ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES, 45, 4, 2009, p. 483-498

Eddy-Mean Flow Interaction and Its Association with Bonin High: Comparison of July and August

Sun-Seon Lee and Kyung-Ja Ha Division of Earth Environmental System, Pusan National University, Busan, Korea (Manuscript received 18 June 2008; in final form 7 September 2009)

Abstract

The intraseasonal time dependency of eddy-mean flow interaction is examined during boreal summer using the inter- annual variability of Bonin high index (BHI) for 44 years (1958-2001). The perturbation kinetic energy (PKE), localized Eliassen-Palm (E-P) fluxes and barotropic energy conversion (BEC) are analyzed with a comparison in July and August. The results in strong July (SJ) years (> 1.0σ in normalized BHI) show that the 200 hPa PKE, particularly PKE of high frequency (HF; 2-9 day) eddy, is enhanced along the westerly jet, while eddy activities are reduced over the western North Pacific. This implies that the strong Bonin high is related to the weak eddy activity over that region. The meridional shear of strong westerly jet associated with the BEC from eddy to basic flow makes anticyclonic circulation over the south of Japan. In terms of the localized E-P flux, eddies tend to accelerate and decelerate the westerly flow over the north and south of the East Asian jet, respectively. Consequently, the barotropically unstable effect by the meridional shear of time-mean zonal flow and the deceleration of mean westerly flow by actions of transients are responsible for the anti- cyclonic circulation over the south of Japan in SJ years. The contributions of low frequency (LF; 10-60 day) eddy to the effects of BEC and localized E-P flux are larger than HF eddy. However, the characteristics of PKE, BEC and localized E-P flux appeared in SJ years are not found in strong August (SA) years, indicating that the development of Bonin high during August is not closely related to the local eddy-mean flow interaction. This interaction during July is more effective in developing the Bonin high than during August.

Key words: Eddy-mean flow interaction, perturbation kinetic energy, barotropic energy conversion, eliassen-Palm (E-P) fluxes, bonin high

1. Introduction Kawamura and Murakami, 1995; Limpasuvan and Hartmann, 1999; Fukutomi and Yasunari, 2002), A full interaction among the stationary wave, E-vector (e.g., Hoskins et al., 1983; Matthews and zonal mean flow, low frequency variability, and Kiladis, 1999; Hu et al., 2008), and barotropic energy transient eddy is required in the dynamics of the conversion (BEC) (e.g., Hartmann and Maloney, tropical-midlatitude teleconnection (Pan and Li, 2001; Moon and Ha, 2003). Hoskins et al. (1983) 2008). They reported that synoptic eddy forcing has suggested that much of the low frequency variabilities a positive feedback to the midlatitude low- frequency of atmospheric circulation in the Northern Hemisphere flow based on the results from both barotropic and are closely connected with the disturbances which baroclinic model. Some previous studies have derive the energy obtained from the barotropically investigated the eddy-mean flow interaction or eddy unstable time-mean flow. Moon and Ha (2003) activity through the various approaches: the divergence indicated that the eddy-mean flow interaction over of Eliassen-Palm (E-P) flux (e.g., Trenberth 1991; the extratropics, in turn, influences the time-mean flow by changing westerly flow. On the other hand, the East Asian summer monsoon Corresponding Author: Kyung-Ja Ha, Department of (EASM) has a complex space-time structure of Atmospheric Environmental Sciences, Pusan National baroclinic and barotropic systems. How does the University, Busan 609-735, Korea. Phone : +82-51-510-2177, Fax : +82-51-515-1689 eddy-mean flow interaction influence the atmospheric E-mail: [email protected] circulation related to the EASM variability? Kawamura 484 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES and Murakami (1995) investigated the influences of through the perturbation kinetic energy (PKE), summer mean monsoon flow on the 30-60 day BEC and localized E-P flux using the high- perturbations through the barotropic and baroclinic frequency (HF; 2-9 day) and low- frequency interactions. They found that the 30-60day waves tend (LF; 10-60 day) eddies. Section 5 summarizes our to reinforce the low-level convergence of the mean results. monsoon flow. In addition, Fukutomi and Yasunari (2002) showed that the forcing of transient eddies on 2. Data the time-mean flow for June-July is different from that for August. The results proposed in this study are based on the A fundamental question in this study is whether the European Centre for Medium-Range Weather Forecasts interaction between eddy and large scale time-mean (ECMWF) 40-yr reanalysis (ERA-40) data which flow is related to the EASM circulation, especially covers the period from mid-1957 to mid-2002 with in the variation of Bonin high. Understanding of the a spatial resolution of 2.5° longitude by 2.5° (Uppala eddy-mean flow interaction and its effect on climate et al., 2005). The upper and low-level winds, geopotential is necessary to study the characteristics of circulations height and outgoing longwave radiation (OLR) for associated with the interannual variability of the the period 1958-2001 (44 years) are analyzed. In EASM. In this study, the features of wave activity and addition, the National Oceanic and Atmospheric eddy-mean flow interaction are investigated by the Administration (NOAA) interpolated OLR data is comparison of July and August. applied. For rainfall, the Climate Research Unit This paper is organized as follows: The data (CRU) TS 2.1 Global Climate Dataset is used. This used are presented in section 2. We then describe data is comprised of 1224 monthly time series the characteristics of July and August climate (1901-2002) of climate variables, covering the global such as circulation and rainfall related to the land surface and excluding Antarctica, at 0.5 degrees variation of the Bonin high in section 3. In section resolution (Mitchell and Jones, 2005). 4, the eddy-mean flow interaction is interpreted

Fig. 1. The interannual variability of normalized (a) July BHI and (c) August BHI. Difference of 500 hPa geopotential height (b) between SJ and WJ years and (d) between SA and WA years. The shadings indicate values significant at the 95% confidence level. The rectangular box in (b) indicates the area of BHI. 30 November 2009 Sun-Seon Lee and Kyung-Ja Ha 485

Fig. 2. First two leading eigenvectors and their associated time series obtained from the EOF analysis of the 500 hPa geopotential height during (a)(b) July and (c)(d) August for 44 years (1958-2001).

3. Comparison of July and August climate particular, Ha and Lee (2007) suggested that the interannual variability of Bonin high is significantly a. Interannual variability of the Bonin high associated with the intensity of summer rainfall over the EASM region. Therefore, in this study, we focused The development and variation of highs such as the characteristics of local eddy-mean flow interaction the Okhotsk, Bonin, and north Pacific are closely related to the variation of Bonin high during boreal related to the EASM. In relation to the interannual summer. variations of summer climate over the EASM region, First of all, the Bonin high index (BHI) is defined several dominant patterns of large-scale atmospheric by the 500 hPa geopotential height averaged over the circulation fields have been examined. Nakamura region [140°-145°E, 25°-30°N] during July and and Fukamachi (2004) investigated the development August, following Ha and Lee (2007). Figure 1 of the Okhotsk high related to cool summer over shows the year-to-year variability of normalized BHI eastern Japan and Wakabayashi and Kawamura (2004) for 44 years. To examine the characteristics of circulations extracted teleconnection patterns linked with the related to the Bonin high intensity, the composite variations of the Okhotsk high that are possibly analysis is carried out by the strong versus weak associated with the anomalous summer climate. In Bonin high years. The strong July (hereafter ‘SJ’) and 486 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES weak July (hereafter ‘WJ’) years are defined when related to Bonin high during July and August have normalized July BHI are above 1.0 standard deviation a fundamental similarity: strong westerly jet at upper and below -1.0 standard deviation, respectively. The level over the EASM region and significant anticyclonic strong August (hereafter ‘SA’) and weak August circulation at low-level over the western North (hereafter ‘WA’) years are defined in the same manner. Pacific (WNP). Six years are selected for each case. It is found that In spite of these similarities, some distinct characteristics the Bonin high has negative relationship with the of circulation patterns are seen in two months. For Okhotsk high during July and August (Figs. 1b and upper level circulation, the significant westerly wind d). In reasonable agreement with Ha et al. (2009), over the EASM region is seen from 100°E to the date July exhibits a meridional teleconnection of Pacific- line during July, whereas that is seen from the east Japan pattern (Nitta, 1987). of Korea to 170°E during August. Contrary to the Furthermore, the principal modes of 500 hPa EASM region, subtropical westerly wind at 200 hPa geopotential height during the period 1958-2001 are during August is more closely related to the Bonin obtained by the empirical orthogonal function (EOF) high than during July. And, the significant easterly and analysis. Figure 2 describes the first two leading northerly at 850 hPa are more dominant over the eigenvectors and their corresponding principal components WNP during August than that during July. of the 500 hPa geopotential height during July and August for 44 years over the region, [20°-35°N, c. Convection and rainfall 122.5°-195°E]. Overall, the dominant patterns during July are similar to those during August. The We look into the characteristics of convection and first eigenvector (EOF 1) for July indicates that the rainfall in strong and weak years to compare the variations of Bonin high and north Pacific high are relationship between variations of EASM and Bonin closely linked to each other. For August, the EOF 1 high. The SJ and SA (WJ and WA) years are also presents the similar relationship between Bonin characterized by suppressed (enhanced) convection and north Pacific high during July in spite of over the WNP including the BHI region (Fig. 4), northeastward shift of the north Pacific high center. implying the development of Bonin high. The difference Particularly, the BHI is well represented by the first of OLR anomaly between SJ and WJ years is more principal components (PC 1) for July and August. apparent around Korea as well as over the WNP The correlation coefficient between PC 1 and the BHI compared to that between SA and WA years. This is during July (August) is 0.68 (0.82), which is significant coincident with the results of Lu (2004), which a at the 95% confidence level. stronger East Asian westerly jet stream in upper On the other hand, the second eigenvector (EOF troposphere corresponds to suppressed convection 2) of both July and August indicates that the Bonin in the WNP in July. In order to show the validity using high is modulated independently from the north the OLR in the reanalysis product, we also examined Pacific high in some part of the variance. For both the anomalous pattern of OLR measured from NOAA July and August, the variances of the first two leading polar-orbiting satellites. The anomalous pattern of modes account for about 60% of the total variance. NOAA interpolated OLR is very similar to that from ERA-40 although the magnitude of anomaly is not b. Upper and low-level circulations exactly same (not shown). In conjunction with the anomalous OLR patterns, In relation to the variability of Bonin high, the spatial distributions of rainfalls in July and August characteristics of circulation at upper and low-levels are investigated. Since the CRU rainfall data is only are investigated. Figure 3 exhibits the correlation available over land, rainfall anomalies over the coefficient between the circulation and BHI during continental EASM region are shown in Fig. 5. During July and August. At first glance, the circulation July, Korea as well as around Yangtze River in China 30 November 2009 Sun-Seon Lee and Kyung-Ja Ha 487

Fig. 3. Correlation coefficients (a)(d) between 200 hPa zonal wind and BHI, (b)(e) between 850 hPa zonal wind and BHI, and (c)(f) between 850 hPa meridional wind and BHI during July (left panels) and August (right panels). The shadings represent the values significant at 95% confidence level. have abundant rainfall when the Bonin high is strong. transients with time scales in the 10-60 day and in the Particularly, significant positive rainfall anomaly is 2-9 day, respectively. Several previous studies indicated seen along the western edge of anticyclone (Fig. 5c). the different effects of LF and HF eddies on the state However, August rainfall over China is not associated of atmospheric circulation through the interaction with the variation of Bonin high although rainfall with time-mean flow (e.g., Hoskins et al., 1983; Mak difference over Korea is still significant. and Cai 1989; Cai et al., 2007).

4. Eddy-mean flow interaction a. Perturbation kinetic energy

In this section, the eddy-mean flow interaction is The PKE is considered as the intensity measure of investigated in terms of PKE, localized E-P fluxes atmospheric eddy evolution (Khokhlov et al., 2004). and BEC. In order to compare the eddy behaviors of Using the PKE, the overall wave activities during different time scales, we apply the band-pass filter July and August related to the variability of Bonin analysis. The LF and HF eddies are defined as the high are compared. The PKE is computed as follow. 488 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES

Fig. 4. Anomalies of OLR in (a) SJ, (b) WJ, (d) SA, and (e) WA years, the difference of OLR anomaly (c) between SJ and WJ, and (f) between SA and WA years. Shaded areas in (c) and (f) represent the values significant at the 95% confidence level.

1 and WA years. Overall, the maximum amplitudes of PKE = (u'2 + v'2 ) (1) 2 PKE of LF eddy are found further downstream of time-mean jet, indicating an enhancement of wave Where, u and v are the time-mean zonal and activity over those regions. For PKE of HF eddy, meridional component winds in each year and u´ and large values are seen in the exit region of the strong v´ are transient parts (either LF or HF eddies), westerly jet streak. This is consistent with the results respectively. In this study, time-mean indicates the presented in previous studies (e.g., Blackmon et al., monthly mean. 1977; Cai et al., 2007). The LF (HF) eddy behavior at 200 hPa in SJ, WJ, When the Bonin high is strongly developed during SA and WA years are shown in Fig. 6 (Fig. 7). The July, HF eddy activity is enhanced along westerly jet. East Asian westerly jet is superimposed upon the However, the PKE of both HF and LF eddies over the PKE. The maximum of westerly jet is seen over north Bonin high region become weakened in SJ years. For China and Korea in both SJ and SA years, whereas the eddy activity at 850 hPa, these characteristics are jet intensity over Korea is reduced evidently in WJ also seen, implying that strong Bonin high is related 30 November 2009 Sun-Seon Lee and Kyung-Ja Ha 489

Fig. 5. Same as Fig. 4 except for rainfall anomaly. to weak eddy activity over that region (not shown). the eddy-mean flow interaction was found from the It is found that wave activity during August is barotropic growth of eddy energy. Fukutomi and different from that during July. PKE of HF eddy is Yasunari (2002) indicated that the barotropic conversion very strong along the upper level jet in WA years, and of kinetic energy from eddies into the time- mean negative differences of HF eddy PKE are dominant flow is most notable in the East Asia and the and significant over the Sea of Okhotsk and around subtropical western Pacific during summer. Meanwhile, Korea and Japan. it is found that the Bonin high has equivalent barotropic structure (Enomoto et al., 2003; Ha and Lee, 2007). b. Barotropic energy conversion Therefore, the BEC at 200 hPa was investigated in order to find the characteristics and differences of As aforementioned, in several previous studies, eddy-mean flow interaction during two months in 490 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES

Fig. 6. LF eddy PKE (m2 s‐2)(shading) and zonal wind (contour) at 200 hPa in (a) SJ, (b) WJ, (d) SA, and (e) WA years, the difference of PKE (c) between SJ and WJ, and (f) between SA and WA years. Thick solid lines in (a), (b), (d) and (e) correspond to 20 and 30 m s-1. Shaded areas in (c) and (f) represent the values significant at the 95% confidence level. strong and weak years. Simmons et al. (1983) suggested ∂u ∂u terms: − u'v' and (v'2 −u'2 ) . Time evolution that the dominant processes by which the growing ∂y ∂x disturbances extract kinetic energy from the basic of PKE is computed according to Simmons et al. state can be described in terms of two conversion (1983).

∂PKE (u'2 −v'2 ) 1 ∂u u'v' ∂ u 1 ∂v = − [ − v tanφ] − [cosφ ( ) + ] ∂t a cosφ ∂λ a ∂φ cosφ cosφ ∂λ (2) (u'2 −v'2 ) 1 ∂u u'v' ∂ u = − ( ) − [cosφ ( )] a cosφ ∂λ a ∂φ cosφ 30 November 2009 Sun-Seon Lee and Kyung-Ja Ha 491

Fig. 7. Same as Fig. 6 except for HF.

Where, φ and λ denote latitude and longitude, BEC from LF eddy are described in Fig. 8 to compare respectively. An overbar (prime) indicates a time- the contribution of two terms on the right hand side mean (transient parts), and a is the radius of the earth. to energy conversion. The BECX (Figs. 8a and b) is When the eddy gets contribution from the basic flow, dominant over the entrance and exit regions in jet energy conversion is positive. The first term on the stream and much smaller than the BECY (Figs. 8d right-hand side (hereafter ‘BECX’) is associated and e). It is found that the energy conversion is more with the effect of zonal inhomogeneity of the time- dependent on the barotropically unstable effect by mean flow and second term on the right-hand side meridional shear of time-mean zonal flow than the (hereafter ‘BECY’) is related to the effect by meridional effect by zonal inhomogeneity of time-mean flow as shear of time-mean flow. indicated by Moon and Ha (2003) and Kosaka et al. To begin with, the BECX and BECY as well as total (2009). 492 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES

Fig. 8. Distributions of BECX (left panels), BECY (middle panels) and total BEC (right panels) from LF eddy in (a)(d)(g) SJ years, (b)(e)(h) WJ years and (c)(f)(i) its difference. The shading interval is 4×10‐4 m2 s‐3. The zonal wind at 200 hPa (contour) are superimposed and thick solid lines correspond to 20 and 30 m s‐1. The light and dark shadings in (c), (f), and (i) represent the values significant at the 90% and 95% confidence level, respectively.

In terms of total BEC, the positive and negative Figure 10 shows the total BEC from LF and HF energy conversions are seen to the north and south eddies during August. The locations of negative and of upper level jet in both SJ and WJ years, respectively positive centers for total BEC are different from July. (Figs. 8g and h). This implies that mean flows give In SA and WA years, the east (positive) - west energy to LF eddies over the north of jet and LF eddies (negative) pairs of BEC related to LF eddy are seen act as an energy source for the mean flow over the over Korea and Japan instead of the north (positive) south of jet. Moreover, negative energy conversion - south (negative) pairs along jet. In conjunction with is conspicuous in the vicinity of Korea and difference HF eddy, the maximum of negative energy conversion of BEC over that region is significant. Hence, in SJ moves eastward compared to July. The significant years, the favorable position for meridional shear of difference of BEC from LF and HF eddies with zonal flow related to the BEC from LF eddies to negative sign is located in the region along 40°N time-mean flow can make the anticyclonic circulation between 150°E and 170°E. In addition, notable negative over the south of Japan and it may be attributable to differences of BEC related to LF or HF eddies are not the development of Bonin high. For HF eddy, the found around the Bonin high region. As a result, the positive energy conversion is not dominant compared BEC during July is more effective in maintaining the to LF eddy (Fig. 9). However, the significant negative Bonin high than that during August. Concerning the energy conversion difference around Japan and the strengthening/weakening of the Bonin high, Kosaka WNP indicates that HF eddy can contribute the et al. (2009) examined the energetics of wave-like development of Bonin high. teleconnection pattern. They showed the strong BEC 30 November 2009 Sun-Seon Lee and Kyung-Ja Ha 493

zonal flow. The barotropic zonal component of the localized E-P flux vector is

1 E = [ (v'2 − u'2 ),−u'v']cosφ (3) u 2

The localized E-P flux and the time evolution of PKE share two common terms. If u'2 > v'2 , the eddies are zonally elongated. The other term, u' v' , indicates the eddy momentum transport. Moreover, with this

definition of barotropic zonal component of Eu, Trenberth (1991) showed that the effect of eddies on the zonal-mean flow is as follow.

∂u 1 = ∇ • E (4) ∂t cosφ u

When localized E-P flux is divergent, the transient disturbance is tending to accelerate the mean westerly flow.

Figure 11 shows the distribution of horizontal Eu vectors and its divergence (positive) at 200 hPa related to LF eddy. In SJ years, the remarkable

Fig. 9. Distributions of total BEC from HF in (a) SJ years, convergence of Eu is seen in the vicinity of Korea and (b) WJ years and (c) its difference. The shading interval Japan, indicating the deceleration of the mean westerly is 4×10‐4 m2 s‐3. The zonal wind at 200 hPa (contour) are superimposed and thick solid lines correspond to 20 and flows by the actions of LF transients. Furthermore, 30 ms‐1. The light and dark shadings in (c) represent the the large divergence related to the acceleration of values significant at the 90% and 95% confidence level, time-mean flow is found over the north of Korea in respectively. SJ years. These opposite effects can make the favorable condition for the strong meridional shear over northern flank of the western core of the Asian of zonal mean flow. This is consistent with the result jet (40-60°E). The BEC between time-mean flow and obtained from the BEC from LF eddy.

LF eddy in SJ and SA years is similar to the results And, Eu vectors point southward and southwestward by Kosaka et al. (2009). from around Japan. Since Eu vectors are parallel to the group velocity of eddies relative to the mean flow c. Localized E-P flux (Trenberth, 1986), these vectors represent southward and southwestward eddy energy propagation in the The E-P flux has been widely used as a tool for direction of weakening the monsoonal westerly. In diagnosis of the atmospheric zonal mean states addition to the effect by the meridional shear of (Tanaka et al., 2004). Trenberth (1986) defined the time-mean zonal flow related to the BEC as shown localized E-P fluxes by three-dimensional vector Eu in Fig. 8, the strong convergence of localized E-P flux and suggested the use of a localized E-P flux as a and trapped westerly by the actions of LF transients diagnostic of the impact of transient eddies on the may contribute to the development of the Bonin high 494 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES

Fig. 10. Distributions of total BEC from LF (left panels) and HF (right panels) eddies in (a)(d) SA years, (b)(e) WA years, and (c)(f) its difference. The shading interval is 4×10-4 m2 s‐3. The zonal wind at 200 hPa (contour) are superimposed and thick solid lines correspond to 20 and 30 ms‐1. The light and dark shadings in (c) and (f) represent the values significant at the 90% and 95% confidence level, respectively. in SJ years. In comparison to LF eddy, strong 5. Summary and discussion convergence of localized E-P flux associated with HF eddy is not dominant over the EASM region in In present study, the eddy-mean flow interaction SJ years (Fig. 12). However, difference is significant related to the variation and development of Bonin over the south of Japan, indicating that HF transients high during July is compared with that during August also contribute the development of Bonin high. for 44 years of 1958-2001 using ECMWF reanalysis During August, on the other hand, the noticeable data. Composite analysis is performed based on the convergence is not found over the EASM region in interannual variability of BHI. In order to compare both SA and WA years. Consequently, this suggests the behaviors of eddies of different time scales, LF that the variation and development of Bonin high and HF eddies are used. In relation to the eddy-mean during August is not closely related to the local flow interaction, PKE, localized E-P fluxes and its eddy-mean flow interaction compared to July. divergence, and BEC at 200 hPa are investigated. In SJ years, the eddy activity at 200 hPa, particularly 30 November 2009 Sun-Seon Lee and Kyung-Ja Ha 495

Fig. 11. The localized E-P flux (m2 s‐2) and its divergence (10‐6 m s‐2)(positive shadings) at 200 hPa related to LF eddy in (a) SJ, (b) WJ, (d) SA, and (e) WA years, the difference of divergence (c) between SJ and WJ, and (f) between SA and WA years. Shaded areas in (c) and (f) represent the values significant at the 90% confidence level.

PKE of HF eddy, is strong along the westerly jet flows) provide energy to mean flows (LF eddies) stream, whereas that over the Bonin high region over the south (north) of upper level jet stream. The became weakened compared to WJ years. The 850 meridional shear of the jet occurred over Korea and hPa PKE is also weak over the WNP in SJ years. This Japan by the BEC effect in SJ years seems to make implies that the strong Bonin high is related to the the anticyclonic circulation. Furthermore, the large weak eddy activity over the WNP in both upper and convergence and divergence of localized E-P flux are low-levels. In terms of the BEC, LF eddies (mean seen over Korea and Japan in SJ years. These opposite 496 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES

Fig. 12. Same as Fig. 11 except for HF. effects can contribute to the strong meridional shear August is not significantly related to the eddy-mean of jet and be responsible for the anticyclonic circulation flow interaction compared to those during July. over south of Japan. In particular, contributions of LF Results presented here show the local eddy-mean eddy to the effects of BEC and localized E-P flux are flow interaction during July is more effective in larger than HF eddy. developing the Bonin high than during August. In SA years, however, the characteristics such as According to Enomoto et al. (2003), an equivalent beneficial negative BEC and eddy forcing by convergence barotropic ridge over Japan is formed by the propagation of localized E-P flux are not found, indicating that the of stationary Rossby waves along the Asian jet and variability and development of Bonin high during their accumulation in the jet exit region near Japan. 30 November 2009 Sun-Seon Lee and Kyung-Ja Ha 497

This is the Silk Road pattern. Kosaka et al. (2009) Meteor. Soc., 129, 157-178. indicated that the Silk Road pattern can be intensified _____, H. Endo, Y. Harada, and W. Ohfuchi, 2009: and maintained through its interaction with the mean Relationship between high-impact weather events in Japan and propagation of Rossby waves along the Asian Asian jet. In addition, Enomoto (2004) found that the jet in July 2004. J. Meteor. Soc. Japan, 87, 139-156. interannual variability of the Bonin high is regulated Fukutomi, Y., and T. Yasunari, 2002: Tropical-extra- by the intensity of Asian jet. This means that eddy- tropical interaction associated with the 10-25-day oscil- mean flow interaction related to wave train pattern lation over the Western Pacific during the Northern along the Asian jet is partly responsible for the summer. J. Meteor. Soc. Japan, 80, 311-331. variation of Bonin high. It needs further study to Ha, K. J., and S. S. Lee, 2007: On the interannual variability understand the linkage between the Silk Road pattern of the Bonin high associated with the East Asian summ- er monsoon rain. Climate Dyn., 28, 67-83. and eddy activity of different time scales. _____, S. K. Park, and K. Y. Kim, 2005: On interannual Meanwhile, summer climate over the EASM region characteristics of climate prediction center merged is influenced by disturbances of Meiyu/Changma /Baiu analysis precipitation over the Korean Peninsula during front. Partly, the Bonin high shows clear difference the summer monsoon season. Int. J. Climatol., 25, related to the Changma intensity (Ha et al., 2005) and 99-116. has two peaks at onset and retreat of strong Changma _____, K. S. Yun, J. G. Jhun, and J. Li, 2009: Circulation period (Ha and Lee, 2007). Enomoto et al. (2009) also changes associated with the interdecadal shift of Korean August rainfall around late 1960s. J. Geophys. showed the relationship between extreme weather Res., 114, D04115, doi:10.1029/2008JD011287. events in Baiu season over Japan and intensification Hartmann, D. L., and E. D. Maloney, 2001: The Madden– of Bonin high associated with propagation of Rossby Julian Oscillation, barotropic dynamics, and North waves. In relation to the activity of front, the transient Pacific tropical cyclone formation. Part II: stochastic eddy behaviors and its effects on the time-mean flow barotropic modeling. J. Atmos. Sci., 58, 2559-2570. may be changed. Further investigation on the linkage Hoskins, B. J., I. N. James, and G. H. White, 1983: The between atmospheric eddy behaviors and front shape, prepagation, and mean-flow interaction of large-scale weather systems. J. Atmos. Sci., 40, 1595- activity will be performed in the future work. 1612. Hu, Y., D. Yang, and J. Yang, 2008: Blocking systems over Acknowledgments. This work was supported by a an aqua planet. Geophys. Res. Lett., 35, L19818, grant from the Korean Ministry of Environment as doi:10.1029/2008GL035351. "Eco-technopia 21 project" and the second stage of Kawamura, R., and T. Murakami, 1995: Interaction be- the Brain Korea 21 Project. tween the mean summer monsoon flow and 45-day tran- sient perturbations. J. Meteor. Soc. Japan, 73, 1087- REFERENCES 1114. Khokhlov, V. N., A. V. Glushkov, and I. A. Tsenenko, Blackmon, M. L., J. M. Wallace, N.-C. Lau, and S. L. 2004: Atmospheric teleconnection patterns and eddy Mullen, 1977: An observational study of the northern kinetic energy content: wavelet analysis. Nonlin. Processes hemisphere wintertime circulation. J. Atmos. Sci. 34, Geophys., 11, 295-301. 1040–1053. Kosaka, Y., H. Nakamura, M. Watanabe, and M. Kimoto, Cai, M., S. Yang, H. M. Van den Dool, and V. E. Kousky, 2009: Analysis on the dynamics of a wave-like tele- 2007: Dynamical implications of the orientation of at- connection pattern along the summertime Asian jet mospheric eddies: a local energetics perspective. based on a reanalysis dataset and climate model Tellus, 59A, 127-140. simulations. J. Meteor. Soc. Japan, 87, 561-580. Enomoto, T., 2004: Interannual variability of the Bonin Limpasuvan, V., and D. L. Hartmann, 1999: Eddies and high associated with the propagation of Rossby waves the annular modes of climate variability. Geophys. Res. along the Asian jet. J. Meteor. Soc. Japan, 82, 1019- Lett., 26, 3133-3136. 1034. Lorenz, D. J., and D. L. Hartmann, 2003: Eddy-zonal flow _____, B. J. Hoskins, and Y. Matsuda, 2003: The formation feedback in the Northern Hemisphere winter. J. mechanism of the Bonin high in August. Quart. J. Roy. Climate, 16, 1212–1227. 498 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES

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