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Project OC5
Quantifying Last Glacial Maximum ocean circulation by state estimation
A. Paul, M. Losch,
S. Mulitza, G. Lohmann, M. Schulz, A. Govin, E. Huang, A. Kloss, T. Kurahashi-Nakamura
The Last Glacial Maximum (LGM, ~19,000- 23,000 years before present) cold period is a means for evaluating the response of the climate system to large perturbations. Further, for this period the best proxy-data coverage is available and forcing functions, boundary conditions and the climate response are “relatively well known” (Jansen et al. 2007). However, while IPCC-type coupled climate models are generally tuned to be consistent with historical and present-day data, they give ambiguous results for the LGM. For example, the Atlantic meridional overturning rates are estimated as both stronger and weaker than and as strong as today by different models (Otto-Bliesner et al. 2007). We propose to reduce this uncertainty by employing systematic state estimation techniques: In practice, models are often tuned to reproduce observations by adjusting individual parameters and repeating simulations in an ad-hoc iteration. We want to overcome this crude tuning procedure and use variational techniques (the so-called adjoint method) and sequential filtering as well as statistical methods (e.g., Monte Carlo methods) to combine proxy data with a numerical ocean model. These techniques take into account the large uncertainties associated with both model and data. The goal is the best estimate of the LGM ocean circulation that is dynamically consistent (within prior error estimates) with model and data. Further, these techniques can be applied to a wide range of problems and have a great potential of integrating previous and ongoing activities in MARUM. They will help exploiting the wealth of paleo-proxy data collected by MARUM, and they are tools for identifying geographic locations where taking new ocean sediment cores have the largest impact on determining the ocean circulation. Ultimately, the Project aims at establishing a “Glacial Ocean Atlas”.
Figure 1: Atlantic meridional overturning circulations simulated by PMIP2 coupled atmosphere-ocean models for (top) modern and (bottom) Last Glacial Maximum conditions (after Otto-Bliesner et al. 2007, recomputed from original model output)
Reference:
MARGO Project Members* (2009): Constraints on the magnitude and patterns of ocean cooling at the Last Glacial Maximum. Nature Geoscience, 2, 127-132, doi:10.1038/ngeo411
*) Waelbroeck C, Paul A, Kucera M, Rosell-Melé A, Weinelt M, Schneider R, Mix AC, Abelmann A, Armand L, Bard E, Barker S, Barrows TT, Benway H, Cacho I, Chen MT, Cortijo E, Crosta X, de Vernal A, Dokken T, Duprat J, Elderfield H, Eynaud F, Gersonde G, Hayes A, Henry M, Hillaire-Marcel C, Huang CC, Jansen E, Juggins S, Kallel N, Kiefer T, Kienast M, Labeyrie L, Leclaire H, Londeix L, Mangin S, Matthiessen J, Marret F, Meland M, Morey AE, Mulitza S, Pflaumann U, Pisias NG, Radi T, Rochon A, Rohling EJ, Sbaffi L, Schäfer-Neth C, Solignac S, Spero H, Tachikawa K, Turon JL
Figure 2: This map shows the reconstructed LGM sea-surface temperature anomaly, computed as the difference between the Last Glacial Maximum (LGM, between 19,000 and 23,000 years before present) and present day, in units of °C for the Northern Hemisphere winter season (July-August-September). Negative anomalies (blue) denote regions that according to the MARGO reconstruction were colder than today, positive anomalies (yellow) point to regions that might have been warmer than today. The squares mark the geographic locations of the sediment cores that were investigated by the MARGO project using a variety of methods for temperature reconstruction. Dark grey areas in the ocean stand in for those regions to which no anomaly could be assigned because the nearest data points were too far (more than 2000 km) away. For illustration, contour lines on land indicate the extent of the continental ice sheets. The grid lines are 30° of longitude and latitude apart.
Figure 3: Sensitivity of vertically integrated primary productivity to wind stress increase in the nested modeling system UVic-ROMS (Giraud and Paul, in prep.). (a) and (b): Standard Last Glacial Maximum (LGM) simulation minus present-day (PD) simulation. (c) and (d): LGM simulation with doubled wind stress over the Northwest African coast minus PD simulation. White areas indicate results that are statistically not significant (student t-test, =0.05). Maps (a) and (c) show the relative change between the LGM and PD simulations with respect to the PD simulation, evaluated at a given geographical location. Curves (b) and (d) show the same relative change sampled along the 1000 m isobath indicated as a solid black line in (a) and (b). Symbols denote sediment core data: Circles refer to the data by Harris et al. (1996), Sicre et al. (1996), Sicre et al. (2001) and Hartmann et al. (2006). Squares refer to the data by Eberwein and Mackensen (2008).