Thermohaline Experiment


In Climate science

Details about the experiment follow. This sub-experiment is being led by Nick Faull (climate science) and Tolu Aina (computing).
The experiment is not a prediction of the likelihood of a reduction in the strength of the THC. In this experiment we impose a reduction of the THC consistent with earlier experiments with the Hadley Centre coupled model and study the atmospheric response. The current phase of the project uses the Hadley Centre atmospheric model in conjunction with a simplified thermodynamic ("slab" ocean) which comprises a single layer ocean with prescribed heat and salinity transports. We impose surface fields which reflect the fully-coupled model's response to an imposed THC slowdown. The experiment is thus consistent with previous coupled model work with the same model at the same resolution. The essence of our THC experiment is to look at how the atmosphere would respond to such changes in the ocean, given a THC slowdown. Because (at this stage) we are only using a slab ocean (this will change in later phases of the project) we cannot yet probe feedbacks between the dynamical responses of the atmosphere back on to the ocean.
Motivation of the THC Experiment
Full Description of the THC experiment
In essence, our methodology extends the climateprediction.net project, using distributed computing to build a perturbed-physics ensemble simulation of the range of responses to increase CO2, to recent studies by Dong and Sutton (2002) and Palmer (2002) examining the response of a particular atmosphere model to changes in THC. Under climateprediction.net, a current climate-resolution atmospheric general circulation model (the standard version of the HadAM3 model, with longitude/latitude resolution of 3.75 by 2.5 degrees and 19 levels in the vertical) is coupled to a simple thermodynamic “slab” ocean model. The properties of the model atmosphere are then changed using some combination of perturbations to over 20 parameters (including switches that allow entire model sub-systems to be activated or deactivated) and the resulting perturbed model is distributed to a volunteer participant in the project to be run on a personal computer.
The standard climateprediction.net experiment comprises a 15-year integration during which sea-surface temperatures are strongly relaxed back to observed present-day conditions and the Ocean Heat Flux Convergence (OHFC) field is calculated to balance the resulting down-welling surface heat flux from the atmosphere. This is followed by a 15-year “control” phase with pre-industrial CO2 levels, and a 15-year “doubled-CO2” phase in which CO2 levels are increased to 560ppmv. A standard diagnostic set comprising approximately 0.5Gb of output is archived on the participants personal computer while a set of summary diagnostics (~5Mb per experiment) is automatically uploaded to one of the projects’ servers at the end of the experiment. These summary diagnostics provide an indication of the range of behaviour across the ensemble as well as highlighting particularly interesting parameter combinations for which we can either request the full archive to be uploaded (if the participant’s internet connection allows) or repeat the run in-house.
In this revised experiment, an additional OHFC perturbation will be applied at the end of the doubled-CO2 phase of the standard experiment to represent the change in net ocean-atmosphere surface heat flux that might result from a substantial reduction in the THC and the model integrated for a further 15 years. Although most models will not all have reached equilibrium by the end of this period, feedbacks in a slab-ocean set-up are sufficiently close to linear in the temperature change for the degree of disequilibrium to be inferred from the time-history and energy budget of the response. As an additional check, however, the OHFC perturbation phase will be started from the end of the 14th year of the doubled-CO2 phase thereby ensuring a 1 year overlap between the end of one phase and the beginning of the next. With an initial condition ensemble this overlap will provide information about linearity of the temperature response when the OHFC perturbation is applied.
Palmer (2002) performed an experiment in which this OHFC response-pattern was scaled to correspond to a ~50% reduction in THC magnitude from HadCM3 control values and obtained a ~2K global mean cooling in response, as compared to Vellinger and Wood (2002) who induced a near-complete THC shut-down associated with a ~4K global mean cooling. We propose initially using double Palmer (2002)’s perturbation, although the perturbed-physics ensemble set-up makes imposing a variety of OHFC perturbations very straightforward, and sensitivity of response to the imposed pattern and amplitude will be explored in the course of this project. Following Palmer (2002), the imposed OHFC perturbation is seasonally varying.
Comparison of the mean precipitation response in the region of the North Atlantic where Vellinga and Wood (2002) imposed a freshwater forcing to induce an THC shutdown reveals that the magnitude (and possibly even the sign) of the atmospheric hydrological-cycle feedback on THC changes is highly sensitive to underdetermined aspects of atmosphere model formulation. Under this project, we will use diagnostics returned from the perturbed-physics ensemble to identify parameter combinations which result in atmospheres that are (1) sensitive to CO2 forcing; (2) sensitive to THC changes with the precipitation response giving a positive feedback and (3) relatively realistic in terms of their base climate. We will assess to what extent current observations allow us to constrain the ensemble in these “pathological” parameter-space directions and provide a range of representative statistical models of possible atmospheric responses to internal ocean variability, CO2 and THC changes for use as upper boundary conditions in ocean THC predictability studies.
Figure 1.(a) DJF mean values of the OHFC anomaly imposed in the climateprediction.net client to simulate the impact of a ~100% reduction in AMOC; (b) Temperature response to this OHFC perturbation averaged over years 8-15 of a 15-year simulation following the perturbation; (c) Precipitation response from the same model-version as (b); (d) Precipitation response from another model-version with different, but also physically plausible, values of model parameters. 

No hay comentarios:

Publicar un comentario