Cyclonic and anticyclonic contributions to atmospheric energetics

Figure 2. Separated contributions from cyclonic and anticyclonic vortices to Eulerian statistics. a–b, 
Contributions to climatological-mean V’V’300 (m
2
/s
2
, colors) separately from cyclonic (a) and anticyclonic 
(b) vortices over the midwinter (24Jan) North Pacific. Contours denote climatological-mean total V’V’300 
from all vortices. c-d, Same as in Figs. 2a–b, respectively, but for U’V’300 (m
2
/s
2
, colors). Contours denote 
climatological-mean U300 (m/s). e–f, Same as in Figs. 2a–b, respectively, but for heat flux V’T’850 (K m/s). 
Contours denote climatological-mean total V’T’850 (K m/s) from all vortices.

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Scientific Reports
| (2021) 11:13202 | 
https://doi.org/10.1038/s41598-021-92548-7
www.nature.com/scientificreports/
V’T’850 relative to their counterpart for cyclonic vortices, are not well represented if relative vorticity is used 
in place of curvature (Supplementary Fig. S6). The discrepancies are attributable to shear vorticity, because 
the decomposed Eulerian statistics based on shear vorticity exhibit consistent and even stronger biases (Sup-
plementary Fig. S7).
Westerly acceleration by cyclonic and anticyclonic vortices. 
The converging/diverging fluxes of 
heat and momentum by cyclonic and anticyclonic vortices imply their feedback forcing onto the climatologi-
cal-mean westerlies
25
,
26
. To quantify this, three-dimensional eddy momentum flux convergence (divergence) is 
calculated as the westerly acceleration (deceleration) by eddies (see Methods for details), and its meridional sec-
tions for the western North Pacific [150°E-180°] are shown in Figs. 
3
a and b for cyclonic and anticyclonic eddies, 
respectively. Consistently with Figs. 
2
c–d, both cyclonic and anticyclonic vortices exert westerly deceleration 
(flux divergence) around the midwinter Pacific jet core (at 200-hPa) and acceleration (flux convergence) to its 
north through their poleward westerly momentum flux (Figs. 
3
a–b), although the contribution from cyclonic 
vortices is weaker. In fact, the westerly acceleration by cyclonic vortices occurring on the northern flank of the 
jet exhibits a much shallower structure (Fig. 
3
a). The near-surface westerly acceleration by cyclonic vortices 
reaches nearly 3 m/s a day, which is twice as strong as its anticyclonic counterpart and enough to replenish the 
climatological low-level westerlies within 3 days. This acceleration associated with the diverging upward and 
poleward E-P flux is due to the enhanced low-level poleward heat flux and equatorward momentum flux by 
cyclonic vortices. The latter diverges from the center of the Aleutian Low (AL), a semi-persistent surface oce-
anic low-pressure system, as marked with zero zonal wind in Fig. 
3
a. This diverging westerly momentum flux 
(or converging E-P flux) yields strong lower-tropospheric westerly deceleration near the AL center, reflecting 
the tendency for poleward moving cyclones to be distorted meridionally under the strong cyclonic shear of the 
westerlies
27
. The overall picture obtained from our analysis is that the poleward transport of westerly momentum 
from the upper-tropospheric core of the climatological-mean jet driven by the Hadley Cell is contributed to 
more by anticyclonic vortices. The transported westerly momentum is then transferred downward to maintain 
the near-surface westerlies around 40°N along the southern fringe of the AL, which is mainly by cyclonic vorti-
ces through their enhanced poleward heat transport. The near-surface westerly acceleration occurs also through