North Atlantic Oscillation Mode Research Articles

A 1500‐year record of North Atlantic storm flooding from lacustrine sediments, Shetland Islands (UK)

ABSTRACTSevere storm flooding poses a major hazard to the coasts of north‐western Europe. However, the long‐term recurrence patterns of extreme coastal flooding and their governing factors are poorly understood. Therefore, high‐resolution sedimentary records of past North Atlantic storm flooding are required. This multi‐proxy study reconstructs storm‐induced overwash processes from coastal lake sediments on the Shetland Islands using grain‐size and geochemical data, and the re‐analysis of historical data. The chronostratigraphy is based on Bayesian age–depth modelling using accelerator mass spectrometry 14C and 137Cs data. A high XRF‐based Si/Ti ratio and the unimodal grain‐size distribution link the sand layers to the beach and thus storm‐induced overwash events. Periods with more frequent storm flooding occurred 980–1050, 1150–1300, 1450–1550, 1820–1900 and 1950–2000 ce, which is largely consistent with a positive North Atlantic Oscillation mode. The Little Ice Age (1400–1850 ce) shows a gap of major sand layers suggesting a southward shift of storm tracks and a seasonal variance with more storm floods in spring and autumn. Warmer phases shifted winter storm tracks towards the north‐east Atlantic, indicating a possible trend for future storm‐track changes and increased storm flooding in the northern North Sea region.

Paleoenvironmental variability and anthropic influence during the last 7300 years in the western Mediterranean based on the pollen record of Cartagena Bay, SE Spain

In this paper, we conduct a palynological analysis of a high-resolution Holocene record from Cartagena Bay, southeast of the Iberian Peninsula, to establish paleoenvironmental variability of coastal areas in the western Mediterranean region at a centennial-scale over the last 7300 years. Statistical analysis of four palynozones allows reconstruction of paleotemperature and paleohumidity conditions. Pinus , steppic, xerophilous, and Mediterranean taxa persisted continuously through the record, and only during periods of increased humidity did deciduous and Mediterranean taxa expand (Zones II, subzone IIIb). Cooler and dry conditions favored the development of Cupressaceae and scrubs between 7300 and 7000 yr cal BP. The mid-Holocene (Northgrippian) mesophytic optimum took place between 6800 and 4000 yr cal BP during which time a Mediterranean climate was present and open forest developed, predominantly consisting of Mediterranean taxa and deciduous trees. The gradual rise in aridity in the Meghalayan (4000–1700 yr cal BP) led to Mediterranean forest being replaced by steppic and xerophilous vegetation, a change related mostly to a decrease in summer insolation, with superimposed centennial-scale variability in humidity. In parallel with forest degradation caused by increasing aridity, the record shows marked evidence of human influence since 4000 yr cal BP, which accelerated the progression of open landscapes from the Chalcolithic onwards, this change being especially marked during the Roman period. Significant denudation of the landscape can be attributed to the use of fire, as well as due to agriculture and grazing, with a major contributor being intense metallurgical and mining activity in the area. The Cartagena Bay record reported herein shows centennial-scale oscillations in humidity and temperature that correlate with well-known climatic events during the late Holocene in the western Mediterranean region, synchronous with variability in solar and atmospheric dynamics. The alternation of persistent North Atlantic Oscillation modes is likely to have played a key role in regulating humid–arid periods. • Pollen content allowed the paleoenvironmental reconstruction during the Holocene. • Centennial-scale oscillations in humidity & temperature correlate with climatic events. • The mid-Holocene mesophytic optimum took place between 6800 and 4000 yr cal BP. • Aridity and xerophytization occurred between 4000 and 1700 yr cal BP. • Human influence accelerated the progression of open landscapes since the Chalcolithic.

Regional perspectives in Eurasian snow - Indian monsoon relationship: An observational study

Relationship between Indian summer monsoon rainfall and Eurasian snow in the preceding seasons is studied extensively. However, the spatial structures of this snow-monsoon relation have rarely been investigated. The present study is aimed at diagnosing nature of observed linkage between Eurasian winter snow and spatial distribution of following summer monsoon rainfall over India. Eurasian snow association with spatial distribution of rainfall over India features a tri-polar pattern of significant negative-positive-negative correlations over the northern, east-central and peninsular regions of India respectively. The physical mechanism underlying snow-monsoon connection is envisaged through two pathways. One of the channels is through Eurasian winter snow directly affecting the monsoon by modulating large scale circulation, while the other way is indirectly through Tropical Eastern Pacific (TEP) Sea Surface Temperatures (SSTs). Enhanced Eurasian wintertime snow is associated with warm TEP SSTs from preceding winter through to the following summer, leading to anomalous large scale zonal circulation during summer monsoon season, thereby affecting rainfall over India. The spatial facets of snow-monsoon relationship remain unaltered by Indian Ocean Dipole and North Atlantic Oscillation modes.

Great Blue Hole (Lighthouse Reef, Belize): A continuous, annually-resolved record of Common Era sea surface temperature, Atlantic Multidecadal Oscillation and cyclone-controlled run-off

The Holocene (<11.7 kyr BP) is characterized by several periods of distinct climate changes. Some of these climate variations had extensive effects on mankind and coincided with demises of extinct high civilizations. Annually-resolved climate reconstructions will certainly play an increasingly important role in public perception, when applying these past patterns to the recent climate debate. The future consequences of the ongoing climate crisis are still challenging to predict, due to the lack of comprehensive, annually-resolved and continuous sea-surface temperature (SST) data. Our 8.55 m long sediment core from the bottom of the Great Blue Hole (Lighthouse Reef, Belize) provides an annually-resolved, continuous and unique south-western Caribbean climate record for the last 1885 years. The varved, “lake-like” sinkhole successions of marine carbonates encompass approximately the entire Common Era (0 CE-modern), a time window, which is key for studying climatic variations and their effects on human society. Our SST record is based on stable isotopes (δ18O) and molecular proxy applications (TEX86). Throughout the Common Era, oxygen isotopes (δ18O) and TEX86 data imply a general SST rise of 0.5 °C and 1.3 °C, respectively, within the south-western Caribbean. The modulation of SST within the Common Era time series likely operated on two different time levels: (1) Solar (e.g., “Gleissberg Cycles”) and volcanic activity triggered climate changes, which in turn induced responses of the Atlantic Multidecadal Oscillation (AMO), the North Atlantic Oscillation (NAO) and the El-Niño-Southern Oscillation (ENSO). Their changing mode of action has been identified as a likely origin of the primary, i.e., centennially scaled SST variability. We suspect long-term positive AMO and NAO modes as the primary key control mechanisms of the Dark Ages Cold and Medieval Warm Period SST patterns. ENSO mode modulation likely exerted primary control on regional SST variability during the Little Ice Age and the Modern Global Warming. (2) Our δ18O data further indicate a striking secondary control on multi-decadal time scales: δ18O variations occur with 32–64 years periodicity. This signal is clearly evidence of SST modulation controlled by AMO phase changes (50–70 years) over almost the entire Common Era. Our carbon isotope record (δ13C) exhibits two remarkable negative anomalies and a long-term up-core decreasing trend. The first excursion (drop of 0.5‰) occurred with the onset of the Medieval Warm Period, which is reconstructed to be a peak time in south-western Caribbean tropical cyclone (TC) activity. This overlap is stressing a potential context between TC activity, enhanced coastal run-off and increased soil-erosion reflected by 13C-depleted carbon isotopes. A second anomaly (>1900 CE) is more likely the result of the “Suess Effect” (anthropogenic impact of the Industrial Revolution on carbon isotopes composition) than another reflection of a TC peak activity interval.

Wintertime Arctic Oscillation and North Atlantic Oscillation and their impacts on the Northern Hemisphere climate in E3SM

The characteristics of the wintertime Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) and their impacts on climate variability over the Northern Hemisphere are important metrics for evaluating a climate system model. Observational analyses reveal that the horizontal and vertical structures in the AO and NAO exhibit a meridional dipole and a large-scale barotropic pattern between the Arctic and mid-latitudes. Historical model simulations from the Energy Exascale Earth System Model (E3SM-HIST) are used to identify how well it captures these major climate modes. It is found that the simulated AO and NAO modes have spatial structures similar to the observed features. In addition, the observed frequency bands in the AO and NAO-related time variability are captured well in the E3SM-HIST simulation. Associated with the positive phase in wintertime AO and NAO, zonal flow and warm advection in mid-latitude continents are enhanced, along with stronger cold flow from enhanced northerly winds over high latitudes. These features are linked to the atmospheric circulation pattern reflected by lower SLP anomalies over the Arctic and higher SLP anomalies over the mid‐latitudes. In E3SM-HIST, these spatial associations and main structural features are analogous to those in observations. In the time-height evolution related to winter AO and NAO modes, it can also be seen that the simulations reproduce the downward propagating patterns in observations. Nevertheless, the vertical structures associated with AO and NAO in E3SM-HIST exhibit substantial biases in the lower stratosphere. The cause of these stratospheric biases is investigated using the strength of climatological stratospheric polar vortex (SPV) and wave activity fluxes. The results herein suggest that E3SM-HIST has a reasonable skill in reproducing the observed characteristics related to the winter AO and NAO, although there exist systematic biases in the associated climate variability.

Targeted Observations for Improving Prediction of the NAO Onset

Based on the viewpoint that the North Atlantic Oscillation (NAO) has an intrinsic timescale of approximate two weeks and can be treated as an initial value problem, targeted observations for improving the prediction of the onset of NAO events are investigated by using the conditional nonlinear optimal perturbation (CNOP) method with a quasi-geostrophic model. The results show that flow-dependent sensitive areas for the prediction of NAO onset are mainly located over North Atlantic and its upstream regions. Targeted observations over the main sensitive areas could improve NAO onset prediction in most cases (approximately 75%) due to reduced errors in anomalous eddy vorticity forcing (EVF) projection in the typical NAO mode. Moreover, a flow-independent sensitive area is determined based on the winter climatological flow, which is located over North America and its adjacent ocean. The NAO onset prediction can also be improved by targeted observations over the flow-independent sensitive area, but the skill improvement is somewhat lower than that derived from observations over the flow-dependent sensitive area. The above results indicate that targeted observations over sensitive areas identified by the CNOP method can help to improve the onset prediction of NAO events.

Subseasonal Week 3–5 Surface Air Temperature Prediction During Boreal Wintertime in a GFDL Model

AbstractWith a Geophysical Fluid Dynamics Laboratory (GFDL) coupled model, the subseasonal prediction of wintertime (December–February) surface air temperature (SAT) is investigated through the analysis of 11‐year hindcasts. Significant subseasonal week 3–5 correlation skill exists over a large portion of the global land domain, and the predictability originates primarily from the eight most predictable SAT modes. The first three modes, identified as the El Niño‐Southern Oscillation mode, the North Atlantic Oscillation mode, and the Eurasia Meridional Dipole mode, can be skillfully predicted more than 5 weeks in advance. The North Atlantic Oscillation and Eurasia Meridional Dipole modes are strongly correlated with the initial stratospheric polar vortex strength, highlighting the role of stratosphere in subseasonal prediction. Interestingly, the Madden‐Julian Oscillation is not essential for the subseasonal land SAT prediction in the Northern Hemisphere extratropics. The spatial correlation skill exhibits considerable intraseasonal and interannual fluctuations, indicative of the importance to identify the time window of opportunity for subseasonal prediction.

Winter precipitation changes during the Medieval Climate Anomaly and the Little Ice Age in arid Central Asia

The strength of the North Atlantic Oscillation (NAO) is considered to be the main driver of climate changes over the European and western Asian continents throughout the last millennium. For example, the predominantly warm Medieval Climate Anomaly (MCA) and the following cold period of the Little Ice Age (LIA) over Europe have been associated with long-lasting phases with a positive and negative NAO index. Its climatic imprint is especially pronounced in European winter seasons. However, little is known about the influence of NAO with respect to its eastern extent over the Eurasian continent.Here we present speleothem records (δ13C, δ18O and Sr/Ca) from the southern rim of Fergana Basin (Central Asia) revealing annually resolved past climate variations during the last millennium. The age control of the stalagmite relies on radiocarbon dating as large amounts of detrital material inhibit accurate 230Th dating. Present-day calcification of the stalagmite is most effective during spring when the cave atmosphere and elevated water supply by snow melting and high amount of spring precipitation provide optimal conditions. Seasonal precipitation variations cause changes of the stable isotope and Sr/Ca compositions. The simultaneous changes in these geochemical proxies, however, give also evidence for fractionation processes in the cave. By disentangling both processes, we demonstrate that the amount of winter precipitation during the MCA was generally higher than during the LIA, which is in line with climatic changes linked to the NAO index but opposite to the higher mountain records of Central Asia. Several events of strongly reduced winter precipitation are observed during the LIA in Central Asia. These dry winter events can be related to phases of a strong negative NAO index and all results reveal that winter precipitation over the central Eurasian continent is tightly linked to atmospheric NAO modes by the westerly wind systems.

Appraising timing response of paleoenvironmental proxies to the Bond cycle in the western Mediterranean over the last 20 kyr

The timing of climate responses to the Bond cycle is investigated in the western Mediterranean. Periodicities had been previously reported in a marine sediment record from this region spanning the last 20 kyr, and registered by diverse paleoenvironmental proxies, in particular those associated with terrigenous input, redox conditions, productivity, sea surface temperature (SST) and salinity. Further cross-spectral analyses on these time series reveal leads–lags in the 1400 year climate cycle. Considering as reference a terrigenous input proxy (the K/Al ratio), all the paleoenvironmental proxies displayed time shifts varying from ca. 700 year to ca. 350 year. SST and salinity variations show a first leaded response with the inflow of cold and less salty Atlantic waters. Followed by a time lead of 525 year, progresively arid conditions with an increase of eolian dust transport to the area, given by the Zr/Al signal, are observed. The intensification of dust transport could have triggered a latest biological response, lead by 350 year, with an increase of productivity, as suggested by the Ba/Al ratio. Lastly changes in the Mediterranean thermohaline circulation, indicated by a selected redox proxy (the U/Th ratio), are observed. These results support that the oceanic response triggered the atmospheric response to the Bond cycle in the western Mediterranean. Changes in the North Atlantic Oscillation mode and in the Inter-Tropical Convergence Zone migrations with variations in the monsoon activity or Saharan winds system, are considered as main forcing mechanisms, with a complex relationship of the involved phenomena.

A late-Holocene multi-proxy record from the northern Norwegian margin: Temperature and salinity variability

To elucidate the natural variability of Atlantic and Coastal water, a late-Holocene multi-proxy analysis is performed on a marine sediment core from the northern Norwegian margin. This includes planktic foraminiferal fauna and their preservation indicators, stable isotopes (δ18Oc, δ13C), sub-surface temperature (SSTMg/Ca) and salinity (SSS) records based on paired Mg/Ca and δ18Oc measurements of Neogloboquadrina pachyderma and transfer function–derived sub-surface temperatures (SSTTransfer). The record shows a general cooling with subtle fluctuating palaeoceanographic conditions, here attributed to shifting North Atlantic Oscillation (NAO) modes. Period I (ca. 3500–2900 cal. yr BP) is strongly influenced by Coastal water and stratified water masses, possibly correlating to negative NAO conditions. During period II (ca. 2900–1600 cal. yr BP), dominating warm Atlantic water might be linked to a positive NAO mode and the Roman Warm Period. A renewed influence of Coastal water is observed throughout period III (ca. 1600–900 cal. yr BP). Stable and colder SST values potentially correlate to the Dark Ages and are here attributed to negative NAO conditions. Within period IV (ca. 900–550 cal. yr BP), the core site experienced a stronger influence of Atlantic water which might be because of the positive NAO conditions correlating to the ‘Medieval Warm Period’. Additionally, an inverse correlation in Atlantic water influence between the eastern and western Atlantic Ocean is observed throughout periods II, III and IV. This Atlantic oceanographic see-saw pattern is attributed to an opposite climatic response to changing NAO conditions arguing for a coupling between ocean and atmosphere.

The complexity of millennial-scale variability in southwestern Europe during MIS 11

Climatic variability of Marine Isotope Stage (MIS) 11 is examined using a new high-resolution direct land–sea comparison from the SW Iberian margin Site U1385. This study, based on pollen and biomarker analyses, documents regional vegetation, terrestrial climate and sea surface temperature (SST) variability. Suborbital climate variability is revealed by a series of forest decline events suggesting repeated cooling and drying episodes in SW Iberia throughout MIS 11. Only the most severe events on land are coeval with SST decreases, under larger ice volume conditions. Our study shows that the diverse expression (magnitude, character and duration) of the millennial-scale cooling events in SW Europe relies on atmospheric and oceanic processes whose predominant role likely depends on baseline climate states. Repeated atmospheric shifts recalling the positive North Atlantic Oscillation mode, inducing dryness in SW Iberia without systematical SST changes, would prevail during low ice volume conditions. In contrast, disruption of the Atlantic meridional overturning circulation (AMOC), related to iceberg discharges, colder SST and increased hydrological regime, would be responsible for the coldest and driest episodes of prolonged duration in SW Europe.

Daily precipitation in Northern Iberia: Understanding the recent changes after the circulation variability in the North Atlantic sector

AbstractTrends in the characteristics of daily precipitation over Northern Iberia (NIB) are analyzed after 14 daily records covering the last 40 years plus seven century‐length time series. Results reveal an evolution to drier conditions with subregional variations: in western and central subregions, the decrease in both the occurrence and the intensity of wet days results in a statistically significant decline of total precipitation. Contrary to other regions in the Iberian Peninsula, the lack of correlation between the North Atlantic Oscillation (NAO) index and the observed rainfall anomalies within NIB suggests that the dominance of a positive NAO mode from the late 1970s to 2002 cannot explain the trends. The land‐sea and mountain distribution creates sharp rainfall transitions depending on the pathways of the moisture‐laden winds. Consequently, NIB and other mountainous regions in the margins of the European continental water divide are sensitive to small changes in the NAO pressure centers and can exhibit both positive and negative rainfall anomalies for each NAO mode. A novel methodology for identifying changes in the NAO modes, consistent with the observed pressure/precipitation anomalies at both continental and regional scales, is presented. After the disaggregation of each dominant NAO wet season into two categories, the main rainfall changes in the NIB region can be explained in the context of the NAO variability during the last decades. The reported (sub)regional rainfall differences stress the need of caution when using NAO reconstructions, based in site specific rainfall anomalies, for the interpretation of the past climate precipitation variability in areas of complex terrain.

Late Holocene precipitation and temperature changes in Northern Europe linked with North Atlantic forcing

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 66:37-48 (2015) - DOI: https://doi.org/10.3354/cr01331 Late Holocene precipitation and temperature changes in Northern Europe linked with North Atlantic forcing Tomi P. Luoto1,2,*, Liisa Nevalainen2 1Division of Geology and Geochemistry, Department of Geosciences and Geography, University of Helsinki, PO Box 64 FI-00014, Finland 2Division of Aquatic Sciences, Department of Biological and Environmental Science, University of Jyvaskyla, PO Box 35 FI-40014, Finland *Corresponding author: tomi.luoto@helsinki.fi ABSTRACT: Long-term paleoclimatic records are needed for understanding natural variability in the context of climate change. Two lake sediment cores were collected from eastern Finland from sites considered sensitive for precipitation and temperature, respectively, over the last ~3000 yr. The paleoclimatic reconstructions based on fossil Chironomidae (Insecta: Diptera) assemblages were considered against a North Atlantic Oscillation (NAO) reconstruction to disentangle possible forcing mechanisms. Generally low effective precipitation was reconstructed between ~3000 and 1500 cal yr BP (calibrated years before present). A distinct stream flow event occurred between ~2400 and 2200 cal yr BP, and the highest effective precipitation occurred ~1300 cal yr BP. The Medieval Climate Anomaly (MCA) between ~1200 and 800 cal yr BP was dry, but the Little Ice Age (LIA) between ~700 and 200 cal yr BP had fluctuating precipitation. The temperature reconstruction depicted a late Holocene cooling trend, which ended at ~1300 cal yr BP when the warm MCA began. The MCA was followed by the cold LIA and the last 100 yr have been characterized by rapid warming. Present temperatures are higher than during any time period of the late Holocene. A general trend of positive NAO mode with elevated summer temperatures and dry hydroclimatic conditions prevailed during the MCA and a negative mode with reduced temperatures during the LIA. However, the NAO-hydroclimate relationship over eastern Scandinavia was not straightforward during the LIA, which varied in moisture dynamics. The records from this present study provide a unique climate archive from the crossing point of southern and northern and eastern and western air masses. KEY WORDS: Chironomidae · Climate change · Eastern Finland · North Atlantic Oscillation · Paleoclimatology Full text in pdf format PreviousNextCite this article as: Luoto TP, Nevalainen L (2015) Late Holocene precipitation and temperature changes in Northern Europe linked with North Atlantic forcing. Clim Res 66:37-48. https://doi.org/10.3354/cr01331 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 66, No. 1. Online publication date: October 06, 2015 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2015 Inter-Research.

Decadal wave power variability in the North‐East Atlantic and North Sea

AbstractEstimation of the long‐term behavior of wave climate is crucial for harnessing wave energy in a cost‐effective way. Previous studies have linked wave heights to the north‐south atmospheric pressure anomalies in the North Atlantic, suggesting that the wave climate fluctuates as a response to changes in zonal circulation in the atmosphere. We identify changes in wave power in the North‐East Atlantic that are strongly correlated to the dominant pressure anomalies, the North Atlantic Oscillation (NAO), and other modes. We present a reconstructed wave power climate for 1665–2005, using a combination of known and proxy indices for the NAO and other modes. Our reconstruction shows high interannual and multidecadal variability, which makes wave energy prediction challenging. This variability should be considered in any long‐term reliability analysis for wave energy devices and in power scheme economics.

Sensitivity of two Iberian lakes to North Atlantic atmospheric circulation modes

The North Atlantic Oscillation (NAO) exerts a major influence on the climate of the North Atlantic region. However, other atmospheric circulation modes (ACMs), such as the East Atlantic (EA) and Scandinavian (SCAND) patterns, also play significant roles. The dynamics of lakes on the Iberian Peninsula are greatly controlled by climatic parameters, but their relationship with these various ACMs has not been investigated in detail. In this paper, we analyze monthly meteorological and limnological long-term datasets (1950–2011 and 1992–2011, respectively) from two lakes on the northern and central Iberian Peninsula (Sanabria and Las Madres) to develop an understanding of the seasonal sensitivity of these freshwater systems to the NAO, EA and SCAND circulation modes. The limnological variability within Lake Sanabria is primarily controlled by fluctuations in the seasonal precipitation and wind, and the primary ACMs associated with the winter limnological processes are the NAO and the SCAND modes, whereas only the EA mode appears to weakly influence processes during the summer. However, Lake Las Madres is affected by precipitation, wind and, to a lesser extent, temperature, whereas the ACMs have less influence. Therefore, we aim to show that the lakes of the Iberian Peninsula are sensitive to these ACMs. The results presented here indicate that the lake dynamics, in some cases, have a higher sensitivity to variations in the ACMs than single local meteorological variables. However, certain local features, such as geography, lake morphology and anthropic influences, are crucial to properly record the signals of these ACMs.

Ecosystem response to human- and climate-induced environmental stress on an anoxic coastal lagoon (Etoliko, Greece) since 1930 AD

To better constrain the effects of anthropogenic impact on coastal wetlands with respect to natural variability, we here analyze annually laminated sediments from Etoliko lagoon (western Greece, Mediterranean Sea) spanning the last ~80 years. Sub-decadal-scale palynomorph (pollen and dinoflagellate cyst) and seasonal-scale palynomorph (microfacies and μ-XRF) analyses were carried out to investigate the evolution of the aquatic environment and the surrounding terrestrial ecosystem. Based on a robust age model, which was developed using varve counting and 137Cs dating, our results indicate that land-use changes have altered the vegetation dynamics and led to eutrophication of the aquatic environment particularly from the early 1980s onwards. In agreement with instrumental data and reports of fish mass mortality events, our varve composition and high-resolution element scanning data suggest that the ecosystem has been under unprecedented pressure since 1990 AD. In particular, the enhancement of anoxic conditions due to human-induced eutrophication is linked to high accumulation rates of organic matter, an increased presence of bacteria in sediment microfacies, and a decrease in the Fe/Mn ratio in the sediment. In addition, a change in varve type from calcite- to aragonite-dominated in 1983 and a higher Sr concentration during the 1990s indicate an increasingly saline aquatic environment. Comparison with meteorological data suggests that lower precipitation during a persistent positive North Atlantic Oscillation mode along with a gradual increase in mean summer temperature since the 1980s may have enhanced the saline conditions. These findings demonstrate that climate change can intensify the human impact on aquatic ecosystems. In conclusion, our analytical approach provides a valuable tool for evaluating the degree of degradation of Etoliko lagoon and the effectiveness of implemented management plans on the aquatic ecosystem, indicating that the efforts to restore its water circulation have only weakly contributed towards an environmental recovery.

Impact of late-Holocene aridification trend, climate variability and geodynamic control on the environment from a coastal area in SW Spain

A detailed pollen analysis has been carried out on two sediment cores taken from a marsh area located in the Doñana National Park, southwestern Spain. The studied sedimentary sequences contain a similar late Holocene record of vegetation and climate and show a progressive aridification trend since at least 5000 cal. yr BP, through a decrease in forest cover in this area. Long-term vegetation changes shown here (semi-desert expansion and Mediterranean forest decline) paralleled declining summer insolation. Decreasing summer insolation most likely impacted negatively on tree growing season as well as on winter precipitation in the area. Superimposed on the long-term aridification trend were multi-centennial scale periods characterized by forest reductions or increases in arid and halophytic plants that can be interpreted as produced by enhanced droughts and/or by local geodynamic processes. These are centered at ca. 4000, 3000–2500, and 1000 cal. yr BP, coinciding in timing and duration with well-known dry events in the western Mediterranean and other areas but could have also been generated by local sedimentary or geodynamic processes such as a marine transgression in a subsidence context and extreme wave events (EWEs). The alternation of persistent North Atlantic Oscillation modes probably played an important role in controlling these relatively humid–arid cycles.

Climate and anthropogenic factors influencing an estuarine ecosystem from NW Iberia: new high resolution multiproxy analyses from San Simón Bay (Ría de Vigo)

Two sedimentary sequences (coastal and subtidal) were studied in San Simón Bay (Ría de Vigo), situated on the Atlantic coast of NW Iberia. The coastal record is a shallowing upward sequence which evidences a locally-developed low marsh, situated below the current beach, and dated at the second half of the 4th century. During the following decades this low marsh was progressively replaced by an alder swamp which formed on it. This suggests an apparent stabilisation or slow-down of the relative sea-level (RSL), in this site, at the beginning of the Dark Ages (DA). The subtidal sequence studied reflects the main changes in the landscape, the hydrological conditions, climate and RSL affecting this part of NW Iberia during the last 1250 years. Evidence of changing dinocysts content in the sediment reveals that two centennial or decadal-scale episodes existed of shelf marine waters more intensely penetrating inside the bay: between the 15th-18th centuries and at ca 1800–1930 AD. Besides, we related different proxies with the occurrence of four main climatic stages, namely the previously described Dark Ages (DA, ca 350–750 AD), the Mediaeval Climatic Anomaly (MCA, ca 750–1100 AD) and the Little Ice Age (LIA. ca 1500–1930 AD); in addition we propose a regional MCA/LIA transition (ca 1100–1500 AD) that it has not been previously described. Our environmental characterization indicates a persistent North Atlantic Oscillation (NAO) negative mode domain in Ría de Vigo during the MCA, but this became weaker during the LIA and, probably, also during the earlier DA. NAO mode become more irregular during the MCA/LIA transition, generally persisting in dominant negative mode except for a phase of minor upwelling intensification, at ca 1150–1350 AD, which mainly affected the external parts of the ria. We postulate that an almost simultaneous phase (ca 1100–1350 AD) of stronger continental contribution in the sediments may be related to increasing storm intensities, probably linked to a reinforcement of the Easter Atlantic (EA) pattern; and also that the intertidal/supratidal ecosystems inside San Simón Bay may have extended further in the past, at least towards the end of the 5th century, and between ca 1050–1350 AD and ca 1450–1750 AD. A number of local historical references are consistent with our palaeoecological data and so support the chronology proposed as well as many of the environmental changes reconstructed. This good agreement will help in the interpretation of other analogous sequences extending back in time.

Middle- to late-Holocene storminess in Brittany (NW France): Part II – The chronology of events and climate forcing

This study focuses on the recurring climate conditions required for the largest storms occurring in NW France (Brittany). It is based on the analysed records of storm events along Western Brittany coast (see Part I). In this manuscript (Part II), storm recurrence is explored along with forcing mechanisms. Periods of more frequent storm events over the two last centuries are analysed first in order to link these events with possible forcing mechanisms (North Atlantic Oscillation (NAO) and Atlantic Multidecadal Oscillation (AMO) modes) triggering the most destructive storms. Then, palaeostorm events are discussed at the Holocene scale, from 6000 yr BP to present, to verify the forcing mechanisms. Most recorded events appear to be linked with cooling episodes, mostly in winter, a transition to or from a negative winter NAO mode, a positive AMO mode. Extreme storms occur immediately prior to the ‘Medieval Warm Period’ (MWP). Maximum effects are reached prior to the onset of the MWP and during the Maunder and Dalton solar minima. Low storm activity occurred during the Spörer Minimum linked to an acceleration of the Atlantic Meridional Overturning Circulation (AMOC). Main storm triggers seem to correspond to a positive AMO mode with an unstable jetstream configuration driving a negative NAO. In this study, four specific weather configurations were defined to explain each type of recorded storminess. The strongest storms correspond to low AMO and decennial-negative NAO modes (e.g. ‘Little Ice Age’), or high AMO in association with dominant low NAO modes, as during the early Middle Age and present-day period. Fresh or warm oceans in association with a positive NAO mode are stormy but with very low sting storms frequency. Although in agreement with the orbital forcing and the Holocene glacial history, increasing storm frequency and intensity is most probably partly biased by continuous sea-level rise and resulting erosion.

Flood magnitudes in the Tagus River (Iberian Peninsula) and its stochastic relationship with daily North Atlantic Oscillation since mid-19th Century

Summary This paper presents a new methodological approach in the analysis of the influence of the North Atlantic circulation on the flood magnitude of the Tagus River, the largest Atlantic draining river of the Iberian Peninsula. This methodology is based on Correspondence Analysis viewed as a qualitative regression tool. A daily scale database (sea level atmospheric pressure, river discharge, rainfall) was used for this study. The selected streamflow station, Vila Velha de Rodao (Portugal, near the Spanish border), holds the longest continuous daily river discharge register of the Iberian Peninsula (over 160 years, since 1852). The annual maximum flood magnitudes were classified into seven categories according to their specific recurrence intervals (T). The qualitative regression approach was used to relate annual peak flood occurrence with the North Atlantic Oscillation (NAO) index mode (positive or negative) registered, during the preceding 40 days (divided in 8 successive 5-day periods). Large floods (categories 1–2 of T > 50 years and category 3, T ∼25–50 years) were found to be associated with a very high frequency of the negative NAO mode during the initial 20–25 days (within a total 40 days period length) before de flood peak. The lack of significant association of these flood categories with a predominant NAO mode during the immediately preceding 15 days, prior to the flood, suggest a time lag of 15 days before the peak. Minor flooding (category 7, T 50 years), indicating that catastrophic events will likely occur under the NAO negative daily and seasonal patterns defined in this work. The robustness of the model was assessed by the phenomenological analysis of misclassified Flood/NAO events. Results also indicate that flood management practises may benefit from improved climate scenarios and forecasting of the NAO index.