The Role of Multiscale Interaction in Synoptic-Scale Eddy Kinetic Energy over the Western North Pacific in Autumn

3750 J O U R N A L O F C L I M A T E VOLUME 27 The Role of Multiscale Interaction in Synoptic-Scale Eddy Kinetic Energy over the Western North Pacific in Autumn CHIH-HUA TSOU Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan HUANG-HSIUNG HSU Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan PANG-CHI HSU International Pacific Research Center, University of Hawai i at Manoa, Honolulu, Hawaii (Manuscript received 27 June 2013, in final form 23 January 2014) ABSTRACT This study formulates a synoptic-scale eddy (SSE) kinetic energy equation by partitioning the original field into seasonal mean circulation, intraseasonal oscillation (ISO), and SSEs to examine the multiscale interactions over the western North Pacific (WNP) in autumn. In addition, the relative contribution of synopticmean and synoptic-iso interactions to SSE kinetic energy was quantitatively estimated by further separating barotropic energy conversion (CK) into synoptic-mean barotropic energy conversion (CK S2M ) and synoptic- ISO barotropic energy conversion (CK S2ISO ) components. The development of tropical SSE in the lower troposphere is mainly attributed to CK associated with multiscale interactions. Mean cyclonic circulation in the lower troposphere consistently provides kinetic energy to SSEs (CK S2M. 0) during the ISO westerly and easterly phases. However, CK S2ISO during the ISO westerly and easterly phases differs considerably. During the ISO westerly phase, the enhanced ISO cyclonic flow converts energy to SSEs (CK S2ISO. 0). The magnitude of the downscale energy conversion from mean and ISO to SSEs is related to the strength of the SSEs. During the ISO westerly phase, a stronger SSE extracts more kinetic energy from mean and ISO circulation. This positive feedback between SSE-mean and SSE ISO interactions causes further strengthening of SSEs during the ISO westerly phase. By contrast, upscale energy conversion from SSEs to ISO anticyclonic flow (CK S2ISO, 0) was observed during the ISO easterly phase. The weaker SSE activity during the ISO easterly phase occurred because the mean circulation provides less energy to SSEs and, at the same time, SSEs lose energy to ISO during the ISO easterly phase. The two-way interaction between the ISO and SSEs has considerable effects on the development of tropical SSEs over the WNP in autumn. 1. Introduction Tropical synoptic-scale eddies (SSEs) are closely related to the large-scale environmental flows in which they are embedded. Numerous studies have shown the influence of a large-scale environment on the activity of SSEs and tropical cyclones (Gray 1979; McBride 1995; Harr and Elsberry 1995; Ritchie and Holland 1999). Corresponding author address: Huang-Hsiung Hsu, Research Center for Environmental Changes, Academia Sinica, 128 Academia Rd., Section 2, Taipei 11529, Taiwan. E-mail: hhhsu@gate.sinica.edu.tw The warm sea surface temperature (SST), high moisture content, and strong positive low-level vorticity and convergence associated with a monsoon trough provide a favorable environment for the growth and development of tropical storms over the western North Pacific (WNP; Gray 1979; McBride 1995; Holland 1995; Elsberry 2004). Intraseasonal oscillation (ISO) is a crucial large-scale factor in modulating SSE activity (Nakazawa 1986; Liebmann et al. 1994; Maloney and Hartmann 2001; Maloney and Dickinson 2003; Camargo et al. 2007). The genesis and track of tropical cyclones are affected by large-scale circulation and tend to occur in clusters during ISO westerly or convective phases (Nakazawa 1986; DOI: 10.1175/JCLI-D-13-00380.1 Ó 2014 American Meteorological Society

15 MAY 2014 T S O U E T A L. 3751 Camargo et al. 2007; Kim et al. 2008; Ko and Hsu 2009; Chen et al. 2009). SSEs and tropical cyclones over the WNP are considerably stronger during ISO active phases than during ISO suppressed phases (Liebmann et al. 1994; Maloney and Hartmann 2001; Maloney and Dickinson 2003; Zhou and Li 2010; Hsu et al. 2011). Barotropic energy conversion from the mean flow to SSEs may be a vital mechanism for the formation and development of tropical synoptic-scale disturbances (Lau and Lau 1992; Maloney and Dickinson 2003; Hsu et al. 2009). By partitioning the energy forms into time mean and synoptic-scale transient eddies, Lau and Lau (1992) noted that baroclinic energy conversions and barotropic energy conversions can intensify the 3 10-day transient eddies associated with tropical cyclogenesis. Maloney and Dickinson (2003) found that barotropic and baroclinic energy conversions increase during the Madden Julian oscillation (MJO) westerly phase, a state favorable for eddy growth and tropical cyclone formation. Although the occurrence and intensity of SSEs may be related to the mean and ISO circulation, the response of SSEs to the large-scale environment is not necessarily passive. Straub and Kiladis (2003) suggested that westward-propagating synoptic-scale disturbances and tropical cyclones may compose a substantial portion of the ISO signal. By comparing the differences between the original field and the field with tropical cyclones removed, Hsu et al. (2008) indicated that tropical cyclones substantially contribute to the seasonal mean, intraseasonal, and interannual variability of the 850-hPa vorticity along the tropical cyclone tracks. Hsu et al. (2009) indicated that SSEs convert kinetic energy to the seasonal mean circulation as they propagate to the subtropical and midlatitude WNP. Recent theoretical and observational studies demonstrated the importance of upscale feedback from SSEs in modulating large-scale flows, which oscillate on an intraseasonal time scale (Majda and Biello 2004; Biello and Majda 2005; Majda and Stechmann 2009; Zhou and Li 2010; Hsu and Li 2011). Previous studies have suggested that a two-way interaction occurs between large-scale flows and SSEs. However, using diagnostic tools that partition energy forms into mean and SSE only (Lau and Lau 1992; Maloney and Hartmann 2001), distinguishing between relative contribution from mean SSE interaction and ISO SSE interaction is difficult. Hsu et al. (2009) and Hsu et al. (2011) further separated the mean and transient eddies into three components: the seasonal mean (or low-frequency state with periods longer than 90 days), ISO, and synoptic eddies. Hsu et al. (2009) indicated that the seasonal mean monsoon trough and westerly jet in summer are favorable for eddy barotropic energy conversion from the seasonal mean flow to ISO and SSEs over the tropical WNP. However, the interaction between the SSEs and ISO was not addressed by Hsu et al. (2009). Conversely, Hsu et al. (2011) focused on the differences in SSE ISO interaction during the various phases of ISO. The interaction between SSEs and mean flow was not thoroughly addressed by Hsu et al. (2011); therefore, the SSE-mean flow interaction during ISO active and suppressed phases was not directly estimated. Whether the magnitude of the downscale energy conversion from mean flow to SSE during ISO active and suppressed phases differs is unclear. Although the interaction between SSEs and largescale environments in summer has recently received considerable attention, the multiscale interaction among seasonal mean, ISO, and SSEs over the WNP in autumn remains unclear. Both the mean states and ISO exhibit various characteristics when the season changes from summer to autumn. It is crucial to determine the manner in which the change in large-scale circulation influences an SSE through multiscale interaction processes in autumn, and the relative contribution from SSE-mean interaction and SSE ISO interaction to SSE kinetic energy. A quantitative diagnostic tool is required to estimate the multiscale interaction among the seasonal mean, ISO, and SSEs. Thus, an energetic diagnostic tool was developed in this study to explore these issues in detail. The remainder of this paper is organized as follows: section 2 introduces the datasets and analytical method; section 3 presents the three-dimensional SSE kinetic energy budget equations partitioned into three components: the seasonal-mean, ISO, and SSEs; section 4 provides the characteristics of large-scale circulations and SSE over the WNP in autumn; section 5 presents the multiscale interaction processes for generating and maintaining the eddy kinetic energy (EKE) of SSEs; and, finally, section 6 provides concluding remarks. 2. Data The National Centers for Environmental Prediction National Center for Atmospheric Research (NCEP NCAR) reanalysis data (Kalnay et al. 1996) from 1979 to 2007 were used to examine large-scale circulations and eddy energetics over the WNP. The horizontal resolutions of these thermodynamic and dynamic variables were 2.58 longitude by 2.58 latitude. These data were utilized at 17 pressure levels, ranging from 1000 to 10 hpa. The monthly Reynolds SST at a resolution of 28 3 28 (Reynolds and Smith 1994) was also used in this study. An orthonormal wavelet transform (Daubechies 1988) was applied to separate the transient eddies into ISO with periods of 10 90 days and SSE with periods of less than 10 days. The Daubechies wavelet transform is similar to the Haar wavelet transform that has been widely applied to the

3752 J O U R N A L O F C L I M A T E VOLUME 27 atmospheric phenomena (Torrence and Compo 1998). Both Daubechies and Haar wavelets have the capability of localizing signals both in time and frequency simultaneously. However, Daubechies wavelet transform improves the Haar wavelet transform and produces smoother scaling function and wavelets by using longer wavelets. The 6-hourly best-track data of tropical cyclones published by the Joint Typhoon Warning Center (JTWC 2008) were obtained to examine tropical storm (TS) activity over the WNP in autumn. Only tropical cyclones with a maximum wind speed exceeding 17 m s 21 (intensity of TSs) were considered in calculating TS activity. 3. Diagnostic energy equations A primitive equation (PE) form of the threedimensional EKE partitioned into three components (the seasonal mean, ISO, and SSEs) was derived to examine the SSE-mean and SSE ISO interactions over the WNP. First, we partitioned the kinetic energy into the time domain to investigate the longitudinal variation over the WNP. An arbitrary variable A can be expressed as A 5 A ~ 1 A 0 I 1 A0 S, (1) where the symbol ; represents the time mean from September to November, and 0 is the deviation (eddy) from the September November time mean. The total transient eddies were further partitioned into ISO with periods of 10 90 days and SSEs with periods of less than 10 days. The ISO and SSEs are denoted by subscripted I and S, respectively. The SSE kinetic energy (KS 0 ) can be expressed as KS 0 5 1 2 (u02 S 1 y02 S ), (2) where u and y are the zonal and meridional wind fields, respectively. To derive the SSE kinetic energy budget equation, we first multiplied the horizontal momentum equations by u 0 S and y0 S and added the two resulting equations to form the SSE kinetic energy equation. A time average over a 10-day interval was subsequently applied to the resulting equations, assuming that the inner product between SSEs and ISO mean flow approaches zero after integration over a 10-day period. The budget equations of SSE kinetic energy for an open system can be derived as K 0 S t 52V 0 S [(V0 ) 3 $ 3 ] ~ V 2 V 0 S [(V0 S 1 V0 I ) 3 $ 3 ]V0 2 R P T0 S v0 S 2 V 3 $ 3 K0 S 2 $ 3 (V0 S f0 S ) 1 D, (3) where represents the time mean over a 10-day period, t is time, V is the horizontal velocity vector, and $ is the horizontal gradient operator (the suffix 3 represents the three-dimensional components). The T is temperature, v 5 dp/dt is vertical velocity, P is pressure, R is the gas constant, f is geopotential, and D is the dissipation by frictional and subgrid-scale effects. For simplicity, the budget equations of SSE kinetic energy for an open system are written as follows: KS 0 5 CK t S2M 1 CK S2ISO 1 CE 1 BK 1 Bf 1 D, (4) where CK S2M 52V 0 S [(V0 S ) 3 $ 3 ] ~ V 52u 02 S CK S2ISO 52V 0 S [(V0 S 1 V0 I ) 3 $ 3 ]V0 I ~u x 2 ~u u0 S y0 S y 2 ~y u0 S y0 S x 2 y02 S 52u 02 u 0 I S x 2 u 0 u0 S y0 I S y 2 y 0 u0 S y0 I S x 2 y 0 y02 I S y 2 u0 S v0 S 2 y 0 S I y 0 u0 I x 2 y 0 y0 S y0 I I y 2 u 0 u0 S v0 I I p 2 y 0 y0 S v0 I I p, u 0 I ~y y 2 ~u u0 S v0 S p 2 y0 S v0 S y 0 I p 2 y0 S v0 S p 2 u0 S u0 I ~y p, u 0 I x 2 u 0 u0 S y0 I I y CE 52 R P T0 S v0 S, BK 52V 3 $ 3 K 0 S, Bf 52$ 3 (V 0 S f0 S ). and The physical mechanisms of SSE energy generation and conversion terms in Eq. (4) can be explained as follows. CK S2M represents the barotropic energy conversion between mean and SSEs. Similarly, CK S2ISO represents the barotropic energy conversion between ISO and SSEs. The sum of CK S2M and CK S2ISO has been referred to as the eddy barotropic energy conversion (CK) in previous studies (Maloney and Hartmann 2001; Hsu et al. 2009). The CE is referred to as the eddy baroclinic energy conversion from eddy available potential energy to eddy kinetic energy. This term

15 MAY 2014 T S O U E T A L. 3753 is identical to the generation of eddy kinetic energy in a closed system. For an open domain, CE can be regarded as the sum of the generation of eddy kinetic energy and Bf, which represents the boundary flux from SSE geopotential. The BK indicates the boundary flux term of SSE kinetic energy caused by the total wind fields, including mean flow and eddies. The D represents the eddy kinetic energy dissipation caused by frictional and subgrid-scale effects. In the framework of kinetic energy, scale interaction occurs only through CK, whereas CE involves only the eddy. The focus of this study is the source of SSE kinetic energy (KS 0 ) associated with SSE formation and development, particularly the scale interaction processes associated with CK S2M and CK S2ISO.ThenewKS 0 budget equation enabled us to examine the SSE-mean and SSE ISO interactions by estimating CK S2M and CK S2ISO separately. Thus, we focused on the KS 0 generation and conversion processes, including CK S2M, CK S2ISO,andCE. 4. Large-scale circulation and TS activity in autumn Although this study focused on autumn, introducing the seasonal migration from summer to autumn is valuable. Figure 1 shows the climatological large-scale circulation superimposed on the TS frequency for July August (summer) and September November (autumn) from 1979 to 2007. The large-scale circulations over the WNP undergo considerable seasonal variation. The monsoon trough in summer (Fig. 1a) is replaced by a strong cyclonic circulation over the WNP in autumn (Fig. 1d). At the southern flank of the cyclonic circulation, a low-level westerly appears and stretches to the central Pacific along 58 108N (Fig. 1d). Hence, the zonal wind convergence region between the westerly associated with the cyclonic circulation and the easterly from the subtropical high shifts southeastward from summer to autumn (Figs. 1b and 1e). The region with the warmest SSTs also shifts southeastward (Figs. 1a and 1d). With the eastward extension of a region with warm SSTs and low-level convergence, the frequency of TSs over the region from 1508E to the date line substantially increases in autumn (Figs. 1b and 1e). These results indicate that the cyclonic circulation (monsoon trough) in autumn (summer) and low-level convergence are vital factors for TS formation and development (Holland 1995; Sobel and Bretherton 1999; Kuo et al. 2001; Tam and Li 2006). In addition to the low-level cyclonic vorticity associated with low-level cyclonic circulation along 58 158N, the large-scale ascending motion associated with diabatic heating may provide favorable conditions for the formation of TSs (Figs. 1c and 1f). The primary TS tracks in summer and autumn are located to the north of the low-level cyclonic vorticity and large-scale ascending motion (Figs. 1c and 1f), because TSs tend to propagate northward after formation. It is interesting to note that the large-scale cyclonic vorticity, ascending motion, and TS formation regions extend southeastward from the tropical western Pacific toward the central Pacific in autumn (Figs. 1c and 1f). The EKE values for SSEs and ISO at 850 hpa from September to November 1979 2007 are shown in Fig. 2. Comparing the distribution of SSE kinetic energy in Fig. 2a and the TS frequency in Fig. 1e shows that the primary TS track over the WNP coincides with the local SSE kinetic energy maximum in the South China Sea and the Philippine Sea. The activity of subtropical and midlatitude SSEs is also vigorous in autumn (Fig. 2a). The source of SSE kinetic energy in midlatitudes is mainly contributed by the baroclinic energy conversion (Fig. 5), which is beyond the scope of this study. We will focus on the energy conversion processes for the growth of tropical SSEs in the WNP. The strongest ISO eddy kinetic energy is collocated with the seasonal mean cyclonic circulation at 850 hpa (Fig. 2b). The propagation of ISO is investigated by the lag-regression analysis. The ISO index used for lagged regression is defined as the temporal coefficients of the leading empirical orthogonal function (EOF) of the 10 90-day-filtered 850-hPa zonal wind fields over the domain (08 308N, 1008E 1808). Figure 3 shows the wavelet spectrum of the ISO index from September to November in 1991and 2004 to illustrate the dominant periodicity of the ISO index. Apparently, the primary signal of the ISO index for these two years is in the 30 50-day band (Figs. 3a and 3b). In addition to the 30 50-day oscillation, the signal in the 10 20-day band is strong but with smaller amplitude (Figs. 3a and 3b). The coexistence of the 30 60- and 10 20-day oscillations over the WNP is also found in summer (e.g., Lau et al. 1988; Ko and Hsu 2009). The characteristics of the wavelet spectrum for other years from 1979 to 2007 were similar to these two years with different amplitude (not shown). Figure 4 depicts the horizontal distribution of lagged regression coefficient between the ISO index and ISO vorticity at 850 hpa in autumn from lag 220 to lag 0 day approximately representing a half-cycle of the ISO index. Note that lag 0 pattern (Fig. 4c) reflects the structure of leading EOF. At lag 220 day, an ISO wave train appeared in the region from the equatorial western Pacific to the subtropical WNP and East Asia (Fig. 4a). The strongest ISO cyclonic vorticity band was centered near the equator with an anticyclonic belt on its north side (Fig. 4a). This feature bears some similarities to the

3754 J O U R N A L O F C L I M A T E VOLUME 27 FIG. 1. Distribution of (a) 850-hPa streamlines and SST (shaded, 8C), (b) 850-hPa wind fields (vector, m s 21 ) and TS frequency (shaded, numbers per 58 latitude longitude per season period), (c) 850-hPa vorticity (shaded, 10 25 s 21 ) and 400-hPa v (contour, 10 22 Pa s 21 ) for summer. (d) (f) As in (a) (c), but for autumn. ISO in summer (Hsu and Weng 2001; Tsou et al. 2005). However, some discrepancies are identified. The autumn ISO circulation cell is zonally elongated, contrasting to the northeast southwest tilt in the summer ISO. Compared to the northward northwestward propagation in summer (Hsu and Weng 2001), the ISO propagation in autumn is less evident (Figs. 4a c). During lag 220 to lag 210 days (Figs. 4a and 4b), the ISO cyclonic (anticyclonic) vorticity band moved slightly northwestward (northward). By lag 0 days (Fig. 4c), the ISO cyclonic vorticity band arrived in the South China Sea WNP and reached maximum intensity with an enhanced anticyclonic belt on its south side. The wave structure at lag 0 days is similar to the wave structure at lag 220 days, but with the opposite sign. The enhanced cyclonic vorticity band was located to the north side of the strongest ISO kinetic energy

15 MAY 2014 T S O U E T A L. 3755 FIG. 2. Distribution of eddy kinetic energy at 850 hpa for (a) synoptic-scale eddies and (b) ISO in autumn (m 22 s 22 ). (Fig. 2b) and coincided with the TS frequency maximum in autumn (Fig. 1e). The cyclonic circulation in the WNP is characterized by multiscale signals. The relationship among the mean, ISO, and SSEs in autumn over the WNP regarding SSE energetics is presented in the following section. 5. Eddy energetic results a. Eddy barotropic and baroclinic energy conversion To further explore the processes responsible for the SSE kinetic energy in autumn, the source of the SSE kinetic energy was examined. Figure 5 shows the vertically integrated eddy barotropic (CK) and baroclinic (CE) energy conversions in autumn. Both CK and CE are positive in tropical regions where the SSEs and TSs areactive(figs. 5a and 5b). These results are similar to those of previous energetic studies of summertime synoptic-scale disturbances over the WNP (Lau and Lau 1992; Maloney and Dickinson 2003; Hsu et al. 2009). These studies demonstrated that baroclinic energy conversion and barotropic energy conversion are the major energy sources of summertime synoptic-scale disturbances over the WNP. In contrast to the confinement of the large energy conversion to the west of 1508E, where TSs are active during summer (Lau and Lau 1992; Hsu et al. 2009), the positive eddy barotropic and baroclinic energy conversions in autumn extend southeastward

3756 J O U R N A L O F C L I M A T E VOLUME 27 FIG. 3. The wavelet power spectrum of the ISO index for the years (a) 1991 and (b) 2004. The ISO index is defined as the leading EOF of the 10 90-day-filtered 850-hPa zonal wind fields over the domain (08 308N, 1008E 1808). The shaded values are at the 1, 2, 5, and 10 normalized variances. Variances. 99% confidence are shown with a black line. from the Philippine Sea (1308 1508E) to 1708E (Figs. 5a and 5b). These features are consistent with the southeastward shift of SSEs (Fig. 2a) and the TS frequency (Fig. 1e) in autumn. Although positive baroclinic conversion prevails in the subtropical and extratropical WNP, negative values of barotropic conversion appear in the region north of 258N(Fig. 5a). This indicates that, when SSEs propagate northward into the subtropical region, they convert EKE to large-scale circulation in agreement with the energetic results in summer (Hsu et al. 2009). The upscale conversion of SSE kinetic energy plays a crucial role in the maintenance of large-scale circulation in the subtropical and midlatitude regions (Fig. 5a). By contrast, this upscale energy conversion process may be unfavorable for subsequent TS development in the subtropical and midlatitude regions (Fig. 5a). This is consistent with the fact that TS frequency is considerably reduced north of 258N FIG. 4. Lagged regression coefficients between the ISO index described in Fig. 3 and ISO vorticity at 850 hpa in autumn (10 27 s 21 ) at lag (a) 220, (b) 210, and (c) 0 days. Regression coefficients greater than the 95% significance level are shaded. (Fig. 1e), although CE remains positive in the region. CE may play an important role on the maintenance and late development of tropical SSEs when they propagate into the subtropical and midlatitude region. Zonal-vertical cross sections of eddy barotropic and baroclinic energy conversions averaged over 108 158N along the maximal TS frequency region are shown in

15 MAY 2014 T S O U E T A L. 3757 FIG. 5. Distribution of vertically integrated (a) eddy barotropic and (b) eddy baroclinic energy in autumn (10 21 Wm 22 ). Fig. 6. Eddy barotropic conversion has a maximal value at low levels and coincides with the enhanced low-level cyclonic circulation over 1208 1508E in autumn (Fig. 6a). In contrast to barotropic energy conversion, eddy baroclinic energy conversion increases substantially at the upper levels (Fig. 6b), which is consistent with the results of Lau and Lau (1992) and Hsu et al. (2009). The eddy latent heat release in the upper troposphere is more efficient for eddy baroclinic energy conversion. Compared to CE, which occurs in the upper troposphere and in higher latitudes, CK occurs in regions in which both SSEs and ISO are active. This collocation implies that the enhancement and eastward extension of the positive CK associated with multiscale interaction plays a crucial role in SSE growth in the lower troposphere in autumn (Fig. 6a). The relative contribution of CK from SSE-mean and SSE ISO interactions in the lower troposphere is presented in the following section. b. Synoptic mean and synoptic ISO interaction Numerous studies have indicated the dependence of synoptic eddy activity on ISO phases (Liebmann et al. 1994; Maloney and Hartmann 2001; Maloney and Dickinson 2003; Hsu et al. 2011). To diagnose the scale interaction processes in various ISO phases, the ISO phases were classified into westerly and easterly phases according to the leading empirical orthogonal function of the 10 90- day-filtered 850-hPa wind fields over the domain (08 308N, 1008E 1808). The ISO westerly (easterly) phase is

3758 J O U R N A L O F C L I M A T E VOLUME 27 FIG. 6. Vertical cross sections of (a) eddy barotropic and (b) eddy baroclinic energy conversion along 108 158N inautumn (10 25 m 2 s 23 ). defined as the PC 1 time series with a value greater (less) than one standard deviation (minus one standard deviation). For the ISO westerly and easterly phase composites, 429 and 428 days were selected, respectively. Figure 7 depicts the composite ISO circulation and SSE kinetic energy for ISO westerly and easterly phases at 850 hpa. ISO circulations in autumn are characterized by a cyclonic and anticyclonic flow during westerly and easterly phases, respectively (Figs. 7a and 7b). In the presence of the ISO cyclonic circulation, the SSE kinetic energy in the subtropical area is considerably stronger during the westerly phase than that during the easterly phase. The presence of higher SSE kinetic energy during the westerly phase has been noted by the previous studies of summertime SSE (Lau and Lau 1992; Maloney and Hartmann 2001; Hsu et al. 2011). Figure 8 depicts zonal-vertical cross sections of eddy barotropic and baroclinic energy conversions averaged over 108 158N along the maximal TS frequency region for ISO westerly and easterly phases. Similar to the climatological pattern shown in Fig. 6, the change of SSE kinetic energy at low levels was mainly contributed by CK (Figs. 8a and 8c), while CE is the major energy source of SSE kinetic energy at the upper levels (Fig. 8b and 8d) during ISO westerly and easterly phases. However, both CK and CE are substantially larger during ISO westerly phase than easterly phase. Compared to the easterly phase, the presence of higher CK associated with multiscale interaction at low levels is more favorable for the SSE growth during the ISO westerly phase. Consistent with the enhancement of SSE kinetic energy, CK is stronger along 58 158N during the westerly phase (Figs. 9a and 9d). This implies that the combined SSE-mean and SSE ISO interactions convert more eddy kinetic energy to SSEs during the ISO westerly phase. This feature was also observed in previous studies on tropical SSE kinetic energy in summer (Maloney and Hartmann 2001; Hsu et al. 2011). The partitioning of CK into CK S2M and CK S2ISO enabled us to further investigate the relative contribution of synoptic mean and synoptic ISO scale interactions to SSE kinetic energy. Figures 9b and 9e displays the CK S2M during the ISO westerly and easterly phases. The maximum CK S2M conversion occurs to the north of the convergence regions where the maximum meridional shear of the mean wind exists. The distributions during both phases are similar but with larger amplitude during the westerly phase (Figs. 9b and 9e). This implies that, when a tropical SSE grows during the westerly phase, it extracts more kinetic energy from the seasonal mean cyclonic circulation. Similar to the CK S2M, the maximum CK S2ISO conversion appears to the north of the ISO convergence regions where the maximum meridional shear of the ISO wind occurs during the ISO westerly phase (Fig. 9c). SSE gains kinetic energy from the SSE-mean and SSE ISO interactions over the tropical WNP, where the mean and ISO cyclonic circulation occur during the ISO westerly phase (Figs. 9b and 9c). Tropical SSEs are enhanced during the westerly phase in autumn. Both CK S2M and CK S2ISO occur in the region of meridional easterly shear, where SSEs can extract kinetic energy from the background flow. The reasons for this occurrence are shown in Fig. 10. During the easterly phase, a SSE still extracts kinetic energy from the climatological mean flow but with weaker magnitude than in the westerly phase. Because the mean flow remains the same, the weaker conversion is attributed to weaker eddies in the easterly phase. By

15 MAY 2014 T S O U E T A L. 3759 FIG. 7. Distribution of synoptic-scale eddy kinetic energy (shaded, J m 22 ) and ISO winds (vector, m s 21 ) at 850 hpa during the ISO (a) westerly and (b) easterly phase. contrast, SSEs provide kinetic energy for ISO over the tropical WNP during the ISO easterly phase, which is characterized by an ISO anticyclonic flow (Fig. 9f). This indicates that upscale energy conversion from SSEs to ISO occurs during the ISO easterly phase. In general, the magnitude of upscale energy conversion is smaller than the downscale energy conversion. This upscale feature was therefore masked in CK. Similarly, upscale energy conversion may be masked in previous energetic studies focusing in summer because CK, which represents the combined effect of CK S2M and CK S2ISO, was examined (Lau and Lau 1992; Maloney and Hartmann 2001). A two-way interaction between the ISO and SSE occurs over the tropical WNP in autumn. This feature is not revealed in previous energetic studies in summer since the separation of CK into CK S2M and CK S2ISO is not performed in previous energetic studies (Maloney and Hartmann 2001; Hsu et al. 2009). On the other hand, Hsu et al. (2011) explored this issue based on different approach and suggested the upscale feedback from the synoptic scale to the ISO in summer. During the ISO westerly phase, ISO provides energy to the development of SSEs, whereas SSEs lose energy to ISO during the ISO easterly phase. Conversely, the total conversion remains positive in the active SSE region because of the larger CK S2M. The cancellation between CK S2M and CK S2ISO partially explains the smaller conversion rate in the easterly phase. Another interesting point to be mentioned is that tropical SSE may lose energy to the mean and ISO circulation during the ISO westerly and easterly phases as they propagate northward to the subtropical North Pacific. This indicates that the tropical SSE plays a crucial role in maintaining the mean and ISO circulation over the subtropical North Pacific. The maintenance and late development of tropical SSEs is mainly contributed by CE when they propagate into the subtropical and midlatitude region. To examine the relative importance of the eddy momentum transport associated with eddy barotropic energy conversion, each term of CK S2M and CK S2ISO was examined. Figures 10 and 11 show the spatial distributions of the largest four terms of CK S2M and CK S2ISO at 850 hpa during the ISO westerly and easterly phases.

3760 J O U R N A L O F C L I M A T E VOLUME 27 FIG. 8. Vertical cross sections of (a) eddy barotropic and (b) eddy baroclinic energy conversion along 108 158N in autumn for the ISO westerly phase. (c),(d) As in (a),(b), but for the easterly phase. (Unit is 10 25 m 2 s 23 ). The other terms are not shown in Figs. 10 and 11 because of the considerably smaller amplitudes. The positive CK S2M over the tropical WNP is mainly attributed to the term 2u 0 S y0 S ( ~u/ y) during the ISO westerly and easterly phases (Figs. 10a,b,e,f). The 2u 02 S ( ~u/ x) term is also positive with comparable magnitude. This term is related to the strong zonal mean wind convergence ( ~u/ x, 0) located between 1408 and 1708E, where the westerly from cyclonic circulation and the easterly from subtropical high converge (Figs. 10a and 10e). However, in autumn, 2uS 02( ~u/ x) is largely compensated by 2y02 S ( ~y/ y) (Figs. 10d and 10h). The direct comparison of these results with the previous energetic studies in summer is not available, since CK instead of CK S2M is examined in previous energetic studies (Maloney and Hartmann 2001). It is interesting to note that the physical processes in summer are also dominated by these two terms and 2u 02 ( u/ x), which have the same mathematical expression as those in CK S2M except replacing the seasonal mean winds with the large-scale flow (Lau and Lau 1992; Maloney and Hartmann 2001). The momentum transport (u 0 S y0 S ) of synoptic eddies is positive because SSEs over the WNP are characterized by a northeast southwest-tilted wave train (not shown). The sign of 2u 0 S y0 S ( ~u/ y) is determined by the meridional shear of mean circulation. The positive CK S2M along the TS track in the lower troposphere is induced by the cyclonic shear ( ~u/ y, 0) associated with the strengthened mean cyclonic circulation during the ISO westerly and easterly phases (Figs. 10b and 10f). However, the

15 MAY 2014 T S O U E T A L. 3761 FIG. 9. Composites of eddy barotropic energy conversion (shaded, 10 25 m 2 s 23 ) and winds (vector, m s 21 ) at 850 hpa for (a) CK and total winds, (b) CK S2M and seasonal mean winds, and (c) CK S2ISO and ISO winds during the ISO westerly phase. (d) (f) As in (a) (c), but for the easterly phase. magnitude of these two downscale energy conversion processes is also related to the strength of SSEs. During the ISO westerly (easterly) phase, a strong (weak) SSE with larger (smaller) eddy kinetic energy (u 02 S ) and stronger momentum flux (u 0 S y0 S ) along the TS track extracts more (less) kinetic energy from the mean circulation during the ISO westerly (easterly) phase. Similarly, the positive CK S2ISO over the tropical WNP during the ISO westerly phase is mainly attributed to the terms 2uS 02 ( u0 I / x) and 2u0 S y0 S ( u0 I / x) (Figs. 11a and 11b). The first term is related to the strong zonal mean wind convergence ( u 0 I / x, 0) located at the exit region of the ISO westerly jet (Fig. 11a). However, this term is largely compensated by 2y 02 S ( y0 I / y) in autumn (Fig. 11d). Thus, CK S2ISO is dominated by the second term, which is induced by the cyclonic shear ( u 0 I / x, 0) associated with the strengthened cyclonic circulation and momentum transport (u 0 S y0 S ) of synoptic eddies during the westerly phase (Fig. 11b). By contrast, both terms are negative over the tropical WNP during the ISO easterly phase, which is characterized by an ISO anticyclonic circulation (Figs. 11e and 11f). Instead of gaining energy from ISO anticyclonic circulation, SSEs provide energy to maintain the ISO anticyclonic circulation during the ISO easterly phase. The enhancement of SSE increases the magnitude of SSE kinetic energy (u 02 S ) and SSE momentum transport (u 0 S y0 S ). These conditions are favorable for SSEs to extract more EKE from mean and ISO cyclonic circulation during the ISO westerly phase. The positive feedback between synoptic-mean and synoptic-iso cyclonic circulation interactions provides a favorable environment for the growth of SSEs during the ISO westerly phase. SSEs become stronger during the ISO westerly phase. Conversely, SSEs gain less eddy kinetic energy from mean circulation during the ISO easterly phase. In addition, SSEs convert eddy kinetic energy to ISO during the ISO easterly phase. Therefore, SSEs develop slowly

3762 J O U R N A L O F C L I M A T E VOLUME 27 FIG. 10. Individual components (a) 2u 02 S ( ~u/ x), (b) 2u0 S y0 S ( ~u/ y), (c) 2u0 S y0 S ( ~y/ x), and (d) 2y02 S ( ~y/ y) ofck S2M (shaded, 10 25 m 2 s 23 ) and seasonal mean winds (vector, m s 21 ) at 850 hpa during the ISO westerly phase. (e) (h) As in (a) (d), but for the ISO easterly phase. and become weaker during the ISO easterly phase because of this upscale energy conversion. 6. Summary and discussion The large-scale circulations over the WNP undergo considerable seasonal variations from summer to autumn. The monsoon trough over the WNP in summer is replaced by a strong cyclonic circulation in autumn. The multiscale signals, including seasonal mean, ISO, and SSEs are substantial near the mean cyclonic circulation over the WNP in autumn. A SSE kinetic energy diagnostic equation was derived to examine the effects of multiscale interactions on the SSE kinetic energy. Instead of separating the original fields into only mean and SSEs (Lau and Lau 1992; Maloney and Hartmann 2001), the original fields in the SSE kinetic energy equation were partitioned into three components: the seasonal mean, ISO, and SSE. This approach enabled us

15 MAY 2014 T S O U E T A L. 3763 FIG. 11. Major components (a) 2u 02 S ( u0 I / x), (b) 2u0 S y0 S ( u0 I / y), (c) 2u0 S y0 S ( y0 I / x), and (d) 2y02 S ( y0 I / y) ofck S2ISO (shaded, 10 25 m 2 s 23 ) and ISO winds (vector, m s 21 ) at 850 hpa during the ISO westerly phase. (e) (h) As in (a) (d), but for the easterly phase. to quantify the relative contribution of SSE-mean and SSE ISO interactions to SSE growth or decay by further separating the barotropic energy conversion (CK) into synoptic-mean barotropic energy conversion (CK S2M ) and synoptic-iso barotropic energy conversion (CK S2ISO ) components. The mean cyclonic circulation and warm SSTs over the WNP establish a favorable environment for CK in the lower troposphere and CE in the upper troposphere from the Philippine Sea (1308 1508E) to the date line in autumn. The growth of tropical SSE in the lower troposphere is primarily attributed to CK associated with the scale interaction between large-scale environments and SSE, whereas the development in the upper troposphere is mainly attributed to CE. These results are similar to those of previous energetic studies on summertime synopticscale disturbances over the WNP (Lau and Lau 1992; Maloney and Dickinson 2003; Hsu et al. 2009). In autumn, positive CK and CE extend from the Philippine Sea (1308 1508E) to 1708E. This explains the substantial increase in SSE and TS activity over the region from 1508E to the date line in autumn.

3764 J O U R N A L O F C L I M A T E VOLUME 27 FIG. 12. Schematic diagram illustrates the barotropic energy conversions between synoptic-scale eddy and mean flow (CK S2M ) and between synoptic-scale eddy and ISO (CK S2ISO ) during the ISO (a) westerly and (b) easterly phase. Gray arrows denote the mean flow in autumn. Orange (blue) streamlines indicate cyclonic (anticyclonic) ISO circulations. Green ovals with solid (dashed) lines represent the cyclonic (anticyclonic) eddy flows. The novel SSE kinetic energy diagnostic tool enabled us to further examine the multiscale interaction and quantify the relative contribution of SSE-mean and SSE ISO interactions by further separating CK into CK S2M and CK S2ISO components. Figure 12 illustrates the relative contribution of CK S2M and CK S2ISO to the SSE kinetic energy and the characteristics of mean circulation, ISO, and SSE over the WNP during the ISO westerly (Fig. 12a) and easterly (Fig. 12b) phases. The CK S2M is mainly attributed to the mean (ISO) wind shear term 2u 0 S y0 S ( ~u/ y) because of the cancellation between two large terms: 2u 02 S ( ~u/ x) and2y02 S ( ~y/ y). Tropical SSEs are stronger during the ISO westerly phase than during the ISO easterly phase, and are characterized by a northeast southwest-tilted eddy during the ISO westerly and easterly phases (Fig. 12). Such a SSE structure (u 0 S y0 S. 0) associated with the mean cyclonic circulations and zonal wind convergence in autumn are favorable for synoptic-mean barotropic energy conversion (CK S2M. 0) during ISO westerly and easterly phases (Fig. 12). Conversely, SSEs decay in the region of anticyclonic meridional shear. Thus, mean circulation provides (receives) eddy kinetic energy to (from) SSEs in 58 208N (208 308N) during both ISO phases. Positive energy conversion is larger during the ISO westerly phase because of larger eddy amplitude and momentum flux. However, CK S2ISO differs substantially during the ISO westerly and easterly phases (Fig. 12). During the ISO westerly phase (Fig. 12a), the enhanced ISO cyclonic flow is beneficial for the downscale energy conversion from ISO to SSE (CK S2ISO. 0). This is particularly evident in the southern part of the ISO cyclonic circulation, where cyclonic meridional shear occurs. Thus, both mean and ISO flow provide eddy kinetic energy to the growth of SSEs (CK S2M and CK S2ISO. 0). When a SSE develops, the SSE momentum transport (u 0 S y0 S ) increases and extracts more kinetic energy from mean and ISO circulation. This positive feedback between SSE-mean and SSE ISO interactions during the ISO westerly phase causes further CK S2M and CK S2ISO energy conversion. Therefore, tropical SSEs are substantially enhanced during the ISO westerly phase. In contrast to the positive feedback described above, SSEs lose eddy kinetic energy to ISO anticyclonic circulation (CK S2ISO, 0) between 108 and 258N during the ISO easterly phase (Fig. 12b). This indicates that upscale energy conversion from SSEs to ISO occurs in the region in which SSEs are most active during the ISO easterly phase. In general, the upscale energy conversion from SSEs to ISO (CK S2ISO, 0) is smaller than the downscale energy conversion from mean to SSEs (CK S2M. 0). Total kinetic energy conversion remains positive during the ISO easterly phase; however, it is weaker than that in the ISO westerly phase. Therefore, SSE grows slowly. This upscale energy conversion process may have been overlooked in previous energetic studies in summer (Lau and Lau 1992; Maloney and Hartmann 2001), because CK, which represents the combined effect of CK S2M and CK S2ISO, is always positive. Thus, based on the previous energetic viewpoint, the considerable reduction in SSE activity during the ISO easterly phase is primarily attributed to the unfavorable large-scale environments, including mean and ISO circulations for downscale energy conversion. From the novel energetic viewpoint presented in this study, SSEs grow slowly during the ISO easterly phase because mean circulation converts less energy to SSEs and, at the same time, SSEs lose energy to ISO. The response of tropical SSEs to large-scale environments is not passive. Recent theoretical and observational studies have shown the importance of upscale feedback from SSEs in modulating large-scale flows, which oscillate on an intraseasonal time scale in summer. This study demonstrated that two-way interaction between the ISO and SSEs plays a crucial role in the activity of SSEs over the tropical WNP in autumn. The two-way interaction between ISO and SSEs causes

15 MAY 2014 T S O U E T A L. 3765 difficulty in simulating and predicting the intensity of SSEs, including predicting TSs over the tropical WNP through the current medium-resolution climate models. The novel diagnostic approach and results can also be used to evaluate the capability of a high-resolution model in simulating the multiscale interaction in the tropical WNP. It has been recognized that ENSO has a strong impact on the interannual variability of the activity of SSEs and TS by modulation of the large-scale environments where they are embedded (e.g., Chia and Ropelewski 2002; Wang and Chan 2002; Seiki and Takayabu 2007). Recently modeling and energetic studies by Lau and Nath (2006) and Hsu et al. (2009) showed that ENSO events could also influence the strength of ISO by modulating the seasonal circulations in which the ISO are embedded. 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