Trends in Lower Tropospheric Relative Humidity from 1979 to 2013

A recent study suggests that global warming due to increased CO2 may have contributed to the observed worldwide droughts in the recent decades (Lau and Kim 2015), while other studies showed that natural variabilities exert a large impact on observed droughts (e.g. Dai 2013). We analyze a number of reanalysis datasets to identify the long-terms trends in the lower tropospheric relative humidity from 1979 to 2013 and assess the relative roles of anthropogenic warming and internal climate variabilities. A significant drying trend is found in lower tropospheric relative humidity over subtropical eastern Pacific and some continental regions in all reanalysis products, with varying magnitudes. A multi-variate linear regression method is employed to assess the relative contributions of various climate processes. Our analysis shows that the decadal drying trends in relative humidity are closely associated with circulation changes, resulting from combined effects of global warming and natural climate variabilities.