Abstract
Abstract. Large-scale tropospheric mixing and Lagrangian transport properties have been analyzed for the long-term period 1979–2014 in terms of the finite-time Lyapunov exponents (FTLEs). Wind field reanalyses from the European Centre for Medium-Range Weather Forecasts were used to calculate the Lagrangian trajectories of large ensembles of particles. Larger values of the interannual and intra-annual mixing variabilities highlight the El Niño Southern Oscillation, the storm track, or the Intertropical Convergence Zone among other large-scale structures. The mean baroclinic instability growth rate and the mean atmospheric river occurrence show large correlation values with the FTLE climatology as an indication of their influence on tropospheric mixing in the midlatitudes. As a case study, the role that land-falling atmospheric rivers have on large-scale tropospheric mixing and the precipitation rates observed in Saharan Morocco and the British Isles has been analyzed. The atmospheric river contribution to tropospheric mixing is found to decrease from 15 % in Saharan Morocco to less than 5 % for the UK and Ireland regions, in agreement with their contribution to precipitation that is 40 % larger in the former than in the latter region.
Highlights
Large-scale tropospheric mixing and transport barriers to air masses play an important role in characterizing weather
We have studied the transport of air masses in terms of their finite-time Lyapunov exponents (FTLEs) from a climatological point of view
Mean Lyapunov exponents show a zonal localization; large values in the midlatitudes for both hemispheres, while the lowest FTLE values were observed in the intertropics
Summary
Large-scale tropospheric mixing and transport barriers to air masses play an important role in characterizing weather. Tropospheric or atmospheric rivers (ARs) have been shown to play a key role in extratropical tropospheric dynamics (Newel et al, 1994; Zhu and Newell, 1998; Gimeno et al, 2016) These structures are narrow and elongated filaments that transport moisture from the tropics into the midlatitudes over a period of a few days once a baroclinic structure develops. Baroclinicity is one of the main mechanisms that addresses the transport of air masses within the troposphere in the midlatitudes (Lindzen and Farrell, 1980; Hoskins and Valdes, 1990) These regions are dominated by cyclone and anticyclone activity that increase tropospheric mixing, in contrast to tropical and subtropical latitudes. To address the first objective, we investigate the long-term variability in tropospheric mixing using the FTLE and focusing on the role that large-scale structures with a timescale of days play in global horizontal transport in the lower troposphere. The contribution of land-falling ARs to tropospheric mixing was found to decrease from 15 % in Saharan Morocco to 5 % for the British Isles, in agreement with a larger contribution to precipitation in the southern region
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