A definition of regional scale is difficult, as different definitions are often implied in different contexts. For example, definitions can be based on geographical, political or physiographic considerations, considerations of climate homogeneity, or considerations of model resolution. Because of this difficulty, an operational definition is adopted in this chapter based on the range of "regional scale" found in the available literature. From this perspective, regional scale is here defined as describing the range of 104 to 107 km2. The upper end of the range (107 km2) is also often referred to as sub-continental scale, and marked climatic inhomogeneity can occur within sub-continental scale regions in many areas of the globe. Circulations occurring at scales greater than 107 km2 (here referred to as "planetary scales") are clearly dominated by general circulation processes and interactions. The lower end of the range (104 km2) is representative of the smallest scales resolved by current regional climate models. Scales smaller than 104 km2 are referred to as "local scale".
Given these definitions, the climate of a given region is determined by the interaction of forcings and circulations that occur at the planetary, regional and local spatial scales, and at a wide range of temporal scales, from sub-daily to multi-decadal. Planetary scale forcings regulate the general circulation of the global atmosphere. This in turn determines the sequence and characteristics of weather events and weather regimes that characterise the climate of a region. Embedded within the planetary scale circulation regimes, regional and local forcings and mesoscale circulations modulate the spatial and temporal structure of the regional climate signal, with an effect that can in turn influence planetary scale circulation features. Examples of regional and local scale forcings are those due to complex topography, land-use characteristics, inland bodies of water, land-ocean contrasts, atmospheric aerosols, radiatively active gases, snow, sea ice, and ocean current distribution. Moreover, climatic variability of a region can be strongly influenced through teleconnection patterns originated by forcing anomalies in distant regions, such as in the El Niño-Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) phenomena.
The difficulty of simulating regional climate change is therefore evident.
The effects of forcings and circulations at the planetary, regional and local
scale need to be properly represented, along with the teleconnection effects
of regional forcing anomalies. These processes are characterised by a range
of temporal variability scales, and can be highly non-linear. In addition, similarly
to what happens for the global Earth system, regional climate is also modulated
by interactions among different components of the climate system, such as the
atmosphere, hydrosphere, cryosphere, biosphere and chemo-sphere, which may require
coupling of these components at the regional scale.
Therefore, a cross-disciplinary and multi-scale approach is necessary for a full understanding of regional climate change processes. This is based on the use of AOGCMs to simulate the global climate system response to planetary scale forcings and the variability patterns associated with broad regional forcing anomalies (see Chapter 9). The information provided by the AOGCMs can then be enhanced to account for regional and local processes via a suitable use of the regionalisation techniques discussed in this chapter.
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