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  • View in gallery

    (left) The 850-hPa wind (arrow; m s−1) and vorticity (contour; 10−5 s−1) at 1200 UTC 27 Aug 1997 and (right) the 850-hPa wind (arrow; m s−1) and vorticity (contour; 10−6 s−1) averaged over JAS in 1997: (a),(d) reanalysis; (b),(e) TC field; and (c),(f) TC field removed. Solid and dashed lines denote positive and negative vorticity, respectively.

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    Seasonal mean (a) EKE (m2 s−2) and (d) EKETC (m2 s−2) over JAS during 1970–2010, and the seasonal TC occurrence frequency over JAS is shown in the top-right corner in (d). (b),(c) EKE anomaly (m2 s−2) and (e),(f) EKETC anomaly (m2 s−2) at 850-hPa averaged over JAS of the composite (b),(e) El Niño year and (c),(f) La Niña year; TC occurrence frequency in the composite El Niño and La Niña years is shown in the top-right corner of (e) and (f), respectively. Light (dark) shades indicate areas where the negative (positive) difference is statistically significant at the 95% confidence level by a t test.

  • View in gallery

    Seasonal mean (a) KmKe (10−5 m2 s−3) and (d) KmKeTC (10−5 m2 s−3) over JAS during 1970–2010, and the seasonal TC genesis frequency over JAS is shown in the top-right corner of (d). (b),(c) KmKe anomaly (10−5 m2 s−3) and (e),(f) KmKeTC anomaly (10−5 m2 s−3) at 850-hPa averaged over JAS of the composite (b),(e) El Niño year and (c),(f) La Niña year; TC genesis frequency in the composite El Niño and La Niña years is shown in the top-right corner of (e) and (f), respectively. Light (dark) shades indicate areas where the negative (positive) difference is statistically significant at the 95% confidence level by a t test.

  • View in gallery

    Composite differences between El Niño and La Niña years at 850 hPa: (a) , (b) , (c) , (d) , (e) , (f) , (g) , and (h) (10−5 m2 s−3). Light (dark) shades indicate areas where the negative (positive) difference is statistically significant at the 95% confidence level by a t test.

  • View in gallery

    Composite wind (streamlines) and relative vorticity (shading; 10−6 s−1) anomalies at 850 hPa in (a) El Niño and (b) La Niña years; dark (light) dots mark the positive (negative) anomaly of TC genesis frequency binned into 5° × 5° grid box. Regions enclosed by solid (dashed) thick contours and crosses within dots indicate the positive (negative) relative vorticity and TC genesis frequency anomalies are statistically significant at the 95% confidence level by a t test, respectively.

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Contributions of Barotropic Energy Conversion to Northwest Pacific Tropical Cyclone Activity during ENSO

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  • 1 College of Meteorology and Oceanography, PLA University of Science and Technology, Nanjing, China
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Abstract

The contribution of barotropic energy conversion to tropical cyclone (TC) activity over the western North Pacific (WNP) during warm and cold phases of El Niño–Southern Oscillation (ENSO) is investigated by separating TC vortices from reanalysis data and using a linearized eddy kinetic energy tendency equation. By comparing the characteristics of TC disturbances with synoptic-scale disturbances, it is found that the modulation of ENSO on the WNP TC intensity is presented more objectively by using TC kinetic energy (EKETC) than eddy kinetic energy (EKE). Barotropic energy conversion (KmKe) into TC disturbances (KmKeTC) is an effective indicator in detecting the barotropic energy source of low-level cyclone genesis and maintenance during the ENSO cycle. However, its dynamical processes play different roles. Shear in large-scale zonal wind and convergence in large-scale meridional wind provide direct barotropic energy source for TC genesis, but make effects in different regions of the WNP. In contrast, convergence in large-scale zonal and shear in large-scale meridional wind exert little influence on TC genesis during ENSO.

Corresponding author address: Zhong Zhong, College of Meteorology and Oceanography, PLA University of Science and Technology, Zhong Hua Men Wai, Nanjing 211101, China. E-mail: zhong_zhong@yeah.net

Abstract

The contribution of barotropic energy conversion to tropical cyclone (TC) activity over the western North Pacific (WNP) during warm and cold phases of El Niño–Southern Oscillation (ENSO) is investigated by separating TC vortices from reanalysis data and using a linearized eddy kinetic energy tendency equation. By comparing the characteristics of TC disturbances with synoptic-scale disturbances, it is found that the modulation of ENSO on the WNP TC intensity is presented more objectively by using TC kinetic energy (EKETC) than eddy kinetic energy (EKE). Barotropic energy conversion (KmKe) into TC disturbances (KmKeTC) is an effective indicator in detecting the barotropic energy source of low-level cyclone genesis and maintenance during the ENSO cycle. However, its dynamical processes play different roles. Shear in large-scale zonal wind and convergence in large-scale meridional wind provide direct barotropic energy source for TC genesis, but make effects in different regions of the WNP. In contrast, convergence in large-scale zonal and shear in large-scale meridional wind exert little influence on TC genesis during ENSO.

Corresponding author address: Zhong Zhong, College of Meteorology and Oceanography, PLA University of Science and Technology, Zhong Hua Men Wai, Nanjing 211101, China. E-mail: zhong_zhong@yeah.net

1. Introduction

The interannual variability of tropical cyclone (TC) activity in the western North Pacific (WNP), including genesis location, frequency, intensity, track, and life span, has been examined by many authors (Lander 1994; Harr and Elsberry 1991, 1995; Chan 2000; Sobel and Maloney 2000; Wang and Chan 2002; Wu et al. 2004; Camargo and Sobel 2005; Chen et al. 2006; He and Jiang 2011; Zhan et al. 2011a,b; Ha and Zhong 2012). El Niño–Southern Oscillation (ENSO) strongly modulates TC genesis frequency and location in the WNP, which exhibit distinct spatiotemporal characteristics, corresponding to different ENSO phases. In general, the WNP TC genesis shows a dipole pattern in El Niño years, with high genesis frequency in the southeastern WNP and low frequency in the northwestern WNP, while the pattern is reversed in La Niña years (Chan 2000; Chia and Ropelewski 2002; Wang and Chan 2002; Camargo and Sobel 2005; Chen et al. 2006; Yang and Jiang 2008; Zhan et al. 2011a).

Significant difference of TC activity in warm and cold ENSO years is explained by the shift of the East Asian monsoon trough and by the activity of summer teleconnection wave trains in the WNP, both associated with ENSO (Wu et al. 2009). Barotropic energy is the main energy source of TC disturbances during ENSO (Shapiro 1978; Maloney and Hartmann 2001). Zhan et al. (2011a) investigated the modulation of ENSO on the WNP TC by the barotropic energy conversion into synoptic-scale disturbances using a linearized eddy kinetic energy tendency equation in terms of barotropic eddy–mean flow interaction. The method can be described as follows: an environmental variable is decomposed into a basic state (defined as an 11-day running mean, and denoted with an overbar) and an eddy component (defined as the residual, and denoted with a prime). Then, according to Lau and Lau (1992), Seiki and Takayabu (2007), and Zhan et al. (2011a), the equation about the basic state can be written as
e1
where
e2
The first term on the right-hand side of Eq. (1) can be written in Cartesian coordinates as
e3
where K′ is the eddy kinetic energy (EKE); u and υ are the zonal and meridional winds, respectively; V is the three-dimensional wind vector; Vh is the horizontal wind vector; R is the gas constant for dry air; p is air pressure; ω is vertical velocity; T is air temperature; and Φ is geopotential. The first term on the right-hand side of Eq. (1) represents the barotropic energy conversion from the mean to synoptic-scale disturbances including TCs (KmKe). The second and third terms denote the advection of EKE by the environmental flow and by the eddy flow, respectively. The fourth term is the conversion from eddy available potential energy to EKE. The fifth term corresponds to the divergence of the eddy geopotential flux, and dissipation is represented by the last term D. KmKe has contributions from four dynamical process terms that are determined by both the large-scale flow and the structure of the disturbance as shown in Eq. (3). The four contributions are barotropic energy conversions due to the zonal convergence and the meridional shear of zonal wind , and to the zonal shear and the meridional convergence of meridional wind .

EKE and KmKe represent all the synoptic disturbances including TCs, and can be calculated with reanalysis data (Zhan et al. 2011a); but it should be noticed only part of synoptic disturbances can develop into TCs, and EKE and KmKe cannot represent the TC kinetic energy (EKETC) and the barotropic energy conversion into TC disturbances (KmKeTC), respectively. Therefore, we consider the separation of EKETC and KmKeTC from EKE and KmKe to identify the contributions of barotropic energy conversion in TC genesis and development over the WNP during ENSO. Moreover, after the TCs are separated from the background flow, the objective representation of TC intensity in the WNP during warm and cold ENSO years by phases of EKETC is examined. Additionally, the utility of KmKeTC as an indicator of a barotropic energy source for TC activity during ENSO is also examined. To examine the above issues, we will separate TC vortices from the reanalysis data and then the barotropic energy conversion terms will be calculated with the TC wind fields to analyze the cause of anomalous TC activity over the WNP during warm and cold years.

This paper is organized as follows. Data and details of methodology used in this study are described in section 2. EKETC, KmKeTC, and its dynamical process terms in the composite warm and cold ENSO years are discussed in section 3. The conclusions are presented in section 4.

2. Data and methodology

We use the best-track dataset, including 6-hourly TC position, obtained from the Regional Specialized Meteorological Center (RSMC) of the Japan Meteorological Agency (JMA), monthly Niño-3.4 index from the Climate Prediction Center, and wind field data from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis during the period 1970–2010 to study the relationship between TC activity in the WNP (including the South China Sea) and large-scale circulation during TC peak season from July to September (JAS). El Niño and La Niña years are defined by the averaged Niño-3.4 index over JAS, and the five largest (smallest) values corresponding to the five El Niño (La Niña) years are 1972, 1982, 1987, 1997, and 2002 (1973, 1975, 1988, 1999, and 2010). The composite analysis method is applied in this study, and a two-tailed Student’s t test at 95% confidence level is used to examine the statistical significance for the composite analysis.

Although latent heat release is important to TC intensification, the barotropic energy conversion is an essential energy source for the developing tropical depressions in the lower troposphere (Shapiro 1978; Maloney and Hartmann 2001). Lau and Lau (1992) pointed out that the processes concerning available potential energy conversion in Eq. (1) mainly extend throughout the mid- to upper troposphere from 500 to 200 hPa, and KmKe is mainly concentrated in the lower troposphere. Following Zhan et al. (2011a), we hypothesize that the barotropic energy conversion provides a significant extra energy source for the synoptic-scale disturbances, which contribute to the TC genesis and development when other conditions are favorable for TC activity during ENSO. Therefore, we examine the large-scale wind at 850 hPa, and focus on the contributions of KmKe/KmKeTC to the EKE/EKETC tendency of the first term in Eq. (1).

To derive EKE (EKETC) and KmKe (KmKeTC), and further examine their differences between all the synoptic-scale and TC disturbances, we first use the NCEP–NCAR reanalysis to calculate EKE, KmKe, and its dynamical process terms based on the linearized eddy kinetic energy tendency equation, which include all synoptic-scale disturbances. Second, the TC-related wind field is separated from the reanalysis by the method of removing TC vortices from the analysis data proposed by Low-Nam and Davis (2001). Therefore, EKETC, KmKeTC, and its dynamical processes associated with TC disturbances can be directly calculated by the TC wind field. The TC vortices are separated from the reanalysis data after the time when each vortex reaches tropical storm intensity [10-min-averaged maximum sustained wind speed ≥34 kt (~17.5 m s−1) on the JMA scale] based on the best-track dataset. Removal of TC vortices from the reanalysis data is a sophisticated practice in hurricane simulation and forecast (Kurihara et al. 1993), and so in the study of TC influences on climate variability (Zhong and Hu 2007; Hsu et al. 2008). We derive the 850-hPa horizontal TC wind field from the reanalysis data by the two-step method that has been used in TC bogussing scheme for the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). The position of the observed TC is identified by the nearest grid point of the reanalysis based on the TC best-track dataset, and the analysis TC is considered as the TC vortex domain. This is determined by searching the maximum vorticity point within a prescribed radial distance from the center of the observed TC. Then, the reanalysis wind field in the first-guess vortex area is decomposed into TC component and background component. The horizontal wind field of the reanalysis data can be decomposed into nondivergent wind, velocity potential, and the residual, and the streamfunction corresponding to nondivergent wind and the velocity potential are calculated based on the best-track dataset at each time. The sum of the nondivergent wind and the velocity potential wind are regarded as TC wind components (Low-Nam and Davis 2001). It should be mentioned that, although the reanalysis data with 2.5° × 2.5° spatial resolution seems relatively coarse for the TC wind field, it is able to represent the interannual and interseasonal variations of the WNP TC activity for TC climatology studies. Hsu et al. (2008) revealed that the identification of TC vortices from the reanalysis data with different spatial resolution was done well not only in case studies but also in seasonal means, by comparing the TC wind fields between the reanalysis data with spatial resolutions of 2.5° and 1.125°. Because the results in this study are dependent on the TC field after the procedure of separating TC vortices from the reanalysis, the wind fields are illustrated in Fig. 1 to demonstrate the procedure is effective in separating TCs from its environmental field. The 850-hPa wind and vorticity fields on 1200 UTC 27 August 1997 are shown in Figs. 1a–c. Figure 1a shows three TCs embedded in the positive latitudinal vorticity band near 20°N, where there was a summer monsoon trough. The separated wind and vorticity fields of three isolated TC vortices are shown in Fig. 1b. After having removed TCs from the reanalysis, the feature of weaker positive latitudinal vorticity is still obvious (Fig. 1c), suggesting the summer monsoon trough is effectively retained in the background field. To show the method is effective at depicting the characteristics of seasonal wind field, the 850-hPa wind and vorticity fields over JAS in 1997 are given in Figs. 1d,e. Compared to the seasonal mean (Fig. 1d), the wind and vorticity fields without TCs are weaker in the southeastern WNP (Fig. 1f), and the reduction is due to seasonal-averaged TCs (Fig. 1e). The TC field appears the positive vorticity in the southeastern WNP, indicating intense TC activity in this region during the TC peak season in 1997. This corresponds to the modulation by the strongest El Niño case in the twentieth century on the TC activity over the WNP. Therefore, the procedure does not only separate TC wind field, but it also effectively retains the characteristics of large-scale circulation, which indicates that the results obtained by this method are objective and reliable for the study.

Fig. 1.
Fig. 1.

(left) The 850-hPa wind (arrow; m s−1) and vorticity (contour; 10−5 s−1) at 1200 UTC 27 Aug 1997 and (right) the 850-hPa wind (arrow; m s−1) and vorticity (contour; 10−6 s−1) averaged over JAS in 1997: (a),(d) reanalysis; (b),(e) TC field; and (c),(f) TC field removed. Solid and dashed lines denote positive and negative vorticity, respectively.

Citation: Monthly Weather Review 141, 4; 10.1175/MWR-D-12-00084.1

3. Results

Figure 2 shows EKE and EKETC seasonal mean and anomalies over JAS in the composite El Niño and La Niña years. The maximum seasonal mean EKE is located around 25°N, 130°E and north of 35°N (Fig. 2a). In El Niño years, the anomaly of EKE is positive in a northwest–southeast-oriented band (0°–30°N, 120°E–180°), with a positive center near 10°N, 153°E (Fig. 2b). In La Niña years, the negative anomaly exhibits a similar spatial distribution with weaker amplitude, and the anomalous center is located near 21°N, 136°E (Fig. 2c). The maximum seasonal mean EKETC is concentrated around 23°N, 130°E (Fig. 2d), which exhibits a similar spatial pattern to the seasonal TC occurrence frequency (the TCs’ tracks appeared in each 5° longitude by 5° latitude grid box) over JAS shown in the inset of Fig. 2d, suggesting that EKETC is closely related to the TC occurrence frequency and is able to represent TC kinetic energy more objectively. In El Niño years, the positive EKETC anomaly is mainly concentrated in the northwestern WNP with larger amplitude than that of EKE (Fig. 2e), while in La Niña years, the anomalous EKETC is very similar to that during warm years in both spatial distribution and quantity but with the opposite phase, and the evident negative anomaly is also located in the northwestern WNP (Fig. 2f). The pattern of EKETC is similar to the anomalous distribution of TC occurrence frequency in the extreme ENSO years (Figs. 2e,f). In general, the significant positive (negative) anomaly of EKETC dominates over the entire WNP in warm (cold) years, corresponding to tremendous interannual variability of TC intensity over the WNP during ENSO (Chia and Ropelewski 2002; Wang and Chan 2002; Camargo and Sobel 2005; Chen et al. 2006). The feature of EKE in this study is consistent with Zhan et al. (2011a), but EKE cannot totally represent EKETC despite EKETC being a substantial part of EKE, and the anomalous spatial distribution and amplitude of EKETC are different from those of EKE. Most previous studies have revealed that there is a significant positive correlation between the genesis frequency of intense TC over the WNP and the ENSO index (Wang and Chan 2002; Camargo and Sobel 2005; Chen et al. 2006; Zhan et al. 2011a). This is because more TCs formed in the southeastern WNP would be intensified as moving northward and westward over the ocean to reach intense TCs in El Niño, while the opposite situation occurs in La Niña. Thus, the maximum positive (negative) EKETC anomaly is mainly localized over the northwest WNP during El Niño (La Niña) years as shown in Figs. 2e and 2f. Furthermore, the amplitude of the EKETC anomaly in El Niño years is slightly larger than the negative anomaly in La Niña years. This implies that the El Niño has stronger influences on the WNP TC activity than La Niña dose. Therefore, the TC intensity in the extreme ENSO phases can be presented more objectively by EKETC than the EKE.

Fig. 2.
Fig. 2.

Seasonal mean (a) EKE (m2 s−2) and (d) EKETC (m2 s−2) over JAS during 1970–2010, and the seasonal TC occurrence frequency over JAS is shown in the top-right corner in (d). (b),(c) EKE anomaly (m2 s−2) and (e),(f) EKETC anomaly (m2 s−2) at 850-hPa averaged over JAS of the composite (b),(e) El Niño year and (c),(f) La Niña year; TC occurrence frequency in the composite El Niño and La Niña years is shown in the top-right corner of (e) and (f), respectively. Light (dark) shades indicate areas where the negative (positive) difference is statistically significant at the 95% confidence level by a t test.

Citation: Monthly Weather Review 141, 4; 10.1175/MWR-D-12-00084.1

Figure 3 shows KmKe and KmKeTC seasonal means and anomalies over JAS in the composite warm and cold years. In El Niño years, the significant positive KmKe anomaly is located in the region of 10°–20°N, 130°–165°E, of which east and west anomaly centers are near 15°N, 140°E and 12°N, 155°E, respectively, and the east is stronger than the west (Fig. 3b). In contrast, the overall anomalous KmKe in the cold year exhibits an opposite pattern relative to that in warm year (Fig. 3c). The seasonal mean KmKeTC does not display a clear pattern (Fig. 3d), of which amplitude is much less than that of KmKe, and it is apparently different from the seasonal TC genesis frequency shown in the inset of Fig. 3d. A possible reason is that the seasonal mean KmKeTC, which is composed of the large-scale flow convergences and shears in the monsoon trough region, could be offset by the distinct change of the background wind field associated with the clear interannual variation of the monsoon trough influenced by ENSO. Additionally, the maximum seasonal mean EKETC is centered around 25°N over the western WNP (Fig. 2d), which exhibits little consistent match with the seasonal KmKeTC. This indicates that KmKeTC may not be the most important contributor to the EKETC budget in the seasonal mean, while other processes produce substantial EKETC over the entire WNP. However, in the extreme ENSO years, the anomaly of KmKeTC exhibits evident regional discrepancy that remarkably differs from the seasonal mean. In El Niño years, anomalous KmKeTC contains significant positive (negative) anomaly in the southeastern (northwestern) WNP (Fig. 3e). This indicates enhanced (reduced) TC activity in the respective region, but its amplitude is less than that of KmKe (Fig. 3b). The anomalous KmKeTC in La Niña years is similar to the pattern in warm years, but with the opposite sign (Fig. 3f). The dipole pattern of KmKeTC anomaly in the extreme ENSO years resembles that of TC genesis frequency shown in the inset of Figs. 3e,f. Thus, it is suggested that the barotropic energy conversion plays an important role in the regional discrepancy of TC activity in the WNP during ENSO. To illustrate KmKeTC is a better indicator in detecting the barotropic energy source of TC activity than the KmKe, we calculate the root-mean-square errors (RMSEs) of normalized KmKe/KmKeTC anomalies and that of normalized TC genesis frequency anomaly over the entire WNP (5°–40°N, 100°E–180°) in the extreme ENSO years (Table 1). The RMSE of KmKeTC is 125.2 (105.0) in the composite El Niño (La Niña) year, which is 45.3% (55.9%) of the RMSE of KmKe, and the composite values are highly representative of individual years. Furthermore, the RMSE of normalized KmKe/KmKeTC anomalies and that of normalized TC occurrence frequency anomaly exhibits similar results as in Table 1, that is, the RMSE of anomalous KmKeTC to TC occurrence frequency is about 70% of KmKe’s. This suggests that KmKeTC is a more effective indicator than KmKe in detecting the barotropic energy source of TC genesis and maintenance during warm and cold ENSO phases. It should be mentioned that, although barotropic conversion is not the main energy source to TC disturbances, it exerts significant impact on TC activity over the WNP during ENSO, which can be effectively diagnosed by the KmKeTC. Additionally, we also calculated the tendency of EKETC on the left-hand side of Eq. (1), and results shows that the quantity of KmKeTC is much larger than that of EKETC tendency (not shown), implying that KmKeTC plays an important role in the change of EKETC.

Fig. 3.
Fig. 3.

Seasonal mean (a) KmKe (10−5 m2 s−3) and (d) KmKeTC (10−5 m2 s−3) over JAS during 1970–2010, and the seasonal TC genesis frequency over JAS is shown in the top-right corner of (d). (b),(c) KmKe anomaly (10−5 m2 s−3) and (e),(f) KmKeTC anomaly (10−5 m2 s−3) at 850-hPa averaged over JAS of the composite (b),(e) El Niño year and (c),(f) La Niña year; TC genesis frequency in the composite El Niño and La Niña years is shown in the top-right corner of (e) and (f), respectively. Light (dark) shades indicate areas where the negative (positive) difference is statistically significant at the 95% confidence level by a t test.

Citation: Monthly Weather Review 141, 4; 10.1175/MWR-D-12-00084.1

Table 1.

The RMSE of normalized KmKe and KmKeTC anomalies to normalized TC genesis frequency anomaly in El Niño and La Niña years.

Table 1.

Figure 4 shows the composite differences of four dynamical process terms of KmKe and KmKeTC between El Niño and La Niña years. A significant positive anomaly of is located around 12°N, 160°E (Fig. 4a), but the anomaly of barotropic conversion into TC disturbances induced by convergence in large-scale zonal wind () is not obvious in the entire WNP (Fig. 4b), implying that has little direct effect on the WNP TC activity. A positive (negative) anomaly exists around 10°–20°N, 125°–160°E (15°–25°N, 120°–125°E) as shown in Fig. 4c, and the anomaly of barotropic energy conversion into TC disturbances induced by shear in large-scale zonal wind () also shows extensive negative anomaly north of 15°N and a positive anomaly centered around 12°N, 155°E (Fig. 4d). Although the evident negative anomaly of is located around 20°N, 130°E (Fig. 4e), barotropic energy conversion into TC disturbances induced by shear in large-scale meridional wind () shows no significant anomaly over the WNP (Fig. 4f). Figure 4g shows a dipole anomaly of with a negative (positive) anomaly over the northwestern (southeastern) WNP. The anomalous pattern of barotropic energy conversion into TC disturbances induced by convergence in large-scale meridional wind () is similar to that of with a smaller amplitude. In particular, it is mainly localized west of 150°E, showing negative anomaly over the northwest WNP and a positive anomalous center around 12°N, 140°E (Fig. 4h). Wang and Chan (2002) revealed that background vorticity in the lower troposphere induced by meridional shear in large-scale zonal wind can increase moisture convergence and potential vorticity entrained into TC, which would help spin up TC vortices, and our result supports and extends the previous study in terms of the barotropic energy conversion during ENSO. It should be noted that the spatial pattern of KmKeTC is highly consistent with and , indicating that they make the significant and synergetic effect to KmKeTC by different dynamical processes. In addition, the significant is mainly distributed east of 150°E, but that of is localized west of 150°E. This indicates that the distinct dynamical process works in different regions of the WNP, and their combined contributions lead to the enhanced (suppressed) TC disturbances in the southeastern (northwestern) WNP in El Niño, as well as the opposite situation in La Niña. Moreover, the anomalies of and are closely related to the activity of East Asian monsoon trough during ENSO, which strongly modulates the genesis location and intensity of TCs over the southeastern WNP. Zhan et al. (2011a) considered that both convergence and shear have contributions to the northwest–southeast-oriented dipole pattern of TC genesis anomaly in the WNP by examining different dynamical processes of barotropic energy conversion into synoptic-scale disturbances. Our study further suggests that the shear in large-scale zonal wind and the convergence in large-scale meridional wind play important roles in TC genesis and maintenance, which contribute to TC disturbances by the direct barotropic energy conversion, but the convergence in large-scale zonal wind and the shear in large-scale meridional wind exert limited influence on TC activity during ENSO. Figure 5 shows wind and relative vorticity anomalies at 850 hPa, as well as the anomaly of TC genesis frequency in the composite El Niño and La Niña years. It is found that the significant anomalies of TC genesis frequency are mainly located in the regions where the significant and anomalies occur. In El Niño (La Niña) years, the positive (negative) anomaly of TC genesis in the southeastern WNP is generally attributed to the shear in large-scale zonal wind due to the enhanced equatorial anomalous westerly (easterly) resulting from the warm (cold) equatorial central-eastern Pacific sea surface temperature anomaly (SSTA). On the other hand, the negative (positive) anomaly of TC genesis in the northwestern WNP is closely related to the large-scale zonal wind convergence in the western flank of the atmospheric circulation associated with the strengthening (weakening) of the WNP subtropical high. Therefore, the WNP TC genesis is strongly modulated by the anomaly of large-scale background flow in terms of barotropic energy conversion into eddy disturbances.

Fig. 4.
Fig. 4.

Composite differences between El Niño and La Niña years at 850 hPa: (a) , (b) , (c) , (d) , (e) , (f) , (g) , and (h) (10−5 m2 s−3). Light (dark) shades indicate areas where the negative (positive) difference is statistically significant at the 95% confidence level by a t test.

Citation: Monthly Weather Review 141, 4; 10.1175/MWR-D-12-00084.1

Fig. 5.
Fig. 5.

Composite wind (streamlines) and relative vorticity (shading; 10−6 s−1) anomalies at 850 hPa in (a) El Niño and (b) La Niña years; dark (light) dots mark the positive (negative) anomaly of TC genesis frequency binned into 5° × 5° grid box. Regions enclosed by solid (dashed) thick contours and crosses within dots indicate the positive (negative) relative vorticity and TC genesis frequency anomalies are statistically significant at the 95% confidence level by a t test, respectively.

Citation: Monthly Weather Review 141, 4; 10.1175/MWR-D-12-00084.1

4. Conclusions

In this study, we have investigated EKETC, KmKeTC, and its dynamical process terms using the linearized eddy kinetic energy tendency equation by separating TC vortices from the NCEP–NCAR reanalysis to identify the barotropic energy source of TC disturbances. The main conclusions are as follows. EKETC is calculated directly using the TC wind field after the separating vortices procedure. The distribution of EKETC exhibits a similar spatial pattern to the TC occurrence frequency over JAS in the seasonal mean. Moreover, ENSO exerts a stronger impact on the TC activity than on other synoptic-scale disturbances in warm and cold years. Besides, it is indicated that the El Niño has stronger influences on the WNP TC activity than La Niña does. Therefore, the TC intensity in the extreme ENSO phases is presented more objectively by EKETC than the EKE. The barotropic energy conversion plays important role in the regional discrepancy of TC genesis in the WNP during ENSO. The RMSE of anomalous KmKeTC to TC genesis (occurrence) frequency is only about 50% (70%) of KmKe’s, indicating that KmKeTC is better at depicting the anomaly of TC genesis and development in terms of barotropic energy conversion into TC disturbances, and can be considered an effective indicator in detecting the barotropic energy source of low-level cyclone over the WNP during warm and cold ENSO phases. In particular, the dynamical processes of barotropic energy conversion play different roles. The shear in large-scale zonal wind and the convergence in large-scale meridional wind provide direct barotropic energy source for TCs, but they make effects in different regions of the WNP. The significant is mainly distributed east of 150°E associated with the anomalous zonal wind induced by the equatorial central-eastern Pacific SSTA, but is localized west of 150°E closely related to the shift of the WNP subtropical high. Their combined contributions lead to the enhanced (suppressed) TC disturbances in the southeastern (northwestern) WNP in El Niño, and exert the opposite impacts on La Niña.

Acknowledgments

The authors are grateful to Dr. Patrick A. Harr and two anonymous reviewers for their constructive and useful comments. Additional editorial comments made by Dr. Patrick A. Harr are also appreciated. This work was jointly supported by the National Key Basic Research Program of China (2013CB956203) and the National Natural Science Foundation of China (41175090).

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