projections suggest a likely increase in the number of
extremely severe tropical cyclones in response to Indian
Ocean warming, particularly in the Arabian Sea, while
changes in frequency remain uncertain (Chap. 8).
Indian Ocean SSTs have a role in regulating the surface air
temperatures over the Indian subcontinent (Chowdary et al.
2014). Associated with the basin-wide warming and frequent
El Niños, the frequency and duration of heat waves have
increased over the Indian subcontinent (Rohini et al. 2016).
Rising ocean temperatures have also resulted in instances of
marine heat waves in the Indian Ocean. Marine heat waves are
similar to heat waves over the land, with periods of extremely
high ocean temperatures that persist for days to months (Collins et al. 2019). Recent marine heat waves, including the one
in 2016 that co-occurred with an extreme El Niño event of
2015–16, resulted in mass bleaching of coral reefs and
adversely impacted aquaculture industries along the Indian
Ocean rim countries (Collins et al. 2019). Satellite observations reveal that the intensity of marine heat waves have
increased and that they have very likely doubled in frequency
over 1982–2016. Climate projections indicate that at the high
emissions RCP 8.5 GHG scenario, a one-in-100-day marine
heat wave event (with pre-industrial CO 2 levels) is very likely
to become a one-in-four-day event by 2031–2050 and a
one-in-two-day event by 2081–2100 (Collins et al. 2019).
The impact of Indian Ocean warming is reflected in the
sea-level changes, as thermal variations have dominated
these changes in recent decades (Chap. 9). The largest
sea-level changes were observed along the northern and
eastern coasts of the Bay of Bengal (Chap. 9). Tide gauge
data corroborates the increasing sea level among the coastal
regions of the Indian Ocean, such as Mumbai, Kochi,
Visakhapatnam on the Indian coast and Durban, Fremantle,
Port Hedland on the Australian coast. The consistent
increase in sea level is attributed to the thermal expansion of
sea water, due to a basin-wide surface warming in the Indian
Ocean. Besides, the observed changes in the wind circulation in the Indian Ocean has modulated the ocean heat
transport, and distributed the heat across the basin, resulting
in a large thermosteric response in the sea level (Chap. 9).
Beyond the regional climate, Indian Ocean warming has
global and remote impacts also. The Indian Ocean has
contributed to more than 21% of the global oceanic heat
uptake over the last two decades and contributed strongly to
the temporary slowdown in global warming during 1998–
2013 (Lee et al. 2015; Cheng et al. 2017). Climate model
experiments indicate that the rapid warming of the Indian
Ocean is strengthening the Atlantic meridional overturning
circulation (Hu and Fedorov 2019; Cherchi 2019). The
basin-wide warming could modulate the Pacific climate,
affect the North Atlantic oscillation and enhance the positive
Southern Annular Mode, and may cause West Sahel and
Mediterranean droughts (Beal et al. 2019). Atmospheric
blocking triggered by tropical convection in the Indian and
Pacific oceans can cause persistent anticyclonic circulation
that not only leads to severe drought but also generates
marine heat waves in the adjacent ocean. Warming in the
Indian Ocean and associated deep convection is found to
trigger droughts in South America and marine heat waves in
the adjoining South Atlantic (Rodrigues et al. 2019).
A strong negative IOD event in 2016 strongly impacted East
African rainfall, with some regions recording below 50% of
normal rainfall, leading to devastating drought, food insecurity and unsafe drinking water for over 15 million people
in Somalia, Ethiopia and Kenya (Collins et al. 2019).
10.4.2 Consequences of Indian Ocean Warming
on Ocean Biogeochemistry
The biogeochemical properties of the Indian Ocean are distinct
from other ocean basins for mainly two reasons—the land
boundary in the north due to Indian subcontinent and the large
amplitude of seasonally reversing monsoon cycle. Climate
driven physical fluctuations are expected to impact the marine
ecosystem substantially by modifying the biotic and abiotic
environments, which can lead to severe repercussions for the
oceanic primary production. The warming of SSTs in the
western Indian Ocean leads to increased stratification in the
basin. Roxy et al. (2016) show a declining trend in marine
phytoplankton in the western Indian Ocean (high confidence),
by 30% in the observations during 1998–2013 and 20% in the
CMIP5 simulations during 1950–2015 (medium confidence).
This significant decline in phytoplankton is attributed to
enhanced stratification of the oceanic water column as a result
of rapid surface warming, thereby suppressing the mixing of
nutrients from subsurface layers into the surface. Downward
trends in primary production over the Indian Ocean can be
detrimental to the marine food web and the fishing industry,
especially the economically valuable tuna industry (Lee et al.
2005). However, careful gathering of in situ observations in
the open ocean is needed to build a substantial understanding
of the bio-physical interactions in the Indian Ocean.
Approximately 30% of the historical anthropogenic CO 2
emissions have been absorbed by the oceans since the
pre-industrial era (e.g. Canadell et al. 2007). The increasing
oceanic uptake of CO 2 has changed seawater chemistry and
resulted in ocean acidification, with profound impacts on biological ecosystems in the upper ocean. Long-term increasing
trends in ocean acidification, consistent with the increase in
atmospheric CO 2, are evident over the past several decades (Dore
et al. 2009).
10 Indian Ocean Warming
199
extremely severe tropical cyclones in response to Indian
Ocean warming, particularly in the Arabian Sea, while
changes in frequency remain uncertain (Chap. 8).
Indian Ocean SSTs have a role in regulating the surface air
temperatures over the Indian subcontinent (Chowdary et al.
2014). Associated with the basin-wide warming and frequent
El Niños, the frequency and duration of heat waves have
increased over the Indian subcontinent (Rohini et al. 2016).
Rising ocean temperatures have also resulted in instances of
marine heat waves in the Indian Ocean. Marine heat waves are
similar to heat waves over the land, with periods of extremely
high ocean temperatures that persist for days to months (Collins et al. 2019). Recent marine heat waves, including the one
in 2016 that co-occurred with an extreme El Niño event of
2015–16, resulted in mass bleaching of coral reefs and
adversely impacted aquaculture industries along the Indian
Ocean rim countries (Collins et al. 2019). Satellite observations reveal that the intensity of marine heat waves have
increased and that they have very likely doubled in frequency
over 1982–2016. Climate projections indicate that at the high
emissions RCP 8.5 GHG scenario, a one-in-100-day marine
heat wave event (with pre-industrial CO 2 levels) is very likely
to become a one-in-four-day event by 2031–2050 and a
one-in-two-day event by 2081–2100 (Collins et al. 2019).
The impact of Indian Ocean warming is reflected in the
sea-level changes, as thermal variations have dominated
these changes in recent decades (Chap. 9). The largest
sea-level changes were observed along the northern and
eastern coasts of the Bay of Bengal (Chap. 9). Tide gauge
data corroborates the increasing sea level among the coastal
regions of the Indian Ocean, such as Mumbai, Kochi,
Visakhapatnam on the Indian coast and Durban, Fremantle,
Port Hedland on the Australian coast. The consistent
increase in sea level is attributed to the thermal expansion of
sea water, due to a basin-wide surface warming in the Indian
Ocean. Besides, the observed changes in the wind circulation in the Indian Ocean has modulated the ocean heat
transport, and distributed the heat across the basin, resulting
in a large thermosteric response in the sea level (Chap. 9).
Beyond the regional climate, Indian Ocean warming has
global and remote impacts also. The Indian Ocean has
contributed to more than 21% of the global oceanic heat
uptake over the last two decades and contributed strongly to
the temporary slowdown in global warming during 1998–
2013 (Lee et al. 2015; Cheng et al. 2017). Climate model
experiments indicate that the rapid warming of the Indian
Ocean is strengthening the Atlantic meridional overturning
circulation (Hu and Fedorov 2019; Cherchi 2019). The
basin-wide warming could modulate the Pacific climate,
affect the North Atlantic oscillation and enhance the positive
Southern Annular Mode, and may cause West Sahel and
Mediterranean droughts (Beal et al. 2019). Atmospheric
blocking triggered by tropical convection in the Indian and
Pacific oceans can cause persistent anticyclonic circulation
that not only leads to severe drought but also generates
marine heat waves in the adjacent ocean. Warming in the
Indian Ocean and associated deep convection is found to
trigger droughts in South America and marine heat waves in
the adjoining South Atlantic (Rodrigues et al. 2019).
A strong negative IOD event in 2016 strongly impacted East
African rainfall, with some regions recording below 50% of
normal rainfall, leading to devastating drought, food insecurity and unsafe drinking water for over 15 million people
in Somalia, Ethiopia and Kenya (Collins et al. 2019).
10.4.2 Consequences of Indian Ocean Warming
on Ocean Biogeochemistry
The biogeochemical properties of the Indian Ocean are distinct
from other ocean basins for mainly two reasons—the land
boundary in the north due to Indian subcontinent and the large
amplitude of seasonally reversing monsoon cycle. Climate
driven physical fluctuations are expected to impact the marine
ecosystem substantially by modifying the biotic and abiotic
environments, which can lead to severe repercussions for the
oceanic primary production. The warming of SSTs in the
western Indian Ocean leads to increased stratification in the
basin. Roxy et al. (2016) show a declining trend in marine
phytoplankton in the western Indian Ocean (high confidence),
by 30% in the observations during 1998–2013 and 20% in the
CMIP5 simulations during 1950–2015 (medium confidence).
This significant decline in phytoplankton is attributed to
enhanced stratification of the oceanic water column as a result
of rapid surface warming, thereby suppressing the mixing of
nutrients from subsurface layers into the surface. Downward
trends in primary production over the Indian Ocean can be
detrimental to the marine food web and the fishing industry,
especially the economically valuable tuna industry (Lee et al.
2005). However, careful gathering of in situ observations in
the open ocean is needed to build a substantial understanding
of the bio-physical interactions in the Indian Ocean.
Approximately 30% of the historical anthropogenic CO 2
emissions have been absorbed by the oceans since the
pre-industrial era (e.g. Canadell et al. 2007). The increasing
oceanic uptake of CO 2 has changed seawater chemistry and
resulted in ocean acidification, with profound impacts on biological ecosystems in the upper ocean. Long-term increasing
trends in ocean acidification, consistent with the increase in
atmospheric CO 2, are evident over the past several decades (Dore
et al. 2009).
10 Indian Ocean Warming
199