Response of the summer atmospheric circulation over East Asia to SST variability in the tropical Pacific

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 3: (1) Published online 17 April 9 in Wiley InterScience ( DOI: 1.1/joc.199 Response of the summer atmospheric circulation over East Asia to SST variability in the tropical Pacific Rena Nagata a * and Takehiko Mikami b a Graduate School of Humanities and Sciences, Ochanomizu University, -1-1 Ohtsuka, Bunkyo-ku, Tokyo , Japan b Faculty of Liberal Arts, Teikyo University, 359 Ohtsuka, Hachioji-City, Tokyo , Japan ABSTRACT: General circulation over East Asia and its linkages with sea surface temperature (SST) variability over the tropical Pacific is investigated for the 1958 period. The western edge of the North Pacific subtropical high (NPSH) index (SHI) is defined from 31 (May 31 to June 4) to 49 (August 9 to September ). A southwestward extension of the SHI has been observed since 198. The changes in the NPSH are associated with SST warming in the tropical eastern Pacific and Indian Ocean. On the basis of the SHI, years with western, eastern, southern and northern displacement of the NPSH are defined as WD, ED, SD and ND years. WD and SD years occur after 198. Climatologically, the subsidence is located around 3 N in the western Pacific. This subsidence area corresponds to the NPSH region. Before 4 in WD and SD years, associated with warm SST anomalies, circulation anomalies show an ascending motion over the tropical eastern Pacific and Indian Ocean. This ascending motion induces the anomalous subsidence over the tropical western Pacific and causes the southwestward extension of the NPSH. After 4 (July 15 19), the seasonal evolution of WD years is different from the SD years. After 4 in WD years, associated with large warm SST anomalies over the tropical eastern Pacific and Indian Ocean, the strong anomalous ascending motion strengthens the anomalous subsidence in the western tropical Pacific and leads to the lack of the eastward contraction of the NPSH. In SD years, warm SST anomalies over the tropical eastern Pacific and Indian Ocean weakened after 4. Correspondently, the weakened anomalous ascending motion over these regions provides the weak anomalous subsidence over the tropical western Pacific. The weakened anomalous subsidence leads to the eastward contraction of the NPSH after 4 similar to the climatological evolution. Copyright 9 Royal Meteorological Society KEY WORDS East Asia; subtropical high index; sea surface temperature; North Pacific Ocean; interannual/intraseasonal variability; circulation cell; Indian Ocean Received 14 November 7; Revised 1 March 9; Accepted 4 March 9 1. Introduction Since the summer monsoon rainband over East Asia is located on the northern side of the North Pacific subtropical high (NPSH), the location of the NPSH greatly influences the distribution of the summer monsoon rainfall over East Asia. Several studies have suggested that the northward shifts of the NPSH are closely associated with the onset and retreat of the East Asian summer monsoon (Huang and Sun, 199; Ueda et al., 1995). The meridional and zonal shifts of the NPSH have been studied. Nitta (1987) suggested that associated with the warmer than normal sea surface temperature (SST), convective activity in the western tropical Pacific becomes more intense and an atmospheric Rossby wave is generated, which propagates from the Tropics to the extratropics. As a result, an anticyclonic circulation cell appears over Japan, which leads to hot summers. This is known as the Pacific-Japan (PJ) pattern. Kurihara and Kawahara (1986) pointed out that the anomalous northward displacement * Correspondence to: Rena Nagata, Graduate School of Humanities and Sciences, Ochanomizu University, -1-1 Ohtsuka, Bunkyo-ku, Tokyo , Japan. nagata.rena@ocha.ac.jp of the subtropical high in the midsummer was one of the causes for hot and dry weather over East Asia in the summer of They also suggested that the displacement of the subtropical high is connected with the strong convective activities around the Philippines. Lu (1) and Lu and Dong (1) showed that the convective activities over the tropical western Pacific have a significant impact on the zonal shifts of NPSH. Gong and Ho () suggested that the southwestward extension of the NPSH since 198 is strongly associated with the variations of SST in the eastern tropical Pacific and the tropical Indian Ocean. Ho et al. (4) showed that the interdecadal changes in typhoon tracks are associated with the westward expansion of the subtropical northwestern Pacific high in the late 197s. The NPSH is greatly affected by general circulation variability over the North Pacific. The atmosphere over the North Pacific has the zonal Walker cell and the meridional Hadley and Ferrel cell. Bjerknes (1969) described a zonal circulation cell over the tropical Pacific, which he called the Walker circulation, since this circulation is an important part of the mechanism of Walker s Southern Oscillation (Walker, 193). The Walker circulation cell Copyright 9 Royal Meteorological Society

2 814 R. NAGATA AND T. MIKAMI has an ascending branch in the equatorial western Pacific and a subsidence branch in the equatorial eastern Pacific, which follows upper westerly and lower easterly winds. The Hadley cell is characterized as heated air rising in the tropical region and cool air sinking in the subtropical region. On the other hand, the Ferrel cell is a thermally indirect cell and is characterized as an ascending branch in the extratropics and a subsidence branch in the subtropical region (Oort and Rasmusson, 197; Trenberth et al., ). Most of the analyses and model results suggested that the interdecadal variability of the general circulation in the Northern Hemisphere is the result of the interdecadal variability of the SST over the Tropics (Kashiwabara, 1987; Nitta and Yamada, 1989; Trenberth, 199; Graham, 1994). Since the Walker and Hadley circulations are thermally driven, the SST variations over the tropical region affect these cells. Chou (1994) found an eastward shift of the convective centre from the maritime continents to the central equatorial Pacific and a strengthened Northern Hemisphere Hadley circulation during the April 1987 El Niño episode. Using singular value decomposition (SVD), Lau and Wu (1) showed that the dominant mode is depressed rainfall over the western Pacific and the Maritime Continent stemming from the eastward shift of the Walker circulation during the growth phase of El Niño. Wang () described the anomalous zonal and meridional cells over the Pacific associated with El Niño. The interannual and intraseasonal variations of the NPSH are important in the East Asian summer monsoon. The low-level southwesterly wind along the western periphery of the NPSH transports a large amount of water vapour into the rain zone over East Asia (e.g. Akiyama, 1973; Ninomiya, 1984; Ninomiya and Muraki, 1986). The zonal and meridional shifts of the NPSH also play an important role in the East Asian summer monsoon. An anomalous position of the NPSH can cause summer drought and flood (Huang and Sun, 199; Yasunari, 1997). Therefore, in this study, in order to investigate the interannual and intraseasonal variability of the NPSH, we define the western edge of the subtropical high (subtropical high index; SHI), which describes the zonal and meridional shift of the NPSH. The SHI is defined for each to examine the seasonal evolution of the NPSH. The seasonal evolution is significant in the monsoon system (Murakami and Matsumoto, 1994; Ueda et al., 1995), since anomalous seasonal evolution is responsible for an anomalous summer (Park and Schubert, 1997; Ding and Sun, 1). To reveal the seasonal variability of the NPSH, this study focused on the changes in the seasonal circulation cells over the North Pacific. Some studies have shown the effects of the seasonal change in the tropical SST on the seasonal evolution of circulation patterns over the North Pacific (e.g. Ueda et al., 1995; Ueda and Yasunari, 1996). For a better understanding of the relationship between the seasonal variability of the NPSH and the tropical SST variations, the anomalous years are identified using the SHI and are examined to determine the connection between the seasonal evolution of the circulation cells over East Asia and the seasonal variability of the tropical SST.. Data and method.1. Data The primary dataset used in this study is the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA data on a resolution of.5.5 (Uppala et al., 5). The National Oceanic and Atmospheric Administration (NOAA) Extended reconstructed SST (ERSST) data are used to examine the relationship between the interannual and intraseasonal variations of the subtropical high and the tropical SST (Smith and Reynolds, 3). This dataset has a horizontal resolution of. The following datasets were also used: Niño region SST (Niño 1 + : 1 S, 9 8W, Niño 3: 5 S 5 N, 15 9 W, Niño 4: 5 S 5 N, 16 E 15 W) from the NOAA Climate Prediction Center (CPC) ces/sstoi.indices, the area-averaged SST in the Niño west region ( 15 N,13 15 E) calculated using the ERSST data; and the Southern Oscillation Index (SOI) data from the Climatic Research Unit (CRU) of the University of East Anglia ( uk/cru/data/soi.htm; Ropelewski and Jones, 1987). All the datasets are available from 1958 to. Daily mean outgoing longwave radiation (OLR) data were obtained from the NOAA from June 1974 to (Liebmann and Smith, 1996). These data are in a gridded format of data on a.5.5 resolution. OLR data are unavailable during the period from 17 March to 31 December, Method Some studies have revealed the interannual and interdecadal variations of the NPSH by defining a subtropical high index. Lu (1) and Lu and Dong (1) define an NPSH index using the June to August mean geopotential height at 85-hPa averaged over the west edge of the NPSH and reveal the zonal shift and variability of the NPSH on the interannual time scale. In another study, using the 588 gpm isoline and the zero isotach of the 5-hPa zonal winds, Ding and Sun (1) illustrate the anomalous activities of the NPSH for He and Gong () defined the subtropical high index as the averaged 5-hPa height in the key region, which lies at the west edge of the mean position of the NPSH. Although the zonal and meridional positions of the NPSH greatly influence the East Asian summer temperature and rainfall (e.g. Kurihara and Kawahara, 1986; Huang and Sun, 199; Ding and Sun, 1), most studies focused on the averaged geopotential height over a certain area on examining the western edge of the NPSH variability. In this study, the SHI, which represents both the zonal and meridional variability of the western edge of the NPSH, is defined using the 5-hPa geopotential height data. The NPSH exhibits considerable interannual variability

3 RESPONSE OF EAST ASIAN SUMMER CIRCULATION TO TROPICAL PACIFIC SST 815 at its western extent. Thus, defining the western edge of the NPSH is critical to investigating the variation of the NPSH. This study demonstrates the interannual and intraseasonal variations in the NPSH. In order to examine the intraseasonal variations, the SHI is defined from 31 (May 31 to June 4) to 49 (August 9 to September ). Using the mean geopotential heights at 5-hPa from 5N and E, the SHI is defined. First, we select the grid points of the western edge of the subtropical high. If the following cases are fitted, grid point a is defined as the western edge of the subtropical high (Figure 1): (1) Grid point a is 588 gpm or above, () Grid points b, c and d are below 588 gpm, (3) Grid points e, f or g are 588 gpm or above, (4) Grid points h and h are below 588 gpm. Case 4 prevents from defining a projection of the subtropical high as the SHI. Although several grid points were tried to define the SHI, the grid points h and h are the most suitable case to define the SHI. We here selected the western edge of the subtropical high (grid point a) as the potential SHI grid point. Second, if there are several western edges of the subtropical high a in the region of 5N and E, SHI is selected as follows. (1) If there are some grid points a in Area 1, then the westernmost grid point a is selected as the SHI (Figures and 3). () If there is no grid point a in Area 1 and there are less than or equal to grid points that have height values 588 gpm or above, then the NPSH is considered to have retreated eastward, and the westernmost grid point a in Area 3 is selected as the SHI. (3) If there is no grid point a in Area 1 and there are more than or equal to 3 grid points that have height values 588 gpm or above, then the NPSH is considered to have extended westward and the easternmost grid point a in Area is selected as the SHI. (4) If there is no grid point a in Areas 1 and, 1E 15 E 13E 135 E 14E 145 E 15E 155 E 4N 4N h 35 N 35 N 3N 3N b e 5 N c a f 5 N d g N N 15 N 15 N h 1N 1N 1E 15 E 13E 135 E 14E 145 E 15E 155 E Figure 1. A schematic plot of the western edge of the subtropical high (1984 ). a is the western edge of the subtropical high; b: a+.5 latitude.5 ; c: a.5 ; d: a.5 latitude.5 ; e: a+.5 latitude +.5 ; f : a+.5 ; g: a.5 latitude +.5 ; h: a 1.5 latitude; h : a+ 1.5 latitude. 3E 6E Area 3W 9E 6W Area 1 1E (Area 4) Area 3 9W 15E 1W Figure. The areas of SHI. Area 1: 5 N, 1 15 E, Area : 5 N, 97.5 E, Area 3 (Area 4): 5 N, 15.5 E.5 W. and there are more than or equal to 3 grid points that have height values 588 gpm or above in Area 1, then the NPSH is considered to have extended more westward, and the easternmost grid point a in Area 4 is selected as the SHI. The SHI is a grid point of the western edge of the subtropical high. Thus, the SHI has pieces of information: the and the latitude. In this study, the (latitude) of SHI is regarded as the SHI zonal (meridional) component. Moreover, if the NPSH has extended westward and SHI is in Area 4, the SHI zonal component value is multiplied by 1. For example, the SHI is 5 W, N. If the NPSH has retreated eastward and SHI is in Area 3, the SHI meridional component is and the SHI zonal component is 5. If the NPSH has extended westward and SHI is in Area 4, the SHI meridional component is same as in Area 3 and the SHI zonal component is transformed into 5 1 = 5. The geopotential height 588 gpm contour line is widely used to investigate the variations of the NPSH (e.g. Zhang et al., 1999) and appears over the North Pacific in nearly all years and are always closed contours (Mikami, 1974). In reality, several different values and windows (instead of the.5 window) were considered. The current definition best describes both the interannual and the intraseasonal variation of the NPSH. 3. Interannual and intrseasonal variability of the summer subtropical high in East Asia 3.1. Interannual variability of the SHI The time series of SHI shows interannual, as well as interdecadal, variations both in the zonal and in the meridional component (Figure 4 and ). Figure 4 shows the interannual variations of the seasonally averaged SHI zonal component from 1958 to. The SHI zonal component exhibits weak variability during Although the SHI retreats eastward in the mid-198s, 18 15W

4 816 R. NAGATA AND T. MIKAMI Grid points a are in Area 1. Yes No (1) The westernmost grid point a in Area 1 is selected as the SHI. There are more than or equal to 3 grid points that have height values 588 gpm or above in Area 1. Yes No Grid points a are in Area. () The NPSH has retreated eastward. The westernmost grid point a in Area 3 is selected as the SHI. Yes No (3) The NPSH has extended westward. The easternmost grid point a in Area is selected as the SHI. (4) The NPSH has extended more westward. The easternmost grid point a in Area 4 is selected as the SHI. Figure 3. The schematic diagram of the definition of SHI. SHI zonal component(seasonal mean) SHI meridional component(seasonal mean) Degree Degree 4 - SHI zonal component(anomaly) Year (d) SHI meridonal component(anomaly) Year Figure 4. Time series of the seasonal ( 31 49) mean SHI zonal component, SHI meridional component, SHI zonal component anomalies and (d) SHI meridional component anomalies. The anomaly is the deviation from the long-term mean for The heavy dotted lines in and denote the linear slope for the entire period, and the dotted lines in and (d) denote the standard deviation for the entire period. a westward extension of SHI is observed since 198. Figure 4 shows the time series of the SHI meridional component from 1958 to. A striking feature is a large variability after 198. The SHI meridional component moves southward during 198, except for the mid-198s. These observations suggest that the SHI has shifted southwestward since 198. The westward (southward) trend of the SHI zonal (meridional) component is significant at a significance level of 5% using the Mann Kendall rank statistic (Kendall, 19). In order to reveal whether the NPSH has extended southwestward or just moved into a southwestern place, 588 gpm contours were compared for each period (Figure 5). It is interesting to note that there is a remarkable difference between the periods before and after 198. The 588 gpm contours have expanded after 198 (Figure 5 and (d)) compared with that before 198 (Figure 5 and ). Thus, the SHI has strengthened and extended southwestward since 198. The recent southwestward extension of NPSH is in agreement with Gong and Ho () and Hu (1997). The interannual variation of the SHI zonal and meridional anomalies is shown in Figure 4 and (d). The anomaly is a simple deviation from the long-term mean

5 RESPONSE OF EAST ASIAN SUMMER CIRCULATION TO TROPICAL PACIFIC SST 817 6N N N 5N 4N 4N 3N 3N N N 1N 1N 1E 1E 14E 16E 18 16W 1E 1E 14E 16E 18 16W (d) N 6N 5N 4N 3N N 1N 5N 4N 3N N 1N 1E 1E 14E 16E 18 16W 1E 1E 14E 16E 18 16W Figure 5. Contour lines at 588 gpm in summer ( mean) for , , and (d) for If the index anomaly exceeds one standard deviation, 9.75 (+9.75), the particular year was classified as a westward (eastward) displaced year: WD (ED) year. A similar criterion is applied to the SHI meridional component. If the index anomaly exceeds one standard deviation,. (+.), this year is classified as a southward (northward) displaced year: SD (ND) year. According to these criteria, WD years were 1983, 1995, 1997 and 1998; ED years were 197, 1974, 1984 and 1986; SD years were 1983, 1987, 1991, 1993, 1995, 1997 and 1998; and ND years were 1961, 1967, 1975, 1978, 1984, 1985, 1986 and As for WD and SD years, all of the years are after 198. The result clearly shows that the NPSH has extended southwestward after 198. Some scientists have pointed out that the discontinuity in the reanalysis datasets since 1979 is due to the introduction of the satellite data (e.g. Onogi et al., 1998). It is possible that the remarkable change in both the SHI zonal and meridional components after 198 is caused by the satellite data. However, some studies suggested that the temperature and precipitation show an obvious change after 198 (e.g. Gong and Ho, ). In the next section, the composite analysis is performed to show the intraseasonal variability of the SHI and the relationship between the tropical Pacific SST and SHI variability is explored. 3.. Intraseasonal variations of the SHI and the relationship between SHI and the tropical Pacific SST Figure 6 shows the climatological SHI tracks from 31 (May 31 to June 4) to 49 (August 9 to September ). Climatologically, the SHI locates around 4N 35 N 3N 5 N N E 15 E 13E 135 E 14E 145 E Figure 6. The plots of the SHI ( 31 49) for climatology. Lines indicate the tracks of the SHI from 31 to 49. Numbers denote numbers. The circles indicate the SHI from 31 to 39 and the stars denote the SHI from 4 to 49. the region from E and 3N. It moves northward from 31 (May 31 to June 4) and contracts eastward after 4 (July 15 19). Lu (1) showed that 4 is the time of the eastward contraction of the NPSH. Figure 7 shows the composite SHI tracks from 31 (May 31 to June 4) to 49 (August 9 to September ) for WD, ED, SD and ND years. It indicates that the SHI extends westward (contracts eastward) remarkably for all s in WD (ED) years (Figure 7 31

6 818 R. NAGATA AND T. MIKAMI 4N 3N 41 N 31 WD year 1N 6E 8E 1E 1E 14E 16E 18 16W 4N 41 3N 31 N ED year 1N 6E 8E 1E 1E 14E 16E 18 16W 4N 3N 41 N 31 SD year 1N 6E 8E 1E 1E 14E 16E 18 16W (d) 4N 3N N ND year 1N 6E 8E 1E 1E 14E 16E 18 16W Figure 7. The plots of the SHI ( 31 49) for WD years, ED years, SD years and (d) ND years. Lines indicate the tracks of the SHI from 31 to 49. Numbers denote numbers. The circles indicate the SHI from 31 to 39, and the stars denote the SHI from 4 to and ). The SHI is located west of 13E and shifts southward for WD years, whereas it is observed east of 1 E for ED years. It is interesting that WD years show no obvious eastward contraction after 4 (July 15 19), although it is evident in ED years. As for the SHI meridional components, the positions of SHI in SD years are similar to those in WD years. The southwestward extension of NPSH is observed for all s (Figure 7). However, the seasonal evolution of SD years is different from that in WD years, showing the eastward contraction after 4. ND years, by contrast, exhibits a northward and large meridional variability movement for all s (Figure 7(d)). To examine the relationship between the tropical Pacific SST and the SHI, correlations are computed among the Niño region SST and SOI, and the SHI zonal and meridional components (Table I). The second (third) row of Table I shows the correlation between Niño region SST and SOI for JJA (June August) mean and seasonal averaged (standard deviation) SHI zonal component. The fourth (fifth) row is the same as the second (third), except for the SHI meridional component. One of the interesting features of Table I is that the seasonal averaged SHI zonal component is strongly correlated with the eastern tropical Pacific SST, whereas the SHI meridional component is significantly correlated both with the eastern and central tropical Pacific SST. This suggests a strong coupling between the eastern (and central) tropical SST and the SHI zonal (meridional) component. Therefore, when the eastern (and central) tropical SST is high, the NPSH extends westward (southward). In addition, Table I shows that there is a strong connection between the standard deviation of the SHI zonal component and the eastern tropical Pacific, while the correlation between that of the SHI meridional component and the Niño SST is relatively weak. On the other hand, there is no significant relationship between the SHI and the western tropical Pacific SST. This result contradicts with Nitta (1987) and Kurihara and Kawahara (1986), but the relation between

7 RESPONSE OF EAST ASIAN SUMMER CIRCULATION TO TROPICAL PACIFIC SST 819 Table I. The correlation coefficients between the SHI zonal (meridional) component and the Niño region SST and SOI. NINO1 + NINO3 NINO4 NINO WEST SOI Zonal (avg) Zonal (σ ) Meridional (avg) Meridional (σ ) avg indicates seasonal ( 31 to 49) averaged SHI, and σ denotes the seasonal standard deviation of the SHI. The Niño region SST and SOI is a June August average. Boldface numbers indicate significance above 95%, and bold italic numbers denote significance above 99.9%. W/m W/m OLR Climatology ( 31-39) 4 OLR Climatology ( 49) 4N N S N N S E 9E 1E 15E 18 15W 1W 9W 6E 9E 1E 15E 18 15W 1W 9W w Climatology ( 31-39) (d) w Climatology ( 49) 4N 4N N N S S 6E 9E 1E 15E 18 15W 1W 9W 6E 9E 1E 15E 18 15W 1W 9W 1 m/s 1 m/s (e) 85w Climatology ( 31-39) (f) 85w Climatology ( 49) 4N 4N N N S S 6E 9E 1E 15E 18 15W 1W 9W 6E 9E 1E 15E 18 15W 1W 9 W 9 m/s 9 m/s Figure 8. Climatological (1958 ) circulation patterns in summer for OLR (upper:, ); -hpa wind (middle:, (d)) and 85-hPa wind (bottom: (e), (f)). The left panel is climatology from 31 to 39 and the right panel is that from 4 to 49. The contour interval is W/m, and the shaded areas denote values below 4 W/m for the OLR. the NPSH and the eastern tropical SST corresponds to Gong and Ho () and Angell (1981). 4. Atmospheric circulation changes over the North Pacific asoociated with the tropical SST variation inferred from the SHI 4.1. Composites of anomalous circulation cells In this section, the connection between the NPSH variations and the circulation changes over the North Pacific are considered. Figure 8 shows the climatological (1958 ) OLR, 85- and -hpa winds from 31 (May 31 to June 4) to 39 (July 1 14) and from 4 (July 15 19) to 49 (August 9 to September ) over the North Pacific. As can be seen in Figure 8, a conspicuous feature over the western Pacific is one Hadley cell, with the air rising (low OLR) in the warm pool region near the equator and the subsidence (high OLR) in the mid-latitude (near 3 N). This subsidence area corresponds to the NPSH region. In the central Pacific, one Hadley cell, with the subsidence near 5 N and the ascending motion near the equator and Ferrel cell, with the subsidence near 5 N andthe ascending motion near 3 N are observed (Figure 8).

8 8 R. NAGATA AND T. MIKAMI In the eastern Pacific, the subsidence is located over the equatorial eastern Pacific cold tongue (around ) and the subtropical high region (around 3 N), while the ascending motion is observed in the intertropical convergence zone (ITCZ), around 1 N. Therefore, there are two Hadley cells in the eastern Pacific. Over the Tropics, the easterly (westerly) winds are observed near the equator over the Pacific (Indian Ocean) in the lower-troposphere, and the westerly (easterly) winds that cover the area are from 15E to 14 W (west of 15 E, including the Indian Ocean) in the upper-troposphere (Figure 8 and (e)). Thus, two zonal circulation cells are observed over the Tropics. One is the Walker circulation cell, with the air rising in the western Pacific, flowing eastward in the upper-troposphere, the subsidence in the eastern Pacific, and retuning towards the west in the lower-troposphere. The other is the western Pacific Indian Ocean zonal circulation, with the ascending motion in the western Pacific, flowing westward in the upper-troposphere, the subsidence in the Indian Ocean, and retuning towards the east in the lower-troposphere. As for the climatology from 4 to 49, the features of the circulation cells are the same as from 31 to 39. However, all of the circulation cells move northward, and the cells over the western Pacific retreat eastward (Figure 8, (d) and (f)). This shift corresponds to SHI climatology (Figure 6). To examine the connection between the NPSH variations and the circulation changes over the North Pacific, the OLR and zonal wind differences between WD and ED years were computed (Figure 9). Although weak negative OLR anomalies are observed over the Tropics in s 35, and 39 (June 4, July 5 9 and July 1 14), the western Pacific (1 14 E) shows the positive OLR anomalies (subsidence) in the Tropics and the negative anomalies (ascending motion) in the midlatitude from 31 (May 31 to June 4) to 39 (July 1 14) as in Figure 9 and. The meridional cell in the tropical eastern Pacific (15 14 W) exhibits the anomalous ascending motion in the Tropics and the anomalous subsidence in the mid-latitude. The Indian Ocean (around 6 7E) has the anomalous ascending motion in the Tropics. During that time, the tropical lower-troposphere shows the westerly (easterly) wind anomalies over the tropical Pacific from 15 E to 1 W (the tropical Indian Ocean), while the uppertroposphere shows the easterly (westerly) wind anomalies (Figure 9 and (d)). Hence, the anomalous Walker cell (the western Pacific Indian Ocean zonal circulation) is characterized by the air rising in the eastern Pacific (Indian Ocean), flowing westward (eastward) in the upper-troposphere, the subsidence in the western Pacific, then returning towards the eastern Pacific (Indian Ocean) in lower-troposphere. Associated with these circulation cell anomalies, atmospheric subsidence is located in the western tropical Pacific and the southwestward extension of the NPSH is observed in WD years from 31 to 39 (Figure 7). After 4 (July 15 19), the anomalous ascending motion in the Tropics is strong particularly around 1 1 W and over the Indian Ocean, and the anomalous subsidence over the tropical western Pacific is strengthened compared with that before 4 (Figure 9). Correspondently, the lower-tropospheric westerly (easterly) wind anomalies and the uppertropospheric easterly (westerly) wind anomalies over the tropical Pacific (Indian Ocean) are developed as shown in Figure 9 and (d). The strong anomalous subsidence over the tropical western Pacific leads to the lack of the eastward contraction of the NPSH (Figure 7), which is different from the climatological evolution given in Figure 6. Associated with the strong anomalous subsidence over this region is the SHI that is observed west of 11 E after 4 in WD years (Figure 7). Figure 1 shows the tropical OLR differences between SD and ND years. Similar to the differences noted between WD and ED years (Figure 9), the western (eastern) Pacific shows the anomalous ascending motion in the mid-latitude (the Tropics) and the anomalous subsidence in the Tropics (mid-latitude) from 31 (May 31 to June 4) to 39 (July 1 14). During that time, the anomalous ascending motion in the tropical Indian Ocean is also observed. The tropical Pacific (Indian Ocean) shows the lower-tropospheric westerly (easterly) anomalies and the upper-tropospheric easterly (westerly) anomalies such as the WD year anomaly field (Figure 1 and (d)). These circulation cell anomalies induced the southwestward extension of the NPSH in SD years similar to WD years (Figure 7). After 4 (July 15 19), however, the anomalous ascending motion over the tropical eastern Pacific and Indian Ocean, and the anomalous subsidence over the tropical western Pacific weaken compared with that before 4 and for the WD ED year anomaly field (Figure 1). The anomalous ascending motion changes gradually into the anomalous subsidence from the central to the western tropical Pacific. Comparing the lower- and upper-tropospheric wind anomalies in the WD ED year anomaly field, the SD ND year wind anomalies are weak both over the tropical Pacific and Indian Ocean. The weakened anomalous subsidence over the tropical western Pacific leads to the eastward contraction of the NPSH after 4 in SD years, unlike in WD years (Figure 7 and ). These observations raise the question as to what causes the difference in the seasonal evolution of the NPSH in WD and SD years. The atmospheric circulation changes over the North Pacific are closely related to the SST variations over the Tropics (e.g. Nitta and Yamada, 1989; Wang, ). The interdecadal and intraseasonal variability of the NPSH is affected by the changes in the tropical SST (Ueda and Yasunari, 1996; Hu, 1997; Gong and Ho, ). In the next section, the connection between the seasonal variability of the tropical SST and the seasonal evolution of the circulation cells over the North Pacific is considered.

9 RESPONSE OF EAST ASIAN SUMMER CIRCULATION TO TROPICAL PACIFIC SST N OLR anomaly (WD year-ed year) N OLR anomaly (WD year-ed year) E 6E 9E 1E 15E 18 15W 1W 9W 1E 1E 14E 16E 18 16W 14W 1W 1W -1N 85uw anomaly (WD year-ed year) - - (d) -1N uw anomaly (WD year-ed year) E 6E 9E 1E 15E 18 15W 1W 9W E 6E 9E 1E 15E 18 15W 1W 9W Figure 9. Composite diagram of the time- section anomalies for WD years minus ED years from 31 to 49. averaged between 1 N and the equator for OLR, averaged between 5 and 3 N for OLR, averaged between 1 N and the equator for the 85-hPa zonal wind, and (d) averaged between 1 N and the equator for the -hpa zonal wind. The contour interval is 1 W/m in, ; 1 m/s in ; and m/s in (d). Dashed lines indicate negative anomalies. The shaded areas denote significance at the 95% level according to the t-test. 4.. Relationship between the atmospheric circulation cells and tropical SST Figure 11, and (e) show the composite monthly SST differences (from June to August) between WD and ED years. In June, the SST anomalies over the Pacific show an El Niño pattern with the high SST region over the tropical central and eastern Pacific (Figure 11). Associated with these positive SST anomalies is the anomalous Hadley circulation over the eastern Pacific, with the ascending motion in the Tropics and the subsidence in the mid-latitude from 31 (May 31 to June 4) to 39 (July 1 14) in Figure 9 and. The high SST anomalies are also observed over the tropical Indian Ocean. Correspondingly, the tropical Indian Ocean shows the anomalous ascending motion (Figure 9). The SST pattern in July and August is different from

10 8 R. NAGATA AND T. MIKAMI -1N OLR anomaly (SD year-nd year) 5-3N OLR anomaly (SD year-nd year) E 6E 9E 1E 15E 18 15W 1W 9W 1E 1E 14E 16E 18 16W 14W 1W 1W -1N 85uw anomaly (SD year-nd year) -1N uw anomaly (SD year-nd year) 1 (d) E 6E 9E 1E 15E 18 15W 1W 9W 3E 6E 9E 1E 15E 18 15W 1W 9W Figure 1. Same as in Figure 9, except for the anomalies for SD years minus ND years. that in June, with negative SST anomalies in the central tropical Pacific and positive anomalies over the eastern tropical Pacific, particularly from 1 W tooffthecoast of Peru and Ecuador (Figure 11 and (e)). The tropical Indian Ocean still remains with warm SST anomalies. The positive SST anomalies in August over this region are larger than that in June and July (Figure 11(e)). These positive anomalies correspond to the strong anomalous ascending motion over the tropical eastern Pacific and Indian Ocean after 4 (July 15 19) in Figure 9. For the SST differences between SD and ND years, the SST anomalies over the Pacific exhibits an El Niño pattern and the warm SST anomalies are dominant over the tropical Indian Ocean in June, similar to the WD year anomaly field (Figure 11). These positive SST anomalies correspond to the anomalous ascending motion in the tropical eastern Pacific and Indian Ocean from 31 to 39, similar to WD year anomaly (Figure 1). However, the warm SST anomalies over the eastern and central tropical Pacific are slightly weaker in July

11 RESPONSE OF EAST ASIAN SUMMER CIRCULATION TO TROPICAL PACIFIC SST 83 4N N S 4S SST anomaly for June (WD year-ed year).4 4N N S S 6E 9E 1E 15E 18 15W 1W 9W 6W SST anomaly for June (SD year-nd year) E 9E 1E 15E 18 15W 1W 9W 6W SST anomaly for July (WD year-ed year) 4N 4N N.4.6 N S.. -. S S 4S 6E 9E 1E 15E 18 15W 1W 9W 6W (d) SST anomaly for July (SD year-nd year) E 9E 1E 15E 18 15W 1W 9W 6W (e) 4N N S 4S SST anomaly for August (WD year-ed year) (f).4 4N N S S 6E 9E 1E 15E 18 15W 1W 9W 6W.4. SST anomaly for August (SD year-nd year) E 9E 1E 15E 18 15W 1W 9W 6W Figure 11. Composite differences of the SST in June (upper), July (middle) and August (bottom) between WD years and ED years (,, (e)), and SD years and ND years (, (d), (f)). The contour interval is. C. Dashed lines indicate negative anomalies. The shaded areas denote significance at the 95% level according to the t-test. and August compared with those in June (Figure 11, (d) and (f)). The tropical Indian Ocean also shows weak warm SST anomalies in July and August compared with those in June and for the WD year anomaly. Associated with these weakened SST anomalies is the weak anomalous ascending motion in the tropical eastern Pacific and Indian Ocean after 4 (July 15 19) in Figure 1. The SST patterns in WD and SD year anomalies coincide with the results in Section 3., in which the SHI meridional component has a significant relationship to SST over the central and eastern tropical Pacific, while the SHI zonal component is closely connected with only the eastern tropical Pacific SST (Table I). 5. Discussion and conclusions In this study, the impact of the tropical Pacific SST on the East Asian summer circulation for the period of 1958 was examined. To understand the interannual and intraseasonal variation of the NPSH, the western edge of the NPSH using a mean at the 5-hPa geopotential height data from 31 (May 31 to June 4) to 49 (August 9 to September ) was defined as the SHI. Although the SHI retreats northeastward in the mid-198s, southwestward extension of the SHI is observed since 198. The anomalous years are identified by looking at the positions of the SHI, and they are categorized as westward-, eastward-, southward- and northward-displaced years (WD, ED, SD and ND years). Climatologically, the SHI locates around the region from E to 3N. It moves northward from 31 and contracts eastward after 4 (July 15 19). During WD and SD years, the SHI extension is west of 13 E for all s. However, the seasonal evolution of WD years is different from the SD years. The eastward contraction of the NPSH after 4 (July 15 19) is not observed in WD years, while it is observed in SD years similar to the climatological evolution. The correlation coefficients between SHI and the tropical Pacific SST show that the SHI zonal component has a strong connection with only the eastern tropical SST, while the SHI meridional component is strongly related to both the eastern and central tropical Pacific SST. To identify the connection between the circulation changes over the North Pacific related to the NPSH variations and the tropical SST variability, the effects of the tropical SST on the zonal and meridional circulation anomalies over the North Pacific during WD and SD years were considered. The climatology and anomaly of the circulation cells associated with the tropical SST changes are summarized in Figures 1 and In the mean state (Figure 1) from 31 (May 31 to June 4) to 39 (July 1 14), the western Pacific shows one Hadley cell, with the air rising near the equator and the subsidence (corresponds to the NPSH region) in the mid-latitude. In the central Pacific, one Hadley cell and one Ferrel cell, with

12 84 R. NAGATA AND T. MIKAMI Climatology ( 31-39) Climatology ( 49) 8N 8N 6N 6N 4N N S 3E 6E 9E 1E 15E 18 15W 1W 4N N S 3E 6E 9E 1E 15E 18 15W 1W Figure 1. Schematic representation of the atmospheric circulation cells for the climatology in 31 to 39 and 4 to 49. All cells are moved northward from panel to. WD (SD) year anomaly ( 31-39) WD year anomaly ( 49) 8N 6N 4N N 8N 6N S 3E 6E 9E 1E 15E 18 15W 1W 4N N S 3E 6E 9E 1E 15E 18 15W 1W SD year anomaly ( 49) 8N 6N 4N N S 3E 6E 9E 1E 15E 18 15W 1W Figure 13. Schematic representation of the atmospheric circulation anomaly cells for WD and SD year anomaly ( 31 39), WD year anomaly ( 4 49) and SD year anomaly ( 4 49). sinking around 5 N and rising near the equator and around 3 N are observed. In the eastern Pacific, there are two Hadley cells, with the ascending motion around 1 N and the subsidence at the equator and around 3 N. Over the Tropics, there are two zonal circulations: the Walker circulation and the western Pacific Indian Ocean zonal circulation. After 4 (July 15 19), all of the circulation cells move northward, and the circulation cells over the western Pacific retreat eastward (Figure 1).. In WD year anomaly from 31 to 39, positive SST anomalies are observed over the tropical eastern Pacific and Indian Ocean. Corresponding to these SST anomalies are changes of the atmospheric circulation pattern. The eastern tropical Pacific has the anomalous Hadley cell, with the ascending in the Tropics and the subsidence in the mid-latitude (Figure 13). The Indian Ocean displays the anomalous ascending motion. In the Tropics, zonal circulations show the reversal of the mean state. These circulation anomalies induce the anomalous subsidence over the tropical western Pacific and the anomalous ascending motion is observed in the mid-latitude. Consequently, the NPSH has extended southwestward. After 4, corresponding to the large positive SST anomalies is the strong anomalous ascending motion over the tropical eastern Pacific (particularly 1 1 W) and Indian Ocean (Figure 13). These ascending motions strengthen the anomalous subsidence in the tropical western Pacific and lead to the lack of the eastward contraction of the NPSH in WD years (Figure 7). 3. Although the circulation cell anomalies in SD years are similar to that for the WD year anomaly from 31 to 39 (Figure 13), it is different after 4 (Figure 13). After 4, associated with the weak positive SST anomalies is the weakened anomalous ascending motion over the tropical eastern Pacific and Indian Ocean. Through the weakening of the anomalous ascending motion over these regions the anomalous subsidence over the western tropical Pacific is weaker compared with that before 4 and for the WD year anomaly field. As a result, the eastward contraction of the NPSH is observed in

13 RESPONSE OF EAST ASIAN SUMMER CIRCULATION TO TROPICAL PACIFIC SST 85 SD years (Figure 7). Therefore, it is reasonable to suggest that the seasonal variations of the circulation cells over East Asia related to the NPSH variability are affected by the seasonal evolution of warm SST anomalies in the Tropics. The southwestward extension of the NPSH is observed since 198. WD and SD years occur only after 198. These results suggest that, since 198, the general circulation over East Asia has changed remarkably. The SST over the eastern tropical Pacific and the tropical Indian Ocean has a significant influence on the variation in the circulation cells over the North Pacific. Nitta and Yamada (1989) suggested that, since the late 197s, SST warming is present over the eastern tropical Pacific and the tropical Indian Ocean. Gong and Ho () and He and Gong () have also demonstrated that extension of the NPSH after 198 is associated with the changes in the SST of the eastern tropical Pacific and tropical Indian Ocean. Thus, the main factor for the recent circulation changes over the North Pacific and the NPSH variability is the SST warming over these regions. Although some studies suggested that the influence of the thermal state of the western tropical Pacific on the variations of the NPSH (e.g. Kurihara and Kawahara, 1986; Nitta, 1987), there is no significant relationship between the NPSH and the SST over the western tropical Pacific in this study. It is speculated that the active convective anomalies over the tropical Indian Ocean associated with the recent SST warming suppress the convection over the western tropical Pacific. In Figures 9 and 1, associated with the positive SST anomalies (Figure 11) is the anomalous ascending motion in the tropical Indian Ocean. This anomalous ascending motion induced the strong anomalous subsidence in the tropical western Pacific. Nitta (199) suggested that the SST over the tropical Indian Ocean in 1988 summer is above normal and the centre of the convective region moved westward from the normal position over the tropical western Pacific towards the Bay of Bengal, and this westward shift of the convective centre may result in the lack of convective activity in the western tropical Pacific due to the compensating downward motion. This study focused on tropical forcing and its effect on the atmospheric circulation over East Asia. However, the atmospheric circulation over East Asia has also connections with the circulation pattern in the mid-latitude, for example, the Silk Road pattern (Enomoto et al., 3). In addition to tropical forcing, the effect of the mid-latitude forcing on the interannual and intraseasonal variability of the atmospheric circulation over East Asia should be studied in the future. 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