http://hdl.handle.net/2122/174 2013-05-22T18:15:26Z http://hdl.handle.net/2122/8588 Title: Predictability of the mid-latitude Atlantic meridional overturning circulation in a multi-model system Authors: Pohlmann, H.; Max-Planck-Institut fu ̈r Meteorologie,; Smith, D. M.; Met Office Hadley Centre; Balmaseda, M. A.; ECMWF; Keenlyside, N. S.; Geophysical Institute and Bjerknes Centre, University of Bergen; Masina, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Bologna, Bologna, Italia; Matei, D.; Max-Planck-Institut fu ̈r Meteorologie,; Muller, W. A.; Max-Planck-Institut fu ̈r Meteorologie,; Rogel, P.; CERFACS Abstract: Assessing the skill of the Atlantic meridional overturning circulation (AMOC) in decadal hindcasts (i.e. retrospective predictions) is hampered by a lack of obser- vations for verification. Models are therefore needed to reconstruct the historical AMOC variability. Here we show that ten recent oceanic syntheses provide a common signal of AMOC variability at 45°N, with an increase from the 1960s to the mid-1990s and a decrease thereafter although they disagree on the exact magnitude. This signal corre- lates with observed key processes such as the North Atlantic Oscillation, sub-polar gyre strength, Atlantic sea surface temperature dipole, and Labrador Sea convection that are thought to be related to the AMOC. Furthermore, we find potential predictability of the mid-latitude AMOC for the first 3–6 year means when we validate decadal hindcasts for the past 50 years against the multi-model signal. However, this predictability is not found in models driven only by external radiative changes, demonstrating the need for initialization of decadal climate predictions. 2012-12-31T23:00:00Z http://hdl.handle.net/2122/7629 Title: Marine biogeochemical responses to the North Atlantic Oscillation in a coupled climate model Authors: Patara, L.; CMCC; Masina, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Bologna, Bologna, Italia; Visbeck, M.; IFM-Kiel; Krahmann, G.; IFM-Kiel; Vichi, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Bologna, Bologna, Italia Abstract: In this study a coupled ocean‐atmosphere model containing interactive marine biogeochemistry is used to analyze interannual, lagged, and decadal marine biogeochemical responses to the North Atlantic Oscillation (NAO), the dominant mode of North Atlantic atmospheric variability. The coupled model adequately reproduces present‐day climatologies and NAO atmospheric variability. It is shown that marine biogeochemical responses to the NAO are governed by different mechanisms according to the time scale considered. On interannual time scales, local changes in vertical mixing, caused by modifications in air‐sea heat, freshwater, and momentum fluxes, are most relevant in influencing phytoplankton growth through light and nutrient limitation mechanisms. At subpolar latitudes, deeper mixing occurring during positive NAO winters causes a slight decrease in late winter chlorophyll concentration due to light limitation and a 10%–20% increase in spring chlorophyll concentration due to higher nutrient availability. The lagged response of physical and biogeochemical properties to a high NAO winter shows some memory in the following 2 years. In particular, subsurface nutrient anomalies generated by local changes in mixing near the American coast are advected along the North Atlantic Current, where they are suggested to affect downstream chlorophyll concentration with 1 year lag. On decadal time scales, local and remote mechanisms act contemporaneously in shaping the decadal biogeochemical response to the NAO. The slow circulation adjustment, in response to NAO wind stress curl anomalies, causes a basin redistribution of heat, freshwater, and biogeochemical properties which, in turn, modifies the spatial structure of the subpolar chlorophyll bloom. 2010-12-31T23:00:00Z http://hdl.handle.net/2122/7104 Title: Societal need for improved understanding of climate change, anthropogenic impacts, and geo-hazard warning drive development of ocean observatories in European Seas Authors: Ruhl, H. A.; NOCS; Andrè, M.; UPC; Beranzoli, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Çagatay, M. N.; ITU; Colaço, A.; Univ. Azores; Cannat, M.; IPGP; Dañobeitia, J. J.; CSIC-UTM; Favali, P.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia; Géli, L.; IFREMER; Gillooly, M.; IMI; Greinert, J.; NIOZ; Hall, P. O. J.; Univ. Goteborg; Huber, R.; MARUM; Karstensen, J.; Univ. Kiel; Lampitt, R. S.; NOCS; Larkin, K. E.; NOCS; Lykousis, V.; HCMR; Mienert, J.; Univ. Tromsø; Miranda, J. M.; Univ. Lisboa; Person, R.; IFREMER; Priede, I. G.; Univ. Aberdeen; Puillat, I.; IFREMER; Thomsen, L.; Jacobs Univ. Bremen; Waldmann, C.; MARUM Abstract: Society’s needs for a network of in situ ocean observing systems cross many areas of earth and marine science. Here we review the science themes that benefit from data supplied from ocean observatories. Understanding from existing studies is fragmented to the extent that it lacks the coherent long-term monitoring needed to address questions at the scales essential to understand climate change and improve geo-hazard early warning. Data sets from the deep sea are particularly rare with long-term data available from only a few locations worldwide. These science areas have impacts on societal health and well-being and our awareness of ocean function in a shifting climate. Substantial efforts are underway to realise a network of open-ocean observatories around European Seas that will operate over multiple decades. Some systems are already collecting high-resolution data from surface, water column, seafloor, and sub-seafloor sensors linked to shore by satellite or cable connection in real or near-real time, along with samples and other data collected in a delayed mode. We expect that such observatories will contribute to answering major ocean science questions including: How can monitoring of factors such as seismic activity, pore fluid chemistry and pressure, and gas hydrate stability improve seismic, slope failure, and tsunami warning? What aspects of physical oceanography, biogeochemical cycling, and ecosystems will be most sensitive to climatic and anthropogenic change? What are natural versus anthropogenic changes? Most fundamentally, how are marine processes that occur at differing scales related? The development of ocean observatories provides a substantial opportunity for ocean science to evolve in Europe. Here we also describe some basic attributes of network design. Observatory networks provide the means to coordinate and integrate the collection of standardised data capable of bridging measurement scales across a dispersed area in European Seas adding needed certainty to estimates of future oceanic conditions. Observatory data can be analysed along with other data such as those from satellites, drifting floats, autonomous underwater vehicles, model analysis, and the known distribution and abundances of marine fauna in order to address some of the questions posed above. Standardised methods for information management are also becoming established to ensure better accessibility and traceability of these data sets and ultimately to increase their use for societal benefit. The connection of ocean observatory effort into larger frameworks including the Global Earth Observation System of Systems (GEOSS) and the Global Monitoring of Environment and Security (GMES) is integral to its success. It is in a greater integrated framework that the full potential of the component systems will be realised. 2010-12-31T23:00:00Z http://hdl.handle.net/2122/2661 Title: Effects of NAO variability on the North Atlantic Ocean circulation Authors: Bellucci, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Bologna, Bologna, Italia; Richards, K. J.; International Pacific Research Center, University of Hawaii, Honolulu, Hawaii, USA. Abstract: The ocean response to an idealised North Atlantic Oscillation-like wind stress is evaluated as a function of the atmospheric forcing frequency. In order to elucidate the relationship between internal and forced variability, the NAO is modulated with two specific timescales, 10 and 50 years, which characterise the spectrum of the system’s internal variability. Different timescales of atmospheric variability select distinct sea surface temperature (SST) and large scale circulation patterns. Under a 50 year NAO forcing period, a lagged SST response is excited in the Gulf Stream extension region, which is consistent with the spinup of the gyre circulation. The thermohaline circulation varies in phase with the NAO and shows a strong sensitivity to the forcing frequency: a dipole mode of the overturning is excited by a 10 year modulation of the NAO, while an enhanced overturning response emerges under a 50 year NAO. With low enough lateral mixing the ocean exhibits an irregular response to a regular NAO-like forcing. 2005-12-31T23:00:00Z http://hdl.handle.net/2122/2595 Title: Aegean Sea Water Masses during the Early Stages of the Eastern Mediterranean Climatic Transient Authors: Gertman, I.; Israel Oceanographic and Limnological Research, Haifa, Israel; Pinardi, N.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Bologna, Bologna, Italia; Popov, Y.; Ukrainian Scientific Center of Sea Ecology, Odessa, Ukraine; Hecht, A.; Israel Oceanographic and Limnological Research, Haifa, Israel Abstract: The Aegean water masses and circulation structure are studied via two large-scale surveys performed during the late winters of 1988 and 1990 by the R/V Yakov Gakkel of the former Soviet Union. The analysis of these data sheds light on the mechanisms of water mass formation in the Aegean Sea that triggered the outflow of Cretan Deep Water (CDW) from the Cretan Sea into the abyssal basins of the eastern Mediterranean Sea (the so-called Eastern Mediterranean Transient). It is found that the central Aegean Basin is the site of the formation of Aegean Intermediate Water, which slides southward and, depending on their density, renews either the intermediate or the deep water of the Cretan Sea. During the winter of 1988, the Cretan Sea waters were renewed mainly at intermediate levels, while during the winter of 1990 it was mainly the volume of CDW that increased. This Aegean water mass redistribution and formation process in 1990 differed from that in 1988 in two major aspects: (i) during the winter of 1990 the position of the front between the Black Sea Water and the Levantine Surface Water was displaced farther north than during the winter of 1988 and (ii) heavier waters were formed in 1990 as a result of enhanced lateral advection of salty Levantine Surface Water that enriched the intermediate waters with salt. In 1990 the 29.2 isopycnal rose to the surface of the central basin and a large volume of CDW filled the Cretan Basin. It is found that, already in 1988, the 29.2 isopycnal surface, which we assume is the lowest density of the CDW, was shallower than the Kassos Strait sill and thus CDW egressed into the Eastern Mediterranean. 2005-12-31T23:00:00Z http://hdl.handle.net/2122/2591 Title: Interannual to Decadal Climate Predictability in the North Atlantic: A Multi-Model-Ensemble Study Authors: Collins, M.; Hadley Centre, Met Office, Exeter, United Kingdom; Botzet, M.; Max-Planck-Institut für Meteorologie, Hamburg, Germany; Carril, A. F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Bologna, Bologna, Italia; Drange, H.; Nansen Environmental and Remote Sensing Center, and Bjerknes Centre for Climate Research, Bergen, Norway; Jouzeau, A.; CERFACS, Toulouse, France; Latif, M.; Max-Planck-Institut für Meterologie, Hamburg, and Leibniz-Institut für Meereswissenschaften, Kiel, Germany; Masina, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Bologna, Bologna, Italia; Otteraa, O. H.; Nansen Environmental and Remote Sensing Center, and Bjerknes Centre for Climate Research, Bergen, Norway; Pohlmann, H.; Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada; Sorteberg, A.; Bjerknes Centre for Climate Research, Bergen, Norway; Sutton, R.; Centre for Global Atmospheric Modelling, Reading, United Kingdom; Terray, L.; CERFACS, Toulouse, France Abstract: Ensemble experiments are performed with five coupled atmosphere–ocean models to investigate the potential for initial-value climate forecasts on interannual to decadal time scales. Experiments are started from similar model-generated initial states, and common diagnostics of predictability are used. We find that variations in the ocean meridional overturning circulation (MOC) are potentially predictable on interannual to decadal time scales, a more consistent picture of the surface temperature impact of decadal variations in the MOC is now apparent, and variations of surface air temperatures in the North Atlantic Ocean are also potentially predictable on interannual to decadal time scales, albeit with potential skill levels that are less than those seen for MOC variations. This intercomparison represents a step forward in assessing the robustness of model estimates of potential skill and is a prerequisite for the development of any operational forecasting system. 2005-12-31T23:00:00Z http://hdl.handle.net/2122/764 Title: Spatial and temporal analysis of the seasonal and interannual variability in the tropical Pacific simulated with a coupled GCM Authors: Álvarez García, J. F.; Departamento de Física, Facultad de Ciencias, Universidad de Alcalá, Madrid, Spain; Cabos Narváez, W. D.; Departamento de Física, Facultad de Ciencias, Universidad de Alcalá, Madrid, Spain; Beviá, M. J. O.; Departamento de Física, Facultad de Ciencias, Universidad de Alcalá, Madrid, Spain Abstract: In the first part of this work, the dominant time scales that explain the tropical variability of the first SINTEX simulation (ECHAM4(T30)-ORCA) are identified through a spectral analysis. Higher order spectral analysis is used to examine the interactions among these time scales. The time series analyzed are an average of sea surface temperature over the Niño3 region. The time scales obtained are compared with those identified in another coupled GCM simulation (ECHAM4(T42)-OPYC3). The higher importance of the biannual time scale in this last is explained partly by the strength of the coupling between the annual and the biannual time scales. There is no such strong coupling in the SINTEX simulation. Important differences among the generation of the simulated warm (or cold) event suggest the need of a systematic classification to isolate their relevant features. Therefore in the second part of this work, we address this problem. A space-time cluster analysis is performed on a data set built by collecting the values of the heat content anomalies in the tropical Pacific region, in the fifteen months previous to a peak in the Niño3 Index that has been identified as a ‘warm’ (or ‘cold’) event. In the case of the warm events, three types of generation schemes are found. In two of them, there are anomalies of heat content in the west, north and south of the equator, more than nine months before the events start. In the third case, the anomalies appear and grow in the central equatorial Pacific. Only two types are needed to classify the generation of cold events. Negative sea level height anomalies appear six months before the Niño3 Index reaches the (local) minimum. They are located north of the equator in one of the groups, and south of it in the other. Some of these characteristic traits also appear in observations of warm and cold events. 2002-12-31T23:00:00Z http://hdl.handle.net/2122/753 Title: Scale interactions on diurnal toseasonal timescales and their relevanceto model systematic errors Authors: Slingo, J.; NCAS Centre for Global Atmospheric Modelling, Department of Meteorology, University of Reading, U.K.; Inness, P.; NCAS Centre for Global Atmospheric Modelling, Department of Meteorology, University of Reading, U.K.; Neale, R.; NCAS Centre for Global Atmospheric Modelling, Department of Meteorology, University of Reading, U.K.; Woolnough, S.; NCAS Centre for Global Atmospheric Modelling, Department of Meteorology, University of Reading, U.K.; Yang, G.; NCAS Centre for Global Atmospheric Modelling, Department of Meteorology, University of Reading, U.K. Abstract: Examples of current research into systematic errors in climate models are used to demonstrate the importance of scale interactions on diurnal,intraseasonal and seasonal timescales for the mean and variability of the tropical climate system. It has enabled some conclusions to be drawn about possible processes that may need to be represented, and some recommendations to be made regarding model improvements. It has been shown that the Maritime Continent heat source is a major driver of the global circulation but yet is poorly represented in GCMs. A new climatology of the diurnal cycle has been used to provide compelling evidence of important land-sea breeze and gravity wave effects, which may play a crucial role in the heat and moisture budget of this key region for the tropical and global circulation. The role of the diurnal cycle has also been emphasized for intraseasonal variability associated with the Madden Julian Oscillation (MJO). It is suggested that the diurnal cycle in Sea Surface Temperature (SST) during the suppressed phase of the MJO leads to a triggering of cumulus congestus clouds, which serve to moisten the free troposphere and hence precondition the atmosphere for the next active phase. It has been further shown that coupling between the ocean and atmosphere on intraseasonal timescales leads to a more realistic simulation of the MJO. These results stress the need for models to be able to simulate firstly, the observed tri-modal distribution of convection, and secondly, the coupling between the ocean and atmosphere on diurnal to intraseasonal timescales. It is argued, however, that the current representation of the ocean mixed layer in coupled models is not adequate to represent the complex structure of the observed mixed layer, in particular the formation of salinity barrier layers which can potentially provide much stronger local coupling between the atmosphere and ocean on diurnal to intraseasonal timescales. 2002-12-31T23:00:00Z http://hdl.handle.net/2122/742 Title: Scale interactions in the tropical Atlanticvariability simulated with a coupled GCM Authors: Cabos Narváez, W. D.; Departamento de Física, Facultad de Ciencias, Universidad de Alcalá, Madrid, Spain; Álvarez García, J. F.; Departamento de Física, Facultad de Ciencias, Universidad de Alcalá, Madrid, Spain; Beviá, M. J. O.; Departamento de Física, Facultad de Ciencias, Universidad de Alcalá, Madrid, Spain Abstract: Warm and cold events in the Gulf of Guinea are an important feature in the interannual variability of the tropical Atlantic Ocean, and partly a manifestation of the equatorial Atlantic system's intrinsic variability. Due to the relatively reduced zonal extension of this ocean, the latter variability is comparatively weak and thus strongly modified by other factors at play, either local or remote, like the seasonal cycle or ENSO. We present here an analysis of the tropical Atlantic variability in a 100-year-long chunk of the output of a coupled GCM. Through it, we obtain a better understanding of this variability and of its interactions with the seasonal cycle and with the ENSO signal. Following hints in the observations, we separate warm or cold events of the simulation in a few types, according to their similarities and differences. This classification is carried out as a spatio-temporal cluster analysis of the values, from nine months before up to the peak of the event, of the heat content anomalies. This is an optimal variable to monitor the generation of the events. One of the warm event classes can be explained by ENSO interactions. One of the cold event types can be explained by this influence as well, while the seasonal interactions might explain the characteristics of another of them. 2002-12-31T23:00:00Z