Role of planetary waves in the stratosphere-troposphere coupled variability in the northern hemisphere winter

Abstract. The probability density on a height-meridional plane of negative refractive index squared f(nk2<0) is introduced as a new analysis tool to investigate the climatology of the propagation conditions of stationary planetary waves based on NCEP/NCAR reanalysis data for 44 Northern Hemisphere boreal winters (1958–2002). This analysis addresses the control of the atmospheric state on planetary wave propagation. It is found that not only the variability of atmospheric stability with altitudes, but also the variability with latitudes has significant influence on planetary wave propagation. Eliassen-Palm flux and divergence are also analyzed to investigate the eddy activities and forcing on zonal mean flow. Only the ultra-long planetary waves with zonal wave number 1, 2 and 3 are investigated. In Northern Hemisphere winter the atmosphere shows a large possibility for stationary planetary waves to propagate from the troposphere to the stratosphere. On the other hand, waves induce eddy momentum flux in the subtropical troposphere and eddy heat flux in the subpolar stratosphere. Waves also exert eddy momentum forcing on the mean flow in the troposphere and stratosphere at middle and high latitudes. A similar analysis is also performed for stratospheric strong and weak polar vortex regimes, respectively. Anomalies of stratospheric circulation affect planetary wave propagation and waves also play an important role in constructing and maintaining of interannual variations of stratospheric circulation.

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On the Linkages between the Tropospheric Isentropic Slope and Eddy Fluxes of Heat during Northern Hemisphere Winter

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-11-0187.1 ◽

2012 ◽

Vol 69 (6) ◽

pp. 1811-1823 ◽

Cited By ~ 16

Author(s):

David W. J. Thompson ◽

Thomas Birner

Keyword(s):

Northern Hemisphere ◽

Low Frequency ◽

Lower Troposphere ◽

Heat Fluxes ◽

Daily Data ◽

Synoptic Eddies ◽

Eddy Fluxes ◽

Hemisphere Winter ◽

Northern Hemisphere Winter

Abstract Previous studies have demonstrated the key role of baroclinicity and thus the isentropic slope in determining the climatological-mean distribution of the tropospheric eddy fluxes of heat. Here the authors examine the role of variability in the isentropic slope in driving variations in the tropospheric eddy fluxes of heat about their long-term mean during Northern Hemisphere winter. On month-to-month time scales, the lower-tropospheric isentropic slope and eddy fluxes of heat are not significantly correlated when all eddies are included in the analysis. But the isentropic slope and heat fluxes are closely linked when the data are filtered to isolate the fluxes due to synoptic (<10 days) and low-frequency (>10 days) time scale waves. Anomalously steep isentropic slopes are characterized by anomalously poleward heat fluxes by synoptic eddies but anomalously equatorward heat fluxes by low-frequency eddies. Lag regressions based on daily data reveal that 1) variations in the isentropic slope precede by several days variations in the heat fluxes by synoptic eddies and 2) variations in the heat fluxes due to both synoptic and low-frequency eddies precede by several days similarly signed variations in the momentum flux at the tropopause level. The results suggest that seemingly modest changes in the tropospheric isentropic slope drive significant changes in the synoptic eddy heat fluxes and thus in the generation of baroclinic wave activity in the lower troposphere. The linkages have implications for understanding the extratropical tropospheric eddy response to a range of processes, including anthropogenic climate change, stratospheric variability, and extratropical sea surface temperature anomalies.

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The Role of Warm Conveyor Belts for the Intensification of Extratropical Cyclones in Northern Hemisphere Winter

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-15-0302.1 ◽

2016 ◽

Vol 73 (10) ◽

pp. 3997-4020 ◽

Cited By ~ 34

Author(s):

Hanin Binder ◽

Maxi Boettcher ◽

Hanna Joos ◽

Heini Wernli

Keyword(s):

Northern Hemisphere ◽

Extratropical Cyclones ◽

Cyclone Center ◽

Explosive Cyclones ◽

Conveyor Belts ◽

Upper Level ◽

Hemisphere Winter ◽

Warm Conveyor Belts ◽

Northern Hemisphere Winter

Abstract The role of warm conveyor belts (WCBs) and their associated positive low-level potential vorticity (PV) anomalies are investigated for extratropical cyclones in Northern Hemisphere winter, using ERA-Interim and composite techniques. The Spearman correlation coefficient of 0.68 implies a moderate to strong correlation between cyclone intensification and WCB strength. Hereby, cyclone intensification is quantified by the normalized maximum 24-h central sea level pressure deepening and WCB strength by the WCB air mass associated with the cyclone’s 24-h period of strongest deepening. Explosively intensifying cyclones typically have strong WCBs and pronounced WCB-related PV production in the cyclone center; they are associated with a WCB of type W2, which ascends close to the cyclone center. Cyclones with similar WCB strength but weak intensification are either diabatic Rossby waves, which do not interact with an upper-level disturbance, or cyclones where much of the WCB-related PV production occurs far from the cyclone center and thereby does not contribute strongly to cyclone deepening (WCB of type W1, which ascends mainly along the cold front). The category of explosively intensifying cyclones with weak WCBs is inhomogeneous but often characterized by a very low tropopause or latent heating independent of WCBs. These findings reveal that (i) diabatic PV production in WCBs is essential for the intensification of many explosive cyclones, (ii) the importance of WCBs for cyclone development strongly depends on the location of the PV production relative to the cyclone center, and (iii) a minority of explosive cyclones is not associated with WCBs.

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