Structure Evolution of Intense Arctic Cyclones in August 2016

Arctic cyclones (ACs) are the major hazardous weather systems affecting the Arctic. Intense AC activity induces strong near-surface wind, high ocean wave, and heavy precipitation, causing dramatic environmental and socio-economic impacts. Whether an AC can develop into an intense one is closely related to its structure evolution. Previous studies generally considered that intense summer ACs are characterized by a shift from a baroclinic structure to a barotropic structure around the maximum intensity time. This study investigates the structure evolution of two intense ACs in August 2016. The results indicate that both ACs exhibited similar structure evolution characteristics: ACs underwent two distinct structural transitions, in which a unique transitional structure is identified between the baroclinic structure and the barotropic structure. During the developing stage, ACs featured an asymmetric baroclinic structure, with its central axis tilting towards cold anomalies. The relative vorticity was high along the warm and cold fronts, with the shallow cyclonic circulation in the lower and middle troposphere. During the mature stage, as the tropopause polar vortex (TPV) caught up to ACs, ACs experienced the first structural transition from the baroclinic structure to a transitional structure. With a tropopause fold descending down to 600 hPa over the cyclone center, ACs developed into a deep cyclonic circulation extending from the troposphere to the lower stratosphere. While a warm core related to the TPV dominated in the lower stratosphere, ACs maintained the baroclinic structure in the troposphere, with the decreased tilt of the axis. During the decay stage, ACs underwent the second structural transition from the transitional structure to an axisymmetric barotropic structure, with a cold core throughout the troposphere and a warm core in the lower stratosphere. These results may have implications for advancing the understanding of the structure evolution and intensification mechanisms of intense summer ACs.