Vertical heat transports in the ocean and their effect on time-dependent climate change |
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| Authors: | J M Gregory |
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| Institution: | (1) Hadley Centre for Climate Prediction and Research, Meteorological Office, London Road, Bracknell, Berks. RG12 2SY, United Kingdom E-mail: jmgregory@meto.gov.uk, GB |
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| Abstract: | In response to increasing atmospheric concentrations of greenhouse gases, the rate of time-dependent climate change is determined
jointly by the strength of climate feedbacks and the efficiency of processes which remove heat from the surface into the deep
ocean. This work examines the vertical heat transport processes in the ocean of the HADCM2 atmosphere–ocean general circulation
model (AOGCM) in experiments with CO2 held constant (control) and increasing at 1 per year (anomaly). The control experiment shows that global average heat exchanges
between the upper and lower ocean are dominated by the Southern Ocean, where heat is pumped downwards by the wind-driven circulation
and diffuses upwards along sloping isopycnals. This is the reverse of the low-latitude balance used in upwelling–diffusion
ocean models, the global average upward diffusive transport being against the temperature gradient. In the anomaly experiment,
weakened convection at high latitudes leads to reduced diffusive and convective heat loss from the deep ocean, and hence to
net heat uptake, since the advective heat input is less affected. Reduction of deep water production at high latitudes results
in reduced upwelling of cold water at low latitudes, giving a further contribution to net heat uptake. On the global average,
high-latitude processes thus have a controlling influence. The important role of diffusion highlights the need to ensure that
the schemes employed in AOGCMs give an accurate representation of the relevant sub-grid-scale processes.
Received: 8 July 1999 / Accepted: 17 November 1999 |
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