projections challenging as the attribution of projected
changes requires a clear understanding of projected changes
in ocean wind-driven circulation, steric changes and terrestrial ice storages (Air-sea momentum and heat fluxes variations are associated with spatially varying sea-level changes.
For example, the excess surface latent heat flux associated
with global warming penetrates differentially into the ocean
depending on, for example, the mixed layer depth and circulation. Similarly, climate change induces changes in surface winds, which will further cause sea-level regional
changes). As far as the Indian Ocean is concerned, apart
from information provided in IPCC AR4/5, there is no
independent studies that provide regional sea-level projections
1 and its attribution (i.e. the role of external forcing and
natural variability as well as the respective contributions
from global (regional) ocean steric and mass variations are
still far from precise).
Figure 9.5b shows the spatial distribution of projected
changes in regional sea level in the world oceans, derived
from 21 CMIP models, for the RCP4.5 scenario (which is
mid-range emission scenario (RCP4.5) between the
high/low-end scenarios (RCPs 8.5 and 2.5). These sea-level
projections reveal a clear regional pattern in sea-level
changes, with complex ridge-and-trough patterns superimposed on a generally rising global mean sea level. For
instance, in the Indian Ocean, the highest changes (trends)
are seen in the north and western tropical Indian Ocean with
a mean sea-level change of about 0.25–0.3 m at the end of
the twenty-first century (Fig. 9.5b). It should be noted that
north and western tropical IO is one of the few oceanic
regions where maximum changes are predicted by CMIP
climate models (maximum steric and dynamic sea-level
rise). In a similar study, Carson et al. (2016) showed that
these projected changes in mean sea level for the twenty-first
century are considerably larger than the (projected) noise
(natural variability) everywhere in the Indian Ocean, suggesting that the projected changes in the Indian Ocean mean
sea level seen in Fig. 9.5b would have an anthropogenic
origin. The contribution of thermal expansion to the GMSL
rise by 2100 is estimated to be about 0.19 m for the RCP4.5
(Church et al. 2013a, b), and the slightly higher changes seen
regionally in the western tropical Indian Ocean could be
either related to additional changes from dynamic sea-level
rise or could be resulting from an excess projected warming
of this region. Note that, these projected changes (shown in
Fig. 9.5b) do not include a contribution from ice-melting
(mainly glaciers and ice sheets). Though, the dynamic
adjustment of the world oceans to additional mass input
from ice-melting is complex and occurs over decadal to
century time scales (e.g. Stammer 2008), our current
understandings indicate that the projected regional sea-level
rise from glaciers and ice sheets is comparable to the
changes from thermal expansion and circulation changes
(see Church et al. 2013a, b). This is true for the Indian Ocean
also, which means, combining the effects of steric changes
(shown in Fig. 9.5) with mass addition may be higher than
the values shown for projected Indian Ocean sea-level
changes, shown in Fig. 9.5b.
9.5 Extreme Sea-Level Changes in the Indian
Ocean
One of the main consequences of mean sea-level rise on
human settlements is an increase in flood risk due to an
increase in the intensity and frequency of extreme sea levels
(ESL). Coastal areas become threatened when high tides
coincide with extreme weather events and drive ESL (Wahl
et al. 2017). Extreme weather (climate extremes) contributes
to ESL through wind-waves and storm surges. Storm surge
is an episodic increase in sea level driven by shoreward
wind-driven water circulation and atmospheric pressure.
Wind-waves are generated when wind energy is transferred
to the ocean through surface friction and is transformed into
wave energy fluxes. When waves reach the coast, they
interact with the bathymetry and drive an additional increase
in water levels through wave set-up and run-up. ESLs are
exacerbated by tropical cyclones (TCs), which significantly
intensify wind-waves and storm surge (Peduzzi et al. 2012).
ESL can be defined as a combination of surges/waves,
tides and MSL, where MSL is the mean sea-level. Though
there are several statistical techniques to evaluate ESL, there
is no universally accepted standard or best approach for
broadscale impact and adaptation analysis (Arns et al. 2013).
The IPCC AR5 (Church et al. 2013a, b; Rhein 2014)
included a review of ESL reveal that the recent increase in
observed extremes worldwide has been caused primarily by
an increase in MSL, although the dominant modes of climate
variability (particularly the El Niño Southern Oscillation
(ENSO), Indian Ocean Dipole (IOD), North Atlantic Oscillation (NAO) and other modes) also have a measurable
influence on extremes in many regions. Since the IPCC
AR5, there have been a number of studies relating to ESL.
Wahl (2014) reported rapid changes in the seasonal cycle of
MSL along the Gulf Coast of the United States. The spatial
variability of ESL is found to be considerably lower than the
global mean trend (Vousdoukas et al. 2018). However,
recent studies have shown that global warming will induce
changes in storm surges and wind-waves, while cyclonic
activity may also be affected (Hemer et al. 2013; Woodruff
1
Future projections for the Indian Ocean are based on thermosteric
component (thermal expansion only).
184
P. Swapna et al.
changes requires a clear understanding of projected changes
in ocean wind-driven circulation, steric changes and terrestrial ice storages (Air-sea momentum and heat fluxes variations are associated with spatially varying sea-level changes.
For example, the excess surface latent heat flux associated
with global warming penetrates differentially into the ocean
depending on, for example, the mixed layer depth and circulation. Similarly, climate change induces changes in surface winds, which will further cause sea-level regional
changes). As far as the Indian Ocean is concerned, apart
from information provided in IPCC AR4/5, there is no
independent studies that provide regional sea-level projections
1 and its attribution (i.e. the role of external forcing and
natural variability as well as the respective contributions
from global (regional) ocean steric and mass variations are
still far from precise).
Figure 9.5b shows the spatial distribution of projected
changes in regional sea level in the world oceans, derived
from 21 CMIP models, for the RCP4.5 scenario (which is
mid-range emission scenario (RCP4.5) between the
high/low-end scenarios (RCPs 8.5 and 2.5). These sea-level
projections reveal a clear regional pattern in sea-level
changes, with complex ridge-and-trough patterns superimposed on a generally rising global mean sea level. For
instance, in the Indian Ocean, the highest changes (trends)
are seen in the north and western tropical Indian Ocean with
a mean sea-level change of about 0.25–0.3 m at the end of
the twenty-first century (Fig. 9.5b). It should be noted that
north and western tropical IO is one of the few oceanic
regions where maximum changes are predicted by CMIP
climate models (maximum steric and dynamic sea-level
rise). In a similar study, Carson et al. (2016) showed that
these projected changes in mean sea level for the twenty-first
century are considerably larger than the (projected) noise
(natural variability) everywhere in the Indian Ocean, suggesting that the projected changes in the Indian Ocean mean
sea level seen in Fig. 9.5b would have an anthropogenic
origin. The contribution of thermal expansion to the GMSL
rise by 2100 is estimated to be about 0.19 m for the RCP4.5
(Church et al. 2013a, b), and the slightly higher changes seen
regionally in the western tropical Indian Ocean could be
either related to additional changes from dynamic sea-level
rise or could be resulting from an excess projected warming
of this region. Note that, these projected changes (shown in
Fig. 9.5b) do not include a contribution from ice-melting
(mainly glaciers and ice sheets). Though, the dynamic
adjustment of the world oceans to additional mass input
from ice-melting is complex and occurs over decadal to
century time scales (e.g. Stammer 2008), our current
understandings indicate that the projected regional sea-level
rise from glaciers and ice sheets is comparable to the
changes from thermal expansion and circulation changes
(see Church et al. 2013a, b). This is true for the Indian Ocean
also, which means, combining the effects of steric changes
(shown in Fig. 9.5) with mass addition may be higher than
the values shown for projected Indian Ocean sea-level
changes, shown in Fig. 9.5b.
9.5 Extreme Sea-Level Changes in the Indian
Ocean
One of the main consequences of mean sea-level rise on
human settlements is an increase in flood risk due to an
increase in the intensity and frequency of extreme sea levels
(ESL). Coastal areas become threatened when high tides
coincide with extreme weather events and drive ESL (Wahl
et al. 2017). Extreme weather (climate extremes) contributes
to ESL through wind-waves and storm surges. Storm surge
is an episodic increase in sea level driven by shoreward
wind-driven water circulation and atmospheric pressure.
Wind-waves are generated when wind energy is transferred
to the ocean through surface friction and is transformed into
wave energy fluxes. When waves reach the coast, they
interact with the bathymetry and drive an additional increase
in water levels through wave set-up and run-up. ESLs are
exacerbated by tropical cyclones (TCs), which significantly
intensify wind-waves and storm surge (Peduzzi et al. 2012).
ESL can be defined as a combination of surges/waves,
tides and MSL, where MSL is the mean sea-level. Though
there are several statistical techniques to evaluate ESL, there
is no universally accepted standard or best approach for
broadscale impact and adaptation analysis (Arns et al. 2013).
The IPCC AR5 (Church et al. 2013a, b; Rhein 2014)
included a review of ESL reveal that the recent increase in
observed extremes worldwide has been caused primarily by
an increase in MSL, although the dominant modes of climate
variability (particularly the El Niño Southern Oscillation
(ENSO), Indian Ocean Dipole (IOD), North Atlantic Oscillation (NAO) and other modes) also have a measurable
influence on extremes in many regions. Since the IPCC
AR5, there have been a number of studies relating to ESL.
Wahl (2014) reported rapid changes in the seasonal cycle of
MSL along the Gulf Coast of the United States. The spatial
variability of ESL is found to be considerably lower than the
global mean trend (Vousdoukas et al. 2018). However,
recent studies have shown that global warming will induce
changes in storm surges and wind-waves, while cyclonic
activity may also be affected (Hemer et al. 2013; Woodruff
1
Future projections for the Indian Ocean are based on thermosteric
component (thermal expansion only).
184
P. Swapna et al.