Large XCH 4 anomaly in summer 2013 over northeast Asia observed by GOSAT

Extremely high levels of column-averaged dryair mole fractions of atmospheric methane (XCH4) were detected in August and September 2013 over northeast Asia (∼ 20 ppb above the averaged summertime XCH4 over 2009–2012, after removing a long-term trend), as being retrieved from the Short-Wavelength InfraRed (SWIR) spectral data observed with the Thermal And Near-infrared Sensor for carbon Observation – Fourier Transform Spectrometer (TANSO-FTS) onboard Greenhouse Gases Observing Satellite (GOSAT). Similar enhancements of XCH4 were also observed by the ground-based measurements at two Total Carbon Column Observing Network (TCCON) sites in Japan. The analysis of surface CH4 concentrations observed at three monitoring sites around the Japan archipelago suggest that the extreme increase of XCH4 has occurred in a limited area. The model analysis was conducted to investigate this anomalously high XCH4 event, using an atmospheric transport model. The results indicate that the extreme increase of XCH4 is attributed to the anomalous atmospheric pressure pattern over East Asia during the summer of 2013, which effectively transported the CH4-rich air to Japan from the strong CH4 source areas in east China. The two Japanese TCCON sites, ∼ 1000 km east–west apart each other, coincidentally located along the substantially CH4-rich air flow from east China. This analysis demonstrates the capability of GOSAT to monitor an XCH4 event on a synoptic scale. We anticipate that the synoptic information of XCH4 from GOSAT data contributes to improve our understanding of regional carbon cycle and the regional flux estimation.


Introduction
Atmospheric methane (CH 4 ) is the second important anthropogenic greenhouse gas after carbon dioxide (CO 2 ), contributing about 20 % of the total radiative forcing from the major well-mixed greenhouse gases (Forster et al., 2007).Methane has multiple natural and anthropogenic sources in the Earth's surface while being mainly removed through reaction with hydroxyl radical (OH) in the troposphere and by photolysis in the stratosphere.The atmospheric CH 4 level has more than doubled since the onset of the industrial revolution in the 18th century (Etheridge et al., 1998).Its growth rate has been considerably variable over the past few decades (Dlugokencky et al., 2009).On a global scale, the causes of recent changes in the CH 4 growth rate remain unknown (e.g.Kirschke et al., 2013;Dlugokencky et al., 2009), and on a regional scale, significant discrepancies have been found in the emission estimates between bottom-up and top-down approaches (e.g.Miller et al., 2013).On the other hand, given the larger radiative forcing than carbon dioxide, it has been argued that reducing anthropogenic CH 4 emission might be a mitigation of possible severe impact of global warming (e.g.Hansen and Sato, 2004).Therefore, to elucidate the drivers of changes in atmospheric CH 4 concentrations and to quantify the regional source distributions are challenging tasks.
The temporal variations of observed atmospheric CH 4 are complicated due to various sources on the Earth's surface, interactions between the emission sources and the atmospheric transport, and removal in the atmosphere.To improve the regional CH 4 flux estimates on the Earth's surface, it is needed to better understand the relative contribution of atmospheric transport to the observed variations of atmospheric CH 4 .
In the past decades, the investigations of the spatiotemporal variability in atmospheric CH 4 concentrations and the inverse modelling estimates of surface CH 4 flux estimates had been mainly based on the ground-based measurements including aircraft and shipboard measurements (e.g.Bousquet et al., 2006;Bergamaschi et al., 2010;Miller et al., 2013).However, the current ground-based measurements of CH 4 are still sparse.In the recent years, the measurements from the satellites have been providing the large spatial and temporal coverage to help better understand the variations of atmospheric CH 4 .Greenhouse gases Observing SATellite (GOSAT) was launched in January 2009, providing column-averaged dry-air mole fractions of atmospheric CH 4 (XCH 4 ) that are retrieved from Short-Wavelength In-fraRed (SWIR) solar spectra observed onboard Thermal And Near infrared Sensor for carbon Observation -Fourier Transform Spectrometer (TANSO-FTS) instrument (Yokota et al., 2009;Yoshida et al., 2013).The GOSAT TANSO-FTS aims at measurements of atmospheric CH 4 concentrations in three-month averages with an accuracy of better than 2 % at 100-1000 km spatial resolution (Kuze et al., 2009).GOSAT XCH 4 is preceded by the several previous and on-going satellite projects, for example, the Infrared Atmospheric Sounding Interferometer (IASI, Crevoisier et al., 2009), and the Tropospheric Emission Spectrometer (TES, Wecht et al., 2012) and the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY, Schneising et al., 2011).Among them, XCH 4 retrievals from SCIA-MACHY instrument onboard ENVISAT launched in 2003 was pioneering, but the communication with ENVISAT was lost in April 2012.These satellite data have been used for the inversion studies of surface CH 4 emissions.Most of the satellite-based inversions are focused on the global-scale estimates (e.g.Bergamaschi et al., 2007Bergamaschi et al., , 2009Bergamaschi et al., , 2013;;Fraser et al., 2013;Monteil et al., 2013;Cressot et al., 2014;Houweling et al., 2014;Alexe et al., 2015).Recently the satellite data have been applied for the flux estimation on a regional-and local-scale at a higher spatial resolution.For example, Wecht et al. (2014) compared the multiple observational constraints including GOSAT and TES to optimize methane emission in California.Turner et al. (2015) estimated North American methane emission at a resolution of up to 50 km × 50 km using GOSAT data.Kort et al. (2014) demonstrated that satellite-based observations can quantify localized anthropogenic CH 4 emissions in the southwestern USA using the SCIAMACHY and Total Carbon Column Observing Network (TCCON) data.
Here, we report the extremely high XCH 4 event observed by GOSAT in August and September 2013 over northeast Asia.Similar high-XCH 4 events were also detected by the ground-based measurements at the two Japanese TCCON sites in Tsukuba and Saga.Given the spacing and temporal frequency (3-day recurrence) of GOSAT sampling, along with possible retrieval biases of XCH 4 retrievals, it is interesting that the GOSAT detected the synoptic-scale variation of XCH 4 that is coherent with the ground-based measurements.This GOSAT-detected XCH 4 event suggests the potential of GOSAT XCH 4 analysis in higher temporal and spatial resolution.The capability to capture synoptic-scale variations of atmospheric CH 4 leads to better regional flux estimation because the synoptic-scale variations of atmospheric CH 4 can carry the information on regional surface fluxes.On the other hand, the atmospheric CH 4 concentrations are highly changeable with the atmospheric transport as well as surface fluxes.Toward improving regional flux estimation, it is essential to observe better a synoptic-scale variation of the atmospheric CH 4 and quantify the attribution of such variations.
In this study, we analyse the extremely high XCH 4 observed by GOSAT in the summer of 2013 and investigate the attributions of such a significant increase of XCH 4 .We discuss how capable GOSAT XCH 4 is to monitor synoptic-scale XCH 4 variations.

GOSAT XCH 4
GOSAT is a joint project of the Japanese Ministry of the Environment (MOE), the National Institute for Environmental Studies (NIES) and the Japan Aerospace Exploration Agency (JAXA) to monitor the global distribution of atmospheric CO 2 and CH 4 from space (Yokota et al., 2009).The retrieved XCH 4 , as a part of NIES GOSAT Level 2 (L2) product (v02.xx),has been reported to have a mean bias of −5.9 ppb and mean standard deviation of 12.6 ppb against the XCH 4 at selected TCCON sites (Yoshida et al., 2013).In this study, we analysed NIES GOSAT L2 XCH 4 (v02.21)without any bias correction.The latest data processing and the auxiliary information are described in GOSAT User Interface Gateway, http://data.gosat.nies.go.jp/GosatUserInterfaceGateway/ guig/doc/documents/doc_en_docdist.html.
We analysed GOSAT XCH 4 over two regions in northeast Asia separately (Fig. 1).One is over northeastern China-Korea (115-130 • E, 30-40 • N), and the other is over Japan (130-145 • E, 30-40 • N).The northeastern China-Korea region covers highly populated and industrialized areas with large anthropogenic CH 4 sources in the Eurasia continent.The Japan region has small CH 4 sources, but located downwind of the continental CH 4 emissions.Time series of XCH 4 data from June 2009 to March 2014 over the two regions with monthly means are shown in Fig. 2. It is noted that we used only the XCH 4 over land to minimize possible errors depending on sounding observation mode (Fig. 3).In fact, since a few soundings over ocean around East Asia were retrieved, removed XCH 4 data through this criterion are less than 5 % of the total.A long-term trend component in each XCH 4 dataset derived through a digital filtering of two-year cutoff period (Nakazawa et al., 1997) is also plotted in Fig. 2. To focus the seasonal variations, the trend components were removed, and the detrended XCH 4 time series are further analysed.
The GOSAT XCH 4 retrievals over northeastern China-Korea have clear seasonality with high peaks in summer and low peaks in winter.The summertime high XCH 4 appear to be influenced by the seasonal biogenic CH 4 emissions from rice paddies and natural wetlands underneath in east China and Korea.The summer peak in 2013 was more prominent than the preceding two years, 2011 and 2012.Also, the summertime XCH 4 retrievals over northeastern China-Korea in 2009 and 2010 were relatively high while no significantly high XCH 4 was observed over Japan.Since there is a limited number of retrieval available over Japan for the first two years of the GOSAT operation, it is difficult to discuss the XCH 4 difference over the two regions for 2009 and 2010.We thus leave this topic for a future investigation.
The seasonality of the GOSAT XCH 4 retrieval over Japan is overall similar to northeastern China-Korea.Although the seasonal cycle varies largely year-to-year, XCH 4 retrievals of August and September in 2013 were outstandingly high.Japan is located downwind of strong anthropogenic and natural biogenic CH 4 emissions in the continent, and then the signals of the continental CH 4 emissions are decreased as the air is transported.However, it is noticeable that, in the summer of 2013, the XCH 4 retrievals over both Japan and northeastern China-Korea regions reached the almost same high levels.This comparable XCH 4 levels in the two regions indicates there was a mechanism of fast atmospheric transport in 2013 to bring CH 4 -rich air to Japan with less diffusion than the preceding years.
The number of GOSAT retrievals over Japan increased in 2013 and 2014, compared with those in the previous years.This increase is due to the change of the observation strategy to increase available GOSAT retrievals over Japan.The initial regular schedule, there were fewer soundings over lands, but most soundings were over oceans or land-ocean mixed locations.The soundings over ocean or mixed locations are difficult to be retrieved.As a result, a few retrievals remained over Japan after screening.Aiming at increasing the retrievals over Japan, the observation locations were moved inland from ocean and mixed locations as much as possible.The observation strategy change was made as a concerted decision by GOSAT Project terms among the three agencies NIES, JAXA, and MOE.This observation change was implemented on 6 May 2013.
The spatial maps of GOSAT retrievals we used in this study are shown in Fig. 3.As seen in Fig. 3a, most of the soundings were taken at lower surface elevation; more than 80 % is below 100 m, 95 % is below 500 m.The spatial maps of the retrievals in August and September in 2012 and 2013 are shown in Fig. 3b and c, respectively.Kort et al. (2014) applied the elevation correction to SCIAMACHY XCH 4 over in the southwestern USA.The elevation impact is mainly exhibited over the Rocky Mountains because of low CH 4 air in the stratosphere.Surface elevation/topography could be one of potential biases in GOSAT XCH 4 .However, for the GOSAT data we used, we found no statistically significant correlation with surface elevation (< r = −0.11)and also the GOSAT XCH 4 data sampled at elevation of more than 1000 m is a few (∼ 0.1 % out of total).Therefore, no elevation correction was applied to the GOSAT XCH 4 in this study.In Fig. 3b and c, the observation strategy change mentioned above is noticeable, the coverage of retrievals over Japan was dramatically increased in 2013, compared with 2012.Regarding China and Korea, there is no significant difference between 2012 and 2013.

TCCON XCH 4
Inside the Japan region of this study, ground-based XCH 4 measurements have been conducted at two TCCON sites, Saga (33.24 • N, 130.29 • E) and Tsukuba (36.05 • N, 140.12 • E) as shown in Fig. 1.TCCON is a worldwide network of ground-based high-resolution FTSs, which record spectra of the direct sunlight in the near-infrared, and provides accurate and precise column-averaged dry-mole fractions of atmospheric constituents including CO 2 , CH 4 , N 2 O, HF, CO, H 2 O, and HDO retrieved from these spectra absorbed by them (Wunch et al., 2011).The TCCON XCH 4 measurements have an estimated uncertainty of 7 ppb (2σ ) (Wunch et al., 2010).TCCON data play a critical role in the validation of space-based measurements.The Saga TCCON site is in Kyushu Island, operated by JAXA since June 2011.The Tsukuba TCCON site is located ∼ 50 km north of Tokyo in the Japan main island, operated by NIES since 2009.These two Japanese TCCON sites are apart ∼ 1000 km longitudinally.In this study, we use the TCCON data processed by GGG 2012.
Figure 4 shows XCH 4 retrievals at Saga and Tsukuba TC-CON sites during the period for 2011 to 2014.We processed the both TCCON XCH 4 time series in the same manner with the GOSAT XCH 4 to obtain the long-term trends that are shown in blue lines in Fig. 4. It is interesting that, before the summer 2013, XCH 4 retrievals at Tsukuba overall are lower than at Saga.Since Saga is located closer to the continent than Tsukuba, Saga is considered to be influenced by the continental anthropogenic CH 4 emissions more strongly than Tsukuba.In the summer of 2013, extremely high XCH 4 retrievals both at Saga and Tsukuba were observed, reaching almost a same level.This XCH 4 enhancement observed at the ground-based TCCON sites is coincident with the high XCH 4 observed by GOSAT, and strongly supports our speculation that the CH 4 rich air was transported quickly from the continent to Japan for this period.
To focus on the seasonal and synoptic variations, we compared the detrended XCH 4 time series from GOSAT over Japan and the two Japanese TCCON sites.Figure 5a shows that all the detrended XCH 4 data are overall in phase of seasonal cycle with seasonal amplitude of ∼ 20 ppb.Compared with TCCON XCH 4 , GOSAT XCH 4 shows large short-term variability.In 2013, both GOSAT and TCCON XCH 4 together rapidly increased in August and remained high in September.In 2012, both GOSAT XCH 4 and TCCON XCH 4 show no clear tendency in August, and all of them appear to be upward in September.On average, the XCH 4 level of GOSAT over Japan in August and September 2013 is higher by ∼ 15 ppb than 2012.The XCH 4 levels of both TCCON sites in 2013 are higher by ∼ 20 ppb than 2012.These enhancements of XCH 4 are comparable to their seasonal amplitude.
To examine further how the synoptic variability of GOSAT is correlated with TCCON, we removed the mean seasonal cycles from the detrended XCH 4 time series and took the monthly means (Fig. 5b).Except the months when the retrievals are available for less than 2 days, the correlation coefficients (r) of the monthly means between GOSAT and TC-CON at Saga, and between GOSAT and TCCON at Tsukuba, are 0.81 and 0.61, respectively.These correlation coefficient values exceed the 95 % significance level.Despite the large short-term variability, the synoptic variability of GOSAT over Japan is overall correlated with the TCCON XCH 4 at two Japanese sites.The enhancement of XCH 4 in the summer of 2013 is consistent among GOSAT and TCCON.If the period is limited to May-December 2013, when the number of GOSAT XCH 4 retrievals was increased due to the observation strategy change mentioned earlier, the correlation coefficients (r) between GOSAT and TCCON are improved to be 0.91 with Saga and 0.96 with Tsukuba.This implies that the increase in the observations over Japan improves the capability of GOSAT to detect synoptic variability in XCH 4 .

Ground-based surface CH 4 concentrations
In order to see the relationship between the surface CH 4 concentration and the enhancement of GOSAT XCH 4 over Japan, we analyzed the surface CH 4 concentrations observed at three ground-based monitoring stations in Japan, Cape Ochi-ishi (COI, 43.16 • N, 145.49• E), Ryori (RYO, 39.03 • N, 141.82 • E), and Yonagunijima (YON, 24.47 • N, 123.02 • E).These site locations are shown in Fig. 1.At all the stations, continuous measurements of atmospheric CH 4 are conducted.Cape Ochi-ishi (COI) is a station operated by NIES, which is located at the east tip of Hokkaido island (Tohjima et al., 2002).Ryori (RYO) is located inside the Japan region defined in this study, where the monitoring of surface greenhouse gas concentrations has been conducted by the Japan Meteorological Agency (JMA) as a part of the Global Atmospheric Watch (GAW) program of the World Meteorological Observation (WMO).RYO is on the west coast of the Japan main island, about 300 km north of Tsukuba and far away from direct influences of residential and industrial pollutants.Yonagunijima (YON) is also one of JMA-operated GAW stations, which is located far south of the Japan main island and east of ∼ 110 km of Taiwan.The detailed station Saga Tsukuba GOSAT over Japan descriptions on RYO and YON are found on the JMA website (http://www.data.jma.go.jp/gmd/env/ghg_obs/en/station).
The time series of surface CH 4 concentrations at the three ground-based stations are shown in Fig. 6, with their monthly means and long-term trends.Here we analyzed the afternoon mean CH 4 (averaged hourly CH 4 over 12:00-15:00 local time) from the respective data sets, assuming that the afternoon values are large-scale representative.The observed CH 4 concentrations at all the sites show similar seasonal cycles in timing.Seasonally the CH 4 values are low in July and August, and high in winter to spring.In the winter, the westerly wind prevails and transports the CH 4 -rich air from the continent (mainly anthropogenic CH 4 emitted in east China) to Japan, causing the rise of CH 4 concentrations.In the summer, the southeasterly wind is dominant, bringing clean air to Japan from the Pacific Ocean, where there is no major CH 4 source, so that the surface CH 4 concentrations become low.
In the summer of 2013, unseasonably high CH 4 concentrations were observed at RYO with a sharp increase in the middle of August.The CH 4 concentrations at COI started increasing earlier from its summer minimum than the previous year, 2012.At YON, no significant CH 4 enhancement was seen in 2013 compared with the previous years.Since no similar CH 4 change to RYO and COI was observed at YON, the farthest southwestern island of Japan, this significant CH 4 enhancement event appears to be spatially limited in the area around Japan main island and Hokkaido island.To further examine the summer increase of surface CH 4 concentrations, we compared the detrended CH 4 at RYO and COI for the two years of 2012 and 2013 (Fig. 7).The timing and amplitude of seasonal cycles at RYO and COI overall agree well with each other, except for the summer of 2013.In August and September of 2013, the temporal variations of CH 4 at RYO and COI are different from those in the previous year 2012, when the CH 4 concentrations were low over the summertime and started rising at the end of September.In August 2013, the abrupt CH 4 increase by ∼ 100 ppb was observed at RYO, followed by COI with ∼ 1 week delay.In September, the CH 4 at both sites decreased but stayed in the higher level than 2012.Given that the fact the major CH 4 sources in east China, the sudden large increase of CH 4 in August 2013 is probably caused by unseasonal transport of CH 4 -rich air from the continent to Japan though normally in August the wind with CH 4 -low air from the Pacific Ocean is prevailing over Japan.

Model analysis
The observational data analysis suggested that the atmospheric transport would be a key factor of the extreme enhancement event of XCH 4 and surface CH 4 concentrations in the summer of 2013 over Japan.To investigate how the interannually varying atmospheric transport plays the role in the enhancement of XCH 4 and surface CH 4 , we conducted a forward model simulation using the global atmospheric transport model of National Institute for Environmental Studies (NIES-TM) version 8.1i.
The NIES-TM has a horizontal resolution of 2.5 • × 2.5 • with 32 vertical layers (Belikov et al., 2013).The global wind fields used in this study were obtained from the JMA Climate Data Assimilation System (JCDAS) (Onogi et al., 2007).The planetary boundary layer height data are obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Reanalysis dataset (Dee et al., 2011).In order to examine the impact of time-varying atmospheric transport on the seasonal cycles of atmospheric CH from Global Fire Emissions Database (GFED) v3.1 (van der Werf et al., 2010), wetland, rice paddy emissions and soil sinks from the Vegetation Integrative Simulator for Trace gases (VISIT) (Ito and Inatomi, 2012), and termites (Fung et al., 1991).Except the termites CH 4 emission, all the natural fluxes are seasonal.We used the modelled methane loss and climatological OH fields provided for a model intercomparison project "TransCom-CH 4 " (Patra et al., 2011).
Figure 8 shows the simulated surface CH 4 concentration and XCH 4 fields for August and September in 2012 and 2013.As a common feature, the high levels of XCH 4 and surface CH 4 are found over east China, reflecting the spatial distribution of the strong anthropogenic emissions around Beijing and Shanghai and biogenic CH 4 sources from rice cultivation in the southeastern China.
Different patterns are found in the XCH 4 and surface CH 4 fields between 2012 and 2013.In August 2012, both levels of XCH 4 and surface CH 4 over Japan are as low as those over the Pacific Ocean.In August 2013, higher concentrations of surface CH 4 extended from the northeastern China and the Korean peninsula to the Japan archipelago.The sur-face CH 4 concentration level in 2013 over Japan is increased by 40-60 ppb from the level in 2012.The XCH 4 values over Japan are also enhanced by ∼ 20 ppb while the XCH 4 values over the southeastern China are decreased compared to the 2012 level.The lower concentrations in August 2013 over the southeastern China than 2012 indicate that the northward wind along the coast was so fast that CH 4 was not much accumulated over the CH 4 source area in the southeastern China, but transported away to the north.As a result, the areas of the highest levels of CH 4 and XCH 4 shifted to the northeast, from the southeast China.In September 2013, XCH 4 level over the southeastern China is higher than August, but still lower than the level of September 2012.Also the XCH 4 over Japan remains higher level than that of 2012.The surface CH 4 concentration pattern in September 2013 is almost similar to the one in 2012, but slightly higher values are found over Japan.
Figure 9 shows the time series of modelled XCH 4 for GOSAT and TCCON, compared with the observations.For GOSAT, the modelled XCH 4 co-located with the GOSAT observations are sampled and averaged for comparison.The modelled XCH 4 produce the enhancement in summer 2013, in phase with the observations (Fig. 9).Overall the temporal variations of modelled XCH 4 are correlated with the observations (r = 0.50-0.72).These correlation coefficient values exceeded the 95 % significance level.The seasonal cycles of modelled XCH 4 for GOSAT and TCCON are in good agreement with the observations, while the modelled GOSAT XCH 4 show less short-term variability than the observations.The modelled surface CH 4 concentrations for the three Japanese sites, COI, RYO, and YON, are shown in Fig. 10.Though the modelled seasonal amplitude is slightly smaller than the observed, the modelled CH 4 overall capture the observed synoptic variations, as well as the abrupt increase in August 2013 at COI and RYO.The model was run with cyclo-stationary surface CH 4 fluxes, which are seasonally varying but not inter-annually.Inside the model, only the transport field is varying inter-annually.The modelobservation comparison thus provides supporting evidence that anomalous wind field in 2013 plays a key role in the large XCH 4 event in 2013.

Characteristics of atmospheric circulation in the summer of 2013
Forward modelling gives us insights into the contribution of atmospheric transport on the enhancement of XCH 4 and surface CH 4 concentration in the summer of 2013 over Japan.
Here we examine the 2013 summertime atmospheric transport over the northeastern Asia.Japan's summer climate is governed by the Pacific High (a lower-level high-pressure system) and the Tibetan High (an upper-level high-pressure system).These pressure systems were reported to have been enhanced during July and August 2013 (Tokyo Climate Center News No.34 Autumn 2013, available at http://ds.data.jma.go.jp/tcc/tcc/news).The Pacific High continued to expand westward and largely developed over the western part of Japanese islands including Okinawa.The Tibetan High expanded to the Japan main island in line with the northward meandering of upper-level westerly winds (the subtropical jet stream).The enhanced atmospheric transport from east China to Japan was probably attributed to those anomalously developed high-pressure systems.To see how the 2013 summertime atmospheric transport differs from the mean transport pattern, Fig. 11 shows the wind fields at the surface and at 850 hPa pressure level, from the JCDAS wind fields of August and September in 2013 over East Asia, compared with those of the mean wind fields for the five years of 2009-2013.
At the surface level (Fig. 11a), the mean wind field clearly shows that, in August the southeasterly wind from the Pacific Ocean prevails due to the development of the Pacific High.In September the wind from the continent to Japan start blowing as the Pacific High is retiring.In August 2013, as the Pacific High expanded westward, the air moved northward along the coast of China, turned around the Korean Peninsula, and flowed to Japan.This wind pattern suggests that the CH 4rich air was transported from east China to Japan in 2013, while the clean air is normally transported from the Pacific Ocean.In September 2013, over the Pacific Ocean, south of the Japan main island, easterly wind was still stronger than the normal, but the wind pattern over Japan was almost back to the normal, which can be characterized as a weak convergence of westerly wind from the continent and easterly wind from the Pacific Ocean.This nearly normal wind pattern over northern Japan would lower the CH 4 concentrations at the surface level as observed at RYO and COI.
At the 850 hPa level (Fig. 11b), it is notable that, in August 2013 the air moved over east China along the coast and turned around the Korean Peninsula sharply to the Japan archipelago.The anomalous westerly winds were stronger in the upper levels than near the surface.Given the major CH 4 source distributions in East Asia, the strong northward air flow along the coast could reduce local CH 4 accumulation, but transport the CH 4 -rich air effectively to the north   and then to Japan as turning around the Korean peninsula.In September 2013, the wind speed over Japan was much lower than August, but wind still blows westerly from the continent to Japan.This slower westerly air flow could maintain the higher level of XCH 4 over Japan during the September 2013.
The wind patterns we examined above shows us how the atmospheric transport field in 2013 differed from a climatological field on a regional-scale.In order to narrow down the origins and the upstream patterns of the air masses to the Japanese TCCON sites, we conducted back trajectory analysis using the Stochastic Time-Inverted Lagrangian Transport (STILT) model (Lin et al., 2003), driven by Global Data Assimilation System (GDAS) meteorology (1 • × 1 • ).To obtain the monthly mean features of the upstream, we released 100 particles from the height of 1500 m (approximately ∼ 850 hPa) at Saga and Tsukuba, at every 12:00 noon local time (= 03:00 UT) and travelled backward for 10 days.Every 30 min, the number of particles was counted by a 1 • × 1 • air column and the total number of particles over the 10 day duration was divided by the maximum number per column.Thus, we obtained a normalized daily upstream pattern and averaged them over a month.Figure 12 shows the monthly normalized trajectories for August and September in 2012 and 2013.There are distinct differences in the upstream patterns between 2012 and 2013.The patterns of the summer of 2012 are almost climatological; in August, the wind flows dominantly from the Pacific to the Japan, in September the dominant wind direction is in transition; from southeasterly wind (from the Pacific) to northwesterly wind (from the continent).On the other hand, in August 2013 the air masses reached the Japanese TCCON sites from the west, after travelling over the coastal side of east China.In September 2013, the westerly wind from the continent is still domi-nant, especially for Saga.This backtrajectory result supports that the anomalous wind field in the summer of 2013 brought the CH 4 -rich air from China to Japan, resulting in the high XCH 4 observed at the two Japanese TCCON sites and also by GOSAT over Japan.

Other possible factors
Although we suggest that the atmospheric transport field probably attributes to the enhancement of XCH 4 and CH 4 concentration observed in the summer of 2013, we cannot entirely rule out other possible factors.Here we discuss two factors.One is the surface emission changes.Though the temporal variations in XCH 4 do not necessarily correlate with the surface emissions (e.g.Bloom et al., 2010), the surface emission change is potential to impact on the change in XCH 4 .The second is the contribution of stratospheric methane.
The CH 4 emissions from rice cultivations and wetland in southern China might be enhanced under the hot summer condition in 2013.East Asia around China experienced a hotter summer monsoon season (June-September) by more than 1 • C than the season normal (Tokyo Climate Center News No.34 Autumn 2013), while less than 60 % of the normal precipitation in eastern China was reported.A hot weather condition increases the CH 4 emissions through the enhancement of photosynthesis and methanogenic activity in inundated grounds such as wetlands and rice paddies; while a dry condition reduces the CH 4 emissions from wetlands as the water table levels in the ground become low.Thus, the hot and dry weather conditions have opposite effects on the CH 4 emissions from wetlands.The time delay in the correlation between CH 4 emissions and climate anomalies should be considered as the groundwater plays an important role in wetland CH 4 emissions.Furthermore, since rice cultivation is human-managed, multiple controlling factors on CH 4 emissions from rice paddies should be considered.A further investigation of wetland and rice CH 4 emission changes responding to the climate anomaly in East Asia is needed.
Another possibility is the contribution of stratospheric methane.Saad et al. (2014) presented the analysis that the stratospheric methane causes short-term fractionations in total column averaged CH 4 observed at several TCCON sites.The contribution of stratospheric methane to the anomaly in summer 2013 is supposed to be minor or less influential.Firstly the surface CH 4 concentrations at COI and RYO increased in August 2013 when the XCH 4 anomaly occurred, suggesting the major contributor on the anomaly is in the troposphere.Secondly, the order of the stratospheric methane fractionation is smaller than ∼ 3 ppb, which would not be enough to produce the anomaly of an order of ∼ 20 ppb.

Conclusion
In this study, we have examined the extremely high XCH 4 event over northeast Asia observed by GOSAT in August and September 2013.Similar XCH 4 enhancements in amplitude and timing were observed at the two Japanese TCCON sites, Tsukuba and Saga.Furthermore, during the same period, the ground-based atmospheric CH 4 monitoring sites of Ryori and Ochi-ishi located in the northern part of Japan observed the higher levels of surface CH 4 .In particular, surface CH 4 concentrations at Ryori showed the rapid increase in the middle of August 2013.
Our model analysis indicates that the significant enhancement of XCH 4 and surface CH 4 are mainly attributed to the anomalous atmospheric pressure patterns of Pacific High and Tibetan High over East Asia during the summer of 2013.The CH 4 -rich air was effectively transported to Japan from the major CH 4 source area in east China.The model analysis also indicates that the XCH 4 enhancement occurred in a limited area of the northeastern China to the Japan main island.The two Japanese TCCON sites, ∼ 1000 km apart from each other, happened to be located along the anomalously CH 4 -rich air flow from the Eurasian continent and coincidentally observed the extreme increase of XCH 4 .The GOSAT with 3-day recurrence successfully observed the high-XCH 4 event.This data analysis study demonstrates the capability of GOSAT to monitor the synoptic-scale XCH 4 event in the association with the high-pressure system anomalies.The GOSAT capability to detect synoptic variations could be helpful to quantify the relative contribution of atmospheric transport, leading to better estimation of regional CH 4 fluxes.

Data availability
Surface CH 4 concentration data observed at RYO and YON are accessible at GAW World Data Centre for Greenhouse Gases (WDCGG), http://ds.data.jma.go.jp/gmd/wdcgg.The COI CH 4 data until 2010 are accessible also at WDCGG.The recent COI data for 2011-2013 are available by contacting Y. Tohjima (tohjima@nies.go.jp).NIES GOSAT L2 XCH 4 product data are available upon request through GOSAT GUIG, https://data.gosat.nies.go.jp.TCCON GGG2012 data are available from the Carbon Dioxide Information Analysis Center (CDIAC), http://tccon.ornl.gov/2012.The model data presented in this paper are available upon request from the corresponding author (ishizawa.misa@nies.go.jp).

Figure 2 .
Figure 2. Temporal variations of GOSAT XCH 4 over the two regions of northeast Asia: (a) northeastern China -Korea (115-130 • E, 30-40 • N), and (b) Japan (130-145 • E, 30-40 • N).GOSAT XCH 4 data are shown in gray dot.The monthly means are plotted in red solid circle and line, whereas monthly means in open circles indicate less than two retrievals available per month.Blue lines indicate the long-term trends.The histograms in the bottom show the number of GOSAT XCH 4 data per month.

Figure 4 .
Figure 4. Temporal variations of TCCON XCH 4 at (a) Saga (130.29 • E, 33.24 • N) and (b) Tsukuba (140.12 • E, 36.05 • N), Japan.TCCON XCH 4 data are shown in gray dot, daily means in green dots.The monthly means are plotted in red solid circle and line, whereas monthly means in open circles indicate less than two observation days per month.Blue lines indicate the long-term trends.The histograms at the bottom show the number of observation day per month.

Figure 7 .
Figure 7. Detrended CH 4 for 2012 to 2013 at Ryori (RYO) and Cape Ochi-ishi (COI) in Japan.Long-term components in individual CH 4 time series are removed by low pass digital filter of cutoff frequency of two years.August and September of both 2012 and 2013 are highlighted.

Figure 11 .
Figure 11.Monthly mean wind fields of August and September at (a) surface and (b) 850 hPa.The left panels are the wind fields averaged over the five years of 2009-2013, and the right panels are the monthly mean wind fields of the year 2013.

Figure 12 .
Figure12.Monthly mean 10-day backward trajectories from (a) Saga and (b) Tsukuba at 12:00 noon local time (= 03:00 UT).The trajectories started at an altitude of 1500 m (approximately 850 hPa).100 particles are released every day for a month.To normalize the number density of particles, the particles passed at each 1 • × 1 • grid air column are counted, and the total numbers are divided by the maximum number per grid.