heat fluxes and their components are needed to address the
uncertainties in SST trends (Beal et al. 2019).
The lack of sufficient observation from Indian Ocean for
the upper ocean carbon cycle research is a lacuna at present.
Surface ocean parameters such as ocean partial pressure of
CO 2 (pCO 2 ) and nutrients such as PO 4 and NO 3 are worthy
of observing because they can cascade through the ocean
solubility and biological pumps to constrain the variables
and parameters in the upper ocean carbon cycle yielding
robust estimates of upper ocean carbon budget of the Indian
Ocean (Sreeush et al. 2019b).
There is a mooring network in the northern Indian Ocean,
Ocean Moored Buoy Network for northern Indian Ocean
(OMNI), led by India that also records high resolution ocean
and near-surface meteorological data (Fig. 10.6). The Ministry of Earth Sciences (MoES) of India announced in June
2018 that the data from OMNI outside the Indian EEZ would
be made freely available at data standards similar to that of
RAMA. A coordinated moored network combining RAMA
and OMNI is a priority for successful monitoring of changes
in the Indian Ocean and also for skilful forecasting.
Further, CMIP5 models fail to reproduce the observed
pattern of SST warming in the Indian Ocean, and as a result
they fail to represent the local and remote impact on the
climate system in the models (Saha et al. 2014). Hence, it is
necessary to accurately monitor changes over the Indian
Ocean, and also improve the models to simulate these
changes, for successful future projections of the regional
climate.
Box 10.1 Modes of climate variability in the
Indo-Pacific
El Niño Southern Oscillation (ENSO): ENSO is a
coupled ocean-atmosphere mode of interannual variability with a periodicity of about 4–7 years. El Niño
is the positive phase of ENSO, characterized by
anomalous surface warming of eastern and central
equatorial Pacific, lasting for several months. The
negative phase with cooler (than normal) SST in the
eastern equatorial Pacific is called La Niña. There are
several indices to quantify the strength, nature and
duration of El Niño based on the anomalous SST over
different regions of eastern and central equatorial
Pacific. The atmospheric counterpart to this interannual warming/cooling is known as southern oscillation, which is quantified as the anomalous sea-level
pressure difference between Darwin and Tahiti. The
southern oscillation index quantifies the intensity of
Walker circulation and the ocean atmosphere coupling
associated with El Niño and La Niña. El Niño is found
to weaken the Indian summer monsoon and warm the
Indian Ocean basin, whereas La Niña strengthens the
monsoon and cools the Indian Ocean basin. Different
flavours of El Niño such as east Pacific (cold tongue)
El Niño, central Pacific (warm pool) El Niño (or
Modoki, Ashok et al. 2007) and their regional impacts
are significant over the Indian landmass and Indian
Ocean.
Pacific decadal oscillation (PDO): PDO is a decadal
mode of variability in the north Pacific (north of
20° N), with warm SST anomalies in the eastern and
north Pacific and cold SST anomalies in the central
north Pacific during the positive phase, and vice versa
in the negative phase. In the equatorial Pacific, PDO
imprints similar spatial structure as that of ENSO but
with a longer time scale. When PDO and ENSO are in
the same phase, the impact of ENSO is amplified.
Interdecadal Pacific oscillation (IPO) is similar to PDO
but has a wider spatial structure covering both southern hemisphere and northern hemisphere, with the
pattern of warming and cooling in the north Pacific
similar to that of PDO. The typical cycle of a PDO or
IPO is about 15–30 years, but in some period, one
phase itself may last more than 20–30 years.
Indian Ocean basin mode (IOBM) and Indian
Ocean Dipole (IOD): TIO is characterized by several
modes of climate variability such as the Indian Ocean
basin mode (IOBM, Klein et al. 1999; Xie et al. 2002;
Chowdary and Gnanaseelan 2007), Indian Ocean
Dipole (IOD, Saji et al. 1999; Webster et al. 1999),
and subsurface mode (Sayantani and Gnanaseelan
2015). IOBM is mainly caused by ENSO forcing and
the associated changes in the net heat flux. IOD is an
ocean atmosphere coupled climate mode of variability
(east west) in the tropical Indian Ocean, defined as the
difference in SST anomalies of western (50 °E to 70 °
E;10 °S to 10 °N) and south-eastern (90 °E to 110 °E;
10 °S to equator) equatorial Indian Ocean (Saji et al
1999). In contrast to the SST variability, subsurface
temperature (at thermocline) displays a north–south
mode of variability. Generally, a positive IOD favours
enhanced summer monsoon rainfall over the Indian
subcontinent while negative IOD favours less rainfall.
More details on the ENSO-IOD-monsoon interactions
are given in Chapter 3, Box 3.2.
10.6 Summary
The ocean-atmospheric conditions in the Indian Ocean
region strongly modulate the subcontinental climate. This
chapter has assessed changes in the Indian Ocean in the
202
M. K. Roxy et al.
uncertainties in SST trends (Beal et al. 2019).
The lack of sufficient observation from Indian Ocean for
the upper ocean carbon cycle research is a lacuna at present.
Surface ocean parameters such as ocean partial pressure of
CO 2 (pCO 2 ) and nutrients such as PO 4 and NO 3 are worthy
of observing because they can cascade through the ocean
solubility and biological pumps to constrain the variables
and parameters in the upper ocean carbon cycle yielding
robust estimates of upper ocean carbon budget of the Indian
Ocean (Sreeush et al. 2019b).
There is a mooring network in the northern Indian Ocean,
Ocean Moored Buoy Network for northern Indian Ocean
(OMNI), led by India that also records high resolution ocean
and near-surface meteorological data (Fig. 10.6). The Ministry of Earth Sciences (MoES) of India announced in June
2018 that the data from OMNI outside the Indian EEZ would
be made freely available at data standards similar to that of
RAMA. A coordinated moored network combining RAMA
and OMNI is a priority for successful monitoring of changes
in the Indian Ocean and also for skilful forecasting.
Further, CMIP5 models fail to reproduce the observed
pattern of SST warming in the Indian Ocean, and as a result
they fail to represent the local and remote impact on the
climate system in the models (Saha et al. 2014). Hence, it is
necessary to accurately monitor changes over the Indian
Ocean, and also improve the models to simulate these
changes, for successful future projections of the regional
climate.
Box 10.1 Modes of climate variability in the
Indo-Pacific
El Niño Southern Oscillation (ENSO): ENSO is a
coupled ocean-atmosphere mode of interannual variability with a periodicity of about 4–7 years. El Niño
is the positive phase of ENSO, characterized by
anomalous surface warming of eastern and central
equatorial Pacific, lasting for several months. The
negative phase with cooler (than normal) SST in the
eastern equatorial Pacific is called La Niña. There are
several indices to quantify the strength, nature and
duration of El Niño based on the anomalous SST over
different regions of eastern and central equatorial
Pacific. The atmospheric counterpart to this interannual warming/cooling is known as southern oscillation, which is quantified as the anomalous sea-level
pressure difference between Darwin and Tahiti. The
southern oscillation index quantifies the intensity of
Walker circulation and the ocean atmosphere coupling
associated with El Niño and La Niña. El Niño is found
to weaken the Indian summer monsoon and warm the
Indian Ocean basin, whereas La Niña strengthens the
monsoon and cools the Indian Ocean basin. Different
flavours of El Niño such as east Pacific (cold tongue)
El Niño, central Pacific (warm pool) El Niño (or
Modoki, Ashok et al. 2007) and their regional impacts
are significant over the Indian landmass and Indian
Ocean.
Pacific decadal oscillation (PDO): PDO is a decadal
mode of variability in the north Pacific (north of
20° N), with warm SST anomalies in the eastern and
north Pacific and cold SST anomalies in the central
north Pacific during the positive phase, and vice versa
in the negative phase. In the equatorial Pacific, PDO
imprints similar spatial structure as that of ENSO but
with a longer time scale. When PDO and ENSO are in
the same phase, the impact of ENSO is amplified.
Interdecadal Pacific oscillation (IPO) is similar to PDO
but has a wider spatial structure covering both southern hemisphere and northern hemisphere, with the
pattern of warming and cooling in the north Pacific
similar to that of PDO. The typical cycle of a PDO or
IPO is about 15–30 years, but in some period, one
phase itself may last more than 20–30 years.
Indian Ocean basin mode (IOBM) and Indian
Ocean Dipole (IOD): TIO is characterized by several
modes of climate variability such as the Indian Ocean
basin mode (IOBM, Klein et al. 1999; Xie et al. 2002;
Chowdary and Gnanaseelan 2007), Indian Ocean
Dipole (IOD, Saji et al. 1999; Webster et al. 1999),
and subsurface mode (Sayantani and Gnanaseelan
2015). IOBM is mainly caused by ENSO forcing and
the associated changes in the net heat flux. IOD is an
ocean atmosphere coupled climate mode of variability
(east west) in the tropical Indian Ocean, defined as the
difference in SST anomalies of western (50 °E to 70 °
E;10 °S to 10 °N) and south-eastern (90 °E to 110 °E;
10 °S to equator) equatorial Indian Ocean (Saji et al
1999). In contrast to the SST variability, subsurface
temperature (at thermocline) displays a north–south
mode of variability. Generally, a positive IOD favours
enhanced summer monsoon rainfall over the Indian
subcontinent while negative IOD favours less rainfall.
More details on the ENSO-IOD-monsoon interactions
are given in Chapter 3, Box 3.2.
10.6 Summary
The ocean-atmospheric conditions in the Indian Ocean
region strongly modulate the subcontinental climate. This
chapter has assessed changes in the Indian Ocean in the
202
M. K. Roxy et al.