A Diagnostic Analysis of Air Temperature Anomaly Mode Over China in 2009/2010 Winter Based on Generalized Equilibrium Feedback Assessment (Gefa) Method

DOI:10.16555/j.1006-8775.2015.02.003

Vol.21 No.2 JOURNAL OF TROPICAL METEOROLOGY June 2015

Article ID: 1006-8775(2015) 02-0121-10

A DIAGNOSTIC ANALYSIS OF AIR TEMPERATURE ANOMALY MODE OVER CHINA IN 2009/2010 WINTER BASED ON GENERALIZED EQUILIBRIUM FEEDBACK ASSESSMENT (GEFA) METHOD

JIANG Zhi-hong (江志红)1,WU Yan-zhu (吴燕珠)1, 3,LIU Zheng-yu (刘征宇)1, 2, WEN Na (温娜)1,ZHAO Can (赵灿)1 (1. Nanjing University of Information Science and Technology, Key Laboratory of Meteorological Disaster, Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Ministry of Education, Nanjing 210044 China; 2. Center for Climatic Research & Department Atmosphere & Oceanic Science, University of Wisconsin-Madison, USA; 3. Meteorological Bureau of Suzhou City, Suzhou 215100 China)

Abstract: By using the observed monthly mean data over 160 stations of China and NCAR/NCEP reanalysis data, the generalized equilibrium feedback assessment (GEFA) method, combined with the methods of EOF analysis, correlation and composite analysis, is used to explore the influence of different SST modes on a wintertime air temperature pattern in which it is cold in the northeast and warm in the southwest in China. The results show that the 2009/2010 winter air temperature oscillation mode between the northern and southern part of China is closely related to the corresponding sea surface temperature anomalies (SSTA) and its associated atmospheric circulation anomalies. Exhibiting warming in Northeast China and cooling in Southwest China, the mode is significantly forced by the El Ni觡o mode and the North Atlantic SSTA mode, which have three poles. Under the influence of SSTA modes, the surface northerly flow transported cold air to North and Northeast China, resulting in low temperatures in the regions. Meanwhile, the mid-latitude westerlies intensify and the polar cold air stays in high latitudes and cannot affect the Southwest China, resulting in the warming there. Key words: generalized equilibrium feedback assessment; sea surface temperature anomalies; atmospheric circulation CLC number: P444 Document code: A

anomalies on the monthly and seasonal or larger scale. 1 INTRODUCTION The effect of sea surface temperature anomaly (SSTA) In the winter of 2009/2010, heavy snowfall ap- on the temperature anomalies of Chinese winter has peared in northern Xinjiang, temperature was persistent- long been focused. When the sea surface temperature ly low in most of Northeast China and eastern Inner (SST) is warmer than usual in the tropical Pacific, the Mongolia but much higher in most of Southwest China air temperature is lower in Northeast China but higher and central Northwest China. The evaporation was so in Southwest China (Chen et al.[3]). According to Qu et huge that an unusually severe drought was resulted in al.[4], anomalously higher SSTs in central North Atlantic Southwest China. The abnormal weather seriously af- will be associated with higher air temperature in the fected the societal and economic activity as well as a- winter of the eastern China and vise versa, and the gricultural crops so that it drew widespread attention higher wintertime air temperature in this part of China from the meteorological community at home and abroad is consistent with the interdecadal warming of SST in (Wang et al.[1]; Jia et al. [2]). It is obvious that investiga- the central North Atlantic. Yang et al. [5] discovered that tive study on the causation of such extensive and a negative-phased dipole oscillation of the Indian Ocean long-lasting anomalous climate events is helpful for im- SST is accompanied by larger meridionality of the mid- proving the diagnostic prediction of short-term climatic and higher-latitude circulation, which results in stronger anomalies. winter monsoon in East Asia, lower air temperature in The ocean is a reservoir of energy, a source of wa- Northeast China and higher air temperature in South- ter vapor and a factor for memory in the terrestrial cli- west China. Kang et al.[6] argued that when it is warm in mate system that plays an important role in the climatic northern China and cold in southern China, the simultaneous SST shows a positive anomaly in North Pacific. Received 2013-10-17; Revised 2015-01-27; Accepted Ma et al. [7] argued that the enhanced WPSH (Western 2015-04-15 Pacific Subtropical High) during 1978-2009 may result Foundation item: Specialized Science Project for Public Wel- in anomalous southerlies in southern China. Jiang et al.[8] fare Industries (Meteorology) (GYHY200906016); State Key discovered that the weakened SH (Siberian High) in the Program of National Natural Science Foundation of China (41230528) warm period may change the strength of the East Asian Biography: JIANG Zhi-hong, Ph. D., primarily undertaking re- winter monsoon (EAWM) and further influence the search on climatology. cold-wave duration (CWD) in China. According to Corresponding author: JIANG Zhi-hong, e-mail: zhjiang@ Zhang et al. [9], in the independent positive (negative) nuist.edu.cn phase years of IOD, winter precipitation in the south- 122 Journal of Tropical Meteorology Vol.21 west (except western Yunnan), North China, and North- 2.2 GEFA east China significantly increases (decreases), while The anomalous change of the atmosphere on the rainfall in the Yangtze-Huaihe River valley decreases climate scale is set as (increases). It is obvious that a number of studies have x1(t) shown that lower air temperature in the winter of x (t) Northeast China is linked with lower SST in the central 2 Xt= . North Atlantic as well as the El Ni觡o type of SST in 噎 the Pacific and the dipole pattern of SST in the Indian xi(t) Ocean, but most of them focus on the role of individual Following the principle of GEFA, the change is zones of SSTA without comprehensive consideration of linearly expressed by two parts, one from the feedback the SSTA modes in various ocean basins, letting alone from the underlying surface anomaly Y (including the the estimation of relative contribution by feedback of t interactions from different oceanic zones) and the other SSTA in different oceanic zones. being from within the atmosphere N , i.e., In 1998, Frankignoul et al. put forward the method t x(t)=BY +N of univariate Equilibrium Feedback Assessment (EFA) t t J (1) to extract the feedback of the ocean to the atmospheric, xi(t)=Σbijyj(t)+ni(t) which attracted widespread attention (Liu and Wu[10];Li- j = 1 [11] [12] u et al. ). Recently, Liu et al. expanded the use of y1(t) the method from unitary feedback to multivariable feed- y2(t) back in the atmosphere and suggested a statistical where Yt= , B=I bij J , bij is the coefficient of the method of comprehensive studying the climatic feed- 噎 back, namely the Generalized Equilibrium Feedback As- yJ(t) sessment (GEFA) (Liu et al. [12]) in attempts to separate atmosphere in area i in response to the anomaly of the different oceanic zones from climatic anomalies. With jth underlying surface, which reflects its own contribu- this method, together with verifications of a simplified tion to the atmosphere. thermodynamic coupled model, Liu et al.[11-12] studied the As the change within the atmosphere can be global SSTA and the feedback of SST thermal flux to viewed as white noise on the climate scale, the underly- the atmosphere and demonstrated that the GEFA is suc- ing surface anomaly over a precedent period cannot be cessful in estimating the coefficient of local or non-local affected, i.e., equilibrium feedback prescribed by theoretic models ＜N(t),Y(t-τ)＞=0 (2) and separating the contributions to atmospheric change Then from Eq.(1), an array of feedback coefficients by SSTA and SST modes in different oceanic zones. can be obtained: GEFA was used to study the effect of different oceanic -1 B(τ)=C (τ)C (τ) (3) modes on the precipitation in USA and make compre- xy yy hensive estimates of its hydrological and climatic condi- where Cuv(τ)=u(t)v′(t-τ)/T. tions as influenced by the global SST (Zhong et al.[13]). Based on the selected anomalous fields of the at- Related domestic work is not yet underway on the cli- mosphere and underlying surface, Eq.(3) is used to get mate anomaly in China. the estimated value of B. Usually, the feedback coeffi- On the basis presented above, this work used the cient is estimated by taking the time a month lagged be- GEFA to conduct comprehensive study on the contribu- hind (τ=1). As the accuracy and stability of the estimat- tion of anomalous modes of different oceanic zones and ed B are closely linked with the length of sample, the anomalies of the atmospheric circulation to the anoma- monthly series samples for the winters from December lous pattern of air temperature in China in the 1958 to January 2010 are selected for the computation. 2009/2010 winter to locate key factors that influence The anomalous field of the underlying surface for the them, in an attempt to provide scientific foundation for 2009/2010 winter is used to estimate the response am- improving the diagnostic prediction of monthly climatic plitude BY for the simultaneous atmosphere. For de- anomalies. tailed introduction to the method and the stability of B estimation, refer to Liu et al.[12]. 2 DATA AND METHODS 3 ANOMALOUS AIR TEMPERATURE PAT- 2.1 Data TERN AND ASSOCIATED ANOMALOUS SST The data used in this study include the monthly STRUCTURE IN THE 2009/2010 WINTER OF mean air temperature and the monthly mean reanalysis CHINA from NCEP/NCAR. The meteorological elements are the geopotential height at 500 hPa, zonal wind (u) and 3.1 Extraction of the mode meridional wind (v), with the horizontal resolution at Figure 1 gives the field of anomalous temperature 2.5°×2.5°. The set of data spans from 1958 to 2010. anomaly in the 2009/2010 winter of China. It shows that the air temperature is low in most of the area north No.2 JIANG Zhi-hong (江志红), WU Yan-zhu (吴燕珠), et al. 123 of 40°N in China but high in the area south of it, with a To look into how representative these spatial struc- main center of negative anomaly in eastern Northeast tures can be, this work runs EOF analysis on the month- China and northern Xinjiang and the maximum negative ly field of air temperature anomalies in the winters from anomaly more than 2.5℃ . It also shows that there is 1958/1959 to 2009/2010. Its first mode (figure omitted) warm anomaly in southern Yunnan and western mainly reflects the nation-wide warming trend (that ex- Guangxi with the center of positive anomaly over 3℃. plains 55% of the variance), Fig.2a is the spatial distri- It is cold in the northeast but warm in the southwest bution pattern of the second mode (that explains 14% of part of China in the field of temperature anomaly. the variance), which shows that the temperature anomaly in Northeast and North China is just the opposite to that of Southwest and South China, with the maximum positive center (of more than 1℃) in central Southwest China and the minimum negative center (of more than 2℃) in northern Northeast China. Significant interannual variations are found with the time coefficient of the corresponding second mode (Fig.2b). The distribution and anomalous values of Fig.2a are similar to those of Fig. 1 and the correlation coefficient is 0.8 between their spatial modes. It is low for the first mode of EOF (0.38) but quite high for the second mode (1.1), suggesting that the anomalous temperature field of the 2009/2010 winter of China is controlled by the second EOF mode. Following the time series of the second EOF mode, Figure 1. Field of air temperature in the winter of 2009/2010 a year with the value larger than 0.6 is defined as a typ- relative to the climatological mean of 1971-2000. Dark (light) ical warm year (which is similar to the situation of the shades indicate the area with the absolute values of positive 2009/2010 winter) and the result is 1964, 1965, 1968, (negative) anomalies larger than 1.5℃. 1969, 1986, and 2000, and a year with the value smaller

Figure 2. Spatial distribution (a) and time coefficient variation (b) of the second EOF mode of the anomalous temperature field of the 2009/2010 winter of China. Dark (light) grey shades indicate the area with the absolute values of positive (negative) temperature anomalies larger than 1.0℃. 124 Journal of Tropical Meteorology Vol.21 than -0.6 is defined as a typical cold year (which is just eastern Pacific, a corresponding situation for the El the opposite of the situation of 2009/2010 winter) and Ni觡o type, which has the maximum at 160°W with the the result is 1982, 1988, 1994, 1995, and 2007. Fig.3 center as strong as 2℃. Warm anomalies are consistent gives the distribution of the differences of winter air over the tropical Indian Ocean with two centers at 10°N temperature anomalies between the typical cold and and 30° N respectively and intensity at 0.6℃ . For the warm years, which shows a spatial structure similar to mid-latitudes, the SSTA is warmer in the west than in that of Fig.2, with the correlation coefficient as high as the east in North Pacific with the warm center as strong 0.93. It suggests that the distribution of being cold in as about 1.2℃, which is stronger than the cold center. A the northeast and warm in the southwest of China in the north-south antiphase is over North Atlantic with the 2009/2010 winter is typical of the pattern of the interan- strongest positive center, as much as 1.2℃ , near the nual temperature variation over the 50 years. Its spatial Tropic of Cancer. structure is entirely consistent with that of the second E- Figure 5 gives the variation of the time coefficients OF mode for the winters of the 50 years. For the ease for the second EOF of the winters from 1958/1959 to of description, the anomalous structure of wintertime air 2009/2010 and the distribution of the correlation coeffi- temperature presented above is denoted as CNE-WSN. cients of simultaneous wintertime SST. It is shown that the SST is much positively correlated with the second time series of air temperature EOF in the equatorial Pa- cific east of 160°E, most of the Indian Ocean along and north of the equator and tropical central North Atlantic but negatively correlated with it in central North Pacific. It is seen that the SSTA in the five oceanic areas, namely the equatorial central and eastern Pacific, tropical Indian Ocean, North Atlantic, and North Pacific, is all correlated to some degree with the anomalous temperature pattern in the 2009/2010 winter of China. However, correlation analysis cannot distinguish the relative contribution of different zones of the ocean to the anomalous temperature pattern. Besides, as the SST in these zones may be intercorrelated, the zones may not be independent of the air temperature. Therefore, we introduced the GEFA, a method put forward by Liu et Figure 3. Distribution of the differences in air temperature al. [10], to separate the effects of the SSTA of different anomalies of winter in typical cold and warm years. The areas basins on this pattern and assess the independent contri- of dark (light) grey shades are statistically significant at the bution of these basins to the pattern. 90% level. 4 FEEDBACK OF ANOMALOUS MODES OF 3.2 Spatial structure of SSTA related with the anomalous SSTA FROM DIFFERENT ZONES OF OCEAN pattern of the 2009/2010 winter temperature in China BASED ON GEFA Figure 4 gives the SSTA field of the region north of 20°S in 2009/2010. The most remarkable feature is 4.1 Assessment of the feedback to the 2009/2010 winter the anomalously warm SST in the equatorial central and temperature anomaly pattern

Figure 4. Field of SSTA in the winter of 2009/2010 (from December 2009 to February 2012). Dark (light) grey shades indicate the areas where the absolute SSTA vales of positive (negative) SSTA are larger than 0.6℃. No.2 JIANG Zhi-hong (江志红), WU Yan-zhu (吴燕珠), et al. 125

Figure 5. Coefficients of temporal variation of the second EOF of temperature anomalies and correlation coefficients of SSTA during the winters from 1958/1959 to 2009/2010. Dark (light) grey shades indicate the areas where positive (negative) correlation coefficients are statistically significant at the 90% confidence level.

In view of the stability in computation of GEFA, (labeled as EA2) is for the distribution of north-south the tropical and mid-latitude oceans are divided into dipole bordered by the equator, and the third mode (la- five subsections that do not overlap each other, similar beled as EA3) is for a distribution of triple poles, with to the treatment in Liu et al.[10]. They are the tropical Pa- positive, negative, and positive centers in tropical north- cific (TP, 120°E to 60°W, 20°S to 20°N), tropical Indi- eastern Atlantic, equatorial eastern Atlantic and tropical an Ocean (TI, 35 to 120°E, 20°S to 20°N), equatorial southern Atlantic, respectively (Fig.6c). The first spatial Atlantic (EA, 65°W to 15° E, 20°S to 20°N), North Pa- mode of North Pacific (known as NP1) indicates cific (NP, 120°E to 60°W, 0° to 60°N), and North At- large-scale negative SSTA over the central and eastern lantic (NA, 80° E to 20° W, 20° S to 60° N). Because North Pacific and high SST in the Gulf of Alaska and some of the main physical modes of SSTA can be de- on the coast of North America. The second (known as fined through the EOF mode, the simultaneous SSTA is NP2) points to a pattern in which a positive center in EOF-decomposed for the five zones of the ocean to ob- the high latitudes of North Pacific is associated with a tain respective SST modes as the forcing fields. negative center in tropical and subtropical West Pacific. Figure 6 gives the distribution of the first three The third mode (known as NP3) refers to a situation in spatial modes of EOF for the five zones, whose accu- which the SST is higher in the west than in the east in mulative variance contribution by the first three modes mid-latitude North Pacific (Fig.6d). The first spatial of each of the basins all passes 50% (Table 1). The first mode of North Atlantic (NA1) is for a pattern in which spatial mode of the tropical Pacific (shortened as TP1) the SST is low in the east coast of USA but high in the is featured mainly by a SSTA mode of El Ni觡o with the southeast coast of Canada and west coast of Africa. The positive anomaly in the equatorial central and eastern second mode (NA2) is for a pattern in which the SST is Pacific east of 180°W. The second spatial mode (short- low in extended areas of the mid- and higher- latitudes ened as TP2) is characterized by an east-west dipole of the North Atlantic but high in areas south of it and pattern in the tropical Pacific with the center of positive the west coast of Africa. The third mode (NA3) is a anomaly stretching from the tropical central Pacific to pattern in which the SST is consistently low in the gulf waters off California and the center of negative anomaly region of North Atlantic but high in the areas north of it along the coast of Peru. The third mode (shortened as (Fig. 6e). TP3) is marked by negative SSTA in the tropical Pacif- The second time coefficients of EOF-analyzed win- ic except for a small portion in the equatorial central ter temperature anomalies are selected from 160 Chi-

Pacific (Fig.6a). The first spatial mode of tropical Indian nese stations as the field of atmospheric response (Xt). Ocean (referred to as TI1) exhibits a basin-wide mode The first three EOF modes of the five zones are com- consistent throughout the Indian Ocean, the second bined together to represent a main oceanic forcing field mode (referred to as TI2) shows an anti-phase distribu- to the atmosphere (Yt). Following Eq.(3) and the method tion from northwestern to southern Indian Ocean, and of GEFA, the estimated values (Fig.7a) are determined the third mode (referred to as TI3) is distributed of the CNE-WSN pattern of winter temperature in re- east-west anti-phased in southern Indian Ocean (Fig.6b). sponse to the SST mode of the tropics and the oceanic The first spatial mode of equatorial Atlantic (labeled as Northern Hemisphere, B(τ), and its significance is tested EA2) is for the positive anomaly (Ni觡o mode) in the e- using the Monte Carlo method. quatorial central and eastern Atlantic, the second mode It is known from Fig.7a that the CNE-WSN pattern 126 Journal of Tropical Meteorology Vol.21

Table 1. Percentages of variance contribution and accumulative contribution by the first three EOFs of SST in different basins.

Covariance Tropical Pacific Tropical Indian Equatorial Atlantic North Pacific North contribution/% /TP Ocean/TI /EA /NP Atlantic/NA EOF1 61.11 41.35 37.33 27.52 28.84 EOF2 10.88 11.03 25.50 17.51 12.63 EOF3 5.26 9.53 7.31 11.79 10.40 EOF1+2+3 77.25 61.91 70.14 56.83 51.87

Figure 6. Distribution of the first three spatial modes of EOF of the SSTA in the five basins across the world from 1958/1959 to 2009/2010. a. Tropical Pacific (TP); b. Tropical Indian Ocean (TI); c. equatorial Atlantic (EA); d. North Pacific (NP); e. North At- lantic (NA). The solid (dashed) line is for the positive (negative) value, the contour interval is 0.3℃ and the dark (light) grey shade stands for the area where the absolute value of the positive (negative) SSTA is larger than 0.6℃. No.2 JIANG Zhi-hong (江志红), WU Yan-zhu (吴燕珠), et al. 127

Figure 7. (a) The response coefficient B of GEFA in the CNE-WSN pattern of air temperature for the first modes of SST in the five basins of tropical Pacific (TP1, TP2, TP3), tropical Indian Ocean (TI1, TI2, TI3), equatorial Atlantic (EA1, EA2, EA3), North Pacific (NP1, NP2, NP3), and North Atlantic (NA1, NA2, NA3). The hallow column stands for the areas statistically significant at the 90% confidence level in the unit of ℃/℃; (b) The response amplitude BY for the CNE-WSN pattern of air temperature (Unit: ℃). is significantly responding to the TP2, TP3, TI3 and at 500 hPa under SSTA from different ocean basins NP3, with the values of 0.97℃/℃, 0.92℃/℃, 1.64℃/℃, The pattern of 2009/2010 winter temperature and -1.12℃/℃, respectively. What the values mean is il- anomaly is mainly associated with TP1 and EA3. The lustrated by taking 0.97℃/℃ for example. When the SS- response of air temperature anomaly to SST forcing TA is of the TP pattern, a change of 1℃ in SST is as- must be realized via the anomaly of the general circula- sociated with a 0.97℃ response by the atmospheric tion. Fig.9 presents the anomaly field of 500-hPa geopo- CNE-WSN pattern. tential height in the winter of 2009/2010 in Northern Using the estimated value of B for the response of Hemisphere. It is shown that the main trough of East A- the CNE-WSN pattern and the SSTA field of the sia is much more eastward, generally NW-SE oriented 2009/2010 winter, the amplitude of simultaneous tem- and with large intensity. Negative anomalous centers are perature response in China is estimated under different located in Lake Baikal and the Aleutian region and EOF modes of winter SST (Fig.7b). It shows that the northern cold air follows an eastward route. In the low most significant forcing contribution comes from the latitudes, the western subtropical high is more westward TP1 and TA3, corresponding to the response amplitudes and stronger and the geopotential height is higher than of 0.3℃ and 0.23℃ of the winter temperature pattern. It normal in Southwest China, preventing the water vapor is now clear that the magnitude of contribution due to of the Indian Ocean from transporting to this region and SST mode forcing depends on not only the response co- thus bringing less rain there. It is obvious that it is efficient but also the intensity of the anomaly of the worth studying whether the pattern of anomalous circu- SST forcing. For instance, although the response coeffi- lation presented above is linked with the SST forcing cient of TP1 is moderate, it results in the largest ampli- modes described above. tude in the CNE-WSN pattern of winter temperature in With the 500-hPa geopotential height field as the 2009/2010 since the SSTA for TP1 is quite significant atmospheric response field, the first three EOF modes of (an El Ni觡o year). the five basins are combined to represent the main field With the mode structure of EOF2 taken into ac- of the ocean forcing the atmosphere (Fig.6). Following count for the 2009/2010 winter temperature, Fig.8 gives Eq. (3) for the GEFA, Fig.10 gives the response of the response of the temperature anomaly to the main GEFA by the 500-hPa geopotential height of the SST forcing of TP1 (Fig.8a) and EA3 (Fig.8b) and their 2009/2010 winter to TP1. It is shown that a PNA re- joint response (Fig.8c). TP1 warms Southwest China by sponse occurs at 500 hPa in the Pacific-North America about 0.3℃, Northeast and North China by more than region, and in particular, a “negative-positive” response 0.7℃ while EA3 warms Southwest China by 0.2℃ but is over the eastern Siberia east of Lake Baikal and sub- cools the latter two regions by 0.5℃. By comparing the tropical West Pacific. It is apparent that such response response field under the joint effect of the two modes of geopotential height field to TP1 of the tropical Pacif- (shown in Fig.8c) with the observed anomaly field (Fig. ic is favorable for the prevalence of the westerly wind 1), we found that its contribution is as much as 50% around 40°N in East Asia, which forces polar cold air from the forcing response. Summing up, the CNE-WSN to stagnate over high latitudes, causes a main trough to pattern is mainly caused by joint effects from TP1 and deepen and move westward in East Asia, and channels EA3. the cold air to move eastward to the sea, resulting in the 4.2 Diagnostic analysis of geopotential height anomalies occurrence of cold temperature in Northeast China and 128 Journal of Tropical Meteorology Vol.21

anomalous center of high pressure on the east coast of Asia. Interacting with each other, the two centers bring straight westerlies flows to the area north of 30° N, channeling Siberian cold air to Northeast China and cooling air temperature in the winter of the region. It is shown by summarizing the analysis above that the El Ni觡o type of SST in the central and eastern e- quatorial Pacific causes anomalies in the atmospheric circulation such that an anomalous low pressure appears east of Lake Baikal and an anomalous high pressure occurs in the northwestern Pacific. As a result, the westerlies prevail around 40°N in East Asia to force the polar cold air to stagnate over high latitudes and keep it from coming south to affect the southwest of China. In the meantime, the equatorial Atlantic trio pattern of “positive-negative-positive” SST results in the strengthening of a high over Mongolia and a low in the Aleutian region and brings strong, freezing northerly winds to North China and Northeast China. It is then clear that the El Ni觡o type of SST in the central and eastern e- quatorial Pacific and the equatorial Atlantic trio pattern of “positive-negative-positive” SST are the two main modes for causing simultaneous anomalies of being cold in the Northeast and warm in the Southwest China in the winter of 2009/2010. 5 CONCLUSIONS Using GEFA together with methods of EOF, correlation and composite analysis, this work studies the forcing of the surface air temperature anomaly field and atmospheric circulation of China by the SST field of the 2009/2010 winter: (1) The spatial distribution of the second mode of EOF of the monthly winter air temperature anomalies in China from 1958/1959 to 2009/2010 is similar to that of winter air temperature anomaly field in 2009/2010, suggesting that the latter is mainly contributed by the second mode of EOF. (2) Using the GEFA method to assess the feedback of different basin modes to the anomaly modes of air temperature of the same year, we found that there are Figure 8. Amplitudes of response of (a) TP1, the El Ni觡o SSTA of the El Ni觡o pattern in the equatorial central mode of equatorial central and eastern Pacific, (b) TA3, the and eastern Pacific and SSTA of positive-negative-posi- pattern of three poles (positive-negative-positive) of the equato- tive pattern in the equatorial Atlantic, and the air tem- rial Atlantic, and (c) TP1 and TA3, the joint effect, to the perature in the 2009/2010 winter exhibits a significant CNE-WSN pattern of air temperature. The dark (light) grey CNE-WSN pattern of air temperature anomaly. shade stands for the area where the absolute value of the posi- (3) As shown in the GEFA response of the anoma- tive (negative) amplitudes of response is larger than 0.6℃. lous atmospheric circulation to the SST mode, the atmospheric circulation field of the mid- and higher-latitudes preventing the cold air from advancing further south to is well forced by the SSTA of the wintertime. Due to affect the southwestern part of China. the joint effect of the SSTA of the El Ni觡o pattern for Figure 11 gives the response of the wintertime the equatorial central and eastern Pacific and that of the 500-hPa geopotential height field in 2009/2010 to the e- triple-poles (positive-negative-positive) pattern in the e- quatorial Atlantic TA3, which shows that there is a quatorial Atlantic, the polar cold air is transported to well-defined anomalous center of low pressure in the North China and Northeast China but kept from advanc- Mogolian region in Northern Hemisphere and an ing further south to the southwestern part of China. It is No.2 JIANG Zhi-hong (江志红), WU Yan-zhu (吴燕珠), et al. 129

Figure 9. Fields of geopotential height at 500 hPa for the 2009/2010 winter of Northern Hemisphere.

Figure 10. The response of 500-hPa geopotential height fields (with contours at intervals of 10m ℃-1) to GEFA of the equatorial central and eastern Pacific (TP1). The solid (dashed) curve indicates the positive (negative) value and the shade areas are statistically significant at the 90% level.

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Citation: JIANG Zhi-hong, WU Yan-zhu, LIU Zheng-yu, et al. A diagnostic analysis of air temperature anomaly mode over China in 2009/2010 winter based on generalized equilibrium feedback assessment (GEFA) method [J]. J Trop Meteorol, 2015, 21(2): 121-130.