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Ocean Heat Uptake & THC

PI: Robin Tokmakian
Institution: Naval Postgraduate School
Abstract:
Climate modeling studies (Stouffer et al. 2006) have shown that there is a broad range of responses in the thermohaline circulation patterns due to both heat and freshwater changes on decadal scales. Gregory et al. (2005) suggests that surface heat fluxes are more responsible for the weakening of the thermohaline circulation than freshwater fluxes in a given set of climate models. Understanding the predictability of the ocean and related changes to climate is tightly coupled to our understanding of the uncertainties associated with the processes in the atmosphere and the ocean of which ocean mixing and heat uptake are significant. One metric, used in understanding the fundamental behavior of a climate model, is called the "effective climate sensitivity" and is defined as the equilibrium warming that would be expected based on model behavior at the time of atmospheric carbon dioxide doubling. Tightly related to this quantity is a metric that is used to understand a climate model~s ability to take up heat, the "ocean heat uptake efficiency" or ?, (Gregory and Mitchell 1997, Raper et al. 2002). As these papers describe the relationship between the atmosphere and the ocean, the near surface air temperature change (??a?) relates to the net heat flux such that ?Q + ???a = ?F, where ?Q is the net heat flux, ? is the percentage of heat contributing to the atmospheric warming and ?F is the flux into the ocean. ?F can further be defined as d?H/dt. ?H is the heat content change of the ocean and is equal to ???a, where ? is a coefficient related to the percentage of heat flux being added to the ocean the "ocean heat uptake efficiency" coefficient. Thus ?Q = (? ? ?)??a. The key objective of this proposal is to quantify ? using observational data and compare it to similar quantities computed from AOGCMs to understand how its variability and spatial distribution influences the thermohaline circulation.

While for climate studies the efficiency rate is used to understand the long term changes of the ocean and atmosphere that are associated with CO2 doubling; it could also be used as a measure to quantify the realism associated with heat uptake during 20th century runs of the current generation of climate models. This research will be to calculate quantities of ocean heat uptake efficiency from a set of the current generation IPCC AR4 models and simulations and evaluate their realism and the associated uncertainty in the uptake rates and compare the values to observational estimates.

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