Chapter summary

Atmospheric and oceanic processes and their coupling dominate much of the study of seasonal-to-interannual climate variability. However, land surface processes contribute significantly to climate variability. Soil water is an important determinant of seasonal precipitation forecasts. The recycling of precipitation in evapotranspiration can lead to a positive feedback by which wet soils pump more moisture into the atmosphere, which enhances rainfall and further wets the soil. Conversely, dry soils, with low rates of evapotranspiration, can reduce rainfall. The retention of precipitation by soil and the influence of soil water on subsequent evapotranspiration contribute to and amplify interannual precipitation variability over tropical and middle latitudes. The presence of snow is also an important initial condition required for accurate forecasts. The high albedo of snow-covered surfaces prevents the surface from warming during the day. On warm days, a large portion of net radiation at the surface is used to melt snow. By cooling the surface and reducing the land–ocean temperature contrast, snow can influence summer precipitation in monsoon climates. The seasonal emergence of leaves in spring imparts a discernible signal to air temperature. Greater latent heat flux with leaf emergence cools air temperature.

Soil water

Atmospheric model simulations have routinely demonstrated the importance of soil water, through its effect on evapotranspiration, for climate simulation. These simulations typically manipulate soil water or more generally soil wetness (the effect of soil water on evapotranspiration).