SEASONAL AND SECULAR VARIATIONS OF SUNSHINE DURATION AND NATURAL SEASONS IN JAPAN

INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 23: 1219 1234 (2003) Published online 9 July 2003 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.933 SEASONAL AND SECULAR VARIATIONS OF SUNSHINE DURATION AND NATURAL SEASONS IN JAPAN TOMOSHIGE INOUE* and JUN MATSUMOTO Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan Received 24 October 2002 Revised 6 May 2003 Accepted 12 May 2003 ABSTRACT Seasonal transitions of sunshine duration and their secular changes in Japan are investigated for the period 1951 2000, and the influence of climatic variations on seasonal weather tendencies is discussed. Two seasons with little sunshine in the Baiu and the Akisame seasons are recognized in central and western Japan (CW Japan). In northern Japan, however, only one season with little sunshine in summer is seen. Another season with little sunshine is recognized for the Sea of Japan side and the Southwest Islands during the winter monsoon season. The sunshine duration from spring to autumn in northern Japan decreased in the late 1980s and the 1990s, and the period of little sunshine in summer has become longer since the mid 1980s. In CW Japan, sunshine in late spring in the 1950s and the early 1960s was less than that in other years. A rapid increase in sunshine duration which corresponds with the end of the Baiu rainy season has been delayed since 1980. The period in midsummer with little sunshine has also appeared more frequently than it did before 1980. In the Southwest Islands, sunshine duration has been decreasing almost all through the year. When comparing the differences between the Sea of Japan side and the Pacific Ocean side in winter, the sunshine contrast between them has intensified recently. By using sunshine duration, an objective classification of six natural seasons in CW Japan is conducted. Then, the long-term change of seasonal march in CW Japan is investigated. The end dates of the Baiu season after 1980 have been delayed by about 5 days compared with those before 1980. From the viewpoint of sunshine contrasts between the Sea of Japan side and the Pacific Ocean side, the winter monsoon season in Japan has become longer in recent years, even under the warming trend of winter temperature. Copyright 2003 Royal Meteorological Society. KEY WORDS: Japan; sunshine duration; natural seasons; seasonal transitions; climate variations; East Asian monsoon 1. INTRODUCTION Recently, much attention has been paid to climate changes and variations in relation to global warming. Many studies (e.g. JMA, 1999; Houghton et al., 2001) have reported on the secular changes of annual average temperature, annual total precipitation, etc. Also, the climatological seasonal march of various climatic elements has been clarified (with regard to Japan or East Asia, see Maejima (1967) and Kawamura (1973)). However, there are few studies about long-term variations of the seasonal march of various climatic elements throughout a year from the viewpoint of the concept of the climate year. Most previous studies about the long-term changes of various climate elements used annual mean, seasonal mean (3 month mean), or monthly mean data. Therefore, they could not discuss secular changes in seasonal boundaries whose time scale is shorter than 1 month. More detailed studies, using pentad or daily data, are needed to understand long-term variations of seasonal transition. Yasunari (1986) pointed out that the method of searching for the signals of climatic changes from the seasonal cycle might be effective, especially in the mid- and high-latitudes, because the amplitude of the seasonal cycle is large in general. Therefore, investigating the long-term variations of the * Correspondence to: Tomoshige Inoue, Faculty of Science Building 5, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan; e-mail: inoue@sys.eps.s.u-tokyo.ac.jp Copyright 2003 Royal Meteorological Society

1220 T. INOUE AND J. MATSUMOTO seasonal march of various climatic elements is important to understand the behaviour of climatic changes in detail. Many studies (e.g. Maejima, 1967; Yamakawa 1988) have shown that there are six natural seasons in Japan. These six seasons consist of three rainy seasons known as the Baiu (or Mei-yu) season in early summer and the Akisame (or Shurin) season in early autumn, one snowy season in winter, and three relatively dry seasons between them. The Baiu season and the Akisame season are respectively caused by northward and southward shifts of a polar frontal zone in East Asia, which is related to East Asian summer monsoon. The snowy season in winter is, on the other hand, related to the winter monsoon in East Asia. Matsumoto (1992) examined the seasonal variations of wind fields and convective activities over the vast region in Asia and Australia, and showed that some abrupt changes of the circulation systems and convective activities were seen in transitional seasons almost simultaneously over a wide region, including not only East Asia but also South Asia and northern Australia. Therefore, clarifying the seasonal boundaries in Japan is important for a better understanding of the Asian monsoon system. Yamakawa (1988) investigated recent climatic variations from the viewpoint of the occurrence frequency of prevailing surface pressure patterns over East Asia from 1941 to 1985. As a result, some interesting facts, such as the shortening of midsummer season and the lengthening of winter season, were discovered. But processing weather charts and classifying pressure pattern types includes subjectivity to some extent. In addition, it is necessary to investigate the changes of seasonal transition after the latter half of the 1980s. Nitta and Yamada (1989), Trenberth (1990) and others have pointed out that a climatic jump occurred around 1976 77 at the Pacific Ocean scale. Thus, how this change influenced the weather, or whether it influenced the seasonal march in Japan, must be clarified. As to the secular changes of sunshine duration, the Japan Meteorological Agency (JMA, 1994) reported the secular changes in annual and seasonal (3 months) total sunshine duration from 1898 to 1992. According to JMA (1994), annual total sunshine duration in Japan averaged over 15 stations gradually increased until the 1940s, reaching a local maximum in the mid-1940s. After this peak, it rapidly decreased and reached a local minimum in the mid-1950s, then gradually increased again from the late 1950s to the 1990s. On the other hand, Sato and Takahashi (2001) took note of sunshine duration in midsummer (early August), and showed that sunshine duration averaged for the period 1986 95 was obviously less than that for the period 1959 68. They stated that the delay of the northward movement of the Baiu frontal zone is likely to be associated with the strengthening of the polar airmass around Japan. These two studies show that a secular trend of sunshine duration in a specific season does not necessarily correspond to that of annual total sunshine duration. Therefore, secular variations of the seasonal march of sunshine duration throughout a year must be clarified in order to understand the long-term variations of weather conditions better. In this study, we take note of the seasonal transition of sunshine duration, and investigate the long-term variations of weather conditions throughout a year, and aim to clarify an aspect of climatic variations in the latter half of the 20th century in Japan. Then, natural seasons in central and western Japan (CW Japan) from the viewpoint of sunshine duration are identified, and the secular changes of seasonal boundaries are investigated. Sunshine duration data reflect weather patterns, and thus natural weather seasons in Japan can be determined by seasonal changes of sunshine duration (or rates of sunshine) observational data. Note that these weather seasons in Japan do not necessarily correspond to temperature seasons. Among the observational records that do not include subjectivity, sunshine duration is considered to be the best at expressing the seasonal transition of weather conditions in Japan. Maejima (1967) recognized natural seasons in Japan by using mainly sunshine duration, cloud amount and precipitation as indices of seasonal divisions. However, precipitation is not appropriate, because precipitation values are subject to local effects and it is difficult to recognize the seasonal transition of weather conditions by the effect of extremely large precipitation due to events such as typhoons. The daily record of cloud amount in Japan is an average value of only a few observations per day. In addition, by using two or more elements it becomes difficult to remain objective. Therefore, sunshine duration is investigated in this study to clarify seasonal and secular variations of weather in Japan and long-term variations of natural weather seasons. Of course, seasonal and secular changes of sunshine duration are also important for agriculture and other human activities.

SUNSHINE DURATION IN JAPAN 1221 In Section 2 we describe the data and the analysis procedures used in this study. In Section 3, regional divisions based on climatological seasonal variations of sunshine are shown. In Section 4, seasonal and secular variations of rates of sunshine are described. Based on the results of Section 4, natural seasons in CW Japan and their secular variations are discussed in Section 5. Finally, conclusions and discussion are presented in Section 6. 2. DATA AND ANALYSIS PROCEDURES Data for sunshine duration are obtained from the monthly reports by the Central Meteorological Observatory (the former JMA before 1956) and by JMA for the period 1951 60, from surface daily product (SDP) data CD-ROMs released by JMA for the period 1961 96, and from the annual report CD-ROMs released by JMA for the period 1997 2000. A total of 28 observational stations were selected (see Figure 1) to be well scattered all over Japan. Most of them are in coastal regions in order to prevent topographic effects on the possible duration of sunshine. Data for the period 1951 2000 are available at most stations, but at Naze and four stations in Okinawa Prefecture (i.e. five stations in the Southwest Islands) the analysis period is only 40 years (1961 2000) because of data availability. Where there are missing data for sunshine duration, then data near the station are used in place of the missing data. It is considered that this substitution is not a great problem, because data for sunshine duration are spatially conservative, and because the number of 130E 140E A 40N C 40N B D E 30N 30N F 130E 140E Figure 1. Regional divisions based on the cluster analysis (Ward s method). Region A: the Sea of Japan side of northern Japan (open triangles); Region B: the Sea of Japan side of CW Japan (open circles); Region C: the Pacific Ocean side of northern Japan (closed triangles); Region D: Kanto and Tokai district (filled circles); Region E: the Pacific Ocean side of western Japan (filled squares); Region F: the Southwest Islands (open squares). Small filled circles represent the stations not recognized as one region

1222 T. INOUE AND J. MATSUMOTO missing data is not very large (the average is 0.3 days/year at each station, and 22 days/year is the largest in all stations and years). During the late 1980s, JMA replaced the Jordan sunshine recorders (which use a closed can with a pinhole on one side to let sunlight onto light-sensitive paper) with rotating mirror sunshine recorders (which use a specially designed rotating mirror that reflects the sun s direct component onto a spectrally flat pyroelectric sensor); now, it is the rotating mirror sunshine recorders that are mainly used in Japan. Referring to Katsuyama (1987), the observed values before the replacement are calibrated as S R = 0.8 S J S R = S J 0.5 h/day (S J < 2.5 h/day) (S J 2.5 h/day) where S J is the observed value of daily sunshine duration obtained by the Jordan sunshine recorder before the replacement and S R is the calibrated value corresponding to the rotating mirror sunshine recorder. This relation is determined based on the results of simultaneous observational comparison between the two sunshine recorders. Pentad-mean rates of sunshine (the value of sunshine duration divided by possible duration of sunshine) data are calculated at each station and in each year. Possible duration of sunshine (PDS, h/day) is calculated as PDS = 24 arccos( tan θ tan δ) π where θ (deg) is the latitude of each station and δ (deg) is the solar declination parameter. For leap years, the data for 29 February are regarded as those of 1 March and all the following days are put off by 1 day; the data for 31 December are then omitted. Data in the present study are smoothed by a five-pentad (or 5 year) triangularly weighted running mean. This running mean is described as y n = 1 9 (x n 2 + 2x n 1 + 3x n + 2x n+1 + x n+2 ) where x n is the original value of the nth data and y n is the smoothed value. This running mean is superior to an unweighted running mean, in that it smoothes more effectively and it does not result in phase inversion, which may occur in case of an unweighted running mean (Burroughs, 1978). To save space, the term running mean in the following text means this triangularly weighted running mean. 3. REGIONAL DIVISIONS BASED ON SEASONAL TRANSITION OF SUNSHINE In order to gather the selected stations into several regions, regional divisions based on climatological seasonal variations of rates of sunshine are carried out. Sekiguti (1959) has already done regional divisions in Japan based on seasonal variations of rates of sunshine. However, he used monthly mean data, and the number of stations and years to process climatological data are different in this study. In addition, the stations in the Southwest Islands were not used (perhaps could not be obtained) in his study. Therefore, new regional divisions are carried out based on the pentad-mean climatological data in recent years. First, the pentad-mean rates of sunshine averaged for the 40 years (1961 2000) are normalized at each station, and then a cluster analysis (Ward s method) is performed about the seasonal variation patterns of rates of sunshine. In addition to the results of this cluster analysis, the geographical distribution of the stations is also considered. For example, the cluster that includes Hachijojima and Tanegashima is ignored because these two stations are located geographically apart each other. As a consequence, six regions (Region A: the Sea of Japan side of northern Japan; Region B: the Sea of Japan side of CW Japan; Region C: the Pacific Ocean side of northern Japan; Region D: Kanto and Tokai district; Region E: the Pacific Ocean side of western Japan; Region F: the Southwest Islands) are obtained (Figure 1).

SUNSHINE DURATION IN JAPAN 1223 Figure 2. Seasonal transition of normalized rates of sunshine of the 40 year average (1961 2000) in each region (smoothed by the five-pentad running mean). (a) Region A (the Sea of Japan side of northern Japan); (b) Region B (the Sea of Japan side of CW Japan); (c) Region C (the Pacific Ocean side of northern Japan); (d) Region D (Kanto and Tokai district); (e) Region E (the Pacific Ocean side of western Japan); (f) Region F (the Southwest Islands). The dates on the horizontal axis represent the central dates of each pentad Figure 2 shows the seasonal transitions of the normalized rates of sunshine in the six regions. Among the six regions, three of them (Regions A, B and F) have little sunshine in winter, whereas the other three (Regions C, D and E) have much more. Among the former three regions, Region A (the Sea of Japan side of northern Japan) is characterized by a period in summer with little sunshine, and two sunshine maxima in spring and autumn. In Region B, there are three sunshine maxima in spring (from March to early June), midsummer (from late July to August) and autumn (from mid-october to mid-november), and three minima that correspond to the Baiu season (early summer rainy season from mid-june to mid-july), the Akisame season (early autumn rainy season from September to early October) and winter monsoon season (from late November to February). The seasonal

1224 T. INOUE AND J. MATSUMOTO transition in this region corresponds well to the six seasons in Japan pointed out by Maejima (1967). Region F (the Southwest Islands) features a long period of much sunshine in midsummer (from late June to October). Among the other three regions, Region C (the Pacific Ocean side of northern Japan) does not have much sunshine in summer, the characteristics of which are similar to those in Region A. The seasonal transition pattern in Region D (Kanto and Tokai district) is comparable to that in Region E (the Pacific Ocean side of western Japan), but sunshine in Region D is more than that in Region E during April and May, and less during the Akisame season. In general, the Baiu rainy season is more obvious in the western part of Japan. On the other hand, the Akisame rainy season is more conspicuous in the eastern part, though it is not so active as the Baiu rainy season. This difference reflects the regional divisions between Regions D and E. Typhoons sometimes hit or approach Japan in late summer and the Akisame season, but the influence of typhoons on sunshine duration seems to be relatively small, because their time scale is generally short (a few days at most). The regional divisions in this study roughly correspond to those in the study by Sekiguti (1959), and it is shown that the Southwest Islands form a separate region from the main islands. In Section 4, the seasonal and secular variations of rates of sunshine in each of the six regions are investigated. 4. SEASONAL AND SECULAR VARIATIONS OF SUNSHINE First, pentad-mean values of rates of sunshine averaged for the stations included in each region are calculated. Then, these are smoothed by the five-pentad running mean and the 5 year running mean. Here, these smoothed data describing the seasonal and secular variations of rates of sunshine are shown in the form of isopleth diagrams in each region. Figure 3 shows these isopleth diagrams for each region (see Appendix A). Similar patterns in the diagrams from spring to autumn appeared for northern Japan (Regions A and C) and in CW Japan (Regions B, D and E). This means that the difference of the seasonal and secular variations of sunshine duration between the Sea of Japan side and the Pacific Ocean side are comparatively small except in the winter monsoon season. To obtain the pattern of secular changes in seasonal transitions (except for the winter monsoon season) more clearly, isopleth diagrams averaged for both northern and CW Japan from March to November are created (Figure 4). In northern Japan (Regions A and C), the 1970s and the early 1980s are relatively good sunshine years almost all through the year (Figures 3(a) and (c) and 4(a)). In these years, the North Pacific High, which brings much midsummer sunshine in Japan, might be of influence. The more northward position than usual gives relatively much more sunshine in midsummer even in northern Japan. Since the late 1980s, on the other hand, rates of sunshine have been decreasing, especially from spring to autumn. As a result, the two sunshine maxima (from April to May and around September) have become weak in recent years. In addition, the tendency for little sunshine from June to July has become strong and the period of little sunshine has become longer, especially in the Sea of Japan side. In Region A (Figure 3(a)), the tendency for little sunshine in winter weakened temporally in the 1970s, but it has become strong since the late 1980s. In Region C (Figure 3(c)), on the other hand, a tendency to an increase in rates of sunshine continued throughout the 50 years, especially in January and March. In CW Japan (Regions B, D and E), rates of sunshine in late spring (from late April to early June) in the 1950s and the early 1960s were comparatively less than those after the late 1960s (Figures 3(b), (d) and (e) and 4(b)). This feature was very obvious in Region E (the Pacific Ocean side of western Japan), where rates of sunshine in early May around 1960 were comparable with those in the peak of the Baiu season (late June to early July). It was possible that the Baiu frontal zone appeared earlier than usual in these years. As to the Baiu season (mid-june to mid-july), the minima of rates of sunshine in the 1950s and 1960s were found in the latter half of the season (around early July), and a contrast between little sunshine in early July and much sunshine in late July or early August was more obvious. Since the 1980s, on the other hand, the minima have appeared earlier (around late June) than those in the 1950s and 1960s, and the sunshine contrast between the Baiu season and midsummer has become somewhat unclear. In addition, the dates when rates of

SUNSHINE DURATION IN JAPAN 1225 Figure 3. Isopleth diagrams representing the seasonal and secular variations of rates of sunshine (%) in each region. (a) Region A (the Sea of Japan side of northern Japan); (b) Region B (the Sea of Japan side of CW Japan); (c) Region C (the Pacific Ocean side of northern Japan); (d) Region D (Kanto and Tokai district); (e) Region E (the Pacific Ocean side of western Japan); (f) Region F (the Southwest Islands). Solid and broken lines are drawn at 10% and 5% intervals respectively

1226 T. INOUE AND J. MATSUMOTO Figure 3. (Continued) sunshine rapidly increase in mid- to late-july, which approximately correspond to the end dates of the Baiu season, have been delayed by some days, compared with before the 1970s. These tendencies are obvious by comparing the intervals of the isopleths around the 41st pentad (20 24 July). The weather conditions in midsummer also changed clearly around 1980. Since the 1980s, the period of little sunshine in midsummer has appeared more frequently, compared with the other years. The years 1980, 1993 and 1998 were cool summers in Japan, and of the latter half of the 20th century, 1993 and 1998 were the only years when JMA could not determine the end date of the Baiu season in some regions in Japan. Before the early 1970s, the end of the long sunshine period in midsummer (the onset of the Akisame season) was around mid- to late-august. Since the mid-1980s, on the other hand, it has been delayed and has appeared more frequently in early September. These characteristics (the frequent short sunshine period in midsummer and the delayed onset of the Akisame season after 1980) are recognized, e.g. in Figure 3(d), by comparing the 50% closed isopleths before 1980 with those after 1980 (i.e. the closed isopleths of 50% rate of sunshine before 1980 are longer in the horizontal direction, whereas those after 1980 are longer in the vertical direction). It appears that there is a periodicity of the sunshine maxima in midsummer, and the period is about 10 years (the maxima years appeared around 1955, 1965, 1975, 1985 and 1995). The timing is close to that of the minima in sunspot numbers. It also appears that the years with little midsummer sunshine also have a period of about 11 years (around 1958, 1969, 1981 and 1992). As such, these facts imply that weather conditions in midsummer in CW Japan have a relationship to solar activity. Sato and Takahashi (2001) pointed out the relationship between sunshine duration in midsummer (early August) in Japan and solar activity. JMA (1994) also raised a correlation between the 11 year period of solar activity and those of some climatic elements, but pointed out that the physical mechanisms between them had not been discovered. Quite recently, Sakurai (2002) discovered that the brightness of the sky background observed with a coronagraph at Mt Norikura, central Japan, has the 11 year periodicity of solar activity cycle, and it is possible that the periodicity of sunshine in midsummer is related to this result. But it is still not explained why the periodicity appears only

SUNSHINE DURATION IN JAPAN 1227 Figure 4. As Figure 3, but in (a) northern Japan, and (b) CW Japan, from March to November in midsummer and in CW Japan; thus, further studies are needed to reveal the physical mechanism of this relationship. In winter, rates of sunshine have been increasing over the Pacific Ocean side (Figure 3(d) and (e)) throughout the 50 years. This feature is also seen in Region C (the Pacific Ocean side of northern Japan); thus, it is common over the Pacific Ocean side. Rates of sunshine over the Sea of Japan side of CW Japan (Figure 3(b)) have also increased, but the trend is relatively small. Figure 3(f) shows an isopleth diagram in Region F (the Southwest Islands). The pattern of the diagram is apparently different from those in the other five regions. It can be pointed out that rates of sunshine have been decreasing almost all through the year. There was a temporally, relatively long sunshine season around April between the period of little sunshine in winter and the Baiu season until the 1970s, but it has disappeared since the 1980s. A sunshine minimum in the Baiu season (mid-may to early June) was obvious in the late 1960s, but it has become unclear since the late 1970s. The timing of the abrupt increase in rates of sunshine in late June (corresponding to the end of the Baiu season in this region) was considerably stable throughout the 40 years. The end of the long sunshine period in midsummer has obviously become earlier, especially in the late 1980s and the 1990s. It is remarkable that rates of sunshine in September have decreased. As is well known, and also as seen in Section 3, the sunshine contrast is clear between the Sea of Japan side and the Pacific Ocean side in the winter monsoon season. When the winter monsoon pressure pattern (west high, east low pattern) prevails over Japan, a northwesterly monsoon blows. Clouds form over the Sea of Japan and move to the Sea of Japan side; thus, there is little sunshine over the Sea of Japan side. The Pacific Ocean side, on the other hand, has much sunshine and dry weather prevails, because the backbone mountain ranges block the clouds from the Sea of Japan side. That is why the winter sunshine contrast between the Sea of Japan side and the Pacific Ocean side is remarkable in Japan. To see the intraseasonal and secular variation of the sunshine contrast in winter between the Sea of Japan side and the Pacific Ocean side, isopleth diagrams of differences between the rates of sunshine over the Sea of Japan side and those over the Pacific Ocean side (the Sea of Japan side minus the Pacific Ocean side) from October to April are created for both northern and CW Japan. For northern Japan, the difference values for the rates of sunshine in Region A minus those in Region C are calculated; for CW Japan, the difference values for the rates of sunshine in Region B minus those averaged in Regions D and E are calculated.

1228 T. INOUE AND J. MATSUMOTO Figure 5(a) shows an isopleth diagram of the difference values between the rates of sunshine over the Sea of Japan side and those on the Pacific Ocean side in northern Japan. From a long-term viewpoint, the difference has been expanding, and the sunshine contrast in the late 1990s was the strongest of the 50 years. Figure 5(b) shows this in CW Japan. The duration between the isolines of 30% has been expanding for both directions; thus, it is recognized that the sunshine contrast has become stronger throughout the 50 years. From these figures, it is implied that the occurrence frequency of the winter monsoon pressure pattern has been increasing throughout the 50 years. This corresponds to the result of Yamakawa (1988) for the years 1941 85. The fact that the sunshine contrast between the Sea of Japan side and the Pacific Ocean side has become stronger implies that the period of winter monsoon season has become longer and/or the winter monsoon has become stronger recently. On the other hand, there has been a warming trend of winter temperature in Japan (e.g. JMA, 1999). To clarify the relationship between the sunshine contrast in winter, the strength of winter monsoon, occurrence frequency of winter pressure pattern and winter temperature, further studies are needed. 5. NATURAL SEASONS IN CW JAPAN BASED ON SUNSHINE DATA, AND THEIR SECULAR CHANGES Many studies have been done concerning natural seasons in Japan (e.g. Takahashi, 1942; Sakata, 1950; Maejima, 1967; Kawamura, 1973; Yoshino and Kai, 1977; Yamakawa, 1988). As described before, Maejima (1967) and Yamakawa (1988) recognized six natural seasons (winter, spring, the Baiu season, midsummer, the Akisame (or Shurin) season and (late) autumn) in Japan. As seen in Figure 2, Region B (the Sea of Japan side of CW Japan) has an annual variation pattern of rates of sunshine that corresponds to these six natural seasons. Region D (Kanto and Tokai district) and Region E (the Pacific Ocean side of western Japan) also have seasonal transition patterns of sunshine similar to that in Region B, except that winter has much more sunshine. These facts mean that the six seasons in CW Japan (Regions B, D and E) are recognized by sunshine duration data as weather seasons. Although the seasonal transition of rates of sunshine seems gradual, the maxima and minima of sunshine corresponding to the natural seasons are clearly seen. Therefore, the situation in CW Japan is focused on in this section, and the natural seasons and their secular changes in the period 1951 2000 from the viewpoint of rates of sunshine are investigated and compared with those of Yamakawa (1988). As described in Section 1, Maejima (1967) used several climatic elements based on climatological daily data for his seasonal divisions, and he used weather singularities, as seasonal boundaries. But it is difficult to Figure 5. As Figure 3, but for the difference values between the rates of sunshine (the Sea of Japan side minus the Pacific Ocean side, %) in (a) northern Japan, and (b) CW Japan, from October to April

SUNSHINE DURATION IN JAPAN 1229 divide natural seasons from unsmoothed daily data in each year by using weather singularities, because two or more candidates of seasonal boundary dates often emerge (i.e. it is unclear which singularities correspond to seasonal boundaries in each region). Moreover, it is difficult to be objective when seasonal divisions are performed using several climatic elements. As such, the pentad data of rates of sunshine smoothed by a five-pentad running mean are used in this study to divide the natural seasons in CW Japan. Figure 6 shows the seasonal transition of rates of sunshine from March to November averaged for the 50 years (1951 2000) in CW Japan. One sunshine maximum in midsummer, two minima in the Baiu and the Akisame seasons and relatively large amounts of sunshine in spring and autumn are obvious. Thus, these seasons are recognized by using the sunshine data alone. Then the climatological seasonal boundaries are defined as the time points when the absolute values of the gradient between two successive pentads become a maximum, and the medians of the pentads are defined as the reference values of rates of sunshine (the values shown in Figure 6) used when seasonal divisions are carried out in each year. Based on this definition, the climatological beginning pentad of the Baiu season, midsummer, the Akisame season and autumn is, respectively pentad 33 (10 14 June), pentad 41 (20 24 July), pentad 51 (8 12 September) and pentad 57 (8 12 October). By comparing these reference values with the values of each year/pentad (the isopleth diagram for these values is shown in Figure 4(b)), the seasonal boundary pentads from spring to autumn are determined in each year. For the beginning pentads of winter and spring, the sunshine contrast between the Sea of Japan side and the Pacific Ocean side is used, because this indicates the typical winter monsoon weather pattern in Japan. Figure 7 shows the seasonal transition of the difference values of rates of sunshine between the Sea of Japan side and the Pacific Ocean side from October to April averaged for the 50 years (1951 2000) in CW Japan. Again, climatological seasonal boundaries are defined as the time when the absolute values of the gradient between two successive pentads become a maximum, and the medians of the pentads are defined as the reference values of the difference of rates of sunshine (the values shown in Figure 7) used when seasonal divisions are carried out in each year. Based on this definition, the climatological beginning pentad of winter is pentad 66 (22 26 November) and that of spring is pentad 12 (25 February 1 March). By comparing these reference values with the values of each year/pentad (the isopleth diagram for these values is shown in Figure 5(b)), the beginnings of winter and spring are determined in each year. Figure 8 shows the secular change of natural seasons in CW Japan for the period 1951 2000 determined by the definitions mentioned above. The beginning of the Baiu season in the mid-1950s and the early 1960s Figure 6. Seasonal transition of rates of sunshine (%) of the 50 year (1951 2000) average in CW Japan (smoothed by the five-pentad running mean) from March to November. Thick lines indicate that the absolute values of the gradient between two successive pentads become a maximum (climatological seasonal boundaries), and numerals show reference values used for seasonal divisions in each year. The dates on the horizontal axis are the central dates of the pentads

1230 T. INOUE AND J. MATSUMOTO Figure 7. As Figure 6, but for the difference of rates of sunshine (the Sea of Japan side minus the Pacific Ocean side, %) of the 50 year (1951 2000) average in CW Japan from October to April was apparently earlier than in the other years, and it was as early as mid-may, although Yamakawa (1988) did not point out this feature. The beginning dates of midsummer (the end of the Baiu season) before 1980 were fairly stable, but after 1980 they have become unstable and have been later than before by about 5 days. It appears from Figure 8 that the beginning of midsummer was extremely late (in mid-august) in 1992, but this was the result of the 5 year running mean data (i.e. little sunshine midsummer was actually in 1991 and 1993). The beginning and end dates of the Akisame season seem to fluctuate considerably, because the seasonal sunshine variations around the Akisame season are relatively smaller than those around the Baiu season and winter (see Figure 6). The beginning of the Akisame season was considerably early (appeared in mid-august) around 1980, which is also pointed out by Yamakawa (1988), and he concluded that the Akisame season due to fronts and cyclones became more active for the period 1971 85 than before. Since the mid-1980s, however, the beginning of the Akisame season has returned to be in early September, and it has been later than that of the 50 year average. The boundary between the Akisame season and autumn fluctuated greatly, and a long-term tendency is not recognized. The beginning of winter has become earlier, and the end of winter (the beginning of spring) has become slightly later (especially since the 1970s). Therefore, winter, from the viewpoint of the sunshine contrast between the Sea of Japan side and the Pacific Ocean side, has become longer in recent years, corresponding to the result by Yamakawa (1988). As described in Section 4, it is possible that this result of weather seasons does not correspond to temperature seasons. The period of winter from the viewpoint of sunshine contrast has become longer, though the period of winter by using temperature as an index of seasons might become shorter than before, because winter temperature in Japan has increased over the period 1951 2000; see also JMA (1999). Therefore, more detailed studies concerning relationships between these changes and the winter monsoon are needed. Table I shows the secular variations of seasonal boundary dates in CW Japan. These dates are determined by reading up to one decimal place of the seasonal boundary pentads (central dates of a pentad represent the pentad). For example, if the Baiu midsummer boundary becomes 40.6 [pentad], the seasonal boundary date becomes 20 July, because the central date of pentad 41 is 22 July. Also shown in Table I is the comparison of the present study with the results of Yamakawa (1988). On comparing the latter 20 years (1981 2000) with the previous 30 years (1951 1980), it can be seen that the start of the Baiu season and midsummer have been delayed by 3 to 5 days. The beginning of winter, on the other hand, became 6 days earlier. From the viewpoint of the length of the seasons through the 50 years, shortening of autumn is remarkable (see broken lines in Figure 8). For comparable years (1971 85), we compared our result based on rates of sunshine with that of Yamakawa (1988). For the beginning of the Akisame season, the difference between these two results

SUNSHINE DURATION IN JAPAN 1231 Table I. Dates of seasonal boundaries and the differences between the first 30 years (1951 80) average and the last 20 years average (1981 2000) in CW Japan. Comparison with the results of Yamakawa (1988) is also shown Seasonal boundary 1951 2000 (50 year average) 1951 80 1981 2000 Difference (days) 1971 85 (this study) 1971 85 (Yamakawa, 1988) Winter Spring 25 Feb 25 Feb 26 Feb +1 27Feb 6Mar Spring Baiu 8 Jun 7 Jun 10 Jun +3 9 Jun 9 Jun Baiu Midsummer 20 Jul 18 Jul 23 Jul +5 19 Jul 26 Jul Midsummer Akisame 6 Sep 5 Sep 7 Sep +2 3 Sep 16 Aug Akisame Autumn 7 Oct 7 Oct 6 Oct 1 8 Oct 8 Oct Autumn Winter 21 Nov 24 Nov 18 Nov 6 21 Nov 22 Nov 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC 5 10 15 20 25 30 35 40 45 50 55 60 65 70 WINTER SPRING BAIU MIDSUMMER AKISAME AUTUMN WINTER 5 10 15 20 25 30 35 40 45 50 55 60 65 70 PENTAD NUMBER 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 YEAR Figure 8. Secular changes of natural seasons in CW Japan from the viewpoint of rates of sunshine. Broken lines show the linear regression lines of the seasonal boundaries

1232 T. INOUE AND J. MATSUMOTO is more than 15 days, but the differences of other boundaries are within a week. Yamakawa (1988) defined the Akisame season by using not only the occurrence frequency of the stationary frontal pattern but also that of part of the trough pattern and the typhoon pattern. As described before, the typhoon pattern, which is frequent in late summer and the Akisame season, does not seem to influence pentad-scale sunshine greatly in CW Japan; thus, it is considered that this difference arises. Finally, Table II shows the standard deviations of seasonal boundary dates for the 50 years, to show the stability of the seasonal boundaries throughout the 50 years. Of the six boundaries, the spring Baiu boundary is the boundary that fluctuates most, and it is influenced by the instability in the 1950s and the early 1960s. The beginning and the end of the Akisame season are also unstable, but it is because the Akisame season is a relatively weak phenomenon from the viewpoint of rates of sunshine, as has already been mentioned. The fluctuations of the beginning and the end of winter are fairly small, but they cannot be compared with those in other boundaries, because the definition is different from those in other boundaries. 6. DISCUSSION AND CONCLUSIONS The seasonal march of sunshine duration and its secular change in Japan are investigated for the period 1951 2000, and the influence of climatic variations on seasonal weather tendencies is discussed. The main results in the present study are as follows: 1. Two seasons with little sunshine in the Baiu and the Akisame seasons are recognized in CW Japan, whereas in northern Japan (Hokkaido and Tohoku district) only one season with little sunshine in summer is seen. Another season with little sunshine is recognized over the Sea of Japan side and the Southwest Islands during the winter monsoon season, although stations in the Pacific Ocean side have much more sunshine in winter. Six regions are recognized in Japan, by using seasonal transitions of rates of sunshine. 2. The sunshine from spring to autumn in northern Japan has obviously decreased in the late 1980s and the 1990s, and the period of little sunshine has become longer since the mid-1980s. 3. In CW Japan, a period of relatively little sunshine often appeared in late spring during the 1950s and the early 1960s. The timing of the increase in sunshine duration after the Baiu season has been delayed since 1980, and a period of little sunshine in midsummer has appeared frequently. 4. In the Southwest Islands, sunshine duration has been decreasing almost all through the year; as a result, the end of the much-sunshine season in midsummer has become earlier in recent years, and a temporal fine-weather period around April has disappeared. 5. The sunshine contrast in winter between the Sea of Japan side and the Pacific Ocean side has intensified recently. 6. By using sunshine duration data, an objective classification of natural seasons is performed for the CW Japan. The dates of seasonal boundaries in this study are quite close to those in the previous studies (e.g. Maejima, 1967; Yamakawa, 1988). Secular changes of seasonal boundaries were then investigated. The Table II. Standard deviations of the six seasonal boundaries and the dates of seasonal boundaries of the 50 year average in CW Japan Seasonal boundary Boundary date (50 year average) Standard deviation (days) Winter Spring 25 Feb 6.3 Spring Baiu 8 Jun 8.7 Baiu Midsummer 20 Jul 5.7 Midsummer Akisame 6 Sep 7.9 Akisame Autumn 7 Oct 8.4 Autumn Winter 21 Nov 5.0

SUNSHINE DURATION IN JAPAN 1233 end of the Baiu season after 1980 has been later than before by about one pentad. Around 1980, the beginning of the Akisame season came earlier, but after that it has been later than before the 1970s, and midsummer lasted until early September. The beginning of winter has become earlier and the end of winter has become later; therefore, winter, as defined by the sunshine contrast between the Sea of Japan side and the Pacific Ocean side, has become longer in recent years. What are the physical mechanisms of the climatic variations that explain the secular change of seasonal transitions of sunshine duration revealed in the present study? The time and spatial scales of these variations are relatively large; thus, local-scale air pollution does not seem to be the main reason. It seems that decadal-scale modulations of the seasonal cycle of atmospheric circulation patterns exist in East Asia, and the causes may be natural climatic variations and/or long-term anthropogenic climate changes, such as global warming. Recently, physical mechanisms for decadal-scale climate variations have been proposed (e.g. Nitta and Yamada, 1989; Trenberth, 1990; Nakamura et al., 1997). Nitta and Yamada (1989) pointed out that a climatic jump occurred around 1976 77 on the whole Pacific Ocean scale. They discovered that the tropical sea-surface temperature (SST), especially in the central and eastern Pacific and in the Indian Ocean, had been increasing since the late 1970s, and convective activity in the tropics became more enhanced. The rise of the tropical SST in the central and eastern Pacific means an intensification of the El Niño southern oscillation (ENSO)-like condition, and it also appeared in the recent decrease in Darwin southern oscillation index (SOI; see Figure 7.9 in Houghton et al. (2001)). In ENSO years, cool summers tend to occur in Japan (JMA, 1999). Therefore, the high frequency of little sunshine in midsummer in CW Japan, the intensification of little sunshine in summer in northern Japan, and the early end of midsummer in the Southwest Islands in recent years may be related to this climatic jump that occurred around 1976 77. But the major change in the present study occurs mainly in the early 1980s; thus, the timing does not necessarily correspond to that of Nitta and Yamada (1989). In addition, relationships between the climatic jump and the secular sunshine variations other than in summer are not seen in this study. Including these problems, further studies are needed to answer the physical mechanisms that explain the secular changes of seasonal transitions discovered here. In the present study, Hokkaido and Tohoku district are included in the same regions as the result of the cluster analysis. However, Maejima (1967) regarded them as different regions. The number of the stations per latitude in the present study is relatively small there, and thus detailed regional divisions may be needed in these regions. The question of where the northern limit of the Baiu frontal zone is also unresolved in the present study, and this must also be clarified in the future. It is also important to investigate the secular changes of the seasonal transitions in the other regions and to compare them with the results of this study. By investigating them in the neighbouring area (East, Southeast and South Asian monsoon regions), or in the other extratropical regions that also have remarkable seasonal variations of climatic elements, the nature of climatic variations will become more obvious. ACKNOWLEDGEMENTS This paper is a part of the graduation thesis of the first author submitted to the Department of Geography, the University of Tokyo. We would like to thank all members of the department and the members of the Climate Colloquium for their useful comments and advice. We are also indebted to two anonymous reviewers because their comments are greatly useful to improve this manuscript. Part of this study is supported financially by the Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Culture, Sports, Science and Technology (no. 14208008). APPENDIX A The data for 1951, 1952 (1961, 1962 in the Southwest Islands), 1999 and 2000 cannot be smoothed by the 5 year running mean; thus, they are smoothed by a running mean only with the available data of one side.

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