Climate Dynamics

, Volume 27, Issue 6, pp 593–611 | Cite as

On the climate response of the low-latitude Pacific Ocean to changes in the global freshwater cycle

  • P. D. Williams
  • E. Guilyardi
  • R. T. Sutton
  • J. M. Gregory
  • G. Madec


Under global warming, the predicted intensification of the global freshwater cycle will modify the net freshwater flux at the ocean surface. Since the freshwater flux maintains ocean salinity structures, changes to the density-driven ocean circulation are likely. A modified ocean circulation could further alter the climate, potentially allowing rapid changes, as seen in the past. The relevant feedback mechanisms and timescales are poorly understood in detail, however, especially at low latitudes where the effects of salinity are relatively subtle. In an attempt to resolve some of these outstanding issues, we present an investigation of the climate response of the low-latitude Pacific region to changes in freshwater forcing. Initiated from the present-day thermohaline structure, a control run of a coupled ocean–atmosphere general circulation model is compared with a perturbation run in which the net freshwater flux is prescribed to be zero over the ocean. Such an extreme experiment helps to elucidate the general adjustment mechanisms and their timescales. The atmospheric greenhouse gas concentrations are held constant, and we restrict our attention to the adjustment of the upper 1,000 m of the Pacific Ocean between 40°N and 40°S, over 100 years. In the perturbation run, changes to the surface buoyancy, near-surface vertical mixing and mixed-layer depth are established within 1 year. Subsequently, relative to the control run, the surface of the low-latitude Pacific Ocean in the perturbation run warms by an average of 0.6°C, and the interior cools by up to 1.1°C, after a few decades. This vertical re-arrangement of the ocean heat content is shown to be achieved by a gradual shutdown of the heat flux due to isopycnal (i.e. along surfaces of constant density) mixing, the vertical component of which is downwards at low latitudes. This heat transfer depends crucially upon the existence of density-compensating temperature and salinity gradients on isopycnal surfaces. The timescale of the thermal changes in the perturbation run is therefore set by the timescale for the decay of isopycnal salinity gradients in response to the eliminated freshwater forcing, which we demonstrate to be around 10–20 years. Such isopycnal heat flux changes may play a role in the response of the low-latitude climate to a future accelerated freshwater cycle. Specifically, the mechanism appears to represent a weak negative sea surface temperature feedback, which we speculate might partially shield from view the anthropogenically-forced global warming signal at low latitudes. Furthermore, since the surface freshwater flux is shown to play a role in determining the ocean’s thermal structure, it follows that evaporation and/or precipitation biases in general circulation models are likely to cause sea surface temperature biases.


Subtropical Gyre Freshwater Flux Isopycnal Surface North Pacific Intermediate Water Salinity Structure 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.



Discussions with Pascale Braconnot, Didier Swingedouw, Jean-Claude Dutay, Malcolm Roberts and Thomas Haine were very helpful. Technical model assistance from Daniel Bernie and Silvio Gualdi is gratefully acknowledged. Calculations were performed on the NEC SX-6 at the Deutsches Klimarechenzentrum (DKRZ). We acknowledge funding from the Rapid Climate Change thematic programme of the UK Natural Environment Research Council (award reference: NER/T/S/2002/00442).


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Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  • P. D. Williams
    • 1
  • E. Guilyardi
    • 1
    • 2
  • R. T. Sutton
    • 1
  • J. M. Gregory
    • 1
    • 3
  • G. Madec
    • 4
  1. 1.Centre for Global Atmospheric Modelling, Department of MeteorologyUniversity of ReadingReadingUK
  2. 2.Laboratoire des Sciences du Climat et de l’Environnement (IPSL/LSCE)Gif-sur-YvetteFrance
  3. 3.Hadley Centre for Climate Prediction and ResearchExeterUK
  4. 4.Laboratoire d’Océanographie et de Climat par Expérimentation et Approche Numérique (IPSL/LOCEAN)Université Paris VIParisFrance

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