A calculation of Lyapunov exponents for atmospheric blocking

Lyapunov exponents quantify the average leading order stability properties of a dynamical system. Atmospheric blocking may be considered as an unstable state in zonal atmospheric flow. We report a calculation of Lyapunov exponents for atmospheric blocking using the Dymnikov et. al. conjecture. This conjecture correlates the sum of regional positive Lyapunov exponents with the area integrated enstrophy of the large scale atmospheric flow. Selected case studies are performed to examine this correlation for blocking events occurring during the three year period (2002-2004), both for Northern as well as Southern Hemispheres, using the NCEP gridded re-analysis data. It is noted that the time variability of Lyapunov exponents is suggestive of the role played by instability in large-scale flow during onset, maintenance and decay of blocking. Our diagnostic study thus indicates that Lyapunov exponents along with the geopotential height of atmospheric flow may be used as a stability index for atmospheric blocking.