The summer Arctic sea ice extent (SIE) during the period 1996−2008 had an accelerating trend of r... more The summer Arctic sea ice extent (SIE) during the period 1996−2008 had an accelerating trend of retreat. The difference of SIE between May and September has a significant correlation with the summer Northern Hemisphere Annular Mode (summer NAM). The summer NAM had a significant positive trend during the period 1958−1996, and a weak negative trend during the period 1996−2008. The atmospheric circulation during the recent period had a trend of anticyclonic circulation over the Arctic with easterly wind over the marginal seas. In addition, tropospheric warming was enhanced especially over the Arctic and near the ground surface.
The relation of interannual connection between Asian dust outbreaks and stratosphere-to-troposphe... more The relation of interannual connection between Asian dust outbreaks and stratosphere-to-troposphere transport (STT) in spring was suggested by the dust and tritium variations in the Mount Wrangell ice core, Alaska in Yasunari et al. (2007). However, these impacts on the ice core site in each event scale have not been investigated. Hence, the present paper focuses on the material transport and deposition processes for further understanding these impacts on the ice core. The variations in dust and tritium concentrations in spring in an ice core taken at Mt. Wrangell, Alaska are explained by meteorological analysis and simulation of trajectories associated with Asian dust outbreaks and STT. Material transport and deposition at Mt. Wrangell are examined in two contrasting years (2001 and 2002). Dust and tritium concentrations both reached peak values in the early spring of 2002, while the dust peak occurred in early spring and the tritium peak occurred in late spring in 2001. Six severe East Asian transpacific dust storms over this period are modeled by forward trajectory and meteorologically analyzed. It is found that 5 of 6 events contributed to the ice core record in Alaska. Stratospheric air is also transported to the ice core site in most cases. Tritium deposition is found to have been suppressed in the cases of the 2001 dust storms due to lack of snowfall at appropriate times. Taken the detailed transport and deposition processes after the severe dust storms with atmospheric circulations into account, we can well explain spring dust and tritium variations in the Mount Wrangell ice core.
Los Angeles, CA 90024 Monterey, CA 93943-5006 8a NAME OF FUNDING,SPONSORNG 8b OFFICE SYMBOL 9. PR... more Los Angeles, CA 90024 Monterey, CA 93943-5006 8a NAME OF FUNDING,SPONSORNG 8b OFFICE SYMBOL 9. PROCURE'.IENT INSTRUMENT IDENTIFICATION NUMBER ORGANIZATION It app0 ca b e) N00014-80-K-0947 (0NR) Naval Air Systems Command AIR-330 (suppl to NSF Grant ATM-8218215) 8c ADDRESS (City, State, and ZIP Code) 10 SOURCE OF FUNDING NUMBERS
Recent evidence from both observations and model simulations suggests that an Arctic sea ice redu... more 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.
[1] The seasonal variations of the Northern Hemisphere annular mode (NAM) are investigated throug... more [1] The seasonal variations of the Northern Hemisphere annular mode (NAM) are investigated through empirical orthogonal function analysis of the zonally averaged geopotential height fields for each individual calendar month. Patterns of the winter and summer NAMs differ not only in the geopotential height fields but also in the mean meridional circulation and eddy structure. The summer NAM has a smaller meridional scale and is displaced poleward as compared to the winter NAM. The antinode on the lower-latitude side in the summer NAM is at the nodal latitude of the winter NAM. The summer NAM is more strongly related to surface air temperatures over Eurasia than the original Arctic Oscillation. The summer NAM is a wave-driven internal atmospheric mode that is maintained by both stationary and transient waves. The summer NAM is associated with the Arctic front, polar jet, and storm track around the Arctic Ocean. The winter-to-summer linkage described by M. Ogi et al. can be interpreted as a preferred transition from one polarity of the winter annular mode to the same polarity of the summer annular mode. The spring cryosphere, i.e., snow in Eurasia and sea ice in the Barents Sea, plays a supporting role in this transition. Citation: Ogi, M., K. Yamazaki, and Y. Tachibana (2004), The summertime annular mode in the Northern Hemisphere and its linkage to the winter mode,
We statistically investigated winter-summer climatic connectivity between the winter North Atlant... more We statistically investigated winter-summer climatic connectivity between the winter North Atlantic Oscillation (NAO), and the summer Okhotsk High. We found that the winter NAO persistently influences the surface air temperature, snow cover over the Eurasia continent, sea ice and SST around the Barents Sea region from winter to summer. The warm air temperature anomalies in East Siberia make a preferable condition for upper-level blocking. Warm signals around the Barents Sea region excite the Rossby wave propagating toward the Sea of Okhotsk. In addition, regional persistency of the wintertime local phenomena, and their possible influence on the summertime Okhotsk High is examined. The negative phase of the winter NAO is related to the winter and spring heavy sea ice in the Sea of Okhotsk. The heavy spring sea ice in the Sea of Okhotsk brings about the cold SST around the Sea of Okhotsk. However, since the Okhotsk High does not have correlation with the SST around the Sea of Okhotsk in the previous month, the local connection of the sea ice in the Sea of Okhotsk with the summer Okhotsk High is unlikely. Associated with an enhanced Okhotsk High, cold SST anomalies appear to the east of Japan. This cold SST anomaly is a response to the cold air advection associated with the Okhotsk High. We suggest that the winter NAO remotely regulates both the winter sea ice, and the summer Okhotsk High.
Journal of Geophysical Research: Atmospheres, 2015
Observational analyses using the ERA interim reanalysis and merged Hadley/OI-SST data reveal that... more Observational analyses using the ERA interim reanalysis and merged Hadley/OI-SST 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. ]. Results from numerical simulations using atmosphere-ocean coupled models suggest that Arctic sea-ice variability and the modulation of the AO/NAM are linked .
Abstract This study deals with two teleconnection patterns and the subsequent wave train propagat... more Abstract This study deals with two teleconnection patterns and the subsequent wave train propagations during an East Asian summer. Diagnostic results are as follows: 1) a stationary wave ray with zonal wavenumber 5 approximates the arc path linking the correlation ...
Journal of the Meteorological Society of Japan, 1999
In this study, recent variability of the Eurasian (EU) pattern during 1978-94 and its association... more In this study, recent variability of the Eurasian (EU) pattern during 1978-94 and its association with wave activity flux are investigated. The sign of the wintertime EU index abruptly reversed from positive to negative in winter 1988/89, concurrent with the intensified polar vortex. To investigate the mechanism of the EU pattern in detail, we computed the wave activity flux. When the EU index is positive, the Rossby wave propagation appears from North Europe to East Asia directly. When the EU index is negative the wave activity flux is directed more southeastward to the Middle East. An empirical orthogonal function (EOF) analysis is performed for the wave activity flux to clarify the variations of the Rossby wave propagation over Eurasia. Two dominant modes associated with the EU pattern were detected. The first mode is closely linked with the height anomaly over arctic regions and contributes to a shift in the 1988/89 winter atmosphere. The second mode is related to the other principal teleconnection patterns: the North Atlantic Oscillation (NAO), and Pacific/North American (PNA) patterns. The appearance of the modes are interpreted in terms of wave forcing to the zonal wind, and the meridional wave propagation of Rossby waves. The Rossby wave with wavenumber three, plays an important role in the zonal wind changes of the first mode. The meridional profile of the zonal wind is consistent with the structure of wave activity flux. The positive feedback between the wave activity and the zonal wind is responsible for the dominance of the first EU mode.
High O3 mixing ratios were observed above 6 km over Syowa Station (SYO), Antarctica in mid-Septem... more High O3 mixing ratios were observed above 6 km over Syowa Station (SYO), Antarctica in mid-September 1989 when the tropopause was found to be higher than 10 km. From the results of a 3-dimensional trajectory analysis, it is suggested that the observed high O3 was caused by vertical transport of O3 from the lower stratosphere to the upper troposphere. To examine general features of the transport process associated with high O3 mixing ratios observed in the upper troposphere over SYO, the same trajectory analysis was further performed for high O3 observations at 350 hPa over SYO in 1987-1994. The results imply that the transport of stratospheric air descending in the polar region to SYO plays a very important role in the increase of upper tropospheric O3 over SYO, especially in spring and autumn, while the downward transport of stratospheric O3 in the circumpolar region is important in summer.
The summer Arctic sea ice extent (SIE) during the period 1996−2008 had an accelerating trend of r... more The summer Arctic sea ice extent (SIE) during the period 1996−2008 had an accelerating trend of retreat. The difference of SIE between May and September has a significant correlation with the summer Northern Hemisphere Annular Mode (summer NAM). The summer NAM had a significant positive trend during the period 1958−1996, and a weak negative trend during the period 1996−2008. The atmospheric circulation during the recent period had a trend of anticyclonic circulation over the Arctic with easterly wind over the marginal seas. In addition, tropospheric warming was enhanced especially over the Arctic and near the ground surface.
The relation of interannual connection between Asian dust outbreaks and stratosphere-to-troposphe... more The relation of interannual connection between Asian dust outbreaks and stratosphere-to-troposphere transport (STT) in spring was suggested by the dust and tritium variations in the Mount Wrangell ice core, Alaska in Yasunari et al. (2007). However, these impacts on the ice core site in each event scale have not been investigated. Hence, the present paper focuses on the material transport and deposition processes for further understanding these impacts on the ice core. The variations in dust and tritium concentrations in spring in an ice core taken at Mt. Wrangell, Alaska are explained by meteorological analysis and simulation of trajectories associated with Asian dust outbreaks and STT. Material transport and deposition at Mt. Wrangell are examined in two contrasting years (2001 and 2002). Dust and tritium concentrations both reached peak values in the early spring of 2002, while the dust peak occurred in early spring and the tritium peak occurred in late spring in 2001. Six severe East Asian transpacific dust storms over this period are modeled by forward trajectory and meteorologically analyzed. It is found that 5 of 6 events contributed to the ice core record in Alaska. Stratospheric air is also transported to the ice core site in most cases. Tritium deposition is found to have been suppressed in the cases of the 2001 dust storms due to lack of snowfall at appropriate times. Taken the detailed transport and deposition processes after the severe dust storms with atmospheric circulations into account, we can well explain spring dust and tritium variations in the Mount Wrangell ice core.
Los Angeles, CA 90024 Monterey, CA 93943-5006 8a NAME OF FUNDING,SPONSORNG 8b OFFICE SYMBOL 9. PR... more Los Angeles, CA 90024 Monterey, CA 93943-5006 8a NAME OF FUNDING,SPONSORNG 8b OFFICE SYMBOL 9. PROCURE'.IENT INSTRUMENT IDENTIFICATION NUMBER ORGANIZATION It app0 ca b e) N00014-80-K-0947 (0NR) Naval Air Systems Command AIR-330 (suppl to NSF Grant ATM-8218215) 8c ADDRESS (City, State, and ZIP Code) 10 SOURCE OF FUNDING NUMBERS
Recent evidence from both observations and model simulations suggests that an Arctic sea ice redu... more 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.
[1] The seasonal variations of the Northern Hemisphere annular mode (NAM) are investigated throug... more [1] The seasonal variations of the Northern Hemisphere annular mode (NAM) are investigated through empirical orthogonal function analysis of the zonally averaged geopotential height fields for each individual calendar month. Patterns of the winter and summer NAMs differ not only in the geopotential height fields but also in the mean meridional circulation and eddy structure. The summer NAM has a smaller meridional scale and is displaced poleward as compared to the winter NAM. The antinode on the lower-latitude side in the summer NAM is at the nodal latitude of the winter NAM. The summer NAM is more strongly related to surface air temperatures over Eurasia than the original Arctic Oscillation. The summer NAM is a wave-driven internal atmospheric mode that is maintained by both stationary and transient waves. The summer NAM is associated with the Arctic front, polar jet, and storm track around the Arctic Ocean. The winter-to-summer linkage described by M. Ogi et al. can be interpreted as a preferred transition from one polarity of the winter annular mode to the same polarity of the summer annular mode. The spring cryosphere, i.e., snow in Eurasia and sea ice in the Barents Sea, plays a supporting role in this transition. Citation: Ogi, M., K. Yamazaki, and Y. Tachibana (2004), The summertime annular mode in the Northern Hemisphere and its linkage to the winter mode,
We statistically investigated winter-summer climatic connectivity between the winter North Atlant... more We statistically investigated winter-summer climatic connectivity between the winter North Atlantic Oscillation (NAO), and the summer Okhotsk High. We found that the winter NAO persistently influences the surface air temperature, snow cover over the Eurasia continent, sea ice and SST around the Barents Sea region from winter to summer. The warm air temperature anomalies in East Siberia make a preferable condition for upper-level blocking. Warm signals around the Barents Sea region excite the Rossby wave propagating toward the Sea of Okhotsk. In addition, regional persistency of the wintertime local phenomena, and their possible influence on the summertime Okhotsk High is examined. The negative phase of the winter NAO is related to the winter and spring heavy sea ice in the Sea of Okhotsk. The heavy spring sea ice in the Sea of Okhotsk brings about the cold SST around the Sea of Okhotsk. However, since the Okhotsk High does not have correlation with the SST around the Sea of Okhotsk in the previous month, the local connection of the sea ice in the Sea of Okhotsk with the summer Okhotsk High is unlikely. Associated with an enhanced Okhotsk High, cold SST anomalies appear to the east of Japan. This cold SST anomaly is a response to the cold air advection associated with the Okhotsk High. We suggest that the winter NAO remotely regulates both the winter sea ice, and the summer Okhotsk High.
Journal of Geophysical Research: Atmospheres, 2015
Observational analyses using the ERA interim reanalysis and merged Hadley/OI-SST data reveal that... more Observational analyses using the ERA interim reanalysis and merged Hadley/OI-SST 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. ]. Results from numerical simulations using atmosphere-ocean coupled models suggest that Arctic sea-ice variability and the modulation of the AO/NAM are linked .
Abstract This study deals with two teleconnection patterns and the subsequent wave train propagat... more Abstract This study deals with two teleconnection patterns and the subsequent wave train propagations during an East Asian summer. Diagnostic results are as follows: 1) a stationary wave ray with zonal wavenumber 5 approximates the arc path linking the correlation ...
Journal of the Meteorological Society of Japan, 1999
In this study, recent variability of the Eurasian (EU) pattern during 1978-94 and its association... more In this study, recent variability of the Eurasian (EU) pattern during 1978-94 and its association with wave activity flux are investigated. The sign of the wintertime EU index abruptly reversed from positive to negative in winter 1988/89, concurrent with the intensified polar vortex. To investigate the mechanism of the EU pattern in detail, we computed the wave activity flux. When the EU index is positive, the Rossby wave propagation appears from North Europe to East Asia directly. When the EU index is negative the wave activity flux is directed more southeastward to the Middle East. An empirical orthogonal function (EOF) analysis is performed for the wave activity flux to clarify the variations of the Rossby wave propagation over Eurasia. Two dominant modes associated with the EU pattern were detected. The first mode is closely linked with the height anomaly over arctic regions and contributes to a shift in the 1988/89 winter atmosphere. The second mode is related to the other principal teleconnection patterns: the North Atlantic Oscillation (NAO), and Pacific/North American (PNA) patterns. The appearance of the modes are interpreted in terms of wave forcing to the zonal wind, and the meridional wave propagation of Rossby waves. The Rossby wave with wavenumber three, plays an important role in the zonal wind changes of the first mode. The meridional profile of the zonal wind is consistent with the structure of wave activity flux. The positive feedback between the wave activity and the zonal wind is responsible for the dominance of the first EU mode.
High O3 mixing ratios were observed above 6 km over Syowa Station (SYO), Antarctica in mid-Septem... more High O3 mixing ratios were observed above 6 km over Syowa Station (SYO), Antarctica in mid-September 1989 when the tropopause was found to be higher than 10 km. From the results of a 3-dimensional trajectory analysis, it is suggested that the observed high O3 was caused by vertical transport of O3 from the lower stratosphere to the upper troposphere. To examine general features of the transport process associated with high O3 mixing ratios observed in the upper troposphere over SYO, the same trajectory analysis was further performed for high O3 observations at 350 hPa over SYO in 1987-1994. The results imply that the transport of stratospheric air descending in the polar region to SYO plays a very important role in the increase of upper tropospheric O3 over SYO, especially in spring and autumn, while the downward transport of stratospheric O3 in the circumpolar region is important in summer.
[1] Using the NCEP/NCAR reanalysis dataset and other observations, we show that the summer high-l... more [1] Using the NCEP/NCAR reanalysis dataset and other observations, we show that the summer high-latitude climate in the Northern Hemisphere is influenced by the NAO of the previous winter. We find this influence in the summertime surface air temperature, the geopotential height, the sea surface temperature (SST), sea-ice/continental snow cover extent fields as well as in the zonal mean geopotential height and zonal wind fields. This summertime NAO signal is annular but its meridional scale is much smaller than the winter annular mode. Distinct summer anomalies are located at the nodal latitudes of the winter anomalies. We suggest that the sea-ice, SST and snow cover anomalies provide the memory allowing the winter NAO to affect the summer climate.
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Papers by Koji Yamazaki