Cyclonic and anticyclonic contributions to atmospheric energetics

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Vol.:(0123456789)
Scientific Reports
| (2021) 11:13202
| 
https://doi.org/10.1038/s41598-021-92548-7
www.nature.com/scientificreports
Cyclonic and anticyclonic 
contributions to atmospheric 
energetics
Satoru Okajima
1
*
, Hisashi Nakamura
1
 & Yohai Kaspi
2
Migratory cyclones and anticyclones account for most of the day-to-day weather variability in the 
extratropics. These transient eddies act to maintain the midlatitude jet streams by systematically 
transporting westerly momentum and heat. Yet, little is known about the separate contributions of 
cyclones and anticyclones to their interaction with the westerlies. Here, using a novel methodology 
for identifying cyclonic and anticyclonic vortices based on curvature, we quantify their separate 
contributions to atmospheric energetics and their feedback on the westerly jet streams as represented 
in Eulerian statistics. We show that climatological westerly acceleration by cyclonic vortices acts to 
dominantly reinforce the wintertime eddy-driven near-surface westerlies and associated cyclonic 
shear. Though less baroclinic and energetic, anticyclones still play an important role in transporting 
westerly momentum toward midlatitudes from the upper-tropospheric thermally driven jet core and 
carrying eddy energy downstream. These new findings have uncovered essential characteristics of 
atmospheric energetics, storm track dynamics and eddy-mean flow interaction.
Transient cyclones and anticyclones are a fundamental component of the extratropical climate system, caus-
ing day-to-day weather variations. By systematically transporting heat and westerly momentum, they also act 
to maintain the meridional thermal structure and a midlatitude westerly jet stream in each hemisphere. Their 
occurrence and intensities are thus very important to climate dynamics
1
,
2
and regional extreme weather events
3
–
6
, 
and therefore further investigating their behavior under the current and future climatic conditions is of great 
scientific and societal importance.
Since the late nineteenth century
7
, investigation of transient eddies, especially extratropical cyclones, has 
relied on a “Lagrangian approach”, which tracks individual moving cyclones or anticyclones
8
–
10
. Since gridded 
atmospheric datasets (analyses) became available 45 years ago, however, an “Eulerian approach” has become used 
widely
11
,
12
, as it is readily applicable also to outputs from numerical atmospheric/climate models
13
. This approach 
extracts sub-weekly fluctuations of such meteorological variables as pressure, temperature or wind velocity locally 
through digital high-pass filtering applied to their daily time series, and eddy activity is then measured locally 
as their temporal variance or covariance
11
,
12
. Regions of strong eddy activity thus measured are called “storm 
tracks”, and many studies have investigated the climatological seasonality of storm tracks and their interannual 
or decadal variability based on Eulerian statistics
14
–
16
. Another advantage of the Eulerian approach is its suit-
ability to quantitative dynamical diagnoses, including the energetics based on the Lorenz energy cycle
16
–
18
 and 
the Eliassen-Palm (E-P) flux diagnosis for eddy-mean flow interaction
19
–
21
. The latter can, for example, illustrate 
horizontal propagation of wave packets and associated translation of wave-activity pseudo-momentum. The 
Eulerian statistics can also elucidate the dynamical distinction of an eddy-driven, subpolar westerly jet stream 
from a stronger thermally driven subtropical jet stream
22
,
23
.
The aforementioned advantages of the Eulerian approach arise from treating cyclonic and anticyclonic eddies 
together as deviations from the mean state, which in turn prevents us from targeting individual cyclonic and 
anticyclonic eddies separately. Unlike the Lagrangian approach, the Eulerian statistics thus represent unified 
contributions from cyclones and anticyclones, which has limited our understanding of storm tracks and associ-
ated eddy-mean flow interaction. This study is the first to evaluate and reconstruct separate contributions from 
both cyclonic and anticyclonic eddies onto Eulerian statistics, based on instantaneous identification of cyclonic 
and anticyclonic vortices.
OPEN
1
Research Center for Advanced Science and Technology, The University of Tokyo, Tokyo, Japan. 
2
Department 
of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot, Israel.
*
email: okajima@
atmos.rcast.u-tokyo.ac.jp

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