Phase Of North Atlantic Oscillation Research Articles

The influence of volcanic eruptions on weather regimes over the North Atlantic simulated by ECHAM5/MPI-OM ensemble runs from 800 to 2000 CE

The volcanic fingerprint on the winter North Atlantic atmospheric circulation and climate is analyzed in six ensemble runs of ECHAM5/MPI-OM covering 800–2000 CE, both for equatorial and Northern Hemisphere (NH) eruptions. Large volcanic eruptions influence climate on both annual and decadal time scales due to dynamic interactions of different climate components in the Earth's system. It is well known that the North Atlantic Oscillation (NAO) tends to shift towards its positive phase during winter in the first 1–2 years after large tropical volcanic eruptions, causing warming over Europe, but other North Atlantic weather regimes have received less attention. Here we investigate the four dominant weather regimes in the North Atlantic: The negative and positive phase of NAO as well as the Atlantic Ridge, Scandinavian blocking. The volcanic fingerprint is detected as a change in the frequency of occurrence and anomalies in the wind and temperature fields as well as in the sea ice cover. We observe a strong significant increase in the frequency of Atlantic Ridge in the second year after equatorial eruptions that precede the NAO+ detected in year 3–5 as a result of a strong zonal wind anomalies in year 1–2. Evidence for a stronger polar vortex is detected in years 12–14 where NAO+ is detected both as a frequency increase and in the wind and temperature fields. A short-term response is also detected 2–4 years after NH eruptions. The longterm signal after NH eruptions indicate a weak polar vortex around a decade after an eruption. Although the signal after NH eruptions is weaker our results stress the need for further studies. The simulated atmospheric response recorded in ECHAM5 after volcanic eruptions suggest a more dynamic response than previously thought. The methodology used can also be applied to other forcing scenario, for example for future climate projections where the aim is to search for a long-term climate signal.

Differential North Atlantic control of winter hydroclimate in late Holocene varved sediments of Lake Kortejärvi, eastern Finland

Sediment microfacies, geochemical μ‐XRF and X‐ray density analyses were conducted on varved sediments from Lake Kortejärvi (eastern Finland) covering the last 2700 years. The varves comprise couplets of detrital and organic sub‐layers throughout the complete time‐span. Based on their microfacies and stratigraphical position within a varve as well as comparisons with local discharge and meteorological data, thicker detrital layers are interpreted to reflect intensified snow‐melt floods following more humid winters. Detailed comparisons with monthly to annually resolved North Atlantic Oscillation (NAO) indices back to AD 1049 (901 a BP) suggest that multidecadal increases in snow‐melt layer thickness tend to be connected with a more positive phase of the NAO and, consequently, warmer winters. In contrast, distinct centennial intervals of thicker snow‐melt layers from −40 to 170, 280 to 460 and 1900 to 2300 a BP as well as around 2600 a BP do not consistently correspond to a particular NAO phase, but coincide with extended sea‐ice margins and a colder North Atlantic climate, causing intensified and southward shifted westerly cyclones. Our results point to a differential modification of North Atlantic winter hydroclimate working on varying time scales.

Reconstruction and analysis of erythemal UV radiation time series from Hradec Králové (Czech Republic) over the past 50 years

Abstract. This paper evaluates the variability of erythemal ultraviolet (EUV) radiation from Hradec Králové (Czech Republic) in the period 1964–2013. The EUV radiation time series was reconstructed using a radiative transfer model and additional empirical relationships, with the final root mean square error of 9.9 %. The reconstructed time series documented the increase in EUV radiation doses in the 1980s and the 1990s (up to 15 % per decade), which was linked to the steep decline in total ozone (10 % per decade). The changes in cloud cover were the major factor affecting the EUV radiation doses especially in the 1960s, 1970s, and at the beginning of the new millennium. The mean annual EUV radiation doses in the decade 2004–2013 declined by 5 %. The factors affecting the EUV radiation doses differed also according to the chosen integration period (daily, monthly, and annually): solar zenith angle was the most important for daily doses, cloud cover, and surface UV albedo for their monthly means, and the annual means of EUV radiation doses were most influenced by total ozone column. The number of days with very high EUV radiation doses increased by 22 % per decade, the increase was statistically significant in all seasons except autumn. The occurrence of the days with very high EUV doses was influenced mostly by low total ozone column (82 % of days), clear-sky or partly cloudy conditions (74 % of days) and by increased surface albedo (19 % of days). The principal component analysis documented that the occurrence of days with very high EUV radiation doses was much affected by the positive phase of North Atlantic Oscillation with an Azores High promontory reaching over central Europe. In the stratosphere, a strong Arctic circumpolar vortex and the meridional inflow of ozone-poor air from the southwest were favorable for the occurrence of days with very high EUV radiation doses. This is the first analysis of the relationship between the high EUV radiation doses and macroscale circulation patterns, and therefore more attention should be given also to other dynamical variables that may affect the solar UV radiation on the Earth surface.

Interannual variability of anticyclone activity and temperature extremes in the Black sea region

Manifestations of North Atlantic oscillation (NAO) during its negative and positive phases and during different El Niño (EN) types in the anticyclone parameters and air temperature extreme thresholds in the Black sea region are studied. It is estimated quantitative characteristics of anticyclone frequency anomalies and temperature extremes associated with the NAO phases and the EN types.

Effect of the north Atlantic oscillation on water level fluctuations in lakes of northern Poland

The paper presents an analysis of correlations between water levels in Polish lakes and the rate of the North Atlantic Oscillation (NAO) in the years 1976-2010. The detailed analysis of the spatial variability of the effect of NAO on water levels in lakes concerned 19 lakes with statistically uniform hydrometric material. Two matrices were obtained for each of the lakes, composed of 156 coefficients of correlation calculated between monthly water stages and monthly and seasonal NAO indices. They provided the basis for performing two variants of classification of lakes by Ward’s method. Four typological classes were distinguished for each variant. It was determined that stronger correlations occur in the case of water stages in lakes with seasonal than those with monthly NAO indices. The strongest effect of NAO on water stages is observed in the winter-spring period. Spatial variability of the effect has been recorded, resulting from the climatic conditions of a given region. Lakes located in the south-western part of the studied area constitute an evidently separate group. In the negative phase of NAO, they are distinguished by higher water stages in the winter-spring period. This may be associated with more frequent thaws during winters, and increased supply to lakes in the period.

Late Holocene changes in vegetation and atmospheric circulation at Lake Uddelermeer (The Netherlands) reconstructed using lipid biomarkers and compound‐specific δD analysis

ABSTRACTWe reconstructed middle to late Holocene changes in atmospheric circulation patterns and vegetation in north‐west Europe by applying novel geochemical techniques to the sediment record of Lake Uddelermeer (The Netherlands). A comparison of higher plant‐derived leaf wax n‐alkane distributions archived in the lake sediments with those in living plant material, combined with palynological analysis, indicates that the vegetation immediately surrounding the lake became more open at 3150 cal a BP, while the regional vegetation responded more gradually and ∼650 years later. Our record of the hydrogen isotopic composition of plant leaf waxes (δDwax) shows a deuterium enrichment starting from 3500 cal a BP, which we interpret as a change in atmospheric circulation. A similar δDwax record from nearby Meerfelder Maar (Germany) shows an opposite trend around this time, which could be explained by a change in sea‐level pressure resembling a negative North Atlantic Oscillation phase. This could account for depleted δD values of precipitation at Meerfelder Maar, while confounding factors related to the more maritime position of Uddelermeer cause the opposite shift there.

Responses of ENSO and NAO to the external radiative forcing during the last millennium: Results from CCSM4 and MPI-ESM-P simulations

The decadal–centennial variations of the El Niño–Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) are analyzed, based on the outputs of last millennium (LM) and Historical experiments (850–2005 AD) and control (CTL) experiments from two climate models Community Climate System Model, version 4 (CCSM4) and Max Planck Institute for Meteorology Paleoclimate Model (MPI-ESM-P). Variation of ENSO and NAO among the Medieval Warm Period (MWP, 1050–1150 AD), Little Ice Age (LIA, 1600–1700 AD) and 20th Century Warming (20CW, 1905–2005 AD) are focused. Significant bi-centennial and multi-decadal periods are detected in the ENSO and NAO series, respectively. Both models have exerted statistically-significant contrasts of ENSO and NAO phases among the three typical periods. The model-independent shifts of an El Niño-like state during the MWP, a La Niña-like state during the LIA and an El Niño-like state during the 20CW are captured. However, the NAO series display a model-dependence. CCSM4 features a negative-phased NAO during the LIA, while MPI-ESM-P is characterized by a positive-phased NAO in the same period. The shifts of the ENSO and NAO phases are mainly due to the external radiative forcing, including the solar-volcanic (SV) forcing and greenhouse gases (GHG) forcing.

Flowering of allergenically important plant species in relation to the North Atlantic Oscillation system and thermal time in the Czech Republic

This paper analyses long-term (1960-2015) onset of flowering in 16 native terrestrial plants (11 of them produce important allergens) recorded in different parts of the Czech Republic (southern, central and northern part) in relation to the North Atlantic Oscillation (NAO) index of the preceding winter and thermal data-growing degree-days (GDD) and soil temperature. Flowering occurred significantly earlier following positive winter NAO phases (causing spring to be warmer than normal in Central Europe) in nearly all early-flowering (February, March, April) species; high Pearson correlation values were recorded in, e.g. wood anemone, common snowdrop, goat willow, common hazel and common alder. There was found a difference between the southern and northern part of the country, e.g. in silver birch and pedunculate oak. Out of the later-flowering (May-July) plant species, black elder and meadow foxtail also significantly correlated with the winter NAO index, lime tree correlated less markedly. The best results of a threshold for calculation of GDD to onset of beginning of flowering were found in lime tree-it was 5 degrees C at all three stations. Results of other taxa were more variable (e.g. 4-7 degrees C in goat willow; 6-10 degrees C in silver birch). Pearson correlation coefficients between NAO index and GDD were negative in lime tree at all thresholds (5, 6, 7, 8, 9, 10 degrees C), while goat willow and silver birch were not so uniform (both positive and negative values). Correlation coefficients between phenophase onset and soil temperature (10 cm depth) had the highest values in silver birch, European larch and wood anemone. Stations situated at higher elevation showed negative correlation coefficient with soil temperature in common snowdrop, pedunculate oak, meadow foxtail and lime tree; other values were positive.

Reconstructing Late Holocene North Atlantic atmospheric circulation changes using functional paleoclimate networks

Abstract. Obtaining reliable reconstructions of long-term atmospheric circulation changes in the North Atlantic region presents a persistent challenge to contemporary paleoclimate research, which has been addressed by a multitude of recent studies. In order to contribute a novel methodological aspect to this active field, we apply here evolving functional network analysis, a recently developed tool for studying temporal changes of the spatial co-variability structure of the Earth's climate system, to a set of Late Holocene paleoclimate proxy records covering the last two millennia. The emerging patterns obtained by our analysis are related to long-term changes in the dominant mode of atmospheric circulation in the region, the North Atlantic Oscillation (NAO). By comparing the time-dependent inter-regional linkage structures of the obtained functional paleoclimate network representations to a recent multi-centennial NAO reconstruction, we identify co-variability between southern Greenland, Svalbard, and Fennoscandia as being indicative of a positive NAO phase, while connections from Greenland and Fennoscandia to central Europe are more pronounced during negative NAO phases. By drawing upon this correspondence, we use some key parameters of the evolving network structure to obtain a qualitative reconstruction of the NAO long-term variability over the entire Common Era (last 2000 years) using a linear regression model trained upon the existing shorter reconstruction.

Quasi-Periodicity of the Black Sea Wind Spatial-Temporal Variability and Its Relation with the North Atlantic Oscillation Phases

Quasi-Periodicity of the Black Sea Wind Spatial-Temporal Variability and Its Relation with the North Atlantic Oscillation Phases

Kvaziperiodichnost' prostranstvenno-vremennoy izmenchivosti vetra Chernogo morya i ee svyaz' s fazami Severoatlanticheskogo kolebaniya

Kvaziperiodichnost' prostranstvenno-vremennoy izmenchivosti vetra Chernogo morya i ee svyaz' s fazami Severoatlanticheskogo kolebaniya

Impact of North Atlantic Teleconnection Patterns on Northern European Sea Level

Northern European sea levels show a non-stationary link to the North Atlantic Oscillation (NAO). The location of the centers of the NAO dipole, however, can be affected through the interplay with the East Atlantic (EAP) and the Scandinavian (SCAN) teleconnection patterns. Our results indicate the importance of accounting for the binary combination of the NAO with the EAP/SCAN for better understanding the non-stationary drivers inducing sea level variations along the European coasts. By combining altimetry and tide gauges, we find that anomalously high monthly sea levels along the Norwegian (North Sea) coast are predominantly governed by same positive phase NAO+/EAP+ (NAO+/SCAN+) type of atmospheric circulation, while the Newlyn and Brest tide gauges respond markedly to the opposite phase NAO−/EAP+ combination. Despite these regional differences, we find that coherent European sea level changes project onto a pattern resembling NAO+/SCAN+, which is signified by pressure anomalies over Scandinavia and southern Europe forcing winds to trace the continental slope, resulting in a pile-up of water along the European coasts through Ekman transport. We conclude that taking into consideration the interaction between these atmospheric circulation regimes is valuable and may help to understand the time-varying relationship between the NAO and European mean sea level.

Meteorological effects on Hg wet deposition in a forested site in the Adirondack region of New York during 2000–2015

An analysis of weekly measurement data of mercury (Hg) wet deposition was conducted for Huntington Wildlife Forest (HWF), a forest ecosystem in Upstate New York and a biological Hg hotspot, during 2000–2015. Annual accumulated Hg wet deposition flux was found to decrease at a rate of −0.13 μg m−2 yr−1 (2% yr−1) (p = 0.09), and volume weighted mean (VWM) Hg precipitation concentrations at −0.14 ng L−1 yr−1 (2.5% yr−1) (p = 0.00). In examining data by season, no trends were identified for the two variables. It was found that the North Atlantic Oscillation (NAO) affected Hg wet deposition predominantly in spring, as did the position of the U.S. East Coast trough in summer, which suggests different dominant mechanisms driving Hg wet deposition in different seasons. The impacts of such large scale circulation processes were facilitated via variations in precipitation amounts. This was manifested in spring 2011 with the strongest positive phase of NAO, resulting in the wettest spring with the largest Hg wet deposition flux, and in summer 2007 with the U.S. East Coast trough positioned the farthest out over the Atlantic Ocean, causing the driest summer with the lowest Hg wet deposition flux of the study period. Extreme precipitation amounts in spring could singularly drive the overall long-term trend in Hg wet deposition whereas in summer other factors could just be as important. Similar mechanisms were thought to control the long term variations of Hg wet deposition and precipitation concentrations in all seasons but summer as indicated in their significant correlation in all but summer. Atmospheric concentrations of gaseous oxidized mercury (GOM) and particulate borne mercury (PBM) at HWF over 2009–2015 hardly exhibited correlations with Hg wet deposition or precipitation concentrations. Chemical transport model simulations strongly supported efficient scavenging of oxidized Hg by precipitation resulting in the lowest concentration of GOM in the warm season despite the supposedly largest GOM production. Our findings suggest that over the long run climate change could play an important role in atmospheric deposition of Hg into ecosystems facilitated by precipitation amounts, which is closely linked to variations in large scale circulation.

Symmetric and asymmetric components of anomalous tropospheric-mean horizontal fluxes of latent and sensible heat associated with ENSO events of variable magnitude

The El Niño-Southern Oscillation (ENSO) represents the dominant mode of global climate variability and is inherently nonlinear such that the linearity of the atmospheric response remains an area of ongoing research. The phase of North Atlantic Oscillation (NAO) and Pacific North American (PNA) patterns of intra-annual climate variability are favored to be the same as the phase of ENSO resulting in important climate impacts across Europe and North America. Advanced understanding of the symmetry of this response at global scale using monthly composite analyses of anomalous horizontal sensible and latent heat fluxes at various ENSO event magnitudes quantified from ERA-Interim output (January 1979 through June 2016) will advance impact predictability. A linear relationship between ENSO, PNA, and NAO patterns was identified, particularly for strong ENSO events. The nonlinear component indicated general eastward (westward) shifts in anomalous heat fluxes during El Niño (La Niña) events such that the greatest impacts were implied across North America during Decembers and Januarys of strong El Niño and weak La Niña events. Analyses of anomalous latent heat fluxes indicated spatial patterns consistent with more frequent atmospheric river phenomena, especially during Decembers and Januarys of strong El Niño events. This work demonstrates that the symmetric component of anomalous horizontal, tropospheric-mean heat fluxes corresponding to ENSO events are effective for identifying north-south dipoles of anomalous circulations consistent with PNA or NAO patterns and connections between tropical heat source regions and the PNA and NAO regions. This work also demonstrates the asymmetric component identified differences in anomalous circulation position and whether El Niño or La Niña resulted in larger heat flux anomalies. Therefore, this work provides insight into impacts associated with future ENSO events, especially across North America during strong El Niño and weak La Niña.

Spatiotemporal variability of lightning activity in Europe and the relation to the North Atlantic Oscillation teleconnection pattern

Abstract. Comprehensive lightning statistics are presented for a large, contiguous domain covering several European countries such as France, Germany, Austria, and Switzerland. Spatiotemporal variability of convective activity is investigated based on a 14-year time series (2001–2014) of lightning data. Based on the binary variable thunderstorm day, the mean spatial patterns of lightning activity and regional peculiarities regarding seasonality are discussed. Diurnal cycles are compared among several regions and evaluated with respect to major seasonal changes. Further analyses are performed regarding interannual variability and the impact of teleconnection patterns on convection. Mean convective activity across central Europe is characterized by a strong northwest-to-southeast gradient with pronounced secondary features superimposed. The zone of maximum values of thunderstorm days propagates southwestward along the southern Alpine range from April to July. Diurnal cycles vary substantially between both different months and regions, particularly regarding the incidence of nighttime lightning. The North Atlantic Oscillation (NAO) is shown to have a significant impact on convective activity in several regions, which is primarily caused by variations of the large-scale lifting pattern in both NAO phases. This dynamical effect is partly compensated for by thermodynamical modifications of the pre-convective environment. The results point to a crucial role of large-scale flow in steering the spatiotemporal patterns of convective activity.

The role of climate variability in extreme floods in Europe

Climate variability is shown to be an important driver of spatial and temporal changes in hydrometereological variables in Europe. However, the influence of climate variability on flood damage has received little attention. We investigated the El Niño Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the East Atlantic pattern (EA) during their neutral, positive, and negative phases, to understand their relationships with four flood indicators: Occurrence of Extreme Rainfall, Intensity of Extreme Rainfall, Flood Occurrence, and Flood Damage. We found that positive and negative phases of NAO and EA are associated with more (or less) frequent and intense seasonal extreme rainfall over large areas of Europe. The relationship between ENSO and the Occurrence of Extreme Rainfall and Intensity of Extreme Rainfall in Europe is much smaller than the relationship with NAO or EA, but still significant in some regions. We show that Flood Damage and Flood Occurrence have strong links with climate variability, especially in southern and eastern Europe. Therefore, when investigating flooding across Europe, all three indices of climate variability should be considered. Future research should focus on their joint influence on flood risk. The potential inclusion of seasonal forecasts of indices of climate variability could be effective in forecasting flood damage.

On the Relationship between Spring NAO and Snowmelt in the Upper Southwestern United States

This study examines the relationship between the North Atlantic Oscillation (NAO) and snowmelt in spring in the upper southwestern states of the United States (UP_SW) including California, Nevada, Utah, and Colorado, using SNOTEL datasets for 34 yr (1980–2014). Statistically significant negative correlations are found between NAO averages in the snowmelt period and timings of snowmelt (i.e., positive NAO phases in spring enhance snowmelt, and vice versa). It is also found that correlations between El Niño–Southern Oscillation and snowmelt are negligible in the region. The NAO–snowmelt relationship is most pronounced below the 2800-m level; above this level, the relationship becomes weaker. The underlying mechanism for this link is that a positioning of upper-tropospheric anticyclonic (cyclonic) circulations over the western United States that are associated with development of the positive (negative) NAO phases tends to bring warmer and drier (colder and wetter) spring weather conditions to the region. The temperature variations related with the NAO phases also strongly modulate the snowfall–rainfall partitioning. The relationship between the NAO and spring snowmelt can serve as key information for the warm season water resources management in the UP_SW.

The possible impact of the circumglobal wave train on the wet season dust storm activity over the northern Arabian Peninsula

This study examines the influence of mid-latitude circulation on the wet season (October–May) Dust Storm Activity (DSA) over the northern Arabian Peninsula for the period of 1983–2013, using observational and reanalysis gridded datasets. Dusty and clear days composites of the 200 hPa geopotential height anomalies reveal a pattern in the upper-levels which is strongly correlated with the mid-latitude circumglobal wave train (CGT) and exists 3 days prior to the outbreak of dust storms. The regional surface conditions (such as the mean sea level pressure; low-level temperature and winds, etc.) that favor the DSA over the northern Arabian Peninsula display significant correlations with the CGT on intraseasonal time scale. A time-lagged analysis reveals that the eastward propagating CGT influences the surface pressure over the northern Arabian Peninsula and adjacent areas—hence the DSA. The DSA over the northern Arabian Peninsula is found to be more frequent during the positive phase of the North Atlantic Oscillation (NAO) and the cold phase of the El Nino–Southern Oscillation (La Nina), and less frequent during the negative phase of NAO and the warm phase of the El Nino–Southern Oscillation (El Nino).

Variations of winter precipitation over southeastern China in association with the North Atlantic Oscillation

The synoptic-scale winter precipitation variations over southeastern China (22°–32°N, 105°–125°E) and their association with the North Atlantic Oscillation (NAO) during 1951–2007 are investigated in this paper. The variability of wintertime precipitation is characterized by meridional displacement of its maximum center. Two precipitation regimes, with maximum centers located over the Yangtze and Pearl River basins, are identified via cluster analysis. Time-lagged analyses suggest that the two precipitation regimes are connected with the decaying phases of positive NAO (NAO+) events of different amplitudes. A strong (medium) NAO+ event is defined as one when the maximum amplitude of the NAO index exceeds 1.0 (in the range of 0.7–1.0) for at least 4 consecutive days and drops to less than 0.3 within 7 days following the peak index. After the peak of a strong NAO+, southerly winds expand northward to the Yangtze River (about 30°N), a northeast–southwest-tilted trough migrates to east of Lake Baikal, and cold air intrudes into central eastern China; thus, precipitation is strengthened over the Yangtze River basin where warm and cold air masses converge. In comparison, during the decaying phase of medium NAO+ events, the southerly winds are relatively weak, and precipitation tends to be enhanced at lower latitudes (around 25°N). Further analysis indicates that downstream Rossby-wave propagation may account for the latitudinal expansion of the southerly wind anomalies over the eastern coastal area of China during the decaying phase of NAO+ events of different strengths.

Analysis of water balance and runoff generation in high latitude agricultural fields during mild and cold winters

High-latitude conditions in northern Europe are characterised by short growing seasons (May–August) and long dormant seasons. Alternating mild and freezing conditions lead to variable snow accumulation–melt cycles affecting runoff generation, and consequently the loss of nutrients and sediments from agricultural fields. We assessed water balance in two subsurface drained clayey agricultural fields of different slopes (1% and 5%) in southern Finland to discern changes between mild and cold winters. The water balances of the two field sections were produced with a spatially distributed 3D hydrological model. Simulated snow water equivalent (SWE), drain discharge, tillage layer runoff and groundwater outflow from a 7-year period were examined during the dormant seasons (September–April) in relation to the North Atlantic Oscillation (NAO) index, which characterises phases related to mild and cold winters in northern Europe. Mild periods (positive NAO) were associated with more frequent runoff events, which were sustained throughout mild winters with lower SWE and shorter time of snow cover. Understanding and quantifying the water balance through periods of different weather patterns is essential as climate change is projected to increase the occurrence of positive NAO phases challenging the control of nutrient and sediment losses from agricultural fields.