Modes and mechanisms of global water vapor variability over the twentieth century

The modes and mechanisms of the annual water vapor variations over the twentieth century are investigated based on a newly developed twentieth-century atmospheric reanalysis product. It is found that the leading modes of global water vapor variations over the twentieth century are controlled by global warming, the Atlantic multidecadal oscillation (AMO), and ENSO. On the global scale, the variations in water vapor synchronize with the sea surface temperature, which can be explained by the simple thermal Clausius-Clapeyron theory under conditions of constant relative humidity. However, on regional scales, the spatial patterns of water vapor variations associated with global warming, AMO, and ENSO are largely attributed to the atmospheric circulation dynamics, particularly the planetary divergent circulation change induced by the sea surface temperature changes. In the middle and high latitudes, the transient eddy fluxes and thermodynamics also play significant roles.