Geoscience Reference
In-Depth Information
Oceans, tectonics and climate
SYSTEMS
Ocean-atmosphere-ice sheet coupling is a principal focus of contemporary research and modelling in AOGCMs. Earth's
atmosphere-ocean dynamics are chaotic and sensitive to small disturbances, coupled primarily by heat and freshwater
fluxes, strongly tied to sea surface temperatures (SSTs) and radiatively active trace gases. Seasonal changes take
place mainly in the surface layer, but they are coupled with deep-water
thermohaline circulation
(see below) at several
important locations. This is controlled by the combined effects of heat and freshwater fluxes called the surface
buoyancy fluxwhich is disturbed by changes in precipitation and evaporation, salinity, continental run-off, sea and land
ice changes and the movement of water through narrow straits and over shallow sills in the sea bed.
Some changes are seasonal. For example, surface water may retain buoyancy despite cooling as it moves polewards
through freshwater inflow from humid continents. This may be reversed in winter, when continents retain fresh water
as snow and ice. Brine rejectionor salt expression, as winter sea ice forms in polar regions, further increases salinity
and therefore density. These effects are reversed in spring and summer, and the ocean may exist in quasi-equilibrium.
Less bouyant water is more likely to subside and vice versa, stimulating mixing. In this way, ocean surface circulation
is coupled with deep water through overturning. Other buoyancy changes may not be cyclical, and global warming
creates feedbacks capable of destabilizing ocean circulation, perhaps irreversibly (see box, p. 242).
More general effects of oceans on global climate were explained in earlier chapters. In summary, oceans source
about 86 per cent of water cycled through the atmosphere-lithosphere, and poleward components of ocean
circulation account for 20-40 per cent of global heat transfer. High ocean thermal capacity moderates terrestrial
temperature extremes, influencing the time scales of climatic change. The global extreme ocean surface range is
37ยท5C between waters in the Persian Gulf and Arctic Ocean, compared with terrestrial extreme ranges of 140C
between the Antarctic ice sheet and Sahara desert. Annual ranges within oceans are
f
2C in the tropics, rising to
4
o
C and 8
o
C in polar and mid-latitude waters respectively. Ocean currents produce strong regional temperature
anomalies, as a comparison of sea surface temperatures in both major oceans shows (
Figure 11.8
).
Tectonic processes were not incorporated directly into earlier AOGCMs but their role is now implicit in modelling
ocean, sea ice, land surface and atmospheric chemistry components and is factored into 3-D GCMs. We are also
able to model past climates for geologically reconstructed former land/sea distributions (not always successfully!).
Shuffling continents between polar-equatorial and clustered-dispersed extremes clearly impacts climate by
transforming ocean geometry, and thereby global patterns of energy and moisture exchanges, but tectonic impacts
go further. Modern ocean gyres, moving surface water around the Indian, north and south Atlantic and Pacific Oceans,
developed only after they had widened beyond c. 1,500 km in diameter. The interconnectivity, or otherwise, of ocean
basins affects ocean circulation systems and water chemistry. The previous box explained how changing sea-floor
spreading rates drive eustatic sea-level change; by the same token, they alter land-sea area, and water depth in
important marine gateways between oceans.
Opening and closure of marine gateways depends on plate motion, and four such mid to late Cenozoic events were
probably instrumental in later Cenozoic climate cooling and precursors of the Quaternary Ice Age. First, the equatorial
Tethys Ocean, formed as Pangaea rifted apart and the central Atlantic opened, closed between Africa and Eurasia
c. 37 Ma. As one gateway closed, others opened at almost the same time around the Atlantic entrance to the Arctic
Ocean. Next Drake Passage, separating South America and the Antarctic peninsula, opened after c. 20 Ma and, at
the other end of South America, the Panamanian gateway between the Americas closed by 3.5 Ma. Collectively,
these changes radically altered Earth's wind-driven and thermohaline ocean circulation; imagine a round-the-world
yacht race able to sail around an equatorial ocean, avoiding the dangerous seas, capes and icebergs of the southern
ocean! The equatorial ocean was replaced by the circumpolar southern ocean, and east-west Atlantic-Pacific zonal
exchanges were replaced by meridional north-south circulations in both oceans. Their implications for water mass,
heat and salt transfers between oceans, and their interconnectivity via the sea bed, are the very essence of global
ocean-tectonic-climate coupling.
