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YAN Zheng-Bin, LIN Zhao-Hui, ZHANG He. The Relationship between the East Asian Subtropical Westerly Jet and Summer Precipitation over East Asia as Simulated by the IAP AGCM4.0. Atmospheric and Oceanic Science Letters, 2014, 7(6): 487-492  
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The Relationship between the East Asian Subtropical Westerly Jet and Summer Precipitation over East Asia as Simulated by the IAP AGCM4.0
YAN Zheng-Bin1,2, LIN Zhao-Hui1,3,*, ZHANG He1
1. The International Center for Climate and Environment Sciences (ICCES), Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences, Beijing 100029, China
2. University of Chinese Academy of Sciences, Beijing 100049, China
3. Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044, China
*Corresponding author: LIN Zhao-Hui,lzh@mail.iap.ac.cn
Abstract

Based on a 30-year Atmospheric Model Intercomparison Project (AMIP) simulation using IAP AGCM4.0, the relationship between the East Asian subtropical westerly jet (EASWJ) and summer precipitation over East Asia has been investigated, and compared with observation. It was found the meridional displacement of the EASWJ has a closer relationship with the precipitation over East Asia both from model simulation and observation, with an anomalous southward shift of EASWJ being conducive to rainfall over the Yangtze-Huaihe River Valley (YHRV), and an anomalous northward shift resulting in less rainfall over the YHRV. However, the simulated precipitation anomalies were found to be weaker than observed from the composite analysis, and this would be related to the weakly reproduced mid-upper-level convergence in the mid-high latitudes and ascending motion in the lower latitudes.

Keyword: East Asian subtropical westerly jet; summer precipitation; IAP AGCM4.0; model evaluation
1 Introduction

As one of the key components of the East Asian monsoon system, the East Asian subtropical westerly jet (EASWJ) exhibits notable seasonal evolutions of location and intensity, accompanying the transition of the East Asian monsoon circulation (e.g., Yeh et al., 1959; Tao and Chen, 1987). The association between the EASWJ and climate anomalies over East Asia has previously been demonstrated: many studies have suggested that the meridional displacement of the EASWJ bears a closer relationship to East Asian summer precipitation, an equator ward displacement of the EASWJ causes precipitation to increase over South-Central China during June-August, while a pole ward shift of the summer EASWJ brings heavier precipitation over North China ( Liang and Wang, 1998; Gong et al., 2002; Lu, 2004; Kuang and Zhang, 2006). It is also found that the Yangtze-Huaihe River Valley (YHRV) summer rainfall is closely related to the monthly variation of the EASWJ ( Xuan et al., 2011b). In addition, the location of the EASWJ exhibits a significant variation on the decadal time scale as revealed by Xuan et al. (2011a), who found that, compared to period 1951-1979, the location of the EASWJ moved abnormally south of the climatic mean during 1980-2008, causing a significant increase in rainfall over the YHRV after the 1980s.

Due to the impact of the EASWJ on the East Asian monsoon climate, the capability of climate models in reproducing the observed characteristics of the EASWJ, which is the most important criterion for model performance, has been conducted for several different climate models. For example, Zhang et al. (2008) evaluated the performance of the two versions of the climate system model, the high-resolution version and the medium-resolution one of Model for Interdisciplinary Research on Climate (MIROC_Hires and MIROC_Medres) on simulating the observed features of the EASWJ, which were developed jointed by the Center for Climate System Research (CCSR) of the University of Tokyo, the National Institute for Environmental Studies (NIES), and the Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology (FRCGC) of Japan. They suggested that reasonable reproductions of the meridional heat transport gradient and the surface diabatic heating are the key challenges for improving the EASWJ simulation by the MIROC model. Huang and Liu (2011) analyzed the fundamental features of the EASWJ simulated by Global Atmospheric Model Version 2.1 (AM2.1), a model developed at NOAA's (National Oceanic and Atmospheric Administration) Geophysical Fluid Dynamics Laboratory (GFDL), and found that AM2.1 performs well in capturing the leading modes of the EASWJ, but shows clear deficiencies in simulating the intensity and location of the EASWJ. Currently, Yan et al. (2014) found that the Institute of Atmospheric Physics (IAP) Atmospheric General Circulation Model (AGCM4.0) can capture the main features of the EASWJ, including the intensity and location, but reproduces poorly the interannual variability of the EASWJ. However, whether the current climate system model can reproduce the observed relationship between the EASWJ and the summer precipitation anomalies over East Asia has not yet been fully evaluated, and remains essential to further mode improvement, especially for the improved simulation of summer monsoon rainfall over East Asia.

This paper investigates the capability of IAP AGCM4.0 in simulating the association between the EASWJ and precipitation over East Asia, using a 30-year Atmospheric Model Intercomparison Project (AMIP) type simulation by the IAP AGCM4.0, in order to provide a useful reference for further improvements to the model, and its eventual application to seasonal climate predictions.

2 Model description and experimental design

The climate model used in this study is the IAP AGCM4.0, which was developed by Zhang et al. (2009), and has been evaluated by many studies for its performance in reproducing the observed modern climate (e.g., Sun et al., 2012; Dong et al., 2012). The dynamical core of IAP AGCM4.0 is largely the same as former-generation IAP AGCMs, but with the introduction of some new techniques, such as flexible but permissible substitutions, a flexible leaping-point scheme at high latitudes, a time- splitting method, and a semi-Lagrangian vapor transport scheme. The full physical package from Community Atmospheric Model ver3.1 (CAM3.1), developed by the National Center for Atmospheric Research (NCAR), is adopted in IAP AGCM4.0, with the convection parameterization schemes taken as the modified Zhang- McFarlane scheme (MZM) ( Neale et al., 2008; Richter and Rasch, 2008).

The IAP AGCM4.0 was integrated for 30 years and forced by monthly mean sea surface temperatures (SSTs) from the Hadley Centre Sea Ice and Sea Surface Temperature data set (HadISST1) dataset ( Rayner et al., 2003) covering the period 1979-2008. The observational data used for analysis and comparison were: the monthly atmospheric data from the Japanese 25-year reanalysis (JRA-25) dataset with a horizontal resolution of 1.25°×1.25° ( Onogi et al., 2007); and the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) data with horizontal resolution of 2.5°×2.5° ( Xie and Arkin, 1997) from NOAA.

3 Results
3.1 Interannual variability of summer precipitation and 200 hPa zonal wind

The close relationship between the EASWJ and summer precipitation over East Asia has been demonstrated by many studies (e.g., Xuan et al., 2011a). In this paper, the singular value decomposition (SVD) method ( Bretherton et al., 1992) is adopted to investigate the relationship between the summer zonal wind at the 200 hPa level and precipitation over East Asia.

Figure 1 shows the first SVD mode of the zonal wind at the 200 hPa level ( U200) and precipitation in summer over East Asia for the observation and model. For the observation, the first SVD mode explained 36.8% of the squared covariance, and the U200 pattern was character-ized by a meridional tripole structure with easterly anomalies between 25-40°N and westerly anomalies at the two flanks, with one of the zero line slocated roughly at the same latitudes as the climatological EASWJ cores in June, July, and August (JJA) (Fig. 1a). The corresponding precipitation (Fig. 1b) displayed dry conditions from the YHRV to the south of Japan and wet conditions south of the Yangtze River and lower reaches of the Yellow River. The temporal correlation between the U200 and precipitation time series was 0.94, with the 99% confidence level (Fig. 1c).

Figure 1 The first singular value decomposition (SVD) mode of zonal wind at 200hPa ( U200) and precipitation in summer for the Japanese reanalysis (JRA) (a) U200, (b) precipitation, and (c) the corresponding normalized time coefficient. Panels (d-f) are the same as (a-c), except for the IAP AGCM4.0.

For the model, the first SVD mode explained 22.6% of the squared covariance between the U200 and precipitation over East Asia. The U200 pattern of the model results revealed a similar structure to that shown in Fig. 1a, but the easterly anomaly moved northward with its center between 25-42°N (Fig. 1d). This may have been due to the northward location of the center of the simulated U200 over East Asia (figure not shown). The simulated precipitation pattern was generally comparable to the main features in the observation, except for simulation deficiencies in reproducing the observed dry conditions in the middle reaches of the Yangtze River (Fig. 1e). Furthermore, correlation between U200 and precipitation reached 0.88, with the 99% confidence level (Fig. 1f). Generally, the SVD analysis results for an individual summer month (e.g., July) were similar to the JJA mean results (figures not shown).

As suggested by Xuan et al. (2011a), the EASWJ position index (EAJSI) can be defined as the difference between U200 averaged over two regions: (35-40°N, 90-130°E) and (40-45°N, 90-130°E), with positive values indicating a southward shift of the EASWJ, and negative values a northward shift.

Figure 2 shows the time series of normalized summer EAJSI and the first SVD mode of U200 (PC1 indicating the 1st principal component) for the observation and model. In the observation, the correlation between EAJSI and PC1 was -0.65, revealing that positive values of PC1 denote a northward displacement of the EASWJ, and negative values a southward displacement. The correlation between simulated EAJSI and PC1 was -0.75, which was above the 99% confidence level, demonstrating that the simulated PC1 also illustrated the displacement of the EASWJ. This suggests that the observed relationship between the EASWJ and the PC1 of first SVD mode of U200 and precipitation in summer over East Asia can be well reproduced by the IAP AGCM4.0.

Figure 2 Normalized time series of summer East Asian subtropical westerly jet (EASWJ) position index (EAJSI) (solid line) and the first SVD mode of U200 (dashed line) from 1979 to 2008.

3.2 Relationship between meridional displacement of the EASWJ and summer precipitation anomalies

To further examine the connection between the meridional displacement of the EASWJ and precipitation anomalies over East Asia, a composite analysis was conducted, for positive and negative phases of the EAJSI respectively. Here, only those years with normalized EAJSI values greater than 1 standard deviation (>1 SD) were identified as years with southward shift of East Asian subtropical jet, and those less than -1 standard deviation (<-1 SD) were selected as years with northward displacement of East Asian subtropical jet.

Composite precipitation anomalies in northward- and southward-shifted years of the EASWJ for the observation and IAP AGCM4.0 are shown in Fig. 3. For the observation, the composite results showed, in EASWJ northward-shifted years, negative precipitation anomalies located in the YHRV and positive anomalies at the two flanks (Fig. 3a). When the EASWJ moved southward of the mean, positive precipitation anomalies were clearly seen in the YHRV, especially in the south of Japan (Fig. 3b). Figure 3c shows the precipitation differences between the EASWJ southward-shifted years and northward-shifted years. It was also found that there was more precipitation in the YHRV and the south of Japan and that the maximum precipitation difference was greater than 5 mm d-1. Clearly, an anomalous southward shift of the EASWJ favored rainfall over the YHRV, while an anomalous northward shift resulted in less rainfall over the YHRV.

Figures 3d-f show the results from the IAP AGCM4.0 simulation, and it is clear that the model well captured the characteristics of anomalous precipitation with the meridional displacement of the EASWJ. However, compared to observational results, the simulated anomalous precipitation weakened markedly in EASWJ northward- and southward-shifted years, especially in the south of Japan in EASWJ southward-shifted years, and this could be ascribed to the model's discrepancy in simulating the EASWJ with weaker intensity.

The EASWJ is one of the important components of the East Asian monsoon system, and changes in the position of the EASWJ are closely associated with the anomalies in the Asian monsoon circulation systems that consequently lead to changes in monsoon precipitation over China.

Figure 4a shows the composite differences of 200 hPa wind between southward- and northward-shifted years of the EASWJ according to the observational data. The most significant difference in the mid-high latitudes of East Asia was the strengthened westerly to the south of 40°N, with an anomalous cyclonic circulation centered at about (40°N, 125°E). Furthermore, in the lower latitudes, there was an anomalous anticyclonic circulation to the south of 30°N. The anomalous cyclonic circulation usually accompanied a convergence of air flow, and anomalous anticyclonic circulation a divergence of air flow. Upper- level convergence favors descending motion below it, while upper-level divergence facilitates ascending motion below it. Figure 4c shows the differences of the latitude-pressure cross section of the vertical velocity averaged over 110-140°E between southward- and northward- shifted years of the EASWJ. Corresponding to the mid- high-latitude convergence and lower-latitude divergence in the upper troposphere, anomalous descending and ascending motion were found in mid-high latitudes and lower latitudes, respectively. Corresponding to the anomalous ascending motion around 30°N, anomalous convergence of air flow was clearly observed in 850 hPa wind (Fig. 4b). The lower-level convergence and anomalous ascending motion was most significant at around 30°N, favoring rainfall over the YHRV, therefore, a significant increase in rainfall could be seen over the region during southward-shifted years of the EASWJ, with the maximum difference in excess of 5 mm d-1, compared to that during northward-shifted years (Fig. 3c).

Figure 3 Composite precipitation anomalies in northward- and southward-shifted years of the EASWJ for (a, b) JRA and (d, e) IAP AGCM4.0, and composite differences of precipitation between northward- and southward-shifted years of the EASWJ for (c) JRA and (f) IAP AGCM4.0. Units: mm d-1.

The composite of the IAP AGCM4.0 simulation results are shown in Figs. 4d-f. The model captured these features with a high degree of success, compared to the observation results in Figs. 4a and 4b. In general, IAP AGCM4.0 successfully reproduced the upper-level anomalous cyclonic circulation in the mid-high latitude (Fig. 4d) and the lower-level convergence over the YHRV (Fig. 4e). Nevertheless, there still existed some differences between the simulation and observation. For example, in the upper troposphere, the observed cyclonic circulation was weakly reproduced, and the center of the cyclonic circulation shifted westward in the simulation. Also, in the lower troposphere, the observed relatively strong convergence at 30°N was weakly represented. The bias in reproducing the intensity and location of cyclonic circulation in the upper troposphere caused weakened anomalous ascending motion around 30°N in the simulation, which would ultimately affect the simulation of rainfall. This was coincident with the weakly simulated rainfall in the southward-shifted years of the EASWJ, as shown in Fig. 3e.

Figure 4 Differences in (a) 200 hPa wind, (b) 850 hPa wind, and (c) the latitude-pressure cross section of the vertical velocity averaged over 110-140°E between southward- and northward-shifted years of the EASWJ for JRA. Panels (d-f) are the same as (a-c), exceptfor the IAP AGCM4.0 (units: m s-1 for zonal and meridional wind; -10-3 Pa s-1 for vertical velocity).

4 Summary

The performance of IAP AGCM4.0 in simulating the relationship between the EASWJ and precipitation over East Asia in summer was evaluated through comparisons of the model output against observations. The observation results indicated that, when the EASWJ was located southward of the climatic mean, rainfall increased over the YHRV during summer. Conversely, a northward displacement of the summer EASWJ resulted in less than normal rainfall over this region. Compared to observations, the model was found to be capable of representing anomalous rainfall in the YHRV and its association with the meridional displacement of the EASWJ. However, the simulation showed weakness with respect to anomalous precipitation, especially in the south of Japan in the southward-shifted years of the EASWJ. This discrepancy could be ascribed to the improperly simulated EASWJ- related atmospheric circulation. The weakly reproduced EASWJ leads to a weaker mid-high-latitude convergence and lower-latitude divergence in the upper troposphere. Furthermore, the mid-high-latitude anomalous descending and lower-latitude ascending motion are weaker than observed, resulting in the model simulating weaker anomalous precipitation in EASWJ southward- and northward- shifted years.

The weakly reproduced anomalous precipitation in southward- and northward-shifted years of the EASWJ, including the improperly simulated large-scale circulation, such as weaker upper-level divergence and ascending motion, may be related to the improper representation of physical processes within the model. Therefore, more simulation studies with different physical parameterization schemes will be investigated in the future, in order to understand the associations between the EASWJ and rainfall over East Asia.

Acknowledgements. This research was jointly supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA05110202) and the National Natural Science Foundation of China (Grant Nos. 41175073 and U1133603).

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