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Multi-level assessment of ocean-climate dynamics: a gateway to interdisciplinary training and analysis

Final Report Summary - GATEWAYS (Multi-level assessment of ocean-climate dynamics: a gateway to interdisciplinary training and analysis)

Climate change affects societies and economies world-wide and consciousness of the issue has risen to levels not faintly anticipated twenty years ago. The most visible signs presented in the media are melting alpine glaciers and polar ice caps, including Arctic sea ice, and the accelerating desertification of currently still habitable landscapes such as in the sought-after southern Europe and Mediterranean region. The ocean is a silent player in the global change arena and receives far less attention, if only for a lack of immediately visible change. Ocean acidification is brought out by the media occasionally because it constitutes a potential threat to coral reef growth. Ocean circulation, however, is rarely considered a concern in Global Change debates. It is a major player in climate because ocean currents distribute heat between the climatic zones thus affecting a range of climatic parameters. The Gulf Stream in the North Atlantic is a key example as it carries warm waters from the tropics to the North Atlantic and the Arctic Ocean which drives temperatures in northwest Europe to levels considerably above those that are encountered elsewhere at similar latitudes away from the Gulf Stream influence. But other currents exist as well that receive far less attention. The Agulhas Current off southern Africa is one such current. It is considered the largest ocean current in the southern hemisphere, directly comparable to the Gulf Stream in dimension and climatic significance. The Current originates in the subtropical Indian Ocean and straddles along the eastern shores of Africa, and at the southern tip of the continent, immediately before entering the South Atlantic, it turns around and flows back into the Indian Ocean. This so-called retroflection follows a dynamics of its own and it is one of the most vigorous ocean circulation features encountered by seafarers. But some water escapes the retroflection and enters the South Atlantic in the form of mega-eddies, or so-called ‘Agulhas Rings’, which can be up to 400 km wide and several kilometers deep. The Rings carry warm and salty Indian Ocean water and cause a density anomaly across the South Atlantic. This stimulates dynamical adjustments of the Atlantic Ocean circulation including a strengthening or weakening of the Gulf Stream, depending on the strength of the density anomaly in the south. The significance of this current regime as a potential driver of climate in Europe is increasingly recognized, but several key parameters are not well understood. In a warming world how does the Agulhas Current respond to changing winds? Is there a connection between the Current’s strength and the amount of Agulhas water spilling into the Atlantic? How exactly does this spillage influence the Atlantic Ocean circulation and what are the controls on that influence?

The GATEWAYS project set out to find answers to some of those questions and at the same time provided a team of young researchers with multidisciplinary training in climate change research. It integrated ocean observations and processes with ocean circulation reconstructions and climate modelling. Beyond the influence of the Agulhas Current on the Atlantic circulation it also focused on the Current’s influence on southern African climate, including weather systems and rain fall patterns. GATEWAYS brought together an international group of marine and continental palaeoclimatologists who approached these topics from the perspective of changes in the past. Its concept was built on the recognition that variations of the natural climate in the past were of a magnitude and rapidity far beyond the changes seen over last one hundred years of direct instrumental observations. This should make it easy to detect variations and follow the ambient signals as they propagate with the Agulhas flow. Its research programme focused on the analysis of deep-sea sediments employing analytical techniques allowing to detect and, whenever possible, quantify variations of the Agulhas Current in the past. The reconstructions were combined with computer model experiments that served as reference for comparison with the analytical data.

There were several new and even unexpected outcomes. The palaeo-time series of temperature variability clearly highlighted that the Agulhas Current underwent rhythmic swings at the pace of the global ice age – warm age cycles. While this may not seem overly surprising it came as an unexpected result that the current reversal at the tip of Africa, the Agulhas retroflection, seemed to respond to variations of Antarctic climate that radiated its signals to subtropical latitudes and altered the patterns of westerly winds at mid-latitudes. Recurrent millennium-long episodes of wet climates on land were evidenced by river sediments found offshore and they incurred at the same time as the North Atlantic experienced extremely cold conditions which were stimulated by a weakened or fully collapsed Gulf Stream. These South African rainy phases were stimulated through atmospheric teleconnections triggered in the north that ultimately impacted the Agulhas Current warm water transports which increased evaporation and rainfall. It came as a big surprise that these humid episodes coincided with phases of technological and behavioural innovation in stone age South Africa hence demonstrating a possible linkage between Agulhas Current behaviour and the evolution of modern humans. The computer models did extremely well at simulating the Agulhas Current and its variability on appropriate time and space scales, and they were instrumental in highlighting the sensitivity of the leakage into the South Atlantic to changing winds. Comparisons between the palaeo-data and the model simulations led to significantly improved constraints on the palaeoclimatic interpretation and, most importantly, comparing the palaeo-data with the models the case strengthens of the Agulhas water spillage into the Atlantic influencing the overall Atlantic Ocean circulation including the Gulf Stream.

Of course, GATEWAYS did not answer all questions and there is much to be done still. A broad consensus exists in the climate research community that a fuller understanding of the ocean dynamics and climate sensitivity at all timescales is paramount to making progress with climate predictions. Collaboration between the climate modelling, observational oceanography and palaeo-data communities is essential in this endeavour. GATEWAYS picked up this challenge and demonstrated the extremely high value of research results that these communities can jointly achieve. Hopefully GATEWAYS instilled the kind of curiosity into the hearts of its young researchers that leads them to desire knowledge outside their own field of expertise and seek collaboration across all isles of the climate sciences.

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