Methods JRA-55 reanalysis. We analyzed 6-hourly global fields of atmospheric variables, including SLP and geo-
potential height, air temperature, wind velocity, and diabatic heating rates in the pressure coordinates, obtained
from the Japanese 55-year reanalysis (JRA-55) by the Japan Meteorological Agency (JMA)
43
,
44
for the period
1958/59–2016/17. The JRA-55 has been constructed through a four-dimensional variational data assimilation
(4D-Var) system with TL319 horizontal resolution (equivalent to 55-km) and 60 vertical levels up to 0.1-hPa.
Variables on selected pressure levels are available on a 1.25° × 1.25° grid system.
Extracting transient eddy components. At a particular grid, fluctuations of a given variable with syn-
optic-scale transient eddies whose period is shorter than about a week have been extracted from the 6-hourly
atmospheric reanalysis as its deviations from their low-pass-filtered fields with an 8-day cutoff Lanczos filter (in
this paper, primes denote local deviations from the climatological mean). Local activity of those transient eddies
or their fluxes is evaluated as the variance based on the sub-weekly fluctuations of meridional velocity or the
covariance representing poleward eddy heat flux. Regions of high eddy activity corresponds to “storm tracks”,
along which transient eddies recurrently develop. Climatological-mean fields plotted in Figs.
2
and
3
for a given
midwinter day are calculated after applying a 31-day running mean to daily climatology.
Separation of curvature and shear vorticity. Vorticity can be decomposed locally into shear and cur-
vature terms as follows
45
–
47
where
V denotes scalar wind speed,
n the direction perpendicular to the flow, and
Rs the radius of curvature.
The first and second terms of the RHS in Eq. (
1
) represent shear vorticity and curvature vorticity, respectively,
which can be calculated as
(1)
ζ
= −
∂
V
∂
n
+
V
R
S
(2)
−
∂
V
∂
n
= −
1
V
2
−
uvu
x
−
v
2
v
x
+
u
2
u
y
+
uvv
y
