PUBLICATIONS Journal of Geophysical Research: Atmospheres RESEARCH ARTICLE 10.1002/2013JD021286 Key Points: • The SCS TC genesis experienced a significant decadal enhancement around the mid-1990s • The TC genesis change is related to the decadal variation of SCS ISV activity • The tracks of SCS TCs also show obvious variation around the mid-1990s
Decadal change of South China Sea tropical cyclone activity in mid-1990s and its possible linkage with intraseasonal variability Yao Ha1, Zhong Zhong1,2, Yuan Sun1, and Wei Lu1,2 1
College of Meteorology and Oceanography, PLA University of Science and Technology, Nanjing, China, 2Jiangsu Collaborative Innovation Center for Climate Change and School of Atmospheric Sciences, Nanjing University, Nanjing, China
Abstract
Supporting Information: • Readme • Text S1 • Figure S1 • Figure S2 • Figure S3 • Figure S4 • Figure S5 Correspondence to: Z. Zhong,
[email protected]
Citation: Ha, Y., Z. Zhong, Y. Sun, and W. Lu (2014), Decadal change of South China Sea tropical cyclone activity in mid-1990s and its possible linkage with intraseasonal variability, J. Geophys. Res. Atmos., 119, 5331–5344, doi:10.1002/2013JD021286. Received 29 NOV 2013 Accepted 21 APR 2014 Accepted article online 24 APR 2014 Published online 13 MAY 2014
This study focuses on the decadal variability of tropical cyclone (TC) activity over the South China Sea (SCS) since the 1970s and its possible cause behind. It is found that TC activity over the SCS experiences a significant decadal change around the mid-1990s. Compared to the period from the 1970s to the early 1990s, the number of TCs formed in the SCS remarkably increases from the mid-1990s through the 2000s. In particular, this change of TC genesis is closely related to a decadal shift in atmospheric intraseasonal variability (ISV) that occurred in 1994. The ISV on the 30–60 days time scale over the SCS has been increasing since the mid-1990s, and the increased TC frequency after 1994 is attributed primarily to the active convection induced by the enhancement of the SCS ISV. In addition, the TC activities before the mid-1990s are mostly confined within the SCS basin. However, more TCs form over the SCS and move northeastward since the mid-1990s and finally enter the East China Sea and the Philippine Sea. Anomalies of westerly over the northern SCS after 1994 are responsible for the northeastward moving of TCs.
1. Introduction The South China Sea (SCS) is one of the largest semienclosed marginal seas in the western North Pacific (WNP). Nearly all the tropical cyclones (TCs) formed over the SCS can make landfall either along the East Asian coast or in Vietnam and the Philippines shortly after their genesis, causing a great loss of human life and property damage in these areas. Many previous studies focused on the interannual and interdecadal variations of SCS TC activity, which are modulated primarily by El Niño–Southern Oscillation and Pacific Decadal Oscillation (PDO), respectively [Wang et al., 2007; Chan, 2008; Zuki and Lupo, 2008; Kubota and Chan, 2009; Goh and Chan, 2010; Kim et al., 2010; Wang et al., 2012]. These studies have shown that the TC genesis frequency (TCGF) over the SCS basin significantly decreased from the mid-1970s through the early 1990s compared to that in the earlier epoch from the 1950s to the 1970s, exhibiting a clear interdecadal reduction around the mid-1970s. Various mechanisms have been proposed to interpret this interdecadal TCGF change around the mid-1970s. Goh and Chan [2010] attributed this change to a shift in the PDO. Wang et al. [2013] argued that it is associated with the warming in sea surface temperature at the equatorial Indian Ocean. Change in vertical wind shear associated with the concurrent variation of the East Asian jet stream and western Pacific subtropical high (WPSH) on an interdecadal time scale also help explain the reduction of the SCS TC genesis since the mid-1970s [Wang et al., 2012]. Recent studies revealed a distinct increase in rainfall over southern China after the early 1990s on the interdecadal time scale [Kwon et al., 2005, 2007; Wu et al., 2010]. Chen et al. [2012] demonstrated that in southern China, the contribution of rainfall induced by those TCs formed in the SCS to the summer total rainfall experienced a significant increase since the mid-1990s, implying an upward trend of the SCS TC activity after the 1990s. Meanwhile, Kajikawa et al. [2009] revealed that a decadal change in the SCS atmospheric intraseasonal variability (ISV) occurred during the mid-1990s. The ISV, which is a dominant mode of tropical atmosphere, exhibits strong variations in terms of circulation and convection on subseasonal scales with a complete cycle of 30–60 days. It also affects many weather and climate phenomena, as a critical role in connecting multiscale atmospheric systems [Zhang, 2005, 2013]. The SCS summer monsoon presents strong ISV in the Asian summer monsoon regions [Kemball-Cook and Wang, 2001], and active (inactive) phases of ISV associated with the SCS monsoon are closely related to the passages of enhanced (suppressed) convection [Madden and Julian, 1971; Zhou and Chan, 2005; Li et al., 2012]. The northeastward propagated ISV of the SCS
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summer monsoon, which is often referred to as monsoon surge/stream [Tao and Wei, 2007; Ju et al., 2010], plays a critical role in weather and climate over the SCS and East Asian regions [Zhou and Chan, 2005] and also largely determines the features of the SCS TC genesis and track [Wang et al., 2007; Kajikawa and Wang, 2012]. In view of the different TC best track data sets employed in the previous studies, robustness of the decadal change in the SCS TC activity occurred in the 1990s is still unclear. Considering the close relationship between the ISV and the local synoptic-scale disturbances, the influence of the SCS ISV on the decadal variation of TC activity remains an open problem. Based on the above consideration, this study is designed to reveal the decadal change of TC activity in the mid-1990s over the SCS and to investigate its possible linkage with the SCS ISV. This paper is organized as follows. The data sets and methodology are described in section 2. The decadal change of the SCS TC activity after the 1970s and its association with environmental conditions are presented in section 3. The relationship between the decadal change of TCGF and the ISV over the SCS are discussed in section 4. Finally, section 5 provides concluding remarks.
2. Data and Methodology Two TC best track data sets at 6 h intervals are used in this study. The data sets are provided by the Joint Typhoon Warming Center (JTWC) [2012] and the Regional Specialized Meteorological Center of Japan Meteorological Agency (JMA) [2012], respectively. Based on the two data sets, we examine the TCs formed over the SCS from June to September in the period 1970–2011. Only those TCs reaching tropical storm intensity (maximum sustained wind speed ≥ 17.2 m s1) are selected for this study. To calculate the anomalies of TCGF/TC occurrence frequency (TCOF), each TC genesis/occurrence position is binned into its corresponding 2.5° × 2.5° grid box. The frequency anomaly and the associated significant level are directly marked in each grid box with dots of different sizes, which present the TCGF information in a straightforward way. Other data sets used in this study include the daily outgoing longwave radiation (OLR) data [Liebmann and Smith, 1996], the precipitation reconstruction data [Chen et al., 2002], and the extended reconstructed sea surface temperature [Smith et al., 2008] from the National Oceanic and Atmospheric Administration. Wind and geopotential height fields are extracted from the National Centers for Environmental Prediction-National Center for Atmospheric Research Reanalysis Project data set [Kalnay et al., 1996]. To extract the intraseasonal signal of 30–60 days atmospheric oscillation, the Lanczos band-pass filter [Duchon, 1979] is applied to the daily OLR and 850 hPa wind fields. Moving t test technique is a common method used to quantitatively detect the significance of decadal abrupt changes by testing the mean values of two subseries [Afifi and Azen, 1972]. When setting a datum point for an n sample series, the subseries x1 (x2) of the n1 (n2) samples is obtained before (after) the datum point with a mean value of x 1 ðx 2 Þ and a variance of s21 s22 , and n1 + n2 ≤ n. The relevant test statistic is given by x1 x2 t ¼ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi : n1 s21 þn2 s22 1 1 n1 þn2 2 n1 þ n2 The test statistic t follows a Gaussian distribution with zero mean and unit standard deviation. We test it using a two-tailed test with the critical values of t = ±2.88 and t = ±2.10 at 99% and 95% confidence, respectively. In this study, the moving t test technique and 9 years running mean are used to reveal the variabilities of the SCS TC frequency and ISV activity on the decadal time scale, which is generally considered the variation with a period 8–13 years [Mehta et al., 2001; Zhu and Wang, 2001]. A composite analysis is applied to compare the environmental variables during the active and inactive TC activity epochs, and a two-tailed Student’s t test is used for the statistical significance test of composite anomalies and differences between the two series. In addition, TCs make significant contributions to the seasonal mean and intraseasonal variance of wind field along their tracks [Hsu et al., 2008] and can also enhance the local ISV activity [Mao and Wu, 2008]. In order to eliminate the effect of TCs on the SCS ISV, we calculate the 30–60 days ISV using the filtered 850 hPa wind, in which the TC-related vortices are also removed from the daily reanalysis data. The horizontal TC wind is removed from the reanalysis data at each TC occurrence time using the method of TC bogussing scheme in the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale HA ET AL.
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Model. The position of the observed TC is first identified by the nearest grid point of the reanalysis based on the TC best track data set, and the TC vortex domain is identified from the analysis TC. The reanalysis wind field in the vortex area is then 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. The stream functions corresponding to nondivergent wind and the velocity potential are calculated based on the best track data set at each time. The sum of the nondivergent wind and the velocity potential wind is regarded as TC wind component, and the residual between analysis wind and TC wind is the background field. Although the spatial resolution of the reanalysis seems relatively coarse for the TC wind field, it is able to represent the interannual and intraseasonal variations of the WNP TC activity for TC climatology studies [Zhong and Hu, 2007; Hsu et al., 2008; Ha et al., 2013a, 2013b]. More details of this method can be found in Low-Nam and Davis [2001]. Figure 1. The anomalies of SCS TCGF from June to September during 1970–2011 derived from (a) JTWC and (b) JMA data sets. The red curves in Figures 1a and 1b are the 9 years running means. (c) Moving t test of 9 years for the TCGF based on the two TC best track data sets. The dashed line indicates 99% confidence level.
The genesis potential index (GPI) proposed by Emanuel and Nolan [2004] is calculated over the SCS to examine the combined impact of environmental variables on local TC genesis. The index is defined as
3 3 GPI ¼ 105 η2 ðH=50Þ3 V pot =70 ð1 þ 0:1V shear Þ2 , where η is the absolute vorticity at 850 hPa, H is the relative humidity at 600 hPa, Vpot is the potential intensity, and Vshear is the magnitude of the vertical wind shear between 200 hPa and 850 hPa. Note that TC wind field itself with an excessive positive vorticity is included in the background circulation, especially in the domain of TC activity over the SCS basin during peak TC seasons. Consequently, the TC activity can affect the GPI evaluation. To eliminate this effect of TCs, the η term in GPI is calculated with the TC-removed wind field.
3. Decadal Change of SCS TC Activity After the 1970s and Its Association With Environmental Conditions The 9 years running mean of the SCS TCGF from the mid-1970s through the early 1990s is less than the climatological mean of 1970–2011 based on the analyses of the JTWC data set (Figure 1a). This is consistent with results of previous studies [Wang et al., 2012, 2013]. However, it is noted that the TC number significantly increases around the mid-1990s recorded in both the data sets above 99% confidence level by the Student’s t test (Figures 1a and 1b). Results of moving t test detection show that a significant and decadal abrupt change occurred in 1994, which exceeds 99% confidence level based on the JTWC/JMA data set (Figure 1c). This decadal change clearly indicates that the SCS TCGF becomes much higher since the mid-1990s when compared to that in the previous epoch from mid-1970s to early 1990s. Moreover, this abrupt change is more evidently revealed in the JTWC data set than in the JMA one (Figure 1c). To further examine the decadal features of the SCS TC activity and the associated environmental conditions, we define a decade of inactive
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Figure 2. TC genesis locations (red TC symbol) and tracks (line) from June to September recorded in the JTWC data set during the (a) IAP and (b) AP, and differences of (c) TCGF and (d) TCOF between the AP and IAP. The numbers in Figure 2a denote total TCGF and number of TCs that experience a sharp recurvature in the IAP, respectively; the numbers in Figure 2b denote total TCGF and the number of TCs that enter the East China Sea in the AP, respectively. The blue TC tracks in Figures 2a and 2b denote those TCs experience sharp recurvature and enter the East China Sea, respectively. Circles with white stars in Figures 2c and 2d indicate the differences are significant above 95% confidence level by the Student’s t test.
TC period (IAP; 1984–1993) and a decade of active TC period (AP; 1994–2003) based on the year of abrupt change (i.e., 1994). Locations of TC genesis and TC tracks derived from the JTWC data set are shown in Figures 2a and 2b for the IAP and AP, respectively. It can be seen that TCs are mainly generated over the central northern SCS from June to September, which is consistent with previous studies that the mean location of the SCS TC formation situates north of 15°N in summer [Wang et al., 2007, 2012]. There are 36 TCs formed over the SCS in the AP, which is nearly 3 times compared to that in the IAP (13), and their difference is significant at 99% confidence level. Furthermore, it is interesting to note that while most of TCs in the IAP move westward and northwestward after their formation, three TCs actually move eastward before they experience a sharp recurvature toward west around 120°E to 125°E. Consequently, the TC activity in the IAP is
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Figure 3. As in Figure 2 but recorded in the JMA data set.
confined within the semienclosed basin of the SCS and make landfall over the East Asian coast south of 24°N (Figure 2a). In contrast, the TC activity in the AP makes more frequent and extensive landfall along the coastal areas (Figure 2b), bringing more destructive influences on the coastal regions. In particular, it is noteworthy that seven TCs in the AP move northeastward through either the Luzon Strait or Taiwan Strait and then enter the East China Sea or the Philippine Sea, imposing great impact over the midlatitudinal areas. Figures 2c and 2d show differences of TCGF and TCOF between the AP and IAP, respectively. A significantly positive anomaly of TCGF appears in the northeastern SCS near the Luzon Island, and several positive anomalies of TCOF are located in the northern SCS and west of Taiwan, suggesting that more TCs tend to make landfall over southern China and Indochina Peninsula in the AP. Furthermore, TC genesis and tracks as well as their anomalies between the two periods recorded in the JMA data set exhibit consistent features with those in the JTWC data (Figure 3). These results clearly suggest that a significant decadal change of TC activities over the SCS occurred around the mid-1990s.
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Figure 4. Differences of (a) 500 hPa wind (vector; m s ), (b) GPI (contour), and (c) TC vorticity-removed GPI (contour) between the AP and IAP from June to September. The solid and dashed contours in Figure 4a indicate 5860 geopotential meter averaged from June to September for the composite AP and IAP, respectively. The shaded GPI and bold wind vector denote that the differences are significant above 95% confidence level by the Student’s t test.
The movement of TC is largely dominated by the steering flow in the midtroposphere [Liu and Chan, 2008]. In the IAP, most TCs tend to move westward and northwestward along the southwestern flank of the WPSH, and no TC moves northeastward to enter the East China Sea or the Philippine Sea. In contrast, the anomaly of westerly over the northern SCS favors more northeastward movement of TCs during the AP (7/8 TCs in Figures 2a and 3a). The difference in lower tropospheric wind between the AP and IAP shows a similar pattern of anomalous anticyclone to that in the midlevel (not shown), indicating that the western flank of the WNP anomalous anticyclone dominates over the entire SCS during the AP. This agrees with previous studies that the WNP anomalous anticyclonic circulation is intensified since the late 1970s [Wang et al., 2008; Xie et al., 2010]. The anticyclonic circulation (Figures 4a and 6a) and the conjunct subsidence dry flow (Figures 6d and 6e) over the northern SCS are closely related to the anomalous descending branch of the local Hadley circulation, which is triggered by the sea surface temperature warming over the Maritime Continent and the East Indian Ocean. GPI is considered an effective empirical index to assess climatological TC genesis conditions [Camargo et al., 2007; Emanuel, 2008]. Figure 4b shows the difference in GPI pattern between the AP and IAP. The negative anomaly of GPI is found north of 12°N over the SCS, which is the major area of TC formation. Meanwhile, the positive anomaly of GPI appears over the northern SCS along southern China coast. After removing the absolute vorticity that is associated with TC vortices from the GPI calculation, the negative anomalies of GPI enlarge over the entire northern SCS (Figure 4c). We also calculate the GPI after removing TC wind field from both the absolute vorticity and vertical shear terms, and it is found that the GPI pattern is very similar to that shown in Figure 4c (see the supporting information). In addition, we further detect the SCS environments on TC genesis using a recent modified GPI reported by Tippett et al. [2011], which is based on the Poisson regression methodology to determine the climatic variables for the index. It shows that the epochal difference of the new GPI between the AP and IAP is generally consistent with that of the traditional one as shown in Figure 4c (see the supporting information). These results suggest that the large-scale environmental condition denoted by the GPI over the SCS is unfavorable for TC activity during the AP. To examine the variation of the SCS summer monsoon in the two decades, we define two summer monsoon indices, i.e., the meridional shear vorticity represented by the difference of averaged 850 hPa zonal wind between 5°–15°N, 110°–120°E and 20°–25°N, 110°–120°E [Wang et al., 2009] and the monthly precipitation averaged over 10°N–20°N, 110°–120°E. The temporal series of the two SCS monsoon indices during June– September from 1984 to 2003 are shown in Figure 5, which clearly indicates that both the SCS monsoon indices decrease since the mid-1990s, and a decreasing trend of monsoon intensity also appears in the two decades (Figure 5). This is consistent with the results shown in Figure 4 that an anticyclonic circulation
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anomaly dominates over the SCS after the 1990s, corresponding to the suppressed convection and dry condition over the SCS in the epoch from the 1970s to mid-1990s. We also examine the spatial patterns of 850 hPa wind and precipitation trends over the SCS from 1984 to 2003 (see the supporting information). An anomalous anticyclonic circulation over the SCS Figure 5. The series of SCS monsoon wind index (column) and monthly prebasin is manifested in the spatial pattern cipitation averaged over 10°N–20°N, 110°–120°E (solid curve) from June to of 850 hPa wind trend, accompanied by September for the period 1984–2003. The left/right y axis label is for the wind/precipitation index, and the black and gray dashed lines are the linear the suppressed precipitation over the trends of wind and precipitation indices, respectively. Correlation coefficient central and southern SCS that is between the indices is 0.57 above 99% confidence level. consistent with the decreasing trend of the SCS monsoon indices. Figure 6 shows the differences of environmental variables over the SCS between the AP and IAP. It can be seen that compared to the atmospheric circulation during the IAP, the lower troposphere over the main TC genesis region in the central northern SCS during the AP is dominated by an anomalous anticyclone with an obvious negative vorticity anomaly (Figure 6a), and the negative vorticity becomes more evident when removing those contributed by the TCs from the background field (Figure 6b). Meanwhile, significant anomalies of convergence are found in the higher troposphere (Figure 6c), accompanied by the anomalous subsidence (Figure 6d) and dry
Figure 6. Differences of environmental variables from June to September between the AP and IAP: (a) 850 hPa absolute 6 1 6 1 vorticity (contour; 10 s ), (b) TC-removed 850 hPa absolute vorticity (contour; 10 s ), (c) 200 hPa divergence 6 1 1 (contour; 10 s ), (d) 500 hPa vertical p velocity (contour; Pa s ), (e) 600 hPa relative humidity (contour; percent), and 1 (f) magnitude of vertical wind shear between 200 hPa and 850 hPa (contour; m s ). Dark (light) shades indicate the positive (negative) anomalies are significant above 95% confidence level by the Student’s t test.
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Figure 7. The latitude-time section of 30–60 days filtered OLR anomalies (W m ) averaged from 100°E to 125°E during the (a) IAP and (b) AP, overlapped with the formation times and latitudes of every TC from June to September. The number in parenthesis denotes the TCGF that year.
condition (Figure 6e) in the midlevel over the northern SCS. The vertical wind shear exhibits weak positive anomalies over the northeastern SCS (Figure 6f). These dynamically coherent patterns are also presented in the linear trends of 850 hPa wind field (Figure 5), suggesting that the interannual and decadal variabilities of these environmental variables are highly indicative for the SCS summer monsoon during 1984–2003 [Zhou et al., 2009; Xie et al., 2010]. Apparently, the differences in these environmental variables between the AP and IAP are generally consistent with the distribution of GPI, which are unfavorable for TC genesis and development during the AP, given the fact that the SCS TCGF significantly increased from 1994 to the early 2000s. In addition, the decadal variation of PDO index shows no obvious change around the mid-1990s (see the supporting information). The decadal change of more active TCs over the SCS after 1994 cannot be attributed to the common modulation factors (e.g., local environmental conditions and PDO). Since the changes of large-scale circulation and anomalous environments are unable to explain the decadal variance of SCS TC genesis around 1993/1994, there might exist some other mechanisms that modulate and finally induce the enhancement of TC genesis over the SCS since the mid-1990s.
4. Relationship Between the Decadal Changes of TCGF and ISV Over the SCS Atmospheric ISV makes a substantial contribution to onset and evolution of the East Asian summer monsoon [Yasunari, 1981; Kajikawa and Wang, 2012]. The anomalous variation of ISV associated with the northeastward propagation of the monsoonal convection system can exert prominent influences on TC activities [Li et al., 2001; Zhou and Chan, 2005]. In this section, the relationship between the TC genesis and the ISV activity over the SCS is examined to investigate the possible influence of ISV on the decadal change of SCS TC genesis in the mid-1990s. Figure 7 shows the latitude-time section of 30–60 days filtered OLR anomalies averaged from 100°E to 125°E during 1984–2003. On the whole, the ISV activity in the AP exhibits stronger intensity compared
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Figure 8. (a) The mean standard deviation of OLR anomalies (column; W m ) on 30–60 days time scale averaged over 5°–20°N, 110°–120°E from June to September in the period 1975–2010, and the thick curve is the 9 years running mean. (b) Moving t test of 9 years for the series in Figure 8a, and the dashed line indicates 95% confidence level. (c) Difference of mean standard 2 deviation of OLR anomalies (contour; W m ) on 30–60 days time scale during June to September between the AP and IAP, and shades indicate the differences are significant above 95% confidence level by the Student’s t test.
to that in the IAP. When overlapping with the formation times and latitudes of every TC, it is found that most TCs in the AP tend to form in the active/wet phases of ISV (Figure 7b). This is because the enhanced ISV activity in its active/wet phases can provide abundant moisture and convergence in the lower troposphere, which are favorable for the cyclogenesis and lead to more TC formation in the AP. On the contrary, fewer TCs form in the IAP when the ISV activity is suppressed (Figure 7a). However, it is found that some TCs can still form in the inactive/dry ISV phases in the IAP (e.g., the TC in 1992), implying that under the suppressed ISV conditions, some other environmental variables might play a more important role in cyclogenesis over the SCS compared to the ISV. To quantitatively detect the intensity of the ISV activity, we calculate the mean standard deviation of 30–60 days filtered OLR from June to September, which can represent the seasonal intensity of atmospheric intraseasonal oscillation [Kajikawa and Wang, 2012, Figure 8]. Figure 8a shows the index of the mean standard deviation of filtered OLR averaged over the SCS from 1975 to 2010. We calculate the correlations between the ISV index and the JTWC/JMA TCGF from 1975 to 2010. It is found that their correlation coefficients reach 0.48/0.47, well above 99% confidence level (0.42). This implies that the interannual variation of the SCS TC frequency has a close relationship with that of ISV activity. Note that the SCS ISV index exhibits an obvious decadal change around the mid-1990s; that is, the intensity of ISV after the mid-1990s is stronger than that in the previous decade from the early 1980s to early 1990s. Result of moving t test detection shows that a decadal abrupt change occurred in 1994 with 95% confidence level (Figure 8b), which is consistent with that of the SCS TC activity. The difference in mean standard deviation of filtered OLR between the AP and IAP exhibits significant positive anomalies over the central northern SCS (Figure 8c), suggesting that the SCS ISV activity is distinctly enhanced over most of the SCS since 1994. The increase in the SCS ISV is probably associated with the persistent warming in the local sea surface temperature (see the supporting information). The consistency in variation of the SCS ISV and TCGF on the decadal time scale at least implies a close linkage between the decadal change of TC genesis over the SCS and the ISV activity in the mid-1990s. The enhancement of SCS ISV activity after the decadal change in 1994 is accompanied by enhanced convection and sufficient moisture condition in the active phase of ISV during the AP; thus, more TCs tend to form and develop over the SCS in this time period. Figure 9 shows the box plots of ambient 30–60 days filtered OLR anomalies for each TC genesis in IAP and AP. We calculate this ambient ISV signal by averaging the 30–60 days filtered OLR in the four grids that are closest to the cyclogenesis position at the time of TC genesis. It shows a more concentrated distribution of the samples in the AP than that in the IAP. Comparisons of the results between the AP and IAP show that the mean value of TC-ambient filtered OLR significantly changes HA ET AL.
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(a) OLR IAP
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(b) OLR AP
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0 −4.63
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1984−1993
1994−2003
Figure 9. Box plots of ambient 30–60 days filtered OLR anomalies of each TC genesis in the (a) IAP and (b) AP. Plotted are the inner quartile range (box), the median (horizontal line in the box), the 25th/75th percentile ± 1.5 times the inner quartile range (horizontal line below/above the box), the mean value of all the samples (thick cross and number), and the ambient 30–60 days filtered OLR anomalies of each sample (dot).
by 5.97 W m2, and the median value of TC-ambient filtered OLR in the AP is also lower than that in the IAP (Figure 9). These results suggest that the TC-ambient environment associated with the ISV provides more favorable conditions for cyclogenesis in the AP than in the IAP. To further illustrate the impact of ISV activity on the SCS TC genesis, Figure 10 shows the composite anomalies of 30–60 days filtered OLR and 850 hPa wind fields in the TC genesis days of the two epochs. In the IAP, a weak cyclonic circulation and positive anomalies of OLR are exhibited in the northern SCS, which is the main TC genesis location (Figure 10a). Apparently, the ISV activity does not induce the enhanced convection when TCs are forming during the IAP. In contrast, a cyclonic shear dominates the central northern SCS during the AP, and the remarkable convection induced by the ISV is shown over the entire SCS when TCs are forming (Figure 10b). This indicates that more TCs tend to form in the active/wet phases of ISV activity during the AP. In addition, note that TCs can act as an immediate trigger for local ISV activity along the tracks in the SCS basin [Mao and Wu, 2008]; thus, the current result actually shows the interaction between TCs and ISV based on the reanalysis data that include the TC-related vortices. To further detect the influence of atmospheric background ISV on the TC genesis, we eliminate the effect of TCs on the ISV by calculating the 30–60 days filtered 850 hPa wind, in which the TC-related vortices are removed from the daily reanalysis. The composite anomalies of TC genesis days in the two epochs are shown in Figures 10c and 10d. Compared to the original ISV signal, the environmental wind decreases in both epochs due to the TC activities. In particular, although the environmental flow in the AP appears weaker than that in Figure 10b, the anomalous cyclonic shear is still significant and located over the entire SCS (Figure 10d). This indicates that the ISV of large-scale circulation can bring enhanced convection, and eventually lead to frequent TC genesis over the SCS from the mid-1990s to the early 2000s. Considering the effect of the SCS TC activity on the ISV of OLR, we try to get rid of the influence of TCs and examine the composite anomalies of 30–60 days filtered OLR/850 hPa wind field of 1 day and 2 days before the TC genesis time in the two epochs (Figure 11). For the IAP, the inactive convection activity and weak cyclonic circulations are observed in the northern SCS 1 day and 2 days before TC genesis time (Figures 11a and 11c). In contrast, during the AP, a cyclonic circulation with distinctly enhanced convection dominates the entire SCS no matter 1 day or 2 days before the TC genesis time (Figures 11b and 11d). This is quite similar to the ISV pattern shown in Figure 10d. Overall, these results indicate that after the elimination of TC contribution to the SCS ISV, the local environmental conditions associated with the ISV are still favorable for the cyclogenesis, suggesting that the enhancement of the ISV activity might be a trigger for the brisk activity of TCs over the SCS after 1994. The enhanced convection associated with the ISV activity since the mid-1990s promotes the summer TC genesis over the SCS, and the local ISV activity is largely responsible for the decadal variability of TC activity over the SCS after the 1970s.
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Figure 10. Composites of 30–60 days filtered OLR anomalies (shade; W m ) and 850 hPa wind anomalies (vector; m s ) in each TC genesis time (0 day) of the (a and c) IAP and (b and d) AP, and wind anomalies in Figures 10c and 10d are those of TC removed. Red TC symbols denote the locations of cyclogenesis, and OLR enclosed by solid contours and bold wind vectors denote the differences are significant above 95% confidence level by the Student’s t test.
5. Concluding Remarks In this study, it is found that TC activity over the SCS experiences a significant decadal change in the mid-1990s. The TCGF obviously increases from 1994 through the early 2000s compared to that in the preceding epoch of the mid-1970s to the early 1990s, and both the JTWC and JMA TC best track data sets have recorded the consistent decadal abrupt change occurring in 1994. However, the change of large-scale circulation and environmental conditions over the SCS are unable to explain the decadal change of TC genesis. The present study reveals that a significant decadal change of the SCS ISV activity occurred in 1994, and its intensity started increasing since the mid-1990s on the decadal time scale. The increased TC frequency over the SCS after the
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Figure 11. Composites of 30–60 days filtered OLR anomalies (shade; W m ) and 850 hPa wind anomalies (vector; m s ) 1 day ((a and b) 1 day) and 2 days ((c and d) 2 days) before each TC genesis time of IAP in Figures 11a and 11c and AP in Figures 11b and 11d. Red TC symbols denote the locations of cyclogenesis in the two periods, and OLR enclosed by solid contours and bold wind vectors denote the differences are significant above 95% confidence level by the Student’s t test.
mid-1990s can be largely attributed to the enhanced convection during the active/wet phase of the local ISV activity. After removing the TC-related vortices from the daily reanalysis, we analyze the ISV signals shown in OLR and 850 hpa wind fields. The results indicate that the enhanced ISV activity not only favors frequent TC formation but also plays a dominant role in the decadal variability of TC activity over the SCS after the 1970s. Additionally, most of the TCs are confined within the SCS basin and stay on north westward tracks before the mid-1990s. However, after the mid-1990s more TCs move northeastward and finally enter the East China Sea or the Philippine Sea. Change in large-scale steering flow, which is shown as the anomalous westerly over the northern SCS, is responsible for the northeastward movement of TCs.
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Acknowledgments The data for this paper are available at NOAA’s Earth System Research Laboratory (http://www.esrl.noaa.gov/psd/data/reanalysis/reanalysis.shtml). The authors are grateful to the three anonymous reviewers and Chidong Zhang for their constructive comments. The FORTRAN script of calculating the maximum potential intensity for the GPI [Emanuel and Nolan, 2004] is derived from Kerry Emanuel’s home page (ftp://texmex.mit. edu/pub/emanuel/TCMAX/). This work is sponsored by National Key Basic Research Program of China (grant 2013CB956203), the R&D Special Fund for Public Welfare Industry (Meteorology) (grant GYHY201306025), the National Natural Science Foundation of China (grants 41175090 and 41205075), and the Jiangsu Collaborative Innovation Center for Climate Change.
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