1. IntroductionCopyright 2014, David A. RandallThe nature of the subject The atmosphere circulates. The circulation is global in extent (Fig. 1.1). It carries dry air, energy, three phases of water, momentum, and lots of other things. It is sustained by thermal forcing, which ultimately comes from the Sun. On the average, the Earth intercepts about 340 W m-2 of incident solar energy, of which a tiny fraction is converted to maintain the kinetic energy of the global circulation against friction. Additional, “primordial” energy leaks out of the Earth’s interior, but this rate is only about 0.08 W m-2 (Sclater et al., 1980; Bukowinski, 1999). The thermal forcing is strongly influenced by the atmospheric circulation itself, e.g., as clouds form and disappear. The interactions between the circulation and the heating that drives it are fascinating but complicated. The conservation equations that govern the behavior of the atmosphere can be used to formulate balance requirements that the global circulation must satisfy. For example, in a time average, the net energy flux at the top of the atmosphere must vanish, the rates of evaporation and precipitation must balance, and the total angular momentum of the atmosphere-ocean-solid Earth system must be invariant, apart from the effects of gravitational interactions with the Moon and other extraterrestrial bodies. This check-book approach to the global circulation emphasizes the sources, sinks, and transports of energy, moisture, and angular momentum. We will discuss the global circulation from this classical perspective.It is important to supplement this discussion, however, with descriptions and analyses of the many and varied but inter-related phenomena of the circulation, including such things as the Hadley and Walker circulations, monsoons, stratospheric Sudden Warmings, the Southern Oscillation, subtropical highs, and extratropical storm tracks. One purpose of this course is to introduce these and other phenomena of the global atmospheric circulation.In addition, we will discuss the diabatic and frictional processes that maintain the circulation, and the ways in which these processes are affected by the circulation itself. ! Revised January 14, 2014 8:09 AM! 1An Introduction to the Global Circulation of the AtmosphereIn the Earth’s atmosphere, the circulations of energy and water are closely linked. The global circulation of the Earth’s atmosphere and the atmospheric component of the hydrologic cycle are so thoroughly inter-dependent that a proper discussion of the global circulation must deal in detail with moist processes. One of the aims of this book is to properly include the role of moisture in the global circulation of the atmosphere. In addition, we occasionally compare the global circulation of the Earth’s atmosphere with those of other planets in our solar system. Such comparisons are becoming increasingly useful as our knowledge and understanding of the solar system rapidly expands; they serve to emphasize not only certain similarities among the planetary circulations, but also the numerous ways in which the circulation of the Earth’s atmosphere is, in our experience to date, unique. Figure 1.1: Full disk image, looking down on North America. Many elements of the global circulation can be seen in this picture, including the “inter-tropical” rain band in the eastern North Pacific, midlatitude baroclinic waves, and the low clouds associated with the subtropical highs. ! Revised January 14, 2014 8:09 AM! 2An Introduction to the Global Circulation of the AtmosphereA brief overviewHere is a qualitative, highly simplified overview of the global circulation, just to give you a feeling for the lay of the land.It is conventional and useful, although somewhat arbitrary, to divide the atmosphere into parts. For purposes of this quick sketch, we will divide the atmosphere up vertically and meridionally, only briefly mentioning the longitudinal variations. Most of the solar radiation that is absorbed by the Earth is actually absorbed by the Earth’s surface. Several processes act to transfer the energy from the ocean and land surface into the lower portion of the atmosphere. We will start at the bottom and work our way up. The layer of air that is directly coupled with the Earth’s surface is called the “planetary boundary layer,” or PBL. The top of the PBL is often very sharp and well defined (Fig. 1.2). The depth of the PBL varies dramatically in space and time, but a ball-park value to remember is 1 km. The air in the PBL is turbulent, and the turbulence produces rapid exchanges of “sensible heat” (essentially temperature), moisture, and momentum between the atmosphere and the surface. The most important exchanges are of moisture, upward into the atmosphere via evaporation from the surface, and of momentum, via friction. The “latent heat” associated with the surface moisture flux is a key source of energy for the global circulation, and surface friction is the primary mechanism that dissipates the kinetic energy of the global circulation. Above the PBL is the “free troposphere.” The troposphere actually includes the PBL, so we use the term “free” troposphere to refer to the part of the troposphere that is above the PBL. The free troposphere is characterized by positive but modest static stability, i.e., the potential temperature increases upward at a moderate rate. The depth of the troposphere varies strongly with latitude and season. A turbulent process called “entrainment” gradually incorporates free-tropospheric air into the PBL. Over the oceans, entrainment is relatively slow but steady. Over land, entrainment is promoted by strong daytime heating of the surface, which generates turbulence. As a result, the turbulent PBL rapidly deepens during the day. When the sun goes down, the processes that promote turbulence and entrainment are abruptly cut off, and the PBL reorganizes itself into a much shallower nocturnal configuration, leaving behind a layer of air that was part of the PBL during the afternoon. This diurnal deepening and shallowing of the PBL acts as a kind of “pump,” which captures air from the free