ENSO cycle and climate anomaly in China*

Transcription

1 Chinese Journal of Oceanology and Limnology Vol. 30 o. 6, P. 985-, ESO cycle and climate anomaly in China* CHE Yongli ( 陈永利 ) 1, 2, **, ZHAO Yongping ( 赵永平 ) 1, 2, FEG Junqiao ( 冯俊乔 ) 1, 2, WAG Fan ( 王凡 ) 1, 2 1 Institute of Oceanology, Chinese Academy of Sciences, Qingdao , China 2 Key Laboratory of Ocean Circulation and Waves, Chinese Academy of Sciences, Qingdao , China Received ov. 21, 2011; accepted in principle Feb. 9, 2012; accepted for publication Mar. 29, 2012 Chinese Society for Oceanology and Limnology, Science Press, and Springer-Verlag Berlin Heidelberg 2012 Abstract The inter-annual variability of the tropical Pacific Subsurface Ocean Temperature Anomaly (SOTA) and the associated anomalous atmospheric circulation over the Asian orth Pacific during the El iño-southern Oscillation (ESO) were investigated using ational Centers for Environmental Prediction/ ational Center for Atmospheric Research (CEP/CAR) atmospheric reanalysis data and simple ocean data simulation (SODA). The relationship between the ESO and the climate of China was revealed. The main results indicated the following: 1) there are two ESO modes acting on the subsurface tropical Pacific. The first mode is related to the mature phase of ESO, which mainly appears during winter. The second mode is associated with a transition stage of the ESO developing or decaying, which mainly occurs during summer; 2) during the mature phase of El iño, the meridionality of the atmosphere in the mid-high latitude increases, the Aleutian low and high pressure ridge over Lake Baikal strengthens, northerly winds prevail in northern China, and precipitation in northern China decreases significantly. The ridge of the Ural High strengthens during the decaying phase of El iño, as atmospheric circulation is sustained during winter, and the northerly wind anomaly appears in northern China during summer. Due to the ascending branch of the Walker circulation over the western Pacific, the western Pacific Subtropical High becomes weaker, and south-southeasterly winds prevail over southern China. As a result, less rainfall occurs over northern China and more rainfall over the Changjiang River basin and the southwestern and eastern region of Inner Mongolia. The flood disaster that occurred south of Changjiang River can be attributed to this. The La iña event causes an opposite, but weaker effect; 3) the ESO cycle can influence climate anomalies within China via zonal and meridional heat transport. This is known as the atmospheric-bridge, where the energy anomaly within the tropical Pacific transfers to the mid-high latitude in the northern Pacific through Hadley cells and Rossby waves, and to the western Pacific-eastern Indian Ocean through Walker circulation. This research also discusses the special air-sea boundary processes during the ESO events in the tropical Pacific, and indicates that the influence of the subsurface water of the tropical Pacific on the atmospheric circulation may be realized through the sea surface temperature anomalies of the mixed water, which contact the atmosphere and transfer the anomalous heat and moisture to the atmosphere directly. Moreover, the reason for the heavy flood within the Changjiang River during the summer of 1998 is reviewed in this paper. Keyword : ESO cycle; tropical Pacific Ocean; subsurface ocean temperature anomalies; interannual variability; climate anomaly of China 1 ITRODUCTIO The El iño-southern Oscillation (ESO) is the strongest inter-annual signal of the air-sea coupled system in the tropical Pacific. It not only impacts the oceanic variability of the Pacific, but influences the climate of areas surrounding the Pacific as well as the global climate. Previous studies (Tao and Zhang, 1998; Chen et al., 0; Huang and Chen, 2; i and Sun, 3; Bai and Wu, 3) show that the ESO is closely related to East Asian Monsoon. * Supported by the Knowledge Innovation Program of Chinese Academy of Sciences (o. KZCX2-YW-Q11-02), the CAS Strategic Priority Research Program (o. XDA ), and the ational Basic Research Program of China (973 Program) (o. 2012CB417401) ** Corresponding author: ylchen@qdio.ac.cn

2 986 CHI. J. OCEAOL. LIMOL., 30(6), 2012 Vol.30 During an El iño event, the East Asian Monsoon may be weak, while the opposite occurs during La iña. During the mature phase of El iño, the East Asian winter monsoon to the east of China is weak, consistently resulting in a warmer winter in southern China. During the developing phase of El iño, more rainfall occurs in the region of the Changjiang (Yangtze) River-Huaihe River, less rainfall occurs in northern China and south of the Changjiang River. The result is opposite during the decaying phase of El iño, with flooding occurring in southern China. It has also been pointed out that the ESO cycle may influence the climate of China due to its effect on convection activity in the western Pacific Ocean, because this convection activity can change the atmospheric circulation in the East Asian region (Zhang et al., 1996; Chen and Hu, 3). The ESO event has been extensively taken into account for climate prediction within China. But there are also issues around uncertainty, as the mechanism of how the ESO affects the atmospheric circulation is not clear. For example, why does the positive potential height and anticyclone always appear over Mongolian Plateau in orth China during the mature phase of El iño? And why is there strong southerly flow over the Bay of Bengal? During the decaying phase of El iño, what processes induce the strong summer monsoon and high rainfall in southern China and little rainfall within the Huanghe (Yellow) River- Huaihe River Basins? Of the many strong El iño events, why did the Changjiang River suffer heavy flooding only during the 1998 event? Most previous studies focused mainly on the effect of the ESO influencing summer precipitation via affecting the subtropical high. Is there any influence of ESO on the mid-high latitude circulation and the Indian monsoon? Is the southerly flow that induces the climate anomaly within China and the Indian Ocean anomaly related to the ESO? Previous investigations mostly considered the sea surface temperature anomaly (SSTA) as an indication of oceanic variability. However, the ESO is a significant oceanic event occurring in the subsurface layer in the tropical Pacific, while the SSTA is the result of large scale air-sea interactions, and is largely influenced by the persistent air-sea heat fluxes, wind velocity, precipitation and cloudiness. Therefore, the SSTA could not accurately represent the dynamic processes of the ESO event. This paper aims to analyze the inter-annual variability of the tropical Pacific subsurface temperature anomaly and associated atmospheric circulation over the Asian orth Pacific, and the effect of this variability on the climate and mechanism of the China anomaly. 2 DATA AD METHOD The Oceanic data utilized in this study are predominantly the Simple Ocean Data Assimilation (SODA ) provided by University of Maryland (Carton and Giese, 8). The specific data used in this study are the ocean temperature field data, ranging from S to with a resolution of 0.5 latitude 0.5 longitude grid, divided into 24 layers in the vertical direction within a depth of m. The atmospheric reanalysis data used in this research are from the ational Centers for Environmental Prediction/ ational Center for Atmospheric Research (CEP/CAR) (Kalnay et al., 1996), with the resolution of 2.5 longitude 2.5 latitude 17 vertical levels. Both the SODA and CEP/CAR data span months over Jan., 1958 Dec., 7. Based on the observed fact that the maximum sea temperature anomalies generally appear near the thermocline, the temperature deviation at the thermocline was used here as the subsurface ocean temperature anomalies (SOTA). First we determined the thermocline curved surface using SODA. Second, the temperature deviation was calculated at the thermocline and the one year low pass filter was applied to remove the annual variation. In conjunction, the long term trend was removed. Similar processes were conducted to the CEP/CAR re-analysis data. During the data analysis, the empirical orthogonal function (EOF) analysis for SOTA was performed in the tropical Pacific first, and the two modes of the ESO event were obtained. Subsequently, the combined EOF analysis for SOTA to relative physical data fields was performed, including 700 hpa height, 700 hpa horizontal wind and SSTA. Within the combined EOF analysis, two or more data fields were put into one large space and a time matrix was consequently decomposed via singular value decomposition. Within the combined analysis, the first two modes of SOTA were kept the same as similar ESO modes through controlling the magnitude of relative physical data. Therefore, the modes of that relative physical field have the major ESO message and reflect the influence of the ESO event. Based on the fact that the ESO occurs in the subsurface layer, which is supported by the research results obtained by Zhao et al. (7), the EOF for SOTA was performed to obtain the two modes of the

3 o.6 CHE et al.: ESO and climate anomaly in China 987 a 31.0% 53.0% b 1E 1E 180 1W 1W 1W 100 W 1E 1E 180 1W 1W 1W 100 W 15.4% 11.1% 1E 1E 180 1W 1W 1W 100 W 1E 1E 180 1W 1W 1W 100 W sot 1 iño3 6 sst 40 iño Year Fig.1 a. The first (upper panel) and second (middle panel) EOF mode of SOTA (solid lines show positive values, dashed lines show negative values, counter interval is 0.01 C), and the associated time coefficients (bottom panel, the solid line represents the first mode, the dashed line the second mode, and the solid thick line represents iño3 index); b. Same as (a) but for SSTA Year ESO events in Section 3. The combined EOF for SOTA and the relative physical field data was then performed to obtain the two modes of relative physical fields associating the ESO event in Section 4. In Section 5 the influence of the ESO on the climate of China was analyzed, and the corresponding anomalous atmospheric circulation is shown. By comparing the results with observation facts and correlational research results, the mechanism controlling the influence of the ESO on the climate of China is discussed. In addition, the reason for the occurrence of the heavy flood in the Changjiang River during the 1998 summer is analyzed. Last, this paper ends with the presentation of some conclusions in Section 6. 3 THE ESO CYCLE AD ITS TWO MODES Figure 1 shows the first two EOF modes of SOTA (Fig.1a) and SSTA (Fig.1b) as well as their time coefficients. The first and the second modes of SOTA explain 31% and 15.4% of the total variance, respectively, with the contribution of the first two modes exceeding 46%. The spatial pattern of the first mode is characterized by a zonal dipole structure within, with the axis located around 1W. The western pole is located north of the Equator near 1E, whereas the eastern pole is located at 90 1W. The amplitude of the latter is slightly larger than that of the former. The second EOF mode presents the decaying or developing phase of the ESO. Spectral analysis suggests that these two modes have significant periods of 56 and 44 months, respectively. Correlation analysis shows that the iño3 index (averaged SSTA in 1 90 W, 5 S 5 ) is highly correlated to the first mode in the zero-one month lag, with coefficients as high as The iño3 index is highly correlated with the second

4 988 CHI. J. OCEAOL. LIMOL., 30(6), 2012 Vol.30 Table 1 Annual cycle of Standard deviation of the time coefficients of the first (I) and second (II) mode for SOTA and SSTA Month Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. ov. Dec. SOTA II SSTA mode in the 9-month lag, with coefficients of Each mode higher than the 3 rd order mode contributes less than 4.7%, and the associated time series is not correlated to the iño3 index. That is to say, the ESO event has two SOTA modes. The first and the second EOF modes of SSTA account for 53% and 11.1% of the total variance, respectively. The first mode is the well-known ESO mode. The associated time series is highly correlated to the iño3 index, with coefficients as high as The second mode is characterized by the opposite sign of the central tropical Pacific and the west/east tropical Pacific. However, the second mode and higher order modes do not have reasonable relationships with the ESO. Therefore, the ESO event only has one SSTA mode. The standard deviation of the time coefficients of the SOTA and SSTA EOF modes (Table 1) indicates that the ESO event reaches its peak during winter time, mostly between October January. The decaying and developing phase occurs during summer time, i.e. June August. The first SSTA mode is consistent with that of SOTA, whereas the second mode is not apparent. Therefore, the mature phase of the ESO occurs during fall and winter both in the subsurface and surface of oceans. The ESO developing and decaying phase only occurs within the subsurface ocean during summer time. The ESO is a kind of cycle and has significant influence on the Asian summer monsoon. Because of the importance of the ESO cycle, many studies have investigated its formation mechanism. Schopf and Suarez (1988) proposed the delayed oscillation theory from the point of view of unstable air-sea interaction. Jin (1997) proposed the recharge and discharge oscillation model of the upper layer warm water in the equatorial Pacific Ocean. Many researchers also discussed other similar theories based on equatorial ocean waves. However, these theories are all based on SST observations, and are limited to equatorial oceans. Recently, the observations (Li, 2; Cao et al., 3) show that the ESO event is not just an airsea interaction phenomenon limited to the ocean surface and equatorial ocean, but a big event that occurs within the whole tropical subsurface Pacific Ocean. Zhao et al. (7) further pointed out that the ESO has two modes within the tropical subsurface Pacific Ocean. The essence of the ESO cycle is the tropical Pacific Ocean mixing layer water oscillations around the tropical Pacific basin between the equator and 12, which is caused by the trade winds anomaly and the coupled ocean-atmosphere interaction. From the SSTA distribution corresponding with the quasi-four-year ESO cycle derived by Zhu et al. (3), we can see that during both the La iña decaying phase and the El iño developing phase, the SSTA pattern is similar to what predominates in the mature phase, but with smaller amplitude. In summary, there are indeed two modes of ESO cycle, and the SSTA distribution during the ESO transition phase in summer is similar to the first mode but with a weaker strength, and thus does not separate with the first mode. Based on the above analysis and previous studies, we can conclude that the ESO event has two modes in the subsurface Pacific Ocean. It is more significant to study the ESO cycle using SOTA instead of SSTA from the point view of dynamics. 4 THE ATMOSPHERIC CIRCULATIO AOMALY DURIG THE ESO CYCLE The ESO may change the tropical and mid-high latitude atmospheric circulation, and hence the climate anomaly in the corresponding regions, via Walker and Hadley cell and some kind of Rossby waves. In order to reveal the changes of atmospheric circulations during the ESO as well as its mechanism affecting the climate, combined EOF analysis on vertical velocity, 700 hpa geopotential height, 700 hpa wind and SOTA was carried out over Asia and the orth Pacific Ocean. The anomalous atmospheric circulation fields associated with the ESO were obtained. Figure 2 shows the meridional distribution of the

5 o.6 CHE et al.: ESO and climate anomaly in China 989 a b Height (hpa) 5 S 5 Height (hpa) 5 15 Height (hpa) Height (hpa) Height (hpa) 35 Height (hpa) 55 0 E 1E 180 1W W 0 E 1E 180 1W W Fig.2 Longitude-height map of vertical velocity (10-3 hpa/s, counter interval is 0.005) during the mature phase (a) and decaying phase (b) of the El iño event along different latitude bands (dark and light shaded areas refer to obvious ascent and decent flow, respectively)

6 990 CHI. J. OCEAOL. LIMOL., 30(6), 2012 Vol a 100 E b Height (hpa) E E Fig.3 Latitude-height map of vertical velocity (units are same as in Fig.2) during the mature phase (a) and decaying phase (b) of the El iño event along different longitude bands (dark and light shaded areas refer to obvious ascent and decent flow, respectively) vertical velocity in different latitude bands. Vertical velocity upward represents the ascending branch of Walker cell and Hadley cell, while the downward vertical velocity represents the descending branch. During the mature phase of El iño (Fig.2a), anomalous ascending movement occurs over the central-eastern equatorial Pacific Ocean, whereas descending movement occurs over the western equatorial Pacific Ocean and southern American Amazon River Basin. Obviously, the Walker circulation in the equatorial Pacific Ocean weakens. The ascending movement over the central-eastern Pacific Ocean shifts to the descending movement with the increasing of latitude, whereas the descending movement over the western Pacific Ocean shifts to ascending movement. Along, anomalous descending appears over the central Pacific Ocean, and anomalous ascending movement occurs over the western and eastern Pacific Ocean, respectively. The anomalous zonal circulation occurs in the middle latitude region. The significant upward flow also occurs to the north of the Arabian Sea and the Atlantic Gulf Stream region. During the decaying phase of the El iño, anomalous descending flow occurs over the central equatorial Pacific Ocean. The ascending branch occurs over the western equatorial Pacific Ocean, the eastern Indian Ocean and the Amazon River Basin. These anomalous vertical flows are favorable for the enhancement of the Walker Circulation. The pattern persists with the increasing of latitude to 25. Along, the vertical movement changes significantly. The opposite sign occurs in the central Pacific Ocean (descending) and western Pacific Ocean (ascending). In mid-high latitude, the distribution of anomalous vertical movement is scattered. Figure 3 displays the zonal distribution of the vertical velocity in different longitude bands during the mature and decaying phase of El iño. During the peak of El iño (Fig.3a), an anomalous positive Hadley cell is evident over the central parts of the north Pacific Ocean (1E 1W), with ascending over S, and descending over. An anomalous weak Hadley cell is evident over the western part of the north Pacific Ocean (100 1E), with ascending flow over 0, and descending

7 o.6 CHE et al.: ESO and climate anomaly in China 991 flow around. During the decaying phase of El iño (Fig.3b), anomalous descending flow occurs over the north-central Pacific Ocean, while weak ascending flows are evident over the mid-high latitude north of. The region east of the Asian continent South China Sea-west of the north Pacific Ocean (100 1E) witness anomalous ascending flow, while mid-latitude regions over experience descending flow. Opposite results are experienced during La iña. Wang (2) analyzed the features of atmospheric circulation associated with the ESO, and pointed out that anomalous Walker and Hadley circulations and a mid-latitude zonal cell (MZC) are evident over the Pacific Ocean during the mature phase of the ESO. During this period, there is a phase of contrary abnormal Hadley cell over the central and western parts of the orth Pacific Ocean, with the latter being weaker than the former. Positive abnormal circulation occurs over the central Pacific with ascending evident in the equatorial region and descending in the midlatitude region. egative abnormal circulation occurs over the western Pacific with descending evident over the tropical regions and ascending flow evident in the mid-latitude region. Figure 4 shows the 700 hpa anomalous geopotential height and horizontal wind during the mature phase and decaying phase of El iño. Since the mature phase is usually evident during winter, and decaying phase during summer, the results in January and July are shown to represent winter and summer, respectively. During the mature phase of El iño (Fig.4a), a significant negative geopotential height anomaly and a powerful anomalous cyclonic circulation appears in the north-east parts of the northern Pacific Ocean during winter. The Aleutian low strengthens, with a positive geopotential height and an anticyclone developing over the Asian continent. The ridge of high over Lake Baikal strengthens. The climate of the Mongolian Plateau is controlled by the anticyclone. The northerly wind prevails in the northern regions of China. Meridionality in the mid-high latitude enhances during winter. Over the tropical-subtropical western Pacific Ocean and the eastern Indian Ocean, the geopotential height anomaly is positive, in association with the presence of the anticyclone. Therefore, the intrusion of southwesterly flow to South China becomes strong during winter. During the decaying phase of El iño (Fig.4b), a significant positive geopotential height develops over the subtropical region in the north Pacific, with a a E 100 E 1E 180 1W 100 W b E 100 E 1E 180 1W 100 W Fig.4 The 700 hpa geopotential height (thin solid line) and its anomaly field (thick solid line for positive and thick dashed line for negative. Unit: m), and the anomalous wind vector (m/s) during the mature phase (a) and decaying phase (b) of the El iño event maximum occurring in the vicinity of the subtropical high which extending in southwesterly direction, strengthens the south-southeasterly wind over south eastern China. At the same time, a southerly wind anomaly occurs to the west of the Bay of Bengal, which can shift into the south western region of China. The winter circulation pattern persists in the mid-high latitude, the ridge of Ural strengthens and the positive geopotential height anomaly and anomalous anticyclonic circulation develops over the Mongolian Plateau-Xinjiang region. The northerly wind prevails in the northern regions of China. 5 IMPACT OF THE ESO O THE CLIMATE OF CHIA The change of atmospheric circulation during the ESO event can induce the change of corresponding climate. Similar to the previous analysis, combined EOF analysis was carried out on the precipitation anomaly percentage and the air temperature anomaly of China and SOTA. Results are displayed in Fig.5. During the mature phase of El iño (Fig.5), more rainfall occurs in southern China, Hebei Province and north eastern China, while less rainfall occurs between the Huanghe and Huaihe Rivers Basins. Generally, regions with lower air temperature experience higher rainfall. An exception is that of southern China, where higher air temperatures correspond to wetter

8 992 CHI. J. OCEAOL. LIMOL., 30(6), 2012 Vol.30 1st 1st E E 2nd 2nd E E Fig.5 The precipitation (left panel, percentage, counter interval is ) and air temperature (right panel, C, counter interval is ) anomaly during the mature phase (upper panel) and decaying phase (lower panel) of an El iño event conditions. By comparing the atmospheric field shown in Fig.4, we can see that the wetter region in South China correlates strongly to south westerly flow that carries much moisture from the Bay of Bengal. To the north of China, regions experiencing less rainfall are influenced by the positive height and anticyclone occurring over the northern regions of the Asian continent, and northerly wind prevails over the Huanghe and Huaihe Rivers Basins. The northerly and southerly flows confluence in south eastern China around 25, resulting in a high-precipitation band expanding from southwest northeast within this region. Tao et al. (1998) suggests that during the winter of an El iño year, the winter monsoon is weaker, which is unfavorable for the southward expansion of cold flow, leading to a much wetter climate in South China. Statistical analysis results (Zhao et al., 1989) also indicate that the air temperature of China during winter in an El iño year is higher. These results are consistent with our above analysis. Less rainfall occurs from north of the Changjiang River to the Huanghe River and the Huaihe River and the southern region of northeastern China during the decaying phase of El iño (Fig.5). Heavy rainfall occurs in the Changjiang River basin and the southwestern and eastern regions of Inner Mongolia.

9 o.6 CHE et al.: ESO and climate anomaly in China 993 Regions experiencing lower air temperatures experience higher rainfall. An exception is evident for Guangdong, Guangxi Provinces and southwestern regions of China where the temperature is higher. By comparing geopotential height and wind fields shown in Fig.4, we can see that the heavy rainfall occurring in the southern regions of China is associated with the south-southeasterly flow to the western side of the subtropical high, that the heavy rainfall occurring to the southwestern regions of China result from the southerly flow from Bay of Bengal, and that more rainfall over the eastern regions of Inner Mongolia may be related to the 700 hpa negative height and a weak cyclone. Less rainfall evident in the northern regions of Changjiang River is due to the northerly wind induced by the anticyclone over this region. The northerly and southerly flows confluence in the area along, resulting in much precipitation there. Huang and Chen (2) studied the precipitation distribution of China during different phases of the ESO, and pointed out that corresponding to the decaying phase of El iño, there are always flooding events evident in South China and periods of drought in the northern regions of Changjiang River, Japan and Korea. This is in agreement with the results presented in this paper. Opposite results occur during La iña. The influence of La iña on the climate of China is weaker than that of El iño, as La iña is weaker. Results of combined EOF analysis between the 700 hpa wind anomaly field, the China precipitation anomaly percentage and SOTA suggest that the first and the second modes together with their associated time coefficients can be used to reconstruct the interannual variability of the SOTA, Asian-north Pacific wind, and precipitation in China during ESO cycle. Since 1958, there have been seven strong El iño events (1965/66, 1965/66, 1972/73, 1982/83, 1986/87, 1991/92, 1997/98 and 2/03), six strong La iña events (1964/65, 1970/71, 1973/74, 1974/75, 1988/89, 1998/99). All the peak phases occurred during winter, except for that of 1986/1987. The composite maps of SOTA, 700 hpa wind and precipitation in China are calculated from these extreme years (1986/87 is excluded). Figure 6 displays the SOTA, the 700 hpa wind anomaly and the precipitation anomaly of China during the different phases of the ESO. From Fig.6a, we can see that during the El iño developing period, positive SOTA develops in the equatorial Pacific, and negative SOTA is observed along 12. In conjunction, cyclonic circulation is evident over the central region of the north Pacific and the north China continent, and there is a northerly wind over the Indo- China Peninsula. The Huanghe and Huaihe River Basins witness more rainfall, while south of Changjiang River and northeast China witness less rainfall. During the El iño peak, a positive SOTA occurs over the eastern tropical Pacific Ocean, while a negative SOTA becomes evident in the western part. Anticyclonic circulation is observed over a large area from the east tropical Indian Ocean to south of Japan. The southwesterly from the Indian Ocean controls the climate of southern China. A significant anticyclone dominates the northern regions of China, where a northerly wind prevails. Here, more rainfall occurs in the south eastern regions of China, north of Huabei and part of northeastern China, while drought becomes evident in the central regions of China. During the decaying phase of El iño, the distributions of SOTA and the overlying wind field over the Asian- orth Pacific are opposite to that during the developing period, but with intensified strength. More rainfall occurs in the southern regions of China, the east part of Inner Mongolia and the southwestern regions of China, but less rainfall is evident in the Huanghe River-Huaihe River reaches and southern regions of northeastern China. The result during the La iña event, shown in Fig.6b, is opposite to that during the El iño event. But the strength of SOTA and wind anomaly weakens, especially during the developing and decaying period. Huang et al. (4) studied the 850 hpa wind field and precipitation in China during the different phases of ESO, using composite analysis. Their results strongly support what we have presented in this paper. For example, except in the Indo-China Peninsula, the wind anomaly in the Philippines and the coastal areas of southeast China are in agreement. The anticyclone and cyclone appear in northern China and the Mongolian Plateau during the ESO decaying phase. Even the precipitation patterns within China are similar. These results indicate that the wind anomaly associated with the ESO is indeed the main factor impacting the climate anomaly of China. Huang and Chen (2) analyzed the flood disaster within the Changjiang River valley since 1980, and determined the climatic background of its onset. During the decaying phase of the El iño, the SOTA in the western tropical Pacific Ocean is cold, lowfrequent oscillations come from the Bay of Bengal and the Indo-China Peninsula, cold air activity occurs and

10 994 CHI. J. OCEAOL. LIMOL., 30(6), 2012 Vol.30 S a. El iño S 1E 1E 1E 180 1W 1W 1W 100 W 80 W E 100 E 1E 180 1W 100 W E132 S S 1E 1E 1E 180 1W 1W 1W 100 W 80 W S S 1E 1E 1E 180 1W 1W 1W 100 W 80 W E 100 E 1E 180 1W 100 W E 100 E 1E 180 1W 100 W E E132 S b. La iña S 1E 1E 1E 180 1W 1W 1W 100 W 80 W E 100 E 1E 180 1W 100 W E132 S S 1E 1E 1E 180 1W 1W 1W 100 W 80 W E 100 E 1E 180 1W 100 W E132 S S 1E 1E 1E 180 1W 1W 1W 100 W 80 W E 100 E 1E 180 1W 100 W E132 Fig.6 a. The SOTA (left column, C), the 700 hpa anomalous wind (middle column, m/s) and the precipitation anomaly (%) within China (right column) during the developing phase (upper row), mature phase (middle row) and decaying phase (lower row) of El iño; b. Same as (a), but for La iña the positive snow cover anomaly in the Tibetan Plateau persists from the preceding winter-spring. Based on the discussion in the previous sections, during the decaying phase of El iño, the equatorial SOTA is colder. orth China is dominated by an anomalous northerly wind, which is favorable for cold air activity.

11 o.6 CHE et al.: ESO and climate anomaly in China 995 Fig.7 The processes influencing the climate of China due to the El iño represented as a block diagram An anomalous anticyclonic circulation appears over the Philippines and the coastal areas of Southeast China, resulting in a strong East Asian summer monsoon. The southerly wind anomaly over the Bay of Bengal and the Indo-China peninsula favors water transportation from the tropical western Pacific South China Sea tropical eastern Indian Ocean the Changjiang River valley. On the other hand, the Asian winter monsoon is weak during the peak of El iño, and high rainfall is observed in southern China and Tibetan Plateau, resulting in much snow in the preceding winter-spring over the Tibetan Plateau. Therefore, we can conclude that El iño events give rise to a favorable atmospheric environment for the occurrence of high rainfall. ow the question arises whether such anomalous atmospheric circulation appears with the ESO simultaneously by accident or if it is the result of the ESO. In the next section, we will discuss the physical mechanism of the ESO affecting climate of China.

12 996 CHI. J. OCEAOL. LIMOL., 30(6), 2012 Vol.30 6 DISCUSSIO 6.1 The possible mechanism of the ESO affecting the climate of China By data analysis and numerical model experiment, reviewing the research results on the physical processes that the ESO influences within the north Pacific SSTA, Alexander et al. (2) pointed out that during an El iño/la iña event, anomalous Walker Circulation over the Equator appears first due to the heating in the tropical Pacific. The El iño/la iña event then generates an anomalous Hadley cell in the north Pacific. Subsequently, the Hadley cell acts as a Rossby wave and induces the PA like wave (Trenberth et al., 1998). Through changes in the Hadley cell, Walker circulation, Rossby waves and interactions in the mid-high latitude, the atmospheric bridge causing the Aleutian Low is strengthened. This can transport the oceanic energy from the tropical Pacific to the mid-high latitude via the atmospheric bridge, resulting in the variation of the atmospheric circulation there. Forced by the descending flow of the north branch of the Hadley cell as well as PA like waves and midhigh latitude air-sea feedback, the Aleutian low develops during the winter and deepens the Asian Trough. Hence, the associated wave chain over the Asian north Pacific changes. The meridionality of the mid-high latitude circulation increases and the high pressure ridge over Lake Baikal intensifies. By comparing Figs.1 and 2, it can be seen that during the mature phase of El iño, positive SOTA is evident in the equatorial central-eastern Pacific Ocean. The overlying atmosphere is controlled by ascending flow, descending flow occurs over the middle of the northern Pacific Ocean. These two anomalous vertical flows construct an anomalous Hadley cell, and induce the Rossby wave, which transports the oceanic energy in the Equator to the mid-high latitude, enlarging the meridionality of the circulation and intensifying the high ridge of Lake Baikal. The latter leads to a strong anticyclonic anomaly over the Mongolian Plateau, resulting in anomalous north wind prevailing over northern China. It can also be seen from Figs.1 and 2 that the SOTA in the western tropical Pacific Ocean is negative, and anomalous descending flow is evident over the eastern tropical Indian-western tropical Pacific Oceans. The subtropical high intensifies during winter. Concurrently, anomalous anticyclonic circulation emerges from north of the eastern equatorial Indian Ocean to southeastern China and coastal areas, resulting in the southern-southwestern wind prevailing over the Bay of Bengal and southeastern China. The situation during the mature phase of La iña is opposite, but weaker in strength. Anomalous precipitation and air temperature is induced by the above atmospheric circulation anomaly. Observation analysis and study results suggest that summer precipitation over China is significantly influenced by the ESO which occurs mainly during winter. A question emerges: what is the process? Wang et al. (0) pointed out that an anomalous anticyclone would be present at the lower atmosphere levels over the Philippine Sea when the SSTA in the eastern equatorial Pacific Ocean becomes warmer. Subsequently, a positive SSTA appears in the Philippine Sea, whereas a negative SSTA appears about 20 degrees longitude to its east. And the latter intensifies the anticyclone in turn via positive feedback. Xie et al. (9) suggested that the Kelvin wave propagated to the western Pacific Ocean from the tropical Indian Ocean can decrease the sea level pressure, and the resulted divergence flow north of the equator prevents the convection activity in the northwestern Pacific Ocean. Zeng et al. (7) proposed that the anomalous Hadley cell develops during the weak summer monsoon period ( ), which can lead to a wet climate in southern China and less rain to the north of China by weakening the eastern Asian summer monsoon. Results revealed by this paper suggest that during the decaying phase of El iño, because of larger areas of negative SOTA in the equatorial Pacific Ocean, the mid-eastern part of the north Pacific and the subtropical region are controlled by the descending flow anomaly, which intensifies the north Pacific subtropical High. On the other hand, the SOTA in the western region of the north Pacific is positive and the vertical flow anomaly over the western tropical Pacific (WP)-South China Sea (SCS)-tropical eastern Indian Ocean (ETIO) is ascending. As a result, the strength of the subtropical High is weak, with its position moving southward and westward. At the 700 hpa level, the geopotential height anomaly is positive which is associated with the anticyclone over the coastal areas of southeastern China. Descending flow over middle Africa-Sahara and ascending flow over ETIO-SCS-WP are favorable for strong Walker Circulation over the tropical Indian Ocean and Indian summer monsoon. Hence, anomalous wind over southwestern China from the Indo-China peninsula

13 o.6 CHE et al.: ESO and climate anomaly in China 997 and Bay of Bengal is southerly in direction. The winter anomalous meridional atmospheric circulation influenced by the El iño mature phase can persist until summer. Therefore, in the following summer, the ridge of Ural High is intensified, a positive geopotential height and an anticyclone are present over the Mongolian Plateau-Xinjiang, and northerly wind develops in northern China. It is the anomalous vertical flow over the equatorial western Pacific associated with the SOTA variability during the El iño decaying phase and persistence of anomalous circulation in the mid-high latitude during the El iño peak that induces the climate anomaly within China during the summer. The case during La iña is opposite, but with a weaker strength. In summary, the processes that influence the climate of China due to the El iño can be represented with the block diagram in Fig.7: Here, the complete physical processes of China climate that anomaly affected by two modes of the ESO event based on the SOTA data set were obtained. But according to the traditional point of view, the subsurface water cannot make contact with the atmosphere directly. Therefore, how can anomalous energy from the subsurface water be transferred to the atmosphere? Heat transfer at the air-sea boundary is facilitated by transient, small scale air-sea interactions. During the heat transfer, a parameter of the ocean thermodynamics is the sea surface temperature uninfluenced by atmosphere (referred here as RSSTA). Because the ESO is a significant event in the subsurface layer of the tropical Pacific, and driven by a huge ocean wave, the temperature of the mixed layer is continuously renewed according to the variation of the thermocline depth. The surface temperature anomalies of the mixed water (that is RSSTA) also change with it. That means, during the ESO event, the influence of the subsurface water of the tropical Pacific on the atmospheric circulation is realized through the RSSTA, which contacts the atmosphere and transfers the anomalous heat and moisture to the atmosphere directly. Figure 8 show the distribution of the RSSTA associating the ESO event. It has the same pattern as the traditional SSTA mode in the mature period, but the RSSTA displays a special mode with opposite anomalies in the central tropical Pacific and both its sides during the transition period. Comparing with an anomalous Walker and Hadley cell in Figs.2 and 3, it is very clear that the positive RSSTA corresponds to ascending flow over the region and negative RSSTA corresponds to decending flow over the region. a. RSSTA 1st mode 1E 1E 1E 180 1W 1W 1W 100 W b. RSSTA 2nd mode 1E 1E 1E 180 1W 1W 1W 100 W Fig.8 Distribution of the sea surface temperature anomalies (RSSTA) associating the ESO event a. El iño mature period (counter interval is C); b. El iño decaying period (counter interval is C). Obviously it is true that the RSSTA distribution causes anomalous Walker and Hadley Circulation during an ESO event. For a long time it was thought that the SSTA is the only one parameter to represent ocean thermodynamics. In fact, the SSTA is resulted from large scale air-sea interactions. It is influenced by long term heat transfer related to the RSSTA, wind velocity, air temperature, cloudiness and precipitation related atmospheric circulation, as well as the vertical thermodynamic stratification of the upper ocean. Considering the oceanic system only, the SSTA is the RSSTA. However, the SSTA is also affected by atmospheric circulation, so the two are rather different from each other. During winter, the large wind velocity causes subsurface water to mix fully and causes the SSTA to approximate to the RSSTA. Therefore, it sustains the ESO message. However, during summer, owing to the weak wind and the stable stratification of the upper ocean as well as the influence of precipitation and cloudiness, the SSTA is quite different from the RSSTA, and hence it loses the ESO message. Therefore, if the SSTA data is used to research the ESO event, the results will lose a lot ESO message, especially during summer. This is the main reason why the traditional ESO using the SSTA, only exhibits one mature pattern, often during winter.

14 998 CHI. J. OCEAOL. LIMOL., 30(6), 2012 Vol.30 S S S S S S S S Eofsot E 180 1W 100 W Eofsot E 180 1W 100 W Eofsot E 180 1W 100 W Eofsot E 180 1W 100 W E 100 E 1E 180 1W 100 W E 100 E 1E 180 1W 100 W E 100 E 1E 180 1W 100 W E 100 E 1E 180 1W 100 W Drain E E 1E 1 E 1 Drain E E 1E 1 E 1 Drain E E 1E 1 E 1 Drain E E 1E 1 E 1 Fig.9 Reconstructed fields of SOTA, wind anomaly and precipitation of China during El iño decaying phase of July 1992, 1983, 1998, and 1988, the observed precipitations of China are also displayed in the right panel during the same period (unit same as in Fig.6) 6.2 Reviewing the 1998 flood disaster in the Changjiang River valley The ESO has an important impact on summer rainfall in China, particularly during the decaying phase of El iño when the Changjiang River valley usually undergoes flooding events, e.g., in However, flooding does not occur in all years when El iño decaying occurs. By analyzing five heavy flood events (1972/73, 1982/83, 1986/87, 1991/92, 1997/98) since 1958, it is evident that there are close relations between the precipitation anomaly and the time of occurrence of maximum SOTA during the El iño decaying phase. Figure 9 shows the reconstructed field of the tropical Pacific Ocean SOTA, the anomalous wind field over the Asian orth Pacific Ocean and precipitation over China, as well as the simultaneous precipitation observation. These are shown for the El iño decaying period in July 1983, 1988, 1992, and It can be seen that the positions of maximum SOTA are not the same, though all of them occur during the El iño decaying period. During July 1992, the negative SOTA center was located in the western tropical Pacific Ocean. During July 1983, the negative SOTA center was located around the dateline. During July 1998, the negative SOTA center was located at the central-eastern tropical Pacific Ocean. During July 1988, the negative SOTA center reached the equatorial eastern Pacific. From the time series of the second mode of SOTA in Fig.1, we can see that the SOTA mode during July 1998 was the strongest one since Fig.9 shows the associated anomalous wind field and the anomalous anticyclone over the coastal areas of southeastern China and north of the interior land. Figure 9 also shows that the southerly wind anomaly moving to southern China from the Indochina Peninsula-Bay of Bengal is strong. The reconstructed precipitation field is similar to observation data. Heavy rainfall occurs to the south of the lower reaches of the Changjiang River, while less rainfall is observed in the Huang-Huai Region. Much rainfall is also observed in the eastern parts of Inner Mongolia and southwestern China. Compared to that of 1998, the

15 o.6 CHE et al.: ESO and climate anomaly in China 999 SOTA during 1992 was weak. The associated wind anomaly field and the anomalous anticyclonic circulation appearing over the coastal areas of southeastern China and north of the interior land, are both weak. Weak too is the southerly wind anomaly to southern China from the Indochina Peninsula and the Bay of Bengal. Here, the reconstructed precipitation field is similar to the observation data. Heavy rainfall occurs in southeastern China, while less rainfall is observed in the Huang-Huai Region. High rainfall is also observed to the east of Inner Mongolia and southwestern China. The SOTA, wind field and precipitation during July 1983 were also similar to that during 1998, but with a weaker subtropical high. The observed high-precipitation region around the Changjiang River valley moved northward and westward during July Compared to that of 1998, the SOTA in July 1988 occurred during the late El iño decaying phase and the early La iña developing phase. The wind field in the mid-high latitude is reversed. The anomalous anticyclone over northern China and the southerly wind from the Indochina Peninsula Bay of Bengal disappears. The anomalous anticyclone still develops over the coastal areas of south eastern China, but with weaker strength and an eastward retreat. The precipitation pattern was opposite to that during During July 1973, a later El iño decaying period occurred, and the precipitation pattern was similar to that in July The above analysis suggests that the SOTA has different impacts on the atmosphere over China and coastal areas, according to the different periods of the El iño decaying phase. During the early and middle period of the El iño decaying phase, the anomalous wind field and the precipitation field are similar, with the former weaker and the latter stronger. However, during the late period of the El iño decaying phase, the impact of SOTA on precipitation over China is weak, even reversed. By comparing the SOTA, the wind anomaly and precipitation during June August of the above events, it can be seen that the distribution characteristics of the El iño decaying phase was shown in June August 1998, when accumulated precipitation was large leading to tremendous flooding events. Typical characteristics of the El iño decaying phase are also shown in June July 1983, when accumulated precipitation was the second largest. During 1992, characteristics of the El iño decaying phase were only shown during July. The summers of 1988 and 1973 also show few characteristics of the El iño decaying phase, yet the precipitation anomaly during June August was not consistent and the Changjiang River valley actually experienced less rain. Therefore, the two conditions for flood occurrence within the Changjiang River valley are the following: (1) the El iño decaying phase occurs during summer, and the subsurface tropical Pacific should have the SOTA pattern associated with the decaying of El iño during June August; 2) in conjunction, northerly winds prevail to the north of China, while an anomalous anticyclone develops in the coastal area of southeastern China, and an anomalous southerly wind develops in southwestern China. 7 COCLUSIO 1) Our results further verified that within the subsurface of the tropical Pacific, there are two ESO modes evident in the SOTA variability. The primary mode presents an ESO mature phase pattern, which generally appears during winter. The second mode shows a pattern associated with the decaying/ developing phase of the ESO, which usually occurs during summer. These two modes constitute the ESO cycle. 2) During the El iño mature phase, negative anomalous Walker Circulation appears with an ascending branch in the central-eastern equatorial Pacific and a strong descending branch to the west. A positive anomalous Hadley cell is observed in the eastern part of orth Pacific, and an anomalous anticyclone is observed over the Mongolian Plateau and the western Pacific-eastern Indian Ocean. During the El iño decaying phase, an anomalous descending flow is evident over the tropical central Pacific and the mid Africa Sahara. Anomalous ascending flow occurs over the tropical western Pacific-eastern Indian Ocean and the eastern tropical Pacific-South American Amazon River Basin. An anomalous anticyclonic circulation develops over coastal areas of southeastern China and the Mongolian Plateau-Xinjiang. The situation occurring during La iña is opposite, but with weaker strength. 3) The ESO event has a very important influence on the climate anomaly within China. During the El iño mature phase in winter, the meridionality of the atmospheric circulation in the mid-high latitude region is enlarged, anticyclonic wind anomalies dominate over the Mongolian Plateau and northerly winds prevail in northern China resulting in less rainfall. The descending branch of the anomalous Walker Circulation develops over the western Pacific,

16 CHI. J. OCEAOL. LIMOL., 30(6), 2012 Vol.30 an anticyclonic circulation anomaly appears over the coastal regions of southeastern China and southsouthwesterly winds prevail over southern China, resulting in high rainfall over southern China. During the decaying phase of El iño events in summer, due to the persistence of the winter atmospheric meridionality circulation over mid-high latitudes, a northerly wind prevails in the northern part of China, resulting in less rainfall there. Due to the ascending branch of the Walker Circulation over the tropical eastern Indian Ocean-western Pacific, the western Pacific subtropical high weakens, the S-SE flow prevails over the southeastern China and southerly wind dominates over southwestern China from Indochina Peninsula and the Bay of Bengal, resulting in high rainfall over the Changjiang River basin and the southwestern and eastern regions of Inner Mongolia. 4) The ESO events influence the climate of China mainly by heat transportation in the zonal and meridional direction through an atmospheric bridge, which brings the anomalous heat flux from the tropical Pacific to the remainder of globe. During the ESO event, the influence of the subsurface water of the tropical Pacific on the atmospheric circulation may be realized through the sea surface temperature anomalies of the mixed water, which facilitate the SOTA making contact with the atmosphere directly. This induces an anomalous heat flux to the atmosphere and then transfers the anomalous heat flux over the tropical Pacific to other places by the atmospheric bridge. References Alexander M A, Blade I, ewman M et al. 2. The atmospheric bridge: the influence of ESO teleconnections on air-sea interaction over the global oceans. J. Climate, 15 : Bai X Z, Wu A M. 3. Impact of El iño on large-scale circulation of southeast Asian summer monsoon. Chinese Journal of Oceanology and Limnology, 21 (2): Carton J A, Giese B S. 8. A reanalysis of ocean climate using SODA. Mon. Weath. Rev., 136 : Chao J P, Yuan S Y, Chao Q C. 3. The origin of warm water mass in warm pool subsurface of the western tropical Pacific-the analysis of the El iño. Chinese Journal of Atmospheric Sciences, 27 (2): Chen W, Craf H F, Huang H. 0. The interannual variability of East Asian winter monsoon and its relation to the summer monsoon. Adv. Atmos. Sci., 17 : Chen Y L, Hu D X. 3. Influence of heat content anomaly in the tropical western Pacific warm pool region on onset of South China Sea summer monsoon. Acta Metro. Sin., 17 : Huang R H, Chen W, Yang B et al. 4. Recent advance in study of the interaction between the East Asian winter monsoon and ESO cycle. Advances in Atmospheric Sciences, 21 (3): Huang R H, Chen W. 2. Recent progresses in the research on the interaction between Asian monsoon and ESO cycle. Climatic and Environmental Research, 7 (3): Jin F F An equatorial ocean recharge paradigm for ESO. Part I: conceptual model. J. Atmos. Sci., 54 : Kalnay E, Kanamitsu M, Kistler R et al The CEP/ CAR 40-year reanalysis project. Bull. Amer. Meteor. Soc., 77 : , Li C Y. 2. A further study of essence of the ESO. Climatic and Environmental Research, 7 (2): i D H, Sun Z B. 3. Influence of ESO cycle at different phases in summer on the east Asian summer monsoon. Journal of anjing Institute of Meteorology, 23 : Schopf P S, Suarez M J Vacilations in a coupled oceanatmosphere model. J. Atmos. Sci., 45 : Tao S Y, Zhang Q Y Response of the Asian winter and summer monsoon to ESO events. Chinese Journal of Atmosphere Science, 23 (4): Trenberth K E, Branstator G W, Karoly D et al Progress during TOGA in understanding and modeling global teleconnections associated with tropical sea surface temperatures. J. Geophys. Res., 103 : Wang B, Wu R G, Fu X H. 0. Pacific-east Asian teleconnection: how does ESO affect Asian climate. J. Climate, 13 : Wang C. 2. Atmospheric circulation cells associated with the El iño-southern Oscillation. J. Climate, 15 : Xie S P, Hu K, Hafner J et al. 9. Indian Ocean capacitor effect on Indo-western Pacific climate during the summer following El iño. J. Climate, 22 : Zeng G, Sun Z B, Wang W et al. 7. Interdecadal variability of the East Asian summer monsoon and associated atmospheric circulations. Advances in Atmospheric Sciences, 24 (5): Zhang H, Akimasa S, Masahide K Impact of the El iño on the East Asian monsoon, a diagnostic study of the 86/87 and 91/92 events. J. Meteor. Sci. Japan, 74 : Zhao Y P, Chen Y L, Wang F et al. 7. Mixed-layer water oscillations in tropical Pacific for ESO cycle. Sci. i n C hina S eries D : Earth Sciences, 50 (12): Zhao Y P, Chen Y L, Zhang B C et al The influence of the ESO events on the air temperature of South China in winter and spring. Journal of Tropical Meteorology, 5 (4): Zhu Y F, Chen L X, Yu R C. 3. Analysis of the relationship between the China anomalous climate variation and ESO cycle on the Quasi-four-year scale. Journal of Tropical Meteorology, 19 (4):