Nonadiabatic and nonquasigeostrophic sources of upper tropospheric Rossby wave activity

The budget of local (Rossby) wave activity in the quasigeostrophic (QG) limit is largely governed by the convergence of wave activity fluxes: the zonal advective flux by geostrophic flow and the Eliassen-Palm (EP) flux, i.e. radiation stress associated with baroclinic wave packets traveling through an inhomogeneous jet stream. The residual of the budget presumably accounts for nonadiabatic and non-QG sources and sinks of wave activity.  Here, we analyze ERA5 data to evaluate non-QG wave sources in the upper troposphere associated with local diabatic heating and advection of potential vorticity by ageostrophic (divergent) flow driven by remote heat sources.  (The diabatic heating is estimated from the material tendency of potential temperature.)

 

Climatological mean distribution of the upper tropospheric wave activity sources in the Northern Hemisphere reveal a few hot spots:

• Local diabatic sources:  In winter (DJF), large positive values are spotted over the northeast Asia to the Sea of Japan.  Secondary maxima are found in the subtropics of the eastern Pacific and the Atlantic.  In summer (JJA), large positive values are found in the subtropics from the Indian Ocean to the western Pacific and in the subtropics of western Atlantic and North America.  

• Non-QG sources (ageostrophic advection of PV): In winter (DJF), large positive (negative) values are located in the subtropics (midlatitudes) of the eastern Pacific. Somewhat surprisingly, significant secondary maxima (positive values) appear in the midlatitudes of the eastern Pacific and along the Atlantic storm track.  In summer (JJA), large positive (negative) values are centered on the subtropics (midlatitude) of the Eurasian continent but they extend over a broader range of longitudes.

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 present study suggests that the eastward enhancement of wave activity is the result of production of wave activity by the local diabatic sources at QSAR and the subsequent downstream advection by the jet stream.  The enhancement of wave activity west of QSAR during summer on the other hand arises from the combination of diabatic heating and ageostrophic advection of potential vorticity, both enhanced by the Asian summer monsoon in the subtropics of the Eurasian continent.