Projection of future sea level and its variability in a high resolution climate model: Ocean process

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Projection of future sea level and its
variability in a high resolution climate model
Ocean processes and Greenland and Antarctic ice
melt contributions.
UJCC Workshop Nov. 24 2005

• Tatsuo SUZUKI, Hiroyasu Hasumi, Takashi T.
  Sakamoto, Teruyuki Nishimura, Ayako Abe-Ouchi,
  Tomonori Segawa, Naosuke Okada, and Seita Emori

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Introduction

• Sea level change is an important aspect of future
  climate change for human societies and the
  environment.
• Globally averaged sea level rise during 21st
  century is 20cm-37cm (thermal expansion).
• The regional sea level change was not specially
  uniform.

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• The distribution of sea level change is
  principally determined by the ocean density
  structure change (Church et al., 2001).
• The detailed ocean structures, such as density
  fronts and western boundary currents, have not
  been reproduced by the previous coarse-resolution
  CGCMs.
• We compared sea level projections made by CGCMs
  with differing resolutions, focusing on detailed
  features of the distribution of sea level changes
  as predicted by a high resolution model.
• We also checked the impact of high-resolution
  model on the global averaged sea level rise.

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Model for Interdisciplinary Research on Climate
(MIROC) version 3.2

• MIROC_hi
• Atmosphere T106, 56 vertical levels
• Ocean 0.28º0.19º, 48 vertical levels
• MIROC_med
• Atmosphere T42, 20 vertical levels
• Ocean 1.4º0.56º(near equator), 44 vertical
  levels

In both model, the same physics have been used.
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Experiments

• Spin-up
• 109-years integration forced by fixed external
  condition for 1900 for MIROC_hi
• 560-years integration forced by fixed external
  condition for 1850 for MIROC_med
• Control-run
• 100-years integration for MIROC_hi
• 400-years integration for MIROC_med
• 20C3M-run
• Forced by historical data during the 20th century
• A1B-run
• IPCC SRES A1B scenario
• B1-run
• IPCC SRES B1 scenario

Some trends are shown in control runs. So, we
subtract these control trends from 20C3M run and
scenario runs.
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1. Steric contribution (thermal expansion and
haline contraction) to Globally averaged sea
level rise

• estimated indirectly from density changes as the
  equivalent volume change under mass conservation
  as the Boussinesq approximation was adapted in
  the ocean model
• ?H globally averaged sea level rise,
• S surface area of the ocean,
• Z ocean depth, ? in situ density,
• ?? difference from the reference state

2. Contributions of ice-sheet melt estimated
using the methods of Wild et al. 2003
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Globally averaged sea level rise

• The steric contribution during 21st century in
  MIROC_hi is similar to that in MIROC_med.
• The contributions of the Greenland and Antarctic
  ice-sheet exhibited opposite tendencies in both
  models, as in previous estimations Church et
  al., 2001
• The amplitude of ice-sheet melt in MIROC_hi was
  larger than that in MIROC_med

Different sensitivity
A1B run induced global warming of about 4.0?C in
MIROC_hi and 3.4?C in MIROC_med at the end of the
21st century, respectively.
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Why steric contribution in MIROC_hi is similar to
that in MIROC_med in spite of the different
sensitivity?

• Total heat flux into the ocean for the 21st
  century is similar in the both models
• The upper ocean in MIROC_hi warms up more
  than that in MIROC_med (different heat uptake?).

The reasons for these differences are current
problems.
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Distribution of sea level change during 21st
centuryA1B(2081-2100)-20C3M(1981-2000)

• The distribution of Sea level changes in MIROC_hi
  also resembled that in MIROC_med on a large
  scale.
• MIROC_hi presented more detailed ocean structure
  changes under global warming.

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Pacific Ocean
There was a reduced sea level rise north of the
Kuroshio at approximately 150E and an enhanced
sea level rise to the south in MIROC_hi. There
was a reduced sea level rise east of Mindanao
Island that spread to the eastern tropical
Pacific in MIROC_hi.
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Pacific Ocean
Sea level change around the Kuroshio extension is
associated with Kuroshio acceleration caused by
changes in wind stress and the consequential
spin-up of the Kuroshio recirculation (Sakamoto
et al., 2005). Sea level change east of the
Mindanao Island also caused by the changes in
wind forcing.
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Atlantic Ocean
4.5Sv
9.5Sv
Enhanced sea level rise northwest of the North
Atlantic Current (NAC) This sea level change
cannot be explained by changes in wind
forcing. AMOC 14Sv?9Sv (-5Sv) NAC -9.5Sv
Labrador Current -4.5Sv (upper 1000m depth) This
pattern consistent with weakening of the upper
branch of the AMOC (Bryan 1996).
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Southern Ocean
11Sv
Under global warming, the westerly wind is
strengthened. These changes in the wind field
contribute the intensification of the circumpolar
front and acceleration of ACC, which is
associated with the sea level change.
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Sea level variability
RMS of sea level variability during 20th century
20C3M(1981-2000)
Changes in RMS of the sea level variability
during 21st century These changes are caused by
the changes in ocean current speed associated
with the local sea level change.
A1B(2081-2100)-20C3M(1981-2000)
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Summary

• The globally averaged sea level rise during the
  21st century predicted by the two models was
  similar.
• The distribution of sea level changes in MIROC_hi
  also resembled that in MIROC_med on a large
  scale.
• MIROC_hi presented more detailed ocean structure
  changes under global warming.
• The changes in the ocean structure affected not
  only the spatial distribution of sea level rise,
  but also changes in local sea level variability.

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Problems

• Why does the upper ocean in MIROC_hi warms up
  more than that in MIROC_med (different heat
  uptake)?
• The distribution of the sea level change is
  strongly influenced by wind forcing. However, the
  common future is not established in climate model.

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Rossbys deformation radius for the first
baroclinic mode (based on WOA)
Model grid size 1/4o(zonally)
1/6o(meridionally) dx15.4km (at 60o) dy20 km
Km
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Heat flux into ocean
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Sea temperature changes during 21st century (A1B
run) and the trend in control run
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Lat-depth cross-section along date line
GM
No-GM
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Distribution of SSH averaged between 1981-2000

• The distribution of SSH in MIROC_hi is similar to
  that in MIROC_med on a large scale.
• MIROC_hi represents more detailed ocean structure.

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Pacific Ocean
The latitude of the Kuroshio separation (LKS) is
well represented in MIROC_hi. However, the LKS
overshoots to the north in MIROC_med. The
Mindanao dome is also represented in MIROC_hi.
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Atlantic Ocean