Physical mechanism of spring and early summer drought over North America associated with the boreal warming.

Drought during the early vegetation growing season (spring through early summer) is a severe natural hazard in the large cropland over North America. Given the recent increasing severity of climate change manifested as surface warming, there has been a growing interest in how warming affects drought and the prospect of drought. Here we show the impact of boreal warming on the spring and early summer drought over North America using Cyclostationary Empirical Orthogonal Function analysis. Northern Hemispheric warming, the leading mode of the surface air temperature variability, has led to a decrease in precipitation, evaporation and moisture transport over the central plain of North America. From a quantitative assessment of atmospheric water budget, precipitation has decreased more than evaporation and moisture transport, resulting in increased (decreased) moisture in the lower troposphere (land surface). Despite the increased moisture content, relative humidity has decreased due to the increased saturation specific humidity arising from the lower-tropospheric warming. The anomaly patterns of the soil moisture and Palmer Drought Severity Index resemble that of the anomalous relative humidity. Results of the present study suggest a credible insight that drought in the main cropland will intensify if the anthropogenic warming continues, exacerbating vulnerability of drought.