Impact of the Middle and Upper Tropospheric Cooling over Central Asia on the Summer Rainfall in the Tarim Basin, China

Abstract

The changes in summer rainfall over the Tarim Basin, China, and the underlying mechanisms have been investigated using the observed rainfall data at 34 stations and the NCEP–NCAR reanalysis data during the period of 1961–2007. Results show that the summer rainfall over the Tarim Basin, which exhibits a significant increasing trend during the last half century, is closely related to the summer middle and upper tropospheric cooling over central Asia. Mechanism analysis indicates that the middle and upper tropospheric cooling over central Asia results in a location farther south of the subtropical westerly jet over western and central Asia with anomalous southerly wind at lower levels and ascending motion prevailing over the Tarim Basin. Such anomalies in the atmospheric circulations provide favorable conditions for the enhanced summer rainfall over the Tarim Basin. Further analysis suggests that the weakened South Asian summer monsoon (SASM) could be potentially responsible for the middle and upper tropospheric cooling over central Asia. This is largely through the atmospheric responses to the diabatic heating effect of the SASM. A weakened SASM can result in an anomalous cyclone in the middle and upper troposphere over central Asia. The western part of the anomalous cyclone produces more cold air advection, which leads to the cooling. This study suggests indirect but important effects of the SASM on the summer rainfall over the Tarim Basin.

Impacts of SST Warming in tropical Indian Ocean on CMIP5 model-projected summer rainfall changes over Central Asia

“…The anomalous southwesterly wind associated with the trough pattern over CA can further transport the moist air originated from the IO into CA for generating more summer rainfall. This is the so-called twostep process as revealed in Zhao et al (2014). Figure 2b further shows the correlations between volumetrically integrated atmospheric precipitable water from the ERA-40 reanalysis to the same area-averaged SSTs used in Fig.…”

“…Meanwhile, rainfall also has experienced an increased trend over CA (Chen et al 2011). Some observational studies suggested that summer rainfall over CA could be closely related to the SST anomalies in IO, with moist air being transported to CA under suitable circulations (Yang et al 2010;Bothe et al 2012;Zhao et al 2014;Zhou et al 2015). In order to confirm the relationship, we have firstly analyzed the correlations between the summer rainfall over CA and SSTs in IO in 25 CMIP5 models in their historical experiments for the period of .…”

“…Thus, it is not well understood about the combined effects from a warmed IO and a weakened tropical monsoon circulation on the potential rainfall responses over CA. Although observational analysis showed that a weakened south Asian summer monsoon could result in the cooling in middle and upper troposphere over CA and furthermore generate favorable anomalous circulations for generating more summer rainfall in east part of CA (Yang et al 2009;Zhao et al 2014), we do not know if the same processes operate in the models.…”

“…Together, these two enhanced southerly branches are important for conveying tropical moisture into the subtropical region. Although big terrains block the moist air in the east of 70°E, Zhao et al (2014) showed that in between the Tibetan Plateau and the Iranian Plateau most part of valley is below 1500 m and the southerly anomalies can transport the moist air into CA through this valley. To the north of these two anti-cyclonic patterns, there is a cyclonic pattern around the domain of 30-50°N and 40-65°E.…”

“…Studies also showed that rainfall over central and southwest Asia is also derived from tropical moisture sources and is well associated with tropical climate variability (Mason and Goddard 2001;Mariotti et al 2005). Statistical results showed that water vapor from the IO plays an important role in generating summer rainfall over east part of CA, especially in heavy rainfall (Yang and Zhang 2007;Bothe et al 2012;Zhao et al 2014).…”