EARLY ONLINE RELEASE
This is a PDF of a manuscript that has been peer-reviewed and accepted for publication. As the article has not yet been formatted, copy edited or proofread, the final published version may be different from the early online release.
This pre-publication manuscript may be downloaded, distributed and used under the provisions of the Creative Commons Attribution 4.0 International (CC BY 4.0) license. It may be cited using the DOI below.
The DOI for this manuscript is DOI:10.2151/jmsj.2020-071 J-STAGE Advance published date: October 7th, 2020 The final manuscript after publication will replace the preliminary version at the above DOI once it is available. 1 Compound effect of local and remote sea surface temperatures on the
2 unusual 2018 western North Pacific summer monsoon
3
4 Wang-Ling Tseng1, Chi-Cherng Hong2, Ming-Ying Lee3, Huang-Hsiung Hsu1*,
5 and Chi-Chun Chang2
6
7 1Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan
8 2Department of Earth and Life, University of Taipei, Taipei, Taiwan
9 3Central Weather Bureau, Taipei, Taiwan
10
11
12
13
14 August 31, 2019
15
16 ------
17 *Corresponding author: Huang-Hsiung Hsu, Research Center for Environmental
18 Change, Academia Sinica, 128 Academia Road, Section 2, Nankang, Taipei, 115,
19 Taiwan; Email: [email protected]; Tel: +886 2787-5845 1
20 21 Abstract
22 In July and August (JA) 2018, the monsoon trough in the western North Pacific
23 (WNP) was unusually strong, the anticyclonic ridge was anomalously northward-
24 shifted, and enhanced and northward-shifted tropical cyclone activity was observed.
25 Studies have examined the effects of sea surface temperature (SST) anomalies on the
26 North Atlantic (NA), the Indian Ocean (IO), and the tropical North Pacific. However,
27 a synthetic view of SST forcings has yet to be identified. Based on a series of numerical
28 experiments, this study demonstrated that the SST anomaly in the tropical WNP was
29 the key forcing that formed the structure of the observed anomalous phenomena in the
30 monsoon trough. Moreover, the combined effect of the SST anomaly in both the
31 tropical and extratropical WNP resulted in enhanced circulation anomalies in the WNP.
32 The NA SST anomaly also enhanced the monsoon trough in the presence of WNP SST
33 anomaly. By contrast, the individual SST anomaly in the NA, IO, the extratropical
34 WNP, and the subtropical eastern North Pacific could not force the enhanced monsoon
35 trough. We proposed that the local effect of both the tropical and extratropical WNP
36 SST anomaly as the major driver and the remote effect of NA SST anomaly as a minor
37 contributor jointly induced the anomalous circulation and climate extremes in the WNP
38 during JA 2018.
2
39 Keywords: enhanced monsoon trough, northward-shifted subtropical high, sea surface
40 temperature anomaly, summer 2018, western North Pacific
41
3
42 1. Introduction
43 The tropical western North Pacific (WNP) was characterized by active tropical
44 cyclone (TC) activity in July and August (JA) 2018. With 14 TCs, WNP TC activity in
45 JA 2018 was only exceeded by the 16 TCs each in 1967 and 1994 (Japan
46 Meteorological Agency; JMA,
47 https://www.data.jma.go.jp/fcd/yoho/typhoon/index.html) compared with the long-
48 term average of 9.5. The overall TC tracks in JA 2018 shifted north of long-term
49 averaged tracks (Fig. 1a), whereas the mean genesis locations mainly shifted eastward
50 (Fig. 1b) along with the eastward-extended monsoon trough (Fig. 1a). Japan had three
51 TC landfalls, an anomalous number compared with the long-term average of 1.4.
52 Eleven TCs approached Japan (i.e., reached within 300 km of Japanese weather stations,
53 including those on Hokkaido, Honshu, Shikoku, and Kyushu; JMA,
54 http://www.data.jma.go.jp/fcd/yoho/typhoon/statistics/landing/landing.html) in JA
55 2018, a record-high number since 1981. The ratio of TCs (11) affecting Japan to the
56 total WNP TCs (14) was unusually high (79%, 11/14). Northward-shifted TC tracks
57 were linked to unusually high TC activity in the subtropical WNP. As shown in Fig. 2a,
58 the number of TC days north of 30°N in the WNP in JA 2018 was the highest since
59 1981. The northward-expanded monsoon trough in JA 2018 provided a favorable
60 background for the excess stormy conditions and severe rainfall events in Japan (e.g., 4
61 the strong southwesterly and extremely heavy rainfall caused devastating landslides,
62 flooding, and more than 200 deaths in west Japan after Typhoon Prapiroon in early July
63 2018 and Typhoon Jebi in early September, which was the strongest typhoon in 25
64 years and severely affected Japan’s mainland in late August and early September)
65 (Tokyo Climate Center 2018a; Moteki 2019; Sekizawa et al. 2019; Shimpo et al. 2019;
66 Sueki and Kajikawa 2019; Takemi and Unuma 2019; Takemura et al. 2019; Yokoyama
67 et al. 2020).
68 Notably, when the TCs shifted northward in JA 2018, Northeast Asia experienced
69 severe heat wave events (Tokyo Climate Center 2018a; Qian et al. 2019; Shimpo et al.
70 2019; Takaya 2019). Recent studies have associated these abnormal phenomena with
71 anomalous large-scale circulation. A strong northern circumpolar perturbation,
72 characterized by zonally distributed positive 500-hPa geopotential height (GHT)
73 anomalies between 30°N and 60°N circumglobally in the upper troposphere, was
74 observed and associated with the widespread heat wave events in the northern
75 extratropics, including Northeast Asia, North America, northern Europe, and the Arctic,
76 from May to August 2018 (Chen, L et al. 2019; Kobayashi and Ishikawa 2019; Liu et
77 al. 2019; Vogel et al. 2019; Ha et al. 2020). These anomalous events in the WNP were
78 accompanied by an unusually northward shift in the subtropical high and an enhanced
79 monsoon trough (Wang et al. 2019). The anomalous circulation pattern exhibited 5
80 characteristics of the positive phase of the Pacific–Japan (PJ) pattern, which was the
81 strongest since 1981 (Fig. 2b). The PJ pattern, characterized by a tripolar structure of
82 circulation and rainfall, is recognized as a major mode that affects summertime climate
83 variability in East Asia and the WNP (Nitta 1987; Kosaka and Nakamura 2006; Hsu
84 and Lin 2007). Consistent with the record-breaking positive PJ index, the WNP summer
85 monsoon was the most active since 1981 in terms of the WNP monsoon index and the
86 Asia summer monsoon outgoing long-wave radiation (OLR) index (SAMOI, average
87 OLR over 10°N–20°N, 115°E–140°E), as shown in Fig. 2b (Wang et al. 2001; Tokyo
88 Climate Center 2018b). Notably, correlations between any two of the three indices
89 ranged from 0.4 to 0.59 (WNP–PJ 0.4, SAMOI–PJ 0.49, and WNP–SAMOI 0.59)
90 The main causes of these abnormal phenomena remain unclear. First, the North
91 Atlantic (NA) sea surface temperature (SST) tripole that triggered a barotropic Rossby
92 wave pattern in the northern extratropics was suggested to be a factor that caused the
93 northward shift of the WNP subtropical high (Chen, L et al. 2019; Deng et al. 2019;
94 Liu et al. 2019). Second, Chen, L et al. (2019) suggested that the observed reduction in
95 snow cover over the Tibetan Plateau in winter 2017–2018 led to a stronger thermal
96 effect of the Tibetan Plateau in the following early summer, indirectly contributing to
97 the enhancement of the monsoon trough over the Philippine Sea and leading to a strong
98 East Asian summer monsoon. These findings are consistent with those of Hsu and Liu 6
99 (2003), who concluded that diabatic heating over the Tibetan Plateau triggered a wave-
100 like circulation pattern that affected East Asian summer rainfall. In addition, Kobayashi
101 and Ishikawa (2019) suggested the importance of SST anomalies in the tropical Pacific
102 in predicting northern-mid-latitude tropospheric warming. Furthermore, the summer of
103 2018 followed a La Niña, which created favorable conditions for a cyclonic circulation
104 anomaly over the Philippine Sea and strengthened the East Asian summer monsoon
105 (Chen, L et al. 2019). Cooling in the southeastern tropical Indian Ocean (IO) was also
106 suggested (Tao et al. 2012; Gao et al. 2018; Chen, R et al. 2019) to be the main reason
107 for a low-level southwesterly anomaly and an anomalous cyclonic circulation over the
108 WNP. Finally, studies have also suggested that the Pacific Meridional Mode (PMM)-
109 like SST anomalies in the eastern North Pacific played a crucial role in establishing a
110 favorable background flow for active TC conditions and eastward-shifted genesis
111 locations in the WNP (Hong et al. 2018; Stuecker 2018; Qian et al. 2019; Takaya 2019).
112 By contrast, Imada et al. (2019) suggested the effect of human-induced global warming
113 on the July 2018 high-temperature event in Japan.
114 The studies mentioned above have focused either on the anomalous warmth in
115 Northeast Asia and the northward-shifted subtropical high or on the enhanced monsoon
116 trough. The various described mechanisms indicate that the anomalous behavior of the
117 subtropical high and monsoon trough could be viewed as a synthetic response to various 7
118 forcings, the aggregated impact of which is described as a compound effect in this study.
119 Because an SST anomaly is often the major forcing that causes anomalous large-scale
120 circulation, the relative role of observed SST anomalies in different basins during JA
121 2018 should be explored. For example, the possibility that extremely high SSTs in the
122 WNP as a key forcing of anomalous circulations has not been explored. The impact of
123 other SST anomalies compared with the effect of the WNP SST anomaly also warrants
124 exploration. This study used an atmospheric general circulation model (AGCM) to
125 evaluate the relative effects of SST anomalies in the Pacific, NA, and IO on inducing
126 the northward shift in the subtropical high and the enhanced monsoon trough in the
127 WNP during JA 2018. The data and model are described in Section 2. Section 3 presents
128 the results, and Section 4 presents the discussion.
129
130 2. Data and Model
131 The observational data used in this study were the monthly atmospheric fields
132 retrieved from the NCEP/NCAR Reanalysis I (Kalnay et al. 1996), and SSTs were
133 retrieved from the NOAA_ERSST_V4 data (provided by the NOAA/OAR/ESRL
134 Physical Sciences Laboratory, Boulder, Colorado, USA,
135 https://www.esrl.noaa.gov/psd/) (Huang et al. 2015). Precipitation data were obtained
136 from NOAA’s PRECipitation REConstruction Dataset (PREC; Chen et al. 2002). OLR 8
137 data (Liebmann and Smith 1996) were retrieved from NOAA/ESRL
138 (ftp://ftp.cdc.noaa.gov). The anomalies in Figs. 3 and 4 are presented in as percentiles,
139 which are statistical measures indicating the value below which a given percentage of
140 observations in a group of observations falls, to demonstrate the extremity. Here, SST
141 and NCEP atmospheric variables for 1948–2018 and PREC precipitation data for 1951–
142 2018 were ranked from smallest to largest. The 95th and 5th percentiles indicate that
143 an observation was in the highest and lowest 5% of observed values, respectively. The
144 atmospheric component of the NCAR Community Earth System Model (Neale et al.
145 2010) CAM5 with a resolution of 1.9° latitude and 2.5° longitude and 30 vertical levels
146 was used in the numerical experiments to determine the relative effects of the SST
147 anomalies in various regions on the anomalous circulations in the WNP. The target
148 period was JA 2018. The detailed settings of the experiments are described in
149 subsection 3.2.
150
151 3. Results
152 3.1 General circulation characteristics in JA 2018
153 This section presents the characteristics of circulation in JA 2018. Rainfall
154 exceeded the 90th percentile over the South China Sea and the Philippine Sea, whereas
155 it was lower than the 30th percentile in the south over the Maritime Continent and in 9
156 the north over Japan and Korea (Fig. 3a). This dry–wet–dry precipitation pattern
157 exhibited characteristics of the PJ pattern in the positive phase (Nitta 1987; Kosaka and
158 Nakamura 2006) or the tripolar rainfall pattern in the negative phase (Hsu and Liu 2003;
159 Hsu and Lin 2007). Figure 3b presents the moisture flux and convergence anomalies at
160 925 hPa. The enhanced moisture flux and convergence confirmed the substantially
161 strengthened monsoon trough over the Philippine Sea. These observations
162 corresponded to those of the positive phase of the WNP monsoon index and SAMOI as
163 shown in Fig. 2b. The dry–wet–dry rainfall pattern coincided with the warm–cold–
164 warm structure in the 2-m temperature (Fig. 3c). Notably, the heat wave events occurred
165 in the extensive warm belt (exceeding the 95th percentile) over China, Korea, and Japan
166 (Chen, R et al. 2019; Kornhuber et al. 2019; Shimpo et al. 2019; Vogel et al. 2019;
167 Wang et al. 2019; Ha et al. 2020). Likewise, the 500-hPa geopotential height (GHT500)
168 exceeding the 95th percentile between 30°N and 40°N revealed a strong anticyclonic
169 anomaly that manifested in the northward-shifted subtropical high in the WNP (Fig.
170 3d), whereas the region lower than the 30th percentile in the south reflected an enhanced
171 monsoon trough. A comparison between the 1490-m contour of 850-hPa geopotential
172 height (GHT850) in 2018 (red line) and the climatological mean (blue line) verified
173 both the northward-shifted subtropical high and the enhanced monsoon trough.
10
174 As mentioned in the Introduction, TC tracks in JA 2018 tended to shift northward
175 compared with the long-term average track distribution. This northward shift was
176 consistent with the steering flow, which was located between the anticyclonic
177 circulation in the north and the cyclonic circulation in the south (i.e., the region of tight
178 gradients in the 1000–400-hPa mean steering flow shown in Fig. 1a). Most TC tracks
179 in JA 2018 closely followed the western flank of the anticyclonic circulation and moved
180 in a clockwise direction (Fig. 1a). The anomalous circulation in JA 2018 explained the
181 record-breaking TC activity north of 30°N and why Japan was affected by more TCs in
182 JA 2018 (Fig. 2a).
183 To further investigate the influence of the northward-shifted subtropical high and
184 active WNP summer monsoon, we examined the large-scale circulation in a much
185 larger domain (Fig. 4). In the upper troposphere (e.g., 200-hPa GHT anomalies; Fig.
186 4a), a circumpolar pattern consisting of predominantly positive GHT anomalies in the
187 northern extratropics (30°N–60°N) was the dominant feature. By contrast, the negative
188 anomaly in the Philippine Sea and the South China Sea that reflected the enhanced
189 monsoon trough dominated in the lower troposphere (Fig. 4b). Although the
190 circumpolar pattern was weaker, it was still evident in the lower troposphere in the
191 northern extratropics, except over Northeast Asia, reflecting the equivalent barotropic
192 nature of the pattern. By contrast, no notable perturbation in the upper troposphere was 11
193 observed over the active monsoon trough region. In summary, the extratropical
194 anomaly exhibited a top-heavy deep hemispheric structure, whereas the one in the lower
195 latitudes was a shallow regional feature occurring mainly in the subtropical WNP.
196 To explore possible forcing from oceans, the percentiles of SSTs in JA 2018 were
197 shown in Fig. 4c to demonstrate the occurrence of extreme SSTs in various basins. First,
198 an SST tripole in the NA was evident. The feature has been suggested to force a
199 barotropic Rossby wave pattern in the northern extratropics that contributed to the
200 extreme heatwave events in 2018 (Chen, L et al. 2019; Deng et al. 2019; Liu et al. 2019).
201 The extreme positive and negative SST anomalies were above the 95th percentile and
202 below the 5th percentile in JA 2018, respectively, reflecting the record-high amplitude
203 of the NA SST tripole since 1981 (not shown). Second, the particularly warm SST
204 anomalies exceeding the 95th percentile spread from the tropical western Pacific and
205 tilted slightly northward to the subtropical eastern North Pacific (Fig. 4c; blue outlined
206 region marked with 2). This positive SST anomaly belt was south of the negative 1000-
207 hPa GHT anomalies in the western and central subtropical North Pacific. An
208 examination of vertical structure indicated that the negative GHT anomaly in the
209 western Pacific extended up to 500 hPa, whereas the counterpart in the eastern Pacific
210 was shallow and appeared generally below 850 hPa (not shown). This phase
211 relationship suggests that positive SST anomalies forced the anomalous cyclonic 12
212 circulation in the lower troposphere in these regions. The contrast between negative
213 GHT anomalies in the vertical extent appeared consistent with the contrasting
214 atmospheric stability between the subtropical western and eastern North Pacific. The
215 WNP is climatologically characterized by the monsoon trough with active convection
216 and TC activity, whereas the eastern North Pacific is climatologically more stable and
217 prevailed by the subtropical anticyclone and widespread stratocumulus clouds. Thus, a
218 positive SST anomaly of similar strength would trigger deeper convections and larger
219 negative GHT anomalies in the western Pacific than in the eastern Pacific.
220 The OLR and 200-hPa velocity potential anomalies shown in Fig. 4d indicate that
221 strong negative OLR and velocity potential anomalies (implying anomalous upward
222 motion) occurred over the North Pacific between the equator and 30°N, where the
223 positive SST anomaly was observed. This active convection system was likely triggered
224 by the anomalously warm SSTs nearby. In addition to the anomalously active
225 convection in the North Pacific in JA 2018, convection was suppressed in the tropical
226 South Pacific and IO, as indicated by the positive OLR and velocity potential anomalies.
227 These results suggest that the anomalously warm SSTs in the tropical and subtropical
228 North Pacific likely played a crucial role in driving anomalous atmospheric circulation
229 in East Asia and the Pacific and IO region in JA 2018.
13
230 As reviewed in the Introduction, SST anomalies in various basins have been
231 suggested to force the observed anomalous circulation in the WNP. Results from a
232 series of numerical experiments forced by the observed SST anomalies in various
233 basins are presented in the following section to evaluate the relative effects of various
234 SST forcings on the observed anomalous phenomena in the WNP.
235 3.2 Numerical experiments
236 3.2.1 Design of experiments
237 Bearing in mind that AGCMs were more skilful in simulating monsoon circulation
238 than rainfall in the WNP (Wang et al. 2004), we conducted AMIP-type experiments by
239 forcing the CAM5 model with the observed SSTs in various basins from January to
240 August 2018 to assess the effects of the local and remote SST anomalies on the
241 anomalous circulations. By using this one-way AMIP-type approach, we could not
242 understand the complete physical process that caused the extremity in the WNP during
243 JA 2018 or quantify the exact contribution of SST anomalies, but the approach provided
244 valuable information on the effects of the anomalous SSTs and the relative
245 contributions from various ocean basins. Each experiment listed in Table 1 comprises
246 11 members with initial conditions in slightly different perturbations. The control
247 experiment was forced with the observed climatological monthly mean SST and sea ice
248 concentration (SIC) from 1982 to 2010. The global experiment (GL-exp) was forced 14
249 with the global SST anomaly in 2018 to identify the ability of the model to reproduce
250 the observed circulation characteristics. The sensitivity experiments were conducted by
251 forcing the model with the observed monthly mean SST anomaly in January–August
252 2018. Eight sensitivity experiments were conducted with the observed SST anomalies
253 in the following basins as lower-boundary forcing: the tropical WNP (TWP-exp), the
254 whole WNP (WP-exp), the tropical WNP plus the NA (TWP+ATL-exp), the whole
255 WNP plus the NA (WP+ATL-exp), the extratropical WNP to the east of Japan (EJP-
256 exp), the NA (ATL-exp), the tropical eastern North Pacific (ENP-exp), and the eastern
257 IO (IO-exp). The outlined regions 1, 2, 3, 4, 5, and 6 marked in Fig. 4c are the SST
258 anomaly domains prescribed in ATL-exp, TWP-exp, WP-exp, EJP-exp, ENP-exp, and
259 IO-exp. Notably, the climatological mean SIC was prescribed in all SST anomaly
260 experiments. Therefore, the impacts of SIC in 2018 were not evaluated. Anomalies of
261 each experiment subtracted from the control run were analyzed to identify the impacts
262 of the SST anomalies in individual basins.
263 3.2.2 Local SST effects on the WNP circulation
264 As shown in Fig. 5a, the observed circulation in JA 2018 was characterized by
265 zonally distributed positive GHT500 anomalies from 30°N to 60°N, which were
266 associated with a cyclonic anomaly over the WNP monsoon trough area from 100°E to
15
267 150°E and 10°N to 30°N (Fig. 3d). These features formed a wave-like perturbation
268 arching over the extratropical North Pacific from the East Asian coast to the Gulf of
269 Alaska. GHT500, rather than GHT200 and GHT1000, is shown here because GHT500
270 can represent both the positive anomaly over Northeast Asia, where heat waves
271 occurred, and the negative anomaly over the monsoon trough region. The overall
272 pattern was reasonably simulated in the global SST anomaly simulation (GL-exp, Fig.
273 5b); however, some phase shifts and weaker amplitudes (e.g., zonally elongated
274 anomaly) were observed. The relative contributions of SST anomalies in various basins
275 to this anomalous perturbation pattern was explored using eight other SST anomaly
276 sensitivity experiments. As previously discussed, the SST anomaly in the tropical North
277 Pacific likely played a crucial role in triggering the north–south circulation dipole in
278 East Asia and the WNP. This hypothesis was partially supported by TWP-exp forced
279 by the positive SST anomaly in the tropical western Pacific. As shown in Fig. 5c, a
280 negative–positive GHT anomaly was simulated in the WNP, indicating an enhanced
281 monsoon trough (although not statistically significant) and a northward-shifted
282 anticyclonic ridge. In addition, the anomalies arching over the extratropical North
283 Pacific simulated in the global SST anomaly experiment were reasonably reproduced
284 in the TWP experiment, except with a weaker zonally elongated component in the
285 eastern North Pacific. Evidently, some of the anomalies in the observations and GL- 16
286 exp could be attributed to the positive SST anomaly in the TWP, which induced
287 atmospheric perturbations that propagated downstream and spread over the North
288 Pacific. This result is consistent with findings that the PJ pattern is often induced by
289 anomalous convection activity over the Philippine Sea (Nitta 1987; Kosaka and
290 Nakamura 2006; Hsu and Lin 2007; Kosaka and Nakamura 2010; Hirota and Takahashi
291 2012).
292 In addition to the SST anomaly in the tropical western Pacific, positive and
293 negative SST anomalies were present in the extratropical WNP. To examine the
294 possible contribution of extratropical SST anomalies that reflect the warmer SST in the
295 Kuroshio extension and anomalous SST gradient in the region, WP-exp including all
296 SST anomalies west of the date line in the North Pacific was conducted. The result
297 shown in Fig. 5d indicates that including the additional SST anomaly forcing in the
298 extratropical WNP enhanced both the south–north pair of the negative–positive
299 anomaly in the WNP and the wave-like perturbation arching over the extratropical
300 North Pacific. The most significant improvement was the more realistically simulated
301 south–north dipole in the WNP, reflecting an enhanced monsoon trough and a
302 northward-shifted anticyclonic ridge. The amplitudes of simulated perturbations in WP-
303 exp were nearly as large as those in GL-exp, suggesting the dominant forcing effect of
304 local SST anomalies in the WNP. By contrast, if only the positive SST anomaly in the 17
305 extratropical WNP was considered as it was in EJP-exp (Fig. 5e), only the wave-like
306 perturbation over the extratropical North Pacific could be induced and the negative
307 anomaly in the monsoon trough was not simulated. Notably, including the negative SST
308 anomaly in the extratropical WNP yielded a similar result (not shown).
309 Both anomalous SST anomalies in the tropical and extratropical WNP were
310 needed to properly induce the observed enhanced monsoon trough and northward-
311 shifted anticyclonic ridge. We further explored this enhancement; Fig. 6 presents the
312 cross sections of vertical motion in the pressure coordinate and wave activity flux
313 (WAF; Takaya and Nakamura 2001) averaged over 110°E–140°E in the observations,
314 GL-exp, TWP-exp, WP-exp, TWP+ATL-exp, and WP+ATL-exp. GL-exp simulated
315 the major features of observed perturbations (Fig. 6a and 6b), such as the lower-level
316 (upper-level) northward (southward) WAF south of 35°N, the anomalous upward
317 (downward) motion in the tropics (subtropics), and the perturbations north of 35°N.
318 Although the amplitudes of the observed and simulated vertical motion were equivalent,
319 the amplitudes of the simulated WAF were generally weaker than those of the observed
320 WAF and are represented by different unit vectors. A weaker WAF was particularly
321 evident in the upper troposphere, but it was qualitatively similar. This deficiency might
322 have been be an inherent problem in the model, but it could not be exactly understood.
323 The basic features with weaker amplitudes were reproduced in TWP-exp (Fig. 6c) and 18
324 further enhanced in WP-exp after the inclusion of SST anomalies in the extratropical
325 WNP (Fig. 6d). By contrast, these anomalous features were poorly simulated in EJP-
326 exp (not shown). The much-enhanced positive anticyclonic anomalies over the
327 extratropical WNP in WP-exp may have indirectly affected the enhanced monsoon
328 trough, which was relatively weak when forced with the TWP SST anomaly only. The
329 enhanced monsoon trough further triggered convection in the tropical WNP and
330 contributed to a PJ pattern-like response. In other words, the mutual enhancement of
331 the atmospheric responses induced by the SST anomaly in both the tropical and
332 extratropical WNP was likely the key to the improved simulation of the observed
333 anomalous circulations in the WNP. The phenomenon can also be interpreted as follows:
334 The PJ pattern has been identified as an intrinsic mode, which is embedded in the
335 summertime mean flow over the East Asian and WNP region (Kosaka and Nakamura
336 2006; Hsu and Lin 2007; Kosaka and Nakamura 2010; Hirota and Takahashi 2012) and
337 can be induced by various forcings or perturbations. The joint effect of SST forcing in
338 the tropical and extratropical WNP likely provided a stronger forcing and induced a
339 larger PJ-pattern-like response in WP-exp than in TWP-exp and EJP-exp, which
340 considered only SST anomalies in one region.
341
342 3.2.3 Remote SST effects on the WNP circulation 19
343 Liu et al. (2019) reported the contribution of the NA SST tripole to the observed
344 anomalous ridge in the extratropical WNP. The influence of the NA SST anomaly was
345 evaluated in ATL-exp (Fig. 5f) and the combined SST anomaly experiments, namely
346 TWP+ATL-exp (Fig, 5g) and WP+ATL-exp (Fig. 5h). The NA SST tripole-like
347 anomaly appeared to induce a wave-like perturbation in the North Pacific (Fig. 5f)
348 similar to that in the aforementioned experiments but with weaker amplitudes that were
349 statistically significant only over the Aleutian Islands. The impact of the anomaly on
350 the wave-like perturbation was relatively minimal in combination with the SST
351 anomaly in the WNP (i.e., TWP+ATL-exp in Fig. 5g and WP+ATL-exp in Fig. 5h).
352 The major positive impact, however, was the enhancement of the monsoon trough in
353 WP+ATL-exp. Notably, a similar impact was not observed in TWP+ATL-exp (Fig. 5g),
354 suggesting the crucial contribution of the SST anomaly in the extratropical WNP in
355 enhancing the monsoon trough. A comparison of the cross sections shown in Fig. 6c–f
356 revealed a similar contrast; enhancement was only observed when the SST anomaly in
357 the whole WNP was included (i.e., WP+ATL-exp). The enhanced anticyclonic anomaly
358 over extratropical East Asia in WP-exp served as a path for the WAF induced by the
359 NA SST anomaly to further propagate southeastward to the Philippine Sea and
360 increased the mutual enhancement of the south–north dipole discussed previously.
20
361 The wide impact of the NA SST anomaly was further explored in the longitude–
362 height cross section of geopotential height averaged over 30°N–60°N, where the
363 extratropical perturbations over the Eurasian continent were the most evident. The
364 observed perturbation was characterized by a northern circumpolar perturbation
365 (mainly positive anomalies), with the maximum amplitude in the upper troposphere
366 over specific regions (e.g., Europe and Northeast Asia; Fig. 7a). As reported by Liu et
367 al. (2019), the pattern was the combination of a zonal mean increase and a wave-like
368 perturbation that was realistically simulated in GL-exp except over East Asia (Fig. 7b).
369 The observed wave-like perturbation over the Eurasian continent was not particularly
370 evident in TWP-exp (Fig. 7c). Including the SST anomaly in the extratropical North
371 Pacific in WP-exp enhanced the wave-like pattern and zonal component both upstream
372 and downstream (Fig. 7d), making the anomalies more in phase with the anomalies in
373 observation and in the GL-exp. When the NA SST anomaly was added in TWP-exp
374 and WP-exp, the wave-like pattern was not evidently enhanced (Fig. 7e and 7f). This
375 intercomparison of four experiments suggests that the SST anomaly in the extratropical
376 WNP exerted larger effects on the extratropical circulation than did the NA SST
377 anomaly. In summary, the remote influence of the NA SST anomaly reported by Liu et
378 al. (2019) and Chen, L et al. (2019) seemed limited in our series of simulations.
21
379 The SST anomalies in the subtropical eastern North Pacific and the tropical eastern
380 IO have been suggested playing a crucial role in inducing the anomalous circulation in
381 the WNP in 2018 (Tao et al. 2012; Chen, R et al. 2019; Takaya 2019). The impact of
382 the SST anomaly in the subtropical eastern North Pacific (region 5 in Fig. 4c) was
383 examined in the ENP-exp. Evidently, prescribing the SST anomaly in the subtropical
384 eastern North Pacific induced not only wave-like perturbations in the extratropical
385 North Pacific, as in all other experiments, but also positive anomalies near the monsoon
386 trough region that reflected a weakened monsoon trough. Thus, the eastern North
387 Pacific SST anomaly contributed positively to the wave-like perturbations in the
388 extratropical North Pacific but did not seem to enhance the monsoon trough. This result
389 is inconsistent with those of Takaya (2019) and Qian et al. (2019) and will be further
390 discussed in the discussion section. IO-exp considered the impacts of the negative SST
391 anomaly over the eastern tropical IO (Fig. 5j). A comparison between Fig. 5b and 5j
392 indicates that the negative SST anomaly in the tropical eastern IO enhanced the overall
393 circulation pattern in the extratropical North Pacific but had little effect on the monsoon
394 trough. The hypothesis suggested in an empirical diagnostic study (Chen et al. 2019)
395 that the negative SST anomaly in the eastern IO enhanced the monsoon trough could
396 be not confirmed in our study.
397 22
398 3.2.4 Precipitation and TC genesis potential
399 The observed excessive rainfall over the WNP monsoon trough area associated
400 with the low-level (850-hPa) cyclonic circulation anomaly (Fig. 8a) was reasonably
401 simulated, although slightly weaker and shifted westward, in GL-exp and WP-exp, as
402 shown in Fig. 8b and 8d. By contrast, the cyclonic anomaly and excessive rainfall were
403 not properly simulated in TWP-exp (Fig. 8c). This contrast between WP-exp and TWP-
404 exp is consistent with the previous discussion that SST anomaly forcing in the
405 extratropical WNP was an important factor enhancing the anomalous rainfall and
406 circulation in the western portion of the WNP (Fig. 8d), and it yielded a more realistic
407 simulation. However, the circulation and precipitation in 150°E–180°, where the Bonin
408 high prevailed, were less realistic likely because of the inability of the model to simulate
409 the observed zonally elongated positive anomalies over the extratropical North Pacific.
410 As expected, the addition of the NA SST anomaly further enhanced the excessive
411 rainfall and monsoon trough in WP+ATL-exp (Fig. 8f) but not in TWP+ATL-exp (Fig.
412 8e).
413 As mentioned in the Introduction, TC activity in JA 2018 was among the highest
414 since the beginning of the record (Tokyo Climate Center 2018a; Shimpo et al. 2019;
415 Takaya 2019). Our experiments were conducted in coarse spatial resolutions that do not
416 resolve TCs. We therefore evaluated the TC genesis potential index (GPI, Emanuel and 23
417 Nolan 2004; Camargo et al. 2007) to infer the implications of the simulation results in
418 terms of potential TC activity. The overall positive anomaly of the TC genesis
419 potential index (Fig. 9a) revealed favorable environmental conditions (e.g., high SSTs,
420 enhanced cyclonic circulation, and moisture content) for TC genesis in JA 2018. This
421 enhanced GPI environment in the WNP was more realistically simulated in the regions
422 west of 160°E in GL-exp (Fig. 9b) and to some extent in TWP-exp, WP-exp,
423 TWP+ATL-exp, and WP+ATL-exp (Fig. 9c–f). Including the extratropical SST
424 anomaly in the WNP and NA helped extend the region of positive GPI anomalies
425 further northward and yielded a closer resemblance to the observations (Fig. 9e and 9f
426 compared with Fig. 9c and 9d). Figure 9h presents the negative and positive influences
427 of the NA SST anomaly on the GPI anomalies in the tropical and subtropical WNP,
428 respectively, that were reversed to the GPI anomaly in the observations and GL-exp.
429 The SST anomaly in the tropical eastern North Pacific tended to produce a negative
430 GPI anomaly (Fig. 9i) consistent with the simulated anticyclonic anomaly in the
431 monsoon trough region (Fig. 5i). Including the SST anomaly in the tropical eastern IO
432 (IO-exp) also did not seem to contribute positively to the observed GPI anomalies (Fig.
433 9j).
434
435 4. Summary and discussion 24
436 In the WNP during JA 2018, the anomalously northward-shifted Pacific
437 subtropical high and the enhanced monsoon trough resulted in rainfall deficit and
438 record-breaking heat in Northeast Asia and an anomalously active WNP summer
439 monsoon and excessive rainfall in the South China Sea and Philippine Sea. Northward-
440 shifted steering flow and TC tracks resulted in the highest TC activity north of 30°N
441 since 1981. Consistent with these abnormal phenomena, the PJ index and the tripolar
442 rainfall pattern also had record-high amplitudes. In addition, the SST in the tropical
443 WNP and the subtropical eastern North Pacific were above the 95th percentile, and the
444 NA SST tripole was the highest since 1981.
445 This study used the CAM5 AGCM to conduct a series of experiments by forcing
446 the model with the SST anomaly in the global domain, the tropical and extratropical
447 WNP, the subtropical eastern North Pacific, the tropical eastern IO, and NA to
448 determine the relative influences of SST anomalies on these basins. The experiment
449 forced by the global SST anomaly reasonably reproduced the observed wave-like
450 pattern emanating from the tropical WNP to the extratropical WNP and arching over
451 the extratropical North Pacific to the Gulf of Alaska. This result confirmed the key role
452 of anomalous SSTs in causing the observed anomalies during JA 2018. The joint
453 influences of the SST anomalies in both the tropical and extratropical WNP was the
454 major factor that strengthened the WNP summer monsoon and the northward shift of 25
455 the Pacific subtropical high. Regarding the influences of individual SST anomaly, the
456 tropical WNP SST anomaly induced a wave-like pattern resembling the observations
457 but with relatively weak amplitude in the monsoon trough region. The extratropical
458 WNP SST anomaly alone had little effect on the monsoon trough while inducing the
459 wave-like perturbation in the extratropical North Pacific.
460 The anomalous SST anomalies in both the tropical and extratropical WNP were
461 required to properly induce the observed enhanced monsoon trough and northward-
462 shifted anticyclonic ridge. The mutual enhancement of the atmospheric responses
463 induced by the SST anomalies in both the tropical and extratropical WNP was
464 suggested to be the key for the reasonable simulation of the observed anomalous
465 circulations in the WNP. The mutual enhancement can also be interpreted as the
466 amplification of an intrinsic mode (i.e., the PJ pattern) embedded in the summertime
467 mean flow over East Asia and the WNP, which can be induced by various forcings or
468 perturbations (Hsu and Lin 2007; Kosaka and Nakamura 2006 and 2010; Hirota and
469 Takahashi 2012). The joint effect of the SST anomalies in the tropical and extratropical
470 WNP likely provided a stronger forcing that induced a larger PJ-pattern-like response
471 than when the effects of individual SST anomalies were separately considered.
472 In addition to the local effect of the WNP SST anomaly, remote effects of the NA
473 SST tripole, the PMM-like SST anomaly in the subtropical eastern North Pacific (Qian 26
474 et al. 2019; Takaya 2019), and the negative SST anomaly in the tropical eastern IO have
475 been proposed to induce anomalous circulations in the WNP (Du et al. 2011; Tao et al.
476 2012). The relative influences of these SST anomaly were evaluated in a series of SST
477 anomaly sensitivity experiments. All these SST anomalies induced the wave-like
478 perturbation arching over the extratropical North Pacific, but mostly either weakened
479 the monsoon trough or had little effect. The only difference was the NA SST anomaly
480 that contributed to the enhancement of the monsoon trough when the SST anomaly in
481 the WNP was also included in the simulation. The enhanced anticyclonic anomaly over
482 extratropical East Asia in the experiment forced by the WNP SST anomaly served as a
483 path for the wave activity induced by the NA SST anomaly to propagate further
484 southeastward to the Philippine Sea and enhance the monsoon trough.
485 The elongated positive SST anomaly spreading from the tropical WNP to the
486 subtropical eastern North Pacific, referred to as the PMM-like SST anomaly, was
487 reported to enhance the monsoon trough and TC activity in the WNP (Takaya 2019,
488 Qian et al. 2019). Our simulation, however, suggested that the western part of this
489 elongated positive SST anomaly contributed to enhancing the monsoon trough, whereas
490 the eastern part weakened the observed circulation in the whole North Pacific. The
491 effect of the SST anomaly in the eastern North Pacific was consistent with Hong et al.
492 (2018), who identified the distinct influences of ENSO-like and PMM-like SST 27
493 anomalies on mean TC genesis location in the WNP. The cause of the discrepancy
494 between our simulations and Qian et al. (2019) in not understood. Here, we summarize
495 different simulation approaches between the present study and that of Qian et al. (2019).
496 First, different models were used, and our simulations were conducted in a coarse
497 spatial resolution to simulate the large-scale circulation instead of TCs. Second, Qian
498 et al. (2019) did not separately consider the effects of SST anomalies in the western and
499 eastern North Pacific or investigate their relative contributions. Third, Hsu et al. (2008)\
500 and Arakane and Hsu (2020) found that the existence of TCs enhanced the seasonal
501 mean and variability of the monsoon trough. The same effect may exist in the TC-
502 resolving model. In that case, a TC-resolving model might better simulate a strong
503 monsoon trough than a coarse-resolution model would. Further studies are required to
504 confirm this conjecture.
505 Based on the simulation results, we propose the local effect of the SST anomaly
506 in the WNP to be the major driver and the remote effect of the SST anomaly in the NA
507 to be a minor contributor of the anomalous Pacific subtropical high and monsoon trough
508 and the climate extremes in the WNP during JA 2018. The anomalous circulation in the
509 WNP exhibited the characteristics of the PJ pattern, which is an intrinsic mode
510 embedded in the mean flow (Kosaka and Nakamura 2010) and can be induced by
511 forcing in different regions (Hirota and Takahashi 2012). The phenomenon that 28
512 occurred in JA 2018 was likely a manifestation of such a dynamic process, and because
513 of this nature, the compound effect of SST anomalies in different basins led to the
514 extremely anomalous circulation. The same view can be applied to the wave-like
515 pattern, simulated in every SST anomaly sensitivity experiment, that arched over the
516 extratropical North Pacific. Whether this wave-like pattern is an intrinsic mode in which
517 the PJ pattern is embedded warrants further research.
518 Considering that the use of AGCMs was more effective in simulating monsoon
519 circulation than rainfall was in the WNP (Wang et al. 2004), our study focused on the
520 impact of anomalous SST anomalies on the WNP circulation and did not aim to obtain
521 a complete understanding of how the observed anomalous atmosphere-ocean
522 conditions occurred. Although this one-way AMIP-type approach could not elucidate
523 the complete physical process leading to the extremity in the WNP during JA 2018 or
524 help quantify the exact contribution of SST anomaly, it provided valuable information
525 regarding the effects of the anomalous SST on the anomalous circulations and relative
526 contributions of various ocean basins.
527 Finally, regarding model dependency, the results were based on only one AGCM;
528 therefore, the possibility of model dependency, as indicated by Qian et al. (2019), could
529 not be ruled out. Different models may have different degrees of sensitivity and
530 responses to the same forcing. This phenomenon was demonstrated in the 29
531 intercomparison study with numerous models conducted for the Coupled Model
532 Intercomparison Project. A similar sensitivity and spread are likely present in the
533 AMIP-type simulations of seasonal anomalies. Nevertheless, it is worthwhile to present
534 our findings and new interpretation of the extreme events of 2018 by using the CAM5,
535 which has been used in numerous studies and proven to be reliable.
30
536 Acknowledgments. This study was supported by the Ministry of Science and
537 Technology, Taiwan (MOST 109-2123-M-001-004, 107-2119-M-001-010, and 107-
538 2111-M-845-001). We are grateful to the National Center for High-Performance
539 Computing for providing computer facilities. This manuscript was edited by Wallace
540 Academic Editing.
541
31
542 Reference
543 Arakane, S., and H.-H. Hsu, 2020: A tropical cyclone removal technique based on
544 potential vorticity inversion to better quantify tropical cyclone contribution to the
545 background circulation. Climate Dynamics, 54, 3201-3226.
546 Camargo, S. J., K. A. Emanuel, and A. H. Sobel, 2007: Use of a genesis potential index
547 to diagnose ENSO effects on tropical cyclone genesis. Journal of Climate, 20,
548 4819-4834.
549 Chen, L., W. Gu, and W. Li, 2019: Why is the East Asian summer monsoon extremely
550 strong in 2018?—Collaborative effects of SST and snow anomalies. Journal of
551 Meteorological Research, 33, 593-608.
552 Chen, M., P. Xie, J. E. Janowiak, and P. A. Arkin, 2002: Global land precipitation: A
553 50-yr monthly analysis based on gauge observations. Journal of
554 Hydrometeorology, 3, 249-266.
555 Chen, R., Z. Wen, and R. Lu, 2019: Influences of tropical circulation and sea surface
556 temperature anomalies on extreme heat over Northeast Asia in the midsummer of
557 2018. Atmospheric and Oceanic Science Letters, 12, 238-245.
558 Deng, K., S. Yang, A. Lin, C. Li, and C. Hu, 2019: Unprecedented East Asian warming
559 in spring 2018 linked to the North Atlantic tripole SST mode. Atmospheric and
560 Oceanic Science Letters, 12, 246-253. 32
561 Du, Y., L. Yang, and S.-P. Xie, 2011: Tropical Indian Ocean influence on northwest
562 Pacific tropical cyclones in summer following strong El Niño. Journal of Climate,
563 24, 315-322.
564 Emanuel, K., and D. S. Nolan, 2004: Tropical cyclone activity and the global climate
565 system. 26th Conference on Hurricanes and Tropical Meteorolgy, 240-241.
566 Gao, S., L. Zhu, W. Zhang, and Z. Chen, 2018: Strong modulation of the Pacific
567 Meridional Mode on the occurrence of intense tropical cyclones over the Western
568 North Pacific. Journal of Climate, 31, 7739-7749.
569 Ha, K.-J., J.-H. Yeo, Y.-W. Seo, E.-S. Chung, J.-Y. Moon, X. Feng, Y.-W. Lee, and
570 C.-H. Ho, 2020: What caused the extraordinarily hot 2018 summer in Korea?
571 Journal of the Meteorological Society of Japan. Ser. II, 98, 153-167.
572 Hirota, N., and M. Takahashi, 2012: A tripolar pattern as an internal mode of the East
573 Asian summer monsoon. Climate Dynamics, 39, 2219-2238.
574 Hong, C.-C., M.-Y. Lee, H.-H. Hsu, and W.-L. Tseng, 2018: Distinct influences of the
575 ENSO-Like and PMM-Like SST Anomalies on the Mean TC genesis location in
576 the Western North Pacific: the 2015 Summer as an extreme example. Journal of
577 Climate, 31, 3049-3059.
578 Hsu, H.-H., and S.-M. Lin, 2007: Asymmetry of the tripole rainfall pattern during the
579 East Asian summer. Journal of Climate, 20, 4443-4458. 33
580 Hsu, H.-H., C.-H. Hung, A.-K. Lo, C.-C. Wu, and C.-W. Hung, 2008: Influence of
581 tropical cyclones on the estimation of climate variability in the tropical western
582 North Pacific. Journal of Climate, 21, 2960-2975.
583 Hsu, H. H., and X. Liu, 2003: Relationship between the Tibetan Plateau heating and
584 East Asian summer monsoon rainfall. Geophysical Research Letters, 30.
585 Huang, B., V. F. Banzon, E. Freeman, J. Lawrimore, W. Liu, T. C. Peterson, T. M.
586 Smith, P. W. Thorne, S. D. Woodruff, and H.-M. Zhang, 2015: Extended
587 reconstructed sea surface temperature version 4 (ERSST. v4). Part I: upgrades and
588 intercomparisons. Journal of climate, 28, 911-930.
589 Imada, Y., M. Watanabe, H. Kawase, H. Shiogama, and M. Arai, 2019: The July 2018
590 high temperature event in Japan could not have happened without human-induced
591 global warming. SOLA, 15A-002.
592 Kalnay, E., M. Kanamitsu, R. Kistler, W. Collins, D. Deaven, L. Gandin, M. Iredell, S.
593 Saha, G. White, and J. Woollen, 1996: The NCEP/NCAR 40-year reanalysis
594 project. Bulletin of the American meteorological Society, 77, 437-472.
595 Kobayashi, C., and I. Ishikawa, 2019: Prolonged northern-mid-latitude tropospheric
596 warming in 2018 well predicted by the JMA operational seasonal prediction
597 system. SOLA, 15A, 31-36.
34
598 Kornhuber, K., S. Osprey, D. Coumou, S. Petri, V. Petoukhov, S. Rahmstorf, and L.
599 Gray, 2019: Extreme weather events in early summer 2018 connected by a
600 recurrent hemispheric wave-7 pattern. Environmental Research Letters, 14,
601 054002.
602 Kosaka, Y., and H. Nakamura, 2006: Structure and dynamics of the summertime
603 Pacific–Japan teleconnection pattern. Quarterly Journal of the Royal
604 Meteorological Society, 132, 2009-2030.
605 Kosaka, Y., and H. Nakamura, 2010: Mechanisms of meridional teleconnection
606 observed between a summer monsoon system and a subtropical anticyclone. Part
607 I: The Pacific–Japan pattern. Journal of Climate, 23, 5085-5108.
608 Liebmann, B., and C. A. Smith, 1996: Description of a complete (interpolated) outgoing
609 longwave radiation dataset. Bulletin of the American Meteorological Society, 77,
610 1275-1277.
611 Liu, B., C. Zhu, J. Su, S. Ma, and K. Xu, 2019: Record-breaking northward shift of the
612 Western North Pacific subtropical high in July 2018. Journal of the
613 Meteorological Society of Japan. Ser. II, 97, 913-925.
614 Moteki, Q., 2019: Role of typhoon prapiroon (Typhoon No. 7) on the formation process
615 of the Baiu front inducing heavy rain in July 2018 in Western Japan. SOLA, 15A,
616 37-42. 35
617 Neale, R. B., C.-C. Chen, A. Gettelman, P. H. Lauritzen, S. Park, D. L. Williamson, A.
618 J. Conley, R. Garcia, D. Kinnison, and J.-F. Lamarque, 2010: Description of the
619 NCAR community atmosphere model (CAM 5.0). NCAR Tech. Note NCAR/TN-
620 486+ STR, 1, 1-12.
621 Nitta, T., 1987: Convective activities in the tropical western Pacific and their impact on
622 the Northern Hemisphere summer circulation. Journal of the Meteorological
623 Society of Japan. Ser. II, 65, 373-390.
624 Qian, Y., H. Murakami, M. Nakano, P.-C. Hsu, T. L. Delworth, S. B. Kapnick, V.
625 Ramaswamy, T. Mochizuki, Y. Morioka, T. Doi, T. Kataoka, T. Nasuno, and K.
626 Yoshida, 2019: On the mechanisms of the active 2018 tropical cyclone season in
627 the North Pacific. Geophysical Research Letters, 46, 12293-12302.
628 Sekizawa, S., T. Miyasaka, H. Nakamura, A. Shimpo, K. Takemura, and S. Maeda,
629 2019: Anomalous moisture transport and oceanic evaporation during a torrential
630 rainfall event over Western Japan in early July 2018. SOLA, 15A, 25-30.
631 Shimpo, A., K. Takemura, S. Wakamatsu, H. Togawa, Y. Mochizuki, M. Takekawa, S.
632 Tanaka, K. Yamashita, S. Maeda, and R. Kurora, 2019: Primary factors behind the
633 heavy rain event of July 2018 and the subsequent heat wave in Japan. SOLA, 15A,
634 13-18.
36
635 Stuecker, M. F., 2018: Revisiting the Pacific meridional mode. Scientific Reports, 8,
636 3216.
637 Sueki, K., and Y. Kajikawa, 2019: Different precipitation systems between Hiroshima
638 and Keihanshin during extreme rainfall event in Western Japan in July 2018.
639 Journal of the Meteorological Society of Japan. Ser. II, 97, 1221-1232.
640 Takaya, K., and H. Nakamura, 2001: A formulation of a phase-independent wave-
641 activity flux for stationary and migratory quasigeostrophic eddies on a zonally
642 varying basic flow. Journal of the Atmospheric Sciences, 58, 608-627.
643 Takaya, Y., 2019: Positive phase of Pacific Meridional Mode enhanced Western North
644 Pacific tropical cyclone activity in summer 2018. SOLA, 15A, 55-59.
645 Takemi, T., and T. Unuma, 2019: Diagnosing environmental properties of the July 2018
646 heavy rainfall event in Japan. SOLA, 15A, 60-65.
647 Takemura, K., S. Wakamatsu, H. Togawa, A. Shimpo, C. Kobayashi, S. Maeda, and H.
648 Nakamura, 2019: Extreme moisture flux convergence over western Japan during
649 the heavy rain event of July 2018. SOLA, 15A, 49-54.
650 Tao, L., L. Wu, Y. Wang, and J. Yang, 2012: Influence of tropical Indian Ocean
651 warming and ENSO on tropical cyclone activity over the western North Pacific.
652 Journal of the Meteorological Society of Japan. Ser. II, 90, 127-144.
37
653 Tokyo Climate Center, J. M. A., 2018a: Primary factors behind the heavy rain event of
654 July 2018 and the subsequent heatwave in Japan from mid-July onward.
655 ——, 2018b: Summary of the 2018 Asian summer monsoon. 6-9.
656 Vogel, M., J. Zscheischler, R. Wartenburger, D. Dee, and S. Seneviratne, 2019:
657 Concurrent 2018 hot extremes across Northern Hemisphere due to human‐induced
658 climate change. Earth's Future.
659 Wang, B., R. Wu, and K. Lau, 2001: Interannual variability of the Asian summer
660 monsoon: Contrasts between the Indian and the western North Pacific–East Asian
661 monsoons. Journal of Climate, 14, 4073-4090.
662 Wang, B., I.-S. Kang, and J.-Y. Lee, 2004: Ensemble simulations of Asian–Australian
663 monsoon variability by 11 AGCMs. Journal of Climate, 17, 803-818.
664 Wang, S. S. Y., H. Kim, D. Coumou, J. H. Yoon, L. Zhao, and R. R. Gillies, 2019:
665 Consecutive extreme flooding and heat wave in Japan: Are they becoming a norm?
666 Atmospheric Science Letters, 20, e933.
667 Yokoyama, C., H. Tsuji, and Y. N. Takayabu, 2020: The effects of an upper-
668 tropospheric trough on the heavy rainfall event in July 2018 over Japan. Journal
669 of the Meteorological Society of Japan. Ser. II, 98, 235-255.
670
671
38
672 Table captions
673 Table 1. List of experiments. SST anomaly is the observed SST anomaly in 2018
674 relative to the long-term monthly mean for 1982–2010. Long-term mean SIC was
675 prescribed in all experiments. The area number corresponds to the region marked
676 in Fig. 4c.
677
39
678 Figure captions
679 Fig. 1 Large-scale circulation and TC activity in JA 2018. (a) TC tracks in the WNP
680 (black curves) and the 1490-m contour of 850-hPa geopotential height for the
681 climatological mean (blue) and that in 2018 (red). The purple contours indicate the
682 steering flow (i.e., vertically averaged (from 1000 hPa to 400 hPa) mean flow) in
683 JA 2018. The green shaded area is the climatological mean TC density
684 (number/year). (b) Mean TC genesis location for every JA from 1981 to 2018 (blue
685 X). Thick black cross denotes the climatological mean location, and red X marks
686 the mean location in JA 2018.
687
688 Fig. 2 Interannual variations of TC activity and summer circulation indices during
689 1981–2018. (a) Number of TC days north of 30°N in the WNP based on the best
690 track data from the JMA. (b) Time series of the PJ (red), SAMOI (blue), and WNP
691 monsoon (green) index. The PJ index is defined as the difference of 850-hPa
692 geopotential height between (35°N, 155°E) and (22.5°N, 125°E). The WNP
693 monsoon index is defined by Wang et al. (2001), and the SAMOI is defined as the
694 OLR anomaly averaged over (10°N–20°N, 115°E–140°E).
695 Fig. 3 Anomalous precipitation, moisture flux, 2-m temperature (T2m), and GHT500
696 in JA 2018. (a) Percentiles of precipitation in JA 2018 relative to that in the 1951– 40
697 2018 period. (b) Moisture flux (streamline, g kg−1m s−1) and convergence (colored
698 shading, 10−4 g kg−1 s−1) anomalies at 925 hPa in JA relative to the climatology
699 from 1948 to 2018. (c) Percentiles of 2-m temperature in JA 2018 relative to that
700 in the 1948–2018 period. (d) Percentiles of 500-hPa geopotential height in JA 2018
701 relative to that in the 1948–2018 period. Blue and red curves in (d) indicate the
702 1490-m contour of 850-hPa geopotential height for the climatological average and
703 2018, respectively. Precipitation was derived from PREC, whereas the other
704 variables were derived from the NCEP Reanalysis I.
705
706 Fig. 4 Circulation, SST, and OLR features in JA 2018. (a) 200-hPa streamfunction
707 (color shading, 106 m2 s−1) and 200-hPa geopotential height (contours, m)
708 anomalies in JA 2018 relative to those in the 1948–2018 climatological data. (b)
709 Same as (a) but at 1000 hPa. (c) Percentiles of SSTs in JA 2018 relative to those
710 in the 1948–2018 period. The blue box/outline marks the region in which SST
711 anomaly was prescribed as a forcing in the numerical experiments listed in Table
712 1. (d) OLR (color shading, W m−2), 200-hPa velocity potential (contour, 106 m2
713 s−1), and 200-hPa divergent wind (vector) anomalies in JA 2018 relative to the
714 1948–2018 climatological mean.
715 41
716 Fig. 5 Geopotential height anomalies (m) at 500 hPa (a) observed and simulated in (b)
717 GL-exp, (c) TWP-exp, (d) WP-exp, (e) EJP-exp, (f) ATL-exp, (g) TWP+ATL-exp,
718 (h) WP+ATL-exp, (i) ENP-exp, and (j) IO-exp. Colored shading (m) indicates
719 statistically significant points at the 10% level. Contour interval is the same as the
720 color map.
721
722 Fig. 6 Cross sections of the vertical motion (color shading) of anomalies and wave
723 activity flux based on the anomalies streamfunction (WAF, v-flux; vectors)
724 averaged over 110°E–140°E in (a) the observations, (b) GL-exp, (c) TWP-exp, (d)
725 WP-exp, (e) TWP+ATL-exp, and (f) WP+ATL-exp. Colored shading indicates
726 statistically significant points at the 10% level. Contour interval is the same as the
727 color map. Units: Pa s−1 for vertical motion and m2 s−2 for WAF. Unit vectors are
728 6 m2 s−2 and 3 m2 s−2 for the observed and modeled results, respectively.
729
730 Fig. 7 Cross sections of geopotential height anomalies (m) averaged over 30N–60N
731 in (a) the observations, (b) GL-exp, (c) TWP-exp, (d) WP-exp, (e) TWP+ATL-
732 exp, and (f) WP+ATL-exp. Colored shading (m) indicates statistically significant
733 points at the 10% level. Contour interval is the same as the color map.
42
734 Fig. 8 Precipitation (colored shading, mm day−1) and 850-hPa wind (vector, m s−1)
735 anomalies in (a) the observations, (b) GL-exp, (c) TWP-exp, (d) WP-exp, (e)
736 TWP+ATL-exp, and (f) WP+ATL-exp. Only those statistically significant at the
737 10% level are plotted. Contour interval is the same as the color map.
738 Fig. 9 Genesis potential index (Emanuel and Nolan 2004; Camargo et al. 2007) of
739 tropical cyclone: (a) observed and simulated in (b) GL-exp, (c) TWP-exp, (d) WP-
740 exp, (e) EJP-exp, (f) ATL-exp, (g) TWP+ATL-exp, (h) WP+ATL-exp, (i) ENP-
741 exp, and (j) IO-exp.. Colored shading indicates statistically significant points at
742 the 10% level. Contour interval is the same as the color map.
743
744
745
43
746 Table 1. List of experiments. SST anomaly is the observed SST anomaly in 2018
747 relative to the long-term monthly mean for 1982–2010. Long-term mean SIC was
748 prescribed in all experiments. The area number corresponds to the region marked in Fig.
749 4c.
Experiment SST anomaly (region) Area CTL None GL Global SST anomaly TWP Positive SST anomaly in the tropical western North Pacific 2 WP SST anomaly in the western North Pacific (10˚S-65˚N, 110˚E-175˚W) 3 TWP+ATL TWP SST anomaly plus SST anomaly in the North Atlantic (0-80˚N, 0-60˚W) 2+1 WP+ATL WP SST anomaly plus SST anomaly in the North Atlantic (0-80˚N, 0-60˚W) 3+1 EJP SST anomaly to the east of Japan 4 ATL SST anomaly in the North Atlantic (0-80˚N, 0-60˚W) 1 ENP SST anomaly in the tropical eastern North Pacific 5 IO SST anomaly in the eastern Indian Ocean (30˚S-30˚N, 90˚E-120˚E) 6
750
751
752
753
754
755
44
756 Figures
757 758 Fig. 1 Large-scale circulation and TC activity in JA 2018. (a) TC tracks in the WNP
759 (black curves) and the 1490-m contour of 850-hPa geopotential height for the
760 climatological mean (blue) and that in 2018 (red). The purple contours indicate the
761 steering flow (i.e., vertically averaged (from 1000 hPa to 400 hPa) mean flow) in
762 JA 2018. The green shaded area is the climatological mean TC density
763 (number/year). (b) Mean TC genesis location for every JA from 1981 to 2018 (blue
764 X). Thick black cross denotes the climatological mean location, and red X marks
765 the mean location in JA 2018.
45
766
767 Fig. 2 Interannual variations of TC activity and summer circulation indices during
768 1981–2018. (a) Number of TC days north of 30°N in the WNP based on the best
769 track data from the JMA. (b) Time series of the PJ (red), SAMOI (blue), and WNP
770 monsoon (green) index. The PJ index is defined as the difference of 850-hPa
771 geopotential height between (35°N, 155°E) and (22.5°N, 125°E). The WNP
772 monsoon index is defined by Wang et al. (2001), and the SAMOI is defined as the
773 OLR anomaly averaged over (10°N–20°N, 115°E–140°E).
774 46
775 776 Fig. 3 Anomalous precipitation, moisture flux, 2-m temperature (T2m), and GHT500
777 in JA 2018. (a) Percentiles of precipitation in JA 2018 relative to that in the 1951–
778 2018 period. (b) Moisture flux (streamline, g kg−1m s−1) and convergence (colored
779 shading, 10−4 g kg−1 s−1) anomalies at 925 hPa in JA relative to the climatology
780 from 1948 to 2018. (c) Percentiles of 2-m temperature in JA 2018 relative to that
781 in the 1948–2018 period. (d) Percentiles of 500-hPa geopotential height in JA 2018
782 relative to that in the 1948–2018 period. Blue and red curves in (d) indicate the
783 1490-m contour of 850-hPa geopotential height for the climatological average and
784 2018, respectively. Precipitation was derived from PREC, whereas the other
785 variables were derived from the NCEP Reanalysis I.
786 47
787
788 789 Fig. 4 Circulation, SST, and OLR features in JA 2018. (a) 200-hPa streamfunction
790 (color shading, 106 m2 s−1) and 200-hPa geopotential height (contours, m)
791 anomalies in JA 2018 relative to those in the 1948–2018 climatological data. (b)
792 Same as (a) but at 1000 hPa. (c) Percentiles of SSTs in JA 2018 relative to those
793 in the 1948–2018 period. The blue box/outline marks the region in which SST
794 anomaly was prescribed as a forcing in the numerical experiments listed in Table
795 1. (d) OLR (color shading, W m−2), 200-hPa velocity potential (contour, 106 m2
796 s−1), and 200-hPa divergent wind (vector) anomalies in JA 2018 relative to the
797 1948–2018 climatological mean.
798
799
800
48
801
802
803 Fig. 5 Geopotential height anomalies (m) at 500 hPa (a) observed and simulated in (b)
804 GL-exp, (c) TWP-exp, (d) WP-exp, (e) EJP-exp, (f) ATL-exp, (g) TWP+ATL-exp,
805 (h) WP+ATL-exp, (i) ENP-exp, and (j) IO-exp. Colored shading (m) indicates
806 statistically significant points at the 10% level. Contour interval is the same as the
807 color map.
808 49
809 810 Fig. 6 Cross sections of the vertical motion (color shading) of anomalies and wave
811 activity flux based on the anomalies streamfunction (WAF, v-flux; vectors)
812 averaged over 110°E–140°E in (a) the observations, (b) GL-exp, (c) TWP-exp, (d)
813 WP-exp, (e) TWP+ATL-exp, and (f) WP+ATL-exp. Colored shading indicates
814 statistically significant points at the 10% level. Contour interval is the same as the
815 color map. Units: Pa s−1 for vertical motion and m2 s−2 for WAF. Unit vectors are
816 6 m2 s−2 and 3 m2 s−2 for the observed and modeled results, respectively.
817
50
818
819 Fig. 7 Cross sections of geopotential height anomalies (m) averaged over 30N–60N
820 in (a) the observations, (b) GL-exp, (c) TWP-exp, (d) WP-exp, (e) TWP+ATL-
821 exp, and (f) WP+ATL-exp. Colored shading (m) indicates statistically significant
822 points at the 10% level. Contour interval is the same as the color map.
823
51
824
825 Fig. 8 Precipitation (colored shading, mm day−1) and 850-hPa wind (vector, m s−1)
826 anomalies in (a) the observations, (b) GL-exp, (c) TWP-exp, (d) WP-exp, (e)
827 TWP+ATL-exp, and (f) WP+ATL-exp. Only those statistically significant at the 10%
828 level are plotted. Contour interval is the same as the color map.
52
829
830 Fig. 9 Genesis potential index (Emanuel and Nolan 2004; Camargo et al. 2007) of
831 tropical cyclone: (a) observed and simulated in (b) GL-exp, (c) TWP-exp, (d) WP-
832 exp, (e) EJP-exp, (f) ATL-exp, (g) TWP+ATL-exp, (h) WP+ATL-exp, (i) ENP-
53
833 exp, and (j) IO-exp.. Colored shading indicates statistically significant points at
834 the 10% level. Contour interval is the same as the color map.
835
54