SIMULATING PAST WARM CLIMATES
Sensitivity studies and paleoclimate simulations provide valuable tests for current theories, and the state-of-the-art models used to simulate the response of Earth's climate to elevated CO2 levels. Uncertainties in the forecasts provided by climate models are largely associated with the parameterization of sub-grid scale phenomena such as atmospheric convection in the tropics or processes in the deep ocean. One way to constrain these uncertainties is to reproduce and study climatic conditions of the past as suggested by paleodata.
The subsiding flow of the atmospheric Hadley circulation controls dry conditions over vast subtropical bands where the main arid regions of the globe reside. In the context of future changes in the atmospheric hydrological cycle, understanding precipitation changes in the subtropics is of particular importance given the scarcity of water resources in these locations. A major puzzle arises when contrasting the drier conditions in the subtropics predicted by climate models under global warming scenarios against the wetter conditions seen in reconstructions of past warm climates, including the warm, ∼400 ppm CO2, Pliocene.
Modeling results from Burls and Fedorov (2017) address this puzzle and highlight the importance of correctly predicting how the meridional temperature gradient between the tropics and the subtropics will change with global warming. The figure on the left from Burls and Fedorov (2017) illustrates our finding that the weaker meridional sea surface temperature gradients during the Pliocene supported weaker Hadley circulation to the point that the dynamic decrease in moisture divergence from the subtropics more than compensates for the thermodynamic increase to the point that the subtropics become wetter.
The hydrological cycle responce to Pliocene large-scale sea surface temperature forcing has interesting implications for the density driven meridional overturning circulation in the Ocean. Today, relatively fresh surface waters in the subpolar North Pacific prevent deep water formation, however under Pliocene conditions the hydrological cycle changes support much saltier surface water in the region allowing cool winter conditions to give rise to deep convection. The schematic on the right illustrates this result as presented in Burls et al. (2017)
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Burls, N.J., Fedorov, A.V., Sigman, D.M., Jaccard, S.L., Tiedemann, R. and Haug, G.H., 2017: Active Pacific meridional overturning circulation (PMOC) during the warm Pliocene, Science Advances, 3, e1700156.
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Burls, N. J., and A. V. Fedorov, 2014a: What controls the mean east-west sea surface temperature gradient in the equatorial Pacific: the role of cloud albedo, Journal of Climate, 27 (7), 2757-2778, http://dx.doi.org/10.1175/JCLI-D-13-00255.1.
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