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On the basis of daily observations of 39 meteorological stations in the Tarim River Basin, the variability of drought and wetness has been analysed using the standardized precipitation-evapotranspiration index (SPEI, 1961–2010). The result shows an increasing trend in annual mean SPEI with a significant change in 1986. Although the frequency of moderate and severe droughts decreased after the change, the frequency of extreme drought events increased slightly. But different categories of wetness show a consistent increase in frequency. The return periods of drought and wetness prolonged and shortened, respectively, after 1986. Furthermore, composites of geopotential height and water vapour flux fields at 500 hPa are analyzed for extreme wet and dry months of the warm season (May to October) as well as for the warm seasons of the periods 1961 to 1986 and 1987 to 2010. The difference between composites of extreme wet and dry shows that the water vapour supply during wet events can be related to transports from the Arabian Sea and the Bay of Bengal. The midlatitude atmospheric circulation plays an important role by transporting moisture from the east into the Tarim River Basin; this is the main reason of the wetter condition in warm seasons after 1986 in the study region

In this study self-organized periodic coherent vortex structures arising in geophysical turbulent flows at low Rossby number are investigated by developing a conceptual model based on an analytical theory of von Kármán vortex streets affected by stratification and differential rotation. In the framework of a quasi-geostrophic (QG) two-layer beta-plane model vortex streets with three different types of vertical structures (barotropic, upper layer and hetonic) are analysed using the point vortex approximation. The streets are found to be exact solutions of the potential vorticity equation and to be characterized by four non-dimensional parameters. Von Kármán streets are semi-localized solutions which form a bridge between vortex pairs (limit of symmetric dilute streets) and two parallel vortex sheets (limit of dense streets). On the beta-plane QG von Kármán streets can only move to the east, i.e. with a speed outside the range of speeds of Rossby waves, so that a dynamical asymmetry in the zonal direction is introduced. A complete classification on a diagram of states shows that critical bounds exist in the parameter space, prescribing for example a maximum distance between vortex rows beyond which no QG vortex streets can be found. Typically a fast and a slow vortex street with different flow structures are found in the region of existence. As a function of distance between vortex rows baroclinic QG vortex streets show a characteristic non-monotonic speed behaviour at scales of the order of the baroclinic Rossby radius. A wide region of possible existence of QG von Kármán streets is found in atmospheric, oceanic and planetary conditions as well as in rotating tank experiments. The theory can be applied to describe the coherent part of turbulent baroclinic intermittent zonal jet-like and frontal flows and provides a scaling for such flows.

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