North Atlantic Weather Regimes Research Articles

Polar Low Motion and Track Characteristics over the North Atlantic

Abstract Polar lows (PLs) are intense mesoscale cyclones over high-latitude oceans. The PL motion and track characteristics over the North Atlantic in the cold season are examined based on ∼1700 PLs during 1979–2016. Our analysis shows that PL motion is mainly controlled by the environmental flow in the lower troposphere. In particular, the steering flow defined over 950–550 hPa within a 450-km radius best matches the PL motion. Meanwhile, 700 hPa is an optimal single level to assess the steering flow. Cluster analysis based on a linear regression mixture model is utilized to describe the PL tracks. Four distinct clusters are identified, and they are characterized by northeastward motion (NE), eastward motion (E), southward motion (S), and slow motion without a dominating direction (SM), respectively. Although PLs in the four clusters have similar lifespans, SM-type PLs have much shorter tracks than the other clusters, due to their slow translation speeds. Further analysis shows that there are no distinctive geographic differences in genesis locations for the four clusters. The track differences can be largely explained by the associated synoptic-scale environmental circulations. Additionally, the S-type PLs tend to develop in a reverse shear environment, the NE-type and E-type PLs are associated with forward shear and left shear environments, respectively, while the SM-type PLs do not show a preference in the environmental shear. The link between the PL tracks and the North Atlantic weather regimes is also investigated. The NAO+ regime is associated with the most frequent PL occurrence over the North Atlantic, while the Scandinavian blocking regime is associated with lowest PL frequency. Significance Statement Polar lows (PLs) are intense mesoscale cyclones and pose hazards to high-latitude coastal areas. A better understanding of polar low motion can help improve polar low forecasts and reduce the hazardous impacts of the storms. Based on a PL track dataset over the North Atlantic basin during 1979–2016, it is found that PL motion is mainly controlled by the environmental flow in the lower troposphere and that the steering flow defined over 950–550 hPa within a 450-km radius best matches the PL motion. In addition, four distinct types of tracks are identified using cluster analysis, and they are characterized by northeastward motion, eastward motion, southward motion, and slow motion without a dominating direction, respectively. The different background flows can largely explain the propagation directions of different track types. Furthermore, the links between the PL motion and the North Atlantic weather regimes are also investigated.

North Atlantic Circulation Regimes and Heat Transport by Synoptic Eddies

AbstractMeridional transport of heat by transient atmospheric eddies is a key component of the energy budget of the middle- and high-latitude regions. The heat flux at relevant frequencies is also part of a dynamical interaction between eddies and mean flow. In this study we investigate how the poleward heat flux by high-frequency atmospheric transient eddies is modulated by North Atlantic weather regimes in reanalysis data. Circulation regimes are estimated via a clustering method, a jet-latitude index, and a blocking index. Heat transport is defined as advection of moist static energy. The focus of the analysis is on synoptic frequencies but results for slightly longer time scales are reported. Results show that the synoptic eddy heat flux is substantially modulated by midlatitude weather regimes on a regional scale in midlatitude and polar regions. In a zonal-mean sense, the phases of the North Atlantic Oscillation do not significantly change the high-latitude synoptic heat flux, whereas Scandinavian blocking and the Atlantic ridge are associated with an intensification. A close relationship between high-latitude (midlatitude) heat flux and Atlantic jet speed (latitude) is found. The relationship between extreme events of synoptic heat flux and circulation regimes is also assessed and reveals contrasting behaviors in the polar regions. The perspective that emerges complements the traditional view of the interaction between synoptic eddies and the extratropical flow and reveals relationships with the high-latitude climate.

Progress towards a probabilistic Earth system model: examining the impact of stochasticity in the atmosphere and land component of EC-Earth v3.2

Abstract. We introduce and study the impact of three stochastic schemes in the EC-Earth climate model: two atmospheric schemes and one stochastic land scheme. These form the basis for a probabilistic Earth system model in atmosphere-only mode. Stochastic parametrization have become standard in several operational weather-forecasting models, in particular due to their beneficial impact on model spread. In recent years, stochastic schemes in the atmospheric component of a model have been shown to improve aspects important for the models long-term climate, such as El Niño–Southern Oscillation (ENSO), North Atlantic weather regimes, and the Indian monsoon. Stochasticity in the land component has been shown to improve the variability of soil processes and improve the representation of heatwaves over Europe. However, the raw impact of such schemes on the model mean is less well studied. It is shown that the inclusion of all three schemes notably changes the model mean state. While many of the impacts are beneficial, some are too large in amplitude, leading to significant changes in the model's energy budget and atmospheric circulation. This implies that in order to maintain the benefits of stochastic physics without shifting the mean state too far from observations, a full re-tuning of the model will typically be required.

North Atlantic weather regimes in δ18O of winter precipitation: isotopic fingerprint of the response in the atmospheric circulation after volcanic eruptions

Equatorial volcanic eruptions are known to impact the atmospheric circulation on seasonal time scales through a strengthening of the stratospheric zonal winds followed by dynamic ocean-atmosphere coupling. This emerges as the positive phase of the North Atlantic Oscillation in the first 5 years after an eruption. In the North Atlantic, other modes of atmospheric circulation contribute to the climate variability but their response to volcanic eruptions has been less studied. We address this by retrieving the stable water isotopic fingerprint of the four major atmospheric circulation modes over the North Atlantic (Atlantic Ridge, Scandinavian Blocking and the negative and positive phases of the North Atlantic Oscillation (NAO − and NAO+)) by using monthly precipitation data from Global Network of Isotopes in Precipitation (GNIP) and 500 mb geo-potential height from the 20th Century Reanalysis. The simulated stable isotopic pattern of each atmospheric circulation mode is further used to assess the retrieved pattern. We test if changes in the atmospheric circulation as well as moisture source conditions as a result of volcanic eruptions can be identified by analyzing the winter climate response after both equatorial and high-latitude North Hemispheric volcanic eruptions in data, reanalysis and simulations. We report of an NAO + mode in the first two years after equatorial eruptions followed by NAO − in year 3 due to a decrease in the meridional temperature gradient as a result of volcanic surface cooling. This emerges in both GNIP data as well as reanalysis. Although the detected response is stronger after equatorial eruptions compared to high latitude eruptions, our results show that the response after high latitude eruptions tend to emerge as NAO − in year 2 followed by NAO + in year 3–4.

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.

The influence of the stratospheric state on North Atlantic weather regimes

Stratosphere–troposphere coupling is often viewed from the perspective of the annular modes and their dynamics. Despite the obvious benefits of this approach, recent work has emphasised the greater tropospheric sensitivity to stratospheric variability in the Atlantic basin than in the Pacific basin. In this study, a new approach to understanding stratosphere–troposphere coupling is proposed, with a focus on the influence of the stratospheric state on North Atlantic weather regimes (during extended winter, November to March). The influence of the strength of the lower‐stratospheric vortex on four commonly used tropospheric weather regimes is quantified. The negative phase of the North Atlantic Oscillation is most sensitive to the stratospheric state, occurring on 33% of days following weak vortex conditions but on only 5% of days following strong vortex conditions. An opposite and slightly weaker sensitivity is found for the positive phase of the North Atlantic Oscillation and the Atlantic Ridge regime. For the North Atlantic Oscillation regimes, stratospheric conditions change both the probability of remaining in each regime and the probability of transitioning to that regime from others. A logistic regression model is developed to further quantify the sensitivity of tropospheric weather regimes to the lower stratospheric state. The logistic regression model predicts an increase of 40–60% in the probability of transition to the negative phase of the North Atlantic Oscillation for a one standard deviation reduction in the strength of the stratospheric vortex. Similarly it predicts a 10–30% increase in the probability of transition to the positive phase of the North Atlantic Oscillation for a one standard deviation increase in the strength of the stratospheric vortex. The stratosphere–troposphere coupling in the European Centre for Medium‐range Weather Forecasts Integrated Forecasting System model is found to be consistent with the re‐analysis data by fitting the same logistic regression model.

Randomly correcting model errors in the ARPEGE-Climate v6.1 component of CNRM-CM: applications for seasonal forecasts

Abstract. Stochastic methods are increasingly used in global coupled model climate forecasting systems to account for model uncertainties. In this paper, we describe in more detail the stochastic dynamics technique introduced by Batté and Déqué (2012) in the ARPEGE-Climate atmospheric model. We present new results with an updated version of CNRM-CM using ARPEGE-Climate v6.1, and show that the technique can be used both as a means of analyzing model error statistics and accounting for model inadequacies in a seasonal forecasting framework.The perturbations are designed as corrections of model drift errors estimated from a preliminary weakly nudged re-forecast run over an extended reference period of 34 boreal winter seasons. A detailed statistical analysis of these corrections is provided, and shows that they are mainly made of intra-month variance, thereby justifying their use as in-run perturbations of the model in seasonal forecasts. However, the interannual and systematic error correction terms cannot be neglected. Time correlation of the errors is limited, but some consistency is found between the errors of up to 3 consecutive days.These findings encourage us to test several settings of the random draws of perturbations in seasonal forecast mode. Perturbations are drawn randomly but consistently for all three prognostic variables perturbed. We explore the impact of using monthly mean perturbations throughout a given forecast month in a first ensemble re-forecast (SMM, for stochastic monthly means), and test the use of 5-day sequences of perturbations in a second ensemble re-forecast (S5D, for stochastic 5-day sequences). Both experiments are compared in the light of a REF reference ensemble with initial perturbations only. Results in terms of forecast quality are contrasted depending on the region and variable of interest, but very few areas exhibit a clear degradation of forecasting skill with the introduction of stochastic dynamics. We highlight some positive impacts of the method, mainly on Northern Hemisphere extra-tropics. The 500 hPa geopotential height bias is reduced, and improvements project onto the representation of North Atlantic weather regimes. A modest impact on ensemble spread is found over most regions, which suggests that this method could be complemented by other stochastic perturbation techniques in seasonal forecasting mode.

Characterizing atmospheric circulation signals in Greenland ice cores: insights from a weather regime approach

Greenland ice cores offer seasonal to annual records of δ18O, a proxy for precipitation-weighted temperature, over the last few centuries to millennia. Here, we investigate the regional footprints of the North Atlantic weather regimes on Greenland isotope and climate variability, using a compilation of 22 different shallow ice-cores and the atmospheric pressure conditions from the twentieth century reanalysis (20CR). As a first step we have verified that the leading modes of winter and annual δ18O are well correlated with oceanic (Atlantic multidecadal oscillation) and atmospheric [North Atlantic oscillation (NAO)] indices respectively, and also marginally with external forcings, thus confirming earlier studies. The link between weather regimes and Greenland precipitation, precipitation-weighted temperature and δ18O is further explored by using an isotope simulation from the LMDZ-iso model, where the 3-dimensional wind fields are nudged to those of 20CR. In winter, the NAO+ and NAO− regimes in LMDZ-iso produce the largest isotopic changes over the entire Greenland region, with maximum anomalies in the South. Likewise, the Scandinavian blocking and the Atlantic ridge also show remarkable imprints on isotopic composition over the region. To assess the robustness and model dependency of our findings, a second isotope simulation from the isotopic model is also explored. The percentage of Greenland δ18O variance explained by the ensemble of weather regimes is increased by a factor near two in both LMDZ-iso and IsoGSM when compared to the contribution of the NAO index only. Similarly, weather regimes provide a net gain in the δ18O variance explained of similar magnitude for the whole set of ice core records. Greenland δ18O also appears to be locally affected by the low-frequency variations in the centres of action of the weather regimes, with clearer imprints in the LMDZ-iso simulation. This study opens the possibility for reconstructing past changes in the frequencies of occurrence of the weather regimes, which would rely on the sensitive regions identified here, and the use of additional proxies over the North Atlantic region.

Impact of the winter North-Atlantic weather regimes on subtropical sea-surface height variability

Interannual variability of subtropical sea-surface-height (SSH) anomalies, estimated by satellite and tide-gauge data, is investigated in relation to wintertime daily North-Atlantic weather regimes. Sea-level anomalies can be viewed as proxies for the subtropical gyre intensity because of the intrinsic baroclinic structure of the circulation. Our results show that the strongest correlation between SSH and weather regimes is found with the so-called Atlantic-Ridge (AR) while no significant values are obtained for the other regimes, including those related to the North Atlantic Oscillation (NAO), known as the primary actor of the Atlantic dynamics. Wintertime AR events are characterized by anticyclonic wind anomalies off Europe leading to a northward shift of the climatological wind-stress curl. The latter affects subtropical SSH annual variability by altered Sverdrup balance and ocean Rossby wave dynamics propagating westward from the African coast towards the Caribbean. The use of a simple linear planetary geostrophic model allows to quantify those effects and confirms the primary importance of the winter season to explain the largest part of SSH interannual variability in the Atlantic subtropical gyre. Our results open new perspectives in the comprehension of North-Atlantic Ocean variability emphasizing the role of AR as a driver of interannual variability at least of comparable importance to NAO.

North-Atlantic dynamics and European temperature extremes in the IPSL model: sensitivity to atmospheric resolution

The variability of the European climate is mostly controlled by the unstable nature of the North-Atlantic dynamics, especially in wintertime. The intra-seasonal to inter-annual fluctuations of atmospheric circulations has often been described as the alternation between a limited number of preferential weather regimes. Such discrete description can be justified by the multi-modality of the latitudinal position of the jet stream. In addition, seasonal extremes in European temperatures are generally associated with an exceptional persistence into one weather regime. Here we investigate the skill of the IPSL model to both simulate North-Atlantic weather regimes and European temperature extremes, including summer heat waves and winter cold spells. We use a set of eight IPSL experiments, with six different horizontal resolutions and the two versions used in CMIP3 and CMIP5. We find that despite a substantial deficit in the simulated poleward peak of the jet stream, the IPSL model represents weather regimes fairly well. A significant improvement is found for all horizontal resolutions higher than the one used in CMIP3, while the increase in vertical resolution included in the CMIP5 version tends to improve the wintertime dynamics. In addition to a recurrent cold bias over Europe, the IPSL model generally overestimates (underestimates) the indices of winter cold spells (summer heat waves) such as frequencies or durations. We find that the increase in horizontal resolution almost always improves these statistics, while the influence of vertical resolution is less clear. Overall, the CMIP5 version of the IPSL model appears to carry promising improvements in the simulation of the European climate variability.

Simulation of the Madden- Julian Oscillation and its teleconnections in the ECMWF forecast system

A series of 46-day ensemble integrations starting on the 15th of each month from 1989 to 2008 has been completed with the European Centre for Medium-Range Weather Forecasts (ECMWF) forecast system. The Madden– Julian Oscillation (MJO) simulated by the hindcasts is diagnosed using an index based on combined empirical orthogonal functions (EOFs) of zonal winds at 200 and 850 hPa and outgoing long-wave radiation. Results indicate that the dynamical model is able to maintain the amplitude of the MJO during the 46 days of integrations and the model displays skill for up to about 20 days to predict the evolution of the MJO. However, the MJO simulated by the model has a too slow eastward propagation and has difficulties crossing the Maritime Continent. The MJO teleconnections simulated by the ECMWF forecast system have been compared to reanalyses. In the Tropics, the impact of the MJO on precipitation is generally consistent with reanalysis. In the Northern Extratropics, the MJO simulated by the model has an impact on North Atlantic weather regimes, but with a smaller amplitude than in reanalysis which can be partly explained by the too slow eastward propagation of the simulated MJO events. The impact of the MJO on the probabilistic skill scores has been assessed. Results indicate that the MJO simulated by the model has a statistically significant impact on weekly mean probabilistic skill scores in the Northern Extratropics, particularly at the time range 19– 25 days. At this time range, the reliability of the probabilistic forecasts over Europe depends strongly on the presence of an MJO event in the initial conditions. This result confirms that the MJO is a major source of predictability in the Extratropics in the sub-seasonal time-scale. Copyright © 2010 Royal Meteorological Society

Understanding the West African monsoon variability and its remote effects: an illustration of the grid point nudging methodology

General circulation models still show deficiencies in simulating the basic features of the West African Monsoon at intraseasonal, seasonal and interannual timescales. It is however, difficult to disentangle the remote versus regional factors that contribute to such deficiencies, and to diagnose their possible consequences for the simulation of the global atmospheric variability. The aim of the present study is to address these questions using the so-called grid point nudging technique, where prognostic atmospheric fields are relaxed either inside or outside the West African Monsoon region toward the ERA40 reanalysis. This regional or quasi-global nudging is tested in ensembles of boreal summer simulations. The impact is evaluated first on the model climatology, then on intraseasonal timescales with an emphasis on North Atlantic/Europe weather regimes, and finally on interannual timescales. Results show that systematic biases in the model climatology over West Africa are mostly of regional origin and have a limited impact outside the domain. A clear impact is found however on the eddy component of the extratropical circulation, in particular over the North Atlantic/European sector. At intraseasonal timescale, the main regional biases also resist to the quasi-global nudging though their magnitude is reduced. Conversely, nudging the model over West Africa exerts a strong impact on the frequency of the two North Atlantic weather regimes that favor the occurrence of heat waves over Europe. Significant impacts are also found at interannual timescale. Not surprisingly, the quasi-global nudging allows the model to capture the variability of large-scale dynamical monsoon indices, but exerts a weaker control on rainfall variability suggesting the additional contribution of regional processes. Conversely, nudging the model toward West Africa suppresses the spurious ENSO teleconnection that is simulated over Europe in the control experiment, thereby emphasizing the relevance of a realistic West African monsoon simulation for seasonal prediction in the extratropics. Further experiments will be devoted to case studies aiming at a better understanding of regional processes governing the monsoon variability and of the possible monsoon teleconnections, especially over Europe.

Summer interactions between weather regimes and surface ocean in the North-Atlantic region

This study aims at understanding the summer ocean-atmosphere interactions in the North Atlantic European region on intraseasonal timescales. The CNRMOM1d ocean model is forced with ERA40 (ECMWF Re-Analysis) surface fluxes with a 1-h frequency in solar heat flux (6 h for the other forcing fields) over the 1959–2001 period. The model has 124 vertical levels with a vertical resolution of 1 m near the surface and 500 m at the bottom. This ocean forced experiment is used to assess the impact of the North Atlantic weather regimes on the surface ocean. Composites of sea surface temperature (SST) anomalies associated with each weather regime are computed and the mechanisms explaining these anomalies are investigated. Then, the SST anomalies related to each weather regime in the ocean-forced experiment are prescribed to the ARPEGE Atmosphere General Circulation Model. We show that the interaction with the surface ocean induces a positive feedback on the persistence of the Blocking regime, a negative feedback on the persistence of the NAO-regime and favours the transition from the Atlantic Ridge regime to the NAO-regime and from the Atlantic Low regime toward the Blocking regime.

North Atlantic weather regimes response to Indian‐western Pacific Ocean warming: A multi‐model study

The extratropical large‐scale atmospheric circulation is often described in terms of a few preferred and recurrent patterns referred to as weather regimes. Here, we investigate the influence of the observed Indian and western Pacific Ocean (IP) warming over the last decades, on the frequency of occurrence of North Atlantic weather regimes. A multi‐model approach is adopted in which five different atmospheric general circulation models are forced with idealized sea surface temperature patterns mimicking the IP warming. Despite some discrepancies, three models suggest a stronger occurrence of the Zonal regime when IP is warm, compensated by less frequent Greenland Anticyclone regimes, consistently with the observed positive trend of the North Atlantic Oscillation. The other two models simulate instead an increase in the frequency of occurrence of the Atlantic Ridge regime. Variance decomposition into stationary and transient waves suggest two mechanisms at work in the individual models. The AR regime favoured occurrence is associated with a strong transient wave activity along the wave guide in the North Pacific and downstream in the North Atlantic. The Zonal favoured excitation is interpreted as an indirect response to changes in the Tropical Atlantic associated with a global alteration of the Walker cell.

North Atlantic ultra high frequency variability: An introductory survey

This study attempts to depict east Atlantic cyclones specificity using ECMWF 1984–1994 cold season analyses. Due to the strong low frequency variability in this area, a climatology of transient events has to take the “background” weather regime into account. North Atlantic weather regimes have therefore been recovered through cluster analysis techniques. An objective criterion defining the occurrence of subsynoptic cyclones is then designed. It uses the high frequency signature of vorticity. The events thus retained include frontal waves, cold air cyclogenesis and other small scale developments. The preferred location of these events is found to be quite distinct from the classical storm-tracks. They appear downstream and slightly south of them, reaching the north-western part of Europe during Zonal weather regimes, and a zone centred some 500 km north of the Azores during Greenland Anticyclone regimes. In order to gain some idea of the various types of subsynoptic cyclones captured by the criterion, a one-point selection of cases was performed and the resulting vorticity fields were partitioned by a cluster analysis. Two types of waves out of three clusters are documented. A Type 1 is shown to be a modified baroclinic development, with reduced scale and features well accounted for by recent theoretical work on frontal waves. A Type 2 composite, with its typical warm front, is more difficult to set into an existing theoretical framework. It seems connected to the presence of an actively diffluent background flow. DOI: 10.1034/j.1600-0870.1995.00112.x