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In the tropics, habitat fragmentation alters forest-climate interactions in diverse ways. On a local scale (<1 km), elevated desiccation and wind disturbance near fragment margins lead to sharply increased tree mortality, altering canopy-gap dynamics, plant-community composition, biomass dynamics, and carbon storage. Fragmented forests are also highly vulnerable to edge-related fires, especially in regions which have periodic droughts or strong dry seasons. At landscape to regional scales (10-1,000 km), habitat fragmentation may have complex effects on forest-climate interactions, with important consequences for atmospheric circulation, water cycling, and precipitation. Positive feedbacks among deforestation, regional climate change, and fire could pose a serious threat for some tropical forests, but the details of such interactions are poorly understood.

The chapter begins with a phenomenological treatment of the observed atmospheric circulation. It then goes on to discuss how the barotropic model arises as a so-calledbalanced model of the slow, vorticity-driven dynamics, from the more general shallowwater model which also admits inertia-gravity waves. This is important because large-scale atmospheric turbulence exhibits aspects of both balanced and unbalanced dynamics. Because of the first-order importance of zonal flows in the atmospheric general circulation, the large-scale turbulence is highly inhomogeneous, and is shaped by the nature of the interaction between zonal flows and Rossby waves described eloquently by Michael McIntyre as a wave-turbulence jigsaw puzzle. This motivates a review of the barotropic theory of wave, mean-flow interaction, which is underpinned by the Hamiltonian structure of geophysical fluid dynamics.

This chapter starts by exploring some basic principles of weather. It then describes California’s Mediterranean climate from the standpoint of atmospheric circulation. This is followed by short-term weather conditions associated with fire spread, and climate variability and its possible role in fire regimes. The Mediterranean climate results from seasonal changes in global circulation, including California’s marginal position to the jet stream and the presence of cold, upwelling ocean waters offshore. The weather influences fire outcomes by altering vegetation fuel moisture and the efficiency of heat transfer in combustion. The El Niño/Southern Oscillation and the Pacific Decadal Oscillation influence the amount and distribution of water vapor that is evaporated into the air, condensed into clouds, and rained back to earth. The effect of precipitation variability is modulated by patch structure in which changes in regional fire hazard result in only finite portions of stands achieving flammability thresholds.

Regional variations in South America’s weather and climate reflect the atmospheric circulation over the continent and adjacent oceans, involving mean climatic conditions and regular cycles, as well as their variability on timescales ranging from less than a few months to longer than a year. Rather than surveying mean climatic conditions and variability over different parts of South America, as provided by Schwerdtfeger and Landsberg (1976) and Hobbs et al. (1998), this chapter presents a physical understanding of the atmospheric phenomena and precipitation patterns that explain the continent’s weather and climate. These atmospheric phenomena are strongly affected by the topographic features and vegetation patterns over the continent, as well as by the slowly varying boundary conditions provided by the adjacent oceans. The diverse patterns of weather, climate, and climatic variability over South America, including tropical, subtropical, and midlatitude features, arise from the long meridional span of the continent, from north of the equator south to 55°S. The Andes cordillera, running continuously along the west coast of the continent, reaches elevations in excess of 4 km from the equator to about 40°S and, therefore, represents a formidable obstacle for tropospheric flow. As shown later, the Andes not only acts as a “climatic wall” with dry conditions to the west and moist conditions to the east in the subtropics (the pattern is reversed in midlatitudes), but it also fosters tropical-extratropical interactions, especially along its eastern side. The Brazilian plateau also tends to block the low-level circulation over subtropical South America. Another important feature is the large area of continental landmass at low latitudes (10°N–20°S), conducive to the development of intense convective activity that supports the world’s largest rain forest in the Amazon basin. The El Niño–Southern Oscillation phenomenon, rooted in the ocean-atmosphere system of the tropical Pacific, has a direct strong influence over most of tropical and subtropical South America. Similarly, sea surface temperature anomalies over the Atlantic Ocean have a profound impact on the climate and weather along the eastern coast of the continent. In this section we describe the long-term annual and monthly mean fields of several meteorological variables.

The climate is changing from human activities. This has major implications for the future and for human society and ecosystems, including birds. However, it is often masked by natural variability and there is great weather-related variability. This chapter reviews observed changes in climate, with a focus on changes in surface climate including variations in major patterns (modes) of climate variability and teleconnections. Of particular importance are changes in extremes. It describes how natural and anthropogenic drivers of climate change are assessed using climate models and concludes with a brief summary of future projected changes in climate and their impacts.

This chapter discusses mercury cycling and contamination in the Alaskan arctic. It provides a framework for investigating the behavior and fate of many contaminants in polar regions. It looks into the role atmospheric circulation and chemistry in affecting the transport and deposition of both inorganic and organic substrates on local and global scales. It analyzes a series of reports by the Arctic Monitoring and Assessment Program in determining mechanisms that transport contaminants, including mercury, to the arctic and identifies their effects on the arctic ecosystem.

Wherever the wind blows, some of its energy is dissipated — converted to entropy — by the shearing of air against air; this happens at all elevations. However, the losses are far greater in the lower-most layer because of friction with the surface — the drag of moving air as it passes across land or water. Drag also affects the direction of the wind, making atmospheric circulation far more complicated than it is aloft. This chapter discusses the following: surface winds; vertical movements of the air; water vapor and energy transfers; storms; how atmospheric energy is dissipated; and the energy in a rainstorm.

Tectonism is the science of Earth movements and the rocks and structures involved therein. These movements build the structural framework that supports the stage on which surface processes, plants, animals and, most recently, people pursue their various roles under an atmospheric canopy. An appreciation of this tectonic framework is thus a desirable starting point for understanding the physical geography of South America, from its roots in the distant past through the many and varied changes that have shaped the landscapes visible today. Tectonic science recognizes that Earth’s lithosphere comprises rocks of varying density that mobilize as relatively rigid plates, some continental in origin, some oceanic, and some, like the South American plate, amalgams of both continental and oceanic rocks. These plates shift in response to deep-seated forces, such as convection in the upper mantle, and crustal forces involving push and pull mechanics between plates. Crustal motions, augmented by magmatism, erosion, and deposition, in turn generate complex three-dimensional patterns. Although plate architecture has changed over geologic time, Earth’s lithosphere is presently organized into seven major plates, including the South American plate, and numerous smaller plates and slivers. The crustal mobility implicit in plate tectonics often focuses more attention on plate margins than on plate interiors. In this respect, it is usual to distinguish between passive margins, where plates are rifting and diverging, and active margins, where plates are either converging or shearing laterally alongside one another. At passive or divergent margins, such as the present eastern margin of the South American plate, severe crustal deformation is rare but crustal flexuring (epeirogeny), faulting, and volcanism occur as plates shift away from spreading centers, such as the Mid-Atlantic Ridge, where new crust is forming. Despite this lack of severe postrift deformation, however, passive margins commonly involve the separation of highly deformed rocks and structures that were involved in the earlier assembly of continental plates, as shown by similar structural legacies in the facing continental margins of eastern South America and western Africa. At active convergent margins, mountain building (orogeny) commonly results from subduction of oceanic plates, collision of continental plates, or accretion of displaced terranes.