Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Meteorological Society  

<title>Abstract</title>We propose a new scheme based on geopotential height fields to detect cutoff lows starting in the preexisting trough stage. The intensity and scale derived from the proposed scheme will allow for a better understanding of the cutoff low life cycle. These cutoff lows often accompany mesoscale disturbances, causing adverse weather-related events, such as intense torrential rainfall and/or tornadoes. The proposed scheme quantifies the geometric features of a depression from its horizontal height profile. The height slope of a line intersecting the depression bottom and the nearest tangential point (optimal slope) locally indicates the intensity and scale of an isolated depression.

The strength of the proposed scheme is that, by removing a local background height slope from a geopotential height field, the cutoff low and its preexisting trough are seamlessly detected as an identical depression. The distribution maps for the detected cutoff lows and preexisting troughs are illustrated along with their intensities, sizes, and local background flows estimated from snapshot height fields. We conducted climatological comparisons of cutoff lows to determine the utility of the proposed scheme.

DOI： 10.1175/mwr-d-20-0255.1

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Other Link： https://journals.ametsoc.org/downloadpdf/journals/mwre/aop/MWR-D-20-0255.1/MWR-D-20-0255.1.xml

Language：English   Publishing type：Research paper (scientific journal)   Publisher：METEOROLOGICAL SOC JAPAN  

This study found that regional snowfall distributions in a Japan-Sea side area of Japan are controlled by intra-seasonal jet variability, particularly the 10-day-timescale quasi-stationary Rossby waves across the Eurasian continent and the atmospheric blocking over the East Asian region. This study was mainly focused on the Niigata area, which is representative of heavy snowfall areas in Japan. Based on previous studies, three types of dominant snowfall distributions were defined: (1) the plain (P) type, which is characterized by heavy snowfall events predominant in coastal regions of the Niigata area; (2) the mountain (M) type, which occurs in the mountainous regions; and (3) the PM type, which occurs across the whole Niigata area.Our results revealed that all distribution types were related to the southward shift of the westerly jet over Japan associated with an intensified trough, i.e., cyclonic anomalies, originating from quasi-stationary Rossby waves along westerly jets over Eurasia (Eurasian jets). The cyclonic anomalies were found to be also related to blocking cyclones because the frequency of blocking events considerably increased in the East Siberian region. The mechanisms leading to trough intensification were different among the events of the three snowfall types. The formation of Siberian blocking with relatively different positions and different paths of quasi-stationary Rossby wave packet propagation along Eurasian jets was evident in the distribution types. Therefore, local-scale snowfall distributions in the Japan-Sea side area are determined by anomalous large-scale circulations, which can be evidently distinguished in the global reanalysis data.

DOI： 10.2151/jmsj.2019-012

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We characterize the differences in the upward planetary-scale wave propagation during observed weak polar vortex (WPV) events between heavy- and light-sea-ice years in the Barents-Kara Sea based on a composite analysis for the period of 1979-2015. Upward wave propagation during WPV events in heavy-ice years is dominated by the wavenumber 1 component. In contrast, WPV events occurring in light-ice years are characterized by stronger wavenumber 2 propagation, which is caused by the tropospheric wavenumber 2 response to sea-ice reduction in the Barents-Kara Sea. The above observed features are supported by an Atmospheric General Circulation Model experiment. Thus, under present climate conditions, Arctic sea-ice loss is a possible factor modulating the wave propagation during the WPV events. We also find that the WPV events in light-ice years have stronger stratosphere-troposphere coupling, followed by colder midlatitude surface conditions particularly over Eurasia.

DOI： 10.1029/2018JD029222

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Publishing type：Research paper (scientific journal)   Publisher：American Geophysical Union (AGU)  

Abstract

Teleconnections between lower and higher latitude regions are widely known in both the Northern and Southern Hemispheres. To broaden our view of these teleconnections, we searched a reanalysis data set for evidence of a teleconnection between the Arctic Oscillation (AO) and the Antarctic Oscillation (AAO), two widely separated circumpolar phenomena. Statistical analysis of the Japanese 55‐year reanalysis data set showed significant in‐phase synchronization between the AO and AAO, particularly in October and February, with a vertical structure extending from the troposphere to the stratosphere. This vertical structure may suggest a stratospheric control, and we did not find a significant signature indicating a tropical ocean control. We also observed decadal‐scale modulation of the synchronicity. Observational evidence implies that the stratospheric meridional circulation may be responsible for AO‐AAO synchronization.

DOI： 10.1029/2018gl081002

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1029/2018GL081002

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Details of the characteristics of upward planetary wave propagation associated with Arctic sea ice loss under present climate conditions are examined using reanalysis data and simulation results. Recent Arctic sea ice loss results in increased stratospheric poleward eddy heat fluxes in the eastern and central Eurasia regions and enhanced upward propagation of planetary-scale waves in the stratosphere. A linear decomposition scheme reveals that this modulation of the planetary waves arises from coupling of the climatological planetary wavefield with temperature anomalies for the eastern Eurasia region and with meridional wind anomalies for the central Eurasia region. Propagation of stationary Rossby wave packets results in a dynamic link between these temperature and meridional wind anomalies with sea ice loss over the Barents-Kara Sea. The results provide strong evidence that recent Arctic sea ice loss significantly modulates atmospheric circulation in winter to modify poleward eddy heat fluxes so as to drive stratosphere-troposphere coupling processes.

DOI： 10.1002/2016GL071893

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Language：Japanese   Publisher：The Japanese Society of Snow and Ice / Japan Society for Snow Engineering  

DOI： 10.14851/jcsir.2017.0_55

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Language：Japanese   Publisher：The Japanese Society of Snow and Ice / Japan Society for Snow Engineering  

DOI： 10.14851/jcsir.2017.0_56

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

We demonstrated atmospheric responses to a reduction in Arctic sea ice via simulations in which Arctic sea ice decreased stepwise from the present-day range to an ice-free range. In all cases, the tropospheric response exhibited a negative Arctic Oscillation (AO)-like pattern. An intensification of the climatological planetary-scale wave due to the present-day sea ice reduction on the Atlantic side of the Arctic Ocean induced stratospheric polar vortex weakening and the subsequent negative AO. Conversely, strong Arctic warming due to ice-free conditions across the entire Arctic Ocean induced a weakening of the tropospheric westerlies corresponding to a negative AO without troposphere-stratosphere coupling, for which the planetary-scale wave response to a surface heat source extending to the Pacific side of the Arctic Ocean was responsible. Because the resultant negative AO-like response was accompanied by secondary circulation in the meridional plane, atmospheric heat transport into the Arctic increased, accelerating the Arctic amplification.

DOI： 10.1002/2016GL070526

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Recent evidence from both observations and model simulations suggests that an Arctic sea ice reduction tends to cause a negative Arctic Oscillation (AO) phase with severe winter weather in the Northern Hemisphere, which is often preceded by weakening of the stratospheric polar vortex. Although this evidence hints at a stratospheric involvement in the Arctic-midlatitude climate linkage, the exact role of the stratosphere remains elusive. Here we show that tropospheric AO response to the Arctic sea ice reduction largely disappears when suppressing the stratospheric wave mean flow interactions in numerical experiments. The results confirm a crucial role of the stratosphere in the sea ice impacts on the midlatitudes by coupling between the stratospheric polar vortex and planetary-scale waves. Those results and consistency with observation-based evidence suggest that a recent Arctic sea ice loss is linked to midlatitudes extreme weather events associated with the negative AO phase.

DOI： 10.1002/2016GL068330

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：METEOROLOGICAL SOC JAPAN  

Synoptic conditions causing an extreme snowfall event in the Kanto-Koshin district occurred on 14-15 February 2014 are investigated through a reanalysis data set. Associated with a developing cyclone passing the south coast of Japan, persistent snowfall exceeding more than 24 hours over the Kofu-Basin resulted in 112 cm snowfall at Kofu. Slow progress of the south-coast cyclone also contributed to the long snowfall duration. An anticyclone developed over the northern Japan (similar to 1032 hPa) also contributed to this extreme snowfall. This anticyclone brought warm and moist air inflow by southeasterlies forming moisture flux convergence over the Kanto-Koshin district on the morning of 14th when snowfall started in the Koshin district in spite that the south-coast cyclone was located to the south of Kyushu. Further, ageostrophic cold northerlies with high pressure extension from the anticyclone by "cold-air damming (CAD)" would suppress warming with the approaching south-coast cyclone and keep snowfall until the morning of 15th. In other four heavy snowfall events at Kofu, snowfall durations were almost 12 hours. Although anticyclone to the north and CAD were identified in each case, the moisture transport from the southeast was not evident and moisture flux convergence was not formed earlier.

DOI： 10.2151/sola.2016-051

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER METEOROLOGICAL SOC  

Contemporary climate science seeks to understand the rate and magnitude of a warming global climate and how it impacts regional variability and teleconnections. One of the key drivers of regional climate is the observed reduction in end of summer sea-ice extent over the Arctic. Here the authors show that interannual variations between the September Arctic sea-ice concentration, especially in the East Siberian Sea, and the maximum Okhotsk sea-ice extent in the following winter are positively correlated, which is not explained by the recent warming trend only. An increase of sea ice both in the East Siberian Sea and the Okhotsk Sea and corresponding atmospheric patterns, showing a seesaw between positive anomalies of sea level pressures over the Arctic Ocean and negative anomalies over the midlatitudes, are related to cold anomalies over the high-latitude Eurasian continent. The patterns of atmospheric circulation and air temperatures are similar to those of the annually integrated Arctic Oscillation (AO). The negative annual AO forms colder anomalies in autumn sea surface temperatures both over the East Siberian Sea and the Okhotsk Sea, which causes heavy sea-ice conditions in both seas through season-to-season persistence.

DOI： 10.1175/JCLI-D-14-00297.1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

This paper examines the possible linkage between the recent reduction in Arctic sea-ice extent and the wintertime Arctic Oscillation (AO)/North Atlantic Oscillation (NAO). Observational analyses using the ERA interim reanalysis and merged Hadley/Optimum Interpolation Sea Surface Temperature data reveal that a reduced (increased) sea-ice area in November leads to more negative (positive) phases of the AO and NAO in early and late winter, respectively. We simulate the atmospheric response to observed sea-ice anomalies using a high-top atmospheric general circulation model (AGCM for Earth Simulator, AFES version 4.1). The results from the simulation reveal that the recent Arctic sea-ice reduction results in cold winters in mid-latitude continental regions, which are linked to an anomalous circulation pattern similar to the negative phase of AO/NAO with an increased frequency of large negative AO events by a factor of over two. Associated with this negative AO/NAO phase, cold air advection from the Arctic to the mid-latitudes increases. We found that the stationary Rossby wave response to the sea-ice reduction in the Barents Sea region induces this anomalous circulation. We also found a positive feedback mechanism resulting from the anomalous meridional circulation that cools the mid-latitudes and warms the Arctic, which adds an extra heating to the Arctic air column equivalent to about 60% of the direct surface heat release from the sea-ice reduction. The results from this high-top model experiment also suggested a critical role of the stratosphere in deepening the tropospheric annular mode and modulation of the NAO in mid to late winter through stratosphere-troposphere coupling.

DOI： 10.1002/2014JD022848

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：METEOROLOGICAL SOC JAPAN  

The influence of large-scale atmospheric blocking over the northwestern Pacific on heavy snowfall events in the Kanto area and on the Pacific Ocean side of northern Japan (PNJ) within the time scale of similar to 10 days was examined through blocking case studies. The past 14 blocking cases over the Pacific, including the blocking during early- to mid-February 2014 that influenced a record-breaking snowfall over the Kanto area, were analyzed using a long-term reanalysis dataset and local meteorological observation station data. Results reveal that blocking over the Pacific causes large precipitation over the Kanto area and the PNJ by shifting cyclone (storm) tracks towards the east coast of Japan from their usual eastward course across the mid-Pacific via the south coast of Japan. Excessive passing of cyclones caused large precipitation in the Kanto area and snowfall in the PNJ. In the blocking cases with heavy snowfall events in the Kanto area, a strong cold-air inflow over Japan also existed in the lower troposphere originating from east Siberia, which initiated synoptic ground cold-air environments in the Kanto area.

DOI： 10.2151/sola.2015-013

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Language：Japanese   Publisher：The Japanese Society of Snow and Ice / Japan Society for Snow Engineering  

DOI： 10.14851/jcsir.2015.0_59

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Language：Japanese   Publisher：The Japanese Society of Snow and Ice / Japan Society for Snow Engineering  

DOI： 10.14851/jcsir.2015.0_57

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Language：Japanese   Publisher：The Japanese Society of Snow and Ice / Japan Society for Snow Engineering  

DOI： 10.14851/jcsir.2015.0_235

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：METEOROLOGICAL SOC JPN  

In the winter of 2009/2010, Japan and the East Asian region experienced a frequent occurrence of cold air outbreaks. Although the winter average temperature in the Japan main islands was slightly positive (+0.81 degrees C for DJF average and +0.71 degrees C for NDJFM average), repeated decline in temperature was notable throughout the season. One explanation for this abnormal winter season is the extremely negative condition of the Arctic Oscillation (AO) that persisted from December to mid-January. However, AO alone does not provide sufficient explanation for the cold air outbreak during November or its intraseasonal periodicity. A case study of the cold air outbreak that reached Japan on Dec. 18 reveals an anomalous ridge forming over the Barents-Kara Sea, which leads to the cold air accumulation over western Siberia. The pressure anomaly subsequently shifted westward to mature into a blocking high which created a wave-train pattern downstream, advecting the cold air eastward towards East Asia and Japan. The sequence of events was also apparent in multiple cases throughout the season. This study suggests that there is a strong and systematic linkage in the intraseasonal timescale between the atmospheric condition over the Barents-Kara Sea and the cold air accumulation over the Eurasian continent, leading to the anomalous cold air outbreak over East Asia and Japan. The mechanism may also provide explanation to extreme winter conditions such as those observed during the winter of 2010/2011.

DOI： 10.2151/sola.2011-007

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Language：English   Publishing type：Part of collection (book)   Publisher：American Geophysical Union  

DOI： 10.1029/2008gm000789

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：METEOROLOGICAL SOC JAPAN  

Using an ensemble of wintertime hindcasts with a high-resolution (T106L60) Atmospheric General Circulation Model (AGCM) forced by observed sea surface temperatures (SSTs) and extending into the stratosphere, we investigate the formation and lifecycle of the Aleutian-Icelandic low Seesaw (AIS) during the 1978 to 1993 period. The AIS has been newly proposed to be an important mode of variability, linking the major wintertime surface lows, the Icelandic Low and the Aleutian Low, in late winter, and thereby linking climate variability over the North Pacific and the North Atlantic. We demonstrate for the first time with a stratosphere-troposphere model, that a coherent, ensemble-mean AIS extension into the stratosphere exists, where its presence modulates ultra-long planetary wave propagation and the polar night jet intensity. The model AIS peaks in February, when the Aleutian and Icelandic Low and-correlation maximizes at -0.59. The AIS provides a new way to describe the El Nino-Southern Oscillation (ENSO) phenomenon influence into the stratosphere, For example, El-Nino conditions correspond to a deeper than normal Aleutian Low, extending its influence into the Icelandic sector as an AIS negative phase (weakened Icelandic Low), hence enhanced planetary wave vertical propagation and a weakened stratospheric polar vortex. This maturation of the AIS in late winter explains the intra-seasonal variability of the stratospheric response to ENSO, which peaks in late winter. Internal model variability is large and enhanced potential predictability is found primarily in the western hemisphere, with a western Atlantic maxima being more pronounced in the stratosphere than in the upper troposphere.

DOI： 10.2151/jmsj.86.817

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：METEOROLOGICAL SOC JAPAN  

Inter-basin links between the North Pacific and North Atlantic in the wintertime upper-tropospheric circulation over the extratropical Northern Hemisphere, including their structure, dominance and seasonal dependence, are examined based on the structures of the leading EOFs for monthly height anomalies for the second half of the last century. Dominant variability in the upper troposphere is found to reflect the corresponding modulations in the strength of the inter-basin dynamical link between the North Pacific and North Atlantic. The first and second EOFs for the upper troposphere represent hemispheric patterns, showing the coherent variability between the North Pacific and North Atlantic. They essentially reflect the respective out-of-phase and in-phase relationships between the Icelandic low (IL) and Aleutian low (AL), as upper-level manifestations of the AL-IL seesaw and the cold ocean-warm land (COWL) pattern, respectively. Dominance of the AL-IL seesaw in the upper-tropospheric leading EOF is a manifestation of a dynamical linkage between the North Atlantic Oscillation and Pacific-North American pattern. The manifestation of the COWL pattern in the second EOF is consistent with a pronounced wintertime warming trend over landmasses and concomitant deepening of the AL and IL observed in the last three decades. The seasonal evolution of the AL-IL seesaw and COWL pattern also manifest themselves in the seasonal modulations in the first and second EOFs of the upper-tropospheric variability, respectively.

DOI： 10.2151/jmsj.85.899

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Language：Japanese   Publishing type：Research paper (scientific journal)  

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Other Link： http://dl.ndl.go.jp/info:ndljp/pid/10607142

Language：English   Publishing type：Research paper (scientific journal)   Publisher：CO-ACTION PUBLISHING  

An analysis of satellite sea-ice records for recent decades reveals a highly coherent spatial and temporal structure of the Northern Hemisphere (NH) wintertime sea-ice variability and its close link to anomalous atmospheric circulation. The dominant mode of the wintertime sea-ice variability is characterized by a double-dipole composed of one dipole over the North Atlantic and the other over the North Pacific, which are mutually correlated interannually. This dominant sea-ice mode is lag correlated with the winter-averaged North Atlantic Oscillation (NAO) index at lags up to two winters when the NAO leads. In the sub-Arctic, significant lead-lag relationships exist between sea-ice extent (SIE) anomalies on regional scales, which are closely associated with atmospheric circulation anomalies. An eastward evolving pattern is identified in regional SIE anomalies from the Labrador to Nordic and farther to the Okhotsk Sea at multi-year time-scales, led by anomalously weak Aleutian and strong Icelandic lows. The results suggest the presence of climate memories over the North Atlantic and Eurasia, which are crucial for recent downward trends in the NH SIE by transforming atmospheric influences into slower changes in sea-ice conditions. The summer Okhotsk high, which leads to a sea-ice reduction along the east Siberian coast and further affects sea-ice conditions over the Arctic Ocean, is a key link between summer Arctic and winter sub-Arctic sea-ice trends. We also conjecture that variations and changes in the NH sea-ice conditions are linked to climate variability in the tropics.

DOI： 10.1111/j.1600-0870.2006.00223.x

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Language：Japanese   Publisher：THE JAPANESE SOCIETY OF SNOW AND ICE  

DOI： 10.14850/jssi.2007.0.194.0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

An intra-seasonal relationship, including a possible feedback, is investigated between the North Atlantic Oscillation (NAO) and a hemispheric-scale seesaw-like pattern dominant in sea ice variability over the wintertime Northern Hemisphere, with one polarity in the Bering and Labrador Seas and the other in the Okhotsk and Greenland-Barents Seas. Statistical analysis using observational data suggests that a particular phase of NAO and anomalous atmospheric circulation in the Pacific during early winter contribute toward the development of the midwinter hemispheric sea-ice seesaw. In contrast, the ice seesaw tends to damp the preexisting NAO signal during late winter, indicating a reversal of the phase relation between them. This NAO damping may be triggered by the Pacific sea-ice anomalies. Results from numerical experiments generally support this notion and further suggests a stationary Rossby wave train emanated from the North Pacific as a dynamical mechanism for damping the NAO signal.

DOI： 10.1029/2006GL026286

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

The influence of the Northern Hemisphere annular mode (NAM) on the El Nino/Southern Oscillation (ENSO) was examined using 41-year reanalysis data and an atmospheric general circulation model (AGCM). Significant lag correlations between the NAM index for spring and the Nino-3 index for the following winter were revealed. An anomalous lower tropospheric westerly associated with modulation of the westerly wind burst (WWB) over the western tropical Pacific in spring often coincided with the positive phase of the NAM and extended eastward during summer and autumn. Then, warm sea surface temperature (SST) anomalies reflecting El Nino appeared over the eastern tropical Pacific in early winter. In an AGCM experiment in which SSTs were fixed as a climatological-mean monthly distribution, the interannual modulation of WWB was significantly associated with NAM variability in spring, supporting the possible influence of the NAM in spring on an ENSO outbreak the following winter.

DOI： 10.1029/2005GL025432

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：METEOROLOGICAL SOC JPN  

The results of the empirical orthogonal function (EOF) analysis applied to the sea-level pressure (SLP) variability in the extratropical Northern Hemisphere is reexamined from viewpoint of statistical independence. For this purpose, "Independent Component Analysis" is introduced, whose effectiveness is demonstrated in its application to a set of simple hypothetical models. While its application to stratospheric variability confirms the relevance of the EOF analysis, its application to the SLP suggests that the two leading components of the EOF analysis are likely superpositions of two statistically independent signals. One of them represents a seesaw-like relationship in the intensities between the surface Aleutian and Icelandic lows, and the other is characterized by a significant long-term trend with the cold ocean-warm land pattern.

DOI： 10.2151/sola.2006-002

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER METEOROLOGICAL SOC  

An interannual seesaw between the intensities of the Icelandic and Aleutian lows and its impact on surface climate observed during the twentieth century are investigated. In a recent period from the late 1960s to the early 1990s, their seesaw relationship was particularly apparent in late winter. The associated anomalies in surface air temperature were significant in many regions over the extratropical Northern Hemisphere except in central portions of the continents. The seesaw also modified the ocean-atmosphere exchange of heat and moisture extensively over the North Atlantic and North Pacific by changing evaporation and precipitation. Since the seesaw formation was triggered by eastward propagation of stationary Rossby wave trains from the North Pacific into the North Atlantic, anomalous circulation over the North Pacific in January was identified as a good precursor for February surface air temperatures in the Euro-Atlantic sector during that period.
The seesaw relationship between the two lows underwent multidecadal modulations during the twentieth century. It was weak in the mid-1950s through the mid-1960s, while it was particularly strong during the preceding period from the 1920s to the 1940s with its impact on surface temperatures as extensive as in the recent period. Although it reached maturity in January, the precursory signal of the seesaw in that early period was also found in the North Pacific one month earlier, which suggests that the formation was through essentially the same mechanisms as in the recent period.

DOI： 10.1175/JCLI3419.1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER METEOROLOGICAL SOC  

The potential predictability associated with the remote influence of midlatitude tropospheric anomalies over the North Pacific or the North Atlantic via a seesawlike interannual oscillation between the surface Aleutian and Icelandic lows (AL and IL, respectively) is investigated. Data from a 24-member ensemble of 50-yr atmospheric general circulation model simulation forced with observed sea surface temperature (SST) conditions are analyzed by separating the total simulated fluctuations into the external component forced by the prescribed SST and the internal component generated by atmospheric internal dynamics. The AL-IL seesaw can be identified in both the external and internal components of the variability. In the external variability, determined through the ensemble mean, the seesaw is gradually formed from December to March through the development of a Pacific-North American (PNA) patternlike wave train, remotely forced by the El Ni (n) over tildeo-Southern Oscillation. The amplitudes of the externally forced North Atlantic anomalies are only about half as large as the North Pacific anomalies. The potential predictability of the Atlantic anomalies, defined as the ratio of the SST-forced variance to the total variance, does not exceed the 20% level. In the internal component of the variability, determined from the deviations of each ensemble member from the ensemble mean, the negative correlation between the AL and IL anomalies is modest but persistent through winter. It is confirmed that, regardless of the polarity of the AL-IL seesaw, the IL anomalies are formed through eastward wave activity propagation of the stationary Rossby wave train emanating from the AL region in the form of what may be called a '' PNAA pattern,'' the extension of the PNA-like wave train into the Atlantic. Thus, the midwinter development of North Pacific anomalies is found to be a necessary, though not sufficient, condition for the seesaw formation. The persistence of the North Pacific anomalies beyond a 1-month time span appears to augment the probability of the seesaw formation by sustaining eastward wave activity propagation to the North Atlantic.

DOI： 10.1175/JCLI3353.1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：METEOROLOGICAL SOC JPN  

Heat transfer coefficients, and relative importance of factors affecting surface turbulent heat flux in sea-ice covered ocean, were investigated using data obtained by rawinsonde observations over, and around, the southwestern region of the Sea of Okhotsk from Jan. to Feb. in 1998, 1999 and 2000. The range of the fluxes estimated by an atmospheric heat budget analysis was large, associated with the ice concentration and synoptic situation. The turbulent heat fluxes from the open water surrounded by the sea ice are always positive (upward). However, the heat flux through sea-ice surface tends to be negative (downward) over sea-ice area associated with the relationship between the air and ice-surface temperature. Using the obtained turbulent heat fluxes and a bulk formula, relationships of turbulent heat fluxes with ice concentrations and atmospheric parameters were investigated, and a representative value of the bulk heat transfer coefficient was obtained. The estimated conditional bulk heat transfer coefficient (1.33 x 10(-3)) in the Sea of Okhotsk shows a smaller value than that in the Arctic region, reflecting the difference of the ice conditions, and thermal conditions in the boundary layer. Through an idea of the insulating coefficient, which measures degrees of the insulating effect of sea ice on the heat transfer, we explained that the sea ice works as a more effective insulator in weaker cold-air outbreaks. From a regression analysis, we found that the primary contribution to the amount of turbulent heat flux is the ice concentration, and second is the air temperature in the boundary layer.

DOI： 10.2151/jmsj.81.1057

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Language：Japanese   Publishing type：Research paper (scientific journal)  

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Other Link： http://dl.ndl.go.jp/info:ndljp/pid/10604742

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Meteorological Society of Japan  

Three-dimensional structure of quasi-stationary circulation anomalies observed in the course of a typical life cycle of an interannual seesaw between the surface Aleutian and Icelandic lows (AL and IL, respectively) is examined by using a reanalysis data set for the last three decades. A diagnosis is applied in a particular framework where the 31-day mean anomalies are regarded as stationary Rossby waves embedded in the zonally varying climatological-mean flow. It reveals that the upward propagation of wave activity into the stratosphere occurs in late winter primarily from the tropospheric anomalies corresponding to the anomalous IL, which develops below the entrance region of the lower-stratospheric polar night jet as a remote influence from the North Pacific. Accordingly, the North Atlantic anomalies exhibit apparent amplification and westward phase tilt with height, whereas the North Pacific anomalies are much more like the external mode. It appears that, in the presence of the zonal wavenumber 1 (k = 1) component of the climatological-mean planetary waves, the polar-night jet over the Pacific is shifted too far north to allow the stationary anomalies associated with the anomalous AL to propagate upward as stationary Rossby waves. Unlike the predominant signal of the Arctic Oscillation (or annular mode) that alters the intensity of the polar vortex, the seesaw modifies the stratospheric planetary-wave patterns in a modest but significant manner. In late winter when the AL is weaker than normal associated with a particular phase of the seesaw, the k = 1 component is masked by the enhanced zonal wavenumber 2 (k = 2) component embedded in the intensified polar-night jet. In contrast, the predominant k = 1 component embedded in the relatively weak polar-night jet dominates over the diminished k = 2 component in late winter, when the AL is stronger associated with the other phase of the seesaw. Our examination of seven late-winter major events of stratospheric sudden warming over the three recent decades suggests that the polarity of the AL-IL seesaw might set up a condition of which planetary wave component (k = 1 or k = 2) is more strongly involved in a late-winter warming event.

DOI： 10.2151/jmsj.80.1051

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：METEOROLOGICAL SOC JAPAN  

Rawinsonde observation was performed in late January through early February in 1998 around the southwestern part of the Sea of Okhotsk to estimate the turbulent fluxes of sensible and latent heat over the ice-covered ocean during the winter monsoon. Upstream cold and stable air mass originated from the Eurasian continent was significantly modified through heat and moisture supply from warm sea surface with offshore cold-air outbreaks, which consequently formed mixed layer characterized by neutral stability over downstream areas. Associated with reduction of the top of the mixed layer height through the observational period, estimated heat fluxes also decreased gradually from 210 W m(-2) to 30 W m(-2). These decrease tendencies of the mixed layer height and estimated heat fluxes may reflect the increase of the insulating effect of the sea-ice cover on heat and moisture exchanges between the atmosphere and ocean.

DOI： 10.2151/jmsj.79.687

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：METEOROLOGICAL SOC JAPAN  

Radio sounding and in-situ observations at land-based (26 January-11 February 1998) and ship stations (4-11 February 1998) were performed over the southwestern Sea of Okhotsk covered with sea ice to estimate turbulent heat flux and investigate characteristics of thermodynamic air mass transformation processes during cold-air outbreaks. Surface heat flux estimations were carried out using two indirect methods; an atmospheric heat budget analysis using the radio sounding data at three stations and a bulk method utilizing meteorological and sea ice thickness data obtained by the ship observations. The estimated total upward turbulent heat flux over the analyzed area was approximately 100 W m(-2) even within the intense cold-air outbreak period. The height of the mixing layer associated with this small amount of the heat flux is only about 1 km, which implies that sea ice acts as an insulating material between the ocean and atmosphere and thus significantly reduces the turbulent heat fluxes. Nevertheless, the upward turbulent heat flux from the ice surface was found to be non-negligible during cold-air outbreaks and it is suggested that the sea ice grows through the sea or ice surface cooling.

DOI： 10.2151/jmsj.79.657

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER METEOROLOGICAL SOC  

Through analysis of observational data for the period of 1973-94, the late-winter formation of an interannual seesaw between the surface Aleutian and Icelandic lows (AL and IL, respectively) is shown to significantly impact the covariance structure of the leading mode of the interannual variability in the geopotential height field over the extratropical Northern Hemisphere. The tropospheric leading mode for early winter (November-January) is characterized by a polar-midlatitude dipole over the Euro-Atlantic sector with a high degree of the annularity, coupled with the anomalous lower-stratospheric polar vortex. Over the North Pacific, no significant anomalies are associated with this mode. After the formation of the AL-IL seesaw, however, the dipole no longer dominates in the upper-tropospheric variability. The dipole signature is masked in late winter (February-April) by the predominant combined signature of the so-called Pacific-North American pattern and a meridional dipole over the northwestern Atlantic as an upper-level manifestation of the seesaw. Though somewhat less pronounced, the leading mode of the near-surface variability is modified accordingly in late winter by the superposition of the distinct signature of the AL-IL seesaw. The annularity of the leading mode of the tropospheric variability is thus reduced in late winter, particularly at the upper levels. Nevertheless, because of the particular geographical alignment between the anomalous AL and IL, their seesaw changes the zonal wind coherently between the two ocean basins, yielding a strong projection on the meridional plane whose latitudinal profile is almost indistinguishable from the counterpart of the Arctic-midlatitude dipole.
It is argued that what is called the Arctic oscillation in some recent literature, defined as the leading mode of the sea level pressure variability for the entire cold season, may be interpreted as a superposition of the AL-IL seesaw upon a dominant signal of the Arctic-midlatitude dipole. The corresponding leading mode for the upper troposphere primarily represents the variability associated with the seesaw. It is also argued that the late-winter tropospheric variability over the North Atlantic may not necessarily be associated with the Arctic-midlatitude dipole. The remote influence of the North Pacific variability accounts for as much as 30%-50% of the variance in the vicinity of the IL for the data period considered.

DOI： 10.1175/1520-0442(2001)014<4512:ISBTAA>2.0.CO;2

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

An atmospheric general circulation model was used to investigate the influence of Okhotsk sea ice on large-scale atmospheric circulation. Significant differences in the model responses between heavy and light ice cover are evident not only around the Sea of Okhotsk but also downstream towards North America in the form of a stationary wavetrain in the troposphere. Because the wave activity flux associated vith the wavetrain emanates downstream from the Sea of Okhotsk, this remote response is regarded as a stationary Rossby wave excited thermally through an anomalous surface heat flux over the Sea of Okhotsk. The wavetrain is thermally reinforced downstream by the anomalous heat flux associated with wind anomalies over the ocean surface, which are induced by the wavetrain itself.

DOI： 10.1029/96GL03474

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：METEOROLOGICAL SOC JAPAN  

We investigated the interannual variation of the sea ice over the Sea of Okhotsk by using radar observation data and gridded ice-coverage data for the years 1969-1994. It became evident that the abrupt decrease of sea ice over the southern part of the Sea of Okhotsk occurred in 1989, and that this reduced-ice condition has continued since then. In addition, the winter time Aleutian low has abruptly weakened since 1989. According to the lag-correlation analysis, this weakening has brought about the reduction in extent of the ice-coverage.

DOI： 10.2151/jmsj1965.74.4_579

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：National Institute of Polar Research  

Variations of air temperature in Hokkaido during winter are characterized by the presence of two minima in ten-day normals; days 21-30 (end of January) and days 41-50 (middle of February). We name this tendency the "W-phenomenon", which is significant in the northern coastal area of Hokkaido facing the Sea of Okhotsk. The second minimum occurs when the ice cover is most extensive along the Okhotsk coast of Hokkaido. We examine this relation using rotated principal component analysis (RPCA). The RPCA results show that the second mode is a factor of the ice cover that is related to the second minimum. However, the correlation coefficient between rotated principal components of the second mode and ice concentration is rather low, -0.56. The RPCA results also show that the wind field is another factor that governs the "W-phenomenon". The relationship between the rotated principal components and wind directions shows that the drop in temperature in Hokkaido is associated with not only heavy ice cover but also northeasterly wind. Two mechanisms of the northeasterly wind system are expected : weak winds from an anticyclone over Okhotsk ice cover off Hokkaido and strong winds from a cyclone over the Pacific Ocean off the southeast coast of Hokkaido. Both these cases are possibly caused by the heavy ice cover in the Sea of Okhotsk. In the former mechanism, the wind directly carries cold air flow cooled over the sea ice. In the latter, the heavy ice cover prevents cyclones from moving into the Sea of Okhotsk.

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Language：Japanese   Publisher：日本気象学会  

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Language：Japanese   Publisher：新潟大学災害・復興科学研究所  

豪雨・豪雪や突風などの災害をもたらす顕著大気現象を迅速に解析する「準リアルタイム気象解析システム」の構築を目指して，領域メソ気象モデルを用いて2012年8月6日に新潟で発生した突風事例の再現実験を試験的に行った．モデル計算結果には，日本海上を発達しながら東に移動するメソ大気擾乱が佐渡島付近に移動してくる様子がみられた．このメソ大気擾乱は，佐渡島の西岸と東岸のそれぞれの地形に沿う北寄りの風を作り，それらの風は佐渡島の南沖で収束していた．新潟の突風はこの収束により急発達した積雲が引き起こした顕著大気現象と推測された．しかし，モデル計算では新潟の突風の再現にはいたっておらず，今後は，より高解像度の海面水温と気象のデータを用いてモデルの境界条件・初期値を作成し，数値実験の精度向上をはかっていく予定である．

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The Sea of Okhotsk is a shallow mid-latitude marginal sea bordered by the Asian Continent to its west and north. A strong thermal contrast forms in summer between the cool sea surface and the heated landmass, while the thermal gradient is reversed in winter. Under the influence of the continuous spill of extremely cold air form the continent and fresh water supplied by the Amur River, the Okhotsk region becomes the southernmost ice-covered maritime region in the Northern Hemisphere. With regard to how these seasonal characteristics of the surface thermal condition and their interannual variability in and around the Okhotsk region can be related to the variability in hemispheric-scale atmospheric circulation, we discuss the possible in fluence of the anomalous sea-ice extent on large-scale atmospheric circulation and the formation of the summertime cool surface Okhotsk High, based on data analysis and numerical experiments.

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平成11年度共同利用研究集会「北日本の気象と海象」（1999年8月18日～19日, 研究代表者：児玉安正）の講演要旨Atmospheric and oceanographic phenomena around the northern part of Japan(Abstracts of scientific symposia held at Otsuchi Marine Research Center in 1999)

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