Cite this Article
CHEN Dong, WANG Hui-Jun, LI Guo-Ping, 2013: Change in Spring Meridional Circulation and Its Relation to Summer Typhoon Activities. Atmospheric and Oceanic Science Letters, 6(3): 144-148  
Permissions
Copyright©2013, Institute of Atmospheric Physics, Chinese Academy of Sciences
Change in Spring Meridional Circulation and Its Relation to Summer Typhoon Activities
CHEN Dong1,2,3,4*, WANG Hui-Jun3,4, LI Guo-Ping1
1 College of Atmospheric Sciences, Chengdu University of Information Technology, Chengdu 610225, China 2 Graduate University of Chinese Academy of Sciences, Beijing 100049, China 3 Climate Change Research Center, Chinese Academy of Sciences, Beijing 100029, China 4 Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
* Corresponding author: XU Zhong-Feng, E-mail:xuzhf@tea.ac.cn
Fund:This research was supported by the National Natural Science Foundation of China (Grant No. 41130103).
Abstract

This study documents the decadal changes of the spring meridional circulation (SMC) over 110°E-165°E and the relationship between the SMC and summer (June-July-August-September) typhoon activity over the Western North Pacific (WNP) during 1948-2010. The authors found that the SMC was changed after 1969. Before its change, the SMC had no clear relation with the summer typhoon number over the WNP (TNWNP), but after the change, it has become positively correlated with the TNWNP, with a correlation coefficient of 0.57 between them (above the 99% confidence level). It was observed that after the SMC was changed, the positive tropical sea surface temperature anomaly associated with the SMC was shifted from the Equatorial Eastern Pacific (El Niño) to the equatorial middle Pacific (El Niño Modoki); at the same time, the Pacific decadal oscillation (PDO) pattern over the North Pacific, which is associated with the SMC, was enhanced. The SMC and the TNWNP are both modulated by the El Niño Modoki after 1969, so the relationship between them becomes significant.

Key words: spring meridional circulation; summer typhoon activity; Modoki
1 Introduction

The Western North Pacific (WNP) is a region with frequent typhoon genesis throughout the year. Previous analyses have indicated many relationships between typhoon activities and related atmospheric circulations and sea surface temperature (SST), including stratospheric quasi-biennial oscillation and El Niño southern oscillation and so on \* MERGEFORMAT ( Camargo and Sobel, 2005; Chan, 1985, 1995; Chia and Ropelewski, 2002; Keqin and Neumann, 1986; Lander, 1994; Li, 1984; Pan, 1982; Wang and Chan, 2002; Wu and Lau, 1992; Wu et al. , 2004; Zhou and Cui, 2008, 2011; Zhou et al. , 2008). These studies have demonstrated that tropical SST and tropical circulation can affect typhoon activities. In addition, a number of studies ( Chen, 1965; Fan and Wang, 2004, 2006; Fan, 2007; Wang and Fan, 2007) have indicated that high-latitude circulations and North Pacific sea ice can also influence the genesis of typhoons.

Besides, Tao et al. (1962) and Xu and Gu (1978) indicated that typhoon genesis is associated with the tropical meridional flow patterns over the West Pacific during summer. Chen et al. (2001) found that the East Asia summer monsoon circulation can replace the Northern Hemisphere Hadley cell in July. It have been announced by Zhou and Cui (2008) that the spring global Hadley Cell can also influence the summer typhoon activity. Qin and Wang (2010) found that the variation in intensity of local Hadley cells is inherently connected with the Pacific SST, and the intensity of these cells in the monsoon zone is in negative correlation with the SST of the Niño region during summer and winter. Li and Mao (2011) indicated that the subtropical meridional circulation in East Asia is associated with the rainfall over Yangtze River basin.

In this paper, based on the abovementioned studies, we demonstrate the decadal changes of the spring meridional circulation (SMC) over 110°E-165°E and the relationship between the SMC and typhoon activity over the WNP during the period 1948-2010; We also provide a preliminary physical explanation for the mechanism.

2 Datasets and methods

The datasets used in this study included the following: (1) the typhoon number over the WNP (TNWNP) from the Joint Typhoon Warning Center (JTWC) and (2) the horizontal and vertical wind, geopotential height, SST, and sea-level pressure from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis data, with a horizontal resolution of 2.5° latitude × 2.5° longitude ( Kalnay et al. , 1996). Additionally, precipitation data from the Climatic Research Unit with a horizontal resolution of 0.5° latitude × 0.5° longitude were used. All analyses were confined within a common data period of 1968-2010. In this study, we used the Saffir-Simpson scale to identify tropical storms and typhoons/hurricanes. The criterion for tropical storms was the wind speed in the range of 39-73 miles per hour or 62.8-117.5 km h-1. When the wind speed is more than 74 miles per hour or 119.1 km h-1, it is recognized as a typhoon.

The mass stream function ψwas calculated from the NCEP/NCAR reanalysis datasets using a superposition computation scheme ( Wang, 1994), and local mass stream function was calculated following the method of Li et al. (2011). The El Niño Modoki Index (EMI), as defined by Ashok (2007), is as follows:EMI = [SSTA]A - 0.5 × [SSTA]B - 0.5 × [SSTA]C . (1)The square bracket in Eq. (1) represents the area-averaged sea surface temperature anomaly (SSTA) over each of the regions A (10°S-10°N, 165°E-140°W), B (15°S- 5°N, 110°W-70°W), and C (10°S-20°N, 125°E-145°E), respectively.

3 Analysis and results
3.1 Change of the SMC

Figure 1 shows the distribution of climatic mass stream function from March to August over 110°E–165°E. In March, there are two local Hadley circulations in the Southern and Northern Hemispheres; after April, a nega-tive cell grows over 15°N–30°N and replaces the Northern Hemisphere local Hadley cell, which is similar to the results of Chen et al. (2001). The negative cell becomes strongest in June and July, and then weakens gradually after August. The whole process corresponds to the gen-eration and development of the East Asia summer mon-soon circulation. We thus define the central value of the negative cell as the index of spring meridional monsoon circulation intensity (SMCI), and the spring circulation is calculated as the average of the values from March to May (MAM) (Fig. 2). Figure 3a shows the normalized time series of the SMCI; there was an obvious decadal change in the late 1960s. The Mann-Kendal test result of the time series of SMCI indicated that the SMCI was changed in 1969 (Fig. 3b).

Figure 1 Climatic mass stream function over 110°E-165°E (units: 1010kg s-1): (a) March, (b) April, (c) May, (d) June, (e) July, and (f) August.

3.2 Relationship between the SMC and typhoon activities and the possible mechanism

The time series of the summer typhoon number and SMCI are displayed in Fig. 3c. All the time series have been normalized. The correlation coefficients between the TNWNP and the SMCI were -0.36 (which could not pass the 95% confidence level test) and 0.57 (passed the 99% confidence level test), respectively, for the periods 1948-1969 and 1970-2010, indicating a close relationship between the SMCI and the TNWNP after the changes in SMCI.

Previous studies ( Hu, 1997; Li et al. , 2001; Wu and Wang, 2002; Gong and Ho, 2002; Fu et al. , 2009) indicated that the SST changes, which took place in the 1970s, can influence the interdecadal shift in the East Asian summer monsoon significantly. Zhu et al. (2011) found the shift of Pacific decadal oscillation (PDO) can affect the atmospheric circulations and summer precipitation in East China through the air-sea interaction in the Pacific. He and Jiang (2011) also obtained that the interdecadal variations of the relationships between tropical cyclone over the WNP and large scale circulation are closely re- lated to PDO.

In order to investigate why the connection between the SMCI and the TNWNP changed around 1969, we plotted the regression coefficients between the SMCI and SST from spring to summer in two epochs, before and after 1969 (Fig. 4). A noticeable difference was found over the North Pacific from spring to summer. In spring, negative anomaly in the West Pacific subtropical region was significant during 1948-1969 (Fig. 4a), and became stronger and extended to the East Pacific during 1970-2010. Significant positive anomaly had been found at the west coast of North America and the Equatorial Eastern Pacific during 1948-1969. However, during 1970-2010, significant positive anomaly was extended from north to south along the west coast of North America and the equatorial central Pacific. The pattern in summer is very similar to that in spring, for the ocean has a “long memory. ” To sum up, the SST pattern related to the SMC were non-signifi- cant PDO and El Niño from spring to summer before 1969, whereas it became significant PDO (consistent with He and Jiang’s result (2011)) and El Niño Modoki after 1969. Therefore, we assume that the more occurrences of El Niño Modoki pattern, which is related to changes in the SMC that took place around 1969, may be the reason why SMC became closely related to the TNWNP after 1969.

To confirm our assumption, we investigated the atmospheric circulation, which includes the dynamic parameters for typhoon genesis, in association with the EMI. We computed the regression coefficients of sea level pressure (SLP), horizontal wind ( U- V wind), and relative vorticity upon the EMI. Distributions of the SLP and U- V wind anomalies to positive EMI were plotted based on the data from 1970 to 2010 (Fig. 5). A significant negative anomaly of SLP appears over the WNP. Meanwhile, an anomaly cyclone at 925 hPa dominated the WNP (Fig. 5a), which could enhance the monsoon trough. The positive relative vorticity anomalies dominated the WNP at 850 hPa (Fig. 5b), whereas the negative relative vorticity anomalies dominated at 200 hPa (figure not shown), which was consistent with the cyclonic anomalies (Fig. 5a). These conditions are favorable to the genesis and development of typhoons.

Figure 2 Climatic mass stream function of spring (MAM) over 110°E-165°E (units: 1010kg s-1).

4 Summary and discussion

In conclusion, this study found that the SMC was changed around 1969. Before the change, the SMC had no obvious relation with the TNWNP, but after the change, the connection between the SMC and the typhoon activi- ties became significant (the correlation coefficient between them being 0.57 during 1970-2010).

According to our analyses, we found that the relationship between the SST and the SMC also changed before and after 1969. First, the PDO pattern related to the SMC was enhanced from spring to summer over the North Pacific during 1970-2010 relative to that during 1948-1969, and the PDO’s changes can affect the typhoon genesis ( He and Jiang, 2011). Second, over the tropic Pacific, the SST distribution associated with the SMC changed from El Niño type to El Niño Modoki type in spring and summer of 1969. The positive El Niño Modoki can induce a reduction in SLP over the WNP and produce a cyclonic anomaly, which can enhance the monsoon trough, and then generate a positive and a negative anomaly of relative vorticity in the low and high troposphere, respectively. All these changes are the most important dynamic factors for the genesis and development of typhoons. Overall, the SMC and the typhoon activity have both been modulated by the SST anomaly of El Niño Modoki type after 1969. Therefore, the SMC have a close relationship with the genesis of summer typhoons over the WNP, then it have some value for the summer typhoons prediction. The detailed interation between the PDO and typhoon activity need further investigation.

Figure 3 Time series of (a) the Spring Meridional Circulation Index and (b) its Mann-Kendal test result; (c) Time series of the SMCI (dashed line) and the TNWNP (solid line) during 1948-2010. All the time series have been normalized. The thick vertical line approximates the changing point.
Figure 4 Spatial distribution of the regression coefficients between the SST and the SMCI: (a) spring (MAM) SST and SMCI during 1948-1969; (b) spring (MAM) SST and SMCI during 1970-2010; (c) summer (JJAS) SST and SMCI during 1948-1969; and (d) summer (JJAS) SST and SMCI during 1970-2010. Shaded for the 95% (light color) and 99% (deep color) confidence level.
Figure 5 Spatial distribution of the regression coefficients of the SLP, U-V wind, and relative vorticity upon the EMI in summer during 1970-2010: (a) SLP, U-V wind, and EMI, and (b) relative vorticity and EMI. Shaded for the 95% (light color) and 99% (deep color) confidence level.

References
1 Ashok, K. , S. K. Behera, S. A. Rao, et al. , 2007: El Ni?o Modoki and its possible teleconnection, J. Geophys. Res. , 112, C11007, doi: 10. 1029/2006JC003798.
2 Camargo, S. J. , A. H. Sobel, 2005: Western North Pacific tropical cyclone intensity and ENSO, J. Climate, 18, 2996-3006.
3 Chan, J. C. L. , 1985: Tropical cyclone activity in the northwest Pacific in relation to the El-Ni?o Southern Oscillation phenomenon, Mon. Wea. Rev. , 113, 599-606.
4 Chan, J. C. L. , 1995: Tropical cyclone activity in the western North Pacific in relation to the stratospheric Quasi-Biennial oscillation, Mon. Wea. Rev. , 123, 2567-2571.
5 Chen, L. S. , 1965: The summertime middle and hig latidudes flow patter in Asia and the western North Pacific typhoon tracks, Acta Meteor. Sinica(in Chinese), 35, 476-485.
6 Chen, Y. J. , J. Jian, H. Zhang, 2001: The interannual variability of meidional circulation from 1961 to 1997 and its relation to SST anomaly, Chinese J. Atmos. Sci. (in Chinese), 25(1), 79-88.
7 Chia, H. H. , C. F. Ropelewski, 2002: The interannual variability in the genesis location of tropical cyclones in the northwest Pacific, J. Climate, 15, 2934-2944.
8 Fan, K. , 2007: North Pacific sea ice cover, a predictor for the Western North Pacific typhoon frequency, Sci. China Ser. D-Earth Sci. , 50, 1251-1257.
9 Fan, K. , H. J. Wang, 2004: Antarctic oscillation and the dust weather frequency in North China, Geophys. Res. Lett. , 31, L10201, doi: 10. 1029/2004GL019465
10 Fan, K. , H. J. Wang, 2006: Interannual variability of Antarctic Oscillation and its influence on East Asian climate during boreal winter and spring, Sci. China Ser. D-Earth Sci. , 49(5), 554-560, doi: 10. 1029/2004GL019465.
11 Fu, J. J. , S. Li, D. H. Luo, 2009: Impact of global SST on decadal shift of East Asian summer climate, Adv. Atmos. Sci. , 26, 192-201.
12 Gong, D. Y. , C. H. Ho, 2002: Shift in the summer rainfall over the Yangtze River valley in the late 1970s, Geophy. Res. Lett. , 29, doi: 10. 1029/2001GL0145.
13 He, P. , J. Jiang, 2011: Effect of PDO on the relationships between large scale circulation and tropical cyclone activity over the western north Pacific, J. Meteor. Sinica(in Chinese), 31(3), 266-273.
14 Hu, Z. Z. , 1997: Interdecadal variability of summer climate over East Asia and its association with 500 hPa height and global sea surface temperature, J. Geophys. Res. , 102, 19403-19412.
15 Kalnay, E. , M. Kanamitsu, R. Kistler, et al. , 1996: The NCEP/ NCAR 40-year reanalyses project, Bull. Amer. Meteor. Soc. , 77, 437-471.
16 Keqin, D. , C. J. Neumann, 1986: The relationship between tropical cyclone motion and environmental geostrophic flows, Mon. Wea. Rev. , 114, 115-122.
17 Lander, M. A. , 1994: An exploratory analysis of the relationship between tropical storm formation in the western North Pacific and Enso, Mon. Wea. Rev. , 122, 636-651.
18 Li, C. Y. , 1984: El Ni?o and the typhoon activity, Chinese Sci. Bull. (in Chinese), 14, 1087-1089.
19 Li, P. , Q. L. Miao, 2011: Characteristics of subtropical meridional circulation in East Asia and their influences on precipitation over Yangtze river basin, Meteor. Disaster Reduction Res. (in Chinese), 34(1), 24-31.
20 Li, T. , Y. Zhang, C. P. Chang, et al. , 2001: On the relationship between Indian Ocean sea surface temperature and Asian summer monsoon, Geophys. Res. Lett. , 28, 2843-2846.
21 Pan, Y. H. , 1982: The impact of the thermal conditions in the tropical eastern Pacific on the typhoon frequency, Acta Meteor. Sinica(in Chinese), 40, 25-33.
22 Qin, Y. J. , P. X. Wang, 2010: Local Hadley cells and their relationship with the Pacific sea surface temperature anomaly, J. Trop. Meteor. (in Chinese), 26(2), 138-146.
23 Tao, S. Y. , S. Y. Xu, Q. Y. Guo, 1962: Characteristics of zonal and meridional circulation patterns in tropical and subtropics during boreal summer, Acta Meteor. Sinica(in Chinese), 32, 91-103.
24 Wang, B. , J. C. L. Chan, 2002: How strong ENSO events affect tropical storm activity over the Western North Pacific, J. Climate, 15, 1643-1658.
25 Wang, H. J. , K. Fan, 2007: Relationship between the Antarctic oscillation and the western North Pacific typhoon frequency, Chinese Sci. Bull. , 52, 561-565.
26 Wang, P. X. , 1994: Diagnosis of mean meridional circulation of the model atmosphere in the GCM with low resolution vertically, J. Nanjing Inst. Meteor. (in Chinese), 17(2), 200-204.
27 Wu, G. X. , N. C. Lau, 1992: A GCM simulation of the relationship between tropical-storm formation and Enso, Mon. Wea. Rev. , 120, 958-977.
28 Wu, M. C. , W. L. Chang, W. M. Leung, 2004: Impacts of El Ni?o-Southern Oscillation events on tropical cyclone landfalling activity in the western North Pacific, J. Climate, 17, 1419-1428.
29 Wu, R. G. , B. Wang, 2002: A contrast of the East Asian summer monsoon-ENSO relationship between 1962-77 and 1978-93, J. Climate, 15, 3266-3279.
30 Xu, J. M. , M. R. Gu, 1978: The relationship between the circulation features and the typhoon genesis over the western Pacific in summer, Chinese J. Atmos. Sci. (in Chinese), 2, 174-178.
31 Zhou, B. T. , X. Cui, 2008: Hadley circulation signal in the tropical cyclone frequency over the western North Pacific, J. Geophys. Res. , 113, doi: 10. 1029/2007JD009156.
32 Zhou, B. T. , X. Cui, 2011: Sea surface temperature east of Australia: A predictor of tropical cyclone frequency over the western North Pacific?Chinese Sci. Bull. , 56, 196-201.
33 Zhou, B. T. , X. Cui, P. Zhao, 2008: Relationship between the Asian-Pacific oscillation and the tropical cyclone frequency in the western North Pacific, Sci. China Ser. D-Earth Sci. , 51(3), 380-385.
34 Zhu, Y. L. , H. J. Wang, W. Zhou, et al. , 2011: Recent changes in the summer precipitation pattern in East China and the background circulation, Climate Dyn. , 36, 1463-1473.