Extending the instrumental record of North Atlantic Ocean climate variability

Climate variability affects nearly all natural systems and human activities. The northward flow of Atlantic surface waters that are transported into the Nordic Seas plays a fundamental role for heat transport to north-western Europe and the Arctic and for the meridional overturning circulation, which is an essential part of the large-scale ocean circulation. Understanding, and possibly forecasting climate changes, is of great importance. We need to identify the natural trends and potential cycles, how they interact, and the feedbacks that affect them to be able to separate the climate signal resulting from human impact from the natural background variability. However, in particular, information about the marine climate system is limited. In the North Atlantic and surrounding regions, such as Europe and the Arctic, the available instrumental observational data of the surface ocean and the atmosphere extend over only the last 100-150 years. Hence, the development of tools and data sets able to reconstruct the marine climate system back through time is urgently needed. In ECHO, we use the annual growth bands of the extremely long-lived bivalve species Arctica islandica to make long time series of bivalve growth at annual to sub-annual resolution back through time. Such time series will help us to study in detail processes in the Norwegian Sea along the axis of the inflowing Atlantic waters. Bivalve shells were collected on the Faroe Shelf. In total, close to 150 shells have been prepared for cross-dating. The chronology construction is based on 39 specimens (9 live-collected and 30 sub-fossil) and results in a master chronology, which covers the period from AD 1625-2013. The comparison between the growth index of the chronology and instrumental data suggests that the phytoplankton dynamics on the Faroe Shelf, as well as in the wider Faroese region, strongly influence the variability in A. islandica shell growth. The data show that most of the year-to-year variability in the growth rate is the result of on-shelf phytoplankton dynamics, superimposed on more extended periods of consistently increased or decreased phytoplankton concentrations in the wider Faroese region. Stable oxygen isotope-based temperature reconstructions also support the hypothesis that on-shelf phytoplankton dynamics strongly influence the shell growth. These reconstructions reveal a temperature signal that is related to spring and summer sea-surface temperatures, matching the timing of the phytoplankton bloom on the Faroe Shelf. An annually-resolved time series of bottom water temperatures has been produced for the period AD 1657-2009. The stable oxygen isotope-based reconstructed temperatures for this period show an overall trend of increasing temperatures towards today, which is interpreted to reflect the transition from relatively cold conditions during the Little Ice Age (LIA) towards the modern warming of Atlantic waters. Superimposed on this overall trend are several periods of warmer and cooler temperatures. From a geographical perspective, the water temperatures on the Faroe Shelf largely reflect the signal that is commonly observed in the wider Northern Hemisphere.

ECHO has been further integrating internationally leading expertise in Nordic Seas oceanography, biology, geochemistry, paleoclimate and molluscan sclerochronology. Within ECHO we have developed a tool that can genuinely extend the marine instrumental record in an area where the processes occurring here have potential global impact. We have also provided early-stage researchers with opportunities to produce outstanding data within an international network of first-class scientists. The long paleotemperature record we have developed within ECHO will be imperative in further work, for example within the PAGES 2k network project MULTICHRON that steams from ECHO.

The scope of ECHO is to provide a definitive breakthrough in our capacity to observe the long-term (century to millennial scale) development of key aspects of North Atlantic Ocean dynamics at annual to sub-annual resolution. In ECHO, we aim at developing molluscan sclerochronology and scleroclimatology to study the inflow of Atlantic water into the Nordic Seas at annual to sub-annual resolution on decadal to centennial time scales. The inflow of warm and saline Atlantic Water plays a key role in the global thermohaline circulation as well as being of fundamental importance for the oceans north of the Greenland-Scotland Ridge and the regional climate of northern Europe through the heat and salt advected northwards by the inflow (Hansen and Østerhus, 2000). Quantitative projections and predictions of future climate at various time scales are increasingly demanded from the scientific community, policymakers and other stakeholders. Climate models are used to make projections, but deficiencies in how they represent key aspects of climate variability remain a challenge. The shortness of available observations is, however, inadequate for an understanding of changes on longer, multi-decadal, time scales. Hence, there is a need of an alternate approach, capable of incorporating knowledge of climate system behaviour on longer time scales than those covered in instrumental observations. Knowledge of the natural climate variability is dependent on proxy records of environmental and climate variables recoded by natural archives. Marine proxy records for the ocean are both sparsely distributed and are poorly resolved in time. The identification and development of proxies for studying key ocean processes at annual to sub-annual resolution that can extend the marine instrumental record is therefore a clear priority for marine climate science.