A New Interpretation of Vortex-Split Sudden Stratospheric Warmings in Terms of Equilibrium Statistical Mechanics

摘要

Vortex-split sudden stratospheric warmings (S-SSWs) are investigated by usingthe Japanese 55-year Reanalysis (JRA-55), a spherical barotropicquasi-geostrophic (QG) model, and equilibrium statistical mechanics. The QGmodel reproduces well the evolution of the composite potential vorticity (PV)field obtained from JRA-55 by considering a time-dependent effective topographygiven by the composite height field of the 550 K potential temperature surface.The zonal-wavenumber-2 component of the effective topography is the mostessential feature required to observe the vortex splitting. Thestatistical-mechanics theory predicts a large-scale steady state as the mostprobable outcome of turbulent stirring, and such a state can be computedwithout solving the QG dynamics. The theory is applied to a disk domain, whichis modeled on the north polar cap in the stratosphere. The equilibrium state isobtained by computing the maximum of an entropy functional. In the range ofparameters relevant to the winter stratosphere, this state is anticyclonic. Bycontrast, cyclonic states are quasi-stationary states corresponding to saddlepoints of the entropy functional. The theoretical calculations are comparedwith the results of the quasi-static experiment in which the wavenumber-2topographic amplitude is increased linearly and slowly with time. The resultssuggest that S-SSWs can be qualitatively interpreted as the transition from thecyclonic quasi-stationary state toward the anticyclonic equilibrium state. Thepolar vortex splits during the transition toward the equilibrium state. Withoutany forcing such as radiative cooling, the anticyclonic equilibrium state wouldbe realized sufficiently after an S-SSW.