Ocean - sea-ice - atmosphere teleconnections between the Southern Ocean and North Atlantic during the Holocene

The joint Indo-Norwegian research project OCTEL aimed to explore the ocean, sea-ice and atmosphere interactions both in the Southern Ocean and the northern North Atlantic in order to assess the manifestation of interhemispheric teleconnections and their influence on climate during last 11 700 years (the Holocene) with a special focus on the last 2 000 years. The first two objects of the project were to 1) improve our understanding on the sea surface temperature and sea-ice variability in the Southern Ocean and northern North Atlantic over the last 10 000 years, and 2) analyze the relation of oceanic and atmospheric variability captured in the marine and ice core data both in the southern and northern hemispheres. These objects were performed by analyzing fossil diatoms and generating quantitative reconstructions of sea surface temperature (SST) and sea-ice using statistical transfer function methods from five marine sediment cores taken from Svalbard, the North Atlantic and Southern Ocean. The reconstructions were compared with existing marine and ice core data in order to analyze the relation of oceanic and atmospheric variability both in the southern and northern hemispheres. In Svalbard, first time in history, we generated two SST and sea ice reconstructions using diatoms. The record from Krossfjorden demonstrates the trends of clear warming and decreasing sea ice during the last 60 years, and indicates that increased CO2 forcing together with ocean-atmosphere interaction were behind these trends. The reconstructions from a longer Holocene sediment core from Kongsfjorden indicate that the warmest temperatures with less sea ice prevailed in Svalbard 10600-9000 years ago indicating the Holocene Climate Optimum. Combined results from Krossfjorden and Kongsfjorden indicate that here the modern ocean surface conditions (temperatures, sea ice) have now reached the levels of the Holocene Climate Optimum. This is a significant result - in Svalbard recent human caused climate change already seems to correspond to the maximum values of natural climate change of the Holocene. In the North Atlantic, findings based on the 6100 years long SST reconstruction from South of Iceland highlight that atmospheric circulation changes are likely to cause pronounced variations in the exchange of heat in the North Atlantic, which may have consequences for and global ocean circulation and the North Atlantic climate. In the Southern Ocean, the 14000 years long SST reconstruction indicate that the warmer and stable conditions during the Holocene Climate Optimum 12000-9000 years ago correspond with Antarctic ice core temperature records, supporting that atmospheric and ocean temperatures varied in phase. The last 9000 years show a slow cooling trend with greater variability which may be connected with changes in the high latitude atmospheric circulation and Southern Ocean convection. In order to perform objects 3) assess the existence and magnitude of teleconnections between the Southern Ocean and the North Atlantic on different time-scales, and 4) model the dynamics of the ocean-sea ice-atmosphere system and teleconnections in the Atlantic Ocean, the achieved new data combined with previous climate data from different regions were compared by different methods and used for modelling. In comparisons on interhemispheric climate variability, the timing of the Holocene Climate Optimum is almost synchronous in the Southern Ocean and the Arctic Ocean (where the optimum prevailed over 9000 years ago), whereas it was delayed in the northern North Atlantic (the optimum 9500-6000 years ago). Main reasons for the Holocene Climate Optimum and differences between the regions were higher solar insolation and changes on ocean and atmospheric circulation. In the Southern Ocean and North Atlantic the climate optimum was a stable 3000 years long time interval, but it was characterized by higher climate variability including short-lived cold periods in the high Arctic. These results indicate that the Arctic is climatically more sensitive region, probably due to stronger ocean - sea ice - atmosphere interactions. In the northern North Atlantic, the climate variability is driven by variations of the Atlantic Meridional Overturning Circulation (AMOC) and atmospheric circulation. Modelling results indicate that AMOC tend to lead SST in the northern North Atlantic by several years. In the last object of the project, "5) Inform policy makers and the public of the project's results, and on the impact of regional climate changes to global climate change through teleconnections", the project has increased knowledge on the climate change in different regions and how the ocean - sea-ice - atmosphere systems of the Northern and Southern hemispheres are coupled together at different time-scales among the public (including schoolchildren). This work will continue after ending of the project.

In climate science, the project has increased knowledge about the complex climate system and climate change in different regions. The project has produced quantitative climate reconstructions which can be directly used in climate modelling. This enables more precise climate projections for the future, which will be beneficial to society's adaptation strategies. In addition to scientific community, the project has increased the knowledge on the complicated climate system and climate change among the public (including schoolchildren). The realization of these facts have long-term effect on the society.

The joint Indo-Norwegian research project OCTEL will contribute to PAGES (Past Global Changes) network in addressing past changes in the Earth System in a quantitative and process-oriented way in order to improve predictions of future climate and environment. OCTEL aims to explore the ocean, sea-ice and atmosphere interactions both in the Southern Ocean and the northern North Atlantic in order to assess the manifestation of interhemispheric teleconnections and their influence on climate during last 11 700 years (the Holocene) with a special focus on the last 2 000 years. As our knowledge on these questions are still very limited, the project will employ synchronized high-resolution marine data along with ice core data from high latitudes to help narrow this knowledge-gap. - OCTEL will produce new high-resolution SST and sea-ice reconstructions from the Southern Ocean for the Holocene and from the northern North Atlantic for the last two millennia. - OCTEL is a multidisciplinary and cost-effective project using already available ocean surface and ice core data, and combining paleo-reconstructions with modern modelling techniques. - The project will produce a large amount of new knowledge for the past ocean surface conditions and the climate system in general, and thus the project will potentially gain a high scientific impact in the climate community. OCTEL will inform policy makers and the public of the project's results on the impact of regional climate changes to global climate change through teleconnections. The joint project meets Norway's research policy both in polar research for 2014-2023 and in Antarctica for 2013-2022, as well as the agreement between the governments of India and Norway on cooperation in the fields of science and technology.