towards the sea-level variability in the Indian Ocean and
account for around 30% of the total interannual variability
(Deepa et al. 2018a, b).
Decadal sea-level variability is driven primarily by the
variations in the surface wind forcing over the Indo-Pacific
Ocean (Lee and McPhaden 2008; Nidheesh et al. 2013; Han
et al. 2017). The decadal sea-level variability of the
south-western tropical Indian Ocean region known as the
thermocline ridge region of the Indian Ocean or open ocean
upwelling region (Vialard et al. 2009) is found to be associated with decadal fluctuations of surface wind stress (Li
and Han 2015; Deepa et al. 2018a, b). Decreasing sea-level
trends were noted in this region from the 1960s and are
driven by the changes in the surface winds associated with
combined changes in the Indian Ocean Hadley and Walker
cells, which is partly attributable to the rising levels of
atmospheric greenhouse gases (Han et al. 2010). In the
southern tropical Indian Ocean region, decadal ENSO contribution dominates and the Pacific influence via Indonesian
Throughflow (ITF; known as the oceanic bridge between the
Indian and Pacific Oceans) mainly accounts for sea-level
variability in the south-east Indian Ocean region (Han et al.
2018; Deepa et al. 2018a, b). However, Nidheesh et al.
(2017) have noted that the representation of Indian Ocean
decadal sea-level variability in observation-based sea-level
products (reanalyses and reconstructions) is not consistent
across the products due to poor observational sampling of
this basin as compared to other tropical oceans. Hence, the
salient features of Indian Ocean decadal sea-level variability,
briefly summarized above from various studies that used any
single of those sea-level products or OGCMs, need to be
considered with caution.
Multi-decadal sea-level variability in the Indian Ocean is
dominated by the thermosteric changes forced by changes in
the winds. For example, Swapna et al. (2017) have shown
that the multi-decadal rise in north Indian Ocean sea level is
caused by the weakened summer monsoon circulation,
which reduces the southward ocean heat transport and results
in an increased heat storage and thermosteric sea-level rise of
about 3.3 mm year
−1 during 1993–2015 in the north Indian
Ocean (Swapna et al. 2017).
9.4 Future Mean Sea-Level Change
9.4.1 Global
Sea level has been rising over the past century, and the rate
has accelerated in recent decades. The onset of modern
sea-level rise coincided with increasing global temperature
(e.g. Kemp et al. 2011a, b); sea-level rise over the coming
centuries is perhaps the most damaging side of rising temperature. The Intergovernmental Panel on Climate Change’s
(IPCC) Fifth Assessment Report (AR5) provided an
assessment of projected global sea-level rise till the end of
the twenty-first century (up to 2100) forced by different
emission scenarios (Taylor et al. 2012). Projected sea-level
rise under each scenario is the sum of individual contributions from steric changes and melting of glaciers and ice
caps, the Greenland ice sheet, the Antarctic ice sheet and
contribution from land water storage. These projections are
derived from the co-ordinated modelling activities under the
Coupled Model Intercomparison Project (CMIP) of the
World Climate Research Programme (WCRP). CMIP5
provides projections of future climate on two time scales,
near term (up to about 2035) and long term (up to 2100 and
beyond). The near-term simulations (simulation over 10–
30 years) are initialized with observed ocean state and sea
ice, and the long-term simulations are initialized from the
end of freely evolving simulations of the historical period
(carried out by atmosphere-ocean global climate models—
AOGCMs or Earth system models). Climate projections in
CMIP models are carried out with specified concentrations
of atmospheric greenhouse gases (known as ‘Representative
Concentration Pathways’—RCPs). The sea-level projections
described here are based on a ‘mitigation scenario’ (RCP4.5,
in which the anthropogenic emission leading to radiative
forcing is limited to 4.5 Wm
−2 in the year 2100) which is a
mid-range scenario between a higher (RCP8.5) and lower
(RCP2.5) scenarios (see Moss et al. 2010). Although the
performance of CMIP5 models in simulating the sea level
has substantially increased compared to previous versions,
these models still do not account for the net ocean mass
changes induced by melting ice sheets and glaciers (Flato
et al. 2013). Consequently, it is not possible to evaluate the
‘total’ sea-level rise directly from CMIP sea-level simulations. However, the dynamic sea-level changes are given
directly, and the methods by which the mass contributions
are estimated (also the method of uncertainty calculation) are
given in Church et al. 2013a, b (refer to their Supplementary
material).
The observed global mean sea-level rise about 1–2 mm
year
−1 for 1900–2000 as inferred from tide gauges (see
Fig. 9.5a) is within the range of hindcasts by CMIP models
over the historical period (1870–2005; Church et al. 2013a,
b), giving confidence in future projections from those
models. Figure 9.5 provides the central estimates and likely
ranges of projected evolution of GMSL for the twenty-first
century for two emission scenarios (RCP8.5 corresponding
to high emission and RCP2.6 corresponding to a very low
emission scenario). Combining paleo data with historical
tide gauge data confirms that the rate of sea-level rise has
increased from a low rate of change during the pre-industrial
period (of order tenths of mm year
−1 ) to rates of about
2 mm year
−1 over the twentieth century, with a likely continuing acceleration during the twenty-first century
182
P. Swapna et al.
account for around 30% of the total interannual variability
(Deepa et al. 2018a, b).
Decadal sea-level variability is driven primarily by the
variations in the surface wind forcing over the Indo-Pacific
Ocean (Lee and McPhaden 2008; Nidheesh et al. 2013; Han
et al. 2017). The decadal sea-level variability of the
south-western tropical Indian Ocean region known as the
thermocline ridge region of the Indian Ocean or open ocean
upwelling region (Vialard et al. 2009) is found to be associated with decadal fluctuations of surface wind stress (Li
and Han 2015; Deepa et al. 2018a, b). Decreasing sea-level
trends were noted in this region from the 1960s and are
driven by the changes in the surface winds associated with
combined changes in the Indian Ocean Hadley and Walker
cells, which is partly attributable to the rising levels of
atmospheric greenhouse gases (Han et al. 2010). In the
southern tropical Indian Ocean region, decadal ENSO contribution dominates and the Pacific influence via Indonesian
Throughflow (ITF; known as the oceanic bridge between the
Indian and Pacific Oceans) mainly accounts for sea-level
variability in the south-east Indian Ocean region (Han et al.
2018; Deepa et al. 2018a, b). However, Nidheesh et al.
(2017) have noted that the representation of Indian Ocean
decadal sea-level variability in observation-based sea-level
products (reanalyses and reconstructions) is not consistent
across the products due to poor observational sampling of
this basin as compared to other tropical oceans. Hence, the
salient features of Indian Ocean decadal sea-level variability,
briefly summarized above from various studies that used any
single of those sea-level products or OGCMs, need to be
considered with caution.
Multi-decadal sea-level variability in the Indian Ocean is
dominated by the thermosteric changes forced by changes in
the winds. For example, Swapna et al. (2017) have shown
that the multi-decadal rise in north Indian Ocean sea level is
caused by the weakened summer monsoon circulation,
which reduces the southward ocean heat transport and results
in an increased heat storage and thermosteric sea-level rise of
about 3.3 mm year
−1 during 1993–2015 in the north Indian
Ocean (Swapna et al. 2017).
9.4 Future Mean Sea-Level Change
9.4.1 Global
Sea level has been rising over the past century, and the rate
has accelerated in recent decades. The onset of modern
sea-level rise coincided with increasing global temperature
(e.g. Kemp et al. 2011a, b); sea-level rise over the coming
centuries is perhaps the most damaging side of rising temperature. The Intergovernmental Panel on Climate Change’s
(IPCC) Fifth Assessment Report (AR5) provided an
assessment of projected global sea-level rise till the end of
the twenty-first century (up to 2100) forced by different
emission scenarios (Taylor et al. 2012). Projected sea-level
rise under each scenario is the sum of individual contributions from steric changes and melting of glaciers and ice
caps, the Greenland ice sheet, the Antarctic ice sheet and
contribution from land water storage. These projections are
derived from the co-ordinated modelling activities under the
Coupled Model Intercomparison Project (CMIP) of the
World Climate Research Programme (WCRP). CMIP5
provides projections of future climate on two time scales,
near term (up to about 2035) and long term (up to 2100 and
beyond). The near-term simulations (simulation over 10–
30 years) are initialized with observed ocean state and sea
ice, and the long-term simulations are initialized from the
end of freely evolving simulations of the historical period
(carried out by atmosphere-ocean global climate models—
AOGCMs or Earth system models). Climate projections in
CMIP models are carried out with specified concentrations
of atmospheric greenhouse gases (known as ‘Representative
Concentration Pathways’—RCPs). The sea-level projections
described here are based on a ‘mitigation scenario’ (RCP4.5,
in which the anthropogenic emission leading to radiative
forcing is limited to 4.5 Wm
−2 in the year 2100) which is a
mid-range scenario between a higher (RCP8.5) and lower
(RCP2.5) scenarios (see Moss et al. 2010). Although the
performance of CMIP5 models in simulating the sea level
has substantially increased compared to previous versions,
these models still do not account for the net ocean mass
changes induced by melting ice sheets and glaciers (Flato
et al. 2013). Consequently, it is not possible to evaluate the
‘total’ sea-level rise directly from CMIP sea-level simulations. However, the dynamic sea-level changes are given
directly, and the methods by which the mass contributions
are estimated (also the method of uncertainty calculation) are
given in Church et al. 2013a, b (refer to their Supplementary
material).
The observed global mean sea-level rise about 1–2 mm
year
−1 for 1900–2000 as inferred from tide gauges (see
Fig. 9.5a) is within the range of hindcasts by CMIP models
over the historical period (1870–2005; Church et al. 2013a,
b), giving confidence in future projections from those
models. Figure 9.5 provides the central estimates and likely
ranges of projected evolution of GMSL for the twenty-first
century for two emission scenarios (RCP8.5 corresponding
to high emission and RCP2.6 corresponding to a very low
emission scenario). Combining paleo data with historical
tide gauge data confirms that the rate of sea-level rise has
increased from a low rate of change during the pre-industrial
period (of order tenths of mm year
−1 ) to rates of about
2 mm year
−1 over the twentieth century, with a likely continuing acceleration during the twenty-first century
182
P. Swapna et al.