Large-scale features of Pliocene climate: results from the Pliocene Model Intercomparison Project

Climate and environments of the mid-Pliocene warm period (3.264 to 3.025 Ma) have been extensively studied. Whilst numerical models have shed light on the nature of climate at the time, uncertainties in their predictions have not been systematically examined. The Pliocene Model Intercomparison Project quantifies uncertainties in model outputs through a coordinated multi-model and multi-model/data intercomparison. Whilst commonalities in model outputs for the Pliocene are clearly evident, we show substantial variation in the sensitivity of models to the implementation of Pliocene boundary conditions. Models appear able to reproduce many regional changes in temperature reconstructed from geological proxies. However, data/model comparison highlights that models potentially underestimate polar amplification. To assert this conclusion with greater confidence, limitations in the time-averaged proxy data currently available must be addressed. Furthermore, sensitivity tests exploring the known unknowns in modelling Pliocene climate specifically relevant to the high latitudes are essential (e.g. palaeogeography, gateways, orbital forcing and trace gasses). Estimates of longer-term sensitivity to CO2 (also known as Earth System Sensitivity; ESS), support previous work suggesting that ESS is greater than Climate Sensitivity (CS), and suggest that the ratio of ESS to CS is between 1 and 2, with a "best" estimate of 1.5

Ontong Java Plateau, Leg 130: Synopsis of major drilling results

Sixteen holes were drilled at five sites on the northeastern flank of Ontong Java Plateau during Leg 130 (Sites 803 through 807). All of these sites are near the equator, but are at different depths (803: 02°26.0\u27N, 3410 m; 804: 01°00.3\u27N, 3861 m; 805: 01°13.7\u27N, 3188 m; 806: 00°19.1\u27N, 2520 m; and 807: 03°36.4\u27N, 2805 m). One of our goals was to obtain a depth transect of Neogene carbonate deposition for reconstructing the history of ocean climate, chemistry, and productivity, and for understanding the origin of acoustic reflectors. Another goal was to achieve considerable penetration into basement for elucidating the origin of the plateau. All sites yielded multiple Neogene sequences, which were cored using the advanced hydraulic piston corer (APC) to the ooze-chalk transition (10-14 Ma) and with the extended core barrel (XCB) below that. Sites 803 and 807 were drilled to basement and yielded incomplete Paleogene and Cretaceous sections. Penetration into basement was 25 m at Site 803 and 149 m at Site 807; 98 m of basalt was recovered. In all, we cored 5889 m, taking 639 cores. Of the record 4822 m recovered, 55% was taken with the APC, 39% with the XCB, and 6% with the rotary core barrel (RCB). All sites except Site 804 were logged. Neogene sedimentation rates were found to vary by more than a factor of 2, with a striking maximum in the latest Miocene to early Pliocene and a strong minimum in the Pleistocene. Fluctuations in carbonate content on the millionyear scale are highly coherent among depths over the last 12 m.y., perhaps less so before that. Many acoustic reflectors appear synchronous with carbonate reduction events (CREs) and other paleoceanographic events. Other reflectors are tied to diagenesis (e.g., the ooze-chalk transformation, which is diachronous). Recovery of the Cretaceous/Tertiary (K/T) boundary at Sites 803 and 807 demonstrates the presence of a deep carbonate-compensation depth (CCD) across the transition: one sequence is calcareous, the other is not. Because the K/T sections occur below and above major hiatuses, we postulate that special conditions for preservation existed during the transition. In addition, there is evidence of volcanic activity at that time. The basalts cored at Sites 803 and 807 are predominantly olivine-bearing and were erupted during the mid-Cretaceous. At Site 807, pillow lavas buried sediments. One thick flow (at about 28 m) was penetrated here, apparently a flood basalt. Magnetic paleolatitudes suggest that the Ontong Java Plateau has moved coherently with the Pacific Plate since the Early Cretaceous

Mediterranean Sea response to climate change in an ensemble of twenty first century scenarios

The Mediterranean climate is expected to become warmer and drier during the twenty-first century. Mediterranean Sea response to climate change could be modulated by the choice of the socio-economic scenario as well as the choice of the boundary conditions mainly the Atlantic hydrography, the river runoff and the atmospheric fluxes. To assess and quantify the sensitivity of the Mediterranean Sea to the twenty-first century climate change, a set of numerical experiments was carried out with the regional ocean model NEMOMED8 set up for the Mediterranean Sea. The model is forced by air–sea fluxes derived from the regional climate model ARPEGE-Climate at a 50-km horizontal resolution. Historical simulations representing the climate of the period 1961–2000 were run to obtain a reference state. From this baseline, various sensitivity experiments were performed for the period 2001–2099, following different socio-economic scenarios based on the Special Report on Emissions Scenarios. For the A2 scenario, the main three boundary forcings (river runoff, near-Atlantic water hydrography and air–sea fluxes) were changed one by one to better identify the role of each forcing in the way the ocean responds to climate change. In two additional simulations (A1B, B1), the scenario is changed, allowing to quantify the socio-economic uncertainty. Our 6-member scenario simulations display a warming and saltening of the Mediterranean. For the 2070–2099 period compared to 1961–1990, the sea surface temperature anomalies range from +1.73 to +2.97 °C and the SSS anomalies spread from +0.48 to +0.89. In most of the cases, we found that the future Mediterranean thermohaline circulation (MTHC) tends to reach a situation similar to the eastern Mediterranean Transient. However, this response is varying depending on the chosen boundary conditions and socio-economic scenarios. Our numerical experiments suggest that the choice of the near-Atlantic surface water evolution, which is very uncertain in General Circulation Models, has the largest impact on the evolution of the Mediterranean water masses, followed by the choice of the socio-economic scenario. The choice of river runoff and atmospheric forcing both have a smaller impact. The state of the MTHC during the historical period is found to have a large influence on the transfer of surface anomalies toward depth. Besides, subsurface currents are substantially modified in the Ionian Sea and the Balearic region. Finally, the response of thermosteric sea level ranges from +34 to +49 cm (2070–2099 vs. 1961–1990), mainly depending on the Atlantic forcing

Influence of bottom topography on integral constraints in zonal flows with parameterized potential vorticity fluxes

An integral constraint for eddy fluxes of potential vorticity (PV), corresponding to global momentum conservation, is applied to two-layer zonal quasi-geostrophic channel flow. This constraint must be satisfied for any type of parameterization of eddy PV fluxes. Bottom topography strongly influence the integral constraint compared to a flat bottom channel. An analytical solution for the mean flow solution has been found by using asymptotic expansion in a small parameter which is the ratio of the Rossby radius to the meridional extent of the channel. Applying the integral constraint to this solution, one can find restrictions for eddy PV transfer coefficients which relate the eddy fluxes of PV to the mean flow. These restrictions strongly deviate from restrictions for the channel with flat bottom topography