11.5.1 Key Processes
Central and northern regions of North America are under the influence of mid-latitude cyclones. Projections by AOGCMs (Chapter 10) generally indicate a slight poleward shift in storm tracks, an increase in the number of strong cyclones but a reduction in medium-strength cyclones over Canada and poleward of 70°N. Consequent with the projected warming, the atmospheric moisture transport and convergence is projected to increase, resulting in a widespread increase in annual precipitation over most of the continent except the south and south-western part of the USA and over Mexico.
The southwest region is very arid, under the general influence of a subtropical ridge of high pressure associated with the thermal contrast between land and adjacent ocean. The North American Monsoon System develops in early July (e.g., Higgins and Mo, 1997); the prevailing winds over the Gulf of California undergo a seasonal reversal, from northerly in winter to southerly in summer, bringing a pronounced increase in rainfall over the southwest USA and ending the late spring wet period in the Great Plains (e.g., Bordoni et al., 2004). The projection of smaller warming over the Pacific Ocean than over the continent, and amplification and northward displacement of the subtropical anticyclone, is likely to induce a decrease in annual precipitation in the south-western USA and northern Mexico.
The Great Plains Low-Level Jet (LLJ) is a dynamical feature that transports considerable moisture from the Gulf of Mexico into the central USA, playing a critical role in the summer precipitation there. Several factors, including the land-sea thermal contrast, contribute to the strength of the moisture convergence during the night and early morning, resulting in prominent nocturnal maximum precipitation in the plains of the USA (such as Nebraska and Iowa; e.g., Augustine and Caracena, 1994). The projections of climate changes indicate an increased land-sea thermal contrast in summer, with anticipated repercussions on the LLJ.
Interannual variability over North America is connected to two large-scale oscillation patterns (see Chapter 3), ENSO and the NAO/AO. The MMD model projections indicate an intensification of the polar vortex and many models project a decrease in the arctic surface pressure, which contributes to an increase in the AO/NAO index; the uncertainty is large, however, due to the diverse responses of AOGCMs in simulating the Aleutian Low (Chapter 10). The MMD model projections indicate a shift towards mean El-Niño like conditions, with the eastern Pacific warming more than the western Pacific; there is a wide range of behaviour among the current models, with no clear indication of possible changes in the amplitude or period of El Niño (Chapter 10).