Cyclonic and anticyclonic contributions to atmospheric energetics

Scientific Reports
| (2021) 11:13202 | 
https://doi.org/10.1038/s41598-021-92548-7
www.nature.com/scientificreports/
Results
Identification of cyclonic and anticyclonic vortices. 
In the Northern Hemisphere, a cyclone (anticy-
clone) accompanies counterclockwise (clockwise) circulation. It is not difficult to identify its rotation center at 
a near-surface level, where the background westerlies and associated pressure gradient are weak. In the upper 
troposphere, however, identifying the rotation center is generally more difficult, because cyclonic (anticyclonic) 
circulations often appear as open pressure troughs (ridges) associated with a meandering westerly jet. An upper-
level geopotential height field is therefore not suited for determining “centers”, because of its strong meridional 
gradient across the jet stream. A vorticity field may be another possibility, but the center detection actually fails 
due to strong shear vorticity along the westerly jet as schematically illustrated in Supplementary Fig. S1a.
To circumvent the aforementioned challenges in identifying pressure troughs and ridges (or cyclonic and 
anticyclonic vortices, respectively) on both sides of a meandering jet (Supplementary Fig. S1b), a new method 
developed here relies on curvature or curvature vorticity calculated from horizontal winds (see Methods). Sup-
plementary Figs. S1c and S1d show snapshots of curvature fields in the upper and lower troposphere, respectively. 
Positive and negative curvatures correspond well to upper-tropospheric pressure troughs and ridges, respectively, 
and the corresponding near-surface cyclones/troughs and anticyclones/ridges as well. These troughs and ridges 
are represented with comparable magnitudes of curvature between the upper and lower troposphere, as an 
advantage of the use of curvature over other measures (Supplementary Fig. S2). In fact, relative vorticity is less 
effective in capturing those upper-tropospheric troughs and ridges (e.g., a ridge along ~ 165°E in Supplementary 
Fig. S2), due to the shear vorticity included in the full relative vorticity (Supplementary Fig. S2c). In addition, 
an upper-tropospheric cut-off low around [50°N, 175°W] coincides with a well-defined maximum of curvature 
(Fig. S1c), which cannot be captured successfully by any of the other measures (Supplementary Fig. S2). Again, 
this example demonstrates an advantage of using curvature, in addition to its straightforward physical mean-
ing since its reciprocal is simply the radius of curvature and thus corresponds to the horizontal size of an eddy.