李宏一 博士

Department of Geophysical Sciences, University of Chicago

邀請人: 陳毅軒

Abstract:
In the earth’s atmosphere, the primary source of planetary wave activity is at the surface in the midlatitudes, and wave activity propagates upwards and then transports meridionally in the upper troposphere. Eventually, wave activity is dissipated in various routes, e.g., friction, mixing, radiative damping, or driving general circulations, indicating the negative divergence of wave activity flux in the interior of the atmosphere to balance wave dissipation. The quasi-geostrophic (QG) theory approximates the divergence of wave activity flux to eddy potential vorticity (PV) flux so that one would expect negative eddy PV flux, i.e., a downgradient eddy PV flux. However, upgradient eddy PV flux has been observed at the poleward flank of the subtropical jet in the upper troposphere during the cold season (e.g., Birner et al. 2013), suggesting the generation of waves instead of dissipation. By analyzing ERA5 data, we partition the eddy PV flux into contributions from geostrophic advection and ageostrophic advection. It is found that the eddy PV flux associated with ageostrophic advection is strongly positive around the core of the subtropical jet in the winter season, whereas the eddy PV flux associated with geostrophic advection is negative. The maintenance of upgradient ageostrophic eddy flux remains speculative, likely due to a torque exerted by nonadiabatic forcing.We generalize the local wave activity budget (Huang and Nakamura 2016) by incorporating non-QG and nonadiabatic wave sources. The non-QG eddy PV flux matches meridional divergent wind, indicating its relation to meridional overturning circulations (Hadley and Ferrel cells). Previously it has been recognized that the climatological mean wave activity is enhanced on the eastern side of the column water vapor maximum (quasi-stationary atmospheric river or QSAR) at 30°N during the cold season but on both sides during summer (Lee and Mitchell 2021). The local wave budget analysis suggests that the eastward enhancement of wave activity results from the production of wave activity by the local nonadiabatic sources at QSAR and the subsequent downstream advection by the jet stream. We also apply the vertically-integrated wave budgets to examine extra-tropical storm tracks. By reconstructing the budgets driven by a fixed transport velocity and damping rate evaluated from the seasonal climatology but suppressing the positive residual values, we find that the diabatic sources contribute to 26% and 20%, respectively, of North Atlantic and North Pacific storm track activities in winter.