Interdecadal variability in the thermal difference between western and eastern China and its association with rainfall anomalies

This study investigated the spring thermal difference between western and eastern China and its association with the rainfall anomalies using station and reanalysis data from 1960 to 2006. The spring thermal difference between western and eastern China underwent an obvious interdecadal shift around 1979. The thermal difference between western and eastern China was small during 1960–1978, which strengthened the southwesterly wind anomalies in line with the thermal wind. This enhanced the East Asian summer monsoon. In addition, the increase in rainfall over North China and decrease in rainfall over the Yangtze River were associated with the strong East Asian summer monsoon. However, during 1979–2006, the thermal difference between western and eastern China was large, which strengthened the northeasterly wind anomalies in line with the thermal wind. This supports the notion of weakened East Asian summer monsoon associated with the decrease in rainfall over North China and increase in rainfall over the Yangtze River.


Introduction
The East Asian summer monsoon is an important subsystem of the global climate system. The monsoon is related to changes in the land-sea thermal contrast. Previous studies mainly focused on the ocean thermal change and land surface processes to study the East Asian summer monsoon. In the ocean thermal change, many have studied the thermal effect of the Western Pacific warm pool and El Niño-Southern Oscillation (ENSO) cycle. East Asian summer monsoon anomalies were associated with the thermal variability of the Pacific Sea Surface Temperature (SST) anomalies (e.g. Huang et al., 1999;Chang et al., 2000aChang et al., , 2000bWang, 2001;Wu and Wang, 2002;Zhou and Huang, 2003;Zhou and Chan, 2007;Ding et al., 2008). In addition, the thermal state of the warm pool and its overhead convective activities play an important role in the interannual variation of the East Asian summer monsoon (Huang and Li, 1987;Nitta, 1987;Huang and Sun, 1992). In terms of land surface processes, the effect of land-air interaction on the Asian summer monsoon has been studied extensively (e.g. Webster, 1983;Yeh et al., 1984;Meehl, 1994;Douville and Royer, 1996;Yang and Xu, 1994;Yang and Lau, 1998;Xu et al., 2006;Wu and Kirtman, 2007;Zhou and Huang, 2010). Many studies also pointed out that the Qinghai Plateau has a huge heating effect on atmospheric and thermal anomalies, and strongly affects the East Asian summer monsoon (e.g. Ye and Gao, 1979;Li et al., 2003;Zhao and Qian, 2007). More recently, the effect of the thermal state of the arid and semiarid Northwest China on the East Asia climate change was studied. One of the regions with the strongest surface sensible heat flux is in Northwest China, which is called a warm underlying surface, whose variability may impact local and remote climate Huang, 2008, 2010;Zhou et al., 2009). Moreover, Gao et al. (2008) numerically simulated the thermal anomaly in Northwest China that affects the local and surrounding atmospheric circulation anomalies. Many ecosystems with different surface characteristics, e.g. Gobi desert and the humid monsoon regions, characterize China. Northwest China is an arid and semiarid region with strong solar radiation, sparse vegetation, and strong surface sensible heat flux. In contrast, eastern China is a monsoon region with high precipitation and weak surface sensible heat flux. Based on station data, Qu et al. (2011) andWen et al. (2014) concluded that the surface air temperatures, precipitation, and temperature differences were not the same between eastern and western China. Zhou and Huang (2014) pointed out the significant difference in surface sensible and latent heat fluxes between western and eastern China using the ERA-40 reanalysis datasets. The surface radiation fluxes were also found to have the same characteristics based on CMIP5 data (Zhou and Du, 2016). It is well established that the monsoon is mainly caused by the   196019651970197519801985199019952000200519601965197019751980198519901995200020051960196519701975198019851990UB UF 1995200020051960196519701975198019851990199520002005   thermal contrast between land and sea. The thermal differences between the arid areas of Northwest China and the humid monsoon regions of eastern China inevitably contribute to the local and remote atmospheric circulation. Moreover, the temperature difference was related to the rainfall and circulation anomalies in China (Zhou and Huang, 2006;Zhou, 2015).

Datasets
This study used daily observations of the land surface temperature (T s ) and the surface air temperature (

Interdecadal variability of the spring thermal difference between western and eastern China
The sensible heat flux was predominantly determined from the difference between the surface temperature (T s ) and surface air temperature (T a ) difference, T s -T a (Fan et al., 2004;Zhou, 2009). Thus, we used this land-air temperature difference to study the thermal difference between western and eastern China and its association with the East Asian summer monsoon. Previous studies show there was an obvious difference in T s -T a between western and eastern China (Wen et al., 2014;Zhou and Wen, 2016). According to station location from Figure 1, the region average in T s -T a of western and eastern China was calculated, respectively. Figure 2 shows the mean T s -T a anomalies in western and eastern China in spring. The T s -T a anomalies were mostly negative in western China before the late 1970s and largely positive after the late 1970s (Figure 2(a)). However, for eastern China (Figure 2(b)), the T s -T a anomalies were mostly positive before the late 1970s but largely negative after the late 1970s. The data suggest that the spring thermal state in eastern and western China underwent an opposite interdecadal shift around the late 1970s. Moreover, Δ(T s -T a ) was used to represent the thermal difference between western and eastern China, where Δ(T s -T a ) = (T s -T a |west) − (T s -T a |east).

Interdecadal variability of rainfall in China
The temperature difference was related to the rainfall anomalies in China (Zhou and Huang, 2006;Zhou, 2015). To investigate the association of the thermal difference anomaly with the summer rainfall variability in China, summer rainfall anomalies, water vapor flux and moisture, and vertical velocity were examined before and after 1979.
During 1960-1978, positive rainfall anomalies appeared over North China and negative anomalies appeared over the Yangtze River (Figure 3(a)). Both these anomalies were related to the East Asian summer monsoon circulation. The southwestward anomalous water vapor fluxes along the coast of eastern China substantially enhanced the moisture supply to North China from the South China Sea. This consequently increased the summer rainfall over North China (Figure 4(a)). Moreover, the increase in the rainfall was associated with water vapor flux convergence and ascending motion over North China. On the other hand, the decrease in the rainfall was associated with water vapor flux divergence and sinking motion over the Yangtze River (Figure 4(c) and (e)). However, during 1979-2006, negative rainfall anomalies also appeared over North China and positive anomalies appeared over the Yangtze River (Figure 3(b)). This was related to the weakening of the East Asian summer monsoon and the dominant northeasterly anomalies over East Asia. The northeastward anomalous water vapor fluxes were not contributed to transport into North China (Figure 4(b)), which were helpful for a decrease in rainfall over North China. Moreover, tropospheric moisture divergence and sinking motion occurred over North China (Figure 4(d) and (f)), whereas tropospheric moisture convergence and ascending motion were observed over the Yangtze River (Figure 4(d) and (f)). The latter contributed to increased rainfall in the region.
To understand the relationship between spring thermal difference between western and eastern China and the East Asian summer monsoon, the summer wind at 850 hPa, temperature at 700 hPa, and SLP were analyzed. Figure 5 shows regression of summer wind at 850 hPa, temperature at 700 hPa, and SLP with respect to the normalized spring thermal difference for the period of 1960-2006. From Figure 5(a), an anticyclonic circulation anomaly appeared over Mongolia. And the anomalous northeasterly winds occurred over eastern China. This may be related to the negative temperature anomalies appeared over Mongolia, which corresponded to positive land-air temperature difference between eastern and western China ( Figure 5(b)). The negative temperature anomalies corresponded to an increased in SLP (Figure 5(c)). This enhanced in pressure difference contributed to an increase the northeasterly wind anomalies in East Asia, and a decrease in the East Asian summer monsoon.

Interdecadal variability of the atmospheric circulation
The summer rainfall anomalies in China and atmospheric circulation anomalies are closely related. In the previous section, the data suggested that the thermal difference underwent an interdecadal shift around 1979. Moreover, the thermal differences were closely related with the East Asian summer monsoon. To understand the thermal difference between western and eastern China and its association with the interdecadal variability in East Asian summer monsoon circulation anomalies, the summer 850 hPa wind, SLP, 700 hPa temperature, and geopotential height anomalies at 500 hPa were analyzed before and after 1979. During 1960During -1978 (Figure 6(a)), cyclonic circulation anomalies appeared over Mongolia. To the east flank of this anomalous cyclone were anomalous southwesterly winds over eastern China. These anomalous winds extended from South to North China, which helped to strengthen the East Asian summer monsoon circulation. From the above analysis, the thermal difference between eastern and western China was found to be relatively small in this period. Based on the thermal wind, the small thermal difference between western and eastern China contributed to strengthening of southwesterly wind in eastern China, which were helpful for strengthening in East Asian summer monsoon. However, during 1979However, during -2006 (Figure 6(b)), anticyclonic circulation anomalies appeared over Mongolia. Moreover, the anomalous northeasterly winds weakened the East Asian summer monsoon circulation. The large thermal difference between western and eastern China was in 1979-2006. Therefore, according to thermal wind, it indicated that large thermal difference between western and eastern China contributed to strengthening of    northeasterly winds, which were helpful for weakening in East Asian summer monsoon. The East Asian summer monsoon anomalies are consistent with previous studies that concluded interdecadal weakening for the East Asian summer monsoon in the late 1970s (e.g. Huang et al., 1999;Chang et al., 2000aChang et al., , 2000bWang, 2001;Wu and Wang, 2002;Zhou and Huang, 2003;Ding et al., 2008).
To analyze in more detail, the effect of the thermal difference between western and eastern China on the East Asian summer monsoon, the temperature, SLP, and geopotential height were examined. During 1960During -1978, at 700 hPa, large positive temperature anomalies appeared over Mongolia (Figure 6(c)). The positive temperature anomalies decreased the SLP (Figure 6(e)) and geopotential height anomalies ( Figure 6(g)). This enhanced the cyclonic circulation anomalies over Mongolia and increased the southwesterly wind anomalies in East Asia. However, during 1979However, during -2006, negative temperature anomalies appeared over Mongolia (Figure 6(d)). The negative temperature anomalies increased the SLP (Figure 6(f)) and geopotential height anomalies (Figure 6(h)). This enhanced the anticyclonic circulation anomalies over Mongolia and increased the northeasterly wind anomalies in East Asia.

Summary and discussion
This study identified the interdecadal variability of the spring thermal difference between western and eastern China and its association with the rainfall anomalies using station and reanalysis data for 1960-2006. The spring Δ(T s -T a ) was used to represent the thermal difference between western and eastern China. The spring Δ(T s -T a ) data suggest an interdecadal shift around 1979 for western and eastern China. During 1960During -1978, large positive temperature anomalies appeared over Mongolia. The positive temperature anomalies contributed to a decrease in pressure in this region, which contribute to an enhanced in cyclonic circulation anomalies over Mongolia. To the east flank of this anomalous cyclone were anomalous southwesterly winds over eastern China, which helped to strengthen the East Asian summer monsoon circulation. The rainfall increase over North China was associated with water vapor flux convergence and ascending motion, whereas the decrease in rainfall over the Yangtze River was associated with water vapor flux divergence and singing motion. However, during 1979-2006, negative temperature anomalies appeared over Mongolia. The negative temperature anomalies increased the pressure and the anticyclonic circulation anomalies over Mongolia. To the east flank of this anomalous anticyclone were anomalous northeasterly winds over eastern China, which were helpful for a weakening in the East Asian summer monsoon circulation. The results suggest tropospheric moisture divergence and sinking motion over North China and tropospheric moisture convergence and ascending motion over Yangtze River that contributed to increased rainfall in this region.
The interdecadal variability of the East Asian summer monsoon was affected by land surface processes and ocean thermodynamics. There are other factors, such as thermal field over the Tibetan Plateau, the snow cover, the Pacific SST, and the middle and high latitudes (e.g. Yang and Lau, 1998;Huang et al., 1999;Chang et al., 2000aChang et al., , 2000bWang, 2001;Wu and Wang, 2002;Zhou and Huang, 2003;Ding et al., 2008), which may affect the atmospheric circulation over East Asia and the rainfall anomalies in China.
It is well known that climate shifts are characterized by changes in the background state of the Pacific Ocean and the ENSO dynamics. The interdecadal variabilities before and after the late-1970s for SST-forced teleconnection are found in many regions (Hare and Mantua, 2000;Karspeck and Cane, 2002;Hartmann and Wendler, 2005;Rodriguez-Fonseca et al., 2011).
Because of the limited data, it cannot be determined whether the change in previous studies as well as in this study is permanent or reversible. In view of the impact of global warming on the tropical sea surface and land surface temperature, it is likely that the East Asian summer monsoon will change in the future owing to global warming.