Effects of irrigation on precipitation in the arid regions of Xinjiang,China

Soil moisture is an important parameter for the interaction between soil and atmosphere. It is the sec- ond important factor that influences climate change, next to sea surface temperature （SST）. Most previous studies focused on the monsoon regions in East China, and only a few laid emphases on arid environments. In Xinjiang, which is located in Northwest China, the climate is typically arid and semi-arid. During the past 20 years, the pre- cipitation in Xinjiang has shown a significant increasing trend, and it is closely related to oasis irrigation. This paper aims at discussing whether abnormal soil moisture in spring can be the signal to forecast summer precipitation. The effects of abnormal soil moisture due to farm irrigation in spring in arid environments on regional climate are inves- tigated by using a regional climate model （RegCM3）. The results indicate that positive soil moisture anomaly in irrigated cropland surface in May led to an increase in precipitation in spring as well as across the whole summer. The impact could last for about four months. The effects of soil moisture on the surface air temperature showed a time-lagging trend. The summer air temperature declined by a maximum amplitude of 0.8℃. The increased soil moisture could enhance evaporation and ascending motion in the low troposphere, which brought in more precipi- tation. The soil moisture affected regional weather and climate mainly by altering the surface sensible and latent heat fluxes.

Characterizing the Winter Concurrent Variation Patterns of the Subtropical and Polar-Front Jets over East Asia

In this study, the concurrent variation relationships between the East Asian subtropical jet （EASJ） and polar-front jet （PFJ） over the East Asian land mass in the winter season on different timescales are identified and the impacts of the jet concurrent variation patterns on the atmospheric circulation in mid-high latitude regions and climate ano- malies in China are examined, using NCEP-NCAR reanalysis data and observational data. The major variability modes of the winter upper-level wind field on interannual timescales are characterized by the meridional shift of the PFJ and out-of-phase variation in the intensity of the subtropical jet and PFJ. On subseasonal and synoptic timescales, the concurrent variation relationships can be categorized into four configuration patterns： a strong （weak） subtropical jet accompanied by a weak （strong） PFJ, or a strong （weak） subtropical jet with a strong （weak） PFJ. The out-of- phase variation [i.e., a stronger （weaker） EASJ and weaker （stronger） PFJ] is found to be more common than the in- phase variation [i.e., a stronger （weaker） EASJ and stronger （weaker） PF J]. These concurrent variation relationships repre- sent the integral structure and variation features of the atmospheric general circulation over East Asia, and have signi- ficant impacts on the weather and climate. The strong subtropical jet/weak PFJ （weak subtropical jet/strong PFJ） pat- tern leads to anomalous negative （positive） geopotential height in midlatitude regions and favors cold （warm） condi- tions, and positive （negative） rainfall anomalies in southern China. For both strong jet configurations, the geopoten- tial height anomaly in the mid-high latitudes shows a northwest-southeast tilted dipole pattern, resulting in northern warm-southern cold temperature anomalies, and positive rainfall anomalies in southern China. For both weak jet situations, positive geopotential height anomalies dominate the East Asian area, and warm conditions occur over most areas in China, corresponding to less negative rainfall anomalies in southern China. The complicated rainfall and temperature anomaly patterns in China can be explained by the concurrent variation relationships between the two jets. A close relationship may exist between the synoptic-scale transient eddy activity （STEA） and the intensity of jet streams, especially for the PFJ. Significantly reduced （strengthened） STEA over the polar-front area is intim- ately associated with a decreased （increased） intensity of the PFJ.