Geoscience Reference
In-Depth Information
2. Tropical Pacific-Indian Ocean Temperature
Anomaly Mode
El Nino and La Nina are defined by the SSTA in the equatorial eastern
Pacific. Actually, when there is the positive (negative) SSTA in the eastern
equatorial Pacific, the negative (positive) SSTA in the western equatorial
Pacific occurs. On the other hand, the so-called Indian Ocean dipole is
defined by the difference of the SSTA in the western equatorial Indian
Ocean from that in the eastern equatorial Indian Ocean, indicating zonal
heat contrast of the Indian Ocean SSTA. Although the name dipole is used,
it actually does not define the mathematic meaning (SSTA distribution with
positive (negative) in the west and opposite in the east).
15
Considering the
close relationship between the Pacific ENSO mode and the Indian Ocean
dipole, the index of the PIM can be defined as the respectively normalized
east-west differences of the equatorial areas in the two oceans. As to the
SSTA, the SSTA of ENSO is stronger than that in the equatorial Indian
Ocean because of the bigger Pacific basin. However, as to the influence of the
SSTA on East Asia, a series of numerical experiments clearly indicate that
the effect of SSTA forcing of the Indian Ocean is stronger than that of the
eastern equatorial Pacific.
21
−
23
So we will define the composite index based
on the normalized dipoles respectively in the Pacific and the Indian Ocean.
In this way, the normalized dipoles are comparable and the composite index
has mathematical and physical base, so does its change. The composite
index
I
com
is defined as
I
com
=
∇T
i
+
∇T
p
,
∇T
i
=
T
1
− T
2
,
∇T
p
=
T
3
− T
4
.
T
4
are denoted, respectively, by the
averaged SSTA in the region (50
◦
E-65
◦
E, 5
◦
S-10
◦
N), (85
◦
E-100
◦
E,
10
◦
S-5
◦
N), (130
◦
W-80
◦
W, 5
◦
S-5
◦
N) and (140
◦
E-160
◦
E, 5
◦
S-10
◦
N). The
symbols
The symbols
T
1
,T
2
,T
3
,and
∇T
p
are normalized.
Figure 1(c) gives the time series of
∇T
i
and
I
com
(tropical Pacific-Indian Ocean
temperature anomaly mode index) and SSTA in Nino
3
.
4
(also called ENSO
index). It can be found that although
I
com
is closely related to ENSO index,
the difference in them is large. The importing Indian Ocean SSTA mostly
makes the
I
com
stronger than the (normalized) ENSO mode, and partly
