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How Does Shifting Pacific Ocean Warming Modulate on Tropical Cyclone Frequency over the South China Sea? GUANGHUA CHEN Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China (Manuscript received 14 October 2010, in final form 14 January 2011) ABSTRACT The different modulation of El Nin˜o Modoki and canonical El Nin˜o events on tropical cyclone (TC) frequency over the South China Sea (SCS) during boreal summer and fall for 1960–2009 is investigated. The bootstrap resampling method and two-sample permutation procedure are applied to simulate sampling distributions and conduct statistical tests, respectively. Results from the hypothesis testing indicate that the above-normal TC frequency over the SCS occurs during June–August (JJA) for the El Nin˜o Modoki years, whereas the below-normal TC frequency is significant during September–November (SON) for the canonical El Nin˜o years. The remarkably opposite modulations can be attributed to the different large-scale circulation anomalies, which are consistent with Matsuno–Gill-type responses to the tropical heating source/sink over the western North Pacific (WNP) and Maritime Continent for two kinds of Pacific Ocean warming events. In response to a broad-scale convection anomaly over the WNP during JJA for El Nin˜o Modoki, a zonally elongated cyclonic anomaly dominates the WNP and SCS, leading to enhanced TC activity. In contrast, during SON for the canonical El Nin˜o, a markedly strengthened cooling source centered in the Maritime Continent induces an anticyclonic anomaly over the SCS, resulting in suppressed TC activity.
1. Introduction It has long been known that the El Nin˜o–Southern Oscillation (ENSO) can exert significant influence on tropical cyclone (TC) activity in various ocean basins (e.g., Chan 1985; Dong 1988). Based on the Nin˜o-3.4 sea surface temperature anomaly (SSTA), much has been done on understanding the relationship between ENSO and TC activity over various ocean basins (e.g., Chan 2000; Saunders et al. 2000; Chia and Ropelewski 2002; Wang and Chan 2002; Wu et al. 2004; Chen et al. 2006). The canonical El Nin˜o (CEN, or traditional El Nin˜o) involves temperature anomalies in the eastern Pacific (e.g., Rasmusson and Carpenter 1982). However, in the last two decades nontraditional El Nin˜o events were observed, in which the usual place of the temperature anomaly is not affected, but an anomaly arises in the central Pacific. Ashok et al. (2007) has identified this new episode of warming in the central Pacific Ocean, referred
Corresponding author address: Dr. Guanghua Chen, Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, P.O. Box 2718, Beijing 100190, China. E-mail:
[email protected] DOI: 10.1175/2011JCLI4140.1 Ó 2011 American Meteorological Society
to as El Nin˜o Modoki (ENM; Modoki is Japanese for similar, but different). In contrast to CEN with temperature anomalies in the Eastern Pacific, ENM is characterized by warm SSTA in the central tropical Pacific flanked by below-normal SSTA on its eastern and western sides. The two types of El Nin˜o can lead to distinct climatic and synoptic variability in the global region. For example, different from the impacts of the CEN events, ENM can give rise to different geographical distributions of dry/wet conditions along the Pacific rim (Weng et al. 2007) and significantly influence the storm track activity in the midlatitudes of the Southern Hemisphere and thus reduce the storm-associated rainfall over southeastern Australia (Ashok et al. 2009). Relatively little research has been conducted concerning the impact of the two types of El Nin˜o events on TC activity. Kim et al. (2009) found significant differences between the impacts of the two types of Pacific Ocean warming on TC activity in the North Atlantic. In contrast to eastern Pacific warming, central Pacific warming is associated with a greater-than-average TC frequency and increasing landfall potential along the Gulf of Mexico coast and Central America. Chen and Tam (2010) documented that ENM can exert a coherent influence on TC activity over the whole western North Pacific (WNP)
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basin during the period of June–August (JJA), as opposed to an inhomogeneous relationship for CEN events. In contrast to WNP, TC frequency over the South China Sea (SCS) during boreal summer (JJA) is comparable to that during boreal fall [September–November (SON)], both accounting for approximately 85% of annual total. However, hitherto there is no study distinguishing the impact of ENM on the TC activity over the SCS from that of CEN in terms of seasonal variability. Besides, many previous studies adopted the Nin˜o-3.4 SST anomaly index for identifying ENSO episodes. Because of the geographical location of the Nin˜o-3.4 region (58S–58N, 1708–1208W), the use of this index possibly picks up SSTA signals associated with both ENM and CEN, thus mixing up impacts related to the two different types of Pacific Ocean warming and causing TC prediction bias on an interannual or seasonal scale. Therefore, by using different indices to distinguish the two types of El Nin˜o events, this study attempts to examine the seasonal variation in the modulations of two kinds of El Nin˜o events on TC frequency over the SCS and proposes physical mechanisms responsible for the modulations.
2. Data and methodology The TC dataset, which spans the period from 1960 to 2009, was obtained from the International Best Track Archive for Climate Stewardship (IBTrACS) v03r01 dataset, which was achieved by merging storm information from all of the Regional Specialized Meteorological Centers and other international centers into one product so as to overcome data availability issues. Since most TCs over the SCS are observed from June to November (JJASON), the present study focuses on the summer (JJA) and fall (SON) seasons to explore the seasonal variation in the impacts related to two kinds of El Nin˜o. Tropical cyclones considered in this study include tropical depressions, tropical storms, and typhoons. The monthly sea surface temperature from the Hadley Center with a 18 3 18 horizontal resolution and the National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) reanalysis with a 2.58 3 2.58 horizontal resolution were used as observations. The monthly outgoing longwave radiation (OLR) data from the National Oceanic and Atmospheric Administration satellites on a 2.58 latitude–longitude grid from 1980 to 2009 was used as a proxy for tropical convection (Liebmann and Smith 1996). The canonical ENSO can be defined by the Nin˜o-3 SST index, which is a SSTA averaged over 58S–58N, 1508–908W without the application of 5-month smoothing. The ENSO Modoki phenomenon is quantified by the ENSO Modoki index (EMI) constructed by Ashok et al.
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(2007), which is computed based on the expression EMI 5 [SSTA]C 2 0.5 [SSTA]E 2 0.5 [SSTA]W, where [SSTA]C, [SSTA]E, and [SSTA]W represent the SSTA averaged over the regions 108S–108N, 1658E–1408W; 158S–58N, 1108–708W; and 108S–208N, 1258–1458E, respectively. After removing the linear trends of the indices, the JJASON seasons during which the Nin˜o-3 index and EMI are larger than one standard deviation are identified as the ENM events (forming in 1966, 1977, 1990, 1994, 2002, and 2004) and CEN events (forming in 1963, 1969, 1972, 1976, 1982, 1987, 1991, and 1997).
3. Modulation of the two kinds of El Nin˜o on TC frequency To examine the impact of the two kinds of El Nin˜o on TC frequency over the SCS, following the procedure of Chu and Wang (1997), the bootstrap resampling method is applied to construct the confidence intervals about the mean TC frequency, and the two-sample permutation procedure is used to test whether the TC frequency in El Nin˜o years differs significantly from that in non– El Nin˜o years. The nonparameteric bootstrap technique allows inferences about a sample to be drawn without assumptions being made on the underlying probability (Wilks 1995). 10 000 bootstrap replications are generated, then the 95% confidence intervals are given by the 250th (10 000 3 0.05/2) ranked mean and 9750th ranked mean [10 000-(10 000 3 0.05/2)]. The permutation test is based on the following hypotheses: H0: mi 5 mj ; H1: mi 6¼ mj , where the subscripts i and j refer to the El Nin˜o and non– El Nin˜o batches. Following the definition of Chu and Wang (1997), the statistic U is defined as U5
(ni 1 nj 2 2)1/2 (xi 2 xj ) [(ni 2 1) vari 1 (nj 2 2) varj /kk]1/2 (1/ni 1 kk/nj )
,
where n is the sample size, x the mean, and var the variance. The factor kk is the population variance ratio. Likewise, 10 000 values of the U statistic are calculated from the resampled data. If the actual statistic value falls outside of the 95% of the reference distribution, the permutation test rejects the null hypothesis and is rejected at the 5% test level. Table 1 lists some summary statistics of TC frequency. The mean TC frequency over the SCS during JJA and SON for the two kinds of Pacific Ocean warming all lie within the confidence interval at 95%. The large confidence interval during JJA for ENM years and the small one during SON for CEN signify, respectively, more TC occurrence in the early season of ENM and less TC
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TABLE 1. Mean of the annual TC frequency from observations and corresponding 95% confidence intervals based on bootstrap resampling method, and U statistics of TC frequency from observations and corresponding 95% confidence intervals from simulations based on a permutation test for El Nin˜o Modoki (ENM) and canonical El Nin˜o (CEN) during JJA and SON. Bold (asterisk) denotes significance at the 5% level for U statistic.
ENM_JJA CEN_JJA ENM_SON CEN_SON
Mean TC frequency
Corresponding 95% confidence interval
U statistic
Corresponding 95% confidence interval
3.50 3.11 2.25 1.88
(2.50, 4.67) (1.57, 3.86) (1.29, 3.37) (0.44, 2.78)
1.54* 0.98 20.87 21.42*
(21.13, 1.22) (21.23, 1.43) (21.21, 1.53) (21.17, 1.32)
occurrence in the late season of CEN. Results from the permutation test, a two-sample test related to the bootstrap, confirm that the actual U value during JJA of ENM years (SON of CEN years) lies above (below) the 95% confidence interval, suggesting that the mean TC frequency during JJA of ENM years (SON of CEN years) is significantly more (less) than that in non-ENM years (non-CEN years). In contrast, the TC frequency during JJA between CEN and non-CEN years, as well as during SON between ENM and non-ENM years, is not significantly different at the 5% test level. Because the two kinds of Pacific Ocean warming can exert an opposite modulation on TC frequency over the SCS and, moreover, the modulations exhibit seasonality, it is necessary to examine the different circulation characteristics corresponding to the two kinds of El Nin˜o events. Figures 1a and 1b display the composite SSTA and 850-hPa wind anomalies during JJA for the ENM and CEN years. The composites of OLR anomalies for two kinds of El Nin˜o events from 1980 onward are also given. For clarity, OLR anomalies are smoothed by applying a T11 truncation to remove smaller-scale features and to highlight the broad-scale convection pattern. Although anomalously warm SST is present in the central Pacific during both ENM and CEN events, the SST warming related to the former extends more meridionally and shifts more westward compared to the counterpart during CEN. Enhanced convection related to ENM dominates a large fraction of tropical WNP, while suppressed convection is mainly confined over the tropical Maritime Continent (Fig. 1a). In contrast, the anomalous convection associated with CEN is found to the east of 1508E. Interestingly, there exist two major regions of convective suppression: one situated just to the north of enhanced convection in the equatorial central Pacific and the other covering a much broader area of the Maritime Continent west of 1508E (Fig. 1b). In response to anomalous OLR associated with the different types of El Nin˜o, the 850-hPa wind anomalies also exhibit markedly different patterns. During the ENM years, low-level westerly anomalies dominate the tropical
WNP and SCS, attaining their maximum near 1358E and meridionally extending to 158N, which results in a largescale cyclonic anomaly (Fig. 1a). The combination of the tropical westerly anomalies and the northeasterly anomalies from the China mainland initiates a cyclonic shear over the SCS, in favor of TC genesis, in agreement with significantly enhanced TCs during this period. Comparatively, during the CEN years, the maximum of tropical westerly anomalies shifts eastward beyond the date line. In response to the zonally elongated suppressed convection over the WNP subtropical region as well as the Maritime Continent, the two anomalous anticyclones prevail to the south of Japan and in the southern part of the SCS, respectively (Fig. 1b). Another distinct feature is the change of direction of wind over the China mainland from northeasterlies during the ENM years to southwesterlies during the CEN years, resulting in the absence of anomalously cyclonic circulation over the SCS. This anomalous circulation pattern is responsible for the insignificant modulation of CEN on TC frequency. Corresponding to the two kinds of Pacific Ocean warming, the circulation and OLR anomalies exhibit remarkable seasonal variation. A direct comparison of the distribution of OLR and wind anomalies during JJA and SON reveals that, during SON, the convective cooling (positive OLR anomaly) over the Maritime Continent is strengthened, shifts more northward, and expands in zonal and meridional directions, while convective heating over the western and central Pacific remarkably shifts eastward and spatially shrinks (Figs. 2a,b). In response to the heating sink and source associated with tropical convection during the late season, the atmospheric circulation anomalies in the ENM and CEN years dramatically vary. In conjunction with the remarkable expansion of the heating sink over the western part of the tropical WNP and eastward-shifting heating source over the eastern part of the tropical WNP, the tropical westerly anomalies penetrate eastward in both of the El Nin˜o events. During the ENM years, the anomalous northerly flow is dominant over the SCS, and anomalously anticyclonic circulation occurs over the Indochina
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FIG. 1. Composites of SSTA (8C) (shading), 850-hPa wind anomalies (vectors, see scale arrow at the upper right; units of scale arrow: m s21), and OLR anomalies (dash/solid contours indicating negative/positive values; contour interval 3 W m22, zero contours omitted) during JJA for (a) El Nin˜o Modoki and (b) canonical El Nin˜o events. OLR anomalies are computed based on data starting from 1980 and with a T11 truncation applied for clarity.
Peninsula and Bay of Bengal (Fig. 2a). In contrast to the heating sink during ENM years, the anomalous cooling, centered to the south of the Philippines, is much stronger and expands more zonally in the CEN years. Concurrently, the scale of convective heating over the central Pacific is contracted. As a result, the original anticyclonic anomaly during JJA over the WNP subtropical region displayed in Fig. 1b disappears, whereas an enhanced anticyclonic circulation anomaly persists over the SCS to the northwest of the cooling center (Fig. 2b), which is responsible for the significantly decreased TC frequency during SON in the CEN years. The above results reveal the important role of the anomalous circulation, consistent with a Matsuno–Gill response to tropical heating source and sink, in initiating and modulating TC activity over the SCS during the different seasons for the two kinds of Pacific Ocean warming.
Additionally, the anomalies of midtropospheric relative humidity and zonal wind vertical shear, as two independent environmental factors favorable for TC genesis, are illustrated in Fig. 3. Over the SCS region, the midlevel environment is much more moist during JJA of ENM, whereas it is much drier during SON of CEN compared with the climatological mean. On the other hand, although the amplitudes of zonal wind vertical shear do not show remarkable differences over the SCS, the increase of zonal wind vertical shear with latitude › ›u .0 ›y ›z during JJA of ENM years is believed to be conducive to the increased TC incidence, consistent with the results by McBride and Zehr (1981) who examined horizontal
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FIG. 2. As in Fig. 1 but for the period SON.
distribution of zonal vertical shear that is favorable for TC activity. Overall, the aforementioned analyses account for how the distinct circulation characteristics corresponding to shifting Pacific Ocean warming modulate on TC frequency over the SCS.
4. Concluding remarks Using the NCEP reanalysis variables and TC dataset for the period 1960–2009, the present study examines the different impacts of ENM and CEN events on TC frequency over the SCS during boreal summer and fall. Different from the Nin˜o-3.4 index, which may mix up eastern Pacific warming and central Pacific warming, EMI and Nin˜o-3 index can distinguish two types of Pacific Ocean warming. The results show that the modulations of shifting Pacific Ocean warming on TC frequency over the SCS exhibit a seasonal variation. ENM has a markedly positive modulation on TC frequency during JJA of ENM years,
while the TC frequency over the SCS is significantly below normal during SON of CEN years. These differences can be attributed to the distinct large-scale circulation anomalies between the two types of El Nin˜o events. An anomalous cyclonic circulation in response to a broad heating source dominates a large fraction of regions over the WNP and SCS during JJA in the ENM years. In contrast, accompanied by the enhancement of the zonally elongated cooling centered in the Maritime Continent during SON in the CEN years, an anomalous anticyclonic circulation persists over the SCS. These atmospheric responses are well consistent with Matsuno– Gill patterns, suggesting a key role of the different distribution of heating source and sink for the two kinds of Pacific Ocean warming in tropical anomalous circulation and hence TC activity over the SCS. Acknowledgments. I thank the two anonymous reviewers for their insightful comments, which strengthened the clarity and usefulness of this paper. This study
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FIG. 3. Composites of 500-hPa relative humidity anomalies (%) (shading) and zonal wind vertical shear anomalies (m s21) between 200 and 850 hPa (vectors)during (a) JJA for El Nin˜o Modoki and (b) SON for canonical El Nin˜o events.
is supported by the National Natural Science Foundation of China (Grant 40905024 and Grant 40921160379) and the Special Scientific Research Project for Public Welfare (Grant GYHY20100602). REFERENCES Ashok, K., S. K. Behera, S. A. Rao, H. Weng, and T. Yamagata, 2007: El Nin˜o Modoki and its possible teleconnection. J. Geophys. Res., 112, C11007, doi:10.1029/2006JC003798. ——, C. Y. Tam, and W. J. Lee, 2009: ENSO Modoki impact on the Southern Hemisphere storm-track activity during extended austral winter. Geophys. Res. Lett., 36, L12705, doi:10.1029/ 2009GL038847. Chan, J. C. L., 1985: Tropical cyclone activity in the northwest Pacific in relation to the El Nin˜o–Southern Oscillation phenomenon. Mon. Wea. Rev., 113, 599–606.
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