Negative NAO Research Articles

Synergistic effect of boreal autumn SST over the tropical and South Pacific and winter NAO on winter precipitation in the southern Europe

We find a positive phase of meridional tripole pattern of boreal autumn sea surface temperature anomalies over the tropical and South Pacific (TSPT+ ) has a synergistic effect with winter negative NAO (NAO−) on the amplitude and occurrence of winter precipitation in southern Europe (SEWP). This synergistic effect is attributed to the linear superposition and nonlinear effect of winter NAO− and preceding autumn TSPT+ . The autumn TSPT+ can persist into winter, and the winter TSPT+ with NAO− can stimulate a greater amount of Rossby wave energy propagating to the North Atlantic-Europe (NAE) region than without NAO−, and thereby synergistically regulating circulation with winter NAO−. Thus, winter NAO− and autumn TSPT+ have a synergistic effect on the northwest-southeast trending positive-negative-positive winter circulation over the NAE region. Driven by the intensified mean circulation, water vapor transports and storm activity over southern Europe are evidently strong, which is in favor of SEWP formation.

Significance of site-specific factors to peat accumulation indicated by differences in Holocene histories of dust delivery and peat deposition at two neighboring locations in the Altai Mountains, northwestern China

Peatlands can archive exceptional paleoenvironmental histories of dust delivery because of their uninterrupted and elevated accumulation rates, yet long-term records of dust deposition and its effect on peat accumulation are rare in the arid and semiarid Asian regions that can be major dust sources. We present here high-resolution Holocene peat accumulation and paleodust records from the Tielishahan (TLSH) and Halashazi (HLSZ) peatlands in the Altai Mountains of northwestern China. We find that although the two peatlands share common histories of external climate forcing and dust delivery, they have significantly different long-term rates of peat accumulations. Peat deposition initiated ~2 kyr earlier in the HLSZ peatland than in the TLSH peatland, exhibiting higher peat accumulation rates and greater mineral dust fluxes during the early Holocene, but it has constantly smaller values from the mid-Holocene to the present. The TLSH peatland has a higher peat accumulation rate and mineral dust flux during the early Holocene, lower values during the mid-Holocene, and increasing values during the latest Holocene. The elemental proxies in the two peat cores indicate that atmospheric deposition is the major factor in controlling delivery of mineral elements in HLSZ peatland, whereas elemental delivery in the TLSH peat record is affected by both the regional atmospheric dust flux and the water table under this sequence.We also find that peat accumulation is closely related to mineral dust deposition, as most Asian locations generally share similar patterns in long-term peat accumulation and dust deposition. We postulate that increased aridity in the dust sources of arid central Asia and stronger westerly winds induced by negative NAO variations are key drivers on greater dust deliveries. We further postulate that local factors, such as site-specific differences in permafrost thawing, the areas of the peatlands, and their hydrology are the main reasons for the asynchronous Holocene histories of these two neighboring peatlands. These local factors should be considered when interpreting long-term paleoclimate records from rates of peat accumulation and paleodust delivery.

Construction of a deltaic strandplain during the Roman period in the Tavoliere di Puglia plain and palaeoclimatic implications

In response to the accidental exhumation of three ancient trees by farmers, we conducted a multidisciplinary study based on the stratigraphic analysis of boreholes, carbon-14 dating, aerial photo interpretation, and analysis of palaeobotanical and archaeological evidences. We reconstructed the formation and evolution during Roman times of a first “continuous” and then “discontinuous” deltaic strandplain at the mouth of the Carapelle Stream in the Tavoliere di Puglia Plain—the second-largest plain in Italy. Ttwo main phases can be recognised in the evolution of the Carapelle deltaic strandplain: 1) a first phase, lasted until ca. the birth of Christ, was characterised by a regular and continuous construction of sand ridges one leaning against the other; 2) a second phase, lasted more or less from the birth of Christ to the termination of the construction of the deltaic strandplain, was characterised by the discontinuous construction of sand ridges/coastal barriers with the isolation of lagoons/ponds, and by evidences of alluvial events. The most probable climatic–environmental scenario to have formed the Roman-period deltaic strandplain implied that: 1) the first phase was triggered by a higher total amount of precipitations, but with less extreme alluvial events; 2) the second phase was triggered by a total amount of precipitation lower than the previous period, but with higher occurrence of extreme alluvial events and/or by extreme alluvial events separated by longer period of low precipitations. This second phase was enhanced by the opening of vegetation. The passage between the first and second phase of the Carapelle deltaic strandplain coincides with the passage from overall negative NAO index to an overall positive NAO index.

Changes in volume and geometry of the Austre Dahlfonna glacier (Spitsbergen island) in 2008–2019

Previously published geodetic mass balance data indicate glacier shrinkage in the Barentsburg area of Svalbard since the beginning of the 20th century on the decadal time scale. However, observations for shorter time spans allowing one to compute the inter-annual variability of the mass balance are scarce. The study presents results of ground-based GNSS and the GPR surveys of the Austre Dahlfonna glacier (2 sq km) located on Spitsbergen island, south of the town of Barentsburg. According to the GPR survey of spring 2019 at 50 MHz frequency, the area-averaged ice thickness was equal to 82 m, while the maximum was 170 m. The results confirm the polythermal structure of the glacier, with a layer of underlying temperate ice. Since the end of the Little Ice Age, the area of Austre Dahlfonna has halved. By comparing the GNSS survey results (the end of the melt season of 2019) with the co-registered archived remote sensing data (ArcticDEM strip of 2013 and S0 Terrengmodell of 2008), it was computed that, within the last 12 years (2008–2019), Austre Dahlfonna lost 16 % of its volume, which corresponds to a geodetic mass balance of –12.05 ± 0.85 m w. e. The mass loss in 2008–2013 (5.22 ± 0.37 m w. e.) was lower than in 2013–2019 (6.83 ± 0.48 m w. e.), which is in agreement with the ongoing direct measurements on the neighboring Austre Grønfjordbreen glacier and with the archipelago-wide mass-balance patterns. We demonstrate that the less intensive glacier mass loss, which occurred in 2005–2012 and was detected previously for the whole archipelago, definitely took place in the Barentsburg area as well. This time interval is characterized by the prevalence of a negative NAO phase (65 % of recurrence), which may indicate more frequent intrusions of colder Arctic air masses. This fact proves that the mass-balance variability of the Barentsburg area glaciers is governed in time spans of 5–10 years by regional-scale factors, presumably by shifts in the atmospheric circulation regimes.

Temporal and Spatial Variability in Contemporary Greenland Warming (1958–2020)

Abstract In this study, 2-m or near-surface air temperature (T2m) products from atmospheric reanalysis ERA5 and the regional climate model RACMO2.3p2 over Greenland are compared with observations from staffed stations and Automated Weather Stations (AWS). The results show the following: 1) Greenland experienced decadal periods of both cooling and warming during 1958–2020, with an inflection point around the mid-1990s, and no significant warming after ∼2005 except in the north and northeast. 2) In the full time series, the magnitude of the warming increases gradually from south to north, with peak warming found along the northeastern coast. 3) The most intense warming occurred in autumn and winter, notably in the northeast. 4) The correlations of T2m with the large-scale circulation indices NAO and GBI are highly significant, but they gradually weaken from southwestern to northeastern Greenland. Under the background of Greenland rapidly warming, the shift from positive to negative NAO (negative to positive GBI) is critical to the sudden warming in Greenland since the mid-1990s.

Evidence for anthropogenic, climatic and oceanographic variability off southwestern Morocco during the last three millennia

In order to obtain a better knowledge of oceanic changes in the Canary Upwelling region during the Late Holocene, marine surface samples and two sediment cores retrieved off southwestern Morocco, GeoB8601–3 and GeoB4223–1, were investigated for organic-walled dinoflagellate cysts (dinocysts). The results were compared with pollen, microcharcoal particles, geochemical records, stable hydrogen and carbon isotope composition (δ 2 H and δ 13 C) of plant-waxes of the same material. The dinocyst results indicate active upwelling prior to 750 BCE and a longer upwelling season between 750 BCE and 950 CE, which is attributed to increased northeast trade winds. After 950 CE, a marked increase of Lingulodinium machaerophorum and Gymnodinium species indicates high fluvial input probably due to a combination of climate (possibly negative NAO) and anthropogenic (terrestrial erosion related to Early Arab conquest) impacts. • We present a new compilation of modern dinocyst distribution off southwestern Morocco. • We investigate Late Holocene dinocyst records from two marine sediment cores. • We employ a multiproxy approach with pollen, microcharcoal and geochemical records. • An interplay of climate and anthropogenic controls were demonstrated.

The role of temperature and the NAO index in the changing snow-related variables in European regions in the period 1900-2010

Snow-related variables are analysed in the present paper in the period 1901‒2010 on the basis of the ERA-20C dataset. Relationships between different snow characteristics, temperature and the NAO index are investigated on monthly, yearly and decadal scales for eight regions within Europe representing different climatic types (i.e. oceanic, continental, polar) to analyse the differences and similarities between them depending on the climatic conditions. According to our results, the ratio of snow (i.e. snowfall compared to total precipitation) can reach 1 in winter in the colder, northern regions, whereas it is about 0.6 in the continental areas of Central Europe, even in the coldest months. During a strong positive phase of NAO more snow falls in the northern regions of Europe due to the large-scale circulation characteristics. When a negative NAO phase occurs, the temperature and snowfall anomalies are the opposite in northern Europe. The highest temperature values generally occurred after 2000, and the snowfall amount was smaller in the first decades of the 21st century compared to the previous decades. The relationship between temperature and snowfall is the strongest in autumn in the colder regions; in spring in the continental areas and in winter in the oceanic climate.

Potential causes of megadroughts in North China in the preindustrial period from the teleconnection perspective

The potential causes of megadroughts over North China (NC) in the preindustrial period were investigated using eight climate models evolved using CMIP5/PMIP3 and multiple proxy data. The full forcing experiments driven by both external forcing and internal variability show that megadroughts in NC were accompanied by extensive decadal drying over the Northern Hemisphere (NH), particularly in most of North America and Europe. The spatial pattern of hydroclimate matched well with the proxy-based illustration. However, such hydroclimate conditions over NH were difficult to reproduce in control experiments driven solely by internal variability. This finding suggests that external forcing may be a crucial factor leading to megadroughts in NC and simultaneous decadal drying over the NH. Moreover, external forcing may lead to climate cooling. Climate cooling weakened the pressure gradient from the West Pacific to East Asia and hence reduced the water vapor to land; meanwhile, local vertical downward movement occurred in the cooling climate background, which was unfavorable to air convection over NC. Both reduced water vapor and weakened air convection led to less precipitation. Meanwhile, there was a negative NAO, which intensified the eastward wave train at mid-latitudes. Consequently, the North American trough and European trough deepened, the northerlies in the post trough pattern intensified, and the water vapor from the high-latitude continent or polar areas was low, which was unfavorable to precipitation. This study implies that the megadrought in NC and simultaneous hydroclimatic spatial pattern over the NH may be underestimated in the CMIP5 climate prediction, which excludes external forcing.

Counteracting Contributions of the Upper and Lower Meridional Overturning Limbs to the North Atlantic Nutrient Budgets: Enhanced Imbalance in 2010

AbstractThe North Atlantic Basin is a major sink for atmospheric carbon dioxide (CO2) due in part to the extensive plankton blooms which form there supported by nutrients supplied by the three‐dimensional ocean circulation. Hence, changes in ocean circulation and/or stratification may influence primary production and biological carbon export. In this study, we assess this possibility by evaluating inorganic nutrient budgets for 2004 and 2010 in the North Atlantic based on observations from the transatlantic A05‐24.5°N and the Greenland‐Portugal OVIDE hydrographic sections, to which we applied a box inverse model to solve the circulation and estimate the across‐section nutrient transports. Full water column nutrient budgets were split into upper and lower meridional overturning circulation (MOC) limbs. According to our results, anomalous circulation in early 2010, linked to extreme negative NAO conditions, led to an enhanced northward advection of more nutrient‐rich waters by the upper overturning limb, which resulted in a significant nitrate and phosphate convergence north of 24.5°N. Combined with heaving of the isopycnals, this anomalous circulation event in 2010 favored an enhancement of the nutrient consumption (5.7 ± 4.1 kmol‐P s−1) and associated biological CO2 uptake (0.25 ± 0.18 Pg‐C yr−1, upper‐bound estimate), which represents a 50% of the mean annual sea–air CO2 flux in the region. Our results also suggest a transient state of deep silicate divergence in both years. Both results are indicative of a MOC‐driven modulation of the biological carbon uptake (by the upper MOC limb) and nutrient inventories (by the lower MOC limb) in the North Atlantic.

Nonlinear Response of Atmospheric Blocking to Early Winter Barents–Kara Seas Warming: An Idealized Model Study

AbstractWintertime Ural blocking (UB) has been shown to play an important role in cold extremes over Eurasia, and thus it is useful to investigate the impact of warming over the Barents–Kara Seas (BKS) on the behavior of Ural blocking. Here the response of UB to stepwise tropospheric warming over the BKS is examined using a dry dynamic core model. Nonlinear responses are found in the frequency and local persistence of UB. The frequency and local persistence of the UB increase with the strength of BKS warming in a less strong range and decrease with the further increase of BKS warming, which is linked to the UB propagation influenced by upstream background atmospheric circulation. For a weak BKS warming, the UB becomes more persistent due to its less westward movement associated with intensified upstream zonal wind and meridional potential vorticity gradient (PVy) in the North Atlantic mid-high latitudes, which corresponds to a negative height response over the North Atlantic high latitudes. When BKS warming is strong, a positive height response appears in the early winter stratosphere, and its subsequent downward propagation leads to a negative NAO response or increased Greenland blocking events, which reduces zonal wind and PVy in the high latitudes from North Atlantic to Europe, thus enhancing the westward propagation of UB and reducing its local persistence. The transition to the negative NAO phase and the retrogression of UB are not found when numerically suppressing the downward influence of weakened stratospheric polar vortex, suggesting a crucial role of the stratospheric pathway in nonlinear responses of UB to the early winter BKS warming.

Temporal variability of flooding events of Guadiana River (Iberian Peninsula) during the middle to late Holocene: Imprints in the shallow-marine sediment record

Sedimentological, geochemical and benthic foraminiferal proxies were used to interpret changes of depositional environments in a mud entrapment inside the Guadiana River paleo-valley, northern Gulf of Cadiz, to understand the temporal variability of fluvial flooding events and to detect patterns of latitudinal climatic variability in western Iberia.The period between ca. 5800 and ca. 1250 cal yr BP was characterized by slowly accumulating coarse-grained sediments, high content of biogenic sand components, and high abundances of shallow-water benthic foraminiferal species. After ca. 1250 cal yr BP, the sedimentary environment was dominated by fine-grained sediments, with high abundances of opportunistic benthic foraminiferal species and successful colonizers, and high values of geochemical ratios indicative of enhanced terrigenous supply. Sedimentation rates increased drastically over the last ca. 500 years.The genesis of these environments was mainly driven by the variable frequency of fluvial flooding events driven by the regional climate variability. Low terrigenous sediment input under dry conditions prevailed in the older sedimentary unit. Flood frequency and terrigenous sediment supply increased during the younger unit. The variability of the North Atlantic Oscillation at centennial scales highly influenced the sediment supply during the last ca. 500 years, with high terrigenous supply during negative NAO conditions. Our results corroborate an N-S gradient along the Atlantic Iberia during middle and late Holocene, with more humid conditions in the northwestern and drier conditions towards the southeast. The gradient weakened over the past ca. 500 years, with increased rainfalls and flood events during the Little Ice Age.

Relationship between Water Temperature of Polish Rivers and Large-Scale Atmospheric Circulation

The objective of the paper consisted in determining the effect of macroscale types of NAO, AO, EA, EAWR, SCAND, and AMO atmospheric circulation on changes in water temperature in Polish rivers. The study has made use of a broad body of hydrometeorological materials covering daily water temperature values for 96 water gauge stations located on 53 rivers and air temperature values for 43 meteorological stations. Percentage shares of positive and negative coefficients of correlation of annual, seasonal, and monthly circulation type indices with air and river water temperature were determined, demonstrating the character of teleconnection. Determinations were made of water temperature deviations in positive and negative phases of the analyzed indices from average values from the years 1971–2015, and their statistical significance ascertained. Research has shown that relations between the temperature of river waters in Poland and macroscale circulation types are not strong, however they are noticeable, sometimes even statistically significant, and both temporally and spatially diverse. NAO, AO, EA, and AMO indices are characterized by a generally positive correlation with temperature, whereas SCAND and EWAR indices are characterized by a negative correlation. Research showed a varying impact of types of atmospheric circulation, with their effectiveness increasing in the winter season. The strongest impact on temperature was observed for the positive and negative NAO and AO phases, when deviations of water temperature from average values are correspondingly higher (up to 1.0 °C) and lower (by a maximum of 1.5 °C), and also for the positive and negative SCAND phases, when water temperature are correspondingly lower (by a maximum of 0.8 °C) and higher (by 1.2 °C) than average values. The strongest impact on water temperature in summer, mainly in July, was observed for AMO. The results point to the complexity of processes determining the thermal regime of rivers and to the possibility of additional factors—both regional and local—exerting an influence on their temporal and spatial variability.

A decision‐tree approach to seasonal prediction of extreme precipitation in eastern China

AbstractSeasonal prediction of extreme precipitation has long been a challenge especially for the East Asian Summer Monsoon region, where extreme rains are often disastrous for the human society and economy. This paper introduces a decision‐tree (DT) method for predicting extreme precipitation in the rainy season over South China in April–June (SC‐AMJ) and the North China Plain in July–August (NCP‐JA). A number of preceding climate indices are adopted as predictors. In both cases, the DT models involving ENSO and NAO indices exhibit the best performance with significant skills among those with other combinations of predictors and are superior to their linear counterpart, the binary logistic regression model. The physical mechanisms for the DT results are demonstrated by composite analyses of the same DT path samples. For SC‐AMJ, an extreme season can be determined mainly via two paths: the first follows a persistent negative NAO phase in February–March; the second goes with decaying El Niño. For NCP‐JA, an extreme season can also be traced via two paths: the first is featured by “non El Niño” and an extremely negative NAO phase in the preceding winter; the second follows a shift from El Niño in the preceding winter to La Niña in the early summer. Most of the mechanisms underlying the decision rules have been documented in previous studies, while some need further studies. The present results suggest that the decision‐tree approach takes advantage of discovering and incorporating various nonlinear relationships in the climate system, hence is of great potential for improving the prediction of seasonal extreme precipitation for given regions with increasing sample observations.

Influence of the North Atlantic Oscillation on δD and δ18O in meteoric water in the Armenian Highland

The stable hydrogen (δD) and oxygen (δ18O) isotope composition of modern meteoric water are influenced by changes in global temperature, Rayleigh distillation processes, and moisture transport history. However, on a global scale the relative influence of these different meteorological variables is spatially heterogeneous. We collected daily precipitation samples from meteorological stations in the Armenian Highlands for a period of one year in order to determine the constraints on δD and δ18O and relationship to broader climatic conditions. These data are coupled with HYSPLIT back-trajectory analysis to determine how these compositions relate to moisture source and transport path. Multivariate analysis shows the strongest relationship between δD and δ18O and climate variables is temperature at the sampling site. Along with temperature, the intensity of the North Atlantic Oscillation, a pattern of atmospheric circulation which determines the strength and location of Northern Hemisphere midlatitude westerlies, is significant (p < 0.05) for both δ18O and δD values of precipitation. This correlation between NAO and isotopes in precipitation is also observed in four nearby GNIP stations from 2008 to 2015. Negative NAO phases are associated with weakened westerlies, which allows precipitation enriched in heavy isotopes from the Mediterranean and southerly sources to penetrate into the Armenian Highlands, whereas positive NAO phases are associated with stronger westerlies bringing more negative precipitation from the Black Sea. This relationship is most significant (p < 0.001) during the winter and spring months (December to May). These data demonstrate that paleo-archives of δD and δ18O in the Armenian Highlands could serve as a tool to measure changes in the NAO in the past, particularly when coupled with independent measurements of temperature.

Atmospheric dynamics associated with exceptionally dusty conditions over the eastern Mediterranean and Greece in March 2018

This study examines the atmospheric dynamics that facilitated frequent and intense dust storms over the eastern Mediterranean in March 2018 via the synergy of satellite observations, ground-based measurements and reanalysis. The negative NAO phase in March 2018 caused cold weather conditions over the central/west Europe and strengthened the temperature gradient between cold European and warm North-African air at all levels from the surface up to 300 hPa. Large negative mean sea-level pressure (MSLP) anomalies over the western Mediterranean and south Europe strengthened the sea-level pressure and geopotential-height gradients across the Mediterranean, resulting in intensification of the westerlies over the west/central Mediterranean and the development of the Sharav cyclones along the north African coast. In addition, enhanced south/southwesterlies dominated over the eastern Mediterranean, associated with cyclonic conditions and intense dust storms on several days in March 2018. These meteorological conditions resulted in an increase of ~0.3–0.4 in AOD (~100%) and ~200 μg m−3 in surface dust concentration over parts of the eastern Mediterranean, compared to the mean March climatology. The vertical aerosol profiles over the eastern Mediterranean show a large increase in the mean extinction coefficient (~200 Mm−1) due to dust loading at altitudes between ~1 and 4 km. The dust radiative impact caused a significant decrease (~−10 Wm−2) in the incoming solar radiation under clear skies and in the latent and sensible energy fluxes over the eastern Mediterranean in March 2018, thus modulating the radiation/energy budget.

Coastal primary productivity changes over the last millennium: a case study from the Skagerrak (North Sea)

Abstract. A comprehensive multi-proxy study on two sediment cores from the western and central Skagerrak was performed in order to detect the variability and causes of marine primary productivity changes in the investigated region over the last 1100 years. The cores were dated by Hg pollution records and AMS 14C dating and analysed for palaeoproductivity proxies such as total organic carbon, δ13C, total planktonic foraminifera, benthic foraminifera (total assemblages as well as abundance of Brizalina skagerrakensis and other palaeoproductivity taxa) and palaeothermometers such as Mg∕Ca and δ18O. Our results reveal two periods with changes in productivity in the Skagerrak region: (i) a moderate productivity at ∼ CE 900–1700 and (ii) a high productivity at ∼ CE 1700–present. During ∼ CE 900–1700, moderate productivity was likely driven by the nutrients transported with the warm Atlantic water inflow associated with a tendency for a persistent positive NAO phase during the warm climate of the Medieval Climate Anomaly, which continues into the LIA until ∼ CE 1450. The following lower and more variable temperature period at ∼ CE 1450–1700 was likely caused by a reduced contribution of warm Atlantic water, but stronger deep-water renewal, due to a generally more negative NAO phase and a shift to the more variable and generally cooler climate conditions of the Little Ice Age. The productivity and fluxes of organic matter to the seafloor did not correspond to the temperature and salinity changes recorded in the benthic Melonis barleeanus shells. For the period from ∼ CE 1700 to the present day, our data point to an increased nutrient content in the Skagerrak waters. This increased nutrient content was likely caused by enhanced inflow of warm Atlantic water, increased Baltic outflow, intensified river runoff, and enhanced human impact through agricultural expansion and industrial development. Intensified human impact likely increased nutrient transport to the Skagerrak and caused changes in the oceanic carbon isotope budget, known as the Suess effect, which is clearly visible in our records as a negative shift in δ13C values from ∼ CE 1800. In addition, a high appearance of S. fusiformis during the last 70 years at both studied locations suggests increased decaying organic matter at the sea floor after episodes of enhanced primary production.

The Importance of a Properly Represented Stratosphere for Northern Hemisphere Surface Variability in the Atmosphere and the Ocean

Major sudden stratospheric warmings (SSWs) are extreme events during boreal winter, which not only impact tropospheric weather up to three months but also can influence oceanic variability through wind stress and heat flux anomalies. In the North Atlantic region, SSWs have the potential to modulate deep convection in the Labrador Sea and thereby the strength of the Atlantic meridional overturning circulation. The impact of SSWs on the Northern Hemisphere surface climate is investigated in two coupled climate models: a stratosphere-resolving (high top) and a non-stratosphere-resolving (low top) model. In both configurations, a robust link between SSWs and a negative NAO is detected, which leads to shallower-than-normal North Atlantic mixed layer depth. The frequency of SSWs and the persistence of this link is better captured in the high-top model. Significant differences occur over the Pacific region, where an unrealistically persistent Aleutian low is observed in the low-top configuration. An overrepresentation of SSWs during El Niño conditions in the low-top model is the main cause for this artifact. Our results underline the importance of a proper representation of the stratosphere in a coupled climate model for a consistent surface response in both the atmosphere and the ocean, which, among others, may have implications for oceanic deep convection in the subpolar North Atlantic.

Greenland monthly precipitation analysis from the Arctic System Reanalysis (ASR): 2000–2012

Arctic System Reanalysis version 1 (ASRv1) forecasts of monthly precipitation over Greenland are compared with gauge-based precipitation measured by the Danish Meteorological Institute (DMI) and precipitation retrieved from the Precipitation Occurrence Sensor System (POSS) at Summit. The ASRv1 precipitation generally agrees with the corrected DMI gauge-based precipitation measured at coastal or near-coastal stations in Greenland, but the corresponding data at Ikerasassuaq and Nuuk are not the case. ASRv1 precipitation at Summit, i.e., in a higher continental environment, is overestimated compared with the POSS observations. The North Atlantic Oscillation (NAO index and ASRv1 precipitation are moderately correlated over northern Greenland, the North Atlantic, and the Greenland Sea regions (0.32–0.49). It is presumed that local wind events have a larger influence on precipitation where smaller correlations occur. Suggested future work to understand discrepancies between ASRv1 and DMI precipitation fields in Greenland coastal regions is to include case studies of local wind events and corresponding precipitation variations utilizing in-situ measurements during both strong positive and negative NAO phases. At high-altitude and inland areas, further observations are needed to confirm the ASRv1 overestimation.

Holocene Hydroclimate Variability in Central Scandinavia Inferred from Flood Layers in Contourite Drift Deposits in Lake Storsjön

Despite the societal importance of extreme hydroclimate events, few palaeoenvironmental studies of Scandinavian lake sediments have investigated flood occurrences. Here we present a flood history based on lithological, geochemical and mineral magnetic records of a Holocene sediment sequence collected from contourite drift deposits in Lake Storsjön (63.12° N, 14.37° E). After the last deglaciation, the lake began to form around 9800 cal yr BP, but glacial activity persisted in the catchment for ~250 years. Element concentrations and mineral magnetic properties of the sediments indicate relatively stable sedimentation conditions during the Holocene. However, human impact in the form of expanding agriculture is evident from about 1100 cal yr BP, and intensified in the 20th century. Black layers containing iron sulphide appear irregularly throughout the sequence. The increased influx of organic matter during flood events led to decomposition and oxygen consumption, and eventually to anoxic conditions in the interstitial water preserving these layers. Elevated frequencies of black layer occurrence between 3600 and 1800 cal yr BP reflect vegetation changes in the catchment as well as large-scale climatic change. Soil erosion during snowmelt flood events increased with a tree line descent since the onset of the neoglacial period (~4000 cal yr BP). The peak in black layer occurrence coincides with a prominent solar minimum ~2600 cal yr BP, which may have accentuated the observed pattern due to the prevalence of a negative NAO index, a longer snow accumulation period and consequently stronger snowmelt floods.

Impact of Ural Blocking on Winter Warm Arctic–Cold Eurasian Anomalies. Part I: Blocking-Induced Amplification

AbstractIn Part I of this study, the impact of Ural blocking (UB) on the warm Arctic–cold Eurasian (WACE) pattern associated with the winter (DJF) arctic sea ice loss during 1979–2013 is examined by dividing the arctic sea ice reduction region into two dominant subregions: the Barents and Kara Seas (BKS) and the North American high-latitude (NAH) region (Baffin and Hudson Bay, Davis Strait, and Labrador Sea). It is found that atmospheric response to arctic sea ice loss resembles a negative Arctic response oscillation with a dominant positive height anomaly over the Eurasian subarctic region. Regression analyses of the two subregions further show that the sea ice loss over the BKS corresponds to the UB pattern together with a positive North Atlantic Oscillation (NAO+) and is followed by a WACE anomaly, while the sea ice reduction in the NAH region corresponds to a negative NAO (NAO−) pattern with a cold anomaly over northern Eurasia.Further analyses reveal that the UB pattern is more persistent during the period 2000–13 (P2) than 1979–99 (P1) because of the reduced middle-to-high-latitude mean westerly winds over Eurasia associated with the intense BKS warming. During P2 the establishment of the UB becomes a slow process because of the role of the BKS warming, while its decay is slightly rapid. In the presence of the long-lived UB that often occurs with the NAO+, the BKS-warming-induced DJF-mean anticyclonic anomaly is intensified and widened and then expands southward during P2 to amplify the WACE pattern and induce the southward displacement of its cold anomaly and the further loss of the BKS sea ice. Thus, midlatitude Eurasian cold events should be more frequent as the sea ice loss continues over the BKS.