ALBERT

Description: Over the Last Glacial Maximum (LGM, ~21ka BP), the presence of vast Northern Hemisphere ice-sheets caused abrupt changes in surface topography and background climatic state. While the ice-sheet extent is well known, several conflicting ice-sheet topography reconstructions suggest that there is uncertainty in this boundary condition. The terrestrial and sea surface temperature (SST) of the LGM as simulated with six different Laurentide Ice Sheet (LIS) reconstructions in a fully coupled Earth System Model (COSMOS) have been compared with the subfossil pollen and plant macrofossil based and marine temperature proxies reconstruction. The terrestrial reconstruction shows a similar pattern and in good agreement with model data. The SST proxy dataset comprises a global compilation of planktonic foraminifera, diatoms, radiolarian, dinocyst, alkenones and planktonic foraminifera Mg/Ca-derived SST estimates. Significant mismatches between modeled and reconstructed SST have been observed. Among the six LIS reconstructions, Tarasov’s LIS reconstruction shows the highest correlation with reconstructed terrestrial and SST. In the case of radiolarian, Mg/Ca, diatoms and foraminifera show a positive correlation while dinocyst and alkenones show very low and negative correlation with the model. Dinocyst-based SST records are much warmer than reconstructed by other proxies as well as Pre-industrial (PI) temperature. However, there are large discrepancies between model temperatures and temperature recorded by different proxies. Eight different PMIP3 models also compared with temperature proxies reconstruction which show mismatches with the proxy records might be due to misinterpreted and/or biased proxy records. Therefore, it has been speculated that considering different habitat depths and growing seasons of the planktonic organisms used for SST reconstruction could provide a better agreement of proxy data with model results on a regional scale. Moreover, it can reduce model-data misfits. It is found that shifting in the habitat depth and living season can remove parts of the observed model-data mismatches in SST anomalies.

Description: Changes in ocean gateway configuration are known to induce basin-scale rearrangements in ocean characteristics throughout the Cenozoic. However, there is large uncertainty in the relative timing of the subsidence histories of ocean gateways in the northern high latitudes. By using a fully coupled General Circulation Model we investigate the salinity and temperature changes in response to the subsidence of two key ocean gateways in the northern high latitudes during early to middle Miocene. Deepening of the Greenland-Scotland Ridge causes a salinity increase and warming in the Nordic Seas and the Arctic Ocean. While warming this realm, deep water formation takes place at lower temperatures due to a shift of the convection sites to north off Iceland. The associated deep ocean cooling and upwelling of deep waters to the Southern Ocean surface causes a cooling in the southern high latitudes. These characteristic impacts in response to the Greenland-Scotland Ridge deepening are independent of the Fram Strait state. Subsidence of the Fram Strait for a deep Greenland-Scotland Ridge causes less pronounced warming and salinity increase in the Nordic Seas. A stronger salinity increase is detected in the Arctic while temperatures remain unaltered, which further increases the density of the North Atlantic Deep Water. This causes an enhanced contribution of North Atlantic Deep Water to the abyssal ocean and on the expense of the colder southern source water component. These relative changes largely counteract each other and cause little warming in the upwelling regions of the Southern Ocean.

Description: Changes in ocean gateway configuration can induce basin‐scale rearrangements in ocean current characteristics. However, there is large uncertainty in the relative timing of the Oligocene/Miocene subsidence histories of the Greenland‐Scotland Ridge (GSR) and the Fram Strait (FS). By using a climate model, we investigate the temperature and salinity changes in response to the subsidence of these two key ocean gateways during early to middle Miocene. For a singular subsidence of the GSR, we detect warming and a salinity increase in the Nordic Seas and the Arctic Ocean. As convection sites shift to the north of Iceland, North Atlantic Deep Water (NADW) is formed at cooler temperatures. The associated deep ocean cooling and upwelling of deep waters to the Southern Ocean surface can cause a cooling in the southern high latitudes. These characteristic responses to the GSR deepening are independent of the FS being shallow or deep. An isolated subsidence of the FS gateway for a deep GSR shows less pronounced warming and salinity increase in the Nordic Seas. Arctic temperatures remain unaltered, but a stronger salinity increase is detected, which further increases the density of NADW. The increase in salinity enhances the contribution of NADW to the abyssal ocean at the expense of the colder southern source water component. These relative changes largely counteract each other and cause a negligible warming in the upwelling regions of the Southern Ocean.

Description: The tectonic opening of the Fram Strait (FS) was critical to the water exchange between the Atlantic Ocean and the Arctic Ocean, and caused the transition from a restricted to a ventilated Arctic Ocean during early Miocene. If and how the water exchange between the Arctic Ocean and the North Atlantic influenced the global current system is still disputed. We apply a fully coupled atmosphere–ocean–sea-ice model to investigate stratification and ocean circulation in the Arctic Ocean in response to the opening of the FS during early-to-middle Miocene. Progressive widening of the FS gateway in our simulation causes a moderate warming, while salinity conditions in the Nordic Seas remain similar. On the contrary, with increasing FS width, Arctic temperatures remain unchanged and salinity changes appear to steadily become stronger. For a sill depth of ~ 1500 m, we achieve ventilation of the Arctic Ocean due to enhanced import of saline Atlantic water through an FS width of ~ 105 km. Moreover, at this width and depth, we detect a modern-like three-layer stratification in the Arctic Ocean. The exchange flow through FS is characterized by vertical separation of a low-salinity cold outflow from the Arctic Ocean confined to a thin upper layer, an intermediate saline inflow from the Atlantic Ocean below, and a cold bottom Arctic outflow. Using a significantly shallower and narrower FS during the early Miocene, our study suggests that the ventilation mechanisms and stratification in the Arctic Ocean are comparable to the present-day characteristics.

Description: The tectonic opening of the Fram Strait (FS) was critical to the water exchange between the Atlantic Ocean and the Arctic Ocean, and caused the transition from a restricted to a ventilated Arctic Ocean during early Miocene. If and how the water exchange between the Arctic Ocean and the North Atlantic influenced the global current system is still disputed. We apply a fully coupled atmosphere-ocean-sea-ice model to investigate stratification and ocean circulation in the Arctic Ocean in response to the opening of the FS during early to middle Miocene. Progressive widening of the FS gateway in our simulation causes a moderate warming, while salinity conditions in the Nordic Seas remain similar. On the contrary, with increasing FS width Arctic temperatures remain unchanged and salinity changes appear to steadily become stronger. For a sill depth of ~1500 m, we achieve ventilation of the Arctic Ocean due to enhanced import of saline Atlantic water through a FS width of ~105 km. Moreover, at this width and depth, we detect a modern-like three-layer stratification in the Arctic Ocean. The exchange flow through FS is characterized by vertical separation of a low salinity cold outflow from the Arctic Ocean confined to a thin upper layer, an intermediate saline inflow from the Atlantic Ocean below and a cold bottom Arctic outflow. Using a significantly shallower and narrower FS during the early Miocene, our study suggests that the ventilation mechanisms and stratification in the Arctic Ocean are comparable to the present-day characteristics.

Description: Cenozoic climate changes have been linked to tectonic activity and variations in atmospheric CO2 concentrations. Here we present Miocene and Pliocene sensitivity experiments performed with the climate model COSMOS. The experiments contain changes with respect to paleogeography, ocean gateway configuration, and atmospheric CO2 concentrations, as well as a range of vertical mixing coefficients in the ocean. For the Mid-Miocene, we show that the impact of ocean mixing on surface temperature is comparable to the effect of the possible range in reconstructed CO2 concentrations. In combination with stronger vertical mixing, relatively moderate CO2-concentrations of 450 ppmv enable global mean surface, deep-water and meridional temperature characteristics representative of Mid-Miocene Climatic Optimum (MMCO) reconstructions. The Miocene climate shows a reduced meridional temperature gradient and reduced seasonality. In the case of enhanced mixing, surface and deep ocean temperatures show significant warming of up to 5-10°C and an Arctic temperature anomaly of more than 12°C. In the Pliocene simulations, the impact of vertical mixing and CO2 is less important for the deep ocean, which we interpret as a different sensitivity dependence on the background state and mixed layer dynamics. We find a significant reduction in surface albedo and effective emissivity for either a high level of atmospheric CO2 or increased vertical mixing. Our mixing sensitivity experiments provide a warm deep ocean via ocean heat uptake. We propose that the mixing hypothesis can be tested by reconstructions of the thermocline and seasonal paleoclimate data indicating a lower seasonality relative to today.

Description: Over the Last Glacial Maximum (LGM, about 21ka BP) and subsequent deglaciation, the presence of vast Northern Hemisphere ice sheets caused abrupt changes in surface topography and background climatic state. While the ice-sheet extent is well known, several conflicting ice-sheet topography reconstructions suggest that there is an existence of uncertainty in this boundary condition. Simulations with a water isotope-enabled fully coupled Earth system model (COSMOS) and five different Laurentide Ice Sheet (LIS) reconstructions of different elevation are used to assess the range of sensitivity of climate modes in response to the uncertainty in this LIS topography. This study reveals that a change in ice sheet height can alter the coupled oceanic and atmospheric climate system during the LGM. A warming anomaly can be found over the region of lower ice sheet height, i.e. North America and lead to a slightly enhanced P-E over the North Atlantic, which also contributes to a weaker ocean circulation in the northern North Atlantic. The continental and sea surface temperature (SST) of the LGM as simulated by climate models have been compared with the subfossil pollen and plant macrofossil based reconstruction and reconstructed from marine temperature proxies. The continental reconstruction shows a similar pattern and in a good agreement with model data. The SST proxy dataset comprises a global compilation of planktonic foraminifera, diatom, radiolarian, dinocyst, alkenones and planktonic foraminifera Mg/Ca-derived SST estimates. Significant mismatches between modeled and reconstructed SST have been observed. Among the five LIS reconstructions, Tarasov LIS reconstructions show the highest correlation with reconstructed SAT and SST. In the case of Radiolarian, Mg/Ca, diatoms and foraminifera show a positive correlation where dinocyst and alkenones show very low and negative correlation with the model. Dinocyst-based SST records are much warmer than reconstructed by other proxies as well than PI temperature. However, large deviations with respect to model temperatures recorded by different proxies remain. Therefore, it has been speculated that considering different habitats depth and growing seasons of the planktonic organisms used for SST reconstruction could provide a better agreement of proxy data with model results on a regional scale and can reduce model–data misfits is determined. It is found that shifting in the habitat depth and living season can remove parts of the observed model–data mismatches in SST anomalies. The findings of this study give a clear reference for PMIP communities to use an appropriate ice sheet reconstructions with a more reliable ocean state as well as indicate that modelled and reconstructed temperature anomalies are to a large degree only qualitatively comparable, thus providing a challenge for the interpretation of proxy data as well as the model sensitivity to orbital forcing.

Description: The Miocene (23.03–5.33 Ma) was a time period with a warmer climate than today. During this period, changes in ocean gateways and atmospheric CO2 levels largely control ocean circulation and climate changes. However, the underlying ocean processes and dynamics are poorly understood and it remains a challenge to simulate Miocene climate key characteristics such as pronounced polar warming and a reduced meridional temperature gradient. By applying state-of-the-art fully coupled atmosphere-ocean-sea-ice model approaches Miocene climate conditions at different atmospheric CO2 concentrations are simulated and thermohaline changes in response to the subsidence of Atlantic-Arctic gateways for various Greenland-Scotland Ridge (GSR) and Fram Strait (FS) configurations are investigated. For a singular subsidence of the GSR, warming and a salinity increase in the Nordic Seas and the Arctic Ocean is detected. As convection sites shift to the north of Iceland, North Atlantic Deep Water (NADW) is formed at cooler temperatures. The associated deep ocean cooling and upwelling of deep waters to the Southern Ocean surface can cause a cooling in the southern high latitudes. These characteristic responses to the GSR deepening are independent of the FS being shallow or deep. An isolated subsidence or widening of the FS gateway for a deep GSR shows less pronounced warming and salinity increase in the Nordic Seas. Arctic temperatures remain unaltered, but a stronger salinity increase is detected, which further increases the density of NADW. The increase in salinity enhances the contribution of NADW to the abyssal ocean at the expense of the colder southern source water component. These relative changes cause a negligible warming in the upwelling regions of the Southern Ocean. For a sill depth of ~1500 m, ventilation of the Arctic Ocean is achieved due to enhanced import of saline Atlantic water through a FS width of ~105 km. Moreover, at this width and depth, a modern-like three-layer stratification in the Arctic Ocean is detected. The exchange flow through FS is characterized by vertical separation of a low salinity cold outflow from the Arctic Ocean confined to a thin upper layer, an intermediate saline inflow from the Atlantic Ocean below and a cold bottom Arctic outflow. These characteristics are comparable to the present-day hydrography, in spite of significantly shallower and narrower FS configurations during the early Miocene, suggesting that the ventilation mechanisms and stratification in the Arctic Ocean have been similar. In simulations with different CO2 levels (280, 450 and 720 ppm) surface temperatures show the best fit to proxy reconstructions for atmospheric CO2 concentrations of 720 ppm, since in particular the high latitude cooling bias becomes least pronounced. For a CO2 increase from 280 to 450 ppm polar amplification is simulated in the northern high latitudes, which is stronger than for the same radiative CO2 forcing from 450 to 720 ppm. At higher CO2 levels the Miocene climate also shows a reduced climate sensitivity, since the warmest Miocene climate scenario with a CO2 level of 720 ppm is characterized by a seasonality breakdown in the Arctic Ocean. A pronounced warming in boreal winter is detected for a CO2 increase from 450 to 720 ppm, in contrast to a moderate boreal summer temperature increase. This change in the seasonal temperature signature is accompanied by a strong sea-ice concentration decline and enhanced moisture availability promotes cloud formation in the summer months. As a consequence the planetary albedo increases and dampens the temperature response to the CO2 forcing at a warmer Miocene background climate.