HOLIVAR2006 Abstracts
North Atlantic climate and ocean variability during the Holocene on multidecadal and longer time scales.
Eystein Jansen
Bjerknes Centre for Climate Research, Allegaten 55, Bergen 5007, Norway
Contact: Eystein Jansen (eystein.jansen@bjerknes.uib.no)
North Atlantic ocean and atmospheric heat transport strongly influence Eurasian and Arctic climates. Variability on decadal and longer time scales, probably associated with prominent climate modes such as the North Atlantic Oscillation (NAO) and the Atlantic Multidecadal Oscillation (AMO), as well as millennial-scale variability, characterise the Holocene. Evidence for these scales of variability exists in marine, terrestrial, and ice-core data, as well as in transient simulations with climate models. Orbital forcing characterises the long-term Holocene evolution, with considerable changes in sea-surface temperatures. There is a large latitudinal and seasonal spread in these changes. The "Early Holocene climate optimum" is a phenomenon restricted to mid-high latitude summer surface ocean conditions. There is no consistent period of Early Holocene warmth across all latitudes and in areas of the tropical ocean there is a warming trend through the Holocene. The further to the North, the shorter this warm phase lasted. Below the surface layer, the Early Holocene in the high latitude North Atlantic is colder than the Late Holocene, indicating that the warming was mainly a result of the direct radiational effect of the orbitally driven summer insolation anomaly, and not a result of stronger advection of warm water towards the North. Current evidence does not indicate a constant pacing of century to millennial variability through the Holocene. There is a clear manifestation of warm conditions in the high latitude areas in the period 1000 AD to 1200 AD, and a generally colder period 1500 AD to about 1900 AD, yet inside these periods climate varied substantially. The onset of Little Ice Age (LIA) conditions was time transgressive in the Nordic Seas and is marked by mean annual temperature reductions and more extensive sea ice cover. The century scale variability during the past 1200 years in the high latitude North Atlantic appears closely coupled with variations in the position of the Intertropical Convergence Zone (ITCZ) and low and high latitude temperatures appear anti-phased on these time scales. In some time series there are indications of coolings related to solar variability, but this picture is not consistent across a range of proxies.
Eystein is a Professor of palaeoceanography at the University of Bergen, and is Director of the Bjerknes Centre for Climate Research through which he interacts with physicists and climate modellers. Much of his research has been on understanding how changes in the large scale circulation of the Atlantic influences climate change. He has made a number of studies on abrupt climate change, when the ocean circulation changed suddenly and large climate changes occurred. Many of these were linked to changes in freshwater supply to the ocean during glacial times. Currently he is most interested in identifying how the tropics and the Arctic may interact and if the natural climate variability in and around the Atlantic is due to the strength of ocean heat transport. He has also made many studies on the origin of the ice ages. Eystein has also been involved in organising international scientific cruises, and serves on the steering committee of the PAGES program on past climate changes and other panels in his field of research. Currently he serves as co-ordinating lead author in the IPCC's 4th assessment report where he co-ordinates the chapter on past climates.