Formation Mechanism for the Anomalous Anticyclonic Circulation over Northeast Asia and the Japan Sea in Boreal Winter 1997/98 and the Spring of 1998

WANG Hai, LIU Qinyu*, and ZHENG Jian

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Formation Mechanism for the Anomalous Anticyclonic Circulation over Northeast Asia and the Japan Sea in Boreal Winter 1997/98 and the Spring of 1998

WANG Hai, LIU Qinyu, and ZHENG Jian

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A robust anomalous anticyclonic circulation (AAC) was observed over Northeast Asia and the Japan Sea in boreal winter 1997/98 and over the Japan Sea in spring 1998. The formation mechanism is investigated. On the background of the vertically sheared winter monsoonal flow, anomalous rainfall in the tropical Indo-Western Pacific warm pool excited a wave train towards East Asia in the upper troposphere during boreal winter of 1997/98. The AAC over Northeast Asia and the Japan Sea is part of the wave train of equivalent barotropic structure. The AAC over the Japan Sea persisted from winter to spring and even intensified in spring 1998. The diagnostic calculations show that the vorticity and temperature fluxes by synoptic eddies are an important mechanism for the AAC over the Japan Sea in spring 1998.

anomalous anticyclonic circulation; Northeast Asia; Japan Sea; wave train; synoptic eddy

1 Introduction

El Ni?o-Southern Oscillation (ENSO) is a coupled ocean-atmosphere phenomenon in the tropical Paci?c. ENSO exerts substantial impacts on short term climate variability on the globe. During the El Ni?o (La Ni?a) maturation in winter, corresponding to the Walker circulation anomaly, the diabatic heating anomaly over the central equatorial Pacific can excite the Pacific North American (PNA) pattern (Wallace and Gutzler, 1981) in the Northern Hemisphere and the Pacific South American (PSA) pattern (Robertson and Mechoso, 2003) in the Southern Hemisphere. Besides the atmospheric diabatic heating anomaly over the central equatorial Pacific, other diabatic heating anomalies appear in both the tropical Western Pacific-East Indian Ocean and the tropical Western Indian Ocean respectively, associated with the Walker circulation anomaly in El Ni?o (La Ni?a) year. Whether they also induce some teleconnection pattern from tropical to middle latitudes in atmosphere troposphere still needs further investigation.

In boreal winter, the Western Pacific (WP) pattern is also a major teleconnection pattern in wintertime lower troposphere (500hPa) and characterized by a meridional dipole of circulation anomalies over the Far East (155?E,60?N) and tropical Western Pacific (155?E, 30?N) (Wallace and Gutzler, 1981). Remote atmospheric responses to ENSO are known to have a considerable projection onto WP pattern in winter (Horel and Wallace, 1981). A crucial process that conveys the impact of El Ni?o on East Asia is an anomalous low-level circulation feature termed Philippine Sea anticyclone (Wang., 1999; Wang., 2000; Lau and Nath, 2000; Wang and Zhang, 2002). Recently，it is demonstrated that the oceanic forcing from the deep tropical eastern Pacific can instigate the Philippine Sea anticyclone at the 850hPa level and rainfall anomaly over East Asia (Lu., 2011).

A new teleconnection pattern named as Indo-Western Pacific and East Asia pattern (IWPEA pattern) in the upper troposphere emitted from the Indo-Western Pacific toward East Asia in boreal winter has been demonstrated through statistical and numerical experiments (Zheng., 2013). If the teleconnection pattern of the atmosphere anomaly is ‘low-frequency flow’ in the upper troposphere, there is the possibility of synoptic eddy and low- frequency flow interaction (SELF feedback) in middle latitudes, where the synoptic eddy in the upper troposphere is very strong (Lau, 1988; Jin., 2006a, b; Jin, 2010).

The 1997/98 El Ni?o is the strongest one since 1950. Besides the stronger PNA teleconnection pattern, there is another weaker teleconnection wave train at the 200hPa level (hereafter H200) in East Asia in the winter 1997/98 (Fig.1a); the wave train in East Asia is induced by ano- malous rainfall in tropical Western Pacific (Zheng., 2013). The positive anomaly of the geopotential height in the upper troposphere over Northeast China and the Japan Sea appears in the boreal winter of 1997/98 (Fig.1a). Besides this typical El Ni?o event, for other three El Ni?o winters (1982/83, 1991/92 and 2002/2003), associated with anomalous rainfall in tropical Western Pacific and Indian Ocean, the winter weaker wave train at the H200 still appears in East Asia (Fig.2a). In spring 1998, there is a stronger AAC over the Japan Sea (Fig.1b), but similar AAC cannot be found over the Japan Sea in the spring of 1983, 1992 and 2003 (Fig.2b) which are El Ni?o second years. Why there is stronger AAC over the Japan Sea only in the spring of 1998?

Fig.1 Anomalous geopotential height at the 200hPa level (contours interval: 30m) and anomalous precipitation (shaded, mmmon-1) in (a) DJF; (b) MAM in 1997/1998. Red dashed line in (a) and (b) denotes the section position in Figs.3(a) and (b).

Fig.2 The average of anomalous geopotential height at the 200hPa level (contours interval: 30m) and anomalous precipitation (shaded, mmmon-1) in (a) DJF; (b) MAM in 1982/1983, 1991/1992 and 2002/2003.

According to the analysis and comparison above, we proposed that the formation mechanism of the AAC over the Northeast Asia and the Japan Sea is the winter wave train induced by tropical rainfall anomaly in the winter 1997/98 and the formation mechanism of the AAC over the Japan Sea should be the synoptic scale eddies’ effect on the low-frequency system in the spring of 1998. Besides the heating induced teleconnection pattern, the synoptic eddy may also regulate and intensify the low-frequency teleconnection pattern when the heating source weakened.

The rest of the paper is organized as follows. Section 2 is a brief description of the data and methods used in this study. In Section 3, the wave train in the upper troposphere will be presented in the winter 1997/98. In Section 4, possible reasons of the AAC formation in the spring of 1998 have been detected. Summary and discussion are given in Section 5.

2 Data and Methodology

The atmospheric data used in this study are from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis dataset on a 2.5?×2.5? grid (Kalnay., 1996). The monthly data are used from January 1980 to December 2009. The daily data used from December 1, 1997 to May 31, 1998 are calculated based on the 30 years (from January 1980 to December 2009) long term mean.

According to Takaya and Nakamura (2001), the wave-activity flux (Eq. (1)) is shown to be parallel to the local three-dimensional group velocity of Rossby waves, and hence to be suited for a snapshot diagnosis of the three-dimensional propagation of wave packets of migratory and stationary eddies on a zonally varying basic flow.,in Eq. (1) represents the monthly mean zonal and meridional wind. P is the pressure normalized by 1000hPa anddenotes the monthly anomalous stream function at the 200hPa level with subscripts,refering to their partial differentials in the zonal and meridional direction.

To measure the feedback of the synoptic eddies onto low-frequency flow, the eddy-induced vorticity and temperature fluxes are defined as follows (Kug., 2010):

, (2a)

3 The AAC in Northeast Asia and the Japan Sea in Winter 1997/98

In winter 1997/98, there is more rainfall over the central equatorial Pacific and tropical Western Indian Ocean and less over the tropical Eastern Indian Ocean-Western Pacific (Fig.1a), which means that besides the atmospheric positive diabatic heating anomaly over the central equatorial Pacific, there is still a positive diabatic heating anomaly center in the Western Indian Ocean and a negative anomaly center in Eastern Indian Ocean-Western Pacific respectively. Corresponding to the rainfall anomaly in Eastern Indian Ocean-Western Pacific and the Western Indian Ocean, the wave train pattern (teleconnection pattern in H200) emitted from the Indo-Western Pacific toward the northeastern China and the Japan Sea appears in winter. This winter teleconnection pattern is induced by the heating anomaly in Eastern Indian Ocean-Western Pacific, and the heating anomaly over the equatorial central Pacific is not important, which has been proved by the analysis through numerical experiments (Zheng,2013). As background circulation, the vertically sheared East Asian winter monsoon should aid in releasing the baroclinic instability energy and in forming a meridional Rossby wave train due to the energy conversion from the heating-induced baroclinic flow anomalies to the barotropic motions near the heating source (Lee, 2009; Wang, 2010; Zheng., 2013).

As a part of the winter meridian teleconnection wave train, the positive anomaly of the geopotential height at the 200hPa level over Northeast Asia and the Japan Sea appears in the winter 1997/98. It is obvious that there is a slightly poleward-tilted anomalous vorticity field in the upper troposphere over East Asia (Fig.3a), which is caused by the vertically sheared winter monsoon and the thermal difference between the land and ocean contributing to this baroclinic structure as well (Zheng., 2013). The vertical section of the wave train along the dashed line in Fig.1a (from 0?, 95?E to 60?N, 130?E) shows the structure of the winter wave train is equivalent barotropic (Fig.3a), and the maximum anomaly of this low-frequency flow appears in the upper troposphere (250–300hPa).

Fig.3 The vertical section of the anomalous vorticity (shaded, ×10-5s-1) and the anomalous geopotential height (contours interval: 15m) along the red dashed line in Fig.1a (from 0?, 95?E to 55?N, 130?E) and Fig.1b (from 0?, 100?E to 60?N, 145?E) during (a) DJF (b) MAM in 1997/1998. The corresponding vectors represent the anomalous wind field with scaling at the bottom right corner.

Corresponding to this winter meridional wave train with equivalent barotropic low-frequency low, there is a positive anomaly of the geopotential height in H200 over Northeast China and the Japan Sea. The low-frequency flow can be demonstrated through the wave-activity flux (Eq. (1)) in H200 (voctors in Fig.4a), which is parallel to the local group velocity of stationary Rossby wave (Takaya and Nakamura, 2001). Contours in Fig.4a and Fig.4b show the vorticity anomalies at the 200hPa and 850hPa level in winter 1997/98 respectively, where the color shading areas represent the rainfall anomaly, which indicates that this winter teleconnection pattern of the H200 may have a significant influence on the winter rainfall anomaly over East Asia. Similar winter precipitation anomaly pattern indicates that tropical oceanic forcing outside the Ni?o regions can also exert significant influence on East Asian climate based on numerical experiments (Lu., 2011). In the lower troposphere (850hPa) the wave-activity fluxes are necessary for the low level anticyclonic circulation (negative voticity) anomaly in tropical Western North Pacific, which is similarto the previous researches about the El Ni?o effect on lower troposphere, such as those by Wang. (1999), Wang. (2000), Wang (2002) and Zhang and Sumi (2002). While in the upper troposphere, the wave-activity fluxes are emitted from the tropics and pointed to Northeast Asia and the Japan Sea, indicating the baroclinic instability energy conversion would benefit the poleward propagation of the Rossby wave in the upper troposphere.

Therefore, due to the anomalous rainfall in Indo-Western Pacific warm pool and vertically sheared boreal winter monsoon, there is a wave train (positive-negative-positive geopotential height anomaly) from tropical western Pacific to East Asia, which is corresponding to the AAC over Northeast Asia and the Japan Sea during the boreal winter of 1997/98.

Fig.4 Horizontal wave-activity flux (vectors) derived by Takaya and Nakamura (1999, 2001), whose scaling is given at the top right corner at the (a) 200hPa and (b) 850hPa levels. Anomalous vorticity (contours) at the (a) 200hPa (solid lines marked from 9 and dashed lines marked from ?3 at 3×10-6s-1 intervals) and (b) 850hPa (solid and dashed lines marked from ±2 at 2×10-6s-1 intervals). The shaded areas represent the anomalous precipitation (mmmon-1) in DJF 1997/1998.

4 The Strongest AAC over the Japan Sea in the Spring of 1998

In the spring of 1998, it is found that the winter AAC centered at the 200hPa level moves southeastwards into the Japan Sea and forms a stronger AAC, which is the strongest atmospheric anomaly center in Northern Hemisphere as the PNA pattern has become weaker due to the decay of El Ni?o (Fig.1b). In the next section, the formation mechanism of the stronger AAC over the Japan Sea in the spring of 1998 will be investigated.

4.1 The Strongest AAC over the Japan Sea in the Spring of 1998

The maximum positive geopotential height anomaly (>120m in H200) over the whole northern hemisphere with barotropic structure (Fig.3b) appears over the Japan Sea, but there is no clear negative geopotential height anomaly to the south of this positive geopotential height anomaly in spring 1998 (Fig.1b and Fig.3b). Therefore, the stronger AAC over the Japan Sea is not a response to the tropical rainfall anomaly in the spring of 1998. It is also proved by horizontal wave-activity flux at the 200 hPa and 850hPa level in the spring of 1998 respectively (vectors in Figs.5a and 5b), because there is not any evident wave-activity flux from tropical to middle latitudes in the upper or lower troposphere in the spring of 1998.

In order to probe the formation mechanism of the stronger AAC over the Japan Sea, local atmospheric processes that may influence the variation of vorticity needs further investigation. Based on the vorticity equation (Eq. (3)), the factors that decide the variation of the vorticity have been derived by an analysis of the order of magnitude and approximation using the small disturbance analysis. By calculating the linearized right hand side terms of Eq. (3), it shows that the first two terms known as advection and the forth term known as divergence/convergence dominate the largest two order of all the factors that may influence the variation of the vorticity. The negative vorticity advection and the divergence of vorticity may be the key factors that lead to the intensification of the AAC over Japan Sea in the spring of 1998.

. (3)

4.2 Role of the Synoptic Eddy

In order to testify the relationship between synoptic eddies and the AAC, we first checked the anomalous daily H200 from December 1, 1997 to May 31, 1998. During the spring (March 1, 1998 to May 31, 1998), there are about 64 days when the positive anomalous geopotential height over the Japan Sea appears, which is far more than the 38 days in DJF. It is found that during the MAM 1998, there are about 15 anomalous synoptic anticyclonic eddies with the period of 2–7 days over the Japan Sea. Therefore, the effect of the synoptic eddies on the low-frequency flow has to be taken into consideration in understanding the formation mechanism of the AAC over the Japan Sea during the spring of 1998.

The sum of the zonal and meridional eddy-induced vorticity flux defined in Eq. (2a) at the 200hPa level is shown in Fig.6a during MAM 1998. It is found that there is a negative anomalous center of the eddy-induced vorticity flux (shading in Fig.6a) locates in the AAC center (contours in Fig.6a) at the 200hPa level over the Japan Sea, which illustrates the synoptic eddies providing the negative vorticity flux that helps to maintain and even intensify the AAC over the Japan Sea by generating the anomalous negative vorticity. Besides, we calculated the divergence/convergence of the eddy vorticity flux as shown in Fig.6c. It shows a divergence to the upstream of the AAC center and a convergence to its downstream. This is coherent to the local weather system as the synop-tic eddy is moving from the west to the east and leads to a convergence of the negative eddy-induced vorticity flux over the Japan Sea near the AAC center. Hence we have the confidence that the geopotential height structure in Fig.3 is maintained by the anomalous synoptic scale eddies.

The synoptic eddies can also feedback to low-frequency flow through eddy-induced temperature flux (Lau and Nath, 1991). Fig.6b shows the sum of zonal and meridional anomalous eddy-induced temperature flux defined in Eq. (2b). There is a positive eddy-induced temperature flux center to the south of the positive geopotential height anomaly. This kind of warm pattern will benefit the strengthening of the local warm high in the upper troposphere compared with that in winter. Thus, the eddy-induced temperature flux also favors reinforcing the positive geopotential height over the Japan Sea in the spring of 1998.

Based on the analysis above, we could understand that the effect of synoptic eddy-induced vorticity and temperature flux could be the dominant formation mechanism of the stronger AAC over the Japan Sea in the spring of 1998. Because the synoptic eddy is stochastic and the rainfall anomaly is weak in intensity over Indo-Western Pacific, the stronger AAC cannot be found in spring of other El Ni?o second year.

5 Summary

Based on the observational data, it is found that there is Anomalous Anticyclonic Circulation (AAC) over Northeast Asia and the Japan Sea in boreal winter 1997/98 and a strongest AAC over the Japan Sea in the spring of 1998. The AAC over Northeast Asia and the Japan Sea is induced by the anomalous rainfall over tropical Indo-Western Pacific in winter 1997/98 during the peak phase of El Ni?o event. The effect of synoptic scale eddy-induced instability at the mid-latitude was finally applied in explaining the intensification of the AAC over the Japan Sea from the winter of 1997/98 to the following spring. The negative vorticity flux and positive temperature flux induced by synoptic eddy jointly help to strengthen the AAC in the upper troposphere.

As the strongest event up till today, the El Ni?o in 1997/98 contributes the most in statistical analysis due to its historical intensity. In the present study, we just take this case as a typical example to understand the formation mechanism of the anomalous atmospheric circulation from winter to spring. With this example, we can understand why there is AAC over the Japan Sea only in the spring of 1998 and why there is no stronger AAC in the spring of other El Ni?o second year due to the weak rainfall anomaly over Indo-Western Pacific and the stochastic synoptic eddy. The specific role of the synoptic scale eddies in regulating the low-frequency system in the middle latitudes still needs further investigation.

Acknowledgements

The authors thank Prof. Feifei Jin for sharing his constructive idea and Prof. Shang-ping Xie, Dr. Chunzai Wang, Dr. Jian Lu for their comments and suggestions. This work is supported by the Ministry of Science and Technology of China (National Basic Research Program of China (Grant No. 2012CB955602)), the National Key Program for Developing Basic Science (Grant No. 2010 CB428904), and the Natural Science Foundation of China (Grant Nos. 40830106, 40921004, 41176006).

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(Edited by Xie Jun)

10.1007/s11802-013-2233-6

ISSN 1672-5182, 2013 12 (2): 312-317

. Tel: 0086-532-66782556 E-mail:liuqy@ouc.edu.cn

(December 3, 2012; revised March 26, 2013; accepted April 1, 2013)

? Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2013