Key Messages
• The tropical Indian Ocean has experienced rapid
basin-wide sea surface temperature (SST) warming, with
an average rise of 1.0 °C (0.15 °C/decade) during 1951–
2015, over which period the global average SST warmed
about 0.7 °C (0.11 °C/decade) (high confidence).
The SST warming is spatially non-uniform and about 90%
of the warming is attributed to anthropogenic emissions.
• The basin-wide non-uniform SST warming trend in the
tropical Indian Ocean is to continue in the future, under
both medium and high emission scenarios (high
confidence)
• The frequency of extreme positive Indian Ocean Dipole
(IOD) events is projected to increase by almost a factor of
three, with one-in-seventeen-year events in the twentieth
century to one-in-six-year by the end of the twenty-first
century (low confidence).
• The heat content of the upper 700 m of the Indian Ocean
has exhibited an increasing trend during 1955–2015 (high
confidence), with spatially non-uniform heating.
• SST warming has very likely contributed to the decreasing
trend observed in oxygen (O 2 ) concentrations in the
tropical Indian Ocean, and the declining trend in pH and
marine phytoplankton over the western Indian Ocean.
These trends are projected to continue with global
warming.
10.1 Introduction
About one third of the global population lives around the
Indian Ocean—many in low-lying coastal regions, small
islands or low-to-middle income nations with low adaptive
capacity—that are especially vulnerable to climate change
impacts. Ocean-atmospheric conditions over the Indian
Ocean regulate the regional weather-climate system over
these regions. Hence, variability and changes in this basin
are of great significance to the food, water and power
security in India and neighbouring countries.
Warm sea surface temperatures (SSTs >28 °C), known as
the Indian Ocean warm pool, occur over a large part of the
tropical Indian Ocean (TIO, 40 °E:115 °E; 30 °S:30 °N),
which is a part of the larger Indo-Pacific warm pool. It
favours deep atmospheric convection (Graham and Barnett
1987) and energizes the global atmospheric circulation,
particularly the Hadley circulation and the Walker circulation thereby modulating the major elements of global climate
such as the Indian monsoon and the El Niño Southern
Oscillation (ENSO). The strong monsoon winds during
June–July–August–September force intense coastal and
open-ocean upwelling in the Arabian Sea, and modulate
evaporation and moisture transport towards India (Izumo
et al. 2008). They also provide a globally significant source
of atmospheric CO 2 (Valsala and Murtugudde 2015), and
foster intense marine primary productivity (Roxy et al.
2016). Unlike the other tropical ocean basins, the Indian
Ocean is landlocked in the north by the vast Asian landmass
and hence the only trans-basin exchanges are from the West
Pacific via the Indonesian Seas, known as the Indonesian
Through flow (ITF), and from the south.
The rate of warming in the tropical Indian Ocean is the
fastest among tropical oceans and accounts for about one
quarter of the increase in global oceanic heat content over
the last two decades (Beal et al. 2019) despite being the
smallest of the tropical oceans (representing only 13% of the
global ocean surface). The Indian Ocean is home to 30% of
the world’s coral reefs and 13% of global wild-catch fisheries. This marine ecosystem, including corals and phytoplankton, and fisheries are being impacted by a rise in heat
waves in the ocean, known as marine heat waves (Collins
et al. 2019). Moreover, an expansion of the Indian Ocean
warm pool is changing the subseasonal weather variability
such as the monsoon intraseasonal oscillation (Sabeerali
et al. 2014) and the Madden-Julian oscillation (Roxy et al.
2019), which originate in the Indian Ocean. This has an
impact on rainfall characteristics, particularly on extreme
rain over the tropics, including India. Heat waves (Chap. 2),
droughts and floods (Chap. 6), tropical cyclones (Chap. 8)
and extreme sea-level changes (Chap. 9) are becoming more
frequent and intense around the Indian Ocean as regional
climate patterns respond to anthropogenic climate change
(Collins et al. 2019). Hence, there is an urgent need to
understand the status and future evolution of the Indian
Ocean warming and its role in influencing the regional climate under a global warming environment.
10.2 Observed and Projected Changes
in Indian Ocean SST
10.2.1 Observed Changes in SST
The oceans have absorbed approximately 93% of the additional heat due to anthropogenic global warming since the
1950s (Cheng et al. 2017) and have resulted in a significant
increasing trend in the global average ocean SST, as evidenced by modern instrumental records (Deser et al. 2010).
Among the oceans, Indian Ocean stands out as one of the
most rapidly warming ocean basins (Gnanaseelan et al.
2017; Beal et al. 2019). The global average rise in SST
192
M. K. Roxy et al.
• The tropical Indian Ocean has experienced rapid
basin-wide sea surface temperature (SST) warming, with
an average rise of 1.0 °C (0.15 °C/decade) during 1951–
2015, over which period the global average SST warmed
about 0.7 °C (0.11 °C/decade) (high confidence).
The SST warming is spatially non-uniform and about 90%
of the warming is attributed to anthropogenic emissions.
• The basin-wide non-uniform SST warming trend in the
tropical Indian Ocean is to continue in the future, under
both medium and high emission scenarios (high
confidence)
• The frequency of extreme positive Indian Ocean Dipole
(IOD) events is projected to increase by almost a factor of
three, with one-in-seventeen-year events in the twentieth
century to one-in-six-year by the end of the twenty-first
century (low confidence).
• The heat content of the upper 700 m of the Indian Ocean
has exhibited an increasing trend during 1955–2015 (high
confidence), with spatially non-uniform heating.
• SST warming has very likely contributed to the decreasing
trend observed in oxygen (O 2 ) concentrations in the
tropical Indian Ocean, and the declining trend in pH and
marine phytoplankton over the western Indian Ocean.
These trends are projected to continue with global
warming.
10.1 Introduction
About one third of the global population lives around the
Indian Ocean—many in low-lying coastal regions, small
islands or low-to-middle income nations with low adaptive
capacity—that are especially vulnerable to climate change
impacts. Ocean-atmospheric conditions over the Indian
Ocean regulate the regional weather-climate system over
these regions. Hence, variability and changes in this basin
are of great significance to the food, water and power
security in India and neighbouring countries.
Warm sea surface temperatures (SSTs >28 °C), known as
the Indian Ocean warm pool, occur over a large part of the
tropical Indian Ocean (TIO, 40 °E:115 °E; 30 °S:30 °N),
which is a part of the larger Indo-Pacific warm pool. It
favours deep atmospheric convection (Graham and Barnett
1987) and energizes the global atmospheric circulation,
particularly the Hadley circulation and the Walker circulation thereby modulating the major elements of global climate
such as the Indian monsoon and the El Niño Southern
Oscillation (ENSO). The strong monsoon winds during
June–July–August–September force intense coastal and
open-ocean upwelling in the Arabian Sea, and modulate
evaporation and moisture transport towards India (Izumo
et al. 2008). They also provide a globally significant source
of atmospheric CO 2 (Valsala and Murtugudde 2015), and
foster intense marine primary productivity (Roxy et al.
2016). Unlike the other tropical ocean basins, the Indian
Ocean is landlocked in the north by the vast Asian landmass
and hence the only trans-basin exchanges are from the West
Pacific via the Indonesian Seas, known as the Indonesian
Through flow (ITF), and from the south.
The rate of warming in the tropical Indian Ocean is the
fastest among tropical oceans and accounts for about one
quarter of the increase in global oceanic heat content over
the last two decades (Beal et al. 2019) despite being the
smallest of the tropical oceans (representing only 13% of the
global ocean surface). The Indian Ocean is home to 30% of
the world’s coral reefs and 13% of global wild-catch fisheries. This marine ecosystem, including corals and phytoplankton, and fisheries are being impacted by a rise in heat
waves in the ocean, known as marine heat waves (Collins
et al. 2019). Moreover, an expansion of the Indian Ocean
warm pool is changing the subseasonal weather variability
such as the monsoon intraseasonal oscillation (Sabeerali
et al. 2014) and the Madden-Julian oscillation (Roxy et al.
2019), which originate in the Indian Ocean. This has an
impact on rainfall characteristics, particularly on extreme
rain over the tropics, including India. Heat waves (Chap. 2),
droughts and floods (Chap. 6), tropical cyclones (Chap. 8)
and extreme sea-level changes (Chap. 9) are becoming more
frequent and intense around the Indian Ocean as regional
climate patterns respond to anthropogenic climate change
(Collins et al. 2019). Hence, there is an urgent need to
understand the status and future evolution of the Indian
Ocean warming and its role in influencing the regional climate under a global warming environment.
10.2 Observed and Projected Changes
in Indian Ocean SST
10.2.1 Observed Changes in SST
The oceans have absorbed approximately 93% of the additional heat due to anthropogenic global warming since the
1950s (Cheng et al. 2017) and have resulted in a significant
increasing trend in the global average ocean SST, as evidenced by modern instrumental records (Deser et al. 2010).
Among the oceans, Indian Ocean stands out as one of the
most rapidly warming ocean basins (Gnanaseelan et al.
2017; Beal et al. 2019). The global average rise in SST
192
M. K. Roxy et al.