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Role of Subtropical Precipitation Anomalies in Maintaining the Summertime Meridional Teleconnection over the Western North Pacific and East Asia RIYU LU National Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, and Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
ZHONGDA LIN Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China (Manuscript received 28 January 2008, in final form 3 September 2008) ABSTRACT The meridional teleconnection patterns over the western North Pacific and East Asia (WNP–EA) during summer have a predominant role in affecting East Asian climate on the interannual time scale. A well-known seesaw pattern of tropical–subtropical precipitation is associated with the meridional teleconnection, and the subtropical precipitation anomaly has been previously viewed as a result of anomalous circulations associated with the teleconnection. In this study, however, the authors suggest that subtropical precipitation anomalies, in turn, can significantly affect large-scale circulations and may be crucial for maintenance of the meridional teleconnection. Diagnosis by using observational and reanalysis data indicates that the meridional teleconnection patterns are clearer in summers when the subtropical rainfall anomalies are greater. The simulated results by a linear baroclinic model indicate that a subtropical heat source, which is equivalent to the diagnosed positive subtropical precipitation anomaly, induces zonally elongated zonal wind anomalies that resemble the diagnosed ones in both the upper and lower troposphere over the extratropical WNP–EA. The simulated results also indicate that the horizontal and vertical structures of circulation responses are insensitive to the locations and shapes of imposed subtropical heat anomalies, which implies the important role of basic flow in circulation responses. This study suggests that, for confidential dynamical seasonal forecasting in East Asia, general circulation models should be required to capture the features of interannual subtropical rainfall variability and basic-state flows in WNP–EA.
1. Introduction The interannual variability of summer climate in the western North Pacific and East Asia (WNP–EA) is dominated by meridional teleconnection (e.g., Lau et al. 2000; Wang et al. 2001; Lu 2004). The meridional teleconnection, at the view of general circulation, is characterized by the zonally elongated anomalies that appear alternately in the meridional direction over this region, in both the lower and upper troposphere. The meridional teleconnection is frequently referred to as the Pacific–Japan (PJ) pattern (Nitta 1987) or East
Corresponding author address: Riyu Lu, Institute of Atmospheric Physics, Chinese Academy of Sciences, P.O. Box 9804, Beijing 100029, China. E-mail:
[email protected] DOI: 10.1175/2008JCLI2444.1 Ó 2009 American Meteorological Society
Asia–Pacific (EAP) pattern (Huang and Sun 1992). The anomalies associated with the meridional teleconnection are mainly located at the longitudinal range from 1108 to 1608E, and between the equator and high latitudes. The circulation anomalies associated with the meridional teleconnection affect the interannual variability of East Asian climate (e.g., Kurihara 1989; Huang 2004; Ogasawara and Kawamura 2007). On one hand, anomalous cumulus convection near the Philippines influences the anomalous East Asian summer climate through the PJ pattern (e.g., Nitta 1987; Kurihara 1989). The PJ pattern tends to be confined and propagates northward in the lower troposphere (Kawamura et al. 1996; Kosaka and Nakamura 2006). In addition, anomalous precipitation over the Philippine Sea results in an anticyclonic (cyclonic) anomaly in the lower troposphere
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to the northwest (Lu 2001; Lu and Dong 2001), thus affecting rainfall in East Asia through modifying water vapor flux into the region. On the other hand, the meridional displacement of the East Asian uppertropospheric westerly jet (EAJ), which is the leading mode of upper-tropospheric zonal wind anomalies over the WNP–EA (Lin and Lu 2005), affects rainfall along the climatological rain belt in East Asia (Liang and Wang 1998; Lau et al. 2000; Lu 2004). The anticyclonic/ cyclonic anomaly and EAJ meridional displacement can be viewed as the components of the meridional teleconnection (Lau et al. 2000; Wang et al. 2001). More features of the meridional teleconnection were recently presented. Kosaka and Nakamura (2006) noticed that the circulation anomalies associated with the meridional teleconnection are elongated zonally with a distinct northward tilt with height. Ogasawara and Kawamura (2007) argued that this zonally elongated structure is favored by the combination of the PJ pattern and west Asia–Japan pattern, which propagates eastward along the upper-tropospheric subtropical westerly jet (Enomoto et al. 2003). Hsu and Lin (2007), on the other hand, emphasized the asymmetry of the meridional teleconnection pattern between the positive and negative phases that are characterized by more and less rainfall in central-eastern China and South Korea and Japan, respectively, and suggested that the teleconnection pattern is more closely associated with circulation anomalies over the WNP in the positive phase but has a stronger connection over the Eurasian continent in the negative phase. These aforementioned recent results suggest a complicated nature of the meridional teleconnection, and the physical mechanisms responsible for the meridional teleconnection are far from being well known. The meridional teleconnection patterns have been previously considered as northward propagation of Rossby waves triggered by the anomalous convective activity over the tropical WNP (e.g., Kurihara and Tsuyuki 1987; Nitta 1987; Huang and Sun 1992). This mechanism, however, cannot provide a satisfactory explanation over the forcing locating south of the critical latitude for stationary Rossby waves in the uppertroposphere and barotropic–baroclinic coupling in which thermally driven baroclinic responses in the tropics generate extratropical barotropic disturbances. Recently, some other mechanisms were proposed. Enomoto et al. (2003) and Enomoto (2004) suggested that the Rossby waves propagating eastward along the Asian jet lead to the formation and variability of the Bonin high, which can be viewed as a local representation of the meridional teleconnection. Kosaka and Nakamura (2006) suggested that the teleconnection
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pattern in its horizontal and vertical structure can gain kinetic energy and available potential energy from the mean flow. In addition, Lu et al. (2006) indicated that this meridional teleconnection is mainly caused by internal atmospheric variability, suggesting that the interaction between the WNP summer monsoon (WNPSM) and the East Asian monsoon may have its own component independent of external forcings such as sea surface temperature (SST) anomalies. There is a well-known seesaw pattern of tropical– subtropical precipitations associated with the meridional teleconnection. That is, an enhanced precipitation in the tropical WNP corresponds to a suppressed precipitation in the subtropical WNP and East Asia, and vice versa. The subtropical precipitation anomaly has been implicitly assumed as a response to anomalous circulations, while the tropical precipitation anomaly viewed as one of the sources for extratropical circulation anomalies. In the current work, however, we will show that as a heat anomaly, the subtropical precipitation anomaly over the WNP–EA, which is induced by circulation anomalies, would in turn affect large-scale circulation. There were few studies on the possible roles of subtropical precipitation anomalies, while the roles of tropical precipitation anomalies were extensively investigated. By using an aqua-planet GCM, Kodama (1999) simulated the role of climatological subtropical heat source in maintaining the subtropical convergence zones. He suggested that subtropical diabatic heating can force significantly atmospheric circulations. A subtropical heating induces a lower-level cyclone along the west/poleward side of the heating, and an accompanying upper-level cyclone on the west/poleward side and an anticyclone on the east/equatorward side of the heating. In his study, the mean flow is zonally symmetric. However, the response of circulation to precipitation may be sensitive to the structure of mean flow, and this sensitivity may be particularly significant for the region of WNP–EA according to the results of Kosaka and Nakamura (2006), who argued that both the horizontal and vertical structures of the mean flow over this region play a crucial role in maintaining the meridional teleconnection pattern. Therefore, to investigate the possible role of subtropical East Asian precipitation anomaly in the meridional teleconnection, some other specific numerical experiments are necessary. A simple numerical model may be a helpful tool to study the role of subtropical precipitation anomaly in circulations under a certain mean flow background, particularly when we consider that the causality between precipitation and circulation is not easily diagnosable through observational diagnoses. Thus, a simple model is utilized in this study to investigate the role of
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subtropical precipitation anomaly in the meridional teleconnection over the WNP–EA. Since the interannual variability in WNPSM and EAJ is characterized by the lower- and upper-tropospheric circulation anomalies, respectively, and since the vertical structure of the mean flow may also be important for the meridional teleconnection pattern (Kosaka and Nakamura 2006), this study uses a baroclinic model to simulate the meridional teleconnection. In this study, we first investigate the structure of the meridional teleconnection and the relationship between the teleconnection and subtropical precipitation anomalies, by using observational and reanalysis data. Then, some specific numerical experiments are performed by using a baroclinic model to illustrate the role of precipitation anomalies in the meridional teleconnection. In section 2, we describe the data, indexes, and baroclinic model used in this study. In section 3, the meridional teleconnection and its association with the intensity of subtropical precipitation anomaly are diagnosed by using the reanalysis and satellite-gauge merged data. In section 4, the role of subtropical precipitation anomaly, as well as of tropical one, is simulated by using the baroclinic model. Section 5 is devoted to a summary.
2. Data, indexes, and model used In this study, the 40-yr European Centre for MediumRange Weather Forecasts (ECMWF) Re-Analysis (ERA40) data from 1958 to 2002 (Uppala et al. 2005) are used. Also used are the precipitation data from 1979 to 2002, derived by the Global Precipitation Climatology Project (GPCP; Huffman et al. 1997; Adler et al. 2003). We repeated all the analyses on precipitation anomalies in this study by using another dataset derived by the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP; Xie and Arkin 1997), and obtained similar results. Only the results from GPCP dataset are given in this paper, partly because this dataset is more reasonable for open oceans (Yin et al. 2004). To focus on interannual variations, the components represented by the Fourier harmonics with periods ranging from 2 to 8 yr are used for analysis throughout this study. For observational diagnosis, we focus on the relationship between the WNPSM and EAJ, which are the southernmost and northernmost dominant components related to the meridional teleconnection, respectively. Furthermore, these two components are located in the lower and upper troposphere, respectively, and thus the relationship between them includes vertical coupling, as well as meridional interaction. The WNPSM index (WNPSMI) and EAJ index (EAJI), defined by Wang
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et al. (2001) and Lu (2004), respectively, are adopted for diagnosis in this study. The WNPSM anomaly is characterized by zonally elongated anticyclonic (cyclonic) anomalies in the lower troposphere over the WNP (Wang et al. 2001). Accordingly, Wang et al. (2001) defined the WNPSMI as the difference of 850-hPa zonal winds between the averages over the two regions—58– 158N, 1008–1308E and 208–308N, 1108–1408E—to measure the interannual variability of the WNPSM. Thus, a positive (negative) WNPSMI indicates cyclonic (anticyclonic) anomaly over the tropical WNP. Lu (2004) measured the meridional displacement of the EAJ, which is the leading mode of upper-tropospheric zonal wind variability over the WNP–EA (Lin and Lu 2005), by the difference between the 200-hPa zonal winds averaged over 308–408N, 1208–1508E and 408–508N, 1208– 1508E. In this paper, this difference is called EAJI, and a positive (negative) EAJI indicates a southward (northward) displacement of the EAJ. We also define an index of subtropical precipitation anomaly, called subtropical precipitation index (STPI), by averaging precipitation anomalies over 308–408N, 1358–1558E, according to the results shown in the next section. The model used in this study consists of primitive equations linearized about the summer [June–August (JJA)] climatology obtained from the ERA-40 for 1961–90. The components of the heating patterns associated with the meridional teleconnection serve as prescribed forcings for the dry version of the linear model. This allows one to investigate the possible roles that the regional heat sources and sinks play in driving the large-scale circulations in the WNP–EA. Particularly, the dry version of the model can prevent the interaction between the tropical and subtropical precipitation anomalies which happens in reality, and thus it is more appropriate than the moist version in analyzing the individual responses of regional heat sources. The model adopts a horizontal resolution of T42 and vertical 20 levels using the sigma (s) coordinate system, and includes a horizontal (vertical) diffusion, Rayleigh friction, and Newtonian damping. The horizontal diffusion has a damping time scales of 6 h for the smallest wave, and the Rayleigh friction and Newtonian damping have a time scale of (0.5 day)21 for s $ 0.9 and 1 day21 for s # 0.03, while (20 day)21 between them. Details of the model formulation are given in Watanabe and Kimoto (2000, 2001). To obtain the linear atmospheric response to forcing, in this study we adopt a time integration method. The integration is continued up to 30 days, and the results at day 21 are only shown as the steady response to a prescribed diabatic heating forcing. The circulation response approaches the steady state approximately after day 15.
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3. Meridional teleconnection and its relationship to the intensity of subtropical precipitation anomaly In summer, a westerly jet stream with maximum zonal velocity of 20;30 m s21 emerges from the North Africa and extends eastward to the North Pacific at 200 hPa, and its axis is located nearly in the zonal direction with a slight northeastward orientation in the western and central Pacific (partly shown in Fig. 1a). The latitude of the axis is about 408N in East Asia. There are westerlies, also with a northeastward orientation, along the northern fringe of the subtropical anticyclone over the North Pacific in the lower troposphere (Fig. 1b). A vertical profile of zonal winds averaged over the western Pacific (Fig. 1c) indicates that 200 hPa is approximately a level of strongest westerly jets, and that the westerlies extend downward to the surface with a slight southward tilt. Figures 2a and 2b show the upper- and lower-tropospheric wind anomalies regressed onto the WNPSMI, respectively. Well-defined meridional teleconnection patterns, with each cell being elongated zonally, appear over the WNP–EA in both the upper and lower troposphere. In the extratropics, the north-positive–southnegative pattern over East Asia in the upper troposphere (Fig. 2a), relative to the position of the climatological EAJ core (Fig. 1a), indicates a poleward displacement of the EAJ. In the tropical WNP, there is a cyclonic anomaly in the lower troposphere (Fig. 2b). These anomalies associated with the meridional teleconnection are in agreement with previous studies (e.g., Lau et al. 2000; Lu 2004; Kosaka and Nakamura 2006). The zonal wind anomalies exhibit a roughly barotropic structure in the extratropics, but a poleward tilt with height is notable (Fig. 2c). This poleward phase tilt has been pointed out by Kosaka and Nakamura (2006), who suggested that the vertically sheared climatological meridional flow over the WNP could be an essential factor for the tilt. In the tropics, the anomalies exhibit a baroclinic structure. Meridional teleconnection patterns appear between the equator and high latitudes in both the upper troposphere and lower troposphere. However, they appear as four cells in the upper troposphere but as three cells in the lower troposphere. In the midtroposphere, the signal of teleconnection pattern tends to be weak in the tropics. This meridional teleconnection pattern suggests that there is not only a strong tropical–extratropical interaction, but also a notable vertical coupling in the WNP–EA. Figure 3 shows the precipitation anomalies regressed onto the WNPSMI and the EAJI, respectively. Both the WNPSMI and the EAJI are significantly associated with a triple pattern of precipitation anomalies in the WNP–
FIG. 1. Climatological JJA-mean upper- and lower-tropospheric horizontal winds and vertical profile of zonal winds. Horizontal winds at (a) 200 hPa and (b) 850 hPa. Zonal winds exceeding 10 m s21 in (a) and 3 m s21 in (b) are shaded. (c) Meridional cross section of zonal winds averaged over 1208–1508E. The thick dashed line indicates the location of maximum westerlies.
EA, with the opposite polarity because of the inverse relationship between the WNPSMI and the EAJI (Tables 1 and 2). In particular, there is a significant precipitation anomaly in the subtropical WNP and central and southern Japan. This subtropical precipitation anomaly is elongated almost zonally but with a slight tilt of southwest– northeast orientation. One major difference between the precipitation anomalies associated with the WNPSMI and EAJI appears over the eastern Indian Ocean and the Maritime Continent.
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FIG. 3. The JJA precipitation anomalies regressed onto (a) the standardized EAJI and (b) the WNPSMI. The EAJI is defined as the difference of zonal wind anomalies at 200 hPa averaged between 308–408N, 1208–1508E and 408–508N, 1208–1508E. The contour interval is 0.5 mm day21, and the contour line of zero is omitted. Shading indicates the regions of 95% significance level.
FIG. 2. The JJA-mean wind anomalies at (a) 200 hPa and (b) 850 hPa regressed onto the standardized WNPSMI. Only wind vectors with zonal wind anomalies at the 95% significance level are shown. (c) Meridional section of zonal wind anomalies averaged over 1208–1508E, regressed onto the standardized WNPSMI. The contour interval is 0.5 m s21, and the contour line of zero is omitted. In (c), the shading indicates significance at the 95% level, and the thick dashed line indicates the location of maximum climatological westerlies.
A WNP cyclonic (anticyclonic) anomaly corresponds to negative (positive) precipitation anomaly in the eastern Indian Ocean and the Maritime Continent (Fig. 3b), but the EAJI does not (Fig. 3a). The concurrent precipitation anomalies over the tropical WNP and the Indian Ocean basin in Fig. 3b can be explained by the SST anomalies in the Indian Ocean. There are negative SST anomalies in the Indian Ocean basin corresponding to positive minus negative WNPSMI (not shown). Previous studies (Watanabe and Jin 2002; Terao and Kubota 2005; Yang et al. 2007; Li et al. 2008) showed that warm SST anomalies in the Indian Ocean basin, which are associated with the El Nin˜o events in the preceding winter, induce enhanced precipitation locally and suppressed precipitation and a lower-tropospheric anticyclonic circulation anomaly in the tropical WNP.
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TABLE 1. Correlation coefficients between the WNPSMI, EAJI, and STPI during the period of 1979–2002. Values in parentheses are partial correlation coefficients between two indexes while controlling a third index.
EAJI WNPSMI
WNPSMI
STPI
20.59* (20.15)
0.72* (0.53*) 20.72* (20.53*)
* Significant at the 99% level.
Based on the result shown in Fig. 3, we define an STPI by averaging precipitation anomalies over 308–408N, 1358–1558E and regress horizontal winds upon this index. The spatial patterns of zonal wind anomalies regressed onto the STPI (Fig. 4) are very similar, with the opposite polarity, to those regressed onto the WNPSMI (Fig. 2), which suggests that the STPI is closely related to both the EAJ meridional displacement and the anticyclonic/cyclonic anomaly in the tropical WNP (see also Table 1). In particular, these zonal wind anomalies exhibit a roughly barotropic structure with a notable poleward tilt with height in the extratropics and a baroclinic structure in the tropics (Fig. 4c; Kosaka and Nakamura 2006). Although the correlation coefficient between the WNPSMI and EAJI is as strong as 20.59, the partial correlation coefficient between these two indexes, after controlling the STPI, drops to 20.15 (Table 1). This suggests that the subtropical precipitation anomaly might play a crucial role in the linkage between the WNPSMI and EAJI. We extend the analysis period from 1979–2002 to 1958–2002, using vertical velocity as a substitute for precipitation, and calculate the (partial) correlation coefficients between the indexes (Table 2). Here, the STPI is replaced by the vertical velocity index (VVELI), which is defined by the 500-hPa vertical velocity (2dp/dt) anomaly averaged over the same area (308–408N, 1358–1558E). The correlation coefficient between the STPI and VVELI from 1979 to 2002 is 0.88. The results shown in Table 2 are in agreement with those in Table 1, suggesting the present results on the relationships between the WNPSMI, the EAJI, and the subtropical precipitation anomaly index (STPI or VVELI) are robust. TABLE 2. Same as Table 1, but for the VVELI and the period of 1958–2002.
EAJI WNPSMI
WNPSMI
VVELI
20.55** (20.27)
0.66** (0.50**) 20.58** (20.35*)
* Significant at the 95% level. ** Significant at the 99% level.
FIG. 4. Same as Fig. 2, but regressed onto the STPI, which is defined as precipitation anomalies averaged over 308–408N, 1358– 1558E.
Figure 5 provides scatter diagrams of the WNPSMI and EAJI for the nonoverlapped cases of strong and weak precipitation anomalies, respectively. The negative relationship between the WNPSMI and EAJI, which shows up as a negative slope, exhibits a remarkable difference between weak and strong STPI (or VVELI) anomalies. For the 12 summers with the strongest STPI,
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FIG. 5. Scatter diagrams of the WNPSMI and EAJI. (a) For the 12 summers with the strongest STPI; (b) for the 12 summers with the weakest STPI; (c) for the 22 summers with the strongest VVELI; and (d) for the 22 summers with the weakest VVELI. Corresponding Rsquared values and case numbers (in parenthesis) are shown at the top right of each diagram.
the R-squared value is 0.46. But for the remaining 12 summers, in which the STPI is weakest, the R-squared value drops to 0.19. Such a difference in the WNPSMI– EAJI relationship between the strong and weak subtropical precipitation anomaly cases can also be seen in Figs. 5c and 5d, which are based on the data of a longer period. The R-squared value is 0.40 for strong VVELI cases, and drops to 0.22 for weak cases. These results demonstrate that the meridional teleconnection, illustrated by the relationship between the EAJ and WNPSM, is closely related to the intensity of subtropical precipitation anomaly. This is somewhat expectable, since, as is well known, an equatorward (poleward) displacement of EAJ and a lower-tropospheric anticyclonic (cyclonic) anomaly over the WNP result in a positive (negative) subtropical precipitation anomaly. However, one may wonder whether the subtropical precipitation anomaly, as a result of the circulation anomalies associated with the meridional teleconnection, can in turn significantly affect the circulation anomalies.
This question, related to the causality between precipitation and circulation, is not easily diagnosable through observational analyses. Therefore, a series of model experiments are designed to investigate the role of precipitation anomalies in affecting circulations in next section.
4. Numerical results by a simple baroclinic model In the tripole pattern of precipitation anomalies associated with the meridional teleconnection pattern of circulation, the tropical center (;158N) is of the strongest amplitude, followed by the subtropical center (;308N), while the mid–high-latitude center (;508N) is of the weakest amplitude (Fig. 3). Actually, the subtropical precipitation in the WNP–EA, which is referred to as the Meiyu/Baiu phenomenon in the rainy season, is predominant not only in climatology (Fig. 6a) but also in interannual standard deviation (Fig. 6b). With a notable intensity, the subtropical precipitation anomaly
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matology, which is obtained from the reanalysis dataset and varies both horizontally and vertically. Kosaka and Nakamura (2006) suggested the importance of both the horizontal and vertical structures of mean flow in maintaining the meridional teleconnection over the WNP–EA.
a. Subtropical heat source
FIG. 6. (a) Climatology and (b) interannual standard deviation of JJA precipitation during the period 1979–2002. The values exceeding 6 mm day21 in (a) and 1.5 mm day21 in (b) are shaded, and the contour intervals are 2 mm day21 in (a) and 0.5 mm day21 in (b).
would have an appreciable effect on large-scale circulations. A simple baroclinic model is utilized to examine qualitatively the role of precipitation anomaly in affecting the horizontal and vertical structures of largescale circulation anomalies over the WNP–EA. We emphasize the role of subtropical precipitation anomaly, although we also show the wind anomalies induced by tropical precipitation anomaly for comparison with the observed results and with the effect of subtropical precipitation anomaly. The simulated results indicate that in comparison with the tropical and subtropical precipitation anomalies, the precipitation anomaly in the mid–high latitudes induces much weaker circulation anomalies (not shown). The mean flow used in the model is taken from the summer mean circulation cli-
Figure 7 shows the prescribed subtropical heat source imposed into the model and the responses of circulations induced by the heat source. The spatial distribution and intensity of the heat source is set to approximately match with those shown in Fig. 3. In the horizontal the heating has a cosine squared profile in an elliptical region (upper panel at the left column). The maximum heat source is set to be at 400 hPa (s 5 0.45, lower panel at the left column). This vertical profile is basically in agreement with the observed profile of precipitation variability, and has been employed in many previous studies (e.g., Rodwell and Hoskins 1996; Annamalai and Sperber 2005). The maximum heating imposed at 400 hPa is 0.5 K day21, which is approximately equivalent to 1.0 mm day21 of anomalous precipitation and thus consistent with Fig. 3. The right column of Fig. 7 shows the simulated horizontal wind responses after imposing the subtropical heat source. This subtropical heat source induces an anticyclonic anomaly at the upper level and a cyclonic anomaly at the lower level. In the upper level, there is also a relatively weak cyclonic anomaly over northeast Asia and the Sea of Okhotsk. In comparison with the heat source, the induced circulation responses occupy a much larger area, extending from East Asia to the central North Pacific and from the tropics to the mid– high latitudes. The zonal wind responses are in a zonally elongated shape in the upper troposphere, but the cyclonic anomaly is notably southwest–northeast oriented in the lower troposphere. This implies the importance of basic flow in affecting the structure of circulation responses over the WNP–EA. In the upper troposphere, the strong westerly anomaly over East Asia and relatively weak easterly anomaly over northeast Asia and the Sea of Okhotsk indicate a southward displacement and intensification of the EAJ. This can also be seen in the meridional–vertical cross section of zonal wind responses (lower panel at the right column of Fig. 7). Actually, the subtropical heat source tends to result in southward displacement of the maximum westerlies in the whole troposphere over the WNP–EA. Over the subtropical WNP and East Asia, in both the upper and lower troposphere, the circulation responses induced by the heat source resemble the anomalies regressed onto the STPI (Fig. 4).
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FIG. 7. (left column) Horizontal distribution (at the level of sigma 5 0.45) and vertical profile of the specific subtropical heat source. Contour interval is 0.1 K day21 for the horizontal distribution of the heat source, and the unit is K day21 for the vertical profile. (right column) Wind anomalies at the (top) upper (sigma 5 0.22) and (middle) lower levels (sigma 5 0.82) and (bottom) meridional section of zonal wind anomalies averaged over 1208–1508E (lower panel) induced by the heat source shown at the left column. The contour interval for wind anomalies is 0.25 m s21, and the zero contour is omitted.
In particular, the subtropical heat anomaly induces the poleward-with-height tilt of zonal wind response (lower panel at the right column of Fig. 7). This tilting response may be closely related to the zonally asymmetric structure of the basic flow. When the zonal-mean flow, rather than the zonally asymmetric flow, is used as the basic state, the subtropical heat anomaly induces an anticyclonic anomaly in the upper troposphere and a cyclonic anomaly in the lower troposphere (results not shown). Both the anticyclonic and cyclonic anomalies are centered at the region of the heat anomaly, and thus the zonal wind response does not exhibit a vertical tilt.
b. Tropical heat sink The horizontal shape of the tropical heat sink is also set to be elliptical, and the vertical profile is the same as
that of the subtropical heat source (left column of Fig. 8). The maximum heat sink imposed at 400 hPa is 1.4 K day21, which is equivalent to 2;3 mm day21 of rainfall. The right column of Fig. 8 shows the wind responses induced by the tropical heat sink. In the upper troposphere, there is a southwesterly anomaly over the Maritime Continent and an easterly anomaly over the tropical WNP. In the extratropics, the wind response is not well organized and not in a zonally elongated shape. The reason for the absence of clear extratropical teleconnections is probably that the tropical heating anomaly is located equatorward of critical latitude for stationary Rossby waves and thus cannot induce propagation of stationary Rossby waves into the extratropics. In the lower troposphere, the circulation response shows a typical Gill (1980) pattern corresponding to an
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FIG. 8. Same as Fig. 7, but for the tropical heat sink with contour interval being 0.3 K day21.
off-equatorial heat sink; that is, there is an anticyclonic anomaly to the northwest of the tropical heat sink as well as a westerly anomaly along the equatorial Pacific. In addition, there is a weak easterly anomaly over the Japan Sea, which may result from the poleward propagation of Rossby waves. The zonal wind anomalies shown in the meridional section (lower panel at the right column) are weak, but such weakness is to some extent artificially caused by the averaging longitudinal scope (1208–1508E) that is located to the east of the strongest tropical and subtropical response to the tropical heating in both the lower and upper troposphere. Besides the limitation of critical latitude for stationary Rossby waves, there is an additional reason for the weakness of extratropical circulation response induced by the tropical heating anomaly in the simulations. In the simulations of this study, the tropical cooling can trigger an anticyclonic anomaly in the lower tropo-
sphere over the WNP, but cannot result in any anomalous subtropical precipitation as in reality, because the model used in this study is a dry version. In reality, the anticyclonic anomaly over the WNP can lead to more subtropical precipitation and thus affect extratropical circulation, as suggested in the preceding subsection.
c. Joint impacts of subtropical heat source and tropical heat sink The subtropical heat source and tropical heat sink coexist over the WNP, which is known as the seesaw pattern of tropical–subtropical precipitation in this region. Therefore, here we show the wind responses induced by the subtropical heat source and tropical heat sink together (Fig. 9) and compare these responses with the observed anomalies shown in Fig. 4. The subtropical heat source and tropical heat sink (left column of Fig. 9) are identical to the heat anomalies imposed in the preceding subsections, respectively. Note that the contour
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FIG. 9. Same as Fig. 7, but for both the subtropical heat source and tropical heat sink. Contour interval is 0.1 K day21 for the subtropical heat source and 0.3 K day21 for the tropical heat sink.
interval of the tropical heat sink is 3 times that of the subtropical heat source in Fig. 9. The subtropical heat source and tropical heat sink together induce strong circulation responses in the western Pacific and East Asia (right column of Fig. 9). In the upper troposphere, southwesterly anomalies appear over the Maritime Continent, and a meridional teleconnection pattern with each cell being zonally elongated appears over the WNP2EA. In the lower troposphere, the teleconnection pattern exhibits a southwest– northeast orientation, extending from the South China Sea to Japan. These circulation responses resemble the extratropical responses induced by the subtropical heat source (Fig. 7) and the tropical responses induced by the tropical heat sink (Fig. 8). The joint effects of subtropical and tropical heat anomalies result in the zonal elongation of upper-tropospheric easterly anomaly and lower-tropospheric westerly anomaly in the subtropics.
The vertical structure induced by the joint heat anomalies (lower panel at the right column of Fig. 9) resembles well that induced by the subtropical heat source (lower panel at the right column of Fig. 7), suggesting that the subtropical heat anomaly plays a crucial role in the vertical structure of the meridional teleconnection over the WNP–EA. In particular, the subtropical heat anomaly is responsible for the notable poleward-with-height tilt of zonal wind responses, which is the unique vertical structure of the meridional teleconnection in the subtropics and midlatitudes, through inducing a subtropical anticyclonic anomaly and midlatitude cyclonic anomaly in the upper troposphere and a subtropical cyclonic anomaly in the lower troposphere (Fig. 7). The wind responses induced jointly by the subtropical heat source and tropical heat sink resemble well the wind anomalies associated with the subtropical precipitation anomaly shown in Fig. 4. In both simulation and
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FIG. 10. Same as Fig. 7, but for a slightly enhanced and southwest–northeast-tilted heat source.
observations, there are teleconnection patterns over the WNP–EA with four centers of action in the upper troposphere and three centers in the lower troposphere. In vertical the zonal wind anomalies exhibit a notable poleward tilt with height, suggesting a southward displacement of the maximum subtropical westerlies over the WNP in both simulation and observations. This resemblance between the simulation and observations suggests that the horizontal and vertical structures of the meridional teleconnection can be explained qualitatively by the subtropical and tropical heat anomalies. Besides the tropical and subtropical heat sources and the basic flow, other factors may also affect the meridional teleconnection. In Fig. 9, the circulation anomalies induced by the tropical and subtropical heat sources resemble well those diagnosed (Fig. 4b) in the preceding section in the lower troposphere, but differ appreciably with the diagnosed ones (Fig. 4a) in the upper troposphere. A major difference between the simulated and
diagnosed upper-tropospheric circulation anomalies is that the simulated easterly anomalies are too weak in the midlatitudes and too strong in the tropics. This difference is likely related to the strong nonlinear advection in the upper troposphere. In addition, the lack of eddy forcing in the model may also lead to the discrepancy in the midlatitudes. Despite these differences in the intensity of circulation anomalies, the good agreement between the structures of simulated and diagnosed circulation anomalies suggests that diabatic heating anomalies play a crucial role in maintaining the large-scale circulations associated with the meridional teleconnection.
d. Insensitivity to the shape of subtropical heat source We have performed some other numerical experiments by imposing heating anomalies with different locations and shapes, and found the responses of circulations are not sensitive to these differences. Here we show the result for only one of these experiments (Fig. 10).
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In this experiment, we add an extra heat source into the subtropical heat source shown in Fig. 7. This extra heat source is centered at 358N, 1508E, with a cosine squared profile in an elliptical region whose area is approximately one-third of that of the subtropical heat source in Fig. 7. The vertical profile of this extra heat source is identical to the previous settings, with the maximum heating imposed at 400 hPa of 0.35 K day21. The superposition of the extra heating adds a southwest– northeast tilt and slight intensification to the subtropical heating. With the extra heating imposed, we attempt to mimic the observed subtropical precipitation anomaly, which tends to be in a southwest–northeast orientation (Fig. 3). Furthermore, this experiment can also allow us to show the sensitivity of circulation responses to subtropical heat anomalies of slightly different shape and intensification through a single experiment. The horizontal and vertical structures of wind responses induced by the southwest–northeast-oriented subtropical heat source (right column of Fig. 10) are very similar to those induced by the zonally oriented subtropical heat source (Fig. 7). The change in orientation of the heat source does not lead to different orientation of the wind responses in both the upper and lower troposphere. After imposing the extra heat source, the induced upper-tropospheric zonal wind anomalies remain as zonally oriented in the WNP–EA, and the cyclonic anomaly in the lower troposphere is in a very similar southwest–northeast orientation. This suggests that the circulation responses are insensitive to the shapes and locations of imposed subtropical heat anomalies, but they may be dependent on the structure of basic flow. With the extra heating imposed, the upper-tropospheric anticyclonic anomaly is extended zonally, and lowertropospheric cyclonic anomaly is extended eastward, in comparison with the circulation anomalies induced by the zonally oriented subtropical heat source, although the heat source is only slightly northward extended in its eastern extent. Moreover, these anticyclonic and cyclonic anomalies are slightly intensified. This suggests that a stronger subtropical heat anomaly may induce stronger circulation responses in a wider area. The influence of subtropical precipitation anomaly on circulations can be recognized as follows, from the above results simulated by the simple model. Compared with its scope, the subtropical precipitation anomaly induces significant zonally elongated zonal wind anomalies over a much wider area in both the upper and lower troposphere. The subtropical precipitation anomaly results in the poleward-tilted circulation anomalies with height and equatorward displaced and intensified upper- and lower-tropospheric westerly jet over the WNP.
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This poleward tilt results from the subtropical anticyclonic anomaly and midlatitude cyclonic anomaly in the upper troposphere and the subtropical cyclonic anomaly in the lower troposphere induced by the subtropical heat source. These horizontal and vertical structures of zonal wind responses are the essential feature of the meridional teleconnection. Therefore, these experimental results indicate that the subtropical precipitation anomaly plays an important role in both the horizontal and vertical structures of the meridional teleconnection over the extratropical WNP.
5. Conclusions and discussion The role of subtropical precipitation anomaly in affecting the summertime meridional teleconnection over the western North Pacific and East Asia (WNP–EA) has been investigated by using observational and reanalysis data and by a linear baroclinic model. This meridional teleconnection is characterized by the zonally elongated westerly and easterly anomalies that appeared alternately in the meridional direction over this region, in both the lower and upper troposphere. These zonal wind anomalies exhibit a baroclinic structure in the tropics and a notable poleward tilt with height in the extratropics. A seesaw pattern between the subtropical and tropical WNP precipitation anomalies is associated with the meridional teleconnection. Using observational and reanalysis data, we found that the relationship between the meridional displacement of EAJ and the anticyclonic/cyclonic anomalies in the tropical WNP, which can be used to illustrate the meridional teleconnection, is intensified in summers when the subtropical precipitation anomaly is greater, and is much weakened without the variability in subtropical rainfall. Therefore, it is suggested that, as a subtropical heat anomaly, subtropical rainfall variability plays a crucial role in maintaining the meridional teleconnection. The effect of subtropical precipitation anomaly on circulations is simulated by a simple dry baroclinic model. A subtropical heat source, which is equivalent to the diagnosed positive subtropical precipitation anomaly, induces significant zonally elongated zonal wind anomalies in both the upper and lower troposphere over a much wider area in comparison with the area of the heat anomaly. The positive subtropical precipitation anomaly results in an equatorward displaced and intensified upper- and lower-tropospheric westerly jet over the WNP. It also leads to the poleward-tilted zonal wind anomalies with height, through inducing a subtropical anticyclonic anomaly and a midlatitude cyclonic
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anomaly in the upper troposphere and a subtropical cyclonic anomaly in the lower troposphere. These horizontal and vertical structures of circulation anomalies are the essential feature of the observed meridional teleconnection in the subtropics and mid–high latitudes. Therefore, it can be concluded that subtropical precipitation anomalies play an important role in both the horizontal and vertical structures of the meridional teleconnection over the subtropical WNP and East Asia. The present results indicate that the horizontal and vertical structures of circulation responses in the subtropical WNP and East Asia are insensitive to the location and shape of imposed subtropical heat anomalies, although they are somewhat sensitive to the intensity of the heat anomalies. The insensitivity to the location and shape of imposed heat anomalies implies the important role of the structure of the basic flow in circulation responses. This provides an explanation for the geographically fixed feature of extratropical anomalies associated with the meridional teleconnection in observations. It should be mentioned that the PJ/EAP pattern has broad frequencies from intraseasonal to interannual time scales, and the features of the tropical–extratropical interaction over the WNP depend significantly on the time scales discussed. In this study, the JJA-mean data are used, and the meridional teleconnection shows as a dominant mode. One should be cautious about the use of the present results to other time scales. This study implies that, for skillful seasonal forecasting in East Asia, one should be fully aware of the importance of the feedback role of subtropical rainfall variability on circulation anomalies. A general circulation model should be required to capture the features of interannual rainfall variability (e.g., standard deviation) and basic-state flows in the subtropical WNP and East Asia for a reasonable dynamical seasonal forecast of climate anomalies in East Asia. Acknowledgments. We thank Prof. M. Watanabe for providing the linear baroclinic model and two anonymous reviewers for their valuable comments. This work was supported by the National Natural Science Foundation of China (Grant 40725016) and by the National Basic Research Program of China (Grant 2006CB403601). REFERENCES Adler, R. F., and Coauthors, 2003: The Version-2 Global Precipitation Climatology Project (GPCP) Monthly Precipitation Analysis (1979–present). J. Hydrometeor., 4, 1147–1167. Annamalai, H., and K. R. Sperber, 2005: Regional heat sources and the active and break phases of boreal summer. J. Atmos. Sci., 62, 2726–2748.
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