Future trends of climatic belts and seasons in China

Transcription

1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 28: (28) Published online 9 January 28 in Wiley InterScience ( Future trends of climatic belts and seasons in China Yundi Jiang, a * Shuyu Wang, b Song Yang, c Wenjie Dong, a Congbin Fu b andtianbaozhao b a National Climate Center, China Meteorological Administration, Beijing, China b Key Laboratory of Regional Climate-Environment for Temperate East Asia, and START Regional Center for Temperate East Asia, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China c Climate Prediction Center, NCEP/NWS/NOAA, Maryland, USA ABSTRACT: In this study, the present and future climate under the Intergovernmental Panel on Climate Change (IPCC) A2 scenario of CO 2 emission are simulated by using the Regional Integrated Environment Modeling System (RIEMS). The simulated climatic belts, climatic seasons, and Yellow River ice phenology in China are compared between the present climate during and the future climate during The long-term trends of the regional climate are also estimated. Compared to , most of the climatic belts in China will shift northward in , by a maximum of of latitude. The southern boundary of the Northern Sub-tropical Belt (NSB) will shift northward significantly, in spite of the little change in its northern boundary. The entire Southern Sub-tropical Belt (SSB) and the Middle Sub-tropical Belt (MSB), as well as the northern boundary of the Warm Extra-tropical Belt (WEB), will also shift northward by 1 2 of latitude. The starting dates of spring and summer will mostly advance, opposite to the delays in the starting dates of autumn and winter. As a whole, the summer in China will grow longer by 26.1 days, while spring, autumn, and winter will become shorter by 6.8, 7.9, and 11.4 days, respectively. In the upper reach of the Yellow River (URYR), the date for enduring sub-zero temperatures will be delayed by 8 days and the date for enduring above-zero temperatures will advance by 5 days. In the lower reach of the river, the date for enduring sub-zero temperatures will be delayed by 4 days and the date for enduring above-zero temperatures will advance by 4 days. Copyright 28 Royal Meteorological Society KEY WORDS global warming; climatic belt; climatic season; future climate in China; regional modelling Received 2 March 27; Revised 7 October 27; Accepted 1 October Introduction As reported by Ren et al. (25), the mean annual surface air temperature in mainland China as a whole has increased by about 1. C since 1951, with a warming rate of about.25 C per decade, and the most significant warming has occurred in winter and spring in northern China and the Tibetan plateau. It is widely anticipated that, as a sequence of global warming, the tendency of the temperature of China to increase will continue in the coming years or decades. Thus, understanding and predicting the long-term trends of climate has become an important issue of the economic development of the country (Yeh and Fu, 1985, 1989; Fu and Wang, 1991; Ye, 1992; Ye and Lu, 2; Ye et al., 21). Wang and Zhao (1995) studied the future climatic trend in China and found that, in the future 5 years, both solar and volcanic activities might lead to cooling of the regional climate, compensating partially the surface warming resulting from the greenhouse effect. However, after 21, the greenhouse effect would gradually become dominant, and the global mean temperature might increase by more than * Correspondence to: Yundi Jiang, National Climate Center, China Meteorological Administration, Beijing, China. jiangyd@cma.gov.cn.6 C by 2, compared to that during In particular, the increase in temperature in East Asia might be slightly greater than the global mean. After 21, precipitation would increase in East Asia, but the summer aridity in northern China might become severer. According to the reports of the Intergovernmental Panel on Climate Change (IPCC, e.g. IPCC, 21), the capability of numerical models for predicting future climate has been improved greatly in recent years. Under all IPCC scenarios of CO 2 emission, the global mean surface temperature is predicted to rise continuously. Previous studies on future climate that employ global models nested with regional climate models have shown advantages of regional climate models in simulating the current climate and projecting the future climate in China (Chen and Fu, 2; Gao et al., 21; 2a,b,c; 24; Xu et al., 2;). Using the Regional Integrated Environment Modeling System (RIEMS), nested with the Australian Global Atmosphere-Ocean Coupling Model (CSIRO-MK.), we simulate the present ( ) climate and predict the future (25 244) climate of China under the condition that the concentration of CO 2 increases continuously. We will compare the present and future climates, focussing on the future trends of climatic belts, climatic seasons, and the Yellow River ice phenology based on the Copyright 28 Royal Meteorological Society

2 1484 Y. JIANG ET AL. Xinjiang Tibetan plateau Inner Mongolia Hebei Shandong Yunnan Fujian 7E 8E 9E 1E 11E 12E 1E Figure 1. Map showing the locations of several provinces that are discussed in the text. The locations of the Yellow River in the north and the Yangtze River in the south are also shown in the map by the thick lines. analysis of simulated results under the IPCC A2 scenario of CO 2 emission, since relevant features have seldom been discussed in the IPCC reports. To facilitate our discussions, we first show in Figure 1 the locations of the Yellow River and the Yangtze River, as well as the locations of several provinces of China that will be referred to later. It should also be pointed out that the prediction of future climate is affected by many factors such as the assumptions of future CO 2 concentration in model simulations, errors of climate models, complexities of the interaction between various climate components, and the regional difference in climate changes. Thus, for prediction of the future climate, we can only project the possible scenarios and the trends of future climate. The rest of the article is organized as follows. In the next section, we briefly describe the model and experiments. In Section, we discuss the future trends of the climatic belts and seasons in China. The future trends of the Yellow River ice phenology are discussed in Section 4. The main features of the results obtained by this study aresummarizedinsection5. 2. Model and experiments 2.1. Regional model The regional model RIEMS applied in this study was developed by the START Regional Committee for Temperate East Asia of the Chinese Academy of Sciences in 1998 (Fu et al., 2). Construction of the model is based on the concept of the general monsoon system, which is considered a complex system consisting of physical, chemical, biological, and social components. The dynamical component of RIEMS is the same as that of the PSU/NCAR meso-scale model MM5-V. The model is equipped with the physical process schemes needed for climate studies such as the radiation scheme and biosphere atmosphere transfer scheme (BATS1e) of the modified Community Climate Model version (CCM), thus allowing the interaction between vegetation and the atmosphere and the climate effect of aerosols. The RIEMS can simulate the temperature in China reasonably well (Fu et al., 24; Xiong et al., 26), and it has been used widely in studies of the East Asian regional climate (Xiong et al., 2; 24), especially in the areas of climate change, vegetation atmosphere interaction, and hydrology atmosphere interaction. The framework of RIEMS is summarized schematically in Figure Experiments IPCC SRES The IPCC third assessment report adopted new greenhouse gas emission scenarios (SRES A1, A2, B1, and B2) to predict future climate change. Under the A2 scenario, the global economy develops diversely, the world population increases constantly, and the emission of greenhouse gases maintains a medium-high level. The CO 2 concentration will increase to 66 ppm in 246 and 81 ppm in 265, from the level of ppm in 196 and 44 ppm in Experiment scheme To simulate the present ( ) and future (25 244) climates, we use the simulated results of CSIRO-MK. under double CO 2 concentrations compared to 196 to drive the RIEMS. The CO 2 concentration in 1984 was 1.1 times of that in 196 and the CO 2 concentration in 244 is 2 times of that in 196. On this basis, we analyse and compare the climates of the two decades and predict the future trends of climatic belts, climatic seasons, and the Yellow River ice phenology.. Future trends of climatic belts and seasons.1. Future trends of climatic belts Global warming has brought about obvious changes in climatic belts and seasons in China in the last 5 years

3 FUTURE CLIMATE IN CHINA 1485 Observation Output of global model Lateral boundary Initialization of soil moisture Hydrologic process MM5 Wind; Temperature; Humidity Moisture scheme Cloud Pressure; Cloud Land surface process Heating rate Temperature; Moisture Ground surface radiative flux Radiative process (Modified CCM) Chemical process Atmosphere-Biosphere two-way coupling Figure 2. Schematic illustration for RIEMSs framework. (Ye et al., 2a,b). The future temperature in China is predicted to continue to increase, which inevitably results in corresponding changes in the climatic belts. Here, we analyse the changes in China climatic belts and focus on the area east of 1 E where these changes mainly occur. In defining the climatic seasons, we refer winter and summer to the coldest and warmest one-fourth period of the year in the long-term (1951 2) mean daily temperature series. Spring and autumn are defined as the time span between winter and summer and the period between summer and winter, respectively. Following Ye et al. (2b), we define climatic belts based on the numbers of days of T 1 C. That is, we define the Cold Extratropical Belt (CEB) as the zone where the temperatures of 1 C appear on 1 or less days in the year. Other climatic belts are classified as follows: Temperate Extra-tropical Belt (TEB), days; Warm Extra-tropical Belt (WEB), days; Northern Sub-tropical Belt (NSB), days; Middle Sub-tropical Belt (MSB), days; and Southern Sub-tropical Belt (SSB), larger than 285 days. Figure shows the changes in climatic belts from the period to In the figure, the climatic belts are delineated by solid black lines for and by shadings for Overall, the climatic belts in China will shift northward in the future. The northward shifts of the northern boundaries of SSB, MSB, and WEB are all pronounced as shown in the figure. However, the northern boundary of NSB will remain almost unchanged, and the area of this climatic belt will be reduced by one half. The entire northern boundary of MSB will move northward by 1 2 of latitude, and its southern boundary (also the northern boundary of SSB) will also shift northward by more than 1 of latitude. That is, the whole MSB moves northward, but its area remains little changed. In addition, the entire northern boundary of WEB will move northward by of latitude, and the area of the climatic belt will enlarge slightly. 65 days SSB 285 days MSB 29 days NSB 218 days WEB 171 days TEB 1 days CEB days Figure. Differences in climatic belts in China between and The climatic belts of are shown by solid black lines and those of are delineated by shadings. The Cold Extra-tropical Belt (CEB), Temperate Extra-tropical Belt (TEB), Warm Extra-tropical Belt (WEB), Northern Sub-tropical Belt (NSB), Middle Sub-tropical Belt (MSB), and Southern Sub-tropical Belt (SSB) are defined based on the numbers of days of T 1 C as shown next to the colour bar.

4 1486 Y. JIANG ET AL. (a) (b) Figure 4. Differences in decadal-mean annual temperature (a; in C) and accumulated temperature (b; in C) in China between and The accumulated temperature is calculated exclusively from the temperatures of 1 C. More specifically, compared to the period , in the period , the northern boundaries along 115 E will shift northward from 27.5 N to 29. N for SSB, from 6. N to 7. N for MSB, and from 42. N to 42.8 N for WEB. However, the northern boundary of NSB, one of two belts that have shifted northward remarkably in the past 5 years, will remain in the same location between and On the contrary, the SSB and MSB in southern China, which have seldom moved in the past 5 years, will shift northward by 1 2 of latitude in compared to Unsurprisingly, the shifts in climatic belts discussed above are consistent with changes in temperatures. It can be seen from Figure 4(a), which shows the difference in decadal-mean annual temperature between the periods and , that temperature increases over almost all areas in the east when the two periods and are compared. This is consistent with the results of Wang and Zhao (1995) who showed that the temperature in East Asia would increase widely by 2. The increase in temperature in the two maximum warming regions in northeast and western China reaches C. Rising temperature is also apparent in a large portion of the Yangtze Huaihe river valleys and part of southwestern China (about C). It can also be seen by comparing Figures 4(a) and that the unresponsive northern boundary of NSB lies in a weak-warming area where increase in temperature is less than 1 C, while the shifting southern boundary of the climatic belt lies in a strong-warming area where temperature increases by more than 1 C. Also, the apparent northward shifts in SSB and MSB are consistent with the increase in temperature in most regions of southern China. Because of the definition of climatic belts (see above), their distribution is consistent with the pattern of accumulated temperature whose changes are related directly to the shifts of climatic belts. Changes in accumulated temperature can also account for the future trends of climatic belts. It can be seen from Figure 4(b) that the accumulated temperature increases obviously from that during to In the east of 1 E, such increase generally exceeds 2 C. The rise in accumulated temperature is most profound in MSB and SSB,

5 FUTURE CLIMATE IN CHINA 1487 (a) 4 1 (b) (c) 4 1 (d) Figure 5. Differences in the starting dates of spring (a), summer (b), autumn (c), and winter (d), between and Positive (negative) values represent postponed (advanced) days. Units are in days. exceeding 25 C (larger than 45 C in some regions). The increase in temperature reaches 25 C inweb but is only about 2 25 C innsb..2. Future trends of climatic seasons Traditionally, December, January, and February are defined as winter; March, April, and May as spring; June, July, and August as summer; and September, October, and November as autumn. If there were no global warming, the traditional definition would work well and one might find no difference between the two definitions. However, due to the warming of the climate system which is unequivocal, as summarized in AR4 of IPCC, the traditional definition of seasons may no longer provide a real timing concept of temperature since it lacks the inclusion of climate change information (IPCC, 27). Here, we present a definition of seasons based on temperature criterion, which is able to objectively and dynamically depict the changes in starting dates and length of seasons as a response to global warming. The redefined seasons are different from the traditionally defined seasons because they may reflect the nature of season variations in both starting points and durations associated with temperature changes, both seasonally and inter-annually, from one location to another, while the traditionally defined seasons are fixed in time for all locations. The definition of seasons given in this study may provide timing of the seasons that contains the information of global warming, which may be more useful for crop planting, risk management, health care, disease control, disaster protection, and other applications Changes in the starting dates of climatic seasons Figure 5 shows the changes in the starting dates of various seasons from to when the CO 2 concentration increases by.77 times that of the former period. For spring (Figure 5(a)), while the starting dates change little in part of southwestern and northwestern China and central western inner Mongolia, they are brought forward in the rest of China. The moving-up days are more than 1 days in part of Xinjiang and between and 1 days in a large part of the country including the northeast and western and central southeastern China. Figure 5 shows that the largest advance in the starting dates of seasons occurs in summer (see Figure 5(b)). Except for part of Fujian province in the southeast, the starting dates of summer are brought forward by more than days in most of China. In a large portion of the country, the summer season advances by more than 1 days. Delays in the starting dates of seasons are most pronounced in autumn (see Figure 5(c)). Such delays are mostly more than days, except part of southeast China. In the northwest, southern southwestern China, and central northern China, the starting dates of autumn are postponed by more than 1 days. The starting dates of winter are postponed by more than days in a major portion of China (Figure 5(d)). In

6 1488 Y. JIANG ET AL. (a) 4 1 (b) (c) 4 1 (d) Figure 6. Differences in the length of seasons (in days) for spring (a), summer (b), autumn (c), and winter (d), between and Positive (negative) values represent lengthening (shortening) of seasons. parts of western and southwestern China, the delays are more than 1 days. Smaller delays in the starting dates of winter appear in Shandong and Hebei provinces and far northwestern China. In brief, the changes in the starting dates of seasons in China are significant, specially the advance of summer season and the delay in autumn. The characteristics of these changes are distinctively larger than 9% of the total areas of China in summer, autumn, and winter, and 8% in spring Changes in the length of climatic seasons Figure 6 shows the changes in the length of seasons, in numbers of days, in the China of compared to It can be seen from Figure 6(a) that the length of spring is shortened in a large part of China. Over central, southwestern, and eastern China, spring season is shortened distinctively by more than 1 days. On the other hand, the length of spring season extends in part of the Northwest, Northeast, and Southeast. Except for a small area in Fujian province (in the southeast), the summer season in China will be prolonged significantly from period compared to (see Figure 6(b)). The majority of the country will experience longer summers by more than 1 days. In southwestern China, like Yunnan province, the summer will be longer by 4 65 days. The changes in the length of autumn exhibit larger regional features (Figure 6(c)). The season will be shortened over about 6% of China, including most of northern China and central southwestern China. However, it will be lengthened in the middle and lower reaches of the Yangtze River, part of the southeast, and near the Tibetan plateau. The winter of China will be shorter than that of (Figure 6(d)). In northeast China, south China, and most of western southwestern China, the decrease in the length of the season is more than 1 days. Relatively, the shortening of the season is less profound in northern China..2.. Statistical characteristics of changes in seasons The above results (Figures 6) have indicated that, under the scenario of global warming, the temperature in China will increase apparently from that of the period to the period. The increase in temperature shifts the climatic belts northward and changes the starting dates and the length of the seasons. However, these changes in climate features are seasonally and regionally dependent. For instance, there always exist opposite climate trends in Fujian province (in southeast China) compared to elsewhere, but the reason for the feature remains unclear. The most prominent features occur for summer whose starting dates are brought forward and duration is lengthened, and for winter whose starting dates are postponed and duration is shortened.

7 FUTURE CLIMATE IN CHINA Starting dates Season Length Spring Summer Autumn Winter Starting dates Season Length Figure 7. Differences in the starting dates and length of seasons averaged for entire China between and for spring, summer, autumn, and winter. Positive (negative) values represent postponed (advanced) days. The numbers in the lower portion specify the values shown by the bars in the upper portion of the figure. Units are in days. Such changes in summer and winter are linked to the changes in the transitional seasons: spring and autumn. For example, the increase or decrease in the length of spring shown in Figure 6(a) is not only associated with the advance or delay in the starting dates of the season shown in Figure 5(a), but also linked to the relative changes in the season starting dates between spring and summer (comparison between Figure 5(a) and (b)). In many regions like the Yangtze River valley (e.g. about N), although the starting dates of spring will be brought forward ( period compared to ; see Figure 5(a)), the length of the season will be shortened regionally (Figure 6(a)) because the starting dates of summer will advance more significantly (Figure 6(b)) than those of spring. Here, we discuss the characteristic features of climate changes averaged over entire China, as shown in Figure 7. Comparing to the projected period of , the spring and summer of China will advance 5.4 and 12.2 days, respectively. On the other hand, the autumn and winter seasons will be delayed by 1.8 and 6. days, respectively. The summer of China will be lengthened remarkably by 26.1 days. However, the spring, autumn, and winter of the country will be shortened by 6.8, 7.9, and 11.4 days, respectively. 4. Future trends of the yellow river ice phenology Ice-jam floods of the Yellow River have occurred frequently in the past decades. Better understanding of the long-term variations of the Yellow River ice phenology is of practical importance for future ice-jam preventions. You and Yang (1995) have considered global warming, long periodic variations of temperature, precipitation, and run-off, and anthropogenic activities the three important factors of the ice phenology, and investigated the impact of environmental changes on the incoming flow and sediment of the lower reach of the Yellow River (LRYR). On average, the Yellow River starts to freeze up after Celsius temperature becomes stably negative by 7 1 days, and to break up after temperature becomes stably positive by 7 1 days. While the freeze and breakup of the river are mainly determined by temperature, they are also affected by other factors such as operations of the alongshore reservoirs. Furthermore, the freeze and breakup dates are different between LRYR and the upper reach of the Yellow River (URYR), and even within LRYR or URYR. Here, we analyse the variations of the periods of persistent positive and negative Celsius temperatures. We define stably positive temperature (SPT) period as the time period when the Celsius temperature changes from negative to positive and remains positive for 1 days. Similarly, we define stably negative temperature (SNT) period as the period when the Celsius temperature changes from positive to negative and continues to be negative for 1 days. Figure 8 shows dates of SPT and SNT for (left of the figure) and (right), and for URYR (upper two panels) and LRYR (lower two panels). In the figure, the dates marked for SNT (SPT) represent the dates immediately after the SNT (SPT) period. For example, the date of 25 November in Figure 8(a), labelled as for SNT, indicates that the Celsius temperature has been negative during November (i.e. the past 1 days up to 25 November). Similarly, the date of 25 March in Figure 8(b), labelled as 25 for SPT, indicates that the Celsius temperature has been positive during March. Compared to , in URYR, the dates of SNT will be postponed in , by an average of 8 days (from 7 to 15 November), as shown in Figure 8(a). On the other hand, the dates of SPT will advance 5 days (Figure 8(b)), from 19 to 14 March. Thus, the URYR freeze days of will be decreased by 1 days compared to For LRYR, the dates of SNT (Figure 8(c)) will be postponed by 4 days on average, from 7 December in to 11 December in , and the dates

8 149 Y. JIANG ET AL. of SPT (Figure 8(d)) will be brought forward by 4 days (from 18 to 14 February). Therefore, the LRYR freeze days will be decreased by 8 days. The above analysis shows that, in both URYR and LRYR, the dates of SNT will be postponed and the dates of SPT will advance when compared to Therefore, the length of the freeze period of the Yellow River will continue to shorten as happened in the past 5 years (Jiang et al., 27), especially in URYR. Owing to the increase in CO 2 concentration, the Yellow River basin will become continuously warmer. However, the increase in temperature exhibits large seasonal differences, with largest changes in winter in URYR. The warming in winter is directly associated with the changes in the freeze and breakup dates of the Yellow River. Furthermore, although temperature is the dominant factor of the changes in Yellow River ice phenology, precipitation may also be linked to these ice phenology changes. Analysis of changes in precipitation indicates that the annual precipitation and the precipitations of summer and winter over the Yellow River basin all decrease in the period compared to (figures not shown). However, how this reduction in precipitation affects the ice phenology of the Yellow River remains an issue of future studies. 5. Summary In this study, we have simulated the present ( ) and future (25 244) climates in China under the IPCC A2 scenario of CO 2 emission using the RIEMS, nested with the CSIRO-MK.. We have analysed the future trends of climatic belts, climatic seasons, and the Yellow River ice phenology. The China climatic belts will shift northward comparing to The SSB and the MSB, which have seldom moved in the past 5 years, will shift (a) URYR in URYR in (b) URYR in (c) LRYR in (d) Dates of SPT in URYR in LRYR in Dates of SPT in LRYR in Dates of SPT in LRYR in Year Figure 8. Dates of SNT and SPT in (left) and (right). The dates of SNT represent the dates immediately after the Celsius temperature changes from positive to negative and remains negative for 1 days. The dates of SPT represent those immediately after the Celsius temperature changes from negative to positive and remains positive for 1 days; (a) and (b) are for the upper reach of the Yellow River (URYR), and (c) and (d) are for the lower reach of the Yellow River (LRYR). (See text for details.).

9 FUTURE CLIMATE IN CHINA 1491 northward by 1 2 latitude. The northern boundary of the WEB will also move northward, with the northernmost boundary at 44.5 N in the 199s and at 46 N during However, the northern boundary of the NSB, one of two belts that have shifted northward remarkably in the past 5 years, will remain unchanged and the area of this climatic belt will be reduced by one half because the northern boundary lies in a weak-warming area, but the southern boundary is located in a strong-warming region. Over nearly all of China, the starting dates of spring and summer will advance, while the starting dates of autumn and winter of the period will be delayed when compared to the period. The characteristics of these changes are distinctive in more than 9% of the area of China for summer, autumn, and winter and more than 8% for spring. In most areas, starting dates will advance 1 days for spring and 1 4 days for summer, but they will be delayed 1 4 days for autumn and 1 days for winter. The country-averaged starting dates of spring and summer will advance 5.4 and 12.2 days, while those of autumn and winter will delay 1.8 and 6 days, respectively. Over entire China, the climatic season will be lengthened by 26.1 days for summer, but shortened by 6.8, 7.9, and 11.4 days for spring, autumn, and winter, respectively. Comparing the period to that of , in the URYR, the date of stably negative Celsius temperature will be delayed by 8 days, while the date of stably positive Celsius temperature will advance by 5 days, shortening the freeze days by 1. In the LRYR, the date of stably negative Celsius temperature will be delayed by 4 days, while the date of stably positive Celsius temperature will be advanced by 4 days, shortening the freeze days by 8. 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