Warming in the TIO has been basin-wide but spatially
non-uniform, with the largest increasing trends seen in the
central equatorial Indian Ocean and lowest warming trends
off the Sumatra and Java coasts. While long-term warming
(1900–2015) is maximum over the western Indian Ocean
(Roxy et al. 2014), warming in recent decades is prominent
over the central equatorial and subtropical south Indian
Ocean (Fig. 10.1a). SST warming trends during the recent
period (2000–2013) also exhibit an interhemispheric difference, with relatively weak warming to the north of 10° S and
accelerated warming to the south of 10° S (Dong and
McPhaden 2016).
10.2.2 Attribution of SST Trends
to Anthropogenic Emissions
The observed surface warming over the Indian Ocean has
been linked to natural and anthropogenic causes. Climate
model simulations show that over 90% of the SST trend since
the 1950s is very likely due to increased anthropogenic
emissions (Du and Xie 2008; Dong and Zhou 2014; Dong
et al. 2014), while the remaining is due to internal variability
(Dong et al. 2014). Among the anthropogenic causes, change
in radiative forcing due to the increased greenhouse gas
concentrations is the major factor (Du and Xie 2008). Over
recent decades, increasing atmospheric pollutants, known as
aerosols, have likely dampened the greenhouse gas forced
warming of the Indian Ocean (Dong and Zhou 2014).
Changes in the tropical circulation, ocean-atmospheric
interaction and dynamics play a role in the observed distribution of the warmer waters in the Indian Ocean. Redistribution of heat from the Pacific Ocean via the Walker circulation
(Roxy et al. 2014) and the ITF (Dong and McPhaden 2016) is
one of the reasons for the observed patterns of warming in the
Indian Ocean. Other than the ITF, Indian Ocean may also be
receiving a warming signal via the deep meridional overturning circulation, and from the Southern Ocean (Gille 2002).
Contrary to expectation, there is a negative trend in the net heat
flux despite the warming trend in the Indian Ocean SSTs
(Rahul and Gnanaseelan 2013). This suggests that local ocean
dynamics and ocean-atmosphere interaction also have a major
role in the observed warming pattern in the Indian Ocean (Lau
and Nath 2000; Du et al. 2009; Rahul and Gnanaseelan 2016;
Pratik et al. 2019; Rao et al. 2012). The period 2000–2013
witnessed strong warming in the southern Indian Ocean south
of 10° S inducing a north–south SST gradient. This interhemispheric gradient is forced primarily by an increased ITF,
from the Pacific into the Indian Ocean, induced by stronger
Pacific trade winds (Dong and McPhaden 2016; Lee et al
2015).
10.2.3 Future Projections of SST Warming
Future projections using CMIP5 simulations clearly show
SST warming in the Indian Ocean with increasing anthropogenic emissions (high confidence). However, SST warming is projected to be non-uniform with regional variations in
the Arabian Sea and the Bay of Bengal. Most models project
a higher SST warming in the Arabian Sea than the Bay of
Bengal. This is consistent with the changes that have been
observed in the Indian Ocean in the last several decades
(Zhao and Zhang 2016).
Changes in SSTs projected for the end of the twenty-first
century show regional and seasonal variability (Cai et al.
2013). The ensemble mean of CMIP5 RCP scenarios indicates stronger warming in the north-western Indian Ocean
and weaker warming off the Sumatra and Java coasts (Zheng
and Xie 2009). The CMIP5 RCP scenarios for the period
2040–2069 and 2070–2099 are shown in Fig. 10.2, where
the stronger warming in the north-western Indian Ocean and
weaker warming in the south-eastern Indian Ocean south of
15° S with respect to the base period of 1976–2005 are
evident, and consistent with Zheng and Xie (2009). Patterns
of warming are similar in both the RCP4.5 and RCP8.5
scenarios, although the magnitude of warming is much larger in the latter scenario (Fig. 10.2). RCP4.5 projects a
warming rate of 0.13 °C/decade in the TIO (Fig. 10.2e),
similar to the current rate of warming (Fig. 10.1c). Meanwhile, RCP8.5 indicates an accelerated warming at the rate
of 0.35 °C/decade. CMIP5 (the ensemble mean) projected
SST rise in the TIO in the near and far future for both the
RCP4.5 and RCP8.5 scenarios are tabulated in Table 10.1.
The strong SST warming trend in the north-west basin
accompanied by an increase in precipitation and weak SST
warming trend in the south-east basin accompanied by
decrease in precipitation drive strong surface easterly wind
anomalies along the equatorial Indian Ocean (Li et al. 2016).
Along with the east–west gradient in SST and precipitation
under global warming scenarios, the thermocline in the east
equatorial Indian Ocean also shoals (a favourable condition
for the formation of IOD) as a result of a weakened Indian
Ocean Walker cell and easterly wind change along the
equator (Zheng and Xie 2009). The shoaling of the thermocline then strengthens the thermocline feedback in this
region. Such a pattern of SST, precipitation and thermocline
feedback would result in more frequent occurrences of
extreme positive IOD (pIOD) events in the future, from one
event every 17.3 years over the twentieth century to one
event every 6.3 years by the end of twenty-first century (low
confidence) (Cai et al. 2014). This suggests increased risks
of climate and weather extremes in regions impacted by
extreme pIOD events.
194
M. K. Roxy et al.
non-uniform, with the largest increasing trends seen in the
central equatorial Indian Ocean and lowest warming trends
off the Sumatra and Java coasts. While long-term warming
(1900–2015) is maximum over the western Indian Ocean
(Roxy et al. 2014), warming in recent decades is prominent
over the central equatorial and subtropical south Indian
Ocean (Fig. 10.1a). SST warming trends during the recent
period (2000–2013) also exhibit an interhemispheric difference, with relatively weak warming to the north of 10° S and
accelerated warming to the south of 10° S (Dong and
McPhaden 2016).
10.2.2 Attribution of SST Trends
to Anthropogenic Emissions
The observed surface warming over the Indian Ocean has
been linked to natural and anthropogenic causes. Climate
model simulations show that over 90% of the SST trend since
the 1950s is very likely due to increased anthropogenic
emissions (Du and Xie 2008; Dong and Zhou 2014; Dong
et al. 2014), while the remaining is due to internal variability
(Dong et al. 2014). Among the anthropogenic causes, change
in radiative forcing due to the increased greenhouse gas
concentrations is the major factor (Du and Xie 2008). Over
recent decades, increasing atmospheric pollutants, known as
aerosols, have likely dampened the greenhouse gas forced
warming of the Indian Ocean (Dong and Zhou 2014).
Changes in the tropical circulation, ocean-atmospheric
interaction and dynamics play a role in the observed distribution of the warmer waters in the Indian Ocean. Redistribution of heat from the Pacific Ocean via the Walker circulation
(Roxy et al. 2014) and the ITF (Dong and McPhaden 2016) is
one of the reasons for the observed patterns of warming in the
Indian Ocean. Other than the ITF, Indian Ocean may also be
receiving a warming signal via the deep meridional overturning circulation, and from the Southern Ocean (Gille 2002).
Contrary to expectation, there is a negative trend in the net heat
flux despite the warming trend in the Indian Ocean SSTs
(Rahul and Gnanaseelan 2013). This suggests that local ocean
dynamics and ocean-atmosphere interaction also have a major
role in the observed warming pattern in the Indian Ocean (Lau
and Nath 2000; Du et al. 2009; Rahul and Gnanaseelan 2016;
Pratik et al. 2019; Rao et al. 2012). The period 2000–2013
witnessed strong warming in the southern Indian Ocean south
of 10° S inducing a north–south SST gradient. This interhemispheric gradient is forced primarily by an increased ITF,
from the Pacific into the Indian Ocean, induced by stronger
Pacific trade winds (Dong and McPhaden 2016; Lee et al
2015).
10.2.3 Future Projections of SST Warming
Future projections using CMIP5 simulations clearly show
SST warming in the Indian Ocean with increasing anthropogenic emissions (high confidence). However, SST warming is projected to be non-uniform with regional variations in
the Arabian Sea and the Bay of Bengal. Most models project
a higher SST warming in the Arabian Sea than the Bay of
Bengal. This is consistent with the changes that have been
observed in the Indian Ocean in the last several decades
(Zhao and Zhang 2016).
Changes in SSTs projected for the end of the twenty-first
century show regional and seasonal variability (Cai et al.
2013). The ensemble mean of CMIP5 RCP scenarios indicates stronger warming in the north-western Indian Ocean
and weaker warming off the Sumatra and Java coasts (Zheng
and Xie 2009). The CMIP5 RCP scenarios for the period
2040–2069 and 2070–2099 are shown in Fig. 10.2, where
the stronger warming in the north-western Indian Ocean and
weaker warming in the south-eastern Indian Ocean south of
15° S with respect to the base period of 1976–2005 are
evident, and consistent with Zheng and Xie (2009). Patterns
of warming are similar in both the RCP4.5 and RCP8.5
scenarios, although the magnitude of warming is much larger in the latter scenario (Fig. 10.2). RCP4.5 projects a
warming rate of 0.13 °C/decade in the TIO (Fig. 10.2e),
similar to the current rate of warming (Fig. 10.1c). Meanwhile, RCP8.5 indicates an accelerated warming at the rate
of 0.35 °C/decade. CMIP5 (the ensemble mean) projected
SST rise in the TIO in the near and far future for both the
RCP4.5 and RCP8.5 scenarios are tabulated in Table 10.1.
The strong SST warming trend in the north-west basin
accompanied by an increase in precipitation and weak SST
warming trend in the south-east basin accompanied by
decrease in precipitation drive strong surface easterly wind
anomalies along the equatorial Indian Ocean (Li et al. 2016).
Along with the east–west gradient in SST and precipitation
under global warming scenarios, the thermocline in the east
equatorial Indian Ocean also shoals (a favourable condition
for the formation of IOD) as a result of a weakened Indian
Ocean Walker cell and easterly wind change along the
equator (Zheng and Xie 2009). The shoaling of the thermocline then strengthens the thermocline feedback in this
region. Such a pattern of SST, precipitation and thermocline
feedback would result in more frequent occurrences of
extreme positive IOD (pIOD) events in the future, from one
event every 17.3 years over the twentieth century to one
event every 6.3 years by the end of twenty-first century (low
confidence) (Cai et al. 2014). This suggests increased risks
of climate and weather extremes in regions impacted by
extreme pIOD events.
194
M. K. Roxy et al.