Near the Eocene's close (∼34 million years ago), the climate system underwent one of the largest shifts in Earth's history: Antarctic terrestrial ice sheets suddenly grew and ocean productivity patterns changed. Previous studies conjectured that poleward penetration of warm, subtropical currents, the East Australian Current (EAC) in particular, caused Eocene Antarctic warmth. Late Eocene opening of an ocean gateway between Australia and Antarctica was conjectured to have disrupted the EAC, cooled Antarctica, and allowed ice sheets to develop. Here we reconstruct Eocene paleoceanographic circulation in the Tasmanian region, using (1) biogeographical distributions of phytoplankton, including data from recently drilled Ocean Drilling Program Leg 189 sites and (2) fully coupled climate model simulations. We find that the EAC did not penetrate to high latitudes and ocean heat transport in the region was not greater than modern. Our results do not support changes in “thermal isolation” as the primary driver of the Eocene‐Oligocene climatic transition.

No abstract

No abstract

A new method for calculating water mass transport between different ocean basins from the velocity fields obtained by numerical models is presented. The method is applied to the velocity field of the Southern Ocean simulated by a primitive equation model (fine resolution Antarctic model). With this method it is possible to judge whether a water mass has been ventilated or not, to estimate how many times it has circled around Antarctica, and to calculate the time it has spent in the Southern Ocean. Calculations have also been undertaken revealing to what extent the changes of temperature, salinity, and density have been caused by mixing and by ventilation. Two major ways to redistribute the water through the Southern Ocean are identified. The first one redistributes 53% of the water and involves an un ventilated direct exchange between the oceans, the second one redistributes 33% by going around Antarctica. It is found that, on average, the water mass makes six circuits before the water is ventilated and subsequently driven to the north by the Ekman transport. A heat transport study is carried out for the Atlantic, showing that the northward heat transport into the Atlantic comes 85% from the Indian Ocean and the rest from the Drake Passage.

Incoming and outgoing solar radiation couple with heat exchange at Earth's surface to drive weather patterns that redistribute heat and moisture around the globe, creating an atmospheric heat engine. Here, we investigate the engine's work output using thermodynamic diagrams computed from reanalyzed observations and from a climate model simulation with anthropogenic forcing. We show that the work output is always less than that of an equivalent Carnot cycle and that it is constrained by the power necessary to maintain the hydrological cycle. In the climate simulation, the hydrological cycle increases more rapidly than the equivalent Carnot cycle. We conclude that the intensification of the hydrological cycle in warmer climates might limit the heat engine's ability to generate work.

The existence of a new route that draws relatively cold waters from the Pacific Ocean to the North Atlantic via the Tasman outflow is presented. The new route materialises with comparable magnitude and characteristics in three independent numerical realisations of the global ocean circulation. Its realism is supported by hydrographic data interpolated via an inverse model. The “Tasman leakage” constitutes a sizeable component of the upper branch of the global conveyor belt and represents an extension to the prevailing views that hitherto emphasised the routes via the Drake Passage and the Indonesian Throughflow [Gordon, 1986].

The meridional overturning cells in the Southern Ocean are decomposed by Lagrangian tracing using velocity and density fields simulated with an ocean general circulation model. Particular emphasis is given to the Deacon Cell. The flow is divided into four major components: (1) water circling around Antarctica in the Antarctic Circumpolar Current (ACC), (2) water leaving the ACC toward the north into the three world oceans, (3) water coming from the north and joining the ACC, mainly consisting of North Atlantic Deep Water (NADW), and (4) interocean exchange between the three world oceans without circling around Antarctica. The Deacon Cell has an amplitude of 20 Sv, of which 6 Sv can be explained by the the east‐west tilt of the ACC, 5 Sv by the east‐west tilt of the subtropical gyre, and the remaining 9 Sv by the differences of the slope and depth of the southward transport of NADW and its return flow as less dense water. The diabatic or cross‐isopycnal Deacon Cell is only 2 Sv.

Calculating a streamfunction as function of depth and density is proposed as a new way of analyzing the thermodynamic character of the overturning circulation in the global ocean. The sign of an overturning cell in this streamfunction directly shows whether it is driven mechanically by large-scale wind stress or thermally by heat conduction and small-scale mixing. It is also shown that the integral of this streamfunction gives the thermodynamic work performed by the fluid. The analysis is also valid for the Boussinesq equations, although formally there is no thermodynamic work in an incompressible fluid. The proposed method is applied both to an idealized coarse-resolution three-dimensional numerical ocean model, and to the realistic high-resolution Ocean Circulation and Climate Advanced Model (OCCAM). It is shown that the overturning circulation in OCCAM between the 200-and 1000-m depth is dominated by a thermally indirect cell of 24 Sverdrups (1 Sv ϵ 10 6 m 3 s Ϫ1 ), forced by Ekman pumping. In the densest and deepest waters there is a thermally direct cell of 18 Sv, which requires a forcing by around 100 GW of parameterized small-scale mixing.