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Abstract
In existing literature, sudden stratospheric warming (SSW) events have been typically defined as displacement or split types. Detailed reexamination of SSW evolution has revealed that an SSW event often alters its type before and after the central day of the warming event. On the basis of this observation, we objectively define three types of SSW using wave amplitude: displacement–displacement (DD) type, displacement–split (DS) type, and split–split (SS) type. The geopotential height (GPH) amplitude of zonal wavenumbers 1 and 2 averaged over 55°–65°N at 10 hPa was used as a criterion for the classification. If the amplitude of zonal wavenumber 1 is larger (smaller) than that of wavenumber 2 before and after the central day of SSW, the event is regarded as a DD (SS) type. If the amplitude of zonal wavenumber 1 is larger than that of wavenumber 2 before the central day but is smaller after that day, the event is regarded as a DS type. The above classification algorithm has been applied to both reanalysis data and model results. We observe that conventional split-type SSW events identified by previous studies can be categorized as either DS- or SS-type events, each type of which exhibits different evolution characteristics. In particular, they are distinctively different during the prewarming period. In the SS type, the characteristics of the conventional split type are more obvious, and the features that differ from those of the DD type are the most robust. The model results generally resemble the reanalysis data, particularly in the DD cases.
Abstract
In existing literature, sudden stratospheric warming (SSW) events have been typically defined as displacement or split types. Detailed reexamination of SSW evolution has revealed that an SSW event often alters its type before and after the central day of the warming event. On the basis of this observation, we objectively define three types of SSW using wave amplitude: displacement–displacement (DD) type, displacement–split (DS) type, and split–split (SS) type. The geopotential height (GPH) amplitude of zonal wavenumbers 1 and 2 averaged over 55°–65°N at 10 hPa was used as a criterion for the classification. If the amplitude of zonal wavenumber 1 is larger (smaller) than that of wavenumber 2 before and after the central day of SSW, the event is regarded as a DD (SS) type. If the amplitude of zonal wavenumber 1 is larger than that of wavenumber 2 before the central day but is smaller after that day, the event is regarded as a DS type. The above classification algorithm has been applied to both reanalysis data and model results. We observe that conventional split-type SSW events identified by previous studies can be categorized as either DS- or SS-type events, each type of which exhibits different evolution characteristics. In particular, they are distinctively different during the prewarming period. In the SS type, the characteristics of the conventional split type are more obvious, and the features that differ from those of the DD type are the most robust. The model results generally resemble the reanalysis data, particularly in the DD cases.
Abstract
Three years of zonally averaged N2O and CH4 data from the SAMS instrument on Nimbus 7 are utilized to investigate the annual and semiannual cycles in long-lived tracer mixing ratios. The annual and semiannual variations are shown to be approximately antisymmetric and symmetric about the equator, respectively. Using the first three components of the annual cycle to estimate the time tendency, the tracer continuity equation is solved diagnostically to obtain the effective transport velocity (i.e., the meridional circulation that can produce the observed seasonal variations in the tracer fields). The resulting circulation is qualitatively in agreement with the diabatic circulations computed by other workers. The present calculations, however, exhibit a stronger equinoctial subsidence in the equatorial upper stratosphere than deduced in other studies as required to produce a “double peak” tracer structure that has the amplitude and vertical extent that is observed.
Abstract
Three years of zonally averaged N2O and CH4 data from the SAMS instrument on Nimbus 7 are utilized to investigate the annual and semiannual cycles in long-lived tracer mixing ratios. The annual and semiannual variations are shown to be approximately antisymmetric and symmetric about the equator, respectively. Using the first three components of the annual cycle to estimate the time tendency, the tracer continuity equation is solved diagnostically to obtain the effective transport velocity (i.e., the meridional circulation that can produce the observed seasonal variations in the tracer fields). The resulting circulation is qualitatively in agreement with the diabatic circulations computed by other workers. The present calculations, however, exhibit a stronger equinoctial subsidence in the equatorial upper stratosphere than deduced in other studies as required to produce a “double peak” tracer structure that has the amplitude and vertical extent that is observed.
Abstract
In this study the vertical convergence of the eddy heat flux, found as a forcing term in the thermodynamic energy equation of the transformed Eulerian mean formulation, is estimated in the troposphere and in the lower stratosphere from climatalogical data. Results show that while the heating rates caused by these eddy effects are small in the stratosphere they may play an important role in tropospheric circulation. The eddy-caused additions to the forcing field are seen as a region of significant cooling in the midaltitudes at the midtroposphere level and of weak heating throughout the tropical region. This net global cooling is important in balancing net global heating. In addition, the heating due to meridional heat flux is found to dominate compared to heating due to the vertical heat flux. To study circulation changes, the residual mean circulation is calculated with and without the estimated eddy heating effects. The added forcing causes additional circulation in each hemisphere that coincides with the primary circulation due to zonal-mean diabatic heating. Therefore, the eddy heat flux convergence has a significant role in enhancing the zonal-mean residual circulation in the troposphere.
Abstract
In this study the vertical convergence of the eddy heat flux, found as a forcing term in the thermodynamic energy equation of the transformed Eulerian mean formulation, is estimated in the troposphere and in the lower stratosphere from climatalogical data. Results show that while the heating rates caused by these eddy effects are small in the stratosphere they may play an important role in tropospheric circulation. The eddy-caused additions to the forcing field are seen as a region of significant cooling in the midaltitudes at the midtroposphere level and of weak heating throughout the tropical region. This net global cooling is important in balancing net global heating. In addition, the heating due to meridional heat flux is found to dominate compared to heating due to the vertical heat flux. To study circulation changes, the residual mean circulation is calculated with and without the estimated eddy heating effects. The added forcing causes additional circulation in each hemisphere that coincides with the primary circulation due to zonal-mean diabatic heating. Therefore, the eddy heat flux convergence has a significant role in enhancing the zonal-mean residual circulation in the troposphere.
