OHC changes also contribute substantially to sea-level rise,
changes in ocean circulation, and energy transfer between
ocean and atmosphere, making it a vital task to estimate and
study historical OHC. Increases in OHC in the recent past
have been attributed to the increase in greenhouse gases in
earth’s atmosphere (e.g. Levitus et al. 2001 and Barnett et al.
2001, 2005). The primary impacts of an increase in OHC are
its effects on marine biodiversity and the melting of glaciers
in the regions like Greenland and Antarctica. Other consequences of OHC rise include declining ocean oxygen
(Schmidtko et al. 2017), bleaching and death of corals
(Hughes et al. 2018), ice shelves directly through bottom
heating, exacerbating marine heat waves (Oliver et al. 2018),
and altered impacts of natural variability such as ENSO,
IOD and Pacific Decadal Oscillation (PDO; e.g. Fasullo
et al. 2018). Over long time periods the ocean’s interior acts
like a capacitor and builds up large heat anomalies. These
persisting large-scale OHC anomalies are a source of ocean
climate predictability from seasonal-to-decadal timescales
(e.g. Smith and Murphy 2007).
10.3.1 Trends and Variability of Indian Ocean
Heat Content
Upper OHC exhibits an increasing trend in the Indian Ocean
since the 1950s (high confidence, Levitus et al. 2009; Xue
et al. 2012; Han et al. 2014). The global OHC700 has risen
at a rate of 2.8 Â 10
22 Joules per decade over the period
1955–2015 (Fig. 10.3a). Over the same period, the rate of
rise in OHC700 in the tropical (30 °S to 30 °N and 40 °E to
115 °E) and north (5 °N to 30 °N and 40 °E to 100 °E)
Indian Ocean basins were more muted at 0.62 Â 10
22 Joules
per decade and 1.0 Â 10
22 Joules per decade, respectively
(Fig. 10.3b). In other words, there was greater net heat
content gain in the north Indian Ocean (NIO) than the TIO
over 1955–2015 despite being a smaller basin in areal extent.
It is important to note that since the year 2000 both the
TIO and NIO have experienced a steep rise in OHC (Cheng
et al. 2017), which is absent in the global OHC signal
(Fig. 10.3a, b). This abrupt increase of OHC in TIO has
accounted for more than 70% of the global ocean heat gain in
the upper 700 m during the same period (e.g. Lee et al. 2015),
despite of representing only about 15% of the global ocean
area. It is very likely that a significant portion of the heat due
to greenhouse warming now resides in the upper 700 m of
the Indian Ocean. Given the fact that the OHC700 in the
Indian Ocean did not increase significantly during 1971–
2000, the rapid increase during the recent period is striking.
In addition, higher variability in the TIO and NIO upper
OHC compared to the global OHC is evident from Fig. 10.3.
Changes in OHC700 in the NIO from 1955 to 2015
(Fig. 10.3c) show wide spatial variations with warming in
the Arabian Sea, Bay of Bengal, equatorial Indian Ocean and
in the region south of 20° S (Fig. 10.3c) (e.g. Nagamani
et al. 2016; Anandh et al. 2018), but a zonally extended
cooling from 20° S to 5° S. Trends in TIO OHC2000 (ocean
heat content in the upper 2000 m) during 2005–2015 are
similar to that of OHC700 (Fig. 10.3b).
There are significant differences in the recent OHC700
trends between the major ocean basins, particularly the Pacific
and Indian Oceans. Despite of large natural variability in
OHC700, increasing trends due to anthropogenic influence are
evident (high confidence). The abrupt increase in the OHC700
during 2003–2012 was not due to surface heating, but due
almost entirely to horizontal heat convergence in the form of an
enhanced ITF (Lee et al. 2015). This Indian Ocean OHC
increase corresponds to a concurrent Pacific Ocean OHC
decrease in the 0–100 m, suggesting a transfer of heat from
Pacific Ocean to Indian Ocean (Liu et al. 2016). The net surface
heat flux into the Pacific Ocean increased greatly during 2003–
2012, consistent with the La Niña-like condition across the
Pacific Ocean. However, the anomalous surface heat uptake
was completely masked by horizontal heat divergence in the
tropical Pacific, which was mainly traced to the increased heat
transport to the Indian Ocean. This has, in fact, inhibited the
increase in tropical Pacific OHC700, which instead shows a
slight decrease during this period.
Among the global oceans, the ensemble-mean trends for
the 700–6000 m layer show the largest increasing trends in
the North Atlantic sub-polar gyre, north Indian Ocean and
Southern Ocean (e.g. Palmer et al. 2017). As there are fewer
observations available in the Indian Ocean between 3000
and 5000 m compared to the other oceans, deep OHC estimates in the Indian Ocean, particularly in the north-western
part, are relatively less accurate (Purkey and Johnson 2010).
Although the Atlantic and Indian oceans contributed to the
global OHC increase at 3500 m depth, the OHC increases in
these two basins have been weak below 4000 m during the
past two decades. The smaller contribution of the Indian
Ocean to the global OHC increase might be due to the
smaller bottom layer volume of the Indian Ocean than that of
the Pacific Ocean (e.g. Kouketsu et al. 2011).
10.3.2 Multidecadal Variability of the Indian
Ocean Heat Content
The observed upper OHC of the Indian Ocean reveals significant decadal variations (Han et al. 2014; Mohapatra et al.
2020). Model experiments suggest that the observed multidecadal trends in OHC are partially associated with anthropogenic forcing. The observed decadal variability in the
basin-wide distribution of sea level and thermal structure in
the Indian Ocean results primarily from forcing by Indian
Ocean winds (Deepa et al. 2019; Srinivasu et al. 2017), with
10 Indian Ocean Warming
197
changes in ocean circulation, and energy transfer between
ocean and atmosphere, making it a vital task to estimate and
study historical OHC. Increases in OHC in the recent past
have been attributed to the increase in greenhouse gases in
earth’s atmosphere (e.g. Levitus et al. 2001 and Barnett et al.
2001, 2005). The primary impacts of an increase in OHC are
its effects on marine biodiversity and the melting of glaciers
in the regions like Greenland and Antarctica. Other consequences of OHC rise include declining ocean oxygen
(Schmidtko et al. 2017), bleaching and death of corals
(Hughes et al. 2018), ice shelves directly through bottom
heating, exacerbating marine heat waves (Oliver et al. 2018),
and altered impacts of natural variability such as ENSO,
IOD and Pacific Decadal Oscillation (PDO; e.g. Fasullo
et al. 2018). Over long time periods the ocean’s interior acts
like a capacitor and builds up large heat anomalies. These
persisting large-scale OHC anomalies are a source of ocean
climate predictability from seasonal-to-decadal timescales
(e.g. Smith and Murphy 2007).
10.3.1 Trends and Variability of Indian Ocean
Heat Content
Upper OHC exhibits an increasing trend in the Indian Ocean
since the 1950s (high confidence, Levitus et al. 2009; Xue
et al. 2012; Han et al. 2014). The global OHC700 has risen
at a rate of 2.8 Â 10
22 Joules per decade over the period
1955–2015 (Fig. 10.3a). Over the same period, the rate of
rise in OHC700 in the tropical (30 °S to 30 °N and 40 °E to
115 °E) and north (5 °N to 30 °N and 40 °E to 100 °E)
Indian Ocean basins were more muted at 0.62 Â 10
22 Joules
per decade and 1.0 Â 10
22 Joules per decade, respectively
(Fig. 10.3b). In other words, there was greater net heat
content gain in the north Indian Ocean (NIO) than the TIO
over 1955–2015 despite being a smaller basin in areal extent.
It is important to note that since the year 2000 both the
TIO and NIO have experienced a steep rise in OHC (Cheng
et al. 2017), which is absent in the global OHC signal
(Fig. 10.3a, b). This abrupt increase of OHC in TIO has
accounted for more than 70% of the global ocean heat gain in
the upper 700 m during the same period (e.g. Lee et al. 2015),
despite of representing only about 15% of the global ocean
area. It is very likely that a significant portion of the heat due
to greenhouse warming now resides in the upper 700 m of
the Indian Ocean. Given the fact that the OHC700 in the
Indian Ocean did not increase significantly during 1971–
2000, the rapid increase during the recent period is striking.
In addition, higher variability in the TIO and NIO upper
OHC compared to the global OHC is evident from Fig. 10.3.
Changes in OHC700 in the NIO from 1955 to 2015
(Fig. 10.3c) show wide spatial variations with warming in
the Arabian Sea, Bay of Bengal, equatorial Indian Ocean and
in the region south of 20° S (Fig. 10.3c) (e.g. Nagamani
et al. 2016; Anandh et al. 2018), but a zonally extended
cooling from 20° S to 5° S. Trends in TIO OHC2000 (ocean
heat content in the upper 2000 m) during 2005–2015 are
similar to that of OHC700 (Fig. 10.3b).
There are significant differences in the recent OHC700
trends between the major ocean basins, particularly the Pacific
and Indian Oceans. Despite of large natural variability in
OHC700, increasing trends due to anthropogenic influence are
evident (high confidence). The abrupt increase in the OHC700
during 2003–2012 was not due to surface heating, but due
almost entirely to horizontal heat convergence in the form of an
enhanced ITF (Lee et al. 2015). This Indian Ocean OHC
increase corresponds to a concurrent Pacific Ocean OHC
decrease in the 0–100 m, suggesting a transfer of heat from
Pacific Ocean to Indian Ocean (Liu et al. 2016). The net surface
heat flux into the Pacific Ocean increased greatly during 2003–
2012, consistent with the La Niña-like condition across the
Pacific Ocean. However, the anomalous surface heat uptake
was completely masked by horizontal heat divergence in the
tropical Pacific, which was mainly traced to the increased heat
transport to the Indian Ocean. This has, in fact, inhibited the
increase in tropical Pacific OHC700, which instead shows a
slight decrease during this period.
Among the global oceans, the ensemble-mean trends for
the 700–6000 m layer show the largest increasing trends in
the North Atlantic sub-polar gyre, north Indian Ocean and
Southern Ocean (e.g. Palmer et al. 2017). As there are fewer
observations available in the Indian Ocean between 3000
and 5000 m compared to the other oceans, deep OHC estimates in the Indian Ocean, particularly in the north-western
part, are relatively less accurate (Purkey and Johnson 2010).
Although the Atlantic and Indian oceans contributed to the
global OHC increase at 3500 m depth, the OHC increases in
these two basins have been weak below 4000 m during the
past two decades. The smaller contribution of the Indian
Ocean to the global OHC increase might be due to the
smaller bottom layer volume of the Indian Ocean than that of
the Pacific Ocean (e.g. Kouketsu et al. 2011).
10.3.2 Multidecadal Variability of the Indian
Ocean Heat Content
The observed upper OHC of the Indian Ocean reveals significant decadal variations (Han et al. 2014; Mohapatra et al.
2020). Model experiments suggest that the observed multidecadal trends in OHC are partially associated with anthropogenic forcing. The observed decadal variability in the
basin-wide distribution of sea level and thermal structure in
the Indian Ocean results primarily from forcing by Indian
Ocean winds (Deepa et al. 2019; Srinivasu et al. 2017), with
10 Indian Ocean Warming
197