The Summer North Atlantic Oscillation: Past, Present, and Future

Journal of Quaternary Science, 2009

The summer North Atlantic Oscillation (SNAO) is strongly associated with July-August climate variability over Europe, especially in northern regions. This association includes drought, where a positive SNAO corresponds to dry conditions over much of northern Europe and wet conditions in southern Europe, but the SNAO/climate association is weaker and less homogeneous in the south. Here we use a dendroclimatological reconstruction of the SNAO for the last 550 a to investigate the SNAO/drought relationship in the past. An association between the SNAO and a regional summer drought index from Sweden suggests that the northern European drought relationship holds back to 1700. In the last 550 a, the relationship between SNAO and drought in the Mediterranean region as a whole is weak, but over the Eastern Mediterranean the relationship is clearer and statistically significant (P < 0.05 level). The Mediterranean relationship is clearest at century scales. An association between the SNAO and Sahel rainfall can clearly be seen on interannual as well as longer timescales in the 20th century. Past droughts in the Sahel, as inferred from historical data, correspond quite well with positive phases of the SNAO on multidecadal timescales back to 1500, the phase expected from instrumental data. The physical reasons for the relationship between Sahel rainfall and the SNAO are, however, not yet understood. This research is a first step towards understanding how the atmospheric circulation over the North Atlantic region affects drought, necessary for forecast future droughts. y The contribution of C. K. Folland was written in the course of his employment at the Met Office, UK and is published with the permission of the Controller of HMSO and the Queen's Printer for Scotland.

Hydrology and Earth System Sciences, 2011

Recent summer heat waves in Europe were found to be preceded by precipitation deficits in winter. Numerical studies suggest that these phenomena are dynamically linked by land-atmosphere interactions. However, there exists as yet no complete observational evidence that connects summer climate variability to winter precipitation and the relevant circulation patterns. In this paper, we investigate the functional responses of summer mean and maximum temperature (June-August, T mean and T max) as well as soil moisture proxied by the self-calibrating Palmer drought severity index (scPDSI) to preceding winter precipitation (January-March, P JFM) for the period 1901-2005. All the analyzed summer fields show distinctive responses to P JFM over the Mediterranean. We estimate that 10 ∼ 15% of the interannual variability of T max and T mean over the Mediterranean is statistically forced by P JFM. For the scPDSI this amounts to 10 ∼ 25%. Further analysis shows that these responses are highly correlated to the North Atlantic Oscillation (NAO) regime over the Mediterranean. We suggest that NAO modulates European summer temperature by controlling winter precipitation that initializes the moisture states that subsequently interact with temperature. This picture of relations between European summer climate and NAO as well as winter precipitation suggests potential for improved seasonal prediction of summer climate for particular extreme events.

Hydrology and Earth System Sciences Discussions

Recent summer heat waves in Europe were preceded by precipitation deficits in winter. Numerical studies suggest that these phenomena are dynamically linked by landatmosphere interactions. However, there is still no clear evidence that connects summer climate variability to winter precipitation and the relevant circulation pattern so far. Using a technique specially designed for detecting directional influences between climatic fields, we investigate the statistical responses of summer mean as well as maximum temperature variability (June-August, T mean and T max) to preceding winter precipitation (January-March, P JFM) for the period 1901-2005. There appear distinctive T mean and T max responses to P JFM over the Mediterranean, where it is most sensitive to land-atmosphere interactions. An analysis of soil moisture proxy (self-calibrating Palmer drought severity index, scPDSI) shows that the P JFM seems to influence summer temperature via soil moisture, and therefore the T mean and T max responses we present here are very likely to be physical hints of water cycle interactions with temperature. We estimate that roughly 10∼20% of the interannual variability of T max and T mean over the Mediterranean is forced by P JFM ; for the scPDSI, these values amount to 20∼25%. Further analysis shows that these responses are highly correlated to the North Atlantic Oscillation (NAO) regime over the Mediterranean. Therefore we suggest that NAO modulates European summer temperature via controlling precipitation that initializes the moisture states of water cycle interactions with temperature. This clear picture of relations between European summer climate and NAO-related precipitation suggests potential for improved seasonal prediction of summer climate in particular extreme events.

Climate Dynamics, 2012

Climate models predict substantial summer precipitation reductions in Europe and the Mediterranean region in the twenty-first century, but the extent to which these models correctly represent the mechanisms of summertime precipitation in this region is uncertain. Here an analysis is conducted to compare the observed and simulated impacts of the dominant large-scale driver of summer rainfall variability in Europe and the Mediterranean, the summer North Atlantic Oscillation (SNAO). The SNAO is defined as the leading mode of July-August sea level pressure variability in the North Atlantic sector. Although the SNAO is weaker and confined to northern latitudes compared to its winter counterpart, with a southern lobe located over the UK, it significantly affects precipitation in the Mediterranean, particularly Italy and the Balkans (correlations of up to 0.6). During high SNAO summers, when strong anticyclonic conditions and suppressed precipitation prevail over the UK, the Mediterranean region instead is anomalously wet. This enhanced precipitation is related to the presence of a strong upper-level trough over the Balkans-part of a hemispheric pattern of anomalies that develops in association with the SNAO-that leads to mid-level cooling and increased potential instability.

Journal of Climate, 2008

CITATIONS 17 READS 35 7 authors, including: Some of the authors of this publication are also working on these related projects: Influence of summer Arctic sea ice reductions on Northern Hemisphere summer climate View project Predictability of Tropical Rainfall View project Adam A. Scaife

Climate Dynamics, 2006

This study investigates the response of wintertime North Atlantic Oscillation (NAO) to increasing concentrations of atmospheric carbon dioxide (CO 2) as simulated by 18 global coupled general circulation models that participated in phase 2 of the Coupled Model Intercomparison Project (CMIP2). NAO has been assessed in control and transient 80-year simulations produced by each model under constant forcing, and 1% per year increasing concentrations of CO 2 , respectively. Although generally able to simulate the main features of NAO, the majority of models overestimate the observed mean wintertime NAO index of 8 hPa by 5-10 hPa. Furthermore, none of the models, in either the control or perturbed simulations, are able to reproduce decadal trends as strong as that seen in the observed NAO index from 1970-1995. Of the 15 models able to simulate the NAO pressure dipole, 13 predict a positive increase in NAO with increasing CO 2 concentrations. The magnitude of the response is generally small and highly model-dependent, which leads to large uncertainty in multi-model estimates such as the median estimate of 0.0061±0.0036 hPa per %CO 2. Although an increase of 0.61 hPa in NAO for a doubling in CO 2 represents only a relatively small shift of 0.18 standard deviations in the probability distribution of winter mean NAO, this can cause large relative increases in the probabilities of extreme values of NAO associated with damaging impacts. Despite the large differences in NAO responses, the models robustly predict similar statistically significant changes in winter mean temperature (warmer over most of Europe) and precipitation (an increase over Northern Europe). Although these changes present a pattern similar to that expected due to an increase in the NAO index, linear regression is used to show that the response is much greater than can be attributed to small increases in NAO. NAO trends are not the key contributor to model-predicted climate change in wintertime mean temperature and precipitation over Europe and the Mediterranean region. However, the models' inability to capture the observed decadal variability in NAO might also signify a major deficiency in their ability to simulate the NAO-related responses to climate change.

Climate Research, 2013

A large ensemble of regional climate model projections was investigated regarding if and when they show an emergence of significant climate change signals in seasonal temperature and precipitation within Europe. The influence of the North Atlantic Oscillation (NAO), as simulated in the projections, was investigated. In most parts of Europe, the projections indicate robust emergence of temperature change in the first 2 decades of the 21st century, typically earlier for summer than for winter. For precipitation changes, signals generally emerge much later than for temperature. For Europe as a whole, the precipitation signals tend to emerge some 40 to 60 yr later than the temperature signals. In some sub-regions, robust signals for precipitation are not found within the studied period, i.e. until 2100. Some sub-regions, notably the Mediterranean area and Scandinavia, show different behaviour in some aspects compared to the ensemble-based results as a whole. NAO has some influence on the temperature change signals, which emerge earlier in winter for some models and regions if NAO is accounted for. For summer temperatures, the influence of NAO is less evident. Similarly, for precipitation, accounting for NAO leads to an earlier emergence in some regions and models. Here, we find an impact for both summer and winter.

Iop Conference Series: Earth and Environmental Science, 2009

This article was submitted without an abstract, please refer to the full-text PDF file.

Climate Dynamics, 2007

Using monthly independently reconstructed gridded European fields for the 500 hPa geopotential height, temperature, and precipitation covering the last 235 years we investigate the temporal and spatial evolution of these key climate variables and assess the leading combined patterns of climate variability. Seasonal European temperatures show a positive trend mainly over the last 40 years with absolute highest values since 1766. Precipitation indicates no clear trend. Spatial correlation technique reveals that winter, spring, and autumn covariability between European temperature and precipitation is mainly influenced by advective processes, whereas during summer convection plays the dominant role. Empirical Orthogonal Function analysis is applied to the combined fields of pressure, temperature, and precipitation. The dominant patterns of climate variability for winter, spring, and autumn resemble the North Atlantic Oscillation and show a distinct positive trend during the past 40 years for winter and spring. A positive trend is also detected for summer pattern 2, which reflects an increased influence of the Azores High towards central Europe and the Mediter-ranean coinciding with warm and dry conditions. The question to which extent these recent trends in European climate patterns can be explained by internal variability or are a result of radiative forcing is answered using cross wavelets on an annual basis. Natural radiative forcing (solar and volcanic) has no imprint on annual European climate patterns. Connections to CO 2 forcing are only detected at the margins of the wavelets where edge effects are apparent and hence one has to be cautious in a further interpretation.

Global Change Biology, 2005

Three European plant phenological network datasets were analysed for latitudinal and longitudinal gradients of nine phenological 'seasons' spanning the entire year. The networks were: (1) the historical first European Phenological Network (1882-1941) by Hoffmann & Ihne, (2) the network of the International Phenological Gardens in Europe , founded by Schnelle & Volkert in 1957 and based on cloned plants, and (3) a dataset that was recently collated during the EU Fifth Framework project POSITIVE, which included network data of seven Central and Eastern European countries. Our study is most likely the first, for over a century, to analyse average onset and year-to-year variability of the progress of seasons across a continent. For early, mid, and late spring seasons we found a marked progress of the seasonal onset from SW to NE throughout Europe, more precisely from WSW to ENE in early spring, then from SW to NE and finally from SSW to NNE in late spring, as exhibited by the relationship between latitudinal and longitudinal gradients. The movement of summer was more south to north directed, as the longitudinal gradient (west-east component) strongly declined or was even of opposite sign. Autumn, as shown by leaf colouring dates, arrived from NE to SW. Possible reasons for the differences among the three datasets are discussed. The annual variability of latitudinal and longitudinal gradients of the seasons across Europe was closely related to the North Atlantic Oscillation (NAO) index; in years with high NAO in both winter and spring, the west-east component of progress was more pronounced; in summer and autumn, the pattern of the seasons may be more uniform.