Journal
CLIMATE DYNAMICS
Volume 32, Issue 2-3, Pages 205-218Publisher
SPRINGER
DOI: 10.1007/s00382-008-0466-7
Keywords
Greenhouse warming; Climate change; Climate model uncertainty; Atlantic meridional mode; Sub-tropical anticyclones; SST dipole mode
Categories
Funding
- Joint Defra and MoD Programme (Defra)
- Program for Climate Model Diagnosis and Intercomparison (PCMDI)
- WCRP's Working Group on Coupled Modelling (WGCM)
- Office of Science, US Department of Energy
- [GA01101]
- [CBC/2B/0417_ Annex C5]
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Through its control on the marine ITCZ, future changes in the tropical Atlantic meridional sea-surface temperature gradient (TAG) could have important impacts, on regional to global scales. We study the inter-model spread of projected TAG trends in response to increasing CO2, using results from 19 coupled GCMs which took part in the IPCC fourth assessment. Some models project substantial changes, with the smallest changes being in boreal autumn. There is substantial uncertainty, with no consistency even in the sign of change, and an ensemble mean close to zero. However, a strong statistical relationship is found between the simulated magnitudes of TAG trends and unforced TAG variability. Models with larger unforced variability in December-February show larger magnitude trends. We speculate that this relationship may be due to an underlying system of feedbacks whose strength varies considerably from model to model (the unforced variability ranges by a factor of 3 amongst these models, and the models exhibit large differences in mean state). We present evidence from further analysis and the literature to suggest which physical mechanisms may be involved. In particular, models projecting larger(smaller) magnitude TAG trends have larger(smaller) SST variability and cooler(warmer) mean SST in not just the Atlantic, but all three tropical/sub-tropical oceans, especially in the southern hemisphere near eastern coasts. These results could assist efforts to understand model errors in present and future tropical climate, and to develop observational constraints on future tropical projections.
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