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Transient and long-term behaviour of the world ocean under global warming

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Bi, Daohua (Dave) (2002) Transient and long-term behaviour of the world ocean under global warming. PhD thesis, University of Tasmania.

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Abstract

The thermohaline circulation (THC) of the oceans plays a crucial role in adjusting
the global thermal and hydrological budget in the climate system. Knowledge
about its stability and change is very important for understanding the evolution of
past climates and assessing possible climate changes in the future. In this study,
we investigate the transient and long-term behaviour of the THC, particularly the
Southern Ocean overturning in the CSIRO climate model, under increasing atmospheric
greenhouse gases (as equivalent CO2 , referred to simply as CO2) following
the IPCC/IS92a scenario to stabilisation at three times preindustrial CO2 (3x CO2)
then continuing at that level of stabilisation.
Firstly the CSIRO ocean model is further developed by modifying the surface boundary
forcing, for the purpose of ensuring a stable and realistic ocean climate to be
used as the initial condition of the ocean for coupled model climate change studies.
The new formulation leads to a significantly improved spinup solution and coupled
control climate of the ocean. The world ocean water mass properties, in particular
the Southern Ocean stratification, the THC, and the Antarctic Circumpolar Current
(ACC) are all in broad agreement with observations.
In the following global warming experiment, pronounced changes of the global THC
occur during the CO2 increase period: the North Atlantic Deep Water Formation
(NADWF) weakens rapidly and the Antarctic Bottom Water Formation (AABWF)
collapses completely before CO2 triples. During the subsequent period of 1100 years
with stabilised 3x CO2, the NADWF intensity shows a tendency to recover gradually
in the upper part of the ocean but the AABWF shows no sign of returning
and the residual deep overturning dies away. Also evident is the change of ACC
transport under CO2 forcing: it increases along with the CO2 increase and keeps
increasing steadily for a few centuries after CO2 tripling. While both the surface
freshening and heating from above are responsible for the weakening of NADWF,
the surface freshening around Antarctica, including the enhancement of precipitation
over evaporation (P - E), runoff from continents and reduction in the sea ice
formation and outflux, suppresses the deep convection off Antarctica and causes the
shutdown of AABWF. The strengthening of the ACC transport is attributable to
the enhanced meridional density contrast across the ACC due to the uneven warming
in the Southern Ocean, both at the surface and in the interior. This change
in density structure leads to an acceleration in the upper layer currents which outweighs
the deceleration in the mid-depth layer caused by the weakening and shutoff
of the AABWF.
Using the Bryan (1984) technique to accelerate the convergence of the deep ocean
towards equilibrium under the 3x CO2 condition, it is found that the global THC
eventually reaches a near-stable state in which the NADWF is fully recovered, the
AABWF is also partly re-established and deep ocean ventilation is activated again.
The recovery of the THC is attributed to the slow but persistent warming in the deep
ocean which gradually destabilizes the water column. After thousands of years, a
stratification structure close to the initial state becomes re-built in the high latitude
Southern Ocean, which allows deep convection and hence overturning off Antarctica
to occur, bringing the system into a new regime. However, this regime needs some
more time to further adjust and settle down to a more stable and slightly different
normal mode solution. This is verified by an extension to the accelerated run for
500 years with the acceleration switched off. This result shows the development
of a possible new quasi-equilibrium for the ocean under long-term global warming
induced by the anthropogenic CO2 increase.
Comparison has been made with the results from an earlier version of the coupled
model which has a clearly different initial climate for the ocean. We conclude that,
for the CSIRO coupled model (mark2), the oceanic response to global warming is
not strongly dependent on the basic state of the ocean. However, some differences in
the oceanic behaviour under CO2 forcing between the two versions are of scientific
interest and have been discussed in detail.

Item Type: Thesis (PhD)
Keywords: Ocean-atmosphere interaction, Ocean-atmosphere interaction, Ocean circulation, Global warming, Climatic changes
Copyright Holders: The Author
Copyright Information:

Copyright 2002 the Author

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2002. Includes bibliographical references

Date Deposited: 25 Nov 2014 00:51
Last Modified: 04 May 2016 00:30
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