Sensitivity of summer precipitation to tropical sea surface temperatures over East Asia in the GRIMs GMP

GEOPHYSICAL RESEARCH LETTERS, VOL. 40, 1824 1831, doi:10.1002/grl.50389, 2013 Sensitivity of summer precipitation to tropical sea surface temperatures over East Asia in the GRIMs GMP Eun-Chul Chang, 1 Sang-Wook Yeh, 2 Song-You Hong, 3 and Renguang Wu 4 Received 19 December 2012; revised 19 March 2013; accepted 19 March 2013; published 15 May 2013. [1] In this study, uncoupled atmospheric general circulation model experiments are conducted to examine the sensitivity of tropical Ocean basins from the Indian Ocean to the tropical Pacific Ocean on the summer precipitation variability over East Asia. It is remarkable that the Indian Ocean basin sea surface temperature (SST) and the tropical Pacific basin SST act on summer precipitation variability over Northeast Asia and southern China quite differently. That is, SST warming in the Indian Ocean largely contributes to the increase in the amount of summer precipitation over East Asia, which is in contrast to the warming of the western tropical Pacific Ocean. Our further analysis indicates that an altered large-scale atmospheric circulation over the western tropical Pacific contributes to contrasting atmospheric motion over East Asia due to the tropics-east Asia teleconnections, which results in changes in the amount of summer precipitation due to the warming of the Indian and western tropical Pacific Oceans. Citation: Chang, E.-C., S.-W. Yeh, S.-Y. Hong, and R. Wu (2013), Sensitivity of summer precipitation to tropical sea surface temperatures over East Asia in the GRIMs GMP, Geophys. Res. Lett., 40, 1824 1831, doi:10.1002/grl.50389. 1. Introduction [2] The variability of summer (June July August, JJA) precipitation over eastern China, Japan, and Korea, where nearly one third of the world s population resides, is largely influenced by atmospheric teleconnections from both the tropical Pacific Ocean and the tropical Indian Ocean [Wang et al., 2000; Lee et al., 2005; Xie et al., 2009; Yun et al., 2010; Shin et al., 2011]. Many studies have been conducted to examine atmospheric teleconnections between the East Asian summer monsoon (EASM) and tropical Ocean basins. In particular, Wang et al. [2000] argued that an anomalous lower-tropospheric anticyclone located in the western North Pacific is a key system bridging the EASM and the tropical Pacific through the so-called Additional supporting information may be found in the online version of this article. 1 Atmosphere and Ocean Research Institute, University of Tokyo, Kashiwa, Japan. 2 Department of Marine Sciences and Convergent Technology, College of Science and Technology, Hanyang University, Ansan, South Korea. 3 Department of Atmospheric Sciences and Global Environment Laboratory, Yonsei University, Seoul, South Korea. 4 Institute of Space and Earth Information Science, Chinese University of Hong Kong, Hong Kong. Corresponding author: S.-W. Yeh, Department of Marine Sciences and Convergent Technology, College of Science and Technology, Hanyang University, Ansan, South Korea. (swyeh@hanyang.ac.kr) 2013. American Geophysical Union. All Rights Reserved. 0094-8276/13/10.1002/grl.50389 1824 Pacific-East Asian atmospheric teleconnection. Since this pioneering work, numerous studies have been conducted to reveal the connections between the variability of the EASM and the tropical Pacific sea surface temperature(sst)variability (e.g., El Niño/Southern Oscillation, ENSO) [Lau and Wang, 2006, and references therein]. For example, Yun et al. [2010] have recently argued that warming of the eastern Pacific can result in stronger convection over the central Pacific and in turn suppress convection over the northwestern Pacific, thereby leading to a Pacific-Japan-like teleconnection from the western Pacific to East Asia. On the other hand, Wang et al. [2001] argued that a strong Indian monsoon, which is associated with a warm Indian Ocean SST, can induce an anomalous anticyclone over northeast China and thereby affect the EASM precipitation. The influence of anomalous Indian monsoon heating on East Asian circulation was also suggested by Wu [2002]. Furthermore, Xie et al. [2009] suggested that the tropical Indian Ocean acts like a capacitor anchoring atmospheric anomalies over the Indo-western Pacific, which eventually influences the EASM. [3] Prior study results suggest that the SSTs of the tropical Pacific and tropical Indian Oceans affect the EASM through atmospheric teleconnections. Building on this work, more recent studies have sought to determine how each Ocean acts differently on the EASM. Meng et al. [2011] performed sensitivity experiments with and without historical SST forcing in the tropical Indian Ocean. They showed that historical warming of the Indian Ocean SST leads to a strengthened Walker circulation and a low-pressure anomaly over the Indian Ocean that eventually affect the amount of precipitation over East Asia. Shin et al. [2011] mapped the sensitivity of EASM precipitation to tropical Pacific SST anomalies by synthesizing the atmospheric general circulation model (AGCM). They argued that the most critical SST changes influencing EASM precipitation occur in the far western Pacific Ocean around the Philippines and the South China Sea. [4] The aforementioned studies do not address the important question of how the different Ocean basins in the tropics affect EASM precipitation. To answer this question, we perform uncoupled AGCM experiments with four sets of historical SSTs prescribed in the global Ocean, the Indian Ocean, the western tropical Pacific, and the eastern tropical Pacific, respectively, for the period of 1950 2010. An additional run in which the climatological (1950 2010) SST is prescribed in the global ocean is also conducted with a simulation period of 90 years. We compare these runs to examine the influence of tropical Ocean SSTs on EASM precipitation. 2. AGCM Experiment and Data [5] The Global/Regional Integrated Model system (GRIMs) global model program (GMP) [Hong et al., 2013] is utilized as

Table 1. Experimental Design for Sensitivity Experiments Experiment Region of the Prescribed Historical SST Integration Period Number of Ensemble Members CNTL Global Ocean 1950 2010 4 IO Indian Ocean 1950 2010 4 WP Western tropical Pacific 1950 2010 4 EP Eastern tropical Pacific 1950 2010 4 CLIM No (all climatological SSTs) 90 years 1 the AGCM in this study. The GRIMs has been originated from the National Centers for Environmental Prediction Global Spectral Model [Kanamitsu et al., 2002], and it has been developed for use in numerical weather prediction, seasonal simulations, and climate research projects, from global to regional scales with improved dynamic core and physical processes. The horizontal resolution is T62 (approximately 200 km), and vertically, 28 layers are used. The Met Office Hadley Centre s sea ice and SST data set [Rayner et al., 2003], which has the resolution of 1 1, is used for the historical SST forcing. [6] Five AGCM experiments are designed as in Table 1. Hereafter we refer to them as the CNTL, IO, WP, and EP experiments, which are forced by the prescribed historical SST in the global Ocean, the Indian Ocean, the western tropical Pacific, and the eastern tropical Pacific, respectively, for the period of 1950 2010 (see Figure 2). The areas outside of the SST regions prescribed for each experiment (i.e., IO, WP, and EP experiment) are given the climatological (1950 2010) SST. The four experiments are integrated for 63 years from 1948 to 2010, and the results for 61 years (1950 2010) are used in this study. The IO, WP, and EP experiments are performed with four ensemble members. The climatological SST run, referred to as the CLIM experiment, is forced by the seasonally cyclical climatological SST (1950 2010) in the global ocean for a 90 year study period. In the present study, all of the data in the CLIM experiment are averaged for the 90 year simulation period. [7] To determine the sensitivity of tropical Ocean SSTs to EASM precipitation, we compare the results of each AGCM experiment. The tropical Indian Ocean region is set to 30 E 120 E, 30 S 25 N, which is the area for which the historical SST is prescribed for the IO experiment. The western Pacific and the eastern Pacific regions are set to 100 E 170 E, 20 S 20 Nand170 E 70 W, 20 S 20 N, respectively. The Indian Ocean and the Western Pacific regions are separately defined along the maritime-continental borderline. To mitigate discontinuity between the regions, the SSTs within 5 of the edge of each prescribed historical SST region are generated by a linear interpolation of the historical and climatological SSTs. 3. Results [8] First, the CNTL is compared with the ERA-40 [Uppala et al., 2005] and the climate prediction center (CPC) 50 year reconstructed precipitation data set [Chen et al., 2002]. Figures 1a 1c display the precipitation simulated in the CNTL for the period of 1950 2010 along with the ERA-40 (1958 2002) and the CPC (1950 2010) precipitation during JJA, respectively. The structure of summer precipitation in the CNTL has some similarities and differences in comparison with the ERA-40 and the CPC. For example, the CNTL reasonably simulates a large amount Figure 1. JJA averaged precipitation (mm d 1 ) from (a) CNTL experiment for 1950 2010, (b) ERA-40 reanalysis for 1958 2002, and (c) CPC reconstructed data for 1950 2010. 1825

Table 2. Pattern Correlation Coefficients of JJA Averaged Precipitation of the CNTL Experiment With Respect to the ERA-40 Reanalysis and CPC Reconstructed Precipitation Data Over East Asia (100 E 150 E, 20 N 50 N) Period Pattern Correlation CNTL versus ERA-40 1958 2002 0.69 CNTL versus CPC 1950 2010 0.70 of precipitation in the southern China and the western subtropical Ocean, which is consistent with both the ERA-40 and the CPC precipitation (Figures 1b and 1c). On the other hand, the amount of JJA precipitation over Korean peninsula, Japan, and the middle China is smaller than the ERA-40 and the CPC precipitation. In spite of this discrepancy, Table 2 indicates that the CNTL reasonably simulates the overall structure of precipitation over East Asia Figure 2. Linear trends of June July August (JJA) averaged SST (K/60 years) prescribed in the (a) IO, (b) WP, and (c) EP experiments for 1950 2010. 1826

Figure 3. Analysis areas for Northeast Asia (110 E 145 E, 30 N 45 N; red box) and Southern China (100 E 120 E, 20 N 30 N; blue box). JJA precipitation anomalies (mm d 1 ) averaged over Northeast Asia from the (a) IO, (b) WP, and (c) EP experiments for the period of 1950 2010. (d f) Same as Figures 3a 3c but averaged over southern China. (100 E 150 E, 20 N 50 N) during JJA in comparison with the ERA-40 and the CPC. In spite of this, the performance of CNTL in simulating the summer rainfall over East Asia on interannual time scales is not skillful, that is, there is no statistical significant relationship in comparison with the ERA-40 and the CPC reconstructed precipitation (see Figure S1 in the auxiliary material). Note that the CNTL experiment is reasonably simulating the interannual variation of summer precipitation over the tropics (Figure S2). Therefore, we focus on the sensitivity of summer precipitation due to a long-term trend of tropical Pacific SST. [9] Figures 2a 2c show linear trends of summer SST prescribed in the IO, WP, and EP experiments, respectively, for the period of 1950 2010. Both the Indian and western tropical Pacific Oceans are dominated by warming; in particular, the warming in the central and southeastern Indian Ocean is prominent. Warming is also evident in the central and eastern tropical Pacific except in the narrow region along the equator in the eastern equatorial Pacific. Overall, however, SST variations of both the tropical Indian and Pacific Oceans during the last 60 years are characterized by a prominent warming trend. Therefore, comparison of the three experiments (i.e., IO, WP, and EP) 1827

Table 3. Correlation Coefficients Between the JJA Averaged SST Anomaly in Each Ocean Basin and the JJA Averaged Precipitation Anomalies Over Northeast Asia and Southern China for the IO, WP, and EP Experiments for the Period of 1950 2010 Experiment Ocean Basin Region for the SST in Each Experiment Correlation Coefficients Northeast Asia Southern China IO 30 E 120 E, 30 S 25 N 0.73* 0.60* WP 100 E 170 E, 20 S 20 N 0.49* 0.76* EP 170 E 80 W, 20 S 20 N 0.24 0.42* *Statistical significance at the 95% confidence level. is also able to discern how the warming of each Ocean basin differently influences the amount of summer precipitation over East Asia. It is noteworthy that the summer rainfall amount over Northeast Asia increases after the mid-1990s and slightly reduces after the mid-2000s; on the other hand, the summer rainfall anomaly shows a distinct change after the mid-1990s over southern China [Wu et al., 2010]. These results indicate that the two regions may have different relationships with the tropics. Therefore, to more accurately examine the details of SST sensitivity on JJA precipitation over East Asia, we divide the region into two areas, namely, Northeast Asia (110 145 E, 30 45 N) and southern China (100 120 E, 20 30 N). The Northeast Asia region is definedbythearea where the variance of summer rainfall anomaly over the Northeast Asia is dominant [Lee et al., 2005]. On the other hand, the southern China region is defined as following the definition of Kwon et al. [2007]andWu et al. [2010], where a significant summer rainfall increase is reported after the mid-1990s. [10] Figures 3a 3c display the summer precipitation variabilities over Northeast Asia simulated in the IO, WP, and EP experiments for the period of 1950 2010, respectively. In the IO experiment, the simulated summer precipitation shows a gradual, but dominant and linear trend of JJA precipitation increasing through time over Northeast Asia. It is also found that the variation of summer precipitation over Northeast Asia is highly positively correlated with the variation of Indian Ocean basin SST (Table 3). This result indicates that the summer precipitation over Northeast Asia responds linearly to the SST forcings in the tropical Indian Ocean. The result in Figure 3a indicates that a warming of Indian Ocean SST significantly contributes to the increase in the amount of JJA precipitation over Northeast Asia. In the WP and EP experiment (Figures 3b and 3c), on the other hand, the variation of simulated summer precipitation is more dominant on the low-frequency time scales than is the IO experiment. Furthermore, the variation of JJA precipitation over Northeast Asia is negatively correlated with the SST variation in both the western tropical Pacific and the central-to-eastern tropical Pacific (Table 3). A correlation analysis and a liner trend analysis (Figure 2) further indicate that, to some extent, the warming of western tropical Pacific SST contributes to the reduction in the amount of summer precipitation over Northeast Asia. In contrast, no clearly evident linear relationship exists between the variation of central-to-eastern tropical Pacific SST and the variation of JJA precipitation over Northeast Asia. [11] These results indicate that the SSTs of the Indian Ocean and the tropical Pacific basins have different impacts on the summer precipitation variability over Northeast Asia. The contrasting impact of SST warming of the Indian and western tropical Pacific Oceans is particularly striking. In addition, the summer precipitation over Northeast Asia linearly responds to SST variations in the Indian Ocean basin, which is in contrast to the response of summer precipitation to SST variation in the central-to-eastern tropical Pacific. The correlation analysis in Table 3 indicates that the linear relationship between Northeast Asian JJA precipitation and the tropical basin SSTs decreases when the location of the basin is shifted from the west to the east. [12] Figures 3d 3f are same as Figures 3a 3c except that JJA precipitation is simulated over southern China in the IO, WP, and EP experiments for the period of 1950 2010. In the IO experiment (Figure 3d), the characteristics of simulated summer precipitation over southern China are similar to those over Northeast Asia. That is, there is a significant increasing trend of simulated JJA precipitation over southern China, and the variation of summer precipitation over southern China is highly positively correlated with SST variation in the Indian Ocean basin (Table 3). Similar to Figure 3a, this result indicates that SST warming in the Indian Ocean basin largely contributes to the increase in the amount of summer precipitation over southern China. In the WP experiment (Figure 3e), on the other hand, the variation of simulated JJA precipitation is similar to that in the IO experiment except that it shows a decreasing trend. This variation is also different from that over Northeast Asia in the WP experiment in which the amount of summer precipitation fluctuates on low-frequency time scales (Figure 3b). As expected, the variation of JJA precipitation over southern China is highly negatively correlated with SST variation in the western tropical Pacific (Table 3). Comparing with the results in Figures 3a and 3b, the influence of SST warming in the Indian and western tropical Pacific Oceans on the amount of JJA precipitation is seen to be more prominent over southern China than over Northeast Asia. [13] In the EP experiment, the variation of simulated summer precipitation over southern China is dominant on low-frequency time scales, which is similar to the result over Northeast Asia (Figure 3c). However, the correlation Table 4. Same as Table 3 but for the Periods of 1950 1979 and 1980 2010 Correlation Coefficients Ocean Basin Region for the Northeast Asia Southern China Experiment SST in Each Experiment 1950 1979 1980 2010 1950 1979 1980 2010 IO 30 E 120 E, 30 S 25 N 0.53* 0.58* 0.57* 0.58* WP 100 E 170 E, 20 S 20 N 0.34 0.42* 0.49* 0.66* EP 170 E 80 W, 20 S 20 N 0.01 0.43* 0.28 0.56* *Statistical significance at the 95% confidence level. 1828

Figure 4. JJA averaged cross-sectional streamlines of differences in zonal and vertical velocities which are averaged for 0 N 20 N in the (a) IO and (b) WP experiments for the period of 1980 2010, respectively, along with the CLIM experiment. Shaded values are vertical velocity differences ( 10 3 Pa s 1 ) that are significant at the 95% confidence level. between the variation of central-to-eastern tropical Pacific SST and the variation of summer precipitation over southern China is greater than that over Northeast Asia (Table 3). This indicates that the JJA precipitation over southern China has a stronger relationship to the variation of central-to-eastern tropical Pacific SST compared to that over Northeast Asia. [14] According to previous studies [Li et al., 2010; Ding et al., 2010; Wu et al., 2012], the EASM exhibits an interdecadal enhanced relationship with both ENSO and tropical SST in the Indian Ocean. For example, Li et al. [2010] presented that the EASM exhibits an interdecadal enhanced relationship with both ENSO and tropical SST in the Indian Ocean, even though the Indian summer monsoon and ENSO relationship breaks down. Ding et al. [2010] also argued that the strengthened relationship between the tropical and northern IO and the EASM after the late 1970s is attributed to the strengthened ENSO-EASM relationship. It is also found that such relationship in the EASM precipitation variability simulated in the IO and WP experiments is found (Table 4). In the EP experiment, in particular, more dramatically enhanced relationship between the EASM rainfall and the ENSO is found. Results from the IO, WP, and EP experiments indicate that aforementioned changes of the relationship between the EASM precipitation and tropical SSTs can be reproduced when each tropical ocean SST is imposed independently. [15] Hereafter we examine why the Indian Ocean SST and the western tropical Pacific Ocean SST differently act on the variation of summer precipitation amount over East Asia based on the analysis of cross-sectional streamlines (Figure 4). Figures 4a and 4b show cross sections of differences of zonal and vertical velocities which are averaged for 0 N 20 N in the IO and WP experiments for the period of 1980 2010, respectively, as well as for the CLIM experiment. It is noted that the climatological (1950 2010) SST is prescribed in the CLIM experiment; therefore, the differences between the two experiments (i.e., IO or WP experiment) for the period of 1980 2010 and the CLIM experiment give an indication of the different influences that SST warming in the Indian and western tropical Pacific Oceans have on summer precipitation variability over East Asia, respectively. [16] Because of a modulation of the Walker circulation due to the warming of the Indian Ocean basin, strong upward and downward motions are evident in the Indian (40 E 80 E) and western tropical Pacific (100 E 140 E) Oceans, respectively, in the IO experiment (Figure 4a). In contrast, there exist strong updrafts over the western tropical 1829

Figure 5. Same as Figure 4 but for meridional and vertical velocities which are averaged for 110 E 130 E. Pacific (Figure 4b) where SST warming is significant in the WP experiment, as shown in Figure 2b. Therefore, the largescale motions over the western tropical Pacific in the IO and WP experiments are nearly opposite; thus, a large contrast exists between the two experiments in terms of the atmospheric circulation over East Asia due to the tropics-east Asia teleconnections (see Figures 5a and 5b). Figures 5a and 5b are same as Figures 4a and 4b except that the cross sections in the meridional direction are averaged for 110 E 130 E, including Northeast Asia and southern China. One can find that downward motions and low-level divergences over the western tropical Pacific (e.g., Figure 4a) lead to strong updrafts and northward humidity transports over 15 N 35 N (Figure 5a), where JJA precipitation is significantly increased in the IO experiment (see Figures 4a and 4d). In contrast, there exist clearly opposite circulations in terms of the zonal and vertical directions between the WP and IO experiments. That is, strong updrafts and convergences over the western tropical Pacific (Figure4b) induce downdrafts over southern China and Northeast Asia (Figure 5b) that, in turn, lead to reduced summer precipitation over East Asia. [17] These results indicate that the contrast of large-scale motion over the western tropical Pacific, which is due to the warming of SSTs in the Indian and western tropical Pacific Oceans, is an important contributor to the variation of summer precipitation amount over East Asia. 4. Summary and Discussion [18] In order to examine how each Ocean basin in the tropics affects the variation of summer precipitation amount over southern China and Northeast Asia, we conducted uncoupled AGCM experiments with four sets of historical SSTs prescribed in the global Ocean (the CNTL experiment), the Indian Ocean (the IO experiment), the western tropical Pacific (the WP experiment), and the eastern tropical Pacific (the EP experiment), respectively, for the period of 1950 2010. In addition, the CLIM experiment, in which the climatological (1950 2010) SST is prescribed in the global ocean, is also conducted for the simulation period of 90 years. [19] It is found that the warming of the Indian Ocean basin contributes to the increase in the amount of summer precipitation over southern China and Northeast Asia. In contrast, the warming of the western tropical Pacific tends to decrease the summer precipitation amount in the same region. Specifically, JJA precipitation over southern China linearly decreases with the warming of the western tropical Pacific, whereas the amount of JJA precipitation over Northeast Asia 1830

changes on the low-frequency time scales under the same conditions. On the other hand, there is a slight linear relationship between the variation of the central-to-eastern tropical Pacific SST and the variation of summer precipitation amount over Northeast Asia, but such a relationship does exist between the SST and JJA precipitation variations over southern China. Overall, it is found that the linear relationships between Northeast Asian summer precipitation and tropical basin SSTs diminish when the location of the Ocean basin is shifted from west to east. Furthermore, the variation of JJA precipitation amount over southern China is more sensitive to the SST variation in the tropics than that over Northeast Asia. [20] The differences in the amount of summer precipitation over East Asia, which arise due to SST warming in the Indian and western tropical Pacific Oceans, principally result from differences in the large-scale atmospheric circulation over the western tropical Pacific. Variations in the large-scale circulation result in contrasting atmospheric motions over East Asia through the tropics-east Asia teleconnections. Therefore, we argue that gradual warming in the Indian Ocean and the western tropical Pacific, which might be associated with global warming [Wang and Mehta, 2008; Cravatte et al., 2009], can cause weakening of summer precipitation variability in association with the EASM. This provides a possible explanation of recent weakening of the Northeast Asian summer monsoon variability experienced under global warming [Zhu et al., 2012]. On the other hand, the observational analysis shows that the summer mean precipitation over East Asia does not exhibit notable trends in the last six decades [Li et al., 2010]. This study is based on idealized uncoupled AGCM experiments forced by SST. 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