• Modeling investigation of northern hemisphere extratropical storm variability and changes in a warming climate

      Basu, Soumik; Zhang, Xiangdong; Bhatt, Uma; Mölders, Nicole; Polyakov, Igor (2014-05)
      Extratropical cyclones are fundamental elements for shaping weather patterns, causing fluctuations of temperatures, bringing rain or snow, and carrying winds to impact daily life. The intensity and number of North Hemisphere extratropical cyclones have demonstrated large interannual variability and long-term changes. To understand the variability and changes, we conducted a modeling investigation using the National Center for Atmospheric Research (NCAR)'s Community Atmosphere Model. Specifically, we examined the effects of two surface forcing factors, including sea surface temperature (SST) associated with El Niño and Arctic sea-ice cover, which represent a major source of natural variability and climate changes. Our modeling investigation indicates that the tropical Pacific SST and Arctic sea ice have significant impacts on Northern Hemisphere mid-latitude and Arctic cyclone activities. The elevated tropical Pacific SST leads to more numerous intense storms over southwestern, southeastern, and northwestern North America, but fewer weaker storms over the northeast. The underlying physical mechanism is enhanced lower tropospheric baroclinicity, which is attributable to a southward shift and an intensification of the subtropical jet. The decreased Arctic sea-ice cover leads to an increased storm activity over the Arctic but a decrease in the mid-latitudes. A corresponding examination of surface climate shows anomalously higher surface air temperature and precipitation when low Arctic sea-ice cover occurs, due to an integrative contribution from an increase in surface sensible and latent heat fluxes and horizontal heat advection. In contrast, reduced Arctic sea ice weakens storm activity and intensifies anticyclones over Eurasia, giving rise to decreased surface air temperature and precipitation. Unlike many other parameters, the Arctic sea ice has shown a dramatic decline in addition to interannual fluctuations. We therefore conducted further modeling experiments to identify the role of this long term sea-ice trend on storm activity. The results show that the long-term decline causes a weakening of overall storm activity but an increase in extreme storm events over the Northern Hemisphere. The atmospheric energetic analysis suggests that the increased conversion rate between transient available potential energy and transient kinetic energy is a leading factor in supporting the increased frequency of extreme storms. Over Eurasia, changes in storm activity are mainly governed by the mean kinetic energy of the atmospheric circulation and its conversion to the transient kinetic energy.