a significant contribution from the ITF transport to the
interior of the south Indian Ocean after 1990 (Han et al.
2014). For the Pacific and Indian Oceans, decadal shifts are
primarily observed in the upper 350 m, likely due to shallow
subtropical circulation, leading to an abrupt increase of OHC
in the Indian Ocean carried by the ITF from the Pacific
Ocean over the last decade (Cheng et al. 2017). During the
slowdown in Pacific SST warming, there was anomalous
warming in the Indian Ocean and an accelerated OHC rise
below 50 m, which is associated with a La Niña-like climate
shift, and an enhanced heat transport of the ITF (Liu et al.
2016). Transmission of the multidecadal signal occurs via an
oceanic pathway through the ITF and is manifested across
the Indian Ocean centred along 12° S as westwardpropagating Rossby waves modulating the thermocline and
subsurface heat content variations (Rahul and Gnanaseelan,
2016; Deepa et al. 2019). Changes in Pacific wind forcing in
recent decades and associated rapid increases in Indian
Ocean subsurface heat content can thus affect the basin’s
leading mode of variability (Ummenhofer et al. 2017). Jin
et al (2018) argued that the western Indian Ocean subsurface
heat content is influenced by Interdecadal Pacific oscillation
(IPO) through wind driven Ekman pumping via the atmospheric bridge, whereas the eastern Indian Ocean is largely
affected through the oceanic pathway via ITF. The recent
negative phase of IPO (1998–2012) enhanced the Pacific
easterlies, which eventually led to export of anomalous heat
from the Pacific towards the Indian Ocean (Gastineau et al
2018). The OHC300-based definition of PDO takes into
account variations throughout the upper ocean and is better
suited to capture various characteristics of PDO variability
(Kumar and Wen 2016). Based on multiple observational
datasets, ocean reanalysis products and an ocean model
simulation, Li et al. (2018) reported the presence of
prominent multidecadal variations in the Indian Ocean
OHC400. They suggested that the upper Indian Ocean first
experienced a heat content increase at the rate of
5.9 ± 2.5 Â 10
21 J decade
−1 during 1965–79, followed by
a decrease at the rate of −5.2 ± 2.5 Â 10
21 J decade
−1
during 1980–96, and subsequently an enhanced rate of
increase of 13.6 ± 1.1 Â 10
21 J decade
−1 from 2000 to
2014. This suggests that the Indian Ocean OHC underwent
tremendous decadal variations which might continue into the
future. CMIP5 simulations have limited skill in capturing
decadal variability in upper OHC during the past 45 years
(Collins et al. 2013; Cheng et al. 2015).
10.3.3 Future Projections of Indian Ocean Heat
Content
Monitoring and understanding OHC change and the role of
circulation in shaping the patterns of increase remain key to
predicting global and regional climate change, and sea-level
rise (Zanna et al. 2019). Under the low-to-medium (RCP4.5)
emissions scenario, half of the energy taken up by the ocean
by the end of the twenty-first century will be in the uppermost 700 m, and 85% will be in the uppermost 2000 m (low
confidence). Future changes in wind and air-sea fluxes, and
ocean transport, likely to have serious implications for
regional sea-level rise and coastal flood risk. Additionally,
the spatial patterns of OHC change under global warming
contribute to the regional sea-level projections (e.g. Slangen
et al 2014). There is a large spread among CMIP5 models in
projections of future changes in OHC, suggesting an urgent
need for further refinement (Cheng et al 2016; Allison et al.
2019).
10.4 Impacts of Indian Ocean Warming
10.4.1 Consequences of Indian Ocean Warming
on Regional Climate
Rapid warming of the Indian Ocean during 1950–2015,
along with substantial changes in land use and anthropogenic aerosols have altered the Indian summer monsoon
(Singh et al. 2019, Chaps. 3 and 6). During 1950–2015,
there has been a significant decline in the summer monsoon
rainfall over central India and parts of north India due to a
reduction in the tropospheric thermal contrast that is associated with the rapid warming of the Indian Ocean (Mishra
et al. 2012; Saha et al. 2014; Roxy et al. 2016). At the same
time, rapid warming in the Arabian Sea has resulted in a rise
in widespread extreme rains over Western Ghats and central
India, since warming induces increased fluctuations in the
monsoon winds, with ensuing episodes of enhanced moisture transport from the Arabian Sea towards the Indian
subcontinent (Roxy et al. 2017). Indian Ocean warming is
also found to reduce rainfall over India during the onset
phase and increase it during the withdrawal phase
(Chakravorty et al. 2016).
In terms of tropical cyclones, the Bay of Bengal region
witnesses more than 80% of the global fatalities associated
with tropical cyclones, while only accounting for 5% of
these storms globally (Beal et al. 2019). Since tropical
cyclones primarily draw their energy from evaporation at the
ocean surface, SST and OHC strongly constrain cyclone
intensity (Rajeevan et al. 2013). Global warming appears to
have increased the intensity of tropical cyclones during the
post-monsoon period in the Bay of Bengal (Chap. 8) and the
pre-monsoon period in the Arabian Sea. For example,
Cyclone Nilofar in 2014 was the first severe tropical cyclone
to be recorded in the Arabian Sea in the post-monsoon
season. Though the cyclone did not make landfall, it produced heavy rainfall along the western coast of India. Future
198
M. K. Roxy et al.
interior of the south Indian Ocean after 1990 (Han et al.
2014). For the Pacific and Indian Oceans, decadal shifts are
primarily observed in the upper 350 m, likely due to shallow
subtropical circulation, leading to an abrupt increase of OHC
in the Indian Ocean carried by the ITF from the Pacific
Ocean over the last decade (Cheng et al. 2017). During the
slowdown in Pacific SST warming, there was anomalous
warming in the Indian Ocean and an accelerated OHC rise
below 50 m, which is associated with a La Niña-like climate
shift, and an enhanced heat transport of the ITF (Liu et al.
2016). Transmission of the multidecadal signal occurs via an
oceanic pathway through the ITF and is manifested across
the Indian Ocean centred along 12° S as westwardpropagating Rossby waves modulating the thermocline and
subsurface heat content variations (Rahul and Gnanaseelan,
2016; Deepa et al. 2019). Changes in Pacific wind forcing in
recent decades and associated rapid increases in Indian
Ocean subsurface heat content can thus affect the basin’s
leading mode of variability (Ummenhofer et al. 2017). Jin
et al (2018) argued that the western Indian Ocean subsurface
heat content is influenced by Interdecadal Pacific oscillation
(IPO) through wind driven Ekman pumping via the atmospheric bridge, whereas the eastern Indian Ocean is largely
affected through the oceanic pathway via ITF. The recent
negative phase of IPO (1998–2012) enhanced the Pacific
easterlies, which eventually led to export of anomalous heat
from the Pacific towards the Indian Ocean (Gastineau et al
2018). The OHC300-based definition of PDO takes into
account variations throughout the upper ocean and is better
suited to capture various characteristics of PDO variability
(Kumar and Wen 2016). Based on multiple observational
datasets, ocean reanalysis products and an ocean model
simulation, Li et al. (2018) reported the presence of
prominent multidecadal variations in the Indian Ocean
OHC400. They suggested that the upper Indian Ocean first
experienced a heat content increase at the rate of
5.9 ± 2.5 Â 10
21 J decade
−1 during 1965–79, followed by
a decrease at the rate of −5.2 ± 2.5 Â 10
21 J decade
−1
during 1980–96, and subsequently an enhanced rate of
increase of 13.6 ± 1.1 Â 10
21 J decade
−1 from 2000 to
2014. This suggests that the Indian Ocean OHC underwent
tremendous decadal variations which might continue into the
future. CMIP5 simulations have limited skill in capturing
decadal variability in upper OHC during the past 45 years
(Collins et al. 2013; Cheng et al. 2015).
10.3.3 Future Projections of Indian Ocean Heat
Content
Monitoring and understanding OHC change and the role of
circulation in shaping the patterns of increase remain key to
predicting global and regional climate change, and sea-level
rise (Zanna et al. 2019). Under the low-to-medium (RCP4.5)
emissions scenario, half of the energy taken up by the ocean
by the end of the twenty-first century will be in the uppermost 700 m, and 85% will be in the uppermost 2000 m (low
confidence). Future changes in wind and air-sea fluxes, and
ocean transport, likely to have serious implications for
regional sea-level rise and coastal flood risk. Additionally,
the spatial patterns of OHC change under global warming
contribute to the regional sea-level projections (e.g. Slangen
et al 2014). There is a large spread among CMIP5 models in
projections of future changes in OHC, suggesting an urgent
need for further refinement (Cheng et al 2016; Allison et al.
2019).
10.4 Impacts of Indian Ocean Warming
10.4.1 Consequences of Indian Ocean Warming
on Regional Climate
Rapid warming of the Indian Ocean during 1950–2015,
along with substantial changes in land use and anthropogenic aerosols have altered the Indian summer monsoon
(Singh et al. 2019, Chaps. 3 and 6). During 1950–2015,
there has been a significant decline in the summer monsoon
rainfall over central India and parts of north India due to a
reduction in the tropospheric thermal contrast that is associated with the rapid warming of the Indian Ocean (Mishra
et al. 2012; Saha et al. 2014; Roxy et al. 2016). At the same
time, rapid warming in the Arabian Sea has resulted in a rise
in widespread extreme rains over Western Ghats and central
India, since warming induces increased fluctuations in the
monsoon winds, with ensuing episodes of enhanced moisture transport from the Arabian Sea towards the Indian
subcontinent (Roxy et al. 2017). Indian Ocean warming is
also found to reduce rainfall over India during the onset
phase and increase it during the withdrawal phase
(Chakravorty et al. 2016).
In terms of tropical cyclones, the Bay of Bengal region
witnesses more than 80% of the global fatalities associated
with tropical cyclones, while only accounting for 5% of
these storms globally (Beal et al. 2019). Since tropical
cyclones primarily draw their energy from evaporation at the
ocean surface, SST and OHC strongly constrain cyclone
intensity (Rajeevan et al. 2013). Global warming appears to
have increased the intensity of tropical cyclones during the
post-monsoon period in the Bay of Bengal (Chap. 8) and the
pre-monsoon period in the Arabian Sea. For example,
Cyclone Nilofar in 2014 was the first severe tropical cyclone
to be recorded in the Arabian Sea in the post-monsoon
season. Though the cyclone did not make landfall, it produced heavy rainfall along the western coast of India. Future
198
M. K. Roxy et al.