[A-CG30] Extratropical oceans and atmosphere

[ACG30-P20] Characteristics of extratropical atmospheric circulation on the isentropic surface in the middle to upper troposphere during the Northern Hemisphere winter

*Masaya Kuramochi¹, Hiroaki Ueda² (1.Graduate School of Science and Technology, University of Tsukuba, 2.Faculty of Life and Environmental Sciences, University of Tsukuba)

Keywords:mass-weighted isentropic mean, isentropic airmass flux, extratropical direct circulation, atmospheric general circulation, planetary wave

The zonal mean meridional circulation diagnosed by the mass-weighted isentropic zonal mean (Z-MIM) exhibits the distinct extratropical direct (ETD) circulation in the winter hemisphere, describing the Lagrangian mean circulation. Based on Z-MIM, the upper warm airmass flux defined between 290–320 K can illustrate regional airmass streams corresponding to the upper branch of the ETD circulation in the Northern Hemisphere winter. In this study, we examined the dynamic structure of ETD circulation using the upper warm airmass flux following the mass-weighted isentropic time mean (T-MIM) framework. The upper warm airmass flux decomposed into the unweighted, stationary eddy, and transient eddy terms showed the most dominant contribution of the transient eddy to the poleward flow in the upper branch of ETD circulation. The stationary eddy contribution was also recognized following the transient eddy term. This result indicates that the poleward thickness larger than the equatorward one is a crucial reason for the ETD circulation. Further analysis focusing on the diabatic genesis/loss of the upper warm airmass amount demonstrated that the upper warm air mass is generated over the midlatitude ocean and lost over the polar continent. The former may be related to the moisture supply from the sea surface and the latter may be associated with radiation cooling. As for the interannual variation, the stationary eddy was predominant and the transient eddy acts to suppress the anomalous ETD circulation, indicating the seesaw variation between the stationary and transient eddies in the zonal mean field.