Figure 1 shows the climatological distributions of the observed and simulated summer SAT in East Asia. In the observation, the SAT in East Asia generally exhibits a decreasing trend from south to north. An extensive area of low SAT is observed over the Tibetan Plateau because of its high altitude, with a climatological value lower than 12°C. Moreover, the climatological summer SAT in western Mongolia and northern Xinjiang is also lower than 12°C, while it is higher than 24°C in Southeast China and the southern part of Xinjiang (Fig. 1a). CESM-DPLE simulates well the decreasing pattern in SAT from south to north, and the high SAT centers over Southeast China and southern Xinjiang, as well as the low SAT centers over the Tibetan Plateau, western Mongolia, and northern Xinjiang (Fig. 1b). Inspection of the difference between the simulation and observation reveals a notable cold bias in most parts of East Asia. The simulated SAT over Southeast China, Northeast China, the Korean Peninsula, and southern Japan is 0–3°C lower than observed, and 3–5°C lower than observed over North China, Southwest China, and southern Xinjiang. Meanwhile, the simulated SAT is 3–5°C higher than observed over most of Xinjiang (Fig. 1c). Here, root-mean-square error (RMSE) and pattern correlation coefficient (PCC) are used to quantify the errors of the simulation. Table 1 shows that the RMSE and PCC for summertime East Asian SAT are 3.24°C and 0.88 respectively, suggesting that CESM-DPLE simulates well the spatial distribution of summer SAT in East Asia.
Figure 1. Climatology of summer surface air temperature (SAT, °C) over East Asia during 1959–2016 from (a) observation, (b) simulation, and (c) simulation minus observation.
SAT (°C) Precipitation (mm) 500-hPa geopotential height (gpm) 850-hPa meridional wind (m s−1) 850-hPa zonal wind (m s−1) PCC 0.88 0.67 0.99 0.72 0.67 RMSE 3.24 284.82 16.99 1.94 1.11
Table 1. The pattern correlation coefficient (PCC) and root-mean-square error (RMSE) of five variables’ climatologies during 1959–2016 between the CESM-DPLE simulation and observations in East Asia (20°–55°N, 90°–120°E). All PCCs exceed the 99% confidence level
Figure 2 shows the observed and simulated climatological mean summer precipitation over China. CESM-DPLE simulates well the observed large-scale pattern of “southeast coast wet–northwest inland dry” and the heavy precipitation above 540 mm over the southeastern Tibetan Plateau, southern part of South China, middle and lower reaches of the Yangtze River, North China, southern part of Northeast China, and the Korean Peninsula (Fig. 2b). The differences between the simulated and observed precipitation in East China indicate that the simulated rainbelt extends too far northward compared to the observation, and the simulated precipitation is lower than observed to the south of the Yangtze River valley and over the Korean Peninsula and southern Japan, but higher than observed to the north of the Yangtze River and over Northeast China. In addition, CESM-DPLE simulates excessive precipitation over the eastern Tibetan Plateau and the Sichuan basin (Fig. 2c). This bias has also been found to exist in previous climate model studies (Jiang et al., 2004, 2016; Zhang et al., 2008; Si et al., 2009). As for this spurious high precipitation center, it may be due to the low horizontal resolution of the climate model (Jiang et al., 2005; Gao et al., 2006; Xu et al., 2010). As Table 1 reveals, the RMSE and PCC for the simulated summertime East Asian precipitation are 284.82 mm and 0.67 respectively, and therefore the skill of CESM-DPLE with respect to the precipitation distribution is generally lower than that for the SAT.
Figure 2. As in Fig. 1, but for summer precipitation (mm).
Since the simulated precipitation and SAT are dynamically related to the large-scale atmospheric circulation, we assess the simulation skill of CESM-DPLE with respect to the summertime atmospheric circulation in East Asia. At 500 hPa, a “one trough and one ridge” geopotential height pattern dominates the mid–high latitudes of Asia, featuring a trough over Lake Balkash and a ridge over East Siberia (Fig. 3a). Meanwhile, a “two high and one low” geopotential height pattern emerges over the mid–low latitudes of Asia. The western North Pacific sub-tropical high (WPSH) generally situates between 20° and 30°N, and its westernmost ridge point lies around 140°E. The other high pressure locates over the Arabian Peninsula. Between the two high pressures, there is an inverted Ω-like trough over India. Generally, CESM-DPLE simulates well the “one trough and one ridge” and “two high and one low” geopotential height patterns over the mid–high latitudes and mid–low latitudes in Asia, respectively (Fig. 3b). However, it simulates a stronger trough over Lake Balkash and a stronger ridge over East Siberia compared with observation. The simulated WPSH is stronger and larger than observed, and extends to the west of its observational position (Fig. 3b). The geopotential height values are higher in the CESM-DPLE simulation than observed in most parts of Asia, except the Tibetan Plateau where the simulated geopotential height is lower than observed. The simulated low geopotential height over the Tibetan Plateau, together with the simulated stronger WPSH, intensifies the pressure gradient between the Tibetan Plateau and WPSH (Fig. 3c), in the region where the East Asian summer monsoon prevails, thus causing a northward advance of the East Asian summer monsoon rainbelt (Si et al., 2009; Tian et al., 2012).
Figure 3. Climatology of summer geopotential height (gpm) at 500 hPa over East Asia during 1959–2016 from (a) observation, (b) simulation, and (c) simulation minus observation. The letter H denotes a center of high geopotential height. The dashed and solid curves denote the trough line and ridge line, respectively.
Next, we examine the simulation of the low-level summer monsoon circulation in East Asia. On the one hand, the simulated southwesterly winds from the Indian Ocean are weaker than observed (Fig. 4), which is largely a result of the weak simulation of the India–Burma trough (Fig. 3); whilst on the other hand, the simulated southeasterly winds from the western North Pacific are stronger than observed, possibly because of the simulated stronger and more westward extended WPSH. Inspection of the difference between the simulation and observation reveals strong southerly winds prevailing over East China, the Korean Peninsula, and Japan, which converge with the northerly winds along Southwest China, North China, and Northeast China (Fig. 4c), leading to a northward advance of the major rainbelt and excessive precipitation over the eastern Tibetan Plateau.
Figure 4. As in Fig. 3, but for summer wind field (m s−1) at 850 hPa. Regions with elevation higher than 1500 m are left blank.
Table 1 shows that the RMSE for the summer 500-hPa geopotential height, 850-hPa meridional wind, and 850-hPa zonal wind over East Asia is 16.99 gpm, 1.94 m s−1, and 1.11 m s−1, respectively. The PCC for these variables is 0.99, 0.72, and 0.67, respectively, all of which exceed the 99% confidence level.