S. L. Weber

Last glacial maximum ocean thermohaline circulation: PMIP2 model intercomparisons and data constraints

The ocean thermohaline circulation is important for transports of heat and the carbon cycle. We present results from PMIP2 coupled atmosphere-ocean simulations with four climate models that are also being used for future assessments. These models give very different glacial thermohaline circulations even with comparable circulations for present. An integrated approach using results from these simulations for Last Glacial Maximum (LGM) with proxies of the state of the glacial surface and deep Atlantic supports the interpretation from nutrient tracers that the boundary between North Atlantic Deep Water and Antarctic Bottom Water was much shallower during this period. There is less constraint from this integrated reconstruction regarding the strength of the LGM North Atlantic overturning circulation, although together they suggest that it was neither appreciably stronger nor weaker than modern. Two model simulations identify a role for sea ice in both hemispheres in driving the ocean response to glacial forcing.

Sahel rainfall variability and response to greenhouse warming

Received 19 April 2005; revised 21 July 2005; accepted 1 August 2005; published 10 September 2005. [1] The NCEP/NCAR re-analyses as well as ensemble integrations with an atmospheric GCM indicate that interannual variations in Sahel rainfall are related to variations in the mean sea level pressure (MSLP) over the Sahara. In turn the MSLP variations are related to the global distribution of surface air temperature (SAT). An increase in SAT over the Sahara, relative to the surrounding oceans, decreases the MSLP over the Sahara, thereby increasing the Sahel rainfall. We hypothesize that through this mechanism greenhouse warming will cause an increase in Sahel rainfall, because the warming is expected to be more prominent over the summer continents than over the oceans. This has been confirmed using an ensemble of 62 coupled model runs forced with a business as usual scenario. The ensemble mean increase in Sahel rainfall between 1980 and 2080 is about 1–2 mm day � 1 (25–50%) during July–September, thereby strongly reducing the probability of prolonged droughts. Citation: Haarsma, R. J., F. M. Selten, S. L. Weber, and M. Kliphuis (2005), Sahel rainfall variability and response to greenhouse warming, Geophys. Res. Lett., 32, L17702,

Sea-level variability in the northwest Atlantic during the past 1500 years: A delayed response to solar forcing?

[1] Numerical experiments with a coupled oceanatmosphere model (ECBilt) have shown that centennial variations in sea level (SL) in the northwest Atlantic may be associated with deep-ocean salinity anomalies generated by solar-forced variations in the North Atlantic overturning circulation. Here we compare simulated SL curves for the Gulf Stream region with reconstructed, late-Holocene SL records from Connecticut (USA). Simulated SL variations lag the solar forcing record by ca. 120 year. This lag is found to be robust over a small number of different experiments. The reconstructed SL curves visually match the solar forcing optimally when lagging it by ca. 125 yr. A quantitative test shows that the correlation is significant, while this result is not sensitive to dating uncertainties. The temporal response pattern of the simulated SL curves compares reasonably well with the reconstructions. INDEX TERMS: 4556 Oceanography: Physical: Sea level variations; 1650 Global Change: Solar variability; 4255 Oceanography: General: Numerical modeling; 4267 Oceanography: General: Paleoceanography; 3030 Marine Geology and Geophysics: Micropaleontology. Citation: van de Plassche, O., G. van der Schrier, S. L. Weber, W. R. Gehrels, and A. J. Wright, Sea-level variability in the northwest Atlantic during the past 1500 years: A delayed response to solar forcing?, Geophys. Res. Lett., 30(18), 1921, doi:10.1029/ 2003GL017558, 2003.