地球環境研究，Vol.22（2020）

Relationship among MJO, westerly wind bursts, tropical
cyclones and mid-latitude baroclinic waves observed in an
idealistic aqua-planet NICAM simulation
YOSHIZAKI Masanori＊
SUZUKI-PARKER Asuka＊

WATARAI Yasushi＊
MORIIZUMI Shin-ichi＊＊

Key words：NICAM, MJO, westerly wind burst, tropical cyclone, mid-latitude baroclinic wave

Hadley circulation is dominant. However, westerly

１．Introduction;

winds are generated due to intense convective activity

In tropics, convective activity of cumulonimbi isof cloud clusters, and intense westerly winds areactive, and disturbances with various horizontal scales

called as westerly wind bursts（WWBs）. The Rossby

and propagating speeds are seen due to diabatic

response of diabatic heating might be a candidate forheating of cloud clusters. Hereafter, disturbances

the formation of westerly winds（Gill 1980）. However,with a horizontal scale greater than 1000 km are

WWBs are also generated in other situations and, thus,

focused on. Madden and Julian （1971）found that

atmospheric motions are complicated.

cloud disturbances having several 1000 km scales－1

Historically speaking, CISK（conditional instability

propagate eastward slowly with a speed of about 5 m s ,of the second kind）（Charney and Eliassen, 1964）

forming over the Indian Ocean and decaying overmechanism was the first proposed mechanism forthe western Pacific Ocean repeatedly with 30-60 day

genesis and development of TCs. Over the western

period（Madden-Julian Oscillation; MJO）
. By analyzing

North Pacific, a number of studies examined the large-

equatorial disturbances using satellite data, Wheeler

scale environmental factors contributing to TC genesis

and Kiladis （1999） showed MJO, fast eastward-（Briegel and Frank, 1997, Ritchie and Holland, 1999,－1

Yoshida and Ishikawa, 2013, Fudeyasu and Yoshida,and slow westward-propagating disturbances as2018, Fudeyasu et al., 2019）. Yoshida and Ishikawa

symmetric equatorial mode, as well as mixed Rossby-（2013）categorized TC genesis into five flow patterns:

gravity wave-like disturbances as anti-symmetricshear line, confluence region, gyre, easterly wave andequatorial one. These disturbances have considerably

preexisting TC, and showed that the most favorable

slow propagating speeds compared with those incase is shear line accounting for 42 percent. Other

the dry atmosphere, indicating that they might bestudies examined TC genesis process at meso-scale.

related to moist properties. In the off-equatorial region

Proposed mechanisms can be categorized to top-from the latitude 5 degrees, tropical cyclones（TCs）

down and bottom-up one. In the top-down assumption,

frequently emerge due to diabatic heating of cloudseveral vortices in the stratiform clouds of cloudclusters and some TCs develop into typhoons whichclusters enhance from middle to lower levels, and－1develop into a TC（Ritchie and Holland 1997, Simpson

propagating disturbances with speeds of 15-25 m s

have wind speeds greater than, for example, 17 m s
in the north-eastern Pacific Ocean.

et al. 1997, Bister and Emanuel 1997）. In the bottom-up

Over the equatorial troposphere, easterly winds

assumption, on the other hand, several intense vortices

generally prevail in the lower troposphere, since thein the lower level associated with cumulonimbus clouds＊＊＊

Faculty of Geo-Environmental Science, Rissho University
Graduate School of Geo-Environmental Science, Rissho University
47

Relationship among MJO, westerly wind bursts, tropical cyclones and mid-latitude baroclinic waves observed in an idealistic aqua-planet NICAM simulation（Yoshizaki・Watarai・Suzuki・Moriizumi）

merge and develop into a TC（Hendricks et al. 2004,

invariant SST forcing given in the aqua-planet NICAM

Montgomery et al. 2006）.

simulation（Iga et al. 2010）. SST is the maximum at

Recently, global cloud-resolving models developed

the equatorial central area（latitude = 0 degree, andwith improvement of the super-computer and so on,

longitude = 0 degree）, and falls in both latitudinal andand one model is a NICAM （Satoh, 2003）, whichlongitudinal directions. Fig. 1b shows a meridional

is an abbreviation of Nonhydrostatic ICosahedral

cross-section of SST forcing at the longitude 0 degree.

Atmospheric Model. This model contains fullSince the SST is symmetric about the equator,

atmospheric dynamics and physical processes,

upward motions are excited in tropics and the Hadley

including radiation, cloud microphysics, and boundary-circulation develops there. In the range between about

layer processes. In particular, the cloud microphysics

10 and 60 degrees in the latitudinal direction in bothprocess solves water vapor, cloud water, cloud ice,

hemispheres, the SST gradient is large and baroclinic

rain, snow, and graupel as prognostic variables, andunstable waves appear there. In higher latitudes

the NICAM can represent temporal variations andgreater than 60 degrees, the SST is nearly constant

structures of cumulonimbus clouds and cloud clusters.

and no disturbance is forced there. Fig. 1c shows a

Nasuno et al.（2007, 2008）simulated fast eastward-

longitudinal cross-section of SST forcing at the latitude

propagating disturbances with a speed of about 17

0 degree. In longitudinal direction, SST is spatially－1（called as super cloud clusters, SCCs） andvaried to follow a sinusoidal curve, with wavenumber 1

westward-propagating ones on the idealistic sea

and the amplitude of 4.5 K（ΔT=4.5）. This SST variation

surface temperature（SST）aqua-planet, forcing thein the longitudinal direction produces the Walker

Hadley circulation in the latitudinal direction. An

circulation. An area denoted by a thick red line（90aqua-planet simulation assumes the earth covered by

W-90 E）is hereafter referred to as the high SST area.

m swater with no land. The model atmosphere is forcedby idealistic, time-invariant SST. Using such models,（b）Data for analysis:

it is anticipated that simple atmospheric motions can

The NICAM simulation was conducted for the total

be reproduced, since complicated real land and seaof 240 days. Analysis was performed for the last 180

distribution, complex uneven terrains and nonlinear

days of simulation, with the first 60 days discarded asinteraction between ocean and atmosphere are

the model spin-up time. An original data for analysis

removed. Then, these outputs are beneficial for deepare 2560 × 1280 horizontal grids（14 km horizontal

understandings, for example, about formation andresolution）and 54 layers in the vertical direction, anddevelopment of disturbances.

rearranged into two data sets. The first dataset is 860

In this study, analyses are made using the× 400 horizontal grids（14 km horizontal resolution）

NICAM aqua-planet outputs under an idealized SST（a rectangle denoted by red broken-lines in Fig. 1a）

distribution. The SST forcing is such that it induces

and 45 layers in the vertical direction, where theHadley and Walker circulations in latitudinal andupper levels are cut off. The second dataset is 360 ×

longitudinal directions, respectively. Since MJO-like

180 horizontal grids（averaged about 100 km horizontal

disturbances and TCs are well simulated, relationships

resolution from the original data）and 54 layers in theamong MJOs, WWBs, TCs, and mid-latitude baroclinic

vertical direction. These datasets have time-series

waves are examined, focusing on the formation of TCs.

data for 3 hour interval about 3 dimensional variables,such as zonal velocity, u; meridional velocity, v; vertical

２．Descriptions of an idealistic aqua-planet
NICAM simulation:（a）Descriptions of SST forcing:
Figure 1a shows the distribution of idealized, time48

velocity, w; potential temperature, ϑ ; diabatic heating,dh; and vertical vorticity, ς; and for one hour interval
about 2 dimensional variables, such as precipitation;and outgoing longwave radiation, OLR.

地球環境研究，Vol.22（2020）

R6_Fig_NICAM_20200120

(b)

(a)

SST(K)

303

276

Latitude

90S

(c)

0

60S

60N

90N

Latitude (Y)SST(K)

303

293

Longitude

180W

High SST
90W

0

90E

2 ΔT

Longitude (X)180E

Fig.1:（a）Horizontal distribution of SST（Iga et al. 2010）,（b）meridional section at the longitudeFig.
1: (a) Horizontal distribution of SST (Iga et al. 2010), (b) meridional section at the
0 degree, and（c）longitudinal section at the latitude 0 degree, where wavenumber 1 andlongitudedegree,(c) longitudinal
sectionat the latitude
0 degree,wavenumber
amplitude0 of
4.5 Kand（ΔT=4.5）
are forced.
A rectangle
denotedby redwhere
broken-lines
in（a）is a
1horizontal
and amplitudeof 4.5 Kthe
(ΔT=4.5)
are forced.
A squareby redby
broken‐lines(a) between
is aarea where
grid resolution
is 14
km. Andenoted
area denoteda thick redinline
90
W
and
90
E
in
（c）
is
a
high
SST
one.
horizontal area where the grid resolution is 14 km. An area denoted by a thick red line

between 90 W and 90 E in (c) is a high SST one.

（c）Classification of propagating disturbancesand vortexes:

speeds greater than 7.5 m s －1 or less than 15 m s －1
are categorized as TCs, and ones with maximum

As shown later in the Section 3, the NICAM

wind speeds exceeding 15 m s － 1 are categorized assimulation produced large-scale disturbances

typhoons. For example, the TC can be represented asaccompanied with diabatic heating in tropics, inthe vorticity painted by a yellow-color in Fig. 5e.

particular, over the high SST area. Since large-scaledisturbances have top-heavy heating profiles in thevertical direction, they generally have a propagation
property, and, in particular, eastward-propagating

３．Results:（a）Classification of disturbances in tropics:

disturbances are likely to appear on the equatorial

Figure 2 shows a longitude-time cross section

beta plane （Yoshizaki, 1991a, 1991b）. Propagatingof precipitation over tropics. In whole areas, bothspeeds of these disturbances in the NICAM simulation

eastward- and westward-propagating disturbances

are determined at a 10 m s－1interval. For example,

are seen. Focusing on high SST areas, eastward-－1

propagating disturbances appear mostly, and, in－1

are denoted as ones having propagation speeds of 15 m s .

particular, intense precipitation areas are found in the

Cyclonic vortexes are divided into three types

range from about 60 W to 5 W. Here, disturbances

according to their maximum wind speeds at the

propagate eastward slowly with a speed of aboutheight of 807 m（model level）; weak cyclonic vortexes,

5 m s － 1. On the low SST areas, on the other hand,TCs, and typhoons. Consider Rankine’s vortexes with

precipitation intensity is weak and most disturbancesmaximum wind speed at the 50 km distant frompropagate westward with a speed of about -15 m s－1.

disturbances with propagation speeds of 10-20 m s

the vortex center. The vortexes with maximum
wind speeds of 7.5 m s－1

Figure 3 is a longitude-time cross section of u at theor less are categorized asheight of 807 m over tropics. In the range from about

weak cyclonic vortexes, ones with maximum wind

120 W to 30 E, westerly winds are seen, while easterly
49

Relationship among MJO, westerly wind bursts, tropical cyclones and mid-latitude baroclinic waves observed in an idealistic aqua-planet NICAM simulation（Yoshizaki・Watarai・Suzuki・Moriizumi）

Fig. 2:winds
Longitude‐timecrossinsection
are dominantother areas. Westerly windsof precipitation (mm h‐1) averaged
are found in the intense diabatic heating on the highbetween 3 S to 3 N in the latitudinal
SST Solid
areas,lines
and,denotein particular, intense westerly winds
direction.
propagation
speeds ofare generated
in disturbances,a range from about 60 W to 30 W.
where a positive (negative) value
Hereafter, WWBs are defined as westerly wind withmeans eastward (westward)
an intensity greater than 10 m s－1．
propagation.

Figure 4 shows a longitude-time cross section of-5 m s-1

5 m s-1

OLR smaller than 220 W m － 2 and WWB areas overtropics. As Fig.2, eastward-propagating disturbances
are remarkable in the smaller OLR areas andrepresentative eastward-propagating ones are denoted
as system A to system J（black dotted lines）. These-15 m s-1

15 m s-1-25 m s-1

25 m s-1

systems have a life cycle to form, develop, and decay
repeatedly with periods of 15-30 days. Since thesesystems have a common propagation speed and alife cycle similar to MJO observed in tropics, they
are called“MJO-like disturbances”. Some MJO-likeFig.2: Longitude-time cross section of precipitation(mm h－1) averaged between 3 S to 3 N in
the latitudinal direction. Solid lines denotepropagation speeds of disturbances,where a positive (negative) value means
eastward (westward) propagation.

disturbances are related to WWBs, but not always.
One example is shown in a black elliptic line in Fig. 4.

図4 赤道域の外向き長波放射（OLR:Outgoing Longwave
Radiation）（カラー，W/m‐2）の
東西‐時間断面図．黒破線と緑破線はMJO‐likeじょう乱，
西向きのじょう乱の時間変化をG示す．黒破線楕円はMJO‐like
じょう乱とは異なるWWB域を
示す．

I

J
I
H

F
E

Fig. 4: Longitude‐of OLR (W m‐2) av
S to 3 N in the lat
H
and WWB areas eJlines.
Broken lines
Calphabets indicate
B
speeds of disturb
Fig. 3: Longitude‐time cross section
positive (negativeF
A eastward (westwa-15 m s-1of zonal wind u (mGs‐1) at the height
15 m s-1of 807 m averaged between 3 S to 3
Note that the inte
-25 m s-1
E
25 m s-1 N in the latitudinal direction.
Soliddifferent from tholines denotepropagation
speedsofFig.4: Longitude-time cross section of OLR (W－2disturbances,mwherea positive between 3 S to 3 N in
) averaged(negative) value
eastward
themeans
latitudinal
direction and WWB areas
Fig.3: Longitude-time cross section of zonal wind(westward) propagation.
encircled by red lines. Broken linesu (m s－1) at the height of 807 m averaged
denoted by alphabets indicate propagation
between 3 S to 3 N in the latitudinal
speeds of disturbances, where a positive
direction. Solid lines denote propagation(negative) value means eastward
speeds of disturbances, where a positive(westward) propagation. Note that the(negative) value means eastward (westward)
interval of longitude is different from those
propagation.
in Figs. 2 and 3.

-5 m s-1

50

5 m s-1

D地球環境研究，Vol.22（2020）

Upper figures in Fig. 5 show time-series of longitude-

direction. This horizontal wind shear helps to produce

vertical cross sections of dh, ϑ, and wind vectors（u,cyclonic vortexes with sizes of several 100 km shownw）at the equator at 9 h in 5, 8, 11, 14, 17 and 20 day.

by circles. In Fig. 5b, a weak cyclonic vortex is found

An MJO-like disturbance denoted by red elliptic lines

around the area （～15 W, ～5 S）, slowly moving

is traced, where high dh areas propagate eastward

southward, and develops into a TC in Fig. 5e. In thewith high cloud-top reaching about 16 km in Figs. 5a,northern hemisphere, a weak cyclonic vortex in Fig.

5b, 5c and 5d. Lower figures of Fig. 5 show horizontal

5d is seen around the area（～15 W, ～5 N）first,

distributions of ς and ϑ, at the height of 5640 m andand develops into a TC, similarly to the southernwind vectors（u, v）at the height of 807 m. The MJO-

hemisphere case. In both cases, intense horizontal windlike disturbance induces intense westerly winds overshear is found where weak cyclonic vortexes develop.

tropics（Fig. 5b）, inducing the WWB as a Rossby

Figure 6 shows a longitude-time cross section of

response of heating in off-equatorial areas on its

TC formation and anticyclonic circulations associatedwestern side（Figs. 5c, 5d）. However, other WWBs

with mid-latitude baroclinic waves. During 180

are similarly excited around TCs where twin cyclones

day simulation, it is summarized that anticyclonic

are found on both hemispheres（Fig. 5e）. The WWB

circulations preceded the formation of TCs in mostdenoted by a black elliptic broken-line in Fig. 5ecases. However, the places of the TC formation are notcorresponds to one in Fig. 4. In this way, WWBs arealways fixed relatively to anticyclonic circulations.

induced by, at least, two ways: Rossby response ofheating and TCs. Therefore, WWBs should not be said
as disturbances, which induce intense westerly winds,
but they rather are produced by disturbances such as
MJO-like ones or TCs.

４．Discussions:（a）Comparison of ΔT=4.5 case with ΔT=0 and
ΔT=2.25 cases:
In Yoshizaki et al.（2012）, the impact of SST was（b）Relationship between WWBs and mid-latitudestudied by changing the longitudinal variations of SST

baroclinic waves, and formation of TCs:

with ΔT=0, 2.25, and 4.5. In the ΔT=0 case（in which

10 several TCs are seen in both hemispheres during

SST is uniform in the longitudinal direction）, fast

180 day simulation. Normally one TC is seen in one

eastward-propagating SCCs with a speed of about 15

hemisphere, while twin cyclones straddling bothm s－1 are dominant in tropics, whereas fast westward-

hemispheres and four TCs occasionally appeared.

propagating ones are also found. In the ΔT=2.25 case,

Here we focus on the relationship between westerly

some differences are seen between high and low SST

wind areas in the tropics, and anticyclonic circulation

areas. On the contrary, in the ΔT=4.5 case, slow eastward-

associated with mid-latitude baroclinic waves. In

propagating disturbances with a speed of about 5 m s－1

the lower figures of Fig. 5, the WWB is indicated by（MJO-like disturbances）appear, and no fast eastward-

red solid arrows, and the anticyclonic circulation is

propagating one like SCC appear. In addition, 10

indicated by dashed arc-shaped arrows. The center ofseveral TCs are generated in both hemispheres during

anticyclonic circulation is denoted by H. Mid-latitude

180 day simulation.

baroclinic waves with large scales of about 5000 km

The SST variation of ΔT=4.5 used in this study seems

repeatedly move eastward on both hemispheres

large compared with the SST observed in the

between 30-40 latitude degrees. The amplitude ofmaritime continent（Indonesia area）and the eastern

mid-latitude baroclinic waves intensifies to produce

Pacific Ocean. In this study, an atmospheric general

anticyclonic circulation around 10-30 latitude degrees.

circulation is induced only by this SST variation.

Along with the WWB, the easterly winds on the

Comparing our study with realistic situation, thermal

equator-sides of the anticyclonic circulation induces

and dynamic forcing such as land-sea contract andstrong horizontal wind shear in the meridional

topography should be added as forcing to atmospheric
51

Relationship among MJO, westerly wind bursts, tropical cyclones and mid-latitude baroclinic waves observed in an idealistic aqua-planet NICAM simulation（Yoshizaki・Watarai・Suzuki・Moriizumi）

(b)

(a)

(c)

H

H
Fig. 5: (Upper) longitude‐vertical cross sections of dh (color, 10‐4 K s‐1 ), θ �contour, K�, and(e) at 9 h in (a) 5, (b) 8, (c)(f)11, (d) 14, (e) 17
wind vectors(d)(u, w) (arrows, m s‐1) at the equatorand (f) 20 day, and (lower) horizontal distributions of ς (color, 10‐4 s‐1) and θ contour, K at
the height of 5640 m, and wind vectors (u, v) (arrows, m s‐1) at the height of 807 m. In theH(continued）

Fig. 5: (Upper) longitude-vertical cross sections of dh (color, 10 － 4 K s － 1), θ (contour, K), and windvectors (u, w) (arrows, m s－1) at the equator at 9 h in (a) 5, (b) 8, (c) 11, (d) 14, (e) 17 and (f) 20
day, and (lower) horizontal distributions of ς (color, 10－4 s－1) and θ (contour, K) at the heightof 5640 m, and wind vectors (u, v) (arrows, m s－1) at the height of 807 m. In the upper figures,dashed red ovals mean areas shown in the line B in Fig. 4. In the lower figures, large-scale
anticyclonic circulations and vortexes are shown by dashed arc-shaped arrows and circles,
respectively. The centers of anticyclonic circulation are denoted by H.
52

地球環境研究，Vol.22（2020）

N

TC in NH
S TC in SH

80 N NTime (day)

70
60

50
40

30

S

S

SN
S
N

S

S

180
170

S

N

40W

N
N
S
S

150

140

S

20W

S
S

160

N
N

N N

130

N N
S S

10
60W

S

S

NN

20
0

N

N

Anticyclonic circulation in NH
Anticyclonic circulation in SH

Time (day)

90

N

0

Longitude

120
110

S

100
20E

90

N

S
N
SN

60W

S

S
40W

S
20W

0

Longitude

20E

6: Longitude‐timecross sectionof TC formations
and anticyclonic
circulationofFig.6: Fig.
Longitude-timecross sectionof TC formations
and anticyclonic
circulationof mid-latitude
mid‐latitude
baroclinic waves.
baroclinic
waves.
motion in the realistic situation. Similarly, by changing

shear instability, resulting in the formation of cyclonic

the topography of the maritime continent using thevortexes and TCs.

NICAM with a 220-km horizontal mesh, Takasuka
et al.（2015）showed that the zonal contrast of latent
heat flux enhances eastward propagation of MJO-likedisturbance. Thus, the ΔT=4.5 is not too large whenreal dynamic and thermal forcing is considered.

５．Summary:Analyses were conducted using an aqua-planet
NICAM simulation with idealized SST forcing. The
SST forcing is such that it induces Hadley and Walker（b） Application of TC formation to realistic
situation:

circulations in latitudinal and longitudinal directions,
respectively. In tropics, MJO-like disturbances, WWBs

Based on the idealistic aqua-planet NICAM

and TCs frequently appeared. WWBs appeared insimulation, this study demonstrated that TCs often

several ways, such as（1）the Rossby response offormed when eastward-propagating anticyclonicdiabatic heating from MJO-like disturbances andcirculation associated with mid-latitude baroclinic

others, and（2）developing TCs. The relationship ofwaves approached the WWBs at appropriate timings.

WWBs and anticyclonic circulations associated with

However, such situations do not frequently occur in the

mid-latitude baroclinic waves was studied for thereal warm period over the Pacific Ocean. Preferable

formation of cyclonic vortexes. It is found that in mostareas for the TC formation in the real situation arecases anticyclonic circulations associated with mid-

the northwestern Pacific Ocean where the monsoon

latitude baroclinic waves first approached WWBs

trough is frequently observed on the eastern part ofand then intensified cyclonic vortexes, sometimes

Indonesia under the planetary-scale Pacific-scale sub-developing into TCs. However, the interaction of mid-high zone. In most cases, the interaction between

latitude baroclinic waves and WWBs rarely occurs in

WWBs and monsoon trough produces the horizontal

real situation. This interaction is considered to appear
53

Relationship among MJO, westerly wind bursts, tropical cyclones and mid-latitude baroclinic waves observed in an idealistic aqua-planet NICAM simulation（Yoshizaki・Watarai・Suzuki・Moriizumi）

as the monsoon trough accompanied with sub-highMadden. R. A. and P. R. Julian, 1971: Detection of a 40-50

zone over the Pacific Ocean, with horizontal scales

day oscillation in the zonal wind in the tropical Pacific. J.

of about 10,000 km. The monsoon trough may work
to induce horizontal shear and help weak cyclonicvortexes to develop into TCs.

Atmos. Sci ., 28, 702-708.
Montgomery, M. T., M. E. Nicholls, T. A. Cram and A. B.
Saunders, 2006: A vortical hot tower route to tropical
cyclogenesis. J. Atmos. Sci ., 63, 355-386.
Nasuno, T., H. Tomita, S. Iga, H. Miura, and M. Satoh, 2007:

Acknowledgement:
The authors thank reviewers for improving thedraft, and Prof. M. Satoh（Atmosphere and Ocean
Research Institute, Univ. Tokyo）and Dr. S. Iga（Research Organization for Information Science andTechnology）for providing the NICAM data.

Multiscale organization of convection simulated with
explicit cloud processes on an aquaplanet. J. Atmos. Sci .,64, 1902-1921.
Nasuno, T., H. Tomita, S. Iga, H. Miura, and M. Satoh, 2008:
Convectively coupled equatorial waves simulated on
an aquaplanet in a global nonhydrostatic experiment. J.

Atmos . Sci ., 65, 1246-1265.
Ritchie, E. A., and G. J. Holland, 1997: Scale interactions

References:during the formation of typhoon Irving. Mon. Wea. Res .,Bister, M. and K. A. Emanuel, 1997: The genesis of125, 1377-1396.

Hurricane Guillermo: TEXMEX analyses and a modelingstudy. Mon. Wea. Res ., 125, 2662-2682.
Briegel, L. M. and W. M. Frank, 1997: Large-scale influenceson tropical cyclogenesis in the western north Pacific.

Mon. Wea. Res ., 125, 1397-1413.
Charney, J. G. and A. Eliassen, 1964: On the growth of thehurricane depression. J. Atmos . Sci. , 21, 68-75.
Emanuel, K. A., 1986: An air-sea interaction theory fortropical cyclones. Part I: Steady-state maintenance. J.

Atmos . Sci ., 43, 585-604.

Ritchie, E. A., and G. J. Holland, 1999: Large-scale patterns
associated with tropical cyclogenesis in the western
Pacific. Mon. Wea. Rev ., 127, 2027-2043.
Satoh, M., 2003: Conservative scheme for a compressible
nonhydrostatic model with moist processes. Mon. Wea.

Rev ., 131, 1033-1050.
Simpson, J. E., E. A. Ritchie, G. J. Holland, J. Halverson
and S. Stewart, 1997: Mesoscale interactions in tropical
cyclone genesis. Mon. Wea. Res ., 125, 2643-2661.
Takasuka, D., T. Miyakawa, M. Satoh, and H. Miura, 2015,

Fudeyasu, H. and R. Yoshida 2018: Western North Pacific

Topographical effects on internally produced MJO-liketropical cyclone characteristics stratified by genesis

disturbances in an aqua-planet version of NICAM. SOLA,

environment, Mon. Wea. Rev ., 146, 435-446.

11, 170-176.

Fudeyasu, H., R. Yoshida, M. Yamaguchi, H. Eito, C. Muroi,

Wheeler, M. and G. N. Kiladis, 1999: Convectively coupled

S. Nishimura, K. Bessho, Y. Oikawa, and N, Koide, 2019:equatorial waves: Analysis of clouds and temperature

Development conditions for tropical storms over thein the wavenumber-frequency domain. J. Atmos . Sci ., 56,

western North Pacific stratified by large-scale flow

374-399.

patterns. J. Meteor. Soc. Japan , Early Online Release.

Yoshida, R. and H. Ishikawa, 2013: Environmental factors

https://www.jstage.jst.go.jp/article/jmsj/advpub/0/

contributing to tropical cyclone genesis over the western

advpub_2020-004/_article/-char/ja/

North Pacific. Mon. Wea. Res ., 141, 451-467.

Gill, A. E. 1980: Some simple solutions for heat-induced

Yoshizaki, M., 1991a: Selective amplification of the eastward-

tropical circulation. Quart. J. Roy. Met. Soc. , 106, 447-462.

propagating mode in a positive-only wave-CISK model on

Hendricks, E. A., M. T. Montgomery, and C. A. Davis, 2004:

an equatorial beta plane. J. Meteor. Soc. Japan , 69, 353-373.

The role of“vortical”hot towers in the formation of

Yoshizaki, M., 1991b: On the selection of eastward-

tropical cyclone Diana（1984）. J. Atmos. Sci ., 61, 1209-

propagating mode appearing in the wave-CISK model1232.

as tropical intraseasonal（30-60-day）oscillations -linear

Iga, S., M. Yoshizaki, M. Satoh, T. Nasuno, H. Tomita, and K.

responses to localized heating moving in the east-west

Yasunaga, 2010: Global aquaplanet experiments using thedirection on the equatorial beta plane-. J. Meteor . Soc.

NICAM new codes（in Japanese）.Proc. Spring Meeting,Japan , 69, 595-608.

Japan Meteor. Soc ., 97, 76.

Yoshizaki, M., K. Yasunaga, S. Iga, M. Satoh, T. Nasuno,

Love, G, 1985: Cross-equatorial influence of winterA. T. Noda, and H. Tomita, 2012: Why do super clusters

hemisphere subtropical cold surges. Mon. Wea. Res ., 113,

and Madden Julian Oscillation exist over the equatorial

1487-1498.

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region? SOLA , 8, 33-36.

地球環境研究，Vol.22（2020）

理想化された海面水温分布の強制のもとでNICAMの水惑星数値実験に見られたMJO、西風バースト、熱帯低気圧、傾圧不安定波の相互関係
吉

﨑

正

憲＊

渡

来

靖＊

鈴木パーカー

明日香＊

森

泉

慎

一＊＊

立正大学地球環境科学部
立正大学大学院地球環境科学研究科＊＊＊

Abstract:
理想化された水温分布の強制のもとでNICAMによる数値実験の結果を用いて、解析を行った。南北にハドレー循
環、東西にウォーカー循環を強制するように、水温分布を与えた。熱帯域ではマッデン・ジュリアン振動（MJO）のような東進するじょう乱が見られ、熱帯低気圧（TC）もしばしば発生した。そこでは、少なくとも二つの方法（熱源によるロスビー応答とTC）によって強い西風域（西風バースト：WWB）が形成された。また、TC発生に関して
WWBと中緯度帯に現れる傾圧不安定波による高気圧性循環との関係を調べた。その結果、多くの場合、まず高気圧性循環がWWBに近づいて、それから弱い低気圧性の渦が現れ、中にはTCまで発達することが分かった。ここで与えた東西方向のSSTの大きさや中緯度帯の傾圧不安定波が果たしてTCを形成するのかについて議論した。
キーワード：NICAM, マッデン・ジュリアン振動、西風バースト、熱帯低気圧、傾圧不安定波

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