Effects of the Tibetan High and the North Pacific High on the Occurrence of Hot Or Cool Summers in Japan

atmosphere

Article Effects of the Tibetan High and the North Paciﬁc High on the Occurrence of Hot or Cool Summers in Japan

Makoto Inoue 1,* , Atsushi Ugajin 2,†, Osamu Kiguchi 1, Yousuke Yamashita 3 , Masashi Komine 4 and Shuji Yamakawa 5

1 Department of Biological Environment, Akita Prefectural University, Akita 010-0195, Japan; [email protected] 2 Graduate School of Bioresource Sciences, Akita Prefectural University, Akita 010-0195, Japan; [email protected] 3 National Institute for Environmental Studies, Tsukuba 305-8506, Japan; [email protected] 4 Department of Biological Production, Akita Prefectural University, Akita 010-0195, Japan; [email protected] 5 College of Humanities and Sciences, Nihon University, Tokyo 156-8550, Japan; [email protected] * Correspondence: [email protected] † Current address: National Institute for Environmental Studies, Tsukuba 305-8506, Japan.

Abstract: In this study, we investigated the effects of the Tibetan High near the tropopause and the North Pacific High in the troposphere on occurrences of hot or cool summers in Japan. We first classified Japan into six regions and identified hot and cool summer years in these regions from a 38-year sample (1980–2017) based on the monthly air temperature. To investigate the features of circulation fields over Asia during hot and cool summers in Japan, we calculated the composite differences (hot summer years minus cool summer years) of several variables such as geopotential Citation: Inoue, M.; Ugajin, A.; height, which indicated significant high-pressure anomalies in the troposphere and lower strato- Kiguchi, O.; Yamashita, Y.; Komine, M.; Yamakawa, S. Effects of the sphere. These results suggest that both the North Pacific and the Tibetan Highs tend to extend to Tibetan High and the North Pacific Japan during hot summer years, while cool summers seem to be associated with the weakening High on the Occurrence of Hot or of these highs. We found that extension of the Tibetan High to the Japanese mainland can lead to Cool Summers in Japan. Atmosphere hot summers in Northern, Eastern, and Western Japan. On the other hand, hot summers in the 2021, 12, 307. https://doi.org/ Southwestern Islands may be due to extension of the Tibetan High to the south. Similarly, the 10.3390/atmos12030307 latitudinal direction of extension of the North Pacific High is profoundly connected with the summer climate in respective regions. Academic Editors: Corrado Camera and Georgios Zittis Keywords: hot summer; cool summer; Tibetan High; North Pacific High; statistical analysis

Received: 19 January 2021 Accepted: 23 February 2021 Published: 26 February 2021 1. Introduction

Publisher’s Note: MDPI stays neutral Meteorological disasters, including drought, a cool summer, or heavy rain, have a with regard to jurisdictional claims in serious effect on crop yields in Japan. Drought causes a fall in rice production due to published maps and institutional affil- inadequate and poorly distributed rainfall, and an exceptionally cool summer brings about iations. low rice yields which can lead to considerable disturbances to rice markets. The Okhotsk High and the North Pacific High dominate the summer weather in East Asia including Japan [1–6]. The “Yamase” wind, a cool and moist air current originating in the Okhotsk High, induces large summer temperature variation, greatly influencing the summer climate in northeastern Japan [3,7,8]. On the other hand, the western stretch (i.e., extension to Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Japan) of the North Pacific High dominates the summer climate in East Asia [9,10], bringing This article is an open access article hot and humid conditions. However, it is difficult to predict the summer weather in Japan distributed under the terms and from the development of only these two highs in the lower and middle troposphere because conditions of the Creative Commons there are many factors, such as the El Niño–Southern Oscillation and phenomena in the Attribution (CC BY) license (https:// upper atmosphere, that may drive temperature variability [11]. creativecommons.org/licenses/by/ Recently, there has been discussion that the Tibetan High is an important factor in 4.0/). determining summer climate in Japan. This high is a warm, gigantic, anti-cyclonic circula-

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tion and covers East Asia from the upper troposphere to the lower stratosphere [12–14], and greatly influences the summer climate in East Asia [15]. This is characterized by a high-pressure system over the Tibetan Plateau, which is also called the South Asian High [16–18]. The temporal analyses revealed that the location of the upper tropospheric westerly jet core near the northern periphery of the Tibetan High changed rapidly from 140◦ E to 90◦ E during the plum rain period over East Asia [19,20]. It was indicated that cool summers are profoundly related to enhancement of the Okhotsk High and weakening of the North Pacific and Tibetan Highs [21]. Yamakawa [22] showed that strengthening of the Tibetan High associated with the eruption of Mt. Pinatubo led to a hot summer in East Asia. The effects of interannual changes in the Tibetan High on the climate were investigated by Nagano et al. [15], who revealed a recent cooling trend over Northern Japan with the weakening of the Tibetan High since 1992. Owada and Ishikawa [23] showed that the center of the Tibetan High was located around the Tibetan Plateau during hot summer in East Asia, whereas it was located near the Iranian Plateau during cool summer. The Japan Meteorological Agency (JMA) reported that, in 2013, Japan experienced an extremely hot summer due to the enhanced Pacific High and the Tibetan High associated with a significantly active Asian monsoon [24]. However, few studies have investigated the characteristics of drought and cool summer years in recent decades. In this study, we clarify the influences of the Tibetan High and North Pacific High on the occurrence of drought and cool summers in Japan by statistical analyses of the atmospheric circulation fields during a 35-year period not only in the lower atmosphere but also near the tropopause. In addition, we examine how the direction of extension of these highs affects the summer climate in each region of Japan.

2. Data and Analysis Methods 2.1. Surface air Temperature data and Definitions of Hot and Cool Summer Years To define hot and cool summer years in Japan, we used monthly surface air temperature data available via the JMA website [25]. We classified Japan into six regions, the Sea of Japan side of Northern Japan (NJJ), the Pacific side of Northern Japan (NJP), Eastern Japan (EJ), Western Japan (WJ), the Northeastern part of the Southwest Islands (SIN), and the Southwestern part of the Southwest Islands (SIS). Table1 and Figure1 list the meteorological stations and show the map of each region, respectively. The selected stations are uniformly distributed over the whole of Japan. For instance, NJJ consists of ten stations located in the Sea of Japan side of Hokkaido prefecture and Tohoku district.

Table 1. Meteorological stations for the six regions of Japan deﬁned in this study.

Region Name of Stations Haboro, Rumoi, Iwamizawa, Sapporo, Esashi, Fukaura, Akita, Sakata, Shinjo, the Sea of Japan side, Northern Japan (NJJ) Yamagata Nemuro, Kushiro, Obihiro, Hiroo, Mutsu, Hachinohe, Miyako, Morioka, Ohunato, the Paciﬁc side, Northern Japan (NJP) Ishinomaki, Sendai, Fukushima, Onahama Mito, Utsunomiya, Maebashi, Tokyo, Katsuura, Kofu, Takada, Toyama, Suwa, Eastern Japan (EJ) Takayama, Fukui, Nagoya, Shizuoka Hikone, Wakayama, Toyooka, Yonago, Hamada, Takamatsu, Matsuyama, Kochi, Western Japan (WJ) Fukuoka, Nagasaki, Kumamoto, Nobeoka, Makurazaki Northeastern part of Southwest Islands (SIN) Yakushima, Naze Southwestern part of Southwest Islands (SIS) Minamidaito, Naha, Ishigakijima, Yonagunijima

Figure2 shows the interannual variation in surface temperature in NJJ, NJP, EJ, WJ, SIN, and SIS during the 38-year period of 1980–2017. The year 1994 had a typical hot summer, in which the surface temperature was higher at most locations [26]. This condition led to rainfall shortage and restrictions of municipal supply service in Japan. On the other hand, temperatures in 1993 and 2003 were lower, and these seriously damaged the Japanese rice crop. The summer weather in 2010 was quite hot, and much of the rice crop Atmosphere 2021, 12, x FOR PEER REVIEW 3 of 14

130°E 135°E 140°E 145°E 125°E 130°E 45°N SIN SIS

30°N

Atmosphere 2021, 12, 307 3 of 13 40°N

was damaged due to extreme heat [27]. For each region, 10 hottest years were deﬁned as

hot summer years, and 10 coolest years3 were5°N deﬁned as cool summer years based on the Atmosphere 2021, 12, x FOR PEER REVIEW 3 of 14 surface temperature data during the studiedNJJ period. The classiﬁcation results for the six NJP 25°N regions are listed in Table2. EJ WJ

130°E 135°E 140°E 145°E 125°E 130°E (a) 45°N (b) SIN SIS Figure 1. Spatial distribution of meteorological stations for each region of (a) the Japanese main- 30°N land and (b) the Southwestern Islands used for the classification of hot and cool summer years in this study. 40°N Figure 2 shows the interannual variation in surface temperature in NJJ, NJP, EJ, WJ, SIN, and SIS during the 38-year period of 1980–2017. The year 1994 had a typical hot summer, in which the surface temperature was higher at most locations [26]. This condition 35°N led to rainfall shortage and restrictionsNJJ of municipal supply service in Japan. On the other NJP 25°N hand, temperatures in 1993 andEJ 2003 were lower, and these seriously damaged the Japa- WJ nese rice crop. The summer weather in 2010 was quite hot, and much of the rice crop was damaged due to extreme heat [27]. For each region, 10 hottest years were defined as hot (a) (b) summer years, and 10 coolest years were defined as cool summer years based on the sur- FigureFigureface 1. temperatureSpatial 1. Spatial distribution distribution data ofduring of meteorological meteorological the studied stations period. forfor eacheach The region region classification of of (a )(a the) the Japanese Japanese results mainland main-for the six re- landandgions and (b ()b theare) the Southwestern listed Southwestern in Table Islands Islands 2. used used for the for classiﬁcation the classification of hot and of coolhot summerand cool years summer in this years study. in this study.

Figure 2 shows the interannual variation in surface temperature in NJJ, NJP, EJ, WJ, SIN, and SIS during the 38-year period of 1980–2017. The year 1994 had a typical hot summer, in which the surface temperature was higher at most locations [26]. This condition led to rainfall shortage and restrictions of municipal supply service in Japan. On the other hand, temperatures in 1993 and 2003 were lower, and these seriously damaged the Japa- nese rice crop. The summer weather in 2010 was quite hot, and much of the rice crop was damaged due to extreme heat [27]. For each region, 10 hottest years were defined as hot summer years, and 10 coolest years were defined as cool summer years based on the surface temperature data during the studied period. The classification results for the six regions are listed in Table 2.

Figure 2. Temporal variation in surface temperature for summer (June–August) in six regions of Japan.Figure The 2. Temporal blue, brown, variation green, gold, in surface orange, andtemperature pink lines showfor summer the surface (June temperature–August) inin NJJ, six NJP,regions of EJ,Japan. WJ, SIN,The andblue, SIS, brown respectively., green, gold, orange, and pink lines show the surface temperature in NJJ, NJP, EJ, WJ, SIN, and SIS, respectively. 2.2. Reanalysis Data for Composite Maps and Analysis Procedure To examine the features of the large-scale circulation associated with higher and lower temperatures in Japan in Northern Hemisphere summers (June, July, and August) from 1980 to 2017, we used monthly reanalysis data (geopotential height, meridional wind, and vertical p velocity) with a horizontal resolution of 2.5◦ × 2.5◦ derived from the National Centers for Environmental Prediction/National Center for Atmospheric Research global atmospheric reanalysis data [28]. In addition, we utilized the monthly precipitation data with a horizontal resolution of 2.5◦ × 2.5◦ provided by the Climate Prediction Center Merged Analysis of Precipitation [29] over the same period. We calculated the difference between hot and cool summer years (i.e., hot summer years minus cool summer years) for Figure 2. Temporal variation in surface temperature for summer (June–August) in six regions of selected variables. In addition, we evaluated the statistical signiﬁcance of these composite Japan. The blue, brown, green, gold, orange, and pink lines show the surface temperature in NJJ, NJP, EJ, WJ, SIN, and SIS, respectively.

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differences using Welch’s t test with the signiﬁcance level at 95%. This method has been used to clarify the features of the upper atmosphere [30,31].

Table 2. Classiﬁcation results for hot and cool summer years in six regions of Japan.

Hot Summer Years Cool Summer Years 1984, 1990, 1994, 1999, 2000, 2010, 2011, 1980, 1981, 1982, 1983, 1986, 1993, the Sea of Japan side, Northern Japan (NJJ) 2012, 2013, 2014 1996, 1998, 2002, 2003 1990, 1994, 1999, 2000, 2004, 2010, 2011, 1980, 1981, 1982, 1983, 1986, 1988, the Paciﬁc side, Northern Japan (NJP) 2014, 2015, 2016 1993, 1996, 1998, 2003 1990, 1994, 2000, 2001, 2002, 2004, 2010, 1980, 1981, 1982, 1983, 1986, 1988, Eastern Japan (EJ) 2011, 2012, 2013 1989, 1992, 1993, 2003 1990, 1994, 2001, 2004, 2005, 2010, 2011, 1980, 1982, 1983, 1986, 1988, 1989, Western Japan (WJ) 2013, 2016, 2017 1992, 1993, 2003, 2015 1990, 1991, 1998, 2001, 2003, 2006, 2007, 1982, 1983, 1985, 1989, 1992, 1993, Northeastern part of Southwest Islands (SIN) 2013, 2016, 2017 1997, 1999, 2000, 2014 1980*, 1988*, 1991, 1998, 2001, 2007, 2013, 1981, 1982, 1984, 1985, 1986, 1989, Southwestern part of Southwest Islands (SIS) 2014, 2015, 2016, 2017 (* joint 10th place) 1992, 1995, 1997, 2000

3. Results 3.1. North Pacific Subtropical High and Tibetan High We first show the features of the atmospheric field over Asia and Eurasia in summer. Figure3 indicates the spatial distribution of geopotential height at several altitudes in the hot (left panels) and cool summer years (right panels). This distribution shows the mean values of ten years in each altitude. The North Pacific subtropical high is centered around the Hawaiian Islands at 500 hPa in the middle troposphere and at 850 hPa in the lower troposphere (Figure3b,c,e,f). This high greatly influences the summer climate in East Asia including Japan [4,5,32]. At 150 hPa in the tropopause, we find the center of the Tibetan High (Figure3a,d) which is a warm, gigantic, anti-cyclonic circulation that covers East Asia from the upper troposphere to the lower stratosphere [12–15]. The Tibetan High extends to the west of India and can be explained as a Rossby wave response to the heating over India [33,34]. As noted in Section1, the Tibetan High has a crucial effect on summertime weather in Japan, though it is hard to discuss the differences between the hot summer years and cool summer years based on the distribution of mean values (i.e., Figure3). We therefore calculate the composite differences between the both years in the next section.

3.2. Extension of the Tibetan High in Hot or Cool Summer Years Now, we shall examine how the occurrence of hot or cool summers in Japan is connected with atmospheric circulation fields in the broad Asian monsoon regions. Figure4a shows the composite differences in geopotential height at 150 hPa between hot and cool summer years in NJJ. We find significant high-pressure anomalies around Japan, which reflects stronger eastward extension (i.e., toward Japan) of the Tibetan High centered near the northern part of India (Figure3a,b) in hot summer years than in cool ones. Striking positive anomalies develop over the Tian Shan Mountains (80◦ E) and west Asia (30–50◦ N), located at the same latitude band as the Japanese islands. On the other hand, low pressure anomalies can be seen in subtropical regions (south of 30◦ N). We here calculated the average and standard deviation of geopotential height at 150 hPa around Northern Japan (35–45◦ N, 60–90◦ E) for hot summer years (10 years) and cool summer years (10 years) in NJJ. The average ± standard deviation of geopotential height during hot summer years and cool summer years were 14,147 ± 22 m and 14,103 ± 20 m, respectively. Thus, the difference between hot and cool summer years was approximately 44 m over Northern Japan. The distribution of geopotential height differences between hot and cool summer years in NJP is similar to that in NJJ (Figure4a,b). This means that eastward extension of the Tibetan High is an important factor in agrometeorology and crop growth in both the Sea of Japan and Pacific sides of Northern Japan. Figure4c shows the geopotential height Atmosphere 2021, 12, 307 5 of 13

differences at 150 hPa between hot and cool summer years in EJ. Signiﬁcant positive anomalies are present in East Asia, including Japan and Mongolia. By contrast, low pressure anomalies are dominant in the subtropics such as southern China. This characteristic is also found in the distribution of geopotential height difference in WJ (Figure4d). We consider the mechanisms of hot and cool summers that occur in Eastern Japan to be extremely similar to those in Western Japan. Figure4e,f show the composite differences between hot and cool summer years in SIN and SIS, respectively. Positive anomalies are seen in most parts of Asia and Eurasia, including Japan. Particularly, for hot summer years in SIS, signiﬁcant high-pressure anomalies develop over the Ogasawara Islands and the Mariana Atmosphere 2021, 12, x FOR PEER REVIEWIslands located southeast of the Japanese mainland. These results suggest that the Tibetan5 of 14 High tends to extend to the south of Japan during hot summers in the Southwest Islands compared to those of Northern, Eastern, and Western Japan.

(a) Z150 (hot summer years) (d) Z150 (cool summer years)

(b) Z500 (hot summer years) (e) Z500 (cool summer years)

FigureFigure 3. 3.Spatial Spatial distribution distribution of of geopotential geopotential height height (in (in m) m) at at ( a(a,d,d)) 150 150 hPa, hPa, ( b(b,e,e)) 500 500 hPa, hPa, and and ( c(c,f,)f) 850 850 hPa hPa for for hot hot summer summer yearsyears (10 (10 years) years) and and cool cool summer summer years years (10 (10 years), years), respectively, respectively, in in the the Sea Sea of of Japan Japan side side of of Northern Northern Japan Japan (NJJ). (NJJ).

3.2. Extension of the Tibetan High in Hot or Cool Summer Years Now, we shall examine how the occurrence of hot or cool summers in Japan is connected with atmospheric circulation fields in the broad Asian monsoon regions. Figure 4a shows the composite differences in geopotential height at 150 hPa between hot and cool summer years in NJJ. We find significant high-pressure anomalies around Japan, which reflects stronger eastward extension (i.e., toward Japan) of the Tibetan High centered near the northern part of India (Figure 3a,b) in hot summer years than in cool ones. Striking positive anomalies develop over the Tian Shan Mountains (80° E) and west Asia (30–50° N), located at the same latitude band as the Japanese islands. On the other hand, low pressure anomalies can be seen in subtropical regions (south of 30° N). We here calculated the average and standard deviation of geopotential height at 150 hPa around Northern Japan (35–45° N, 60–90° E) for hot summer years (10 years) and cool summer years (10 years) in NJJ. The average ± standard deviation of geopotential height during hot summer years and cool summer years were 14,147 ± 22 m and 14,103 ± 20 m, respectively. Thus, the

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(a) (d)

(b) (e)

FigureFigure 4. 4.Composite Composite differencesdifferences in in geopotential geopotential height height (in (in m) m) at at 150 150 hPa hPa between between hot hot and and cool cool summer summer years years in in ( a(a)) the the Sea Sea ofof Japan Japan side side of of NorthernNorthern Japan Japan (NJJ),(NJJ), ((bb)) the the Paciﬁc Pacific side side of of NorthernNorthern Japan Japan (NJP), (NJP), ( (cc)) EasternEastern JapanJapan (EJ),(EJ), ((dd)) WesternWestern Japan Japan (WJ),(WJ), ( e()e) the the Northeastern Northeastern part part of of the the Southwest Southwest Islands Islands (SIN), (SIN), and and (f) the (f) Southwesternthe Southwestern part ofpart the of Southwest the Southwest Islands Islands (SIS). The(SIS). borders The borders of regions of regions that reached that reached a signiﬁcance a significance of 95% of are 95% shown are shown with thick with lines. thick lines.

3.3.3.3. ExtensionExtension ofof thethe NorthNorth PacificPacific HighHigh inin HotHot oror CoolCool SummerSummer YearsYears Next,Next, wewe focus focus on on the the extension extension of of the the North North Pacific Pacific High High centered centered around around the Hawai-the Ha- ianwaiian Islands. Islands. Figure Fig5urea shows 5a shows the composite the composite differences differences in geopotential in geopotential height height at 850 at hPa 850 betweenhPa between hot and hot cooland summercool summer years years in NJJ. in WeNJJ. find We significantfind significant high-pressure high-pressure anomalies anom- aroundalies around Japan, Japan, which which reflects reflects stronger stronger westward westward extension extension (i.e., toward (i.e., toward Japan) ofJapan) the North of the

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North Pacific High centered on Hawaii (Figure 3c,f) in hot summer years than in cool ones. PaciﬁcThe average High centered ± standard on Hawaiideviation (Figure of geopotential3c,f) in hot height summer at years850 hPa than over in coolNorthern ones. Japan The average(35–45° ±N, standard60–90° E) deviationduring hot of summer geopotential years heightand cool at summer 850 hPa overyears Northernwere 1479 Japan ± 8 m ◦ ◦ ± (35–45and 1471N, 60–90± 4 m, E)respectively. during hot The summer difference years between and cool hot summer and cool years summer were 1479 years8 was m ± andabout 1471 8 m over4 m, respectively.Northern Japan. The differenceThe distribution between of hotgeopoten and cooltial summer height anomalies years was aboutin NJP 8is m also over similar Northern to that Japan. in NJJ The (Fig distributionure 5b). Composite of geopotential anomaly height maps anomalies of hot and in cool NJP summer is also similar to that in NJJ (Figure5b). Composite anomaly maps of hot and cool summer years years in EJ and WJ reveal positive anomalies over those two regions, and negative anom- in EJ and WJ reveal positive anomalies over those two regions, and negative anomalies alies around the Okhotsk Sea (Figure 5c,d). Significant high-pressure anomalies can be around the Okhotsk Sea (Figure5c,d). Signiﬁcant high-pressure anomalies can be seen in seen in southern parts of Kyushu district (Figure 5e,f). southern parts of Kyushu district (Figure5e,f). (a) (d)

(b) (e)

FigureFigure 5. 5.Composite Composite differences differences in in geopotential geopotential height height (in (in m) m) at 850at 850 hPa hPa between between hot hot and and cool cool summer summer years years in (a in) NJJ, (a) NJJ, (b) NJP,(b) NJP, (c) EJ, (c ()d EJ,) WJ, (d) ( eW) SIN,J, (e) andSIN, ( fand) SIS. (f) The SIS. borders The borders of regions of regions that reached that reached a signiﬁcance a significance of 95% of are 95% shown are shown with thick with lines. thick lines.

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To clarify the characteristics of the large-scale circulation associated with high pres- sureTo system clarify over the characteristicsJapan, we investigated of the large-scale the distributions circulation of associated geopotent withial height, high pressure vertical systemflow, and over precipitation. Japan, we investigated Here we focus the on distributions the North Pacific of geopotential High extension height, in vertical hot and flow, cool andsummer precipitation. years in NJJ, Here WJ, we and focus SIS on as therepresentatives North Pacific of HighNorthern extension Japan, in Western hot and Japan, cool summerand the yearsSouthwestern in NJJ, WJ, Islands, and SISrespectively as representatives (Figures 6 of–8). Northern Figure 6a Japan, shows Western the composite Japan, anddifferences the Southwestern in geopotential Islands, height respectively at 500 hPa (Figures in NJJ.6– We8). Figurefound6 significanta shows the high composite-pressure differencesanomalies in the geopotential middle troposphere height at 500 over hPa the in Japanese NJJ. We islands, found significant including Hokkaido. high-pressure This anomaliesmeans that in the the North middle Pacific troposphere High extende over thed Japanese to Japan islands, during includinghot summers Hokkaido. in Northern This meansJapan, thatunlike the during North Pacificcool summers. High extended The distribution to Japan during of precipitation hot summers anomalies in Northern shows Japan,negative unlike and duringpositive cool anomalies summers. over The Japan distribution and the Philippines, of precipitation respectively anomalies (Fig showsure 6b). negative and positive anomalies over Japan and the Philippines, respectively (Figure6b) . These are consistent with ascent anomalies around the Philippine Sea at low latitude (10– These are consistent with ascent anomalies around the Philippine Sea at low latitude 25° N) and descent anomalies around Japan (30–40° N) in the troposphere (Figure 6c,d). (10–25◦ N) and descent anomalies around Japan (30–40◦ N) in the troposphere (Figure6c,d) . We consider that hot summers tend to occur in Northern Japan due to extension of the We consider that hot summers tend to occur in Northern Japan due to extension of the North Pacific High over the Japanese mainland. For hot summers in Western Japan, sig- North Pacific High over the Japanese mainland. For hot summers in Western Japan, nificant high-pressure anomalies are seen over the Japanese mainland, as in Northern Ja- significant high-pressure anomalies are seen over the Japanese mainland, as in Northern pan (Figure 7a). However, whether the North Pacific High extends to Hokkaido during Japan (Figure7a). However, whether the North Pacific High extends to Hokkaido during hot summers in Western Japan is unclear, compared to Northern Japan. There are positive hot summers in Western Japan is unclear, compared to Northern Japan. There are positive and negative rainfall anomalies (Figure 7b), which are consistent with the distribution of and negative rainfall anomalies (Figure7b), which are consistent with the distribution ascent anomalies at low latitude (10–20° N) and descent anomalies (20–40° N) in the trop- of ascent anomalies at low latitude (10–20◦ N) and descent anomalies (20–40◦ N) in the osphere, respectively (Figure 7c,d). On the other hand, the analysis results for the South- troposphere, respectively (Figure7c,d). On the other hand, the analysis results for the Southwesternwestern Islands Islands reveal reveal significant significant high high-pressure-pressure anomalies anomalies in the in thebroad broad region region from from the thePhilippine Philippine Sea Seato the to Japanese the Japanese islands islands (Figure (Figure 8a). The8a). descent The descent anomalies anomalies in the low in the and lowmiddle and latitudes middle latitudes (20–30° N) (20–30 associated◦ N) associated with low rainfall with low anomalies rainfall anomaliesare striking are (Fig strikingure 8b– (Figured). These8b–d) results. These indicate results that indicate extension that of extension the North of thePacific North High Pacific to the High south to can the lead south to canwarming lead to and warming drought and in drought the Southwestern in the Southwestern Islands. Islands.

(a) (c)

(b) (d)

10 m s-1 0.02 Pa s-1

FigureFigure 6. 6.Composite Composite differencesdifferences in (a) geopotential height height (in (in m) m) at at 500 500 hPa, hPa, (b (b) )precipitation precipitation (in (in mmmm day day−−1),), and and ( (cc)) vertical vertical p p-velocity-velocity (in (in 1010−3− Pa3 Pa s−1 s) −at1 )500 at 500hPa hPabetween between hot and hot andcool coolsummer summer yearsyears in in NJJ. NJJ. ( d(d)) Latitude-pressure Latitude-pressure cross-sectioncross-section ofof thethe compositecomposite differencesdifferences in vertical p p-velocity-velocity −3 −1 (10(10−3 Pa s−)1 )between between hot hot and and cool cool summer summer years years in NJJ in NJJ at 140° at 140 E. ◦VectorsE. Vectors show show the composite the composite dif- differencesferences in in meridional meridional-vertical-vertical flow ﬂow (see (see legend legend for for units). units). The The borders borders of ofregions regions that that reached reached a a significance of 95% are shown with thick lines. signiﬁcance of 95% are shown with thick lines.

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(a) (c) (a) (c)

(b) (d) (b) (d)

10 m s-1 -1 0.0210 Pa m ss-1 0.02 Pa s-1

FigureFigure 7. CompositeComposite differences differences in in(a) ( ageopotential) geopotential height height (in m) (in at m) 500 at hPa, 500 ( hPa,b) precipitation (b) precipitation (in (in mmFiguremm day 7.− 1Composite),1), and and (c) ( cvertical) differences vertical p-velocity p-velocity in (a) (ingeopotential 10 (in−3 10Pa− s3−1Pa )height at s500−1) (inhPa at m) 500 between at hPa 500 betweenhPa, hot and(b) precipitationcool hot andsummer cool (in summer mm day−1), and (c) vertical p-velocity (in 10−3 Pa s−1) at 500 hPa between hot and cool summer yearsyears in WJ. ( (dd)) Latitude Latitude-pressure-pressure cross cross-section-section of the of the composite composite differences differences in vertical in vertical p-velocity p-velocity years −in3 WJ.− 1(d) Latitude-pressure cross-section of the composite differences in vertical p-velocity (in 10−3 Pa s −) 1between hot and cool summer years in WJ at 140° E. Vectors◦ show the composite (in 10−3 Pa− s1 ) between hot and cool summer years in WJ at 140 E. Vectors show the composite (indifferences 10 Pa s in) betweenmeridional hot-verti andcal cool flow summer (see legend years forin WJ units). at 140° The E. borders Vectors of show regions the thatcomposite reached a differencesdifferencessignificance in of meridional meridional-vertical95% are shown-verti calwith flow ﬂowthick (see (see lines. legend legend for for units). units). The The borders borders of regions of regions that thatreached reached a a signiﬁcancesignificance of 95% 95% are are shown shown with with thick thick lines. lines. (a) (c) (a) (c)

(b) (d) (b) (d)

10 m s-1 -1 0.0210 Pa m ss-1 0.02 Pa s-1

Figure 8. Composite differences in (a) geopotential height (in m) at 500 hPa, (b) precipitation (in FigureFiguremm day 8. − 1Composite),Composite and (c) vertical differences differences p-veloc in ity in(a) ((ingeopotentiala) geopotential10−3 Pa s−1 )height at 500 height (inhPa m) (inbetween at m) 500 at hPa, hot 500 and( hPa,b) precipitationcool (b )summer precipitation (in (in −−1 1 −3 −3−1 −1 mmmmyears day in SIS.), ),and ( andd) (Latitudec) ( cvertical) vertical-pressure p-veloc p-velocity itycross (in- section10 (in 10Pa ofs )Pathe at s500composite) hPa at 500 between differences hPa between hot and in vertical cool hot summer and p-velocity cool summer yearsyears(in 10 − in3 Pa SIS. s− 1() (d dbetween)) Latitude Latitude-pressure hot-pressure and cool cross summer cross-section-section years of the ofin theSIScomposite at composite 140° E.differences Vectors differences show in vertical the in vertical composite p-velocity p-velocity (in 10−3 Pa s−1) between hot and cool summer years in SIS at 140° E. Vectors show the composite (in 10−3 Pa s−1) between hot and cool summer years in SIS at 140◦ E. Vectors show the composite differences in meridional-vertical ﬂow (see legend for units). The borders of regions that reached a signiﬁcance of 95% are shown with thick lines.

Atmosphere 2021, 12, x FOR PEER REVIEW 11 of 14

differences in meridional-vertical flow (see legend for units). The borders of regions that reached a significance of 95% are shown with thick lines. Atmosphere 2021, 12, 307 10 of 13 3.4. Relationships between the Extension of Two Highs and Occurrences of Hot or Cool Summers Figure 9 shows a schematic diagram of the longitudinal and vertical extensions of the 3.4. Relationships between the Extension of Two Highs and Occurrences of Hot or Cool Summers Tibetan High near the tropopause and the North Pacific High in the troposphere during Figure9 shows a schematic diagram of the longitudinal and vertical extensions of the Tibetanhot summers High near in the Japan. tropopause We andsuggest the North that Pacific the enhanced High in the troposphereTibetan High during and North Pacific High hot(i.e., summers a structure in Japan. with We suggest two layers that the over enhanced Japan) Tibetan can High lead and to North adiabatic Pacific High heating when air masses (i.e.,descend a structure in withthe twoatmosphere, layers over Japan) causing can lead hot to summers. adiabatic heating Conversely, when air masses cool summers are linked descend in the atmosphere, causing hot summers. Conversely, cool summers are linked to theto weakeningthe weakening of the Tibetan of the High Tibetan and the High North Pacificand the High. North Pacific High.

lower stratosphere

Tibetan High

troposphere North Pacific High

Tibet Japan Hawaii

Figure 9. Schematic diagram showing the longitudinal and vertical extensions of the Tibetan High andFigure the North 9. Schematic Pacific High diagram during hot summersshowing in the Japan. longitudinal and vertical extensions of the Tibetan High and the North Pacific High during hot summers in Japan. The relationship between the latitudinal direction of extension of the Tibetan High and the region where a hot summer occurs in Japan is summarized in Figure 10a. In hot summer years forThe the Japaneserelationship mainland, between high pressure the latitudinal anomalies were direction seen at 150 of hPa extension over Japan. of the Tibetan High Thisand means the reg thation hot summerwhere in a thehot mainland summer occurs occurs due to in extension Japanto is the summarized north (i.e., the in Figure 10a. In hot Japanese mainland) of the Tibetan High (Figure 10a). By contrast, extension to the south cansummer lead to drought years infor the the Southwestern Japanese Islands. mainland, Next, we high focus pressure on extension anomalies of the North were seen at 150 hPa Pacificover HighJapan. in the This troposphere means (Figure that hot10b). summer In hot summer in the years mainland for Northern occurs Japan, high due to extension to the pressurenorth (i.e anomalies., the atJapanese 500 hPa are mainland) observed in theof the Japanese Tibetan islands. High On the (Fig otherure hand, 10a). hot By contrast, extension summer in the Southwestern Islands seems to occur with high pressure anomalies in the subtropicalto the south zone can including lead theto drought southern part in the of Japan. Southwestern This might mean Islands. that the Next, North we focus on extension Pacificof the High North extends Pacific to the High north duringin the hot troposphere summers in the (Fig Japaneseure 10b). mainland, In hot whereas summer years for North- hotern summers Japan, in high the Southwestern pressure Islandsanomalies occur dueat 500 to extension hPa are to theobserved south of the in North the Japanese islands. On Pacific High (Figure 10b). In other words, the summer climate in each region of Japan is profoundlythe other controlled hand, hot by the summer direction in of extensionthe Southwestern of these two highs. Islands seems to occur with high pressure anomalies in the subtropical zone including the southern part of Japan. This might mean that the North Pacific High extends to the north during hot summers in the Japanese mainland, whereas hot summers in the Southwestern Islands occur due to extension to the south of the North Pacific High (Figure 10b). In other words, the summer climate in each region of Japan is profoundly controlled by the direction of extension of these two highs.

AtmosphereAtmosphere 2021, 12, 2021x FOR, 12 PEER, 307 REVIEW 12 of 14 11 of 13

(a)

Hot summer in the mainland

Tibetan High

Hot summer in the Southwestern Islands

(b)

Hot summer in the mainland

North Pacific High

Hot summer in the Southwestern Islands

Figure 10.Figure Schematic 10. Schematic diagram diagram showing showing the relationships the relationships between between the latitudinal the latitudinal direction direction of exten- of extension of (siona) the of Tibetan (a) the TibetanHigh and High (b) andthe (Northb) the Pacific North PaciﬁcHigh and High the and regions the regions where where hot summers hot summers occur occur inin Japan. Japan.

4. Summary4. Summary This paperThis has paper presented has presented the effects the effects of the ofNorth the North Pacific Pacific High Highand the and Tibetan the Tibetan High High on on occurrencesoccurrences of hot of or hot cool or cool summers summers in Japan. in Japan. Japan Japan was was class classifiedified into into six regions, six regions, and and hot hot andand cool cool summer summer years years in these in these regions regions were were extracted extracted during during a 38 a 38-year-year period period (1980 (1980–2017).– 2017). TheThe features features of of the the large large-scale-scale circulation circulation in in the the troposphere troposphere and and lower lower stratosphere stratosphere and and thethe precipitation precipitation distribution distribution in hot in hot and and cool cool summer summer years years were were investigated investigated using using the the NationalNational Centers Centers for forEnvironmental Environmental Prediction/National Prediction/National Center Center for for Atmospheric Atmospheric Re- Research search reanalysisreanalysis data and the Climate Prediction CenterCenter MergedMerged AnalysisAnalysis of of Precipitation Precipitation data. In data. Ingeneral, general, both both the the Tibetan Tibetan High High and and North North Pacific Pacific High High tended tended to extend to extend to Japan to Japan during hot during summershot summers in Northern in Northern Japan, Japan, Eastern Eastern Japan, Japan, Western Western Japan, Japan, and and the the Southwestern Southwest- Islands. ern Islands.On the On other the other hand, hand, the occurrences the occurrences of cool of summerscool summers were we associatedre associated with with the weakening the weakeningof these of these highs. highs. Thus, Thus the, summerthe summer climate climate in each in each region region of Japan of Japan is profoundly is profoundly controlled controlledby theby the extension extension of theof t Tibetanhe Tibetan High High and and North North Pacific Pacific High. High. It isIt is important important to to monitor

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the behaviors of these highs for accurate prediction of extreme weather disasters such as droughts and cold weather which can seriously damage many crops. Further analyses on these highs from June through August may lead to great beneﬁts to the agricultural and economic activity in summer.

Author Contributions: S.Y. conceived of the presented idea. M.I. took on the project administration and wrote the original draft. A.U. performed formal analysis and investigations. O.K. supervised the findings of this work and contributed to the writing—review and editing. Y.Y., M.K., and S.Y. participated in discussions and contributed to the writing—review and editing. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by Grant-in-Aid for New faculty startup support of Akita Prefectural University. Informed Consent Statement: Not applicable. Data Availability Statement: Data used in this paper are available in a publicly accessible website: Monthly surface air temperature data for each region of Japan are available in the Japan Meteorolog- ical Agency at https://www.data.jma.go.jp/obd/stats/etrn/index.php (accessed on 26 February 2021); Monthly global atmospheric reanalysis data are available from the National Centers for En- vironmental Prediction/National Center for Atmospheric Research at https://psl.noaa.gov/data/ gridded/data.ncep.reanalysis.pressure.html (accessed on 26 February 2021); Monthly precipitation data are available from the Climate Prediction Center Merged Analysis of Precipitation at https://psl.noaa.gov/data/gridded/data.cmap.html (accessed on 26 February 2021). Acknowledgments: We used the Grid Analysis and Display System for drawing part of figures. This study was conducted with financial support from Akita Prefectural University, Japan. Conflicts of Interest: The authors declare no conflict of interest.

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