Interannual Variability and Long-Term Trends in Upper Tropospheric Humidity

Water vapor is the most radiatively active greenhouse gas and the water vapor feedback is a process that significantly amplifies global warming induced by anthropogenic greenhouse gasses. Satellite radiance observations, sensitive to the water vapor and temperature of the upper troposphere, from the past 20 years provide the first global observations of trends in upper tropospheric humidity. Trends in upper tropospheric humidity are strongly positive in the deep tropics, negative in the southern hemisphere subtropics and mid-latitudes, and of mixed sign in the northern hemisphere subtropics and mid-latitudes. Large regional trends are related to changes in transient eddy activity. Increases of transient eddy activity are associated with increased upper tropospheric humidity and areas of decreased transient eddy activity. The trends are shown to be consistent with atmospheric circulation changes observed in the past 20 years including a tendency toward more El Niño-Southern Oscillation warm events and a high index of extratropical circulation annular modes.