Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Masato Mori Last modified date:2021.05.25

Assistant Professor / Division of Earth Environment Dynamics / Research Institute for Applied Mechanics


Papers
1. Description and basic evaluation of simulated mean state, internal variability, and climate sensitivity in MIROC6.
2. Masato Mori, Yu Kosaka, Masahiro Watanabe, Hisashi Nakamura, Masahide Kimoto, A reconciled estimate of the influence of Arctic sea-ice loss on recent Eurasian cooling, Nature Climate Change, 10.1038/s41558-018-0379-3, doi:10.1038/s41558-018-0379-3, 2019.01.
3. The Effects of Natural Variability and Climate Change on the Record Low Sunshine over Japan During August 2017.
4. CLIMATE CHANGE INCREASED THE LIKELIHOOD OF THE 2016 HEAT EXTREMES IN ASIA.
5. Satoru Okajima, Hisashi Nakamura, Kazuaki Nishii, Takafumi Miyasaka, Akira Kuwano-Yoshida, Bunmei Taguchi, Masato Mori, Yu Kosaka, Mechanisms for the Maintenance of the Wintertime Basin-Scale Atmospheric Response to Decadal SST Variability in the North Pacific Subarctic Frontal Zone, JOURNAL OF CLIMATE, 10.1175/JCLI-D-17-0200.1, 31, 1, 297-315, 2018.01, Mechanisms for the maintenance of a large-scale wintertime atmospheric response to warm sea surface temperature (SST) anomalies associated with decadal-scale poleward displacement of the North Pacific subarctic frontal zone (SAFZ) are investigated through the following two ensemble experiments with an atmospheric general circulation model (AGCM): one with climatological-mean SST and the other with positive SST anomalies along the SAFZ prescribed on top of the climatological-mean SST. As actually observed, the simulated January ensemble response over the North Pacific is anticyclonic throughout the depth of the troposphere, although its amplitude is smaller. This response is maintained through energy conversion from the ensemble climatological-mean circulation realized under the climatological SST as well as feedback from anomalous transient eddy activity, suggesting that the response may have characteristics as a preferred mode of variability (or "dynamical mode''). Conversions of both available potential energy and kinetic energy from the climatological-mean state are important for the observed anomaly, while the latter is less pronounced for the model response. Net transient feedback forcing is also important for both the observed anomaly and simulated response. These results imply that a moderate-resolution (similar to 1 degrees) AGCM may be able to simulate a basin-scale atmospheric response to the SAFZ SST anomaly through synoptic-and basin-scale dynamical processes. Weaker PNA-like internal variability in the model may lead to the weaker response, suggesting that misrepresentation of intrinsic atmospheric variability can affect the model response to the SST anomaly..
6. N. Hirota, H. Shiogama, H. Akiyoshi, T. Ogura, M. Takahashi, Y. Kawatani, M. Kimoto, M. Mori, The influences of El Nino and Arctic sea-ice on the QBO disruption in February 2016, NPJ CLIMATE AND ATMOSPHERIC SCIENCE, 10.1038/s41612-018-0020-1, 1, 2018.01, The westerly phase of the quasi-biennial oscillation (QBO) was unexpectedly disrupted by an anomalous easterly near 40 hPa (similar to 23 km) in February 2016. At the same time, a very strong El Nino and a very low Arctic sea-ice concentration in the Barents and Kara Sea were present. Previous studies have shown that the disruption of the QBO was primarily caused by the momentum transport of the atmospheric waves in the Northern Hemisphere. Our results indicate that the tropical waves evident over the Atlantic, Africa, and the western Pacific were associated with extratropical disturbances. Moreover, we suggest that the El Nino and sea-ice anomalies in 2016 account for approximately half of the disturbances and waves based on multiple regression analysis of the observational/reanalysis data and large-ensemble experiments using an atmospheric global climate model..
7. Uchida, Junya, Mori, Masato, Hara, Masayuki, Satoh, Masaki, Goto, Daisuke, Kataoka, Takahito, Suzuki, Kentaroh, Nakajima, Teruyuki, Impact of Lateral Boundary Errors on the Simulation of Clouds with a Nonhydrostatic Regional Climate Model, MONTHLY WEATHER REVIEW, 10.1175/MWR-D-17-0158.1, 145, 12, 5059-5082, 2017.12, A nonhydrostatic, regional climate limited-area model (LAM) was used to analyze lateral boundary condition (LBC) errors and their influence on the uncertainties of regional models. Simulations using the fully compressible nonhydrostatic LAM (D-NICAM) were compared against the corresponding global quasi-uniform-grid Nonhydrostatic Icosahedral Atmospheric Model (NICAM) and a stretched-grid counterpart (S-NICAM). By this approach of sharing the same dynamical core and physical schemes, possible causes of model bias and LBC errors are isolated. The simulations were performed for a 395-day period from March 2011 through March 2012 with horizontal grid intervals of 14, 28, and 56 km in the region of interest. The resulting temporal mean statistics of the temperatures at 500 hPa were generally well correlated between the global and regional simulations, indicating that LBC errors had a minor impact on the large-scale flows. However, the time-varying statistics of the surface precipitation showed that the LBC errors lead to the unpredictability of convective precipitation, which affected the mean statistics of the precipitation distributions but induced only minor influences on the large-scale systems. Specifically, extratropical cyclones and orographic precipitation are not severely affected. It was concluded that the errors of the precipitation distribution are not due to the difference of the model configurations but rather to the uncertainty of the system itself. This study suggests that applications of ensemble runs, internal nudging, or simulations with longer time scales are needed to obtain more statistically significant results of the precipitation distribution in regional climate models..
8. Chiharu Takahashi, Masahiro Watanabe, Masato Mori, Significant Aerosol Influence on the Recent Decadal Decrease in Tropical Cyclone Activity Over the Western North Pacific, GEOPHYSICAL RESEARCH LETTERS, 10.1002/2017GL075369, 44, 18, 9496-9504, 2017.09, Over the past two decades, the number of tropical cyclones (TCs) has decreased markedly in the southeastern part of the western North Pacific (WNP) as a component of the interdecadal variation. This decrease has partially been explained by an internal low-frequency variability of sea surface temperature (SST) in the Pacific, but influences of external forcing remain unclear. Here we show that past changes in sulfate aerosol emissions contributed approximately 60% of the observed decreasing trends in TC genesis frequency in the southeastern WNP for 1992-2011, using multiple simulations by a global climate model. This decrease was mainly attributed to the increased vertical wind shear and decreased low-level vorticity, associated with a trans-basin multidecadal SST change driven by aerosol forcing. The near-future projection shows that the aerosol forcing still has some potential influence on decadal TC change, but the projected decreasing frequency is mainly due to increasing greenhouse gases forcing..
9. Youichi Kamae, Hideo Shiogama, Yukiko Imada, Masato Mori, Osamu Arakawa, Ryo Mizuta, Kohei Yoshida, Chiharu Takahashi, Miki Arai, Masayoshi Ishii, Masahiro Watanabe, Masahide Kimoto, Shang-Ping Xie, Hiroaki Ueda, Forced response and internal variability of summer climate over western North America, CLIMATE DYNAMICS, 10.1007/s00382-016-3350-x, 49, 1-2, 403-417, 2017.07, Over the past decade, anomalously hot summers and persistent droughts frequented over the western United States (wUS), the condition similar to the 1950s and 1960s. While atmospheric internal variability is important for mid-latitude interannual climate variability, it has been suggested that anthropogenic external forcing and multidecadal modes of variability in sea surface temperature, namely, the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO), also affect the occurrence of droughts and hot summers. In this study, 100-member ensemble simulations for 1951-2010 by an atmospheric general circulation model were used to explore relative contributions of anthropogenic warming, atmospheric internal variability, and atmospheric response to PDO and AMO to the decadal anomalies over the wUS. By comparing historical and sensitivity simulations driven by observed sea surface temperature, sea ice, historical forcing agents, and non-warming counterfactual climate forcing, we found that large portions of recent increases in mean temperature and frequency of hot summers (66 and 82 %) over the wUS can be attributed to the anthropogenic global warming. In contrast, multidecadal change in the wUS precipitation is explained by a combination of the negative PDO and the positive AMO after the 2000s. Diagnostics using a linear baroclinic model indicate that AMO- and PDO-related diabatic heating anomalies over the tropics contribute to the anomalous atmospheric circulation associated with the droughts and hot summers over wUS on multidecadal timescale. Those anomalies are not robust during the periods when PDO and AMO are in phase. The prolonged PDO-AMO antiphase period since the late twentieth century resulted in the substantial component of multidecadal anomalies in temperature and precipitation over the wUS..
10. Ryo Mizuta, Akihiko Murata, Masayoshi Ishii, Hideo Shiogama, Kenshi Hibino, Nobuhito Mori, Osamu Arakawa, Yukiko Imada, Kohei Yoshida, Toshinori Aoyagi, Hiroaki Kawase, Masato Mori, Yasuko Okada, Tomoya Shimura, Toshiharu Nagatomo, Mikiko Ikeda, Hirokazu Endo, Masaya Nosaka, Miki Arai, Chiharu Takahashi, Kenji Tanaka, Tetsuya Takemi, Yasuto Tachikawa, Khujanazarov Temur, Youichi Kamae, Masahiro Watanabe, Hidetaka Sasaki, Akio Kitoh, Izuru Takayabu, Eiichi Nakakita, Masahide Kimoto, OVER 5,000 YEARS OF ENSEMBLE FUTURE CLIMATE SIMULATIONS BY 60-KM GLOBAL AND 20-KM REGIONAL ATMOSPHERIC MODELS, BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY, 10.1175/BAMS-D-16-0099.1, 98, 7, 1383-1398, 2017.07, An unprecedentedly large ensemble of climate simulations with high-resolution atmospheric models enables the assessment of probabilistic change by global warming in low-frequency local-scale severe events..
11. Hideo Shiogama, Masahiro Watanabe, Yukiko Imada, Masato Mori, Masayoshi Ishii, Masahide Kimoto, Corrigendum to: An event attribution of the 2010 drought in the South Amazon region using the MIROC5 model: Event attribution of the 2010 Amazon drought (Atmospheric Science Letters, (2013), 14, 3, (170-175), 10.1002/asl2.435), Atmospheric Science Letters, 10.1002/asl.725, 18, 2, 103-104, 2017.02, As it has been reported by Shiogama et al. (, hereafter S16), the first half of 100 member ALL-forcing runs of Shiogama et al. (, hereafter S13) have the well mixed greenhouse gas concentrations of the year 1850 instead of that in the year 2006–2010 by an error. We have redone these experiments with the correct GHGs concentrations (S16). This error correction made little changes in the results and did not affect the conclusions of S13. The corrected Figures and are very similar to that in S13. The probabilities of drought events of ALL more at least as severe as the 2010 record is the same as S13 (1%) (see the first paragraph of the results section in page 172). With the bias corrections of ANSG and ENSO, the probabilities of drought events of ALL more at least as severe as the 2010 record are 14%. <
3%, >
29%, while those were 15% <
5%, >
32% in S13 (see the first paragraph of the right column of page 173). The ratios of ALL/NAT and noCS/ALL are 5.0 <
2.9, 8.0 >
(6.3 <
3.2, 8.7 >
in S13) and 1.1 <
0.9, 1.3 >
(1.0 <
0.8, 1.1 >
in S13), respectively (see the first paragraph of the right column of page 173). This error correction did not affect the other figures. 2 (Figure presented.) The black and 10 gray curves are probability density functions (PDF) of the Jul-Oct precipitation anomalies (%, relative to the 1979–2010 mean) during the 1979–2010 period for the observations and ALL-Long, respectively. The PDF is calculated as a normalized histogram of samples within half-overlapped 6%-width bins. The black vertical line is the 2010 observation. The green, blue, and red curves indicate the PDFs of the ALL, NAT, and noCS runs, respectively. Note that these PDFs were calculated from different numbers of samples: 32 for the observations and ALL-Long, and 100 for ALL, NAT and noCS. 4 (Figure presented.) The dashed green, blue, and red curves are the PDFs of the July-October precipitation anomalies (%) for the ALL, NAT, and noCS runs, respectively, as shown in Figure 1. The solid curves indicate the PDFs with the bias corrections (with the best estimates of the scaling factors) according to the precipitation responses to (a) ANSG, (b) ENSO, and (c) ANSG + ENSO. (d) Solid curves show the PDF estimates with the neutral ANSG + ENSO. Acknowledgements We thank Oliver Angélil who found the errors in the GHGs concentrations. This work was supported by the Program for Risk Information on Climate Change (SOUSEI program) and Grant-in-Aid 26281013 from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Earth Simulator and NEC SX (NIES) were utilized for the simulations. We have no conflict of interest..
12. Chiharu Takahashi, Masahiro Watanabe, Hideo Shiogama, Yukiko Imada, Masato Mori, 21. A persistent Japanese heat wave in early August 2015: Roles of natural variability and human-induced warming, Bulletin of the American Meteorological Society, 10.1175/BAMS-D-16-0157.1, 97, 12, S107-S112, 2016.12, The persistent Japanese heat wave that occurred in early August 2015 was mainly attributed to intraseasonal disturbances including tropical cyclones. Anthropogenic warming contributed to an increase in the probability of occurrence..
13. Hideo Shiogama, Yukiko Imada, Masato Mori, Ryo Mizuta, Daithi Stone, Kohei Yoshida, Osamu Arakawa, Mikiko Ikeda, Chiharu Takahashi, Miki Arai, Masayoshi Ishii, Masahiro Watanabe, Masahide Kimoto, Attributing Historical Changes in Probabilities of Record-Breaking Daily Temperature and Precipitation Extreme Events, SOLA, 10.2151/sola.2016-045, 12, 225-231, 2016.12, We describe two unprecedented large (100-member), longterm (61-year) ensembles based on MRI-AGCM3.2, which were driven by historical and non-warming climate forcing. These ensembles comprise the "Database for Policy Decision making for Future climate change (d4PDF)". We compare these ensembles to large ensembles based on another climate model, as well as to observed data, to investigate the influence of anthropogenic activities on historical changes in the numbers of record-breaking events, including: the annual coldest daily minimum temperature (TNn), the annual warmest daily maximum temperature (TXx) and the annual most intense daily precipitation event (Rx1day). These two climate model ensembles indicate that human activity has already had statistically significant impacts on the number of record-breaking extreme events worldwide mainly in the Northern Hemisphere land. Specifically, human activities have altered the likelihood that a wider area globally would suffer record-breaking TNn, TXx and Rx1day events than that observed over the 2001-2010 period by a factor of at least 0.6, 5.4 and 1.3, respectively. However, we also find that the estimated spatial patterns and amplitudes of anthropogenic impacts on the probabilities of record-breaking events are sensitive to the climate model and/or natural-world boundary conditions used in the attribution studies..
14. Uchida, Junya, Mori, Masato, Nakamura, Hisashi, Satoh, Masaki, Suzuki, Kentaroh, Nakajima, Teruyuki, Error and Energy Budget Analysis of a Nonhydrostatic Stretched-Grid Global Atmospheric Model, MONTHLY WEATHER REVIEW, 10.1175/MWR-D-15-0271.1, 144, 4, 1423-1447, 2016.04, A nonhydrostatic stretched-grid (SG) model is used to analyze the large-scale errors generated by stretching horizontal grids and their influence on a region of interest. Simulations by a fully compressible, nonhydrostatic global atmospheric model, the Nonhydrostatic Icosahedral Atmospheric Model (NICAM), and its SG regional model, stretched-NICAM, were performed for the months of March, April, and May of 2011 using various resolutions and stretching factors. A comparison of week-long accumulative precipitation amounts between the Tropical Rainfall Measuring Mission (TRMM) satellite data and the quasi-uniform and SG simulations showed that a stretched run better represents storms and associated precipitation because the errors generated in the outer regions with coarser grid spacing do not significantly affect the inner domain centered at the focal point. For season-long simulations, in one particular set of stretched runs with the focal point located in the eastern United States, the artificial suppression of baroclinic development of midlatitude eddies in the Southern Hemisphere weakened the eddy-driven polar-front jet (PFJ), which yielded a cold bias at mid- to high latitudes. However, in the Northern Hemisphere, in contrast, the aforementioned changes are less apparent. Therefore, for the SG runs, the mean temperature was maintained at the region of interest, and an increased amount of moderate to heavy precipitation, which is also frequently found in the TRMM data, was observed; thus, the benefits of increased resolution were realized. However, careful attention must be given when applying the SG model because a regional climate response to the change in the large-scale circulations may not be fully accounted for..
15. Yukiko Imada, Hiroaki Tatebe, Masayoshi Ishii, Yoshimitsu Chikamoto, Masato Mori, Miki Arai, Masahiro Watanabe, Masahide Kimoto, Predictability of Two Types of El Nino Assessed Using an Extended Seasonal Prediction System by MIROC, MONTHLY WEATHER REVIEW, 10.1175/MWR-D-15-0007.1, 143, 11, 4597-4617, 2015.11, Predictability of El Nino-Southern Oscillation (ENSO) is examined using ensemble hindcasts made with a seasonal prediction system based on the atmosphere and ocean general circulation model, the Model for Interdisciplinary Research on Climate, version 5 (MIROC5). Particular attention is paid to differences in predictive skill in terms of the prediction error for two prominent types of El Nino: the conventional eastern Pacific (EP) El Nino and the central Pacific (CP) El Nino, the latter having a maximum warming around the date line. Although the system adopts ocean anomaly assimilation for the initialization process, it maintains a significant ability to predict ENSO with a lead time of more than half a year. This is partly due to the fact that the system is little affected by the spring prediction barrier, because increases in the error have little dependence on the thermocline variability. Composite analyses of each type of El Nino reveal that, compared to EP El Ninos, the ability to predict CP El Ninos is limited and has a shorter lead time. This is because CP El Ninos have relatively small amplitudes, and thus they are more affected by atmospheric noise; this prevents development of oceanic signals that can be used for prediction..
16. Masato Mori, Masahiro Watanabe, Hideo Shiogama, Jun Inoue, Masahide Kimoto, Robust Arctic sea-ice influence on the frequent Eurasian cold winters in past decades, NATURE GEOSCIENCE, 10.1038/NGEO2277, 7, 12, 869-873, 2014.12, Over the past decade, severe winters occurred frequently in mid-latitude Eurasia(1,2), despite increasing global- and annual-mean surface air temperatures(3). Observations suggest that these cold Eurasian winters could have been instigated by Arctic sea-ice decline(2,4), through excitation of circulation anomalies similar to the Arctic Oscillation(5). In climate simulations, however, a robust atmospheric response to sea-ice decline has not been found, perhaps owing to energetic internal fluctuations in the atmospheric circulation(6). Here we use a 100-member ensemble of simulations with an atmospheric general circulation model driven by observation-based sea-ice concentration anomalies to show that as a result of sea-ice reduction in the Barents-Kara Sea, the probability of severe winters has more than doubled in central Eurasia. In our simulations, the atmospheric response to sea-ice decline is approximately independent of the Arctic Oscillation. Both reanalysis data and our simulations suggest that sea-ice decline leads to more frequent Eurasian blocking situations, which in turn favour cold-air advection to Eurasia and hence severe winters. Based on a further analysis of simulations from 22 climate models we conclude that the sea-ice-driven cold winters are unlikely to dominate in a warming future climate, although uncertainty remains, due in part to an insufficient ensemble size..
17. Hideo Shiogama, Masahiro Watanabe, Yukiko Imada, Masato Mori, Youichi Kamae, Masayoshi Ishii, Masahide Kimoto, Attribution of the June-July 2013 Heat Wave in the Southwestern United States, SOLA, 10.2151/sola.2014-025, 10, 122-126, 2014.10, A severe heat wave occurred in the southwestern United States (US) during June and July 2013. To investigate the effects of natural variability and anthropogenic climate change on this event, we generated large ensemble simulations of possible weather using the MIROC5A climate model forced by "historical external forcing agents, sea surface temperature (SST) observations and sea ice (SIC) observations" both with and without human influence. It was suggested that both the anthropogenic warming and an atmospheric circulation regime related to the natural variability of SST and SIC made the heat wave event more likely. On the other hand, no significant human influence was found in atmospheric circulation patterns. These results were robust for two different estimates of anthropogenic signals on SST and SIC..
18. Takashi Mochizuki, Masahide Kimoto, Yoshimitsu Chikamoto, Masato Mori, Masahiro Watanabe, Masayoshi Ishii, Error Sensitivity to Initial Climate States in Pacific Decadal Hindcasts, SOLA, 10.2151/sola.2014-009, 10, 39-44, 2014.10, We perform decadal hindcast experiments with initialization every year and assess sensitivity of the hindcasted errors to the errors in the initial climate states. The hindcasted sea surface temperature (SST) over the extratropical North Pacific shows significant impacts of initialization, yet the hindcasted indices of the Pacific decadal variability usually suffer from limited predictability. Our sensitivity analyses reveal that, in the decadal SST hindcasts over the extratropical North Pacific, the annual-to-decadal errors of the Aleutian Low fluctuation before the time of starting hindcast experiments work as a major source of uncertainty through delayed responses of the ocean. As we directly assimilate only the ocean states to the atmosphere-ocean coupled model in initialization, the Aleutian Low fluctuation in the assimilation exhibits large errors even though the SST is well correlated with the observation. These assimilation errors in the Aleutian Low fluctuation are primarily due to the distorted responses of the extratropical atmosphere to the tropical SST changes in the model. A close examination indicates that the observed and assimilated Aleutian Lows are sensitive to the eastern and central equatorial SSTs, respectively. Toward further reducing uncertainty in the Pacific decadal hindcasts, therefore, it should be an effective way to raise quality of initial conditions for the extratropical atmosphere and the tropical atmosphere-ocean coupling..
19. Yukiko Imada, Hideo Shiogama, Masahiro Watanabe, Masato Mori, Masayoshi Ishii, Masahide Kimoto, THE CONTRIBUTION OF ANTHROPOGENIC FORCING TO THE JAPANESE HEAT WAVES OF 2013, BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY, 95, 9, S52-S54, 2014.09, Anthropogenic climate change played a significant role in increasing the probability of events such as the heat wave in Japan in 2013..
20. Yuya Satake, Masaru Inatsu, Masato Mori, Akira Hasegawa, Tropical Cyclone Tracking Using a Neighbor Enclosed Area Tracking Algorithm, MONTHLY WEATHER REVIEW, 10.1175/MWR-D-12-00092.1, 141, 10, 3539-3555, 2013.10, Tropical cyclone (TC) tracking is essential for calculating TC statistics from gridded datasets. A new method for TC tracking is presented here using neighbor enclosed area tracking (NEAT), which is based on the temporal overlap of enclosed areas above a vorticity threshold and differs from the widely used neighbor point tracking (NPT) method. The parameters of cyclone intensity, vertical-shear, and warm-core criteria were intensively tuned for NEAT and NPT. When these criteria were optimized for the typhoon tracks observed in the western North Pacific based on the Japanese 25-yr Reanalysis Project (JRA-25)/Japan Meteorological Agency (JMA) Climate Data Assimilation System (JCDAS) dataset, the NEAT and NPT algorithms captured approximately 85% of typhoons with little qualitative distortion in the spatial distribution and temporal variability of the TC track density. The grid system dependency of the algorithms was tested by applying NEAT and NPT to a high-resolution general circulation model output. The method presented here can also provide realistic statistics on the TC size, the extratropical transition timing, and the meridional heat transport..
21. Contribution of atmospheric circulation change to the 2012 heavy rainfall in southern Japan.
22. Hiroaki Tatebe, Yukiko Imada, Masato Mori, Masahide Kimoto, Hiroyasu Hasumi, Control of Decadal and Bidecadal Climate Variability in the Tropical Pacific by the Off-Equatorial South Pacific Ocean, JOURNAL OF CLIMATE, 10.1175/JCLI-D-12-00137.1, 26, 17, 6524-6534, 2013.09, Delayed negative feedback processes determining intrinsic decadal and bidecadal time scales for the tropical variability in the Pacific are investigated based on climate model experiments. By comparing a control run driven by preindustrial forcing and partial blocking runs driven by the same forcing but with ocean temperature and salinity restored to climatology in selected regions, subsurface oceanic signals of South Pacific origin are shown to precede SST variability in the Nino-3.4 region. Using a linear reduced-gravity ocean model driven only by wind stress changes and an offline tracer model, oceanic wave adjustment triggered by changes of wind stress curl in the South Pacific extratropics is suggested to be essential for the decadal component of the equatorial SST, while slower isopycnal advection of subsurface temperature anomalies from the formation region of South Pacific Eastern Subtropical Mode Water controls the bidecadal component. The intrinsic time scales of the tropical variability are regulated by simple linear ocean dynamics..
23. Thomas C. Peterson, Lisa V. Alexander, Myles R. Allen, Juan A. Anel, David Barriopedro, Mitchell T. Black, Trevor Carey-Smith, Rodrigo Castillo, Julien Cattiaux, Xiaolong Chen, Xianyan Chen, Matthieu Chevallier, Nikolaos Christidis, Andrew Ciavarella, Hylke de Vries, Sam M. Dean, Kirsten Deans, Noah S. Diffenbaugh, Francisco Doblas-Reyes, Markus G. Donat, Buwen Dong, Gary Eilerts, Chris Funk, Gideon Galu, Ricardo Garcia-Herrera, Agathe Germe, Stephen Gill, Luis Gimeno, Virginie Guemas, Stephanie C. Herring, Andrew Hoell, Martin P. Hoerling, Chris Huntingford, Greg Husak, Yukiko Imada, Masayoshi Ishii, David J. Karoly, Masahide Kimoto, Andrew D. King, Thomas R. Knutson, Sophie C. Lewis, Renping Lin, Bradfield Lyon, Neil Massey, Edoardo Mazza, Joel Michaelsen, James Mollard, Masato Mori, Philip W. Mote, Raquel Nieto, Friederike E. L. Otto, Joseph Park, Sarah E. Perkins, Suzanne Rosier, James Rowland, David E. Rupp, David Salas y Melia, Martin Scherer, Hideo Shiogama, Shraddhanand Shukla, Fengfei Song, Sarah Sparrow, Peter A. Scott, Rowan Sutton, William Sweet, Simon F. B. Tett, Ricardo Machado Trigo, Geert Jan van Oldenborgh, Rudolf van Westrhenen, James Verdin, Masahiro Watanabe, Andrew T. Wittenberg, Tim Woollings, Pascal Yiou, Fanrong Zeng, Chris Zervas, Rong Zhang, Tianjun Zhou, EXPLAINING EXTREME EVENTS OF 2012 FROM A CLIMATE PERSPECTIVE, BULLETIN OF THE AMERICAN METEOROLOGICAL SOCIETY, 10.1175/BAMS-D-13-00085.1, 94, 9, S1-S74, 2013.09.
24. Masato Mori, Masahiro Watanabe, Masahide Kimoto, Superrotation and Nonlinear Hadley Circulation Response to Zonally Asymmetric Sea Surface Temperature in an Aquaplanet GCM, JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN, 10.2151/jmsj.2013-A10, 91A, 269-291, 2013.09, The responses of the equatorial zonal wind and the Hadley circulation to the equatorial zonal wavenumber one sea surface temperature (SST) anomaly, T-s*, are examined in an atmospheric general circulation model (AGCM) with an aquaplanet condition. The Hadley cell is weakened as the magnitude of T-s* increases, balancing with a decrease in the zonal-mean diabatic heating over the tropics. The decrease of heating reflects a nonlinear relationship between precipitation and SST; deep convection, such as a super cloud cluster, is significantly suppressed over cold T-s*, whereas is slightly enhanced over warm T-s*. The effective suppression of deep convection is accomplished by the stable boundary layer and the dry subsidence anomaly associated with the Walker cell which is excited by the SST anomaly. And the decreased convection acts to further reinforce the subsidence via thermodynamic balance. Therefore, this positive feedback between large-scale circulation and deep convection determines the nonlinear relationship and controls the strength of the Hadley cell.
In terms of the energetics of the tropical circulation, the Hadley cell has to be weakened to compensate for the lack of energy supply caused by an increase of tropical radiative cooling due to the effective suppression of deep convection over cold T-s*.
We compared the results of our AGCM with that of other 15 aquaplanet AGCMs integrated with the same SST distribution. While the Hadley cell is weakened in all AGCMs when T-s* is added to the zonal uniform SST, there is a large diversity in the strength. This suggests that the difference in the physical parameterization causes a different sensitivity of the Hadley cell response to zonally asymmetric SST. The magnitude of weakening is approximately proportional to the decreased (increased) amount of the deep convective precipitation (the radiative cooling) over the tropics. This strong relationship suggests that the positive feedback also works in other AGCMs. It is considered that the feedback is also important for understanding the formation of a real tropical climate..
25. Masahiro Watanabe, Hideo Shiogama, Yukiko Imada, Masato Mori, Masayoshi Ishii, Masahide Kimoto, Event Attribution of the August 2010 Russian Heat Wave, SOLA, 10.2151/sola.2013-015, 9, 65-68, 2013.09, An extreme heat wave hit western Russia in the summer of 2010. To investigate the contribution of anthropogenic climate change to this event, 100-member ensembles of atmospheric general circulation model (AGCM) experiments, with and without possible human-induced changes in sea surface temperature (SST) and sea ice, were generated. The AGCM can reproduce monthly surface air temperature (SAT) anomalies for the past 30 years over the continental area, indicating a significant influence of the anomalous boundary conditions on the surface climate variability. While the ensemble average does not capture the extremely high SAT anomaly over western Russia observed in August 2010, the ensemble covers the anomaly with the probability of occurrence at 3.3%. Without the anthropogenic change in SST and sea ice, the ensemble fails to capture the observed SAT anomaly, reducing the probability of occurrence to 0.6%. The atmospheric response to the tropical precipitation change associated with anthropogenic SST increase leads to warming over Eurasia through northward temperature advection, consistent with the observed upward SAT trend. Drying of the land surface in spring may also have favored the summer warming over western Russia..
26. Masato Mori, Masahide Kimoto, Masayoshi Ishii, Satoru Yokoi, Takashi Mochizuki, Yoshimitsu Chikamoto, Masahiro Watanabe, Toru Nozawa, Hiroaki Tatebe, Takashi T. Sakamoto, Yoshiki Komuro, Yukiko Imada, Hiroshi Koyama, Hindcast Prediction and Near-Future Projection of Tropical Cyclone Activity over the Western North Pacific Using CMIP5 Near-Term Experiments with MIROC, JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN, 10.2151/jmsj.2013-402, 91, 4, 431-452, 2013.08, In line with the experimental design for near-term climate prediction toward the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, we perform ensembles of initialized decadal hindcast and near-future projection using three versions of the coupled atmosphere ocean model MIROC. In the present study, we explore interannual and multiyear predictability of tropical cyclone (TC) activity in the western North Pacific (WNP) using the initialized hindcasts and examine global warming impacts on TC activity in the near-future on the basis of near-future projections up to 2035.
The hindcasts of year-to-year variation in TC number capture the observed values reasonably well. Moreover, interannual variability of TC genesis and occurrence frequency associated with the El Nino Southern Oscillation are found to be predictable, mainly through better prediction of sea surface temperature (SST) and large-scale vorticity anomalies in the lower troposphere. These results indicate that the models can reproduce the major basic mechanisms that link TC genesis with large-scale circulation. Skillful prediction of TC number is likely difficult on multiyear timescales, at least based on our hindcasts, but through initializations, the three-year-mean hindcasts from 1998 onward reasonably capture observed major characteristics of TC activity associated with Pacific climate shift during the late 1990s.
Near-future projections (2016-2035) suggest significant reductions (approximately 14%) in TC number, particularly over the western part of the WNP, even under scenarios in which projected global warming is less prominent than that at the end of this century. This reduction is likely due to the suppression of large-scale lower tropospheric vorticity and relative humidity and the enhancement of vertical wind shear. The projected SST exhibits a more pronounced warming over the eastern tropical Pacific than over the western region and accompanies the weakening of Walker circulation via redistribution of tropical convection activity, which appears to be responsible for the change in the large-scale fields in the WNP..
27. Hideo Shiogama, Masahiro Watanabe, Yukiko Imada, Masato Mori, Masayoshi Ishii, Masahide Kimoto, An event attribution of the 2010 drought in the South Amazon region using the MIROC5 model, Atmospheric Science Letters, 10.1002/asl2.435, 14, 3, 170-175, 2013.07, We produced 100-member event attribution ensembles during 2009-2012 under all forcing conditions and in two different counterfactual worlds without anthropogenic forcing (mainly greenhouse gases and aerosols) and without aerosol emission changes using the MIROC5 atmospheric general circulation model. It seemed that both human influences and the sea surface temperature (SST) natural variability increased probabilities of the 2010 severe drought in the South Amazon region, and that changes in aerosols emissions had little effect on the drought. It should be noted that our assessments were sensitive to bias corrections according to the relationships between the SST natural variability and precipitation. © 2013 The Authors..
28. Masamitsu Hayasaki, Ryuichi Kawamura, Masato Mori, Masahiro Watanabe, Response of extratropical cyclone activity to the Kuroshio large meander in northern winter, GEOPHYSICAL RESEARCH LETTERS, 10.1002/grl.50546, 40, 11, 2851-2855, 2013.06, We examined possible responses of cyclone activities to the bimodal path states of the Kuroshio Current [i.e., large meander (LM) and non-LM (NLM)] by using the long-term reanalysis data and the 20th century hindcast experiment of a high-resolution atmosphere-ocean coupled model. Compared with a seasonal mean cyclone track frequency for the LM and NLM periods, a primary cyclone track shifts southward in association with the meander of Kuroshio Current. Composite analyses of the hindcast experiment showed remarkable atmospheric responses accompanying the Kuroshio LM. The Kuroshio LM causes a decrease in latent heat flux in the south of Japan and a southward shift of the near-surface baroclinic zone. Distinctive decreases in thermodynamic fluxes inhibit the rapid development of cyclones in the meander region, eventually inducing positive sea level pressure anomalies downstream from that region..
29. Yoshimitsu Chikamoto, Masahide Kimoto, Masayoshi Ishii, Takashi Mochizuki, Takashi T. Sakamoto, Hiroaki Tatebe, Yoshiki Komuro, Masahiro Watanabe, Toru Nozawa, Hideo Shiogama, Masato Mori, Sayaka Yasunaka, Yukiko Imada, An overview of decadal climate predictability in a multi-model ensemble by climate model MIROC, Climate Dynamics, 10.1007/s00382-012-1351-y, 40, 5-6, 1201-1222, 2013.05, Decadal climate predictability is examined in hindcast experiments by a multi-model ensemble using three versions of the coupled atmosphere-ocean model MIROC. In these hindcast experiments, initial conditions are obtained from an anomaly assimilation procedure using the observed oceanic temperature and salinity with prescribed natural and anthropogenic forcings on the basis of the historical data and future emission scenarios in the Intergovernmental Panel of Climate Change. Results of the multi-model ensemble in our hindcast experiments show that predictability of surface air temperature (SAT) anomalies on decadal timescales mostly originates from externally forced variability. Although the predictable component of internally generated variability has considerably smaller SAT variance than that of externally forced variability, ocean subsurface temperature variability has predictive skills over almost a decade, particularly in the North Pacific and the North Atlantic where dominant signals associated with Pacific decadal oscillation (PDO) and the Atlantic multidecadal oscillation (AMO) are observed. Initialization enhances the predictive skills of AMO and PDO indices and slightly improves those of global mean temperature anomalies. Improvement of these predictive skills in the multi-model ensemble is higher than that in a single-model ensemble. © 2012 The Author(s)..
30. Takashi T. Sakamoto, Yoshiki Komuro, Teruyuki Nishimura, Masayoshi Ishii, Hiroaki Tatebe, Hideo Shiogama, Akira Hasegawa, Takahiro Toyoda, Masato Mori, Tatsuo Suzuki, Yukiko Imada, Toru Nozawa, Kumiko Takata, Takashi Mochizuki, Koji Ogochi, Seita Emori, Hiroyasu Hasumi, Masahide Kimoto, MIROC4h-A New High-Resolution Atmosphere-Ocean Coupled General Circulation Model, JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN, 10.2151/jmsj.2012-301, 90, 3, 325-359, 2012.06, A new high-resolution atmosphere-ocean coupled general circulation model named MIROC4h has been developed, and its performance in a 120-year control experiment (including a 50-year spin-up) under the present conditions (the year 1950) is examined. The results of the control experiment by MIROC4h are compared with simulations of preindustrial conditions carried out for the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) using the previous high- and medium-resolution versions of the model, called MIROC3h and MIROC3m, respectively. A major change in MIROC4h is a doubling of the resolution of the atmospheric component to 0.5625 degrees, compared to 1.125 degrees for MIROC3h. The oceanic components of MIROC4h and MIROC3h are eddy-permitting, with a horizontal resolution of 0.28125 degrees (zonal) x 0.1875 degrees (meridional). In MIROC3m, the horizontal resolution is 2.8125 degrees for the atmospheric component and 1.40625 degrees (zonal) x 0.56 degrees-1.4 degrees (meridional) for the ocean component.
Compared with MIROC3h and MIROC3m, many improvements have been achieved; for example, errors in the surface air temperature and sea surface temperature are smaller, there is less drift of the ocean water temperature in the subsurface-deep ocean, and the frequency of heavy rain is comparable to observations. The fine horizontal resolution in the atmosphere makes orographic wind and its effects on the ocean more realistic than those of the former models, and the treatment of coastal upwelling motion in the ocean has been improved. Phenomena in the atmosphere and ocean related to the El Nino and southern oscillation are now closer to observations than was obtained by MIROC3h and MIROC3m. The effective climate sensitivity for CO2 doubling is calculated to be about 5.7 K, which is much larger than the value obtained using the IPCC AR4 models, and is mainly due to a decrease in the low-level clouds at low latitudes..
31. Takashi Mochizuki, Yoshimitsu Chikamoto, Masahide Kimoto, Masayoshi Ishii, Hiroaki Tatebe, Yoshiki Komuro, Takashi T. Sakamoto, Masahiro Watanabe, Masato Mori, Decadal Prediction Using a Recent Series of MIROC Global Climate Models, JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN, 10.2151/jmsj.2012-A22, 90A, 373-383, 2012.02, In line with the experimental design for near-term climate prediction toward the 5th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC-AR5) and the Coupled Model Intercomparison Project Phase 5 (CMIP5), we perform ensembles of initialized decadal hindcast experiments using two recent versions of the Model for Interdisciplinary Research On Climate (MIROC): MIROC4h (T213L56 AGCM and 1/6-1/4 deg. 48 level OGCM) and MIROC5 (T85L40 AGCM and 0.56-1.4 deg. 50 level OGCM). We analyze sets of 10-year-long 9-ensemble hindcasts (3 members by MIROC4h and 6 members by MIROC5) with initialization every five years after 1961 and explore the predictability of decadal climate changes.
The most predictable variation on decadal timescales is the global warming signal due to the favorable response of the models to external forcing. The results of these initialized hindcast experiments using MIROC5 validate our ability to enhance decadal predictability primarily through the initialization, particularly of the Pacific Decadal Oscillation (PDO) for a few years and of the Atlantic Multidecadal Oscillation (AMO) for almost a decade. The initialization has large impacts on the upper ocean temperature hindcasts over the mid- and high latitudes of the North Pacific and the high latitudes of the North Atlantic, where the PDO and AMO signals are observed to be strongest. In contribution to process and assessment studies in IPCC-AR5 and CMIP5, further analysis of our hindcast data (and near-term prediction data) using MIROC4h and MIROC5 is worthwhile. We note that the initialized hindcasts using MIROC4h have predictive skill inferior to the MIROC5 results and that at this stage, fully significant discussions may not be possible due to the small number of ensembles with limited computational resources..
32. Yoshimitsu Chikamoto, Masahide Kimoto, Masayoshi Ishii, Masahiro Watanabe, Toru Nozawa, Takashi Mochizuki, Hiroaki Tatebe, Takashi T. Sakamoto, Yoshiki Komuro, Hideo Shiogama, Masato Mori, Sayaka Yasunaka, Yukiko Imada, Hiroshi Koyama, Masato Nozu, Fei-fei Jin, Predictability of a Stepwise Shift in Pacific Climate during the Late 1990s in Hindcast Experiments Using MIROC, JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN, 10.2151/jmsj.2012-A01, 90A, 1-21, 2012.02, Sea surface temperature (SST) predictability in the Pacific on decadal timescales is examined in hindcast experiments using the coupled atmosphere-ocean model MIROC with low, medium, and high resolutions. In these hindcast experiments, initial conditions are obtained from an anomaly assimilation procedure using the observed oceanic temperature and salinity while prescribing natural and anthropogenic forcing based on the IPCC concentration scenarios. Our hindcast experiments show the predictability of SST in the western subtropical Pacific, the Indian Ocean, and the tropics to the North Atlantic. Previous studies have examined the SST predictability in the Indian Ocean and the Atlantic, but SST predictability in the western subtropical Pacific has not been evaluated. In the western Pacific, the observed SST anomalies in the subtropics of both hemispheres increased rapidly from the early 1990s to the early 2000s. While this SST warming in the western subtropical Pacific is partly explained by global warming signals, the predictions of our model initialized in 1995 or 1996 tend to simulate the pattern of the SST increase and the associated precipitation changes. This large climate change around the late 1990s may be related to phenomena such as the recent increase in the typhoon frequency in Taiwan and the weakened East Asian monsoon reported by recent studies..
33. Hiroaki Tatebe, Masayoshi Ishii, Takashi Mochizuki, Yoshimitsu Chikamoto, Takashi T. Sakamoto, Yoshiki Komuro, Masato Mori, Sayaka Yasunaka, Masahiro Watanabe, Koji Ogochi, Tatsuo Suzuki, Teruyuki Nishimura, Masahide Kimoto, The Initialization of the MIROC Climate Models with Hydrographic Data Assimilation for Decadal Prediction, JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN, 10.2151/jmsj.2012-A14, 90A, 275-294, 2012.02, This paper documents the procedure of ocean data assimilation that initializes the climate models MIROC3m, MIROC4h, and MIROC5 for decadal climate predictions following the CMIP5 protocol, and summarizes the performance of the climate models using this data assimilation. Only anomalies of observed ocean hydrographic data are assimilated using the incremental analysis update method in order to prevent model climate drifts during predictions. In the case of MIROC4h, which has an eddy-permitting ocean model, a spatial smoother is used in calculating analysis increments so that oceanic mesoscale eddies cannot be damped by observational constraints and that they are generated and decay physically in response to the assimilated background state. Globally, the decadal-scale variations of ocean temperatures in the assimilation runs are highly correlated with the observations. Variations of surface air temperature over oceans are also consistent with the observations, but this is not the case in some regions over continents. Atmospheric responses to the SST variations corresponding to the Pacific Decadal Oscillations and the Atlantic Multi-decadal Oscillation are better represented in MIROC4h and MIROC5 than in MIROC3m. The high resolution of MIROC4h and new cloud parameterizations in MIROC5 may contribute to this improvement. Root-mean-squared amplitudes of sea surface height variations associated with oceanic eddies (hereafter, eddy activity) are not suppressed undesirably in the MIROC4h assimilation run and these are comparable with those in the uninitialized runs. In the Kuroshio-Oyashio confluence zone, eddy activity is modulated on a decadal timescale. This modulation is reasonably represented in the assimilation run compared with the observations. in the hindcast experiments, significant decadal prediction skills are found for the North Atlantic, the subtropical North Pacific, and the Indian Ocean. The decadal climate predictions are expected to contribute to the IPCC AR5 and political decision-making for the coming decades..
34. Takahiro Toyoda, Toshiyuki Awaji, Nozomi Sugiura, Shuhei Masuda, Hiromichi Igarashi, Yuji Sasaki, Yoshihisa Hiyoshi, Yoichi Ishikawa, Takashi Mochizuki, Takashi T. Sakamoto, Hiroaki Tatebe, Yoshiki Komuro, Tatsuo Suzuki, Teruyuki Nishimura, Masato Mori, Yoshimitsu Chikamoto, Sayaka Yasunaka, Yukiko Imada, Miki Arai, Masahiro Watanabe, Hideo Shiogama, Toru Nozawa, Akira Hasegawa, Masayoshi Ishii, Masahide Kimoto, Impact of the Assimilation of Sea Ice Concentration Data on an Atmosphere-Ocean-Sea Ice Coupled Simulation of the Arctic Ocean Climate, SOLA, 10.2151/sola.2011-010, 7, 37-40, 2011.07, We have investigated the effects of assimilating sea ice concentration (SIC) data on a simulation of Arctic Ocean climate using an atmosphere-ocean-sea ice coupled model. Our results show that the normal overestimation of summertime SIC in the East Siberian Sea and the Beaufort Sea in simulations without sea-ice data input can be greatly reduced by assimilating sea-ice data and that this improvement is also evident in a following hindcast experiment for 3-4 years after the initialization of the assimilation. In the hindcast experiment, enhanced heat storage in both sea ice and in the ocean surface layer plays a central role in improving the accuracy of the sea ice distribution, particularly in summer. Our detailed investigation suggests that the ice-albedo feedback and the feedback associated with the atmospheric pressure pattern generated by the improved estimation of SIC work more effectively to retain the heat signal after initialization for a coupled atmosphere-ocean-sea ice system prediction. In addition, comparison with field observations confirms that the model fails to produce a realistic feedback loop, which is (presumably) due to inadequacies in both the ice-cloud feedback model and the feedback via the Beaufort Gyre circulation. Further development of coupled models is thus required to better define Arctic Ocean climate processes and to improve the accuracy of their predictions..
35. Takashi Mochizuki, Masayoshi Ishii, Masahide Kimoto, Yoshimitsu Chikamoto, Masahiro Watanabe, Toru Nozawa, Takashi T. Sakamoto, Hideo Shiogama, Toshiyuki Awaji, Nozomi Sugiura, Takahiro Toyoda, Sayaka Yasunaka, Hiroaki Tatebe, Masato Mori, Pacific decadal oscillation hindcasts relevant to near-term climate prediction, PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 10.1073/pnas.0906531107, 107, 5, 1833-1837, 2010.02, Decadal-scale climate variations over the Pacific Ocean and its surroundings are strongly related to the so-called Pacific decadal oscillation (PDO) which is coherent with wintertime climate over North America and Asian monsoon, and have important impacts on marine ecosystems and fisheries. In a near-term climate prediction covering the period up to 2030, we require knowledge of the future state of internal variations in the climate system such as the PDO as well as the global warming signal. We perform sets of ensemble hindcast and forecast experiments using a coupled atmosphere-ocean climate model to examine the predictability of internal variations on decadal timescales, in addition to the response to external forcing due to changes in concentrations of greenhouse gases and aerosols, volcanic activity, and solar cycle variations. Our results highlight that an initialization of the upper-ocean state using historical observations is effective for successful hindcasts of the PDO and has a great impact on future predictions. Ensemble hindcasts for the 20th century demonstrate a predictive skill in the upper-ocean temperature over almost a decade, particularly around the Kuroshio-Oyashio extension (KOE) and subtropical oceanic frontal regions where the PDO signals are observed strongest. A negative tendency of the predicted PDO phase in the coming decade will enhance the rising trend in surface air-temperature (SAT) over east Asia and over the KOE region, and suppress it along the west coasts of North and South America and over the equatorial Pacific. This suppression will contribute to a slowing down of the global-mean SAT rise..
36. Masato Mori, Masahiro Watanabe, Growth and triggering mechanisms of the PNA: A MJO-PNA coherence, JOURNAL OF THE METEOROLOGICAL SOCIETY OF JAPAN, 10.2151/jmsj.86.213, 86, 1, 213-236, 2008.02, A prominent mode of low-frequency variability in the northern extratropical winter known as the Pacific/North American (PNA) teleconnection pattern prevails not only on seasonal but also on intraseasonal timescales. In this study, processes governing the intraseasonal PNA are investigated using daily fields during 1957-2002. The results of the vorticity budget analysis illustrate that the positive phase of the PNA tends to grow by linear processes such as the barotropic energy conversion from the zonally asymmetric climatological flow. For the negative phase of the PNA, nonlinear low-frequency vorticity advection is as important as the linear processes.
Composite life cycle of the PNA shows that at 9 days before the peak a pronounced wave train was observed along the Asian jet stream and it eventually developed to the PNA near the jet exit region. This wave train is found to be excited by divergent winds primarily associated with anomalous convection of the Madden-Julian Oscillation (MJO). Probability density functions of the MJO calculated separately following the polarity of the PNA reveal a phase locking between the PNA and the MJO. When the active (inactive) convection associated with the MJO reaches the Bay of Bengal to the western Pacific, occurrence frequency of the negative (positive) phase of the PNA is the highest. This MJO triggering explains roughly 30% of the total PNA events, suggesting that, even though the PNA may be inherent to the extratropical atmosphere, a specific tropical forcing is of importance to realize the PNA as dominant mode..