Abstract
Analyses of precipitation (1961–2010) from 39 meteorological stations in the Tarim River Basin revealed a trend from dryer towards wetter conditions induced by an increase of the number of wet extremes. A first (1961–1986) and second (1987–2010) period are the basis for a dynamical analysis of changing drought and wetness extremes which are closely related to cyclonic activity over the European continent and circulation anomalies in the Northern Hemisphere mid-latitudes. Wave train, cyclone tracks, water flux and potential vorticity (PV) front analysis of the wet and dry months show the following result: (1) The extreme wet and dry cases in winter and summer are characterized by distinguished wave train patterns upstream of the Tarim River Basin. All wave trains originate in the Atlantic–European sector pointing towards wave train dynamics as one possible mechanism underlying the connection patterns observed. (2) The selected extreme cases show that exceptional precipitation events can be connected to characteristic cyclone tracks and a PV front in the upper troposphere even if cyclone tracks never cross the Tarim Basin. Extremely wet winters are characterized by cyclone tracks close to the western and northern boundary of the Tarim Basin whereas, during extremely dry winters, such cyclone tracks are absent. Wet summers are characterized by long-lived cyclonic anomalies at the north western corner of the Tarim River Basin [see also (3)]. During dry summers such anomalies are absent. (3) On a more local level the hydrological extreme events are linked to special dynamical structures of the upper tropospheric PV front. In winter strong (extreme) precipitation is connected to a strong non-linear wave development or a wave-breaking event over the Tarim River Basin. Together with non-linear wave development moisture and precipitation areas are advected towards the Tarim River Basin. In dry winters the upper tropospheric PV front is much more zonally oriented and wave-breaking is less frequent. Strong precipitation events are connected to strong breaking events and to the formation of long-lived nearly stationary cyclones over or north of the Tarim River Basin during extremely wet summer months.
Highlights
The Tarim River Basin is one of the largest endorheic regions in China and one of the largest inland river basin
Geopotential height fields (500 hPa), quasi-stationary waves, cyclone tracks and potential vorticity (PV) fields are all deduced from ECMWF re-analysis ERA-40 (Uppala et al 2006) retrieved from the World Data Center for Climate (WDCC) in Germany
Wet and dry extremes are distinguished by different geographical locations and different amplitudes of meridional excursions of the isolines of the geopotential height field
Summary
The Tarim River Basin (roughly between 74°–90°E and 35°–43°N) is one of the largest endorheic regions in China and one of the largest inland river basin (covering about 1 million square kilometers). Using geopotential height fields on 500 hPa and atmospheric large-scale ciruclation indices a close connection to mid-latitude circulation variability is found which is in agreement with previous works (Diffenbaugh et al 2013; Zhang et al 2013; Kingston et al 2007; Talaee et al 2014). To further investigate the mechanisms connecting hydrological extremes in the Tarim River Basin and mid-latitude atmospheric dynamics the present paper analyzes the atmosphere as follows: (a) composites of mid-tropospheric geopotential height fields for specific months and the corresponding seasonal quasi-stationary wave patterns, (b) cyclone tracks and (c) upper tropospheric PV fields (Ertel 1942). The connection between hydrological extremes and large-scale mid-latitude dynamics using cyclone tracks, upper tropospheric PV fields and precipitation fields is presented in Sect.
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