1. Introduction
The western North Pacific (WNP) has the highest tropical cyclone (TC) activity in the world. This region exhibits considerable interannual and interdecadal fluctuations (Chia and Ropelewski 2002; Wang and Chan 2002; Camargo and Sobel 2005; Chan 2005). The interdecadal decrease in the annual mean number of TCs since the late 1990s has been the most remarkable interdecadal variation (Liu and Chan 2013; Hsu et al. 2014; He et al. 2015). This interdecadal decrease was attributed to increases in the large-scale vertical shear and geopotential height at 500 hPa (Liu and Chan 2013); these increases were caused by a basin-scale climate regime shift in the Pacific in the mid- to late 1990s (Hong et al. 2016). Although the annual average number of TCs has decreased since the late 1990s, the ratio of the number of major TCs (MTCs) to the total number of TCs (referred to as the MTC ratio) during the typhoon season (June to November) increased substantially. Moreover, the time required for tropical depression to develop into an MTC has decreased due to changes in the interaction between intraseasonal oscillation (ISO) and TCs (Hong et al. 2018).
TC activity in the WNP region is substantially modulated by the 30–60-day MJO (Li and Zhou 2013). The convective MJO phase is associated with the strengthening of the monsoon trough, whereas the nonconvective MJO phase is associated with its weakening. That is, the northeastward-propagating MJO determines the basinwide TC frequency, and the TC frequency increases and decreases in the MJO convective and nonconvective phases, respectively. In addition, the frequencies of TCs and MTCs are significantly correlated with the convective MJO phase. The associated active convection and high ocean heat content create an environment conducive to the occurrence of a category 5 TC (Weng and Hsu 2017). Moreover, the TC frequency and movement in the WNP region are substantially modified by 10–20-day ISO (Ko and Hsu 2009; Li and Zhou 2013; Hong et al. 2018). The 10–20-day ISO is characterized by a wavelike southwest–northwest tilting pattern, which is generally initiated in the Philippine Sea and propagated northwestward to the East China Sea. In contrast to MJO that exerts a basinwide effect on the TC frequency, the 10–20-day ISO modulates the TC in an opposite manner in western and eastern WNP regions, resulting in a northwestward shift in TC genesis locations (Li and Zhou 2013).
The TC–ISO interaction substantially modulates the interdecadal variation in TC activity in the WNP region. An abrupt decrease in the TC–ISO relationship is one of the main factors causing the interdecadal decrease in the TC frequency in the WNP region since the late 1990s (Hsu et al. 2017). This interdecadal decrease in the TC frequency primarily occurred in the eastern flank of the WNP region. By contrast, the frequency and occurrence of TCs in the western flank of the WNP region moderately increased due to changes in the mega–La Niña–like environment (Hong et al. 2016). The mega–La Niña–like condition enhanced and modulated the WNP subtropical high to shift westward, which in turn caused the TC genesis location and ISO activity to shift westward and shrink to a smaller region. The resulting increase in the TC–ISO interaction intensified TCs, leading to the development of MTCs (Hong et al. 2018).
Although several studies have examined the effect of the TC–ISO interaction on the development of MTCs, the seasonality of the TC–ISO interaction and its associated influences have not been fully investigated. The mean TC genesis location is closely connected to the annual cycle, with locations moving northward along with the northward migration of the WNP subtropical high in June, July, and August (JJA) and moving southward with the migration of the WNP subtropical high in September, October, and November (SON). In addition, the features of ISOs are closely associated with the seasonal cycle. The center of ISOs activity move northward from the equator in JJA and return to the equator in SON. In this study, we observed that the magnitude and propagation of ISOs in the WNP region have demonstrated strong difference between JJA and SON since the late 1990s. A possible effect of the seasonal dependence of ISO–TC interactions on the rate of TCs developing into MTCs are discussed.
2. Data and methodology
The observational data used in this study included 1) the daily atmospheric fields (2.5° × 2.5°) retrieved from the NCEP and from the National Center for Atmospheric Research Reanalysis I (Kalnay et al. 1996), 2) interpolated daily data for OLR (Liebmann and Smith 1996), which were used to analyze the tropical ISO activity during the typhoon season, and 3) the daily high-resolution data (0.5° × 0.5°) provided by the NCEP CFSR (Saha et al. 2010) and NCEP CFSv2 (Saha et al. 2014), which were used to calculate the eddy kinetic energy (EKE) budget.
The real-time multivariate index obtained from the Australian Bureau of Meteorology was used to define the amplitude and phase of the MJO (Wheeler and Hendon 2004). In addition, the boreal summer ISO (BSISO) index was used to identify the northward-propagating ISO in the WNP region during the boreal summer (Lee et al. 2013). The BSISO1 was determined by the EOFs 1 and 2, and the BSISO2 was determined by EOFs 3 and 4, respectively. TC data were obtained from the JTWC (available at https://www.metoc.navy.mil/jtwc/jtwc.html?best-tracks) and the Regional Specialized Meteorological Center (RSMC) Tokyo–Typhoon Center (https://www.jma.go.jp/jma/jma-eng/jma-center/rsmc-hp-pub-eg/besttrack.html). The TCs with genesis location over the WNP region (0°–40°N, 100°E–180°) was selected for the analysis. The TC genesis location is defined as location in where the maximum sustained wind reaches 25 kt (1 kt ≈ 0.51 m s−1). The Saffir–Simpson hurricane wind scale (Simpson 1974) was used to categorize the strength of TCs. According to this scale, MTCs are tropical storms with a maximum sustained wind speed of ≥96 kt (≥category 3). TC activity was compared between JJA and SON to investigate the possible seasonality. A regime shift index (RSI) (Rodionov 2004) was used to determine the specific time at which an abrupt change occurred. The cutoff length was set at 14 years during the period 1950–2020, and only significant RSI values (p < 0.05) were identified as a changepoint. The developing stage of MTC was defined as the period the maximum sustained wind speed of TC from 34 kt (category 0) to 96 kt (category 3).
3. Difference of the MTC ratio between JJA and SON
Figure 1 presents a comparison of TC activity in the WNP region between JJA and SON. The 9-yr running-mean annual number of TCs in JJA and SON indicated a gradual increase in the TC genesis number since the early 1980s. The number of TCs in JJA and SON peaked in the early 1990s and decreased thereafter (Fig. 1a). The decrease since the 1990s is consistent with the abrupt decrease in the mean annual number of TCs that occurred in the late 1990s in the WNP region (He et al. 2015; Hsu et al. 2014; Liu and Chan 2013). The time series of the number of MTCs in JJA exhibited an evident distinction compared with that in SON since the early 2000s: The number of MTC in JJA decreased, conversely, it increased in SON. This distinction is also clearly reflected in the MTC ratio (Fig. 1c). The 21-yr sliding correlation coefficient of the MTC ratio between JJA and SON revealed that both was positive correlated during 1970–90. However, the correlation coefficient dropped dramatically since 1990 and turned to negative since 2000 (red line in Fig. 1c). The termination was consistent with the occurrence of changepoint of MTC ratio in SON during the early 2000s. The aforementioned account for the difference of the MTC between JJA and SON. Furthermore, the climate regime index revealed that the MTC ratio abruptly increased considerably in 2002 SON. A sensitive test of cutoff length (e.g., 10, 14, and 15 years) indicated that the changepoint in the early 2000s was robust. The robustness of the changepoint in 2002 was further confirmed by performing a sliding Student’s t test with a 10-yr window (data not shown). Figure 1 was replotted using other TC datasets from RSMC’s track data. The result obtained by replotting RSMC’s track data was consistent with that obtained using JTWC’s data (not shown), corroborating the robust difference of the MTC ratio between JJA and SON in the early 2000s. On the basis of the changepoint, we separated the period into two subperiods: the period one (P1; 1989–2002) and the period two (P2; 2003–16). Table 1 presents a comparison of the number of MTCs between P1 and P2. The MTC ratio in JJA was approximately 30% during the entire period. Overall, the MTC ratio was larger (∼40%) in SON than in JJA. The MTC ratio in SON increased from 38% to 49% from P1 to P2. Moreover, the MTC ratio slightly increased in JJA from P1 to P2, from 28% to 32%; however, the change was not significant. The difference of MTC ratio between JJA and SON was a notable phenomenon: more MTCs occurred in SON in the WNP region in P2 each year (4.9 yr−1) compared with P1 (4.6 yr−1), although the mean annual number of TCs in P2 (9.9 yr−1) was considerably smaller than that in P1 (12 yr−1).
(a) Time series of interannual variation (solid line) and 9-yr running mean (dashed line) of TC number in JJA (black line) and SON (blue line) in the WNP. (b),(c) As in (a), but for the major TC (MTC; ≥category 3), and the ratio of MTC number to total TC number (MTC ratio). The red line in (c) indicates the 21-yr sliding correlation between MTC ratio in JJA and SON. The bar at the bottom of (c) yields the year of MTC ratio in SON experienced a significant about change approximately in 2002. The regime shift index (Rodionov 2004) was used in identifying the changepoint. The JTWC best track data were used for the calculation. Both the values of red line in (b) and bars in (c) correspond to the axis of right side. The black and red y axes in the right side of (c) present the value of values of RSI and 21-yr sliding correlation, respectively.
Citation: Journal of Climate 36, 13; 10.1175/JCLI-D-22-0657.1
Average number of TC and MTC (values in parentheses), and the ratio of TC number to MTC number (MTC ratio) during P1 (1989–2002) and P2 (2003–16) in JJA and SON. The unit of average TC and MTC is number per year.
Figure 2 presents a comparison of genesis and track density (occurrence frequency and shading) for MTCs in JJA and SON between P1 and P2. The MTC genesis location exhibited northwestward movement from P1 to P2 in either JJA or SON. The northwestward movement was especially evident in SON: the mean MTC location shifted from 11.3°N, 156.9°E to 13.0°N, 145.8°E. The northwest shift primarily resulted from the westward extension of the WNP subtropical high since the late 1990s, which was associated with a mega–La Niña–like background state change that was accompanied with a negative PDO phase (Liu and Chan 2008; Matsumura and Horinouchi 2016; Zhao et al. 2018; Hong et al. 2018; Kim et al. 2020). Whereas the mean TC genesis location shifted southeastern (northwestern) relative to the climatology during the El Niño (La Niña) year (Chia and Ropelewski 2002; Wang and Chan 2002; Camargo et al. 2007), ENSO’s influence focused on the interannual time scale and its impact on the regime shift was not evident. Briefly, the PDO phase associated cold SST in the equatorial eastern Pacific forced a Gill-type response of anticyclone anomaly in the WNP, which strengthened the WNP subtropical high. The enhancement of WNP subtropical high suppressed the TC activity and modified the TC’s genesis location to shift northwestward. Consequently, The westward shift of the mean MTC location led to more TC tracks passing through the western flank of WNP; conversely, fewer TC tracks passed through the eastern flank, in P2, causing more landfalls in the Philippines and Taiwan since the late 1990s (Fig. 2); this result is consistent with those reported by Yu et al. (2015). The shading shows the track occurrence for the full life cycle of MTCs. The westward shift of the MTC’s track in P2 was clearly observed in SON (Fig. 2f) but was not evident in JJA (Fig. 2c). If we separate the entire life cycle of MTCs to developing stage and decaying stage, it revealed that the difference between JJA and SON primarily resulted from the developing stage, and the contribution of decaying stage was relatively weak (Fig. 3). Additionally, Fig. 2 was replotted by replacing major TCs with moderate TCs (<category 3). It revealed that the seasonal dependence was not observed in the moderate TCs (not shown).
Spatial distribution of MTC track density (shading) and genesis locations (dots) in JJA during (a) P1 (1989–2002), (b) P2 (2003–16), and (c) the difference of MTC track density between P1 and P2 (P2 minus P1). The definition of P1 and P2 is based on the changepoint in 2002 (Fig. 1c). (d)–(f) As in (a)–(c), but for SON. The plus sign indicates the mean location of MTC’s birth place and the coordinates in the upper-right corner presents the corresponding longitude and latitude.
Citation: Journal of Climate 36, 13; 10.1175/JCLI-D-22-0657.1
(a),(c) As in Figs. 2c and 2f, but for the difference of TC track density between P1 and P2 for the developing stage (days during category 0–3) of MTCs. (b),(d) As in (a) and (c), but for the decaying stage (days after the peak of maximum sustained winds).
Citation: Journal of Climate 36, 13; 10.1175/JCLI-D-22-0657.1
4. Seasonal dependence of TC–ISO relationship
a. ISO and associated SST
TC activity in the WNP region is closely related to the ISO phase (Li and Zhou 2013; Weng and Hsu 2017). The TC–ISO relationship between JJA and SON was compared to investigate whether the increase of MTC ratio in SON is correlated with ISO. Here, ISO was presented using 10–60-day filtered OLR. Because the 10–60-day ISO in the WNP region during the boreal summer was determined using 10–20- and 30–60-day ISOs (Kikuchi and Wang 2009; Lee et al. 2013; Li and Zhou 2013), we separated the ISOs into 30–60- and 10–20-day ISOs, referred to as BSISO1 and BSISO2, respectively (Lee et al. 2013). Subsequently, the composite of 30–60- and 10–20-day filtered OLR during the developing stage of each MTC was calculated. Figure 4 presents a comparison of 30–60-day filtered OLR between P1 and P2 in JJA and SON, respectively.
Composite map of 30–60-day filtered OLR (shading) and 850-hPa wind (vectors) during the developing stage of MTCs in JJA during (a) P1 (1989–2002), (b) P2 (2003–16), and (c) the difference between P1 and P2 (P2 minus P1). (d)–(f) As in (a)–(c), but for the SON. The closed purple circles in (a), (b), (d), and (e) indicates the genesis location of MTC. The red solid (hollow) circles indicate the positive (negative) differences of MTC track density between P1 and P2 (P2 minus P1). The marker size indicates the magnitude of difference. The shading in (c) and (f) indicates that the difference between P1 and P2 exceeds the 95% confidence level based on the Wilcoxon signed-ranks test. Only wind speeds larger than 0.1 m s−1 are plotted in (a), (b), (d), and (e).
Citation: Journal of Climate 36, 13; 10.1175/JCLI-D-22-0657.1
During JJA, the spatial distribution of the major negative 30–60-day OLR anomaly exhibited a zonal structure along the axis of the monsoon trough (∼10°N), extending from the South China Sea (100°E) to western Pacific (∼160°E) in P1. The negative OLR anomaly, accompanied with a cyclonic circulation anomaly, enhanced and shifted eastward with slight northward extend in P2 (comparing Fig. 4b with Fig. 4a). Figures 4a and 4b demonstrate that the MTC (the purple circle) primarily formed in the negative OLR anomaly that was embedded in a large-scale cyclonic circulation anomaly. The difference in the OLR anomaly between P1 and P2 revealed a west–east dipole in the WNP region [i.e., a negative (positive) anomaly in east (west) at 130°E]. This dipole structure resulted from the eastward shift of the negative OLR anomaly from P1 to P2. The pattern of the 30–60-day OLR during P1 during SON resembled that during JJA except that the center of the negative OLR anomaly shifted northward (from 10° to 15°N) and eastward (approximately from 135° to 145°E; comparing Fig. 4d with Fig. 4a). In contrast to JJA, the negative OLR anomaly in SON shrunk and shifted westward approximately from 145° to 130°E in P2 (comparing Fig. 4e with Fig. 4d). In addition, the difference in OLR between P1 and P2 yielded an opposite dipole-like pattern compared with JJA (comparing Fig. 4c with Fig. 4f). Overall, as presented in Fig. 4, the negative OLR anomaly was well coherent with the MTC genesis location and MTC’s pathway during JJA and SON. However, the westward shift and shrinking of the negative OLR anomaly during SON in P2 limited the TC genesis and track in a narrow region, a feature not observed during JJA (comparing Fig. 4f with Fig. 4c). The results of 10–20-day OLR were similar to those of 30–60-day OLR (comparing Fig. 5 with Fig. 4), except that the increase in track density during SON in P2 was more coherent with the negative OLR anomaly. Figures 4 and 5 indicate that the increase in the MTC track in P2 was well coupled with the increase in the negative OLR. The spatial pattern of the difference in the 30–60- and 10–20-day OLR between P1 and P2 revealed an approximated opposite structure for JJA and SON. That is, we observed a negative (positive) OLR anomaly in the Philippine Sea for SON (JJA) but a positive OLR anomaly in the east of 140°E for SON (JJA).
As in Fig. 4, but for 10–20-day filtered OLR (shading) and 850-hPa wind (vectors).
Citation: Journal of Climate 36, 13; 10.1175/JCLI-D-22-0657.1
Although a warm SST is favorable for TC formation, the TC generated upwelling by the air–sea interaction tends to cool the SST, thus resulting in a negative feedback for TC development (Lin et al. 2003, 2008). Figure 6 is identical to Fig. 4 except for the difference in the 30–60- and 10–20-day filtered SST between P1 and P2. The increase in the track density for the MTC primarily occurred where the 30–60-day filtered SST anomaly was positive (Figs. 6a,c). This phenomenon was especially evident during SON: the increase of track density shrunk in the western WNP where a positive SST anomaly occurred, but the increase of TC track spread to the whole WNP region and was not well coupled with SSTA during JJA. Figure 6c indicates that the major track of a MTC was along the positive SST anomaly, a similar argument reported by Weng and Hsu (2017). By contrast, the change of MTC track density was less coupled with the 10–20-day filtered SST compared with 30–60-day filtered SST in SON (comparing Fig. 6c with Fig. 6d). Whereas the change of MTC track was coupled with 10–20-day filtered SST in JJA, the distinction between 10–20- and 30–60-day filtered SST was not as evident as seen in SON (Figs. 6a,b). Because the abrupt increase of MTC ratio in the early 2000s was only identified in SON, we focused our analysis in the 30–60-day filtered SST. A possible cause is the fast propagation of the 10–20-day ISO, resulting in the ocean not having adequate time to respond to the change in ISO-associated atmospheric circulation. We noted that the SST anomaly might be disturbed by the air–sea interaction. The composite of the upper ocean heat content (OHT) was examined instead of SST. The results of SST and OHT yielded similar results (not shown). No substantial difference was observed, indicating that the difference of large-scale thermodynamics presented in Fig. 6 was robust.
(a),(c) As in Figs. 4c and 4f, but for 30–60-day filtered SST (shading). (b),(d) As in (a) and (c), but for 10–20-day filtered SST.
Citation: Journal of Climate 36, 13; 10.1175/JCLI-D-22-0657.1
b. Change in ISO propagation and its effect on the TC–ISO relationship
Because we focused on the development of TCs into MTCs, we investigated the MTC–ISO relationship during the period from category 0–3. Overall, the statistics from category 0–3 were similar to those from genesis to category 3. Because the 10–20-day ISO-associated anomalous OLR and SST exerted an opposite effect on TC development (Figs. 5 and 6), we examined the TC–ISO relationship during 30 to 60 days. Figure 7 presents a comparison of the corresponding 30–60-day ISO (BSISO1) for each MTC between P1 and P2. Here, only ISOs with magnitudes larger than one were included. A significant difference between P1 and P2 for BSISO1 was noted during SON. No significant change was observed during JJA. That is, the percentage of MTCs significantly increased in phases 5 and 6 in SON during P2. It increased from 4.8% to 19.7% and from 5.1% to 12.8% for phase 5 and phase 6, respectively. Notably, the spatial patterns of the phases 5 and 6 of BSISO1, referring to the wet or convective phase, exhibited a negative OLR anomaly in the WNP region with a pattern similar to the difference in the composite 30–60-day OLR between P1 and P2 (Fig. 4f). This finding indicates that the westward shift of the negative 30–60-day OLR anomaly in SON during P2 (Fig. 4f) primarily resulted from phases 5 and 6. The robustness of Fig. 7a was further investigated by calculating the MJO index (Wheeler and Hendon 2004) instead of BSISO1. A consistent result was observed by comparing Fig. 7b with Fig. 7a, indicating that the enhanced relationship between MTC and 30–60-day ISO in SON during P2 substantially contributed to the increase of MTC ratio. We noted that the MJO was generally more active in SON than in JJA, the difference shown in Figs. 7a and 7b was robust when the percentage was replaced by the total MJO active days. That is, the accumulated MJO active days decreased in JJA but increased in SON during P2 (not shown). Whereas the percentage of BSISO1-index and MJO index in phase 7 for JJA increased substantially during the P2 (Figs. 7a,b), the MJO associated convection center was far away the WNP, and therefore, it does not exert substantial impact on the TC development as observed in SON (Fig. 4).
(a) Percentage of each ISO phase (1–8) for 30–60-day ISO (BISIO1) for MTCs during the developing stage for JJA (red bars) and SON (blue bars) in P1 (left bars) and P2 (right bars). (b) As in (a), but for MJO. Here, BSISO1 index (Lee et al. 2013) and MJO index (Wheeler and Hendon 2004) were used to identify the ISO phase, respectively. (c),(d) Composite map of 30–60-day filtered OLR and 850-hPa wind anomalies for BSISO1 phase 5 and phase 6, respectively, during P2. Only the amplitude of BSISO1/MJO ≥ 1.0 was selected for composite. The filled (hatched) bar indicates the difference between P1 and P2 was significant at 95% (90%) confidence level by the Z test (Huang et al. 2011; Li and Zhou 2013).
Citation: Journal of Climate 36, 13; 10.1175/JCLI-D-22-0657.1
The seasonal dependence of MTC–ISO relationship was closely related to the interdecadal change in the propagation of the ISO since the late 1990s. As presented in Fig. 8, the 30–60-day ISO in the WNP region primarily propagated westward in JJA, whereas the ISO in the South China Sea migrated northward during P1. The change in ISO propagation was not evident except that the westward (northward) moving speed in the WNP region (South China Sea) in JJA became smaller than that in P1. The propagation of ISO in SON during P1 yielded a similar result as did that in JJA. During P2. In contrast to JJA, ISO propagation exhibited a marked change in SON: the northwestward propagation enhanced and the propagation speed increased. This result was clearly reflected in the change of the center of OLR: the center of the OLR (red contour in Fig. 8) moved northward in JJA and SON during P1; the OLR kept propagating northward in JJA during P2 but turned from northward to northwestward in SON. Previous studies have reported that the ISO modulates the TC track substantially (Ko and Hsu 2009). This effect was clearly identified in SON: the enhancement of the northwestward propagation of ISO modulated the MTC’s track to a northward recurve in the WNP region. A similar analysis was performed for the 10–20-day period (not shown). Briefly, the change in the propagation feature between P1 and P2 was nonsignificant, and no clear difference between JJA and SON was observed in P2. Thus, the seasonal dependence of TC–ISO relationship depicted in the figure primarily resulted from the 30–60-day ISO activity.
Correlation map of 30–60-day filtered OLR onto OLR index (averaged OLR over 0°–20°N, 120°–140°E) for lag 0 day (shading; interval: 0.1) and 7-day lag (blue contour, OLR index leads) in JJA during (a) P1 and (b) P2. Vectors indicate the propagation of 30–60 day filtered OLR, determining from each grid to the grid, 7 days later having maximum correlation coefficient with respect to the original grid. The black (green) dashed line indicates the climatology (period mean) of the contour of 5880-m geopotential height at 500 hPa. (c),(d) As in (a) and (b), but for the SON.
Citation: Journal of Climate 36, 13; 10.1175/JCLI-D-22-0657.1
The aforementioned results indicate that the significant change in TC–BSISO1 coupled in SON during P2 provided favorable large-scale dynamic and thermodynamic conditions for TC intensification. In particular, the change in the BSISO1-related cyclonic circulation anomaly and negative OLR anomaly shrunk to a smaller region due to the westward shift of the WNP subtropical high. This change combined with the westward shift of the MTC genesis location strengthened the MTC–ISO coupling (Figs. 4f and 7). The westward shift of the MTC genesis location was primarily caused by the westward shift of WNP subtropical high, which was all identified in JJA and SON. However, the ridge of climatological WNP subtropical high in JJA migrated northward compared with SON due to annual cycle (not shown). The northward migration that led the effect of the westward shift of WNP subtropical high on modulating the ISO propagation in JJA was not as evident as seen in SON. In the next section, we discuss the effect of the seasonal dependence of the MTC–ISO interaction on the MTC ratio by the transient eddy kinetic energy budget.
5. Eddy kinetic energy budget
The ISO–TC interaction plays a crucial role in the intensification of TC to MTC (Weng and Hsu 2017; Hong et al. 2018). Here, the ISO–TC interaction means two-way energy cascade between ISO and TC. We used the EKE budget of the tropical synoptic-scale eddy (SSE; time scale ≤ 10 days) (Tsou et al. 2014) to investigate the effect of MTC–ISO interaction on the abrupt increase of the MTC ratio for the SON in P2.
Because we examined the development of TCs to MTCs, the following EKE budget focused on MTCs. Figure 9 presents the spatial distribution of the EKE tendency derived from the composite of each MTC during the developing stage. Overall, the EKE tendency exhibited a pronounced positive distribution in the WNP region, wherein the TC genesis was active. A comparison between P1 and P2 indicated that both exhibited a similar spatial pattern but it shifted eastward in P2 JJA (comparing Fig. 9a with Fig. 9b). This eastward shift was synchronized with the eastward shift of the 30–60-day filtered OLR (Fig. 4c). By contrast, the spatial pattern of the EKE tendency shifted westward in P2 SON, leading to an increase (decrease) in the EKE tendency in the western (eastern) flank of the WNP region (comparing Fig. 9d with Fig. 9e). Notably, the westward shift of the EKE tendency was associated with the westward shift of the TC genesis location (Fig. 2e) and 30–60-day filtered OLR (Fig. 4f) that primarily resulted from the westward extension of the WNP subtropical high in P2. The opposite shift in the EKE tendency between JJA and SON in P2 led to strong seasonal dependence in the change of the EKE tendency between P1 and P2 (Figs. 9c,f).
Spatial distribution of the eddy kinetic energy tendency (10−5 m2 s−3) during the developing stage of MTCs in JJA during (a) P1, (b) P2, and (c) difference between P1 and P2. (d)–(f) As in (a)–(c), but for SON.
Citation: Journal of Climate 36, 13; 10.1175/JCLI-D-22-0657.1
Figures 10 and 11 present the comparison of the right-hand terms in Eq. (1) between P1 and P2. Here, the CK was divided into two terms, CKS–ISO, and CKS–M, to investigate the relative contribution of the scale interaction between TC–ISO and TC–mean flow on the EKE budget. A positive CKS–ISO was accompanied with a 10–60-day filtered cyclonic circulation anomaly in the western WNP region in JJA during P1 (Fig. 10a). This positive CKS–ISO and the accompanied cyclone anomaly both enlarged and extended eastward (comparing Fig. 10b with Fig. 10a) in P2. The change in CKS–ISO in SON was approximately opposite to that in JJA: the positive CKS–ISO and accompanied cyclonic circulation anomaly shrunk and shifted westward to western Pacific from P1 to P2 (Figs. 10d,e). This opposite change, resembling the EKE tendency, led to more favorable coupling of MTC tracks with CKS–ISO in SON than in JJA (comparing Fig. 10f with Fig. 10c). Notably, the spatial distribution of the increase in CKS–ISO in SON during P2 closely fitted with the enhancement of the MTC track in P2 (comparing Fig. 10f with Fig. 2f). Figure 10f indicates that the MTC track and accompanied ISO-related cyclonic circulation anomaly in SON shrunk and shifted westward to the Philippine Sea from P1 to P2. Consequently, the spatial distribution of the increase in CKS–ISO during P2 was well coupled with the enhancement of the MTC track in P2. These results indicate that CKS–ISO considerably contributes to the increase in the EKE tendency, which provides more energy that enables TCs to develop into MTCs. In contrast to SON, the difference in CKS–ISO in JJA was less organized and nearly opposite to that in SON (comparing Fig. 10c with Fig. 10f). In addition, the increase in the MTC track difference between P2 and P1 did not exhibit a similar pattern as did the change in CKS–ISO in SON.
As in Fig. 9, but for barotropic energy conversion term (10−5 m2 s−3) between eddy and ISO [shading; term CKS-ISO in Eq. (1)] and 10–60-day filtered wind at 850 hPa. Wind speed less than 0.2 m s−1was masked in all panels.
Citation: Journal of Climate 36, 13; 10.1175/JCLI-D-22-0657.1
As in Fig. 9, but for barotropic energy conversion term (10−5 m2 s−3) between eddy and mean flow [shading; term CKS–M in Eq. (1)] and mean flow at 850 hPa. Wind speed less than 0.2 m s−1was masked in (c) and (f).
Citation: Journal of Climate 36, 13; 10.1175/JCLI-D-22-0657.1
The difference in CKS–ISO accounted for the intensification of TC and contributed to the difference of the MTC ratio between the JJA and SON. Notably, the spatial pattern and magnitude of CKS–ISO determine by the distribution of 10–60-day circulation and TC location. Whereas the10–60-day cyclone anomaly in JJA also provided a favorable environment for TC development, the increase of TC track density in IP2 distributed scattering compared with that in SON (comparing Fig. 2c with Fig. 2f). Figure 10 revealed that the cyclone anomaly in SON shrunk to a smaller region, which was more coherent with increase of track density than that in JJA (comparing Fig. 10e with Fig. 10c). This difference led to a large and dense of CKS–ISO in the western Pacific during SON.
CKS–M exhibited a zonal distributed positive structure along the WNP monsoon trough for JJA and SON in P1 (Figs. 11a,d). This finding indicates that the TC tended to gain energy from the TC–mean flow interaction. This positive CKS–M weakened substantially in P2 for JJA and SON due to the weakening of the WNP monsoon trough caused by the enhancement and westward extension of the WNP subtropical high (Figs. 11b,e). Thus, the difference in CKS–M between P1 and P2 revealed a broad negative anomaly in JJA and SON (Figs. 11c,f). This finding indicates that the mean state change, the weakening of the WNP monsoon trough in P2 was not favorable for the intensification of TCs. In addition, no evident difference between JJA and SON was observed in CKS–M. Thus, CKS–ISO, which is the dominant term of barotropic energy conversion (CK), contributed to the seasonality of the MTC ratio.
In contrast to CK, the term CE, resembling the EKE pattern, yielded a positive result in the WNP region (comparing Fig. 12 with Fig. 9). As presented in Fig. 12, the seasonality of the EKE tendency, that is, the opposite change in P2 between JJA and SON, was determined by the seasonality of CE (comparing Figs. 12c,f with Figs. 9c,f). This finding was because CE (
As in Fig. 9, but for baroclinic energy conversion term (10−5 m2 s−3) from eddy available potential energy to EKE [shading; term CE in Eq. (1)] at 200 hPa.
Citation: Journal of Climate 36, 13; 10.1175/JCLI-D-22-0657.1
6. Conclusions and remarks
The MTC ratio in the WNP region exhibited strong difference between JJA and SON since the early 2000s. The possible causes were examined, and the main results are summarized as follows:
The total TC genesis number in JJA and SON in the WNP region had decreased simultaneously since the l990s; however, the ratio of TCs developing into MTCs (MTC ratio) exhibited strong seasonal dependence since the early 2000s. The MTC ratio in JJA was nearly unchanged (∼30%) between the preperiod (P1, 1989–2002) and postperiod (P2, 2003–16), but it exhibited a significant increase from 38% to 49% from P1 to P2 in SON.
The mean MTC genesis location in SON shifted westward considerably in P2 in response to the enhancement and westward extension of the WNP subtropical high: it shifted westward approximately from 157° to 146°E in SON from P1 to P2. This westward shift was considerably weaker and nonsignificant in JJA (from 152° to 148°E). The westward shift of the MTC genesis location in SON combined with the westward shift of TC-related ISO enhanced the TC–ISO interaction and consequently contributed to the seasonality of the MTC ratio.
The increase of MTC ratio in SON was associated with the interdecadal change of the 30–60-day ISO activity. The center of MTC accompanied with the negative OLR anomaly in SON shrunk and shifted westward approximately from 145° to 130°E in P2. This westward shift modulated the MTC–ISO interaction to occur in a limited region. This substantially increased the barotropic energy conversion of ISO to TC, which in turn intensified the rate of the development of TCs to MTCs. In contrast to SON, the MTC accompanied with a negative OLR anomaly extended eastward, and the MTC–ISO interaction spread over the region in JJA. The favorable condition for the development of TCs to MTCs observed in SON during P2 was not identified in JJA.
The interdecadal changes in ISO propagation since the early 2000s contributed substantially to the seasonal dependence of the MTC–ISO interaction. The northwestward propagation of the 30–60-day ISO increased considerably in SON during P2, which was not identified in JJA. The increase in the northwestward propagation of ISO modified the MTC track to a northwestward recurve in the WNP. In addition, the ISO-associated negative OLR and MTC mean genesis location shrunk and shifted westward due to the westward extension of the WNP subtropical high. Consequently, the MTC was closely coupled with the ISO wet phase in P2. Only 27% of MTCs occurred in the ISO wet phase in SON in P1, but it increased dramatically to 49% in P2. This increase was not noted in JJA. That is, the seasonal dependence of MTC–ISO interaction in P2 was one of the main factors leading to the increase of the MTC ratio in SON.
The increase in the MTC–ISO interactions in SON during P2 provided a favorable environment for energy conversion from the ISO to TC. The energy budget of SSE indicated that the lower-level barotropic energy conversion between the TC and ISO (CKS–ISO) substantially increased along the MTC track. In addition, the upper-level baroclinic energy conversion from the TC-associated perturbation (CE) increased along the TC track. The increases in CKS–ISO and CE that have been observed in SON but not in JJA since the early 2000s favored the development of MTCs.
The TC genesis number experienced an interdecadal decrease since the late 1990s (Liu and Chan 2013; Hsu et al. 2014; He et al. 2015). However, when the annual mean TC genesis number decreased, the MTC ratio during the typhoon season increased (Hong et al. 2018). Through the EKE budget, Hong et al. (2018) reported that the abrupt increase in the barotropic energy conversion from ISO to TC (CKS–ISO) and the baroclinic energy conversion of the eddy potential energy (CE) both contribute to this unprecedented increase. In this study, we extended the findings of previous studies and further reported that the abrupt increase in the MTC ratio exhibited strong seasonality, which was identified in SON but not in JJA. This seasonality primarily resulted from the substantial westward shift of the mean TC genesis location and ISO activity in SON in response to the westward extension of WNP subtropical high in P2. However, this westward shift was not noted in JJA. The westward extension of the WNP subtropical high does not substantially modulate the TC genesis location and ISO activity in JJA as observed in SON during the annual cycle: the TC genesis location and ISO activity migrated northward in JJA, weakening the effect of the WNP subtropical high on the modulation of the TC genesis location and ISO propagation. Notably, our study revealed that the enhancement of TC–ISO interaction plays a crucial role in increasing the TC developing rate. We applied the EKE budget for the moderate TCs (<category 3). In contrast to MTCs, the CKS-ISO was negative in SON over the WNP region (not shown). That is, the effect of TC–ISO interaction on intensifying the TC was only identified for the category of major TCs.
We noted that the abrupt change of MTC ratio in 2002 was concurrent with the phase change of PDO from positive to negative in the late 1990s to early 2000s, which substantially modified the TC activity in the WNP (Liu and Chan 2008; Zhao et al. 2018; Kim et al. 2020). The negative PDO phase associated with cold SST in the equatorial eastern Pacific may force an anticyclone anomaly in the WNP. That strengthened and shifted the WNP subtropical high westward and consequently suppressed the TC activity and modified the TC’s tracks (Du et al. 2011; Wang et al. 2013; Lin and Chan 2015; Matsumura and Horinouchi 2016). This effect was clearly reflected in Figs. 2 and 8. The westward shift of the MTC genesis location was primarily caused by the westward shift of WNP subtropical high, which was all identified in JJA and SON. However, the ridge of climatological WNP subtropical high in JJA migrated northward compared with SON due to annual cycle (not shown). The northward migration that led the effect of the westward shift of WNP subtropical high on modulating the ISO propagation in JJA was not as evident as seen in SON.
The moving speed of TC was identified as the main factor for the intensification of TCs (Mei et al. 2012; Chang et al. 2020). No difference in the moving speed of TCs was noted between JJA and SON during the development stage of MTC (not shown). This finding indicates that the moving speed of TC is not a key factor contributing to the seasonality of the MTC ratio in P2. Our study revealed that the TC–ISO energy conversion was substantially associated with the WNP subtropical high. However, the future projection of the WNP subtropical high under global warming remains largely uncertain (Chen et al. 2020). The effect of the seasonality of the WNP subtropical high on the modulation of TC activity, ISO activity, and TC–ISO interactions should be investigated.
Acknowledgments.
This study was supported by the National Science and Technology Council (NMTC), Taiwan, under Grants 110-2111-M-845-001, 109-2111-M-845-001, 110-2111-M-845-001, and 109-2121-M-001-004. The authors are grateful to the National Center for High-Performance Computing (NCHC), National Applied Research Laboratories (NARLabs), for providing computer facilities. This manuscript was edited by Wallace Academic Editing.
Data availability statement.
The reanalysis data used in this study, including 1) NCEP Reanalysis I (Kalnay et al. 1996), are available at https://psl.noaa.gov/data/gridded/data.ncep.reanalysis.html. 2) NCEP CFSR (Saha et al. 2010) and NCEP CFSv2 (Saha et al. 2014) can be found at https://rda.ucar.edu/. NOAA Interpolated Outgoing Longwave radiation (OLR) data are available at https://psl.noaa.gov/data/gridded/data.olrcdr.interp.html. TC data were obtained from the 1) JTWC are available at https://www.metoc.navy.mil/jtwc/jtwc.html?best-tracks and 2) the Regional Specialized Meteorological Center (RSMC) Tokyo–Typhoon Center (https://www.jma.go.jp/jma/jma-eng/jma-center/rsmc-hp-pub-eg/besttrack.html). Daily MJO index (Wheeler and Hendon 2004) are available at http://www.bom.gov.au/climate/mjo/, and BSISO index used in this study are calculated following Lee et al. (2013) using NCEP R1 and NOAA OLR dataset.
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