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Edmund K. M. Chang - One of the best experts on this subject based on the ideXlab platform.

  • Impacts of the Madden-Julian Oscillation on Storm Track Activity, Surface Air Temperature, and Precipitation over North America
    Journal of Climate, 2018
    Co-Authors: Cheng Zheng, Minghua Zhang, Edmund K. M. Chang, Wanqiu Wang
    Abstract:

    AbstractIn this study, the intraseasonal variations in Storm-Track activity, surface air temperature, and precipitation over North America associated with the Madden–Julian oscillation (MJO) in bor...

  • variations of northern hemisphere Storm Track and extratropical cyclone activity associated with the madden julian oscillation
    Journal of Climate, 2017
    Co-Authors: Yanjuan Guo, Toshiaki Shinoda, Jialin Lin, Edmund K. M. Chang
    Abstract:

    AbstractThis study investigates the intraseasonal variations of the Northern Hemispheric Storm Track associated with the Madden–Julian oscillation (MJO) during the extended boreal winter (November–April) using 36 yr (1979–2014) of reanalysis data from ERA-Interim. Two methods have been used to diagnose Storm-Track variations. In the first method, the Storm Track is quantified by the temporal-filtered variance of 250-hPa meridional wind (vv250) or mean sea level pressure (pp). The intraseasonal anomalies of vv250 composited for eight MJO phases are characterized by a zonal band of strong positive (or negative) anomalies meandering from the Pacific all the way across North America and the Atlantic into northern Europe, with weaker anomalies of opposite sign at one or both flanks. The results based on pp are consistent with those based on vv250 except for larger zonal variations, which may be induced by surface topography. In the second method, an objective cyclone-Tracking scheme has been used to Track the ...

  • Variations of Northern Hemisphere Storm Track and Extratropical Cyclone Activity Associated with the Madden–Julian Oscillation
    Journal of Climate, 2017
    Co-Authors: Yanjuan Guo, Toshiaki Shinoda, Jialin Lin, Edmund K. M. Chang
    Abstract:

    This study investigates the intraseasonal variations of the Northern Hemispheric Storm Track associated with the Madden–Julian oscillation (MJO) during the extended boreal winter (November–April) using 36 yr (1979–2014) of reanalysis data from ERA-Interim. Two methods have been used to diagnose Storm-Track variations. In the first method, the Storm Track is quantified by the temporal-filtered variance of 250-hPa meridional wind (vv250) or mean sea level pressure (pp). The intraseasonal anomalies of vv250 composited for eight MJO phases are characterized by a zonal band of strong positive (or negative) anomalies meandering from the Pacific all the way across North America and the Atlantic into northern Europe, with weaker anomalies of opposite sign at one or both flanks. The results based on pp are consistent with those based on vv250 except for larger zonal variations, which may be induced by surface topography. In the second method, an objective cyclone-Tracking scheme has been used to Track the extratropical cyclones that compose the Storm Track. The MJO-composite anomalies of the “accumulated” cyclone activity, a quantity that includes contributions from both the cyclone frequency and cyclone mean intensity, are very similar to those based on pp. Further analysis demonstrates that major contribution comes from variations in the cyclone frequency. Further analysis suggests that the intraseasonal variations of the Storm Track can be primarily attributed to the variations of the mean flow that responds to the anomalous MJO convections in the tropics, with possible contribution also from the moisture variations.

  • northern hemisphere winter Storm Track trends since 1959 derived from multiple reanalysis datasets
    Climate Dynamics, 2016
    Co-Authors: Edmund K. M. Chang, Albert M. W. Yau
    Abstract:

    In this study, a comprehensive comparison of Northern Hemisphere winter Storm Track trend since 1959 derived from multiple reanalysis datasets and rawinsonde observations has been conducted. In addition, trends in terms of variance and cyclone Track statistics have been compared. Previous studies, based largely on the National Center for Environmental Prediction–National Center for Atmospheric Research Reanalysis (NNR), have suggested that both the Pacific and Atlantic Storm Tracks have significantly intensified between the 1950s and 1990s. Comparison with trends derived from rawinsonde observations suggest that the trends derived from NNR are significantly biased high, while those from the European Center for Medium Range Weather Forecasts 40-year Reanalysis and the Japanese 55-year Reanalysis are much less biased but still too high. Those from the two twentieth century reanalysis datasets are most consistent with observations but may exhibit slight biases of opposite signs. Between 1959 and 2010, Pacific Storm Track activity has likely increased by 10 % or more, while Atlantic Storm Track activity has likely increased by <10 %. Our analysis suggests that trends in Pacific and Atlantic basin wide Storm Track activity prior to the 1950s derived from the two twentieth century reanalysis datasets are unlikely to be reliable due to changes in density of surface observations. Nevertheless, these datasets may provide useful information on interannual variability, especially over the Atlantic.

  • Northern Hemisphere winter Storm Track trends since 1959 derived from multiple reanalysis datasets
    Climate Dynamics, 2016
    Co-Authors: Edmund K. M. Chang, Albert M. W. Yau
    Abstract:

    In this study, a comprehensive comparison of Northern Hemisphere winter Storm Track trend since 1959 derived from multiple reanalysis datasets and rawinsonde observations has been conducted. In addition, trends in terms of variance and cyclone Track statistics have been compared. Previous studies, based largely on the National Center for Environmental Prediction–National Center for Atmospheric Research Reanalysis (NNR), have suggested that both the Pacific and Atlantic Storm Tracks have significantly intensified between the 1950s and 1990s. Comparison with trends derived from rawinsonde observations suggest that the trends derived from NNR are significantly biased high, while those from the European Center for Medium Range Weather Forecasts 40-year Reanalysis and the Japanese 55-year Reanalysis are much less biased but still too high. Those from the two twentieth century reanalysis datasets are most consistent with observations but may exhibit slight biases of opposite signs. Between 1959 and 2010, Pacific Storm Track activity has likely increased by 10 % or more, while Atlantic Storm Track activity has likely increased by

Xiu-qun Yang - One of the best experts on this subject based on the ideXlab platform.

  • Impacts of the subarctic frontal zone on the North Pacific Storm Track in the cold season: an observational study
    International Journal of Climatology, 2018
    Co-Authors: Yao Yao, Zhong Zhong, Xiu-qun Yang
    Abstract:

    This study revealed that the Storm Track intensifies with the strengthening of the subarctic frontal zone (SAFZ) and moves northwards following the northwards shift of the SAFZ as shown in the figure. The Storm‐Track response to the SAFZ intensity variation is strongest in February and March, while its response to the SAFZ meridional shift is most pronounced in November and December. The Storm‐Track response is relatively weak in January.

  • Seasonal variation of the North Pacific Storm-Track relationship with the Subarctic frontal zone intensity
    Dynamics of Atmospheres and Oceans, 2018
    Co-Authors: Yao Yao, Xiu-qun Yang, Zhong Zhong, Xiaogang Huang
    Abstract:

    Abstract The relationship between the North Pacific Storm Track and the intensity of the subarctic frontal zone (SAFZ) in each season is investigated in this study. Associated with the strengthened SAFZ, the Storm Track in winter characterize a distinct downstream intensification, and a comparable enhancement appears in spring in the northwestern part of the Storm Track; while the Storm-Track anomalies are much weaker in summer and autumn. It is found that the seasonal variation of the Storm-Track anomalous patterns may be attributed to the near-surface baroclinicity and the baroclinic energy conversion (BCEC) anomalies. When the SAFZ intensifies in winter (spring), accompanied by the increased near-surface baroclinicity in the central and eastern (northwestern) part of the Storm Track, more energy is converted form the mean available potential energy to the eddy available potential energy and then transferred to the eddy kinetic energy, which favors the robust downstream (northwestward) enhancement of the Storm Track in winter (spring). While the weaker Storm-Track anomalous patterns occur in summer and autumn, due to small near-surface baroclinicity and BCEC anomalies.

  • an observational study of the north pacific Storm Track impact on the midlatitude oceanic front
    Journal of Geophysical Research, 2017
    Co-Authors: Yao Yao, Zhong Zhong, Xiu-qun Yang
    Abstract:

    A lagged Maximum Covariance Analysis is used to examine the impact of North Pacific Storm-Track activity on midlatitude oceanic frontal intensity in this study. It is found that an enhanced Storm Track tends to intensify the oceanic frontal intensity with a lag of 1–2 months. The forcing effect of Storm-Track anomalies on oceanic frontal intensity is strongest in autumn, followed by that in summer and winter, and it is weakest in spring. Moreover, the mixed layer heat budget analysis suggests that sea surface temperature anomalies (SSTAs) related to oceanic fronts are primarily attributed to the Storm-Track-induced net surface heat flux and Ekman advection anomalies, while contributions of geostrophic advection and entrainment are relatively small. In summer and autumn, the impact of net surface heat flux anomalies on SSTAs plays a more important role than that of Ekman heat transport anomalies. Whereas in winter, Ekman heat transport anomaly forcing is comparable to the net surface heat flux forcing. Anomalous turbulent heat fluxes contribute to generating net surface heat flux anomalies in those three seasons, while the shortwave radiative fluxes make a strong contribution in summer but have little impact in winter. The anomalies of both net surface heat flux and Ekman heat transport are presumed to be associated with Storm-Track-induced surface wind anomalies. Results of the present study provide observational evidences for the positive feedback between the North Pacific Storm-Track activity and midlatitude oceanic frontal intensity.

  • An observational study of the North Pacific Storm‐Track impact on the midlatitude oceanic front
    Journal of Geophysical Research: Atmospheres, 2017
    Co-Authors: Yao Yao, Zhong Zhong, Xiu-qun Yang
    Abstract:

    A lagged Maximum Covariance Analysis is used to examine the impact of North Pacific Storm-Track activity on midlatitude oceanic frontal intensity in this study. It is found that an enhanced Storm Track tends to intensify the oceanic frontal intensity with a lag of 1–2 months. The forcing effect of Storm-Track anomalies on oceanic frontal intensity is strongest in autumn, followed by that in summer and winter, and it is weakest in spring. Moreover, the mixed layer heat budget analysis suggests that sea surface temperature anomalies (SSTAs) related to oceanic fronts are primarily attributed to the Storm-Track-induced net surface heat flux and Ekman advection anomalies, while contributions of geostrophic advection and entrainment are relatively small. In summer and autumn, the impact of net surface heat flux anomalies on SSTAs plays a more important role than that of Ekman heat transport anomalies. Whereas in winter, Ekman heat transport anomaly forcing is comparable to the net surface heat flux forcing. Anomalous turbulent heat fluxes contribute to generating net surface heat flux anomalies in those three seasons, while the shortwave radiative fluxes make a strong contribution in summer but have little impact in winter. The anomalies of both net surface heat flux and Ekman heat transport are presumed to be associated with Storm-Track-induced surface wind anomalies. Results of the present study provide observational evidences for the positive feedback between the North Pacific Storm-Track activity and midlatitude oceanic frontal intensity.

  • numerical experiments of the Storm Track sensitivity to oceanic frontal strength within the kuroshio oyashio extensions
    Journal of Geophysical Research, 2016
    Co-Authors: Yao Yao, Zhong Zhong, Xiu-qun Yang
    Abstract:

    The sensitivity of the North Pacific Storm Track to midlatitude oceanic frontal strength within the Kuroshio/Oyashio Extensions is investigated by applying artificially changed meridional sea surface temperature (SST) gradients in the Weather Research Forecasting model version 3.4. The result of sensitivity experiments further confirms the close relationship between the Storm Track activity and meridional SST gradient; i.e., the Storm Track activity can be intensified as a response to increases in the oceanic frontal strength. In order to better understand the mechanism for the Storm Track intensification due to increased SST gradient, velocity-temperature correlation and local energetics are analyzed. The result indicates that the enhancement of the meridional SST gradient leads to amplitude magnification of eddy meridional velocity and temperature and their phase consistency, suggesting that synoptic-scale eddies tend to approach the optimum structure for the baroclinic energy conversion, which is mainly responsible for the SST front-induced enhancement of Storm Track activity. In order to estimate the impact of the oceanic front on the maintenance of the near-surface baroclinicity, further investigation is made by the composite analysis. With the increase in oceanic frontal strength, the near-surface baroclinicity experiences a slow but strong restoration. The increase in the meridional SST gradient results in the intensification in the cross-frontal differential sensible heat flux, which can more effectively offset the relaxing effect of the transient eddy poleward heat transport.

Yao Yao - One of the best experts on this subject based on the ideXlab platform.

  • Impacts of the subarctic frontal zone on the North Pacific Storm Track in the cold season: an observational study
    International Journal of Climatology, 2018
    Co-Authors: Yao Yao, Zhong Zhong, Xiu-qun Yang
    Abstract:

    This study revealed that the Storm Track intensifies with the strengthening of the subarctic frontal zone (SAFZ) and moves northwards following the northwards shift of the SAFZ as shown in the figure. The Storm‐Track response to the SAFZ intensity variation is strongest in February and March, while its response to the SAFZ meridional shift is most pronounced in November and December. The Storm‐Track response is relatively weak in January.

  • Seasonal variation of the North Pacific Storm-Track relationship with the Subarctic frontal zone intensity
    Dynamics of Atmospheres and Oceans, 2018
    Co-Authors: Yao Yao, Xiu-qun Yang, Zhong Zhong, Xiaogang Huang
    Abstract:

    Abstract The relationship between the North Pacific Storm Track and the intensity of the subarctic frontal zone (SAFZ) in each season is investigated in this study. Associated with the strengthened SAFZ, the Storm Track in winter characterize a distinct downstream intensification, and a comparable enhancement appears in spring in the northwestern part of the Storm Track; while the Storm-Track anomalies are much weaker in summer and autumn. It is found that the seasonal variation of the Storm-Track anomalous patterns may be attributed to the near-surface baroclinicity and the baroclinic energy conversion (BCEC) anomalies. When the SAFZ intensifies in winter (spring), accompanied by the increased near-surface baroclinicity in the central and eastern (northwestern) part of the Storm Track, more energy is converted form the mean available potential energy to the eddy available potential energy and then transferred to the eddy kinetic energy, which favors the robust downstream (northwestward) enhancement of the Storm Track in winter (spring). While the weaker Storm-Track anomalous patterns occur in summer and autumn, due to small near-surface baroclinicity and BCEC anomalies.

  • an observational study of the north pacific Storm Track impact on the midlatitude oceanic front
    Journal of Geophysical Research, 2017
    Co-Authors: Yao Yao, Zhong Zhong, Xiu-qun Yang
    Abstract:

    A lagged Maximum Covariance Analysis is used to examine the impact of North Pacific Storm-Track activity on midlatitude oceanic frontal intensity in this study. It is found that an enhanced Storm Track tends to intensify the oceanic frontal intensity with a lag of 1–2 months. The forcing effect of Storm-Track anomalies on oceanic frontal intensity is strongest in autumn, followed by that in summer and winter, and it is weakest in spring. Moreover, the mixed layer heat budget analysis suggests that sea surface temperature anomalies (SSTAs) related to oceanic fronts are primarily attributed to the Storm-Track-induced net surface heat flux and Ekman advection anomalies, while contributions of geostrophic advection and entrainment are relatively small. In summer and autumn, the impact of net surface heat flux anomalies on SSTAs plays a more important role than that of Ekman heat transport anomalies. Whereas in winter, Ekman heat transport anomaly forcing is comparable to the net surface heat flux forcing. Anomalous turbulent heat fluxes contribute to generating net surface heat flux anomalies in those three seasons, while the shortwave radiative fluxes make a strong contribution in summer but have little impact in winter. The anomalies of both net surface heat flux and Ekman heat transport are presumed to be associated with Storm-Track-induced surface wind anomalies. Results of the present study provide observational evidences for the positive feedback between the North Pacific Storm-Track activity and midlatitude oceanic frontal intensity.

  • An observational study of the North Pacific Storm‐Track impact on the midlatitude oceanic front
    Journal of Geophysical Research: Atmospheres, 2017
    Co-Authors: Yao Yao, Zhong Zhong, Xiu-qun Yang
    Abstract:

    A lagged Maximum Covariance Analysis is used to examine the impact of North Pacific Storm-Track activity on midlatitude oceanic frontal intensity in this study. It is found that an enhanced Storm Track tends to intensify the oceanic frontal intensity with a lag of 1–2 months. The forcing effect of Storm-Track anomalies on oceanic frontal intensity is strongest in autumn, followed by that in summer and winter, and it is weakest in spring. Moreover, the mixed layer heat budget analysis suggests that sea surface temperature anomalies (SSTAs) related to oceanic fronts are primarily attributed to the Storm-Track-induced net surface heat flux and Ekman advection anomalies, while contributions of geostrophic advection and entrainment are relatively small. In summer and autumn, the impact of net surface heat flux anomalies on SSTAs plays a more important role than that of Ekman heat transport anomalies. Whereas in winter, Ekman heat transport anomaly forcing is comparable to the net surface heat flux forcing. Anomalous turbulent heat fluxes contribute to generating net surface heat flux anomalies in those three seasons, while the shortwave radiative fluxes make a strong contribution in summer but have little impact in winter. The anomalies of both net surface heat flux and Ekman heat transport are presumed to be associated with Storm-Track-induced surface wind anomalies. Results of the present study provide observational evidences for the positive feedback between the North Pacific Storm-Track activity and midlatitude oceanic frontal intensity.

  • numerical experiments of the Storm Track sensitivity to oceanic frontal strength within the kuroshio oyashio extensions
    Journal of Geophysical Research, 2016
    Co-Authors: Yao Yao, Zhong Zhong, Xiu-qun Yang
    Abstract:

    The sensitivity of the North Pacific Storm Track to midlatitude oceanic frontal strength within the Kuroshio/Oyashio Extensions is investigated by applying artificially changed meridional sea surface temperature (SST) gradients in the Weather Research Forecasting model version 3.4. The result of sensitivity experiments further confirms the close relationship between the Storm Track activity and meridional SST gradient; i.e., the Storm Track activity can be intensified as a response to increases in the oceanic frontal strength. In order to better understand the mechanism for the Storm Track intensification due to increased SST gradient, velocity-temperature correlation and local energetics are analyzed. The result indicates that the enhancement of the meridional SST gradient leads to amplitude magnification of eddy meridional velocity and temperature and their phase consistency, suggesting that synoptic-scale eddies tend to approach the optimum structure for the baroclinic energy conversion, which is mainly responsible for the SST front-induced enhancement of Storm Track activity. In order to estimate the impact of the oceanic front on the maintenance of the near-surface baroclinicity, further investigation is made by the composite analysis. With the increase in oceanic frontal strength, the near-surface baroclinicity experiences a slow but strong restoration. The increase in the meridional SST gradient results in the intensification in the cross-frontal differential sensible heat flux, which can more effectively offset the relaxing effect of the transient eddy poleward heat transport.

Zhong Zhong - One of the best experts on this subject based on the ideXlab platform.

  • Impacts of the subarctic frontal zone on the North Pacific Storm Track in the cold season: an observational study
    International Journal of Climatology, 2018
    Co-Authors: Yao Yao, Zhong Zhong, Xiu-qun Yang
    Abstract:

    This study revealed that the Storm Track intensifies with the strengthening of the subarctic frontal zone (SAFZ) and moves northwards following the northwards shift of the SAFZ as shown in the figure. The Storm‐Track response to the SAFZ intensity variation is strongest in February and March, while its response to the SAFZ meridional shift is most pronounced in November and December. The Storm‐Track response is relatively weak in January.

  • Seasonal variation of the North Pacific Storm-Track relationship with the Subarctic frontal zone intensity
    Dynamics of Atmospheres and Oceans, 2018
    Co-Authors: Yao Yao, Xiu-qun Yang, Zhong Zhong, Xiaogang Huang
    Abstract:

    Abstract The relationship between the North Pacific Storm Track and the intensity of the subarctic frontal zone (SAFZ) in each season is investigated in this study. Associated with the strengthened SAFZ, the Storm Track in winter characterize a distinct downstream intensification, and a comparable enhancement appears in spring in the northwestern part of the Storm Track; while the Storm-Track anomalies are much weaker in summer and autumn. It is found that the seasonal variation of the Storm-Track anomalous patterns may be attributed to the near-surface baroclinicity and the baroclinic energy conversion (BCEC) anomalies. When the SAFZ intensifies in winter (spring), accompanied by the increased near-surface baroclinicity in the central and eastern (northwestern) part of the Storm Track, more energy is converted form the mean available potential energy to the eddy available potential energy and then transferred to the eddy kinetic energy, which favors the robust downstream (northwestward) enhancement of the Storm Track in winter (spring). While the weaker Storm-Track anomalous patterns occur in summer and autumn, due to small near-surface baroclinicity and BCEC anomalies.

  • an observational study of the north pacific Storm Track impact on the midlatitude oceanic front
    Journal of Geophysical Research, 2017
    Co-Authors: Yao Yao, Zhong Zhong, Xiu-qun Yang
    Abstract:

    A lagged Maximum Covariance Analysis is used to examine the impact of North Pacific Storm-Track activity on midlatitude oceanic frontal intensity in this study. It is found that an enhanced Storm Track tends to intensify the oceanic frontal intensity with a lag of 1–2 months. The forcing effect of Storm-Track anomalies on oceanic frontal intensity is strongest in autumn, followed by that in summer and winter, and it is weakest in spring. Moreover, the mixed layer heat budget analysis suggests that sea surface temperature anomalies (SSTAs) related to oceanic fronts are primarily attributed to the Storm-Track-induced net surface heat flux and Ekman advection anomalies, while contributions of geostrophic advection and entrainment are relatively small. In summer and autumn, the impact of net surface heat flux anomalies on SSTAs plays a more important role than that of Ekman heat transport anomalies. Whereas in winter, Ekman heat transport anomaly forcing is comparable to the net surface heat flux forcing. Anomalous turbulent heat fluxes contribute to generating net surface heat flux anomalies in those three seasons, while the shortwave radiative fluxes make a strong contribution in summer but have little impact in winter. The anomalies of both net surface heat flux and Ekman heat transport are presumed to be associated with Storm-Track-induced surface wind anomalies. Results of the present study provide observational evidences for the positive feedback between the North Pacific Storm-Track activity and midlatitude oceanic frontal intensity.

  • An observational study of the North Pacific Storm‐Track impact on the midlatitude oceanic front
    Journal of Geophysical Research: Atmospheres, 2017
    Co-Authors: Yao Yao, Zhong Zhong, Xiu-qun Yang
    Abstract:

    A lagged Maximum Covariance Analysis is used to examine the impact of North Pacific Storm-Track activity on midlatitude oceanic frontal intensity in this study. It is found that an enhanced Storm Track tends to intensify the oceanic frontal intensity with a lag of 1–2 months. The forcing effect of Storm-Track anomalies on oceanic frontal intensity is strongest in autumn, followed by that in summer and winter, and it is weakest in spring. Moreover, the mixed layer heat budget analysis suggests that sea surface temperature anomalies (SSTAs) related to oceanic fronts are primarily attributed to the Storm-Track-induced net surface heat flux and Ekman advection anomalies, while contributions of geostrophic advection and entrainment are relatively small. In summer and autumn, the impact of net surface heat flux anomalies on SSTAs plays a more important role than that of Ekman heat transport anomalies. Whereas in winter, Ekman heat transport anomaly forcing is comparable to the net surface heat flux forcing. Anomalous turbulent heat fluxes contribute to generating net surface heat flux anomalies in those three seasons, while the shortwave radiative fluxes make a strong contribution in summer but have little impact in winter. The anomalies of both net surface heat flux and Ekman heat transport are presumed to be associated with Storm-Track-induced surface wind anomalies. Results of the present study provide observational evidences for the positive feedback between the North Pacific Storm-Track activity and midlatitude oceanic frontal intensity.

  • numerical experiments of the Storm Track sensitivity to oceanic frontal strength within the kuroshio oyashio extensions
    Journal of Geophysical Research, 2016
    Co-Authors: Yao Yao, Zhong Zhong, Xiu-qun Yang
    Abstract:

    The sensitivity of the North Pacific Storm Track to midlatitude oceanic frontal strength within the Kuroshio/Oyashio Extensions is investigated by applying artificially changed meridional sea surface temperature (SST) gradients in the Weather Research Forecasting model version 3.4. The result of sensitivity experiments further confirms the close relationship between the Storm Track activity and meridional SST gradient; i.e., the Storm Track activity can be intensified as a response to increases in the oceanic frontal strength. In order to better understand the mechanism for the Storm Track intensification due to increased SST gradient, velocity-temperature correlation and local energetics are analyzed. The result indicates that the enhancement of the meridional SST gradient leads to amplitude magnification of eddy meridional velocity and temperature and their phase consistency, suggesting that synoptic-scale eddies tend to approach the optimum structure for the baroclinic energy conversion, which is mainly responsible for the SST front-induced enhancement of Storm Track activity. In order to estimate the impact of the oceanic front on the maintenance of the near-surface baroclinicity, further investigation is made by the composite analysis. With the increase in oceanic frontal strength, the near-surface baroclinicity experiences a slow but strong restoration. The increase in the meridional SST gradient results in the intensification in the cross-frontal differential sensible heat flux, which can more effectively offset the relaxing effect of the transient eddy poleward heat transport.

Hisashi Nakamura - One of the best experts on this subject based on the ideXlab platform.

  • influence of the subtropical high and Storm Track on low cloud fraction and its seasonality over the south indian ocean
    Journal of Climate, 2018
    Co-Authors: Ayumu Miyamoto, Hisashi Nakamura, Takafumi Miyasaka
    Abstract:

    AbstractThe south Indian Ocean is characterized by enhanced midlatitude Storm-Track activity around a prominent sea surface temperature (SST) front and unique seasonality of the surface subtropical Mascarene high. The present study investigates the climatological distribution of low-cloud fraction (LCF) and its seasonality by using satellite data, in order to elucidate the role of the Storm-Track activity and subtropical high. On the equatorward flank of the SST front, summertime LCF is locally maximized despite small estimated inversion strength (EIS) and high SST. This is attributable to locally augmented sensible heat flux (SHF) from the ocean under the enhanced Storm-Track activity, which gives rise to strong instantaneous wind speed while acting to relax the meridional gradient of surface air temperature. In the subtropics, summertime LCF is maximized off the west coast of Australia, while wintertime LCF is distributed more zonally across the basin unlike in other subtropical ocean basins. Although i...

  • arctic summer Storm Track in cmip3 5 climate models
    Climate Dynamics, 2015
    Co-Authors: Kazuaki Nishii, Hisashi Nakamura, Yvan J. Orsolini
    Abstract:

    Model performance and future projection of Arctic summertime Storm-Track activity and associated background states are assessed on the basis of Coupled Model Intercomparison Project Phase 3 (CMIP3)/5 (CMIP5) climate models. Despite some improvement in the CMIP5 models relative to the CMIP3 models, most of the climate models underestimate summertime Storm-Track activity over the Arctic Ocean compared to six reanalysis data sets as measured locally as the variance of subweekly fluctuations of sea level pressure. Its large inter-model spread (i.e., model-to-model differences) is correlated with that of the intensity of the Beaufort Sea High and the lower-tropospheric westerlies in the Arctic region. Most of the CMIP3/5 models project the enhancement of Storm-Track activity over the Arctic Ocean off the eastern Siberian and Alaskan coasts, the region called the Arctic Ocean Cyclone Maximum, in association with the strengthening of the westerlies in the warmed climate. A model with stronger enhancement of the Storm-Track activity tends to accompany stronger land-sea contrast in surface air temperature across the Siberian coast, which reflects greater surface warming over the continent and slower warming over the Arctic Ocean. Other processes, however, may also be likely to contribute to the future changes of the Storm-Track activity, which gives uncertainty in the projection by multiple climate models. Our analysis suggests that further clarification of those processes that influence Storm-Track activity over the Arctic is necessary for more reliable future projections of the Arctic climate.

  • Arctic summer Storm Track in CMIP3/5 climate models
    Climate Dynamics, 2014
    Co-Authors: Kazuaki Nishii, Hisashi Nakamura, Yvan J. Orsolini
    Abstract:

    Model performance and future projection of Arctic summertime Storm-Track activity and associated background states are assessed on the basis of Coupled Model Intercomparison Project Phase 3 (CMIP3)/5 (CMIP5) climate models. Despite some improvement in the CMIP5 models relative to the CMIP3 models, most of the climate models underestimate summertime Storm-Track activity over the Arctic Ocean compared to six reanalysis data sets as measured locally as the variance of subweekly fluctuations of sea level pressure. Its large inter-model spread (i.e., model-to-model differences) is correlated with that of the intensity of the Beaufort Sea High and the lower-tropospheric westerlies in the Arctic region. Most of the CMIP3/5 models project the enhancement of Storm-Track activity over the Arctic Ocean off the eastern Siberian and Alaskan coasts, the region called the Arctic Ocean Cyclone Maximum, in association with the strengthening of the westerlies in the warmed climate. A model with stronger enhancement of the Storm-Track activity tends to accompany stronger land-sea contrast in surface air temperature across the Siberian coast, which reflects greater surface warming over the continent and slower warming over the Arctic Ocean. Other processes, however, may also be likely to contribute to the future changes of the Storm-Track activity, which gives uncertainty in the projection by multiple climate models. Our analysis suggests that further clarification of those processes that influence Storm-Track activity over the Arctic is necessary for more reliable future projections of the Arctic climate.

  • significance of a midlatitude sst frontal zone in the formation of a Storm Track and an eddy driven westerly jet
    Journal of Climate, 2010
    Co-Authors: Takeaki Sampe, Hisashi Nakamura, Atsushi Goto, Wataru Ohfuchi
    Abstract:

    Abstract In a set of idealized “aquaplanet” experiments with an atmospheric general circulation model to which zonally uniform sea surface temperature (SST) is prescribed globally as the lower boundary condition, an assessment is made of the potential influence of the frontal SST gradient upon the formation of a Storm Track and an eddy-driven midlatitude polar front jet (PFJ), and on its robustness against changes in the intensity of a subtropical jet (STJ). In experiments with the frontal midlatitude SST gradient as that observed in the southwestern Indian Ocean, transient eddy activity in each of the winter and summer hemispheres is organized into a deep Storm Track along the SST front with an enhanced low-level baroclinic growth of eddies. In the winter hemisphere, another Storm Track forms just below the intense STJ core, but it is confined to the upper troposphere with no significant baroclinic eddy growth underneath. The near-surface westerlies are strongest near the midlatitude SST front as observe...

  • reproducibility and future projection of the midwinter Storm Track activity over the far east in the cmip3 climate models in relation to haru ichiban over japan
    Journal of the Meteorological Society of Japan, 2009
    Co-Authors: Kazuaki Nishii, Hisashi Nakamura, Takafumi Miyasaka, Yu Kosaka
    Abstract:

    A reanalysis dataset is used to establish the relationship between the year-to-year fluctuations in the midwinter Storm-Track activity over the Far East measured by poleward heat flux associated with subweekly disturbances and the occurrence of the first spring Storm with strong southerly winds over Japan (Haru-Ichiban). Our analysis reveals that its early (delayed) occurrence tends to follow the enhanced (suppressed) winter Storm-Track activity with less (more) apparent minimum in midwinter in the course of the seasonal march. A metric is defined on the basis of the eddy heat flux to measure the reproducibility of the particular seasonal march of the Far East Storm-Track activity simulated in each of the Coupled Model Intercomparison Project Phase 3 climate models under the present climate. Under a particular global warming scenario, ensemble projection based only on the several models that show the highest reproducibility of the Storm-Track activity measured with the particular metric indicates that the future enhancement is likely in the midwinter Storm-Track activity associated with the weakening of the East Asian winter monsoon, implying that Haru-Ichiban is likely to occur earlier in the late 21st century than in the 20th century.