A simple thermodynamic argument suggests that as the water vapor content of the atmosphere increases with global warming dry regions may become drier and wet regions wetter. This enhanced hydrological contrast with global warming can be attributed to changes in the atmospheric water vapor concentration being comparatively larger than those of the moisture advecting winds in the lower atmosphere. This rich-get-richer hypothesis may explain opposite trends with global warming in different regions, for instance between enhanced dryness in the subtropics and enhanced wet condition in the deep tropics, as well as in different seasons, or between an increase in summer dryness and greater wetness in winter in the storm track regions. Simulations of global warming scenarios with comprehensive climate models have validated this rich-get-richer hypothesis, but whether this trend actually exists in Earth’s observational data has remained uninvestigated to this day.
In an article recently published in Nature Geoscience, Chou et al. (2013) investigate changes in the hydrologic cycle using multiple datasets over many regions and in all seasons for the past 30 years. Chou et al. find that a marked increase in the seasonal amplitude of the hydrologic cycle has occurred over the past 30 years, with precipitation being enhanced during the wet seasons and reduced during the dry seasons. This trend appears to operate globally, although trends in the subtropical dry regions remain uncertain. This increase in seasonal amplitude is asymmetric, with enhanced wetness during the wet seasons comparatively larger than enhanced dryness during the dry seasons. This asymmetry leads to an overall increase in the annual-mean precipitation rates, consistent with the global increase in evaporation and precipitation in global warming scenarios.
This study is quite significant as it demonstrates for the first time that the hydrological cycle has indeed become more contrasted during the past decades, which implies that a greater occurrence of summer droughts and winter downpours must have occurred during this period. This study implies that even in regions defined as annually wet, mean drought conditions during the dry season can strain infrastructures and induce significant reductions in water usage.
Chou, C. et al., 2013: Increase in the range between wet and dry season precipitation, Nature Geoscience, 6, 263–267.