Long-term changes in total and extreme precipitation over China and the United States and their links to oceanic atmospheric features

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 34: (2014) Published online 27 April 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: /joc.3685 Long-term changes in total and extreme precipitation over China and the United States and their links to oceanic atmospheric features Fang Wang, a Song Yang, b * Wayne Higgins, c Qiaoping Li a and Zhiyan Zuo d a National Climate Center, China Meteorological Administration, Beijing, China b Department of Atmospheric Sciences, Sun Yat-sen University, Guangzhou, China c Climate Prediction Center, NCEP/NWS/NOAA, College Park, MD, USA d Chinese Academy of Meteorological Sciences, Beijing, China ABSTRACT: We investigate the variations of total and extreme precipitations over China and the United States, focusing on long-term changes. We also explain the features of precipitation by changes in sea surface temperature (SST) and atmospheric circulation. Features of total precipitation and the ratio of extreme to total precipitation are different between China and the United States, and differences occur in both annual and seasonal means. Both total precipitation and precipitation ratio show large seasonal and regional variations over China, but change modestly over the United States Annually, China total precipitation changes insignificantly and the ratio shows only a slight positive trend. However, the US annual total precipitation increases significantly, although the ratio decreases moderately. In China, the ratio exhibits positive trends in all seasons and total precipitation shows small positive trends except a negative trend in fall. The US total precipitation increases remarkably in all seasons except winter when a slight decrease occurs, and the ratio decreases in winter and summer but increases in spring and fall. The change in China precipitation ratio has a strong link to SSTs around the Indian Ocean and the South and East China Seas, and the change in US total precipitation is associated with changes in the Indian Ocean and eastern Pacific SSTs. These relationships become weaker when the trend of total precipitation or precipitation ratio is removed, indicating an impact of SST on the long-term change in precipitation. The trends of US total precipitation and China precipitation ratio are also linked to the long-term changes in atmospheric circulation including the trade wind, the North Pacific anticyclone, and the circulation patterns over Asia. In most cases, the total and extreme precipitations are associated with similar SST and atmospheric patterns, except in China where the annual extreme precipitation is associated with SST and circulation features as is the precipitation ratio. KEY WORDS total and extreme precipitation; atmospheric circulation patterns; SST features Received 19 June 2012; Revised 1 November 2012; Accepted 11 January Introduction The averaged global air temperature has increased by 0.74 C from 1906 to 2005 and the linear warming trend over the last 50 years is 0.13 C per decade, which is as nearly twice as that for the last 100 years (IPCC, 2007). In the context of global warming, increasing attention has been drawn to extreme precipitation in the recent decades (Zhai et al., 2005; Liu et al., 2006; Qian et al., 2007; Curtis, 2008; Li et al., 2010; Qian et al., 2010; Mahajan et al., 2012). One often relates extreme precipitation to global warming, arising from a basic expectation that global warming will lead to large increase in atmospheric water vapour content and change in the hydrological cycle. It is also expected that an increase in the intensity of precipitation extreme is led proportionately by an * Correspondence to: S. Yang, Department of Atmospheric Sciences, Sun Yat-sen University, 135 West Xingang Road, Guangzhou , China. yangsong3@mail.sysu.edu.cn increase in atmospheric water vapour content (Allen and Ingram, 2002; Pall et al., 2007). Accompanied by global warming, both precipitation and precipitation extremes have changed greatly. Significantly increased precipitation has been observed in the eastern parts of North and South America, northern Europe, and northern and central Asia. Meanwhile, the frequency of heavy precipitation events has increased over most land areas, even over some regions where total precipitation decreases, which is consistent with the observed increase in atmospheric water vapour (IPCC, 2007). On a global scale, Frich et al. (2002) found a significant increase in extreme precipitation amount, although the changing spatial patterns were complex. Alexander et al. (2006) also found that precipitation indices exhibited a tendency towards wetter conditions throughout the 20th century, but precipitation extremes were much less coherent than for temperature extremes. On regional scales, many studies on precipitation extremes have 2013 Royal Meteorological Society

2 LONG-TERM CHANGES IN TOTAL AND EXTREME PRECIPITATION 287 emerged in recent years, and they have also shown small spatial coherence. For example, over North America, heavy precipitation has been increasing over the last half of the 20th century (Peterson et al., 2008). For South America, the pattern of trends for precipitation extreme is generally the same as that for total annual rainfall (Haylock et al., 2006). Over China, extreme precipitation generally has positive trends in the Yangtze River basin, southeastern and northwestern China, and southern and northern Tibetan Plateau, but negative trends in the Yellow River basin, northern China, and central Tibetan Plateau (Yao et al., 2008; You et al., 2008, 2010; Dong et al., 2011; also see review by Wang et al., 2012). Heavy precipitation also shows a decrease over the last half century in central Africa (Aguilar et al., 2009), and a decrease in extreme precipitation events occurs in the countries to the west of the Indian Ocean (Vincent et al., 2011). China and the United States are two large countries that are located across the Pacific Ocean. They are characterized by different climate features due to the differences in land sea thermal contrast and ocean atmosphere interaction between the western Pacific and the eastern Pacific, among others. However, previous studies have also demonstrated a link of climate signals across the North Pacific. For example, Lau and Weng (2002) have identified two atmospheric teleconnection patterns linking the summer precipitation between East Asia and the United States on inter-annual time scales. Wang et al. (2001) have also found that there exists a teleconnection pattern between the western North Pacific summer monsoon and the North America climate. Zhang et al. (2005) investigated the relationship between the South Asian high and the climate anomalies in the Asian- Pacific-American sector through changes in the western Pacific subtropical high, the mid-pacific trough, and the Mexican high. Ding and Wang (2005) revealed a near circumglobal teleconnection pattern in the summer midlatitude circulation of the Northern Hemisphere, which is accompanied by rainfall anomalies in both East Asia and North America. Furthermore, Li et al. (2005) compared the features of precipitation over China and the United States and discussed their relationships to largescale atmospheric oceanic patterns across the Pacific Ocean. Zhao et al. (2011) also revealed an inter-decadal relationship of rainfall between Asia and North America, modulated by the so-called Asian-Pacific Oscillation. One may wonder, however, whether the extreme precipitation in the two countries shows consistent changes with the total precipitation. Also, how is extreme precipitation linked to oceanic and atmospheric conditions compared to total precipitation? Given the relationships between China and the United States reviewed above and the availability of longrecorded data in these countries, in this study, we examine the long-term changes in total and extreme precipitations and investigate their links to sea surface temperature (SST) and atmospheric patterns, focusing on the two countries. We first provide a description of the data sets used and analysis methods in Section 2. We then analyse the main results in Section 3 and Section 4 for annual means and seasonal features. Conclusions are given in Section 5. This study focuses on country-mean features, and the regional features within each country will be reported by another study. 2. Data and analysis methods 2.1. Data The following data sets are used in this study: (1) Daily precipitation over China is provided by the National Meteorological Information Center (NMIC) of the China Meteorological Administration. There are totally 917 stations in the data set, and basic quality control has been done by the NMIC. Only the data from 1961 to 2009 are used in this study, and a station is selected only if it has missing data for 3 d or less during the analysis period. According to this criterion, 529 stations are selected. (2) Daily precipitation over the United States is from the US Daily Precipitation Gridded Analysis (Chen et al., 2008). Data resolution is , and time period is from 1948 to (3) The NOAA extended reconstructed SSTs V3b data set, which has a resolution of 2 2 and covers the time from 1854 to present (Smith et al., 2008), is also used. We extract data from 1948 to 2009, to be consistent with the analysis period for most of the data sets applied. (4) Surface air temperature is from the station observation based, global land monthly mean surface air temperature data set (Fan and van den Dool, 2008). The data set, developed at the NOAA Climate Prediction Center, has a latitude longitude resolution for time period starting from (5) Monthly winds and vertical velocity from the NCEP/NCAR Reanalysis (Kalnay et al., 1996) are also employed in this study Precipitation indices The joint World Meteorological Organization Commission for Climatology and World Climate Research Programme project on Climate Variability and Predictability, more specifically the Expert Team on Climate Change Detection, Monitoring and Indices, has defined a total of 27 core indices for both temperature and precipitation. These indices have been used in many studies on climate extremes of the world (e.g. Alexander et al., 2006; You et al., 2010). More details about these indices are available online at In this study, we select two indices to portrait the basic characteristics of total and extreme precipitation. They are: P 95 Extreme precipitation based on 95% percentile. Let R wj be the daily precipitation amount on a wet day

3 288 F. WANG et al. w (R 1.0 mm) in period j (j can be year or season; the same below) and let R wn 95 be the 95th percentile of precipitation on wet days in the period. If W represents the number of wet days in the period, then P 95j = W R wj, where R wj > R wn 95 (1) w=1 P TOT Annual or seasonal total precipitation of wet days. Let R ij be the daily precipitation amount on day i in period j.ifi represents the number of days in j,then P TOTj = I R ij (2) i=1 Beside these two indices, another index is defined to depict the contribution of P 95 to P TOT : R 95p Ratio of extreme precipitation to total precipitation. It measures the relative importance of extreme precipitation: R 95p = P 95 /P TOT 100 (3) We calculate the indices not only for the whole year but also for each season due to the large seasonality of total and extreme precipitations. It should be noted that the extreme precipitation threshold is calculated separately for each season according to the wet days in that season, similar to Yao et al. (2008, 2010). So the annual extreme precipitation cannot be considered as the summation of seasonal extremes. Seasons are defined as those commonly used: December January February (DJF), March April May (MAM), June July August (JJA), and September October November (SON) Interpolation of indices to grids The station-based precipitations over China are unevenly distributed. They are much denser in eastern China but very sparse in the west, especially over the Tibetan Plateau (figure not shown). This feature causes difficulties in calculating the regional averages over China accurately. Although a simple arithmetic average may not affect inter-annual features significantly, it tends to overestimate the regional average from a country-wide perspective, especially for China where there are much denser stations in the east that are often with abundant precipitation. Thus, the angular distance weighting (ADW) algorithm is used to interpolate station indices to regular grids (Alexander et al., 2006) in this study. The ADW algorithm is one of the most appropriate methods for gridding irregularly spaced data (New et al., 2000). The concept of correlation decay distance (CCD) is commonly used before implementation of the ADW method, which can be calculated from a correlation matrix for all the stations in the data sets. More details about CCD calculations can be found in Hofstra and New (2009). Once the CDD is determined, one can select the stations trapped in the search radius (namely CDD) from the target grid point to be interpolated. The distance weight can be defined as w i = ( e x/cdd) m (4) where w i is the weight for station i and x the distance from station i to the target grid point. m is a constant and is set to four in our study (New et al., 2000; Caesar et al., 2006; Hofstra and New, 2009). Then the ADW for each station i out of k contributing stations is (Hofstra and New, 2009): W i = w i 1 + w k [1 cos (θ k θ i )] k k w k, i k (5) where θ is the station s angle to the north relative to the target grid point. According to these weights of W i, one can obtain the interpolation in any grid if there are enough stations in the search radius; otherwise, the value will be set to missing. To avoid more calculation errors, we first calculate all the indices for each station, then interpolate the indices onto a regular latitude longitude grid ( ) using the methods described above Techniques of trend analysis The long-term trends of the indices are estimated by the nonparametric Mann Kendall test and Sen s method (Kendall, 1955; Sen, 1968). Different from the linear regression approach, these methods allow the existence of missing values and do not require the data to conform to any particular distribution. Moreover, the Sen s slope estimation is not greatly affected by any single data error or outlier. Details of these methods can be easily found in Appendix A of Wang and Swail (2001). In this paper, a trend is considered to be statistically significant if it passes the significance test at 95% confidence level. 3. Annual means The amount of extreme precipitation defined as percentile of precipitation is usually significantly correlated with total precipitation, both temporally and spatially. As a consequence, the features of extreme precipitation are very likely to be confused with those of total precipitation when we try to understand the influential factors of extreme precipitation, although extreme precipitation amount is a more intuitive index. However, precipitation ratio, which measures the relative importance of extreme precipitation to total precipitation and is nonlinear to total precipitation, is a better indicator for measuring the importance of extreme precipitation. Therefore, in our study, we mainly focus on the total precipitation and the precipitation ratio over China and the United States, as well as their possible links to oceanic and atmospheric patterns.

4 LONG-TERM CHANGES IN TOTAL AND EXTREME PRECIPITATION 289 Figure 1. Climatology of annual total precipitation (mm; contours) and ratio of extreme precipitation to total precipitation (%; shadings). (a) is for China and (b) is for United States Climatological features Figure 1 shows the climatological distributions of annual total precipitation and precipitation ratio over China and the United States. The total precipitation over China shows an apparent increase from the northwest to the southeast, with the largest amount in Southeast China and the smallest amount in northwest China. In contrast, the total precipitation over the United States shows a general increase from the west to the east, except for the heavy precipitation of the western coastal regions. It should be noted that in the United States the largest total precipitation amount appears in the Pacific Northwest, which is affected by the oceanic climate of the eastern North Pacific. The ratio of extreme precipitation to total precipitation exhibits a more apparent west east distribution over China. In most cases, the largest values of the ratio do not occur in the regions of largest total precipitation amount. The largest ratio appears in the Sichuan Basin in southwest China, and high values also appear in the Huaihe River Valley and western South China, which often suffer from the effect of severe rain storms and subsequent flooding during the rainy season. The ratio over the United States shows a more uniform spatial distribution than that over China. Table 1 provides the country-averaged climatological values of precipitation indices over China and the United States. The annual total precipitation is mm for China, which is smaller than mm for the United States. This feature is much consistent with the result of Li et al. (2005), although different data sets and analysis periods are used. The extreme precipitation shows the same feature as the total precipitation, which is mm over China, smaller than mm over the United States. However, the ratio of extreme to total precipitation of China is 23.3%, which is larger than that of the United States (22.1%). We have also compared the wet days and the extreme wet days between China and the Table 1. Annual indices and their trends. Indices China United States Total precipitation (mm; mm ( 0.01) (13.2*) decade 1 ) Extreme precipitation (mm; mm (2.11) (4.3*) decade 1 ) Ratio (%; % decade 1 ) 23.3 (0.23) 22.1 ( 0.15) Wet days (d; d decade 1 ) 63.3 ( 0.30) 88.9 (2.67**) Extreme wet days (d; d decade 1 ) 3.2 (0.04) 4.6 (0.09) Note: * and ** indicate the trends passing significance test (Student s t-test) at the 95 and 99% confidence levels, respectively. United States and found that the values of both indices in China are smaller than those in the United States, contributing to a larger intensity for both total and extreme precipitations over China, although the total and extreme precipitations in the United States are larger than those in China Trends The variations and long-term trends of annual total precipitation and precipitation ratio are given in Figure 2. Obvious differences can be seen between China and the United States. Over China, the total precipitation almost has no trend and the ratio shows a moderate positive trend of about 0.22% per decade, which is not statistically significant. In contrast, over the United States, the total precipitation increases significantly, with a trend about 13.2 mm per decade. Meanwhile, the precipitation ratio shows a slight decrease by 0.15% per decade. In addition, comparing the trends of countryaveraged indices between the two countries (Table 1) indicates that extreme precipitation amount has positive trends for both China and the United States, but the ratio has opposite trends between the two countries. This feature may be because total precipitation has little trend

5 290 F. WANG et al. Figure 2. Variations of annual total precipitation (mm; black solid lines) and precipitation ratio (%; red solid lines). Shown also are their trends (dashed lines). (a) is for China and (b) is for United States. over China but it increases much faster than extreme precipitation over the United States Links to SST, Ts, and atmospheric circulation To understand the possible links of the long-term changes in total precipitation and precipitation ratio to SST (and surface air temperature over land; Ts) and atmospheric circulation, we take two approaches. First, we examine the differences in SST and atmospheric circulation between the last 20 years and the first 20 years of the analysis period. The selection of 20 years is arbitrary but modifying the number does not change the results significantly. It should also be pointed out that the results obtained from this analysis are similar to those from a linear trend analysis. Secondly, we further examine the possible links of SST and circulation features to the changes in China and US precipitations by analyzing correlation patterns. We compare these patterns for both original and de-trended data to distinguish the long-term changes from the features on inter-annual time scales. As seen from Figure 3, climatologically, the annual SST is higher in the tropical oceans and it decreases from low latitudes to the polar region, with an obvious cold tongue in the eastern tropical Pacific. The lowertropospheric winds are characterized by strong westerly flows in the mid-latitudes and strong trade winds over the tropical oceans. Anti-cyclones appear over the subtropical oceans. The long-term differences in SST and 850-hPa winds between the different decades indicate that the annual SST has changed remarkably. An obvious increase occurs in the tropical and Southern Hemisphere oceans, especially in the Indian Ocean, the eastern Pacific, and the Atlantic Ocean. A decrease in SST can also be seen in portions of the North Pacific and the North Atlantic. These changes in SST are consistent with the results of Wang et al. (2009), who extracted two leading empirical orthogonal function (EOF) modes, the global warming pattern with largest warming in the Indian and Atlantic Oceans and the Pacific decadal variability pattern with large opposite SST anomalies between the central-eastern tropical Pacific and the North Pacific. These SST patterns and their long-term changes may be closely linked to the changes in precipitation, which is discussed below. The changes in 850-hPa winds are consistent with changes in SST. Especially, trade winds have weakened remarkably over the central and eastern tropical Pacific. Figure 4(a) and (b) shows few significant features about the link of the total precipitation over China to SST and atmospheric circulation patterns. The original and de-tended patterns present similar features due to the very small trend of the precipitation, suggesting that the features appear mainly on inter-annual time scale. However, the precipitation ratio of China is strongly correlated with the SSTs from the Indian Ocean to the western Pacific and in the South China Sea, the East China Sea, the tropical Atlantic Ocean, and the waters east of Australia (Figure 4(c)). Corresponding to the SST patterns, strong easterly flow appears from the central Pacific through the Philippine Sea and the South China Sea to the northeast, forming a significant anti-cyclonic pattern over the western Pacific. After the linear trend of precipitation ratio is removed, these features become much weaker (Figure 4(d)), especially in the Indian Ocean. This feature indicates a link of precipitation ratio to SST and 850-hPa winds for their long-term changes. That is, the positive trend of China precipitation ratio is related to the long-term warming in these oceans. It is noteworthy that the wind patterns change only slightly after the precipitation ratio is de-trended. Thus, the relationships of precipitation with atmospheric circulation may be dominated by shorter time scales such as interannual time scale. Here, we have just considered the annual mean features, and more about the physics of long-term link for precipitation to SST and atmospheric conditions will be discussed when seasonal means are analysed in the next section.

6 LONG-TERM CHANGES IN TOTAL AND EXTREME PRECIPITATION 291 Figure 3. Annual-mean SST ( C; shadings) and U850 (m s 1 ; vectors) (a) climatology ( ) and (b) differences between the average of and the average of Over the United States, the total precipitation is significantly related to SST and atmospheric circulation features, which is different from the feature for the total precipitation over China. It can be seen from Figure 5(a) and (b) that the total precipitation has significant relationships with the SSTs in the Indian Ocean and the eastern Pacific. It also has a negative correlation with the North Pacific SST. Particularly, the correlation pattern in the Pacific may be related to the long-term change in the Pacific decadal oscillation (PDO), which has experienced an obvious shift from cool phase to warm phase around 1976 (Bond and Harrison, 2000). In the warm phase, strong warm SST anomalies occur in the central and eastern Pacific and distinct cooling appears in the North Pacific around 40 N, corresponding to a relatively abundant rainy era of the United States, although strong inter-annual variation exists (Figure 2(b)). Meanwhile, the trade winds over the central and eastern Pacific weaken remarkably, leading to higher SST in the tropical eastern Pacific and the ocean domain off North America. These features favour abundant water vapour transferred from the Pacific to eastern United States (passing through Central America and the Gulf of Mexico) and precipitation over the United States. It can also be seen that the US total precipitation have a high correlation to the Indian Ocean SST. After the trend is removed, however, the precipitation SST relationships become apparently weaker in the Pacific and disappear entirely in the Indian Ocean (Figure 5(b)), suggesting again that the relationships discussed above are mainly for the long-term changes in SST and precipitation. Namely, the increasing trend of PDO index in recent decades may play an important role in the positive trend of the total precipitation over United States, which is consistent with the result of Wang et al. (2009), who showed that the SST pattern associated with the Pacific decadal variability is a main forcing to the increasing annual precipitation trend observed over the central United States. Meanwhile, the

7 292 F. WANG et al. Figure 4. Correlations of annual total precipitation (a and b) and extreme precipitation ratio (c and d) over China with SST and Ts (shadings) and with 850-hPa winds (vectors). Left panels are for original precipitation and right panels are for de-trended precipitations. The values that are statistically significant at the 95, 99, and 99.9% confidence levels are 0.282, 0.365, and 0.456, respectively. Indian Ocean warming may also play an important role in the long-term trend of precipitation, which is discussed in the next section. However, is spite of the significant features discussed above for US total precipitation, the long-term change in the extreme to total precipitation ratio in United States is very weakly linked to SST and atmospheric features (see Figure 5(c) and (d)). These features about the respective links of total precipitation and precipitation ratio to oceanic atmospheric conditions are clearly different between China and the United States. 4. Seasonal means Precipitation variations usually present distinct features in different seasons, especially in China where monsoon climate dominates. In this section, the seasonal features of precipitation indices and their links to oceanic and atmospheric conditions are addressed Climatological features Figure 6 provides the climatological distributions of seasonal values of total precipitation and precipitation ratio. Over China, the total precipitation shows a strong annual cycle, a typical feature of monsoon climate. In winter, the cold and dry East Asian winter monsoon prevails, suppressing precipitation. In spring, precipitation increases, especially in southern China. During the summer monsoon season, rain belt marches northward from southern China to the Yangtze River basin and then to northeast China. In fall, accompanied by the withdrawal of summer monsoon, rain belt is pushed back to the south of the Yangtze River and precipitation amount decreases greatly. The precipitation ratio over China also shows a strong annual cycle, with the largest ratio in summer and the smallest in winter. However, its spatial distribution is much different from that of total precipitation, and the spatial structure of total precipitation does not match that of precipitation ratio. In many cases, the regions of large total precipitation are not the regions of large precipitation ratio. It is also interesting that large precipitation ratio always exists in the Sichuan Basin of southwest China except in winter. Over the United States, the seasonal cycle of total precipitation is much smaller except southwest United States because of the influence by the weaker North America summer monsoon. The total precipitation mainly shows a basic distribution with less precipitation in the west and more precipitation in the east. However, it also shows apparent seasonal features in some regions. For example, the largest precipitation in the United States occurs in the Pacific Northwest region in fall and winter and in Florida and its adjacent coastal regions in summer. As for China, the spatial structure does not match between total precipitation and precipitation ratio. The largest US precipitation ratio mainly occurs in summer and fall. Comparing the climatological features of seasonal precipitation indices averaged for the two countries

8 LONG-TERM CHANGES IN TOTAL AND EXTREME PRECIPITATION 293 Figure 5. Correlations of annual total precipitation (a and b) and extreme precipitation ratio (c and d) over United States with SST and Ts (shadings) and with 850-hPa winds (vectors). Left panels are for original precipitation and right panels are for de-trended precipitations. The values that are statistically significant at the 95, 99, and 99.9% confidence levels are 0.25, 0.325, and 0.408, respectively. (Table 2) shows that total precipitation exhibits much stronger seasonality in China than in the United States, which is consistent with the results by Li et al. (2005). This feature of total precipitation is also applicable to the other indices. Except precipitation ratio, all indices over China are generally smaller than those over the United States in all seasons except summer. However, the precipitation ratio over China is larger than that over the United States in all seasons except winter Trends Figure 7 shows the changes in seasonal total precipitation and precipitation ratio, averaged for China and the United States. Over China, total precipitation changes insignificantly in winter, spring, and summer, with only slight positive trends being detected. In fall, it shows a significant negative trend. However, the precipitation ratio over China shows consistent positive trends in all seasons, which are especially significant in winter and spring. In contrast, over the United States, total precipitation shows positive trends in all seasons except winter and the trends are especially significant in summer and fall. However, precipitation ratio shows small longterm changes except the moderate negative trends in winter and summer. A more detailed comparison for the trends of precipitation indices between the two counties is given in Table 2. In fall and winter, total precipitation, extreme precipitation, and precipitation ratio generally show opposite trends between the two countries. In spring and summer, however, the trends have consistent signs in common. Over China, extreme precipitation increases more rapidly than total precipitation, leading to the significant positive trends in winter and spring. Over the United States, the number of wet days increases significantly in summer and fall, consistent with the significant increase in total precipitation. However, the number of extreme wet days does not show obvious trends, indicating increases in moderate and light rains over the United States in these seasons Links to SST, Ts, and atmospheric circulation Figure 8 shows the long-term changes in seasonal SST and 850-hPa winds. Overall, the seasonal means of difference fields exhibit similar futures to the annual mean (see Figure 3). However, some obvious seasonal features can also be noted, which include the largest North Pacific SST cooling in spring, the largest Indian Ocean warming and strongest trade wind weakening over the central and eastern Pacific in summer and fall, and the East Asia summer monsoon weakening in summer. Recently, Zuo et al. (2012a, 2013) have depicted the features of weakening of the large-scale Asian summer monsoon measured by the Webster Yang monsoon index (Webster and Yang, 1992) and revealed the strong links of the weakening monsoon circulation with the long-term changes in precipitation and tropospheric temperature over the Asian continent. In addition, opposite changes

9 294 F. WANG et al. Figure 6. Climatology of seasonal total precipitation and ratio of extreme precipitation to total precipitation for different seasons. Left panels are for China and right panels for the United States. Panels from upper to bottom refer to DJF, MAM, JJA, and SON, respectively. occur in SSTs between the south of Greenland and the east of North America, which are possibly linked to the change in the North Atlantic Oscillation. Below, the SST patterns and the atmospheric circulation patterns that are associated with seasonal precipitation changes for China and the United States are addressed, respectively Total precipitation The SST, Ts, and atmospheric circulation patterns linked to the total precipitation over China are given in Figure 9. In winter, the total precipitation has significant positive relationships to the SSTs from the Indian Ocean to the Maritime Continent and from central to eastern tropical Pacific. After the long-term trend is removed, the high correlation over the Indian Ocean almost disappears, suggesting that the Indian Ocean warming has a close link to the positive trend of winter total precipitation in China, although the trend is not significant. Meanwhile, it is noted that the winter precipitation over China is closely related to ENSO and SST anomalies in the Maritime Continent on inter-annual time scale, which is consistent with the result by Zhou et al. (2010). In spring and summer, the total precipitations only have very small trends, so correlation patterns are mainly for inter-annual time scale, as shown by the small differences between the original and de-trended patterns. It can also be seen that high precipitation-sst correlation exhibits an obvious shift from the Indian Ocean and the Maritime Continent in spring to the western Pacific especially the Philippine Sea in summer, implying a change in possible impacting factors of the precipitation over China. Furthermore, the SST to the east of Australia seems to link to the China precipitation in spring and winter. The regional SST anomalies may affect the Australian high, and winter SST anomalies can persist through the following summer, influencing the summer precipitation in the Yangtze River valley (Zhou, 2011). In fall, high correlations are mainly located in the tropical central Pacific and the North Pacific close to North America, as well as the Atlantic. After the trend of fall total precipitation is removed, high precipitation-sst correlation disappears from the tropical central Pacific and the Atlantic Ocean, suggesting that the significant trend of total precipitation over China is linked to the change in SSTs in these ocean domains for this season (Figure 9(d)). Note that the negative trend shown in Figure 7(d) and the negative correlation shown in

10 LONG-TERM CHANGES IN TOTAL AND EXTREME PRECIPITATION 295 Table 2. Seasonal indices and their trends. Indices China United States DJF Total precipitation (mm; mm decade 1 ) 35.2 (1.81) ( 0.12) Extreme precipitation (mm; mm decade 1 ) 7.4 (0.71) 36.9 ( 0.24) Ratio (%; % decade 1 ) 14.0 (1.2**) 17.2 ( 0.15) Wet days (d; d decade 1 ) 6.5 (0.22) 20.6 (0.41) Extreme wet days (d; d decade 1 ) 0.3 (0.04**) 1.1 ( 0.01) MAM Total precipitation (mm; mm decade 1 ) (0.67) (2.44) Extreme precipitation (mm; mm decade 1 ) 33.2 (0.77) 43.9 (1.15) Ratio (%; % decade 1 ) 18.4 (0.79**) 18.3 (0.04) Wet days (d; d decade 1 ) 15.8 (0.05) 23.6 (0.54*) Extreme wet days (d; d decade 1 ) 0.8 (0.03*) 1.2 (0.02) JJA Total precipitation (mm; mm decade 1 ) (0.44) (3.41*) Extreme precipitation (mm; mm decade 1 ) 76.7 (1.50) 48.7 (0.75) Ratio (%; % decade 1 ) 21.3 (0.34) 19.3 ( 0.20) Wet days (d; d decade 1 ) 27.0 ( 0.22) 25.0 (0.56*) Extreme wet days (d; d decade 1 ) 1.4 (0.01) 1.3 (0.02) SON Total precipitation (mm; mm decade 1 ) ( 3.83**) (6.01*) Extreme precipitation (mm; mm decade 1 ) 29.6 ( 0.84) 43.2 (1.48*) Ratio (%; % decade 1 ) 19.1 (0.22) 18.9 (0.01) Wet days (d; d decade 1 ) 14.2 ( 0.45**) 19.7 (0.83*) Extreme wet days (d; d decade 1 ) 0.7 ( 0.01) 1.0 (0.03) * and ** indicate the trends passing significance test (Student s t-test) at the 95 and 99% confidence levels, respectively. Figure 9(d) are consistent with the Indian Ocean warming in the season shown in Figure 8(d). However, how the Indian Ocean warming is associated with a negative trend of China total precipitation in fall is still unclear. Over the United States, total precipitation shows obvious increases in nearly all seasons, especially the significant changes in summer and fall (Figure 7). The SST, Ts, and circulation features related to the total precipitation are shown in Figure 10. It seems that the total precipitation over United States is affected by the atmospheric circulation anomalies across the North Pacific Ocean, indicating some links between the climate anomalies over China and the United States, affected by the wave-trainlike circulation patterns in the mid-latitudes across the ocean. Particularly, a cyclonic pattern is located over the continental United States in all seasons, although the centre varies with seasons, which favours precipitation over the region. However, these relationships are mainly on inter-annual time scale, which can be seen by comparing the original and de-trended features shown between the left panels and the right panels of the Figure 10. It can also be found that the SST in tropical eastern Pacific has significant positive correlation with the total precipitation over United States, accompanied by apparent weakening of the trade wind over the central and eastern Pacific, due to the long-term changes in precipitation and SSTs. In addition, the SST warming trend in the Indian Ocean and the tropical Atlantic Ocean also has significant relationships with the US precipitation in fall. The long-term trend of total precipitation over United States is closely associated with the tropical SST warming in recent decades. Previous studies have focused on the influences of tropical ocean anomalies on remote climate variability (e.g. Hoerling et al., 2001; Hoerling and Kumar, 2003; Barsugli et al., 2006). Nevertheless, the results were mainly from simulations of atmospheric general circulation models or coupled general circulation models, which need to be confirmed by more observations Precipitation ratio As seen from Figure 7, the precipitation ratio over China has consistent long-term positive trends in all seasons, particularly, the significant trends in winter and spring. Here, we extract the SST and circulation features associated with these long-term changes in precipitation ratio by comparing the original and detrended patterns (Figure 11). In winter, the ratio is significantly related to SSTs in the Indian Ocean, the South China Sea, and the East China Sea, concurrent with an intensification of northeasterly wind and an anomalous cyclonic pattern over the North Pacific. The relationship becomes significantly stronger in spring, especially for the Indian Ocean. At this time, significant correlation also appears in the tropical eastern Pacific Ocean. In summer, the precipitation ratio also has a significant relationship with the SSTs in eastern tropical Pacific and tropical Atlantic Ocean. In fall, the high correlation is mainly located in the South China Sea and the Philippine Sea, which are key areas to the East Asia summer monsoon (Wang et al., 2001). After the precipitation ratio is de-trended, its relationships with SSTs become much weaker in most areas, implying again an apparent link of the long-term changes between the precipitation ratio and the SSTs in these areas. In particular, the tropical ocean warming especially that in the Indian Ocean, the eastern Pacific, and the Atlantic Ocean, as well as the corresponding atmospheric features, may play an important role in the long-term changes in the precipitation ratio over China, particularly for spring.

11 296 F. WANG et al. Figure 7. Variations of seasonal total precipitation (mm; black solid lines) and precipitation ratio (%; red solid lines). Shown also are their trends (dashed lines) for different seasons. Left panels are for China and right for the United States. Panels from upper to bottom refer to DJF, MAM, JJA, and SON, respectively. Over the United States, few features can be found for the link of the long-term changes in precipitation ratio and in SST and atmospheric circulation patterns (figures not shown) due to the similar tendencies of changes in total and extreme precipitations. Although the above analysis has shown an apparent link between the precipitation ratio over China and the tropical SST warming trend especially that of the Indian Ocean, it is indeed a challenge to interpret the possible cause and effect aspect. The change in extreme precipitation is greatly affected by local thermal conditions such as moist-adiabatic temperature lapse rate, upward velocity, and temperature when precipitation extremes occur (O Gormana and Schneiderb, 2009). Nevertheless, the local conditions may be linked to the warming trend of surface air temperature over China, which has significant positive relationship to the extreme precipitation ratio especially in northern China in winter and summer (Figure 11). Large-scale atmospheric circulation is another important prerequisite, which controls transportation of water vapour to China and the activity of cold and warm airs in China and is thus critical to the formation of extreme precipitation. However, how tropical SST warming especially that in the Indian Ocean affects the increasing ratio trend over China is still unclear. Nevertheless, the Indian Ocean warming plays a role in modulating the circulation and precipitation anomalies over China through at least two ways. First, the Indian Ocean warming changes the Walker circulation over the Indo-Pacific Oceans and suppresses precipitation over the tropical western Pacific and the Maritime Continent, contributing to the development of a low-level anticyclone over the Philippine Sea and the South China Sea. This anticyclone increases the precipitation along the East Asian winter monsoon front from December to May (Annamalai et al., 2005). Secondly, the warming induces an atmospheric heat source over South Asia and then generates an anomalous high to its northwest over western-central Asia, which in turn enhances the circumglobal teleconnection (Ding and Wang, 2005) that is significantly associated with the atmospheric circulation and precipitation in East Asia (Yang et al., 2009).

12 LONG-TERM CHANGES IN TOTAL AND EXTREME PRECIPITATION 297 Figure 8. Seasonal-mean SST ( C; shadings) and U850 (m s 1 ; vectors). (a d) Differences between the average of and the average of for different seasons. (a) (d) refer to DJF, MAM, JJA, and SON, respectively. Therefore, it is reasonable to believe that there exists a link between the Indian Ocean warming and the trend of extreme precipitation ratio over China, although further studies, both statistical and modelling, are necessary. In addition, the East Asian monsoon is a vital influential factor for the changes in total precipitation and extreme precipitation over China, which is also closely linked to the second way mentioned above. Thus, monsoon variations may play an important role in the trends of precipitation and precipitation ratio. Recently, Qian et al. (2012) pointed out that land sea heating contrast had increased over the Asian region under the background of global warming because the warming in the high latitudes was greater than that in the lower latitudes and the warming in the continent was greater than that over oceans. The variations of heating contrast cause abnormal thermal winds, explaining the decadal weakening of the East Asian summer monsoon intensity. However, the warming in the northern land of Asia may also affect local thermal conditions and thus extreme precipitation. Thus, we cannot fully interpret precipitation change only by the changes in monsoon intensity and the accompanied change in atmospheric circulation. Finally, it should be noticed that both total and extreme precipitations are affected by multiple complicated and interactive factors within the climate system. Above we have tried to interpret the changes in total precipitation and extreme precipitation ratio through local thermal conditions and atmospheric circulation processes linked to precipitations over the two countries. However, many other factors that are important for total and extreme precipitations such as tropical cyclones also exist. Previous studies (e.g. Barlow, 2011; Chang et al., 2012) showed that the precipitation related to tropical cyclones had important contributions to both total and extreme precipitations over both eastern coastal regions and inlands. However, to what extent tropical cyclones contribute to the trend of precipitations over China and the United States has not been discussed in this article. A feasible way is to address this question when the features of regional precipitation over the two countries are discussed, which will be reported in another paper. 5. Conclusions and further discussions In this article, the long-term changes in extreme precipitation over China and the United States are analysed by using station-based daily precipitation over China and high-resolution daily precipitation analysis over the United States. The relationships of total precipitation and extreme precipitation ratio with SST/Ts patterns and atmospheric circulation features are also examined. The main results are summarized as follows. (1) Features of both total precipitation amount and ratio of extreme precipitation to total precipitation are apparently different between China and the United States, and the differences occur not only in the annual means but also in seasonally averaged values. Both total precipitation and precipitation ratio show

13 298 F. WANG et al. Figure 9. Correlations of seasonal total precipitation over China with SST, Ts and 850-hPa winds. Left panels are for original precipitation and right panels are for de-trended precipitations. Panels from upper to bottom refer to DJF, MAM, JJA, and SON, respectively. large seasonal and regional features over China, while they change relatively more uniformly over the United States. (2) The annual total precipitation over China changes insignificantly and the ratio shows a slight positive trend. However, the annual total precipitation over the United States increases significantly, although the ratio decreases moderately. Seasonally, over China, the ratio exhibits positive trends in all seasons, especially in winter and spring, and the total precipitation shows small positive trends except a significant negative trend in fall. Over the United States, the total precipitation increases remarkably in all seasons, except a slight decrease in winter, and the ratio decreases in winter and summer but increases in spring and fall. (3) The change in precipitation ratio in China has a strong link to the SSTs around the Indian Ocean, the South China Sea, and the East China Sea, while the change in total precipitation over the United States is associated with changes in the SSTs over the Indian Ocean and the eastern Pacific Ocean. These relationships become much weaker when the trend of total precipitation or precipitation ratio is

14 LONG-TERM CHANGES IN TOTAL AND EXTREME PRECIPITATION 299 Figure 10. Correlations of seasonal total precipitation over United States with SST, Ts and 850-hPa winds. Left panels are for original precipitation and right panels are for de-trended precipitations. Panels from upper to bottom refer to DJF, MAM, JJA, and SON, respectively. removed, indicating an apparent link of SST to the long-term changes in precipitation. The trends of precipitation are also linked to the long-term changes in atmospheric circulation including the trade winds, the North Pacific anticyclone, and circulation patterns over the Asian continent. We have noted that the trends of extreme precipitation are also associated with SST and circulation patterns that are much closer to the features associated with total precipitation, as extreme and total precipitation amount often tend to show consistent trends (see Tables 1 and 2). However, the annual extreme precipitation in China is an exception and it has an opposite trend compared to the total precipitation but has a same-sign trend as precipitation ratio. So, the annual extreme precipitation and the precipitation ratio are associated with similar SST and circulation patterns. In this study, we have depicted the distinct features of total and extreme precipitations over China and the United States, the two countries whose climate is modulated by the North Pacific SST, among other factors. We have also attempted to link the changes in precipitations to SST and atmospheric patterns. Particularly, we have

15 300 F. WANG et al. Figure 11. Correlations of seasonal extreme precipitation ratio over China with SST, Ts and 850-hPa winds. Left panels are for original precipitation and right panels are for de-trended precipitations. Panels from upper to bottom refer to DJF, MAM, JJA, and SON, respectively. discussed the possible linkage in the long-term changes by comparing the features revealed by de-trended and non-de-trended data. Secondly, the results shown in this study are based on an analysis of countrywide perspective. Both total precipitation and extreme precipitation, especially the latter, are usually also characterized by large regional features. Thus, an analysis of the precipitations over different climate zones or divisions in the two countries has also been carried out, and the full results of this regional analysis are reported separately. In particular, we have conducted a similar analysis for the total and extreme precipitations averaged over each of eight China domains and 13 US divisions. Suffice it to say that the features of long-term changes in both total precipitation and precipitation ratio, in either China or the United States, vary from one region to another. For example, over southwest China, the total precipitation has experienced a moderate negative trend but the precipitation ratio exhibited a significant positive trend. Over the inter-mountains division of western United States, a significant positive trend is found for total precipitation but a significant negative trend is observed for precipitation ratio. These different trends in precipitations are associated with different features of long-term changes in SST and atmospheric circulation. Furthermore, interpretation of the changes in precipitation especially extreme precipitation, which changes nonlinearly with total precipitation, is undoubtedly a