Geoscience Reference
In-Depth Information
(a)
(b)
34
0.010
0.008
0.3
28
0.006
0
0
0.004
21
0.002
0.2
2
-2
0.000
14
4
-0.002
-4
-0.004
0.1
6
-6
7
-0.006
-8
-0.008
P
Q
-0.010
0
0
4900
10
19
28
37
46
Time (min)
55
64
73
82
92
t
(s)
Figure 9.3. Time-height cross section of the observed zonal mean zonal flow velocity component (panel (a) adapted from Figure
10 in Plumb and McEwan [1978], contour lines in mm / s), compared to the result of the 3D numerical simulation at y = L y / 2
in panel (b), reproduced from Figure 2b in Wedi and Smolarkiewicz [2006], contour lines in m/s. © American Meteorological
Society. Used with permission. According to Plumb and McEwan [1978], the lowest 2 cm in panel (a) could not be observed due
to restrictions of the viewing window. For color detail, please see color plate section.
9.3.MADDEN-JULIAN OSCILLATION
persistent and ubiquitous modeling difficulties with state-
of-the-art global NWP and climate models. The complex-
ity of the processes involved, due to their multiscale nature
ranging from micro to global scales, render the MJO an
intriguing problem in fluid dynamics, not least because
of its appearance as a solitary structure, with its episodic
appearance and its slow (5 m / s), nearly dispersionless
eastward motion.
What makes solitary wave theories with Korteweg-
de Vries (KDV)-like solutions particularly attractive is
that they extend the linear shallow-water theory [ Mat-
suno , 1966], commonly used to explain different modes
of equatorial wave motions, to the weakly nonlinear
regime.Remarkably,mostorallspectralsignalsof satellite-
observedoutgoinglong-waveradiation(OLR),aproxyfor
cloudiness,canbeexplainedviathelineartheoryof equato-
riallytrappedwaves,exceptforthedominantlow-frequency
spectral peak of the MJO [ Wheeler and Kiladis , 1999].
In addition, moist physics and in particular the coupling
of the large-scale flow to the deep convection parametriza-
tion [ Hirons et al. , 2013], or in fact the absence of deep
The MJO [ Madden and Julian , 1971], named after the
scientists who first discovered the anomaly in time series
of atmospheric datasets, is the main intraseasonal atmo-
spheric fluctuation in the equatorial troposphere affecting
weather in large parts of the world. Polynesian seamen
are believed to have used the phenomenon to sail east-
ward in the trade wind-dominated equatorial Pacific 4500
years ago [ Zhang , 2005]. Despite substantial efforts, a
reliable forecasting of the MJO and understanding its
underlying dynamical mechanisms remain key challenges
in atmospheric science. It is not strictly an oscillation as
its period varies and its appearance is episodic [ Hartmann
and Hendon , 2007]. Existing theories stress the impor-
tance of the feedback mechanisms between convection,
large-scale wave dynamics and surface fluxes; see Zhang
[2005] for a comprehensive review. However, while a syn-
thesis of the theories and observations explains important
aspects of the MJO life cycle, a unifying theory is still elu-
sive for the basic mechanism that would also explain the
 
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