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- Author or Editor: Masakazu Taguchi x
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Abstract
This study investigates basic characteristics of stratospheric predictability in the Northern Hemisphere using 1-month hindcast (HC) experiment data from the Japan Meteorological Agency for 1979–2009. The author describes characteristics of forecast properties of spread, error (root-mean-square error), and anomaly correlation, contrasting the stratosphere and troposphere for different seasons and exploring the so-called spread–skill relationship for the winter stratosphere. The properties are defined for each HC set (ensemble forecasts initialized on the same day). The error and anomaly correlation are calculated with the ensemble mean as measures of forecast accuracy. The author also examines the role of stratospheric sudden warmings (SSWs) in variations in forecast accuracy. Results show that, for lead times shorter than about 10–15 days, the accuracy of the HC data is higher on average and more variable in the stratosphere than in the troposphere, especially for the northern winter. This is reflected in larger averages and variability in the predictable time limit, or the characteristic time scale of useful predictions, for the winter stratosphere. The author also reveals that the spread–skill relationship for the northern winter stratosphere is characterized by the existence of notable outliers from their expected linear distribution; the outliers have markedly large errors for given spreads. Most outliers are contributed by HC sets initialized before observed major SSWs. Such HC data fail to reproduce the strength and/or shape of the stratospheric polar vortex, including both onset and recovery phases of SSWs. The HC data tend to yield a too-strong vortex and shorter-than-average predictable limit.
Abstract
This study investigates basic characteristics of stratospheric predictability in the Northern Hemisphere using 1-month hindcast (HC) experiment data from the Japan Meteorological Agency for 1979–2009. The author describes characteristics of forecast properties of spread, error (root-mean-square error), and anomaly correlation, contrasting the stratosphere and troposphere for different seasons and exploring the so-called spread–skill relationship for the winter stratosphere. The properties are defined for each HC set (ensemble forecasts initialized on the same day). The error and anomaly correlation are calculated with the ensemble mean as measures of forecast accuracy. The author also examines the role of stratospheric sudden warmings (SSWs) in variations in forecast accuracy. Results show that, for lead times shorter than about 10–15 days, the accuracy of the HC data is higher on average and more variable in the stratosphere than in the troposphere, especially for the northern winter. This is reflected in larger averages and variability in the predictable time limit, or the characteristic time scale of useful predictions, for the winter stratosphere. The author also reveals that the spread–skill relationship for the northern winter stratosphere is characterized by the existence of notable outliers from their expected linear distribution; the outliers have markedly large errors for given spreads. Most outliers are contributed by HC sets initialized before observed major SSWs. Such HC data fail to reproduce the strength and/or shape of the stratospheric polar vortex, including both onset and recovery phases of SSWs. The HC data tend to yield a too-strong vortex and shorter-than-average predictable limit.
Abstract
This study investigates the predictability of three major stratospheric sudden warmings (MSSWs) of the vortex split type: the Southern Hemisphere case in September 2002 and two Northern Hemisphere cases in January 2009 and February 1989. The author examines changes in the predictability of the MSSWs with lead time, as well as the connection of the predictability to lower-atmospheric features for pre- and post-MSSW periods. The Japan Meteorological Agency (JMA)’s 1-month ensemble hindcast (HC) experiment data are compared to the Japanese 25-year Reanalysis Project (JRA-25)/JMA Climate Data Assimilation System (JCDAS) data.
For the pre-MSSW period, a strong predictability connection is observed among all three cases. Unsuccessful predictions of the MSSWs are characterized by an underestimation (or lack) of the enhanced wave activity in the lower stratosphere, which is further related to the strength and persistence of the upper-tropospheric ridge and trough. The mean zonal wind profile in the upper troposphere is also important for the 2009 case. These results confirm the role of tropospheric wave forcing of the MSSWs in the context of predictability. The characteristic time scale for successful predictions is approximately 10 days–2 weeks, which roughly corresponds to the time scale of the tropospheric wave forcing. No ensemble member successfully predicts the MSSWs with lead times longer than the time scale.
The predictability connection between the stratospheric and tropospheric anomalies is more subtle for the post-MSSW period. In particular, the HC group initialized about 1 week before the MSSWs tends to reproduce the evolution of the stratosphere after the MSSWs well but not that of the troposphere in some cases.
Abstract
This study investigates the predictability of three major stratospheric sudden warmings (MSSWs) of the vortex split type: the Southern Hemisphere case in September 2002 and two Northern Hemisphere cases in January 2009 and February 1989. The author examines changes in the predictability of the MSSWs with lead time, as well as the connection of the predictability to lower-atmospheric features for pre- and post-MSSW periods. The Japan Meteorological Agency (JMA)’s 1-month ensemble hindcast (HC) experiment data are compared to the Japanese 25-year Reanalysis Project (JRA-25)/JMA Climate Data Assimilation System (JCDAS) data.
For the pre-MSSW period, a strong predictability connection is observed among all three cases. Unsuccessful predictions of the MSSWs are characterized by an underestimation (or lack) of the enhanced wave activity in the lower stratosphere, which is further related to the strength and persistence of the upper-tropospheric ridge and trough. The mean zonal wind profile in the upper troposphere is also important for the 2009 case. These results confirm the role of tropospheric wave forcing of the MSSWs in the context of predictability. The characteristic time scale for successful predictions is approximately 10 days–2 weeks, which roughly corresponds to the time scale of the tropospheric wave forcing. No ensemble member successfully predicts the MSSWs with lead times longer than the time scale.
The predictability connection between the stratospheric and tropospheric anomalies is more subtle for the post-MSSW period. In particular, the HC group initialized about 1 week before the MSSWs tends to reproduce the evolution of the stratosphere after the MSSWs well but not that of the troposphere in some cases.
Abstract
The dynamical effect of the stratosphere on the troposphere is investigated in a series of numerical experiments with a simple global circulation model under a perpetual winter condition. One control simulation (CS) and four degraded stratosphere simulations (DS) are performed for three values of topographic amplitude h 0 of 0, 500, and 1000 m to examine the role of forced planetary waves. In DS, the thermal relaxation rate is increased only in the stratosphere to give poor simulations there. A comparison of the tropospheric circulation between CS and DS reveals the stratospheric effect on the troposphere, or tropospheric response to the stratospheric degradation.
The numerical experiments demonstrate that the tropospheric response to the stratospheric degradation depends on h 0. The response is weak for h 0 = 0 and 500 m but remarkably strong for h 0 = 1000 m. The response is projected onto the dominant mode of variability, or annular variability, which is the model counterpart of the Arctic Oscillation, in different ways depending on h 0. The response is essentially projected onto the annular variability for h 0 = 0 and 1000 m. The climatological states shift toward negative (positive) polarity of the mode for h 0 = 0 m (1000 m), characterized by positive (negative) anomalies of geopotential height in high latitudes. The response is almost independent of the annual variability for h 0 = 500 m. A diagnosis based on the transformed Eulerian mean equations shows that the change of the wave driving in the stratosphere can explain the tropospheric response in part, suggesting that the downward control is a mechanism at play.
Abstract
The dynamical effect of the stratosphere on the troposphere is investigated in a series of numerical experiments with a simple global circulation model under a perpetual winter condition. One control simulation (CS) and four degraded stratosphere simulations (DS) are performed for three values of topographic amplitude h 0 of 0, 500, and 1000 m to examine the role of forced planetary waves. In DS, the thermal relaxation rate is increased only in the stratosphere to give poor simulations there. A comparison of the tropospheric circulation between CS and DS reveals the stratospheric effect on the troposphere, or tropospheric response to the stratospheric degradation.
The numerical experiments demonstrate that the tropospheric response to the stratospheric degradation depends on h 0. The response is weak for h 0 = 0 and 500 m but remarkably strong for h 0 = 1000 m. The response is projected onto the dominant mode of variability, or annular variability, which is the model counterpart of the Arctic Oscillation, in different ways depending on h 0. The response is essentially projected onto the annular variability for h 0 = 0 and 1000 m. The climatological states shift toward negative (positive) polarity of the mode for h 0 = 0 m (1000 m), characterized by positive (negative) anomalies of geopotential height in high latitudes. The response is almost independent of the annual variability for h 0 = 500 m. A diagnosis based on the transformed Eulerian mean equations shows that the change of the wave driving in the stratosphere can explain the tropospheric response in part, suggesting that the downward control is a mechanism at play.
Abstract
This study explores the climatological annual cycle of temperature, circulation, and wave driving distributions in the tropical lower stratosphere as produced in a 50-yr simulation of the Whole Atmosphere Community Climate Model (WACCM). The simulation is forced with a climatological sea surface temperature and sea ice condition. The present diagnoses verify the primary balances of the annual cycle in this region, consistent with lower temperatures, stronger residual circulation (upwelling and local meridional outflow), and nearby stronger wave driving for Northern Hemisphere (NH) winter. An in-detail analysis on the wave driving further reveals that the stronger driving, occurring mostly in the northern tropics and subtropics, is contributed by northward and upward propagation (associated with meridional and vertical fluxes of zonal momentum, respectively) of equatorial Rossby waves forced by convective heating, and also by equatorward propagation of NH extratropical planetary and synoptic waves. The results are used to discuss implications about possible factors that may affect the different observations of the wave driving. The present framework and results will be extended to investigate ENSO-induced changes in this region during NH winter in a forthcoming paper.
Abstract
This study explores the climatological annual cycle of temperature, circulation, and wave driving distributions in the tropical lower stratosphere as produced in a 50-yr simulation of the Whole Atmosphere Community Climate Model (WACCM). The simulation is forced with a climatological sea surface temperature and sea ice condition. The present diagnoses verify the primary balances of the annual cycle in this region, consistent with lower temperatures, stronger residual circulation (upwelling and local meridional outflow), and nearby stronger wave driving for Northern Hemisphere (NH) winter. An in-detail analysis on the wave driving further reveals that the stronger driving, occurring mostly in the northern tropics and subtropics, is contributed by northward and upward propagation (associated with meridional and vertical fluxes of zonal momentum, respectively) of equatorial Rossby waves forced by convective heating, and also by equatorward propagation of NH extratropical planetary and synoptic waves. The results are used to discuss implications about possible factors that may affect the different observations of the wave driving. The present framework and results will be extended to investigate ENSO-induced changes in this region during NH winter in a forthcoming paper.
Abstract
This paper presents statistical analyses of possible associations between major stratospheric sudden warming (SSW) and tropospheric blocking events in the Northern Hemisphere (NH) with 49 yr of NCEP–NCAR reanalysis data from 1957/58 to 2005/06. Using a random shuffling or “bootstrap” method, these analyses explore two hypotheses claiming that blocking events occur preferentially and last longer in association with SSWs (pre- and post-SSW periods are considered separately). In the shuffling method, the defined SSWs are randomly redistributed to evaluate the statistical significance of linked cases in the original data. The author’s analyses generally do not support either hypothesis for the pre- or post-SSW period when treating the SSW events all together, suggesting that such associations are not dominant modes of coupling.
Abstract
This paper presents statistical analyses of possible associations between major stratospheric sudden warming (SSW) and tropospheric blocking events in the Northern Hemisphere (NH) with 49 yr of NCEP–NCAR reanalysis data from 1957/58 to 2005/06. Using a random shuffling or “bootstrap” method, these analyses explore two hypotheses claiming that blocking events occur preferentially and last longer in association with SSWs (pre- and post-SSW periods are considered separately). In the shuffling method, the defined SSWs are randomly redistributed to evaluate the statistical significance of linked cases in the original data. The author’s analyses generally do not support either hypothesis for the pre- or post-SSW period when treating the SSW events all together, suggesting that such associations are not dominant modes of coupling.
Abstract
This study investigates ENSO-induced changes in the tropical lower stratosphere for northern winter as simulated by the Whole Atmosphere Community Climate Model (WACCM). A comparison is made between two 3650-day perpetual January experiments forced with La Niña– and El Niño–like sea surface temperature conditions over the equatorial Pacific. The present analysis includes an extension of the diagnostic framework used for the climatological annual cycle in . A comprehensive description of the ENSO-induced changes, together with their heat and zonal momentum budget diagnoses, demonstrates that the changes are consistently characterized by cooling, locally accelerated Brewer–Dobson circulation (upwelling and poleward flow) and strengthened tropical/subtropical wave driving for the El Niño–like condition. The cooling broadly peaks near the equator with general hemispheric symmetry, and the strengthenings in the poleward flow and wave driving take place in both hemispheres. An important role in the strengthened wave driving is played by changes in tropical/subtropical stationary waves. The changes notably include a dumbbell-shaped height pattern over the Pacific or a modulation of equatorial Rossby waves in response to redistributed convective heating with the ENSO-like perturbation.
Abstract
This study investigates ENSO-induced changes in the tropical lower stratosphere for northern winter as simulated by the Whole Atmosphere Community Climate Model (WACCM). A comparison is made between two 3650-day perpetual January experiments forced with La Niña– and El Niño–like sea surface temperature conditions over the equatorial Pacific. The present analysis includes an extension of the diagnostic framework used for the climatological annual cycle in . A comprehensive description of the ENSO-induced changes, together with their heat and zonal momentum budget diagnoses, demonstrates that the changes are consistently characterized by cooling, locally accelerated Brewer–Dobson circulation (upwelling and poleward flow) and strengthened tropical/subtropical wave driving for the El Niño–like condition. The cooling broadly peaks near the equator with general hemispheric symmetry, and the strengthenings in the poleward flow and wave driving take place in both hemispheres. An important role in the strengthened wave driving is played by changes in tropical/subtropical stationary waves. The changes notably include a dumbbell-shaped height pattern over the Pacific or a modulation of equatorial Rossby waves in response to redistributed convective heating with the ENSO-like perturbation.
Abstract
This study investigates the predictability of major stratospheric sudden warmings (MSSWs) with a chief question: how far in advance can MSSWs be forecasted? An average picture and case-to-case variations of the MSSW predictability are revealed by analyzing operational 1-month ensemble prediction data of the Japan Meteorological Agency from 2001/02 to 2012/13 in comparison with the Japanese 55-year Reanalysis Project (JRA-55) data. The variations are further related to planetary wave forcing (PWF) from the troposphere to the stratosphere. A contingency table analysis for nine MSSWs occurring in the period shows that the average percentage of ensemble members that successfully forecast the MSSWs is about 70%, 30%, and 20% for lead times of 5, 10, and 15 days, respectively, when a 3-day time difference between actual and forecasted vortex collapses, or zonal wind reversals, is allowed. Using other measures such as the root-mean-square error and anomaly correlation of 10-hPa geopotential height, forecasts of lead times less than about two weeks are judged to be useful. Results also show large case-to-case predictability variations for lead times of about 10 days. The variations for most MSSWs are explained by the degree to which the strength of PWF, or wave activity flux in the lower stratosphere, is forecasted in comparison to the JRA-55 data. Forecasted PWF in the lower stratosphere is largely determined by that in the upper troposphere for several MSSWs, whereas it is also affected by the planetary wave propagation between the two regions for a few others.
Abstract
This study investigates the predictability of major stratospheric sudden warmings (MSSWs) with a chief question: how far in advance can MSSWs be forecasted? An average picture and case-to-case variations of the MSSW predictability are revealed by analyzing operational 1-month ensemble prediction data of the Japan Meteorological Agency from 2001/02 to 2012/13 in comparison with the Japanese 55-year Reanalysis Project (JRA-55) data. The variations are further related to planetary wave forcing (PWF) from the troposphere to the stratosphere. A contingency table analysis for nine MSSWs occurring in the period shows that the average percentage of ensemble members that successfully forecast the MSSWs is about 70%, 30%, and 20% for lead times of 5, 10, and 15 days, respectively, when a 3-day time difference between actual and forecasted vortex collapses, or zonal wind reversals, is allowed. Using other measures such as the root-mean-square error and anomaly correlation of 10-hPa geopotential height, forecasts of lead times less than about two weeks are judged to be useful. Results also show large case-to-case predictability variations for lead times of about 10 days. The variations for most MSSWs are explained by the degree to which the strength of PWF, or wave activity flux in the lower stratosphere, is forecasted in comparison to the JRA-55 data. Forecasted PWF in the lower stratosphere is largely determined by that in the upper troposphere for several MSSWs, whereas it is also affected by the planetary wave propagation between the two regions for a few others.
Abstract
This study investigates winter forecasts of the northern stratosphere and troposphere using seasonal hindcast experiments of the Japan Meteorological Agency (JMA). A main focus is placed on the seasonal forecasts of the December–February (DJF)-mean northern annular mode (NAM) when the forecasts are initialized in late fall. Results demonstrate that the hindcast data have significant skill for both ensemble-mean and category (probability) forecasts of the NAM but only in the stratosphere. Probability forecasts for DJF major stratospheric sudden warmings (MSSWs) are also suggested to be significant (higher probabilities for actual MSSW years) near the 90% confidence level. The forecast skill of the stratospheric NAM changes with the observed phase of the quasi-biennial oscillation (QBO), although the QBO is not simulated but is only included in initial conditions. The skill is higher for the easterly phase, characterized by hits of negative NAM states (weaker-than-normal polar vortex), whereas it is lower for the westerly phase, reflecting misses of positive NAM states. It is finally shown that a verification score for category forecasts of the stratospheric and tropospheric NAM tends to covary as a whole. The tropospheric forecast skill is significant when the stratosphere has large NAM anomalies in the real world and they are well forecasted. In contrast, the tropospheric forecasts are sometimes poor when the stratospheric forecasts fail to capture observed NAM conditions. It is speculated that stratospheric and tropospheric forecasts could be improved together through the stratosphere–troposphere coupling for such cases, that is, by successfully forecasting anomalous vortex states in the stratosphere.
Abstract
This study investigates winter forecasts of the northern stratosphere and troposphere using seasonal hindcast experiments of the Japan Meteorological Agency (JMA). A main focus is placed on the seasonal forecasts of the December–February (DJF)-mean northern annular mode (NAM) when the forecasts are initialized in late fall. Results demonstrate that the hindcast data have significant skill for both ensemble-mean and category (probability) forecasts of the NAM but only in the stratosphere. Probability forecasts for DJF major stratospheric sudden warmings (MSSWs) are also suggested to be significant (higher probabilities for actual MSSW years) near the 90% confidence level. The forecast skill of the stratospheric NAM changes with the observed phase of the quasi-biennial oscillation (QBO), although the QBO is not simulated but is only included in initial conditions. The skill is higher for the easterly phase, characterized by hits of negative NAM states (weaker-than-normal polar vortex), whereas it is lower for the westerly phase, reflecting misses of positive NAM states. It is finally shown that a verification score for category forecasts of the stratospheric and tropospheric NAM tends to covary as a whole. The tropospheric forecast skill is significant when the stratosphere has large NAM anomalies in the real world and they are well forecasted. In contrast, the tropospheric forecasts are sometimes poor when the stratospheric forecasts fail to capture observed NAM conditions. It is speculated that stratospheric and tropospheric forecasts could be improved together through the stratosphere–troposphere coupling for such cases, that is, by successfully forecasting anomalous vortex states in the stratosphere.
Abstract
A composite analysis is made of 132 stratospheric sudden warming (SSW) events obtained in a 10 000-day integration with a simple global circulation model under a perpetual-winter condition. The analysis confirms general features of the SSWs, such as enhanced upward propagation of planetary wave activity from the troposphere to the stratosphere before the SSWs and downward propagation of warming signals to the lower stratosphere after the events.
A further dynamical diagnosis shows that the tropospheric circulation is quite different between pre-SSW and post-SSW periods in terms of the zonal mean zonal wind, planetary wave, and synoptic-scale waves. In the pre-SSW period, the planetary wave is more active than normal in relation to the tropospheric westerly jet that shifts poleward. In the post-SSW period, on the other hand, the planetary wave is less active, while the mean zonal wind is close to the climatology. Synoptic-scale waves also exhibit anomalous features in both periods corresponding to the anomalous planetary-scale flow. The less active planetary wave in the post-SSW period is a return signal of the SSWs, or a tropospheric response to the SSWs, since the signal disappears as SSWs are absent by increased thermal damping in the stratosphere.
Abstract
A composite analysis is made of 132 stratospheric sudden warming (SSW) events obtained in a 10 000-day integration with a simple global circulation model under a perpetual-winter condition. The analysis confirms general features of the SSWs, such as enhanced upward propagation of planetary wave activity from the troposphere to the stratosphere before the SSWs and downward propagation of warming signals to the lower stratosphere after the events.
A further dynamical diagnosis shows that the tropospheric circulation is quite different between pre-SSW and post-SSW periods in terms of the zonal mean zonal wind, planetary wave, and synoptic-scale waves. In the pre-SSW period, the planetary wave is more active than normal in relation to the tropospheric westerly jet that shifts poleward. In the post-SSW period, on the other hand, the planetary wave is less active, while the mean zonal wind is close to the climatology. Synoptic-scale waves also exhibit anomalous features in both periods corresponding to the anomalous planetary-scale flow. The less active planetary wave in the post-SSW period is a return signal of the SSWs, or a tropospheric response to the SSWs, since the signal disappears as SSWs are absent by increased thermal damping in the stratosphere.
Abstract
This study explores interannual variations (IAVs) of the stratosphere and troposphere during Northern Hemisphere (NH) winter using a 50-yr simulation of Sassi et al. with the Whole Atmosphere Community Climate Model (WACCM). The simulation is forced with observed sea surface temperature (SST) and sea ice distributions from 1950 to 1999. The focus herein is on tropical tropospheric variations correlated with NH stratospheric variations and El Niño–Southern Oscillation (ENSO).
The discussed correlation analysis generally reproduces the following features as obtained in observational studies by Salby and Callaghan: an intensification of the Brewer–Dobson (BD) circulation driven by enhanced planetary wave (PW) drag in the NH stratosphere is accompanied by intensification of the Hadley circulation and anomalous warming of the tropical troposphere. It is further revealed that the tropical tropospheric warming is a reflection of the ENSO variability resulting from a positive correlation between the PW driving/BD circulation and ENSO, whereas the Hadley circulation does intensify with the BD circulation even when ENSO’s effects are removed.
Abstract
This study explores interannual variations (IAVs) of the stratosphere and troposphere during Northern Hemisphere (NH) winter using a 50-yr simulation of Sassi et al. with the Whole Atmosphere Community Climate Model (WACCM). The simulation is forced with observed sea surface temperature (SST) and sea ice distributions from 1950 to 1999. The focus herein is on tropical tropospheric variations correlated with NH stratospheric variations and El Niño–Southern Oscillation (ENSO).
The discussed correlation analysis generally reproduces the following features as obtained in observational studies by Salby and Callaghan: an intensification of the Brewer–Dobson (BD) circulation driven by enhanced planetary wave (PW) drag in the NH stratosphere is accompanied by intensification of the Hadley circulation and anomalous warming of the tropical troposphere. It is further revealed that the tropical tropospheric warming is a reflection of the ENSO variability resulting from a positive correlation between the PW driving/BD circulation and ENSO, whereas the Hadley circulation does intensify with the BD circulation even when ENSO’s effects are removed.
