Abstract
Abstract. The agreement between reanalysis datasets, in terms of the zonal-mean momentum budget, is evaluated during sudden stratospheric warming (SSW) events. It is revealed that there is a good agreement among datasets in the lower stratosphere and troposphere concerning zonal-mean zonal wind, but less so in the upper stratosphere. Forcing terms of the momentum equation are also relatively similar in the lower atmosphere, but their uncertainties are typically larger than uncertainties of the zonal-wind tendency. Similar to zonal-wind tendency, the agreement among forcing terms is degraded in the upper stratosphere. Discrepancies among reanalyses increase during the onset of SSW events, a period characterized by unusually large fluxes of planetary-scale waves from the troposphere to the stratosphere, and decrease substantially after the onset. While the largest uncertainties in the resolved terms of the momentum budget are found in the Coriolis torque, momentum flux convergence also presents a non-negligible spread among the reanalyses. Such a spread is reduced in the latest reanalysis products, decreasing the uncertainty of the momentum budget. It is also found that the uncertainties in the Coriolis torque depend on the strength of SSW events: the SSW events that exhibit the most intense deceleration of zonal-mean zonal wind are subject to larger discrepancies among reanalyses. These uncertainties in stratospheric circulation, however, are not communicated to the troposphere.
Highlights
Sudden stratospheric warming (SSW) events are prime manifestations of coupling between the tropospheric and stratospheric circulations (Baldwin, 2001)
Whenever the average or standard deviation (SD) of multiple reanalyses is taken as a reference, it is performed on a subset including the latest reanalysis products from each center (ERA-Interim, NCEP-CFSR, JRA-55, and MERRA2)
When considering the dominant fluxes of wave activity producing SSW evens, we find that out of seven low-agreement SSW events (LASSWs), four are W1-dominant and one is W2 dominant and out of seven high-agreement SSW events (HASSWs), one is W1-dominant and three are W2-dominant. This seems to indicate that wavenumber-1 wave drag is responsible for larger uncertainties in reanalysis datasets but a detailed analysis reveals that inter-reanalysis spread is not markedly different between W1-dominant and W2-dominant events, suggesting that it is the intensity of wave drag rather than the longitudinal scale of wave activity that is linked to uncertainties among reanalyses
Summary
Sudden stratospheric warming (SSW) events are prime manifestations of coupling between the tropospheric and stratospheric circulations (Baldwin, 2001). A comprehensive comparison of momentum diagnostics in the stratosphere among reanalyses further revealed non-negligible inconsistencies in the zonal-mean momentum equation, resulting primarily from inter-data variability in the residual circulation in the mid-stratosphere (Martineau et al, 2016). Martineau et al (2016) have shown that the ability to explain the stratospheric zonal-mean zonal-wind tendencies using the forcing terms of the zonal-mean momentum equation has improved in the latest reanalysis products and that momentum diagnostics tend to agree better among the latest reanalyses This improvement was demonstrated in the context of the wintertime climatology and different regimes of vortex variability (strong or weak, accelerating or decelerating) with an emphasis on the mid-stratosphere.
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