A Diagnostic Analysis of the Mechanisms for Arctic Cyclone Intensity Evolution

The intensity evolution of Arctic cyclones (ACs) is examined via cyclone parameter space and composite analyses based on approximately 18 000 AC tracks during 1979–2021. Cyclone parameter spaces are defined by various parameters representing the cyclone structure and physical processes relevant to cyclone development. It is shown that intensifying ACs are associated with diabatic heating and characterized by a cold core in both the lower and upper troposphere, as well as a thermally asymmetric and vertically tilted structure. In contrast, the decay phase is associated with diabatic cooling and characterized by a vertically aligned cyclone with reduced horizontal asymmetry. The cyclone parameter space analysis also indicates a warm core in the lower troposphere for a subset of ACs, which may reflect a frontal occlusion. The transition from AC intensification to decay, on average, is marked by a sharp decrease in both upward motion and diabatic heating, along with the vertical alignment of the cyclone structure. Following this transition, an upright cyclone may persist for a long time due to the weak background vertical wind shear, diabatic cooling, and weak Rossby wave energy dispersion. The evolution of ACs can thus be regarded as a two-stage process: a baroclinic development stage aided by diabatic heating, during which the AC evolution may conform with the Norwegian model for midlatitude cyclones, and a slow decay stage of an equivalent barotropic cyclone, which may leave a remnant tropopause polar vortex after the erosion of the surface circulation.