winds (200-hPa streamlines showing upper level circulation), (B) sea level pressure (SLP), (C) surface winds (925-hPa stream lines), (D) large-scale uplift or subsidence (500-hPa vertical velocity), and (E) precipitation for the northern winter / southern summer (December-January-February, DJF). (F-J are the same as in A-E for the northern summer / southern winter (June-July-August, JJA).

FIGURE 1 Features of the present climate of the Western Hemisphere: (A) upper level winds (200-hPa streamlines showing upper level circulation), (B) sea level pressure (SLP), (C) surface winds (925-hPa stream lines), (D) large-scale uplift or subsidence (500-hPa vertical velocity), and (E) precipitation for the northern winter / southern summer (December-January-February, DJF). (F-J are the same as in A-E for the northern summer / southern winter (June-July-August, JJA).

(JJA), the westerlies are weaker and shifted poleward in the Northern Hemisphere (Fig. 1F), while at the surface, the STHs are expanded (Fig. 1G), while coastal western North America is dominated by large-scale subsidence and dry conditions (Figs. 1H and 1I). Onshore flow and large-scale uplift to the east and south of this region mark the wet phase of the northern monsoon. In the tropics, the ITCZ and its accompanying precipitation maximum are shifted northward. In the Southern Hemisphere, the midlatitude westerlies are stronger than in summer (Fig. 1F), and precipitation reaches its seasonal maximum along the southern west coast of South America. Over most of the continent, subsidence prevails (Fig. 1I), and precipitation is consequently high only in the tropics (Fig. 1J).

There are several distinctive asymmetries in the climates of the two hemispheres. The ITCZ-related precipitation maximum is located north of the equator throughout the year, and the seasonal variability of midlatitude precipitation maxima is stronger in the Northern Hemisphere than in the Southern Hemisphere (Figs 1E and 1J). The amplitude of the waves in the westerlies is greater in the Northern Hemisphere than in the Southern Hemisphere (Figs. 1A and 1F), reflecting the influence of the Andes as the single topographic barrier that influences the large-scale wave pattern in the Southern Hemisphere. Other differences in climates are related to the fact that the land area in the temperate latitudes is large in North America, while South America is relatively narrow. Despite these in-terhemispheric differences in circulation and continental outline, the climates of the temperate ecosystem regions are generally similar, with cool, wet winters and warm, dry summers.

The regions discussed here are bordered to the east by the complex of climates determined by elevation developed along the cordillera and toward the equator by the Mediterranean-climate regions of California and central Chile. Toward the east and toward the equator, the monsoonal-climate regions of both continents are encountered (Kousky and Ropelewski, 1997).

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