Observed Dynamic Structure of an Intense Oceanic Cyclone

Abstract

A major extratropical cyclone developed over the East China Sea during the initial phase of the AMTEX'75. Using cross-section analyses the dynamic structure of the cyclone is investigated with an emphasis on obtaining a better understanding of the physical mechanisms controlling the development and maintenance of this intense oceanic system. The dynamic and kinematic variables examined include ageostrophic acceleration, potential and absolute vorticity, and vertical and horizontal motions. During the 24-h period following 1200 UTC 14 February, the surface cyclone moved eastward more than 22° longitude, while the upper-level trough propagated only about 15°. In the same period, the central pressure at the surface dropped more than 20 mb while the areas extent of the closed circulation showed little change. Vertical decoupling of the wave-cyclone system is suggested by this discrepancy in phase speed, and by the temporal evolution of the trough axis and the height fields during the lust 12 h. Latent heat release, which seemed to contribute to the vertical decoupling and the decrease in zonal baroclinicity, tended to strengthen the ascent aloft and convergence below in the vicinity of the cyclone. Strong ascent enhanced the thermally direct east?west circulation across the upper trough, while convergence resulted in further latent heating and consequently larger ageostrophic components and convergence near the surface. Thus, kinetic energy was generated to support the system's development despite the decoupling in the vertical and the weakening of the horizontal baroclinicity. The roles of conservation of potential vorticity and ageostrophic acceleration in the dynamics of this system were strongly influenced by latent heating. Analyses suggested that the observed explosive oceanic cyclogenesis was a result of latent heat enhanced regional baroclinic processes.