Bernd J. Haupt

Climate Response to External Sources of Freshwater: North Atlantic versus the Southern Ocean

Abstract The response of an atmosphere–ocean general circulation model (AOGCM) to perturbations of freshwater fluxes across the sea surface in the North Atlantic and Southern Ocean is investigated. The purpose of this study is to investigate aspects of the so-called bipolar seesaw where one hemisphere warms and the other cools and vice versa due to changes in the ocean meridional overturning. The experimental design is idealized where 1 Sv (1 Sv ≡ 106 m3 s−1) of freshwater is added to the ocean surface for 100 model years and then removed. In one case, the freshwater perturbation is located in the Atlantic Ocean from 50° to 70°N. In the second case, it is located south of 60°S in the Southern Ocean. In the case where the North Atlantic surface waters are freshened, the Atlantic thermohaline circulation (THC) and associated northward oceanic heat transport weaken. In the Antarctic surface freshening case, the Atlantic THC is mainly unchanged with a slight weakening toward the end of the integration. This w...

Meltwater and the global ocean conveyor: northern versus southern connections

Abstract The sensitivity of the ocean circulation to changes in North Atlantic surface fluxes has become a major factor in explaining climate variability. The role of the Antarctic Bottom Water in modulating this variability has received much less attention, limiting the development of a complete understanding of decadal to millennial time-scale climate change. New analyses indicate that the southern deepwater source may change dramatically (e.g., experience a decrease of as much as two thirds during last 800 years). Such change can substantially alter the ocean circulation patterns of the last millennium. Additional analyses indicate that the Southern Hemisphere led the Northern Hemisphere changes in some of the glacial cycles of Pleistocene, implying a seesaw-type oscillation of the global ocean conveyor. The potential for melting of sea ice and ice sheets in the Antarctica associated with global warming can cause a further slowdown of the southern deepwater source. These results demand an assessment of the role of the Southern Ocean in driving changes of the global ocean circulation and climate. Systematic model simulation targeting the ocean circulation response to changes in surface salinity in the high latitudes of both Northern and Southern Hemispheres demonstrate that meltwater impacts in one hemisphere may lead to a strengthening of the thermohaline conveyor driven by the source in the opposite hemisphere. This, in turn, leads to significant changes in poleward heat transport. Further, meltwater events can lead to deep-sea warming and thermal expansion of abyssal water, that in turn cause a substantial sea-level change even without a major ice sheet melting.

Depositional style and subsidence history of the Turpan Basin (NW China)

The Turpan Basin has a complex polycyclic sedimentary and tectonic history from Late Permian to late Tertiary time. Main stratigraphic boundaries are unconformities that bound tectonically induced sedimentary cycles. The depositional style reflects a continental environment exhibiting changes from alluvial fan/fluvial to lacustrine conditions within each cycle. More than 7000 m of clastic sediments accumulated during the evolution of the basin. Paleocurrent analysis reveals a complex pattern of sediment dispersal pathways into the basin. On the southern margin of the basin, the transport direction is always directed from south to north indicating sediment sources in the Jueluotage Shan, while in the northern part the Late Jurassic uplift of the Bogda Shan provided an important source area from the Early Cretaceous onwards with transport directions from north to south. After basin formation during the Late Permian, the Turpan Basin underwent first thermal subsidence and then flexural subsidence. The thermal subsidence took place during the Late Permian and Early Triassic following the period of magmatic activities in this region. The flexural subsidence was throughout the Middle Triassic to early Tertiary induced by collisions and accretions onto the south Asian continental margin of the Qiangtang Block in the Late Triassic/Early Jurassic, the Gangdise Block in latest Jurassic/earliest Cretaceous, and the Indian Subcontinent in the latest Cretaceous/early Cenozoic.

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.

Warm deep-water ocean conveyor during Cretaceous time

Because the deep water is associated with its high-latitude sources, a warm deep ocean durin gMesozoic-Cenozoi ctim ei sa challenge ;ther ei sn ofeasibl ephysica lmechanism that could maintain warm subpolar surface oceans in both hemispheres. The goal of this study is to explore a hypothesis that a warm deep ocean can coexist with relatively cool subpolar (high latitude) sea surface in one hemisphere and a warmer subpolar sea surface in the other. A series of numerical ocean circulation experiments confirms that the ocean meridional circulation can keep the abyssal ocean warm despite the northern subpolar surface water staying relatively cool.

On sensitivity of ocean circulation to sea surface salinity

Abstract Sensitivity of global ocean circulation to details of sea surface boundary conditions has been revisited in a set of numerical experiments. The focus is on the role of sea surface salinity (SSS) asymmetries between different ocean basins. The results indicate that the basin-wide inter-basin salinity contrasts are the key elements of success for many idealized ocean models that have realistically mirrored ocean circulation despite strongly simplified sea surface conditions. We suggest that exact knowledge of the spatial distribution of sea surface salinity may not be crucial to the simulation of the thermohaline ocean circulation (THC) in many process-oriented climate studies. Therefore, simulations based on limited sea surface data, for example in paleoclimate studies, can yield acceptable circulation patterns, as long as the inter-basin surface salinity contrasts are retained. Possible feedbacks of both concurrent northern and southern freshwater impacts and the building up of inter-basin sea surface salinity contrasts are discussed.

Global ocean thermohaline conveyor at present and in the Late Quaternary

Operation modes of the global ocean thermohaline conveyor at present, at the last glacial maximum, and at a subsequent meltwater event (MWE) are revisited using a combination of a global ocean circulation model and a semi-Lagrangian trajectory tracing model. The trajectory tracing model helps to visualize the true three-dimensional water transport that is not accessible within traditional ocean circulation modeling. Our simulations confirm that the glacial mode of the conveyor was substantially weaker as compared to the present day mode. However, the simulations indicate that major changes of the deep global ocean conveyor occurred only at the MWE. These changes led to reversal of the Indian-Atlantic branch of the deep conveyor due to complete cessation of North Atlantic Deep Water production caused by a very localized meltwater impact.

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.