Dan Seidov

North Atlantic ocean circulation during the last glacial maximum and subsequent meltwater event: A numerical model

A numerical model that was forced with reconstructed sea surface temperature and sea surface salinity values of the North Atlantic was used to investigate three major states of the North Atlantic ocean circulation since the last glaciation: the modern state, the last glacial maximum (LGM), and an important meltwater event (MWE) near 14,200 - 13,200 C-14 yr B.P. Preliminary results of numerical experiments show significant differences of the three outlined modes in the northern North Atlantic and especially in the Norwegian Greenland Seas (NGS). The overturning strength of the North Atlantic salinity conveyor belt and northward heat transport decreased somewhat in the LGM and drastically in the MWE case. Accordingly, the North Atlantic Deep Water production decreased by 30% during the LGM and almost ceased at the MWE. The climatologically most important changes occurred during the MWE and are characterized by a reversal in the circulation of the surface water in the northeastern part of the NGS, Implying that the North Atlantic Current did not reach the Norwegian Sea anymore.

Simulated ocean circulation and sediment transport in the North Atlantic during the Last Glacial Maximum and today

Paleocirculation of the North Atlantic Ocean at the last glacial maximum (LGM) is simulated using a large-scale ocean general circulation model (OGCM). The model is driven by glacial sea surface thermohaline conditions and wind stress. For comparison of past and present circulation patterns, a separate run provides the Holocene/modern circulation patterns based on the present day sea surface climatology. The output of the OGCM is then used in a sedimentation model and in a model to trace water parcel trajectories. The sedimentation model reveals the differences in sediment deposition in the North Atlantic linked to past and present circulation regimes. The trajectory-tracing model facilitates a better understanding of the thermocline and deep ocean ventilation, the actual three-dimensional conveying of water, and the role of convection in maintaining the meridional thermohaline overturning. The results of the trajectory-tracing technique indicate stronger subtropical thermocline ventilation during the LGM. For the deep ocean currents, we find severe alteration of three-dimensional water motion in response to weakening of the LGM convection and its retreat to the southwest from its present locations. Ocean circulation models can provide sedimentation studies with information on the circulation regime that proxy data used alone cannot. These is because such models furnish both circulation patterns and the ventilating convection depths.

An intermediate model for large-scale ocean circulation studies

Abstract The design and purposes of an intermediate model are discussed along with fundamentals of the model and results of numerical experiments. The main purposes of the model are reconstructions of the schemes of the ocean large-scale circulation and paleocirculation. For these problems numerical effectiveness is the key factor. A novel feature is the parameterization of the side wall Ekman boundary layers which was introduced to enable the use of geostrophy for calculating baroclinic velocity. This approach of connecting the side frictional layers with a non-viscous interior is not model-specific and can be used in any model employing geostrophy in the interior. The method can facilitate the no-flux and no-slip boundary conditions at the side walls in such models. Preliminary numerical experiments with simple basin geometry and idealized forcing aimed at a comparison with primitive equation and planetary geostrophic models are carried out. A direct comparison with the Geophysical Fluid Dynamics Laboratory (GFDL) primitive equation model was performed for a quantitative test of the proposed model. The results of the experiments are discussed in the context of the applicability of intermediate models for studying ocean climate dynamics.

Last glacial and meltwater interbasin water exchanges and sedimentation in the World Ocean

Modeling the global ocean thermohaline conveyor at present, at the Last Glacial Maximum, and at a subsequent meltwater event is revisited using a combination of an ocean global circulation model and a sediment transport model. The modeled changes of sediment deposition rates, linked to the changes of the global deep-ocean thermohaline circulation, provide a better understanding of the glacial-to-interglacial variability of thermohaline currents, and help to identify the regions of the world ocean that are most sensitive to the glacial and meltwater impacts. In addition to the well-known local changes of the conveyor in the Atlantic Ocean during the last glaciation and subsequent meltwater events, the simulations show the global character of these impacts, detected as far from the North Atlantic as the Indian and the southwestern Pacific Oceans. However, the numerical experiments challenge the idea of a global conveyor-like deep flow strongly connecting the surface waters of northern parts of the North Atlantic and North Pacific Oceans at either glacial or meltwater intervals.

Eddy-Resolving Model of Idealized and Real Ocean Circulation

Abstract The model discussed in this paper is a new version of the eddy-resolving model of the large scale (“general”) ocean circulation, ERGCM, described by Seidov (1985). The model was designed and used for the study of the North Atlantic circulation (Seidov et al., 1985, 1986). Some additional calculations for an idealized geometry and some of the results of the real circulation modelling (Seidov et al., 1985, 1986) are presented in this paper for the purpose of comparison.