Atmosfera, Tempo e Clima

Atmosfera, Tempo e Clima

(Parte 4 de 9)

The success in modelling the life cycle of the midlatitude frontal depression, and its value as a forecasting tool, naturally led to attempts in the immediate pre- Second World War period to apply it to the atmospheric conditions which dominate the tropics (i.e. 30°N – 30°S), comprising half the surface area of the globe. This attempt was doomed largely to failure, as observations made during the air war in the Pacific soon demonstrated. This failure was due to the lack of frontal temperature discontinuities between air masses and the absence of a strong Coriolis effect and thus of Rossby-like waves. Tropical airmass discontinuities are based on moisture differences, and tropical weather results mainly from strong convectional features such as heat lows, tropical cyclones and the intertropical convergence zone (ITCZ). The huge instability of tropical airmasses means that even mild convergence in the trade winds gives rise to atmospheric waves travelling westward with characteristic weather patterns.

Above the Pacific and Atlantic Oceans the intertropical convergence zone is quasi-stationary with a latitudinal displacement annually of 5°or less, but elsewhere it varies between latitudes 17°S and 8°N in January and between 2°N and 27°N in July – i.e. during the southern and northern summer monsoon seasons, respectively. The seasonal movement of the ITCZ and the existence of other convective influences make the south and east Asian monsoon the most significant seasonal global weather phenomenon.

Investigations of weather conditions over the broad expanses of the tropical oceans were assisted by satellite observations after about 1960. Observations of waves in the tropical easterlies began in the Caribbean during the mid-1940s, but the structure of mesoscale cloud clusters and associated storms was recognized only in the 1970s. Satellite observations also proved very valuable in detecting the generation of hurricanes over the great expanses of the tropical oceans.

In the late 1940s and subsequently, most important work was conducted on the relations between the south

Asian monsoon mechanism in relation to the westerly subtropical jet stream, the Himalayan mountain barrier and the displacement of the ITCZ. The very significant failure of the Indian summer monsoon in 1877 had led Blanford (1860) in India, Todd (1888) in Australia, and others, to seek correlations between Indian monsoon rainfall and other climatic phenomena such as the amount of Himalayan snowfall and the strength of the southern Indian Ocean high pressure centre. Such correlations were studied intensively by Sir Gilbert Walker and his co-workers in India between about 1909 and the late 1930s. In 1924 a major advance was made when Walker identified the ‘Southern Oscillation’ – an east–west seesaw of atmospheric pressure and resulting rainfall (i.e. negative correlation) between Indonesia and the eastern Pacific. Other north–south climatic oscillations were identified in the North Atlantic (Azores vs. Iceland) and the North Pacific (Alaska vs. Hawaii). In the phase of the Southern Oscillation when there is high pressure over the eastern Pacific, westwardflowing central Pacific surface water, with a consequent upwelling of cold water, plankton-rich, off the coast of South America, are associated with ascending air, gives heavy summer rains over Indonesia. Periodically, weakening and breakup of the eastern Pacific high pressure cell leads to important consequences. The chief among these are subsiding air and drought over India and Indonesia and the removal of the mechanism of the cold coastal upwelling off the South American coast with the consequent failure of the fisheries there. The presence of warm coastal water is termed ‘El Niño’. Although the central role played by lower latitude high pressure systems over the global circulations of atmosphere and oceans is well recognized, the cause of the east Pacific pressure change which gives rise to El Niño is not yet fully understood. There was a waning of interest in the Southern Oscillation and associated phenomena during the 1940s to mid-1960s, but the work of Berlage (1957), the increase in the number of Indian droughts during the period 1965 to 1990, and especially the strong El Niño which caused immense economic hardship in 1972, led to a revival of interest and research. One feature of this research has been the thorough study of the ‘teleconnections’ (correlations between climatic conditions in widely separated regions of the earth) pointed out by Walker.

Prior to the mid-twentieth century thirty years of record was generally regarded as sufficient in order to define a given climate. By the 1960s the idea of a static climate was recognized as being untenable. New approaches to palaeoclimatology were developed in the 1960s to 1970s. The astronomical theory of climatic changes during the Pleistocene proposed by Croll (1867), and developed mathematically by Milankovitch, seemed to conflict with evidence for dated climate changes. However, in 1976, Hays, Imbrie and Shackleton recalculated Milankovitch’s chronology using powerful new statistical techniques and showed that it correlated well with past temperature records, especially for ocean palaeotemperatures derived from isotopic (180/160) ratios in marine organisms.

Undoubtedly the most important outcome of work in the second half of the twentieth century was the recognition of the existence of the global climate system (see Box 1.1). The climate system involves not just the atmosphere elements, but the five major

The idea of studying global climate through co-ordinated intensive programmes of observation emerged through the World Meteorological Organization (WMO: http://www.wmo.ch/ and the International Council on Science (ICSU: http://www.icsu.org in the 1970s. Three ‘streams’ of activity were planned: a physical basis for long-range weather forecasting; interannual climate variability; and long-term climatic trends and climate sensitivity. Global meteorological observation became a major concern and this led to a series of observational programmes. The earliest was the Global Atmospheric Research Programme (GARP). This had a number of related but semi-independent components. One of the earliest was the GARP Atlantic Tropical Experiment (GATE) in the eastern North Atlantic, off West Africa, in 1974 to 1975. The objectives were to examine the structure of the trade wind inversion and to identify the conditions associated with the development of tropical disturbances. There was a series of monsoon experiments in West Africa and the Indian Ocean in the late 1970s to early 1980s and also an Alpine Experiment. The First GARP Global Experiment (FGGE), between November 1978 and March 1979, assembled global weather observations. Coupled with these observational programmes, there was also a co-ordinated effort to improve numerical modelling of global climate processes.

The World Climate Research Programme (WCRP: http://www.wmo.ch/web/wcrp/wcrp-home.html established in 1980, is sponsored by the WMO, ICSU and the International Ocean Commission (IOC). The first major global effort was the World Ocean Circulation Experiment (WOCE) which provided detailed understanding of ocean currents and the global thermohaline circulation. This was followed in the 1980s by the Tropical Ocean Global Atmosphere (TOGA).

Current major WCRP projects are Climate Variability and Predictability (CLIVAR: http://www.clivar.org/ the

Global Energy and Water Cycle Experiment (GEWEX), and Stratospheric Processes and their Role in Climate (SPARC). Under GEWEX are the International Satellite Cloud Climatology Project (ISCCP) and the International Land Surface Climatology Project (ISLSCP) which provide valuable datasets for analysis and model validation. A regional project on the Arctic Climate System (ACSYS) is nearing completion and a new related project on the Cryosphere and Climate (CliC: http://clic.npolar.no/) has been established.

Reference

Houghton, J. D. and Morel, P. (1984) The World Climate Research Programme. In J. D. Houghton (ed.) The Global Climate, Cambridge University Press, Cambridge, p. 1–1.

box 1.1topical issueGLOBAL ATMOSPHERIC RESEARCH PROGRAMME (GARP) AND THE WORLD CLIMATE RESEARCH PROGRAMME (WCRP) subsystems: the atmosphere (the most unstable and rapidly changing); the ocean (very sluggish in terms of its thermal inertia and therefore important in regulating atmospheric variations); the snow and ice cover (the cryosphere); and the land surface with its vegetation cover (the lithosphere and biosphere). Physical, chemical and biological processes take place in and among these complex subsystems. The most important interaction takes place between the highly dynamic atmosphere, through which solar energy is input into the system, and the oceans which store and transport large amounts of energy (especially thermal), thereby acting as a regulator to more rapid atmospheric changes. A further complication is provided by the living matter of the biosphere. The terrestrial biosphere influences the incoming radiation and outgoing re-radiation and, through human transformation of the land cover, especially deforestation and agriculture, affects the atmospheric composition via greenhouse gases. In the oceans, marine biota play a major role in the dissolu- tion and storage of CO2. All subsystems are linked by fluxes of mass, heat and momentum into a very complex whole.

The driving mechanisms of climate change referred to as ‘climate forcing’ can be divided conveniently into external (astronomical effects on incoming short-wave solar radiation) and internal (e.g. alterations in the composition of the atmosphere which affect outgoing long-wave radiation). Direct solar radiation measurements have been made via satellites since about 1980, but the correlation between small changes in solar radiation and in the thermal economy of the global climate system is still unclear. However, observed increases in the greenhouse gas content of the atmosphere (0.1 per cent of which is composed of the trace gases carbon dioxide, methane, nitrous oxide and ozone), due to the recent intensification of a wide range of human activities, appear to have been very significant in increasing the proportion of terrestrial long-wave radiation trapped by the atmosphere, thereby raising its temperature. These changes, although small, appear to have had a significant thermal effect on the global climate system in the twentieth century. The imbalance between incoming solar radiation and outgoing terrestrial radiation is termed ‘forcing’. Positive forcing implies a heating up of the system, and adjustments to such imbalance take place in a matter of months in the surface and tropospheric subsystems but are slower (centuries or longer) in the oceans. The major greenhouse gas is water vapour and the effect of changes in this, together with that of cloudiness, are as yet poorly understood.

The natural variability of the global climate system depends not only on the variations in external solar forcing but also on two features of the system itself – feedback and non-linear behaviour. Major feedbacks involve the role of snow and ice reflecting incoming solar radiation and atmospheric water vapour absorbing terrestrial re-radiation, and are positive in character. For example: the earth warms; atmospheric water vapour increases; this, in turn, increases the greenhouse effect; the result being that the earth warms further. Similar warming occurs as higher temperatures reduce snow and ice cover allowing the land or ocean to absorb more radiation. Clouds play a more complex role by reflecting solar (short-wave radiation) but also by trapping terrestrial outgoing radiation. Negative feedback, when the effect of change is damped down, is a much less important feature of the operation of the climate system, which partly explains the tendency to recent global warming. A further source of variability within the climate system stems from changes in atmospheric composition resulting from human action. These have to do with increases in the greenhouse gases, which lead to an increase in global temperatures, and increases in particulate matter (carbon and mineral dust, aerosols). Particulates, including volcanic aerosols, which enter the stratosphere, have a more complex influence on global climate. Some are responsible for heating the atmosphere and others for cooling it.

Recent attempts to understand the global climate system have been aided greatly by the development of numerical models of the atmosphere and of climate systems since the 1960s. These are essential to deal with non-linear processes (i.e. those which do not exhibit simple proportional relationships between cause and effect) and operate on many different timescales.

The first edition of this book appeared some thirtyfive years ago, before many of the advances described in the latest editions were even conceived. However, our continuous aim in writing it is to provide a nontechnical account of how the atmosphere works, thereby helping the understanding of both weather phenomena and global climates. As always, greater explanation inevitably results in an increase in the range of phenomena requiring explanation. That is our only excuse for the increased size of this eighth edition.

How have technological advances contributed to the evolution of meteorology and climatology?

Consider the relative contributions of observation, theory and modelling to our knowledge of atmospheric processes.

Books

Allen, R., Lindsay, J. and Parker, D, (1996) El Niño

Southern Oscillations and Climatic Variability, CSIRO, Australia, 405pp. [Modern account of ENSO and its global influences.]

Fleming, J. R. (ed.) (1998) Historical Essays in Meteorology, 1919–1995, American Meteorological Society, Boston, MA, 617 p. [Valuable accounts of the evolution of meteorological observations, theory, and modelling and of climatology.]

Houghton, J. T. et al. eds (2001) Climate Change 2001: The

Scientific Basis; The Climate System: An Overview, Cambridge University Press, Cambridge, 881pp. [Working Group I contribution to The Third Assessment Report of the Intergovernmental Panel on Climate

Change (IPCC); a comprehensive assessment from observations and models of past, present and future climatic variability and change. It includes a technical summary and one for policy-makers.]

Peterssen, S. (1969) Introduction to Meteorology(3rd edn),

McGraw Hill, New York, 3p. [Classic introductory text, including world climates.]

Stringer, E. T. (1972) Foundations of Climatology. An

Introduction to Physical, Dynamic, Synoptic, and Geographical Climatology, Freeman, San Francisco, 586pp. [Detailed and advanced survey with numerous references to key ideas; equations are in Appendices.]

Van Andel, T. H. (1994) New Views on an Old Planet(2nd edn), Cambridge University Press, Cambridge, 439pp. [Readable introduction to earth history and changes in the oceans, continents and climate.]

(Parte 4 de 9)

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