Local Mixing Events in the Upper Troposphere and Lower Stratosphere. Part II: Seasonal and Interannual Variability

Abstract

The Lyapunov diffusivity is used to investigate local isentropic mixing events in the upper troposphere?lower stratosphere (UTLS) region. The diagnostic highlights the seasonal cycle of the longitudinally varying mixing properties, in particular those associated with the monsoon circulations and the westerly ducts in the subtropics. The results are broadly consistent with studies of Rossby wave?breaking frequencies. The mixing structure is shown to be modulated by modes of atmospheric variability. El Niño?Southern Oscillation (ENSO) is found to strongly influence the mixing structure throughout the tropics and subtropics. The subtropical jet is associated with longitudinal bands of mixing minima (isentropic mixing barriers) separated by synoptic-scale regions of strong mixing activity. The greatest ENSO modulation in December?February is confined to the Pacific sector, where the barriers associated with the subtropical jets extend farther into the eastern Pacific, and in the western Pacific a barrier is found at the equator during the positive phase. During June?August, the influence is seen to extend beyond the Pacific region, with the barrier at the subtropical jet in the Southern Hemisphere increasing in strength at all longitudes and with an increase in strength (and isolation of) monsoon-related mixing over Asia and North America. The local influence of the North Atlantic Oscillation in wintertime is investigated. During the positive phase, a double-barrier structure is associated with the subtropical jet, the northern branch crosses the Atlantic toward Scandinavia, and the southern branch tends toward North Africa. The Antarctic Oscillation is shown to influence whether the subvortex region is isolated from midlatitudes.