Investigating the Mechanisms Associated With the Evolutions of Twin Extratropical Cyclones Over the Northwest Pacific Ocean in Mid-January 2011

Twin extratropical cyclones that appeared over the northwest Pacific Ocean in mid-January 2011 experienced significantly different transitions: the northern cyclone dissipated rapidly, whereas the southern cyclone developed into an extreme example of an explosive (i.e., rapidly deepening) extratropical cyclone. This event was reasonably reproduced by the fifth-generation Mesoscale Model, and its associated mechanisms were investigated using piecewise potential vorticity inversion and the Zwack-Okossi vorticity budget. Main results are as follows. (i) The extratropical cyclone-included twin cyclone event showed a Fujiwhara effect, during which the two cyclones orbited cyclonically about a midpoint, approached each other, and finally merged. The twin cyclone interactions enhanced the northern cyclone, but weakened the southern cyclone, by inducing warm advection and cold advection, respectively. (ii) Although upper level tropopause folding contributed the most to deepening of the twin cyclones, significant differences in their deepening rates were due predominantly to the contrast between lower level temperature advection and precipitation-related latent heating. (iii) Stronger upper tropospheric positive absolute-vorticity advection and warm advection associated with an upper level jet, and larger latent heating, made geostrophic vorticity of the southern cyclone increase more rapidly than that of the northern cyclone. (iv) Explosive deepening of the southern cyclone showed insensitive responses to its relative configuration with the upper level jet, whereas its rotation enhancement was faster when the cyclone was embedded within the upper level jet. (v) The upper level forcing had a greater influence on the large-scale flow associated with the twin cyclones, whereas the middle-level and lower level forcings were more dominant close to the cyclone core regions.