Go-occurrence of Northern and Southern Hemisphere blocks as partially synchronized chaos

Teleconnections between the midlatitudes of the Northern and Southern Hemis pheres are diagnosed in National Centers for Environmental Prediction-Natio nal Center for Atmospheric Research reanalysis data and separately in Europ ean Centre for Medium-Range Weather Forecasts reanalysis data. The teleconn ections are manifested as a small but significant tendency for blocking to occur simultaneously in the two hemispheres, though at different longitudes and different relative latitudes, during boreal winters over datasets. One way to explain the correlations between blocking events is as an instan ce of synchronized chaos, the tendency of some coupled chaotic systems to s ynchronize, permanently or intermittently, regardless of initial conditions . As the coupling is weakened, the systems no longer synchronize completely . but small correlations between the states of the coupled systems are obse rved instead. In previous work, such behavior was observed in an idealized coupled-hemisphere model constructed from a midlatitude model due to de Swa rt, which extended the earlier Charney-DeVore spectral truncation of the ba rotropic vorticity equation by including a few extra modes. The direct coup ling of the two midlatitude systems in the coupled-hemisphere model represe nted the exchange of Rossby waves through the upper-tropospheric "westerly ducts" in the Tropics. Significant correlations are found between blocking events, which are chaot ically timed in each hemisphere considered singly, even without several of the idealizations used in the previous study. fna model modified to include an extended tropical region, the correlations are little affected by atten uation and phase shift of the Rossby waves that couple the two midlatitude systems. Variations in the relative longitudes of topographic features in t he two hemispheres leave significant correlations or anticorrelations. The annual cycle, which imposes directionality on the coupling, since the North ern Hemisphere is more strongly forced than the Southern Hemisphere at the times when the hemispheres are coupled, increases the correlations slightly . A two-hemisphere model constructed from a higher-order (wavenumber 3) tru ncation of the barotropic vorticity equation exhibits regime transitions be tween blocked and zonal flow at a more realistic rate in each hemisphere bu t still exhibits interhemispheric correlations.