scholarly journals The North Atlantic variability structure, storm tracks, and precipitation depending on the polar vortex strength

2005 ◽  
Vol 5 (1) ◽  
pp. 239-248 ◽  
Author(s):  
K. Walter ◽  
H.-F. Graf
Keyword(s):  
North Atlantic ◽  
Storm Track ◽  
Polar Vortex ◽  
The State ◽  
Storm Tracks ◽  
The Arctic ◽  
Boreal Winter ◽  
Vortex Strength ◽  
The North ◽  
The North Atlantic

Abstract. Motivated by the strong evidence that the state of the northern hemisphere vortex in boreal winter influences tropospheric variability, teleconnection patterns over the North Atlantic are defined separately for winter episodes where the zonal wind at 50hPa and 65° N is above or below the critical velocity for vertical propagation of zonal planetary wave 1. We argue that the teleconnection structure in the middle and upper troposphere differs considerably between the two regimes of the polar vortex, while this is not the case at sea level. If the polar vortex is strong, there exists one meridional dipole structure of geopotential height in the upper and middle troposphere, which is situated in the central North Atlantic. If the polar vortex is weak, there exist two such dipoles, one over the western and one over the eastern North Atlantic. Storm tracks (and precipitation related with these) are determined by mid and upper tropospheric conditions and we find significant differences of these parameters between the stratospheric regimes. For the strong polar vortex regime, in case of a negative upper tropospheric "NAO" index we find a blocking height situation over the Northeast Atlantic and the strongest storm track of all. It is reaching far north into the Arctic Ocean and has a secondary maximum over the Denmark Strait. Such storm track is not found in composites based on a classic NAO defined by surface pressure differences between the Icelandic Low and the Azores High. Our results suggest that it is important to include the state of the polar vortex strength in any study of the variability over the North Atlantic.

scholarly journals The North Atlantic variability structure, storm tracks, and precipitation depending on the polar vortex strength

2004 ◽  
Vol 4 (5) ◽  
pp. 6127-6148 ◽  
Author(s):  
K. Walter ◽  
H.-F. Graf
Keyword(s):  
North Atlantic ◽  
Storm Track ◽  
Polar Vortex ◽  
The State ◽  
Storm Tracks ◽  
The Arctic ◽  
Ample Evidence ◽  
Vortex Strength ◽  
The North ◽  
The North Atlantic

Abstract. There is ample evidence that the state of the northern polar stratospheric vortex in boreal winter influences tropospheric variability. Therefore, the main teleconnection patterns over the North Atlantic are defined separately for winter episodes in which the zonal mean wind at 50 hPa and 65° N is above or below the critical Rossby velocity for zonal planetary wave one. It turns out that the teleconnection structure in the middle and upper troposphere differs considerably between the two regimes of the polar vortex, while this is not the case at sea level. If the "polar vortex is strong", there exists "one" meridional dipole structure of geopotential height in the upper and middle troposphere, which is situated in the central North Atlantic. If the "polar vortex is weak", there exist "two" such dipoles, one over the western and one over the eastern North Atlantic. Storm tracks (and precipitation related with these) are determined by mid and upper tropospheric conditions and we find significant differences of these parameters between the stratospheric regimes. For the strong polar vortex regime, in case of a negative upper tropospheric "NAO" index we find a blocking height situation over the Northeast Atlantic and the strongest storm track of all. It is reaching far north into the Arctic Ocean and has a secondary maximum over the Denmark Strait. Such storm track is not found in composites based on a classic NAO defined by surface pressure differences between the Icelandic Low and the Azores High. Our results show that it is essential to include the state of the upper dynamic boundary conditions (the polar vortex strength) in any study of the variability over the North Atlantic. Climate forecast based solely on the forecast of a "classic NAO" and further statistical downscaling may lead to the wrong conclusions if the state of the polar vortex is not considered as well.

scholarly journals Northern hemisphere winter storm tracks of the Eemian interglacial and the last glacial inception

2007 ◽  
Vol 3 (2) ◽  
pp. 181-192 ◽  
Author(s):  
F. Kaspar ◽  
T. Spangehl ◽  
U. Cubasch
Keyword(s):  
Northern Hemisphere ◽  
North Atlantic ◽  
Storm Track ◽  
Storm Tracks ◽  
Winter Storm ◽  
The North ◽  
The North Atlantic ◽  
Hemisphere Winter ◽  
Eemian Interglacial ◽  
The Last Glacial

Abstract. Climate simulations of the Eemian interglacial and the last glacial inception have been performed by forcing a coupled ocean-atmosphere general circulation model with insolation patterns of these periods. The parameters of the Earth's orbit have been set to conditions of 125 000 and 115 000 years before present (yr BP). Compared to today, these dates represent periods with enhanced and weakened seasonality of insolation in the northern hemisphere. Here we analyse the simulated change in northern hemisphere winter storm tracks. The change in the orbital configuration has a strong impact on the meridional temperature gradients and therefore on strength and location of the storm tracks. The North Atlantic storm track is strengthened, shifted northward and extends further to the east in the simulation for the Eemian at 125 kyr BP. As one consequence, the northern parts of Europe experience an increase in winter precipitation. The frequency of winter storm days increases over large parts of the North Atlantic including the British Isles and the coastal zones of north-western Europe. Opposite but weaker changes in storm track activity are simulated for 115 kyr BP.

scholarly journals The Basic Ingredients of the North Atlantic Storm Track. Part I: Land–Sea Contrast and Orography

2009 ◽  
Vol 66 (9) ◽  
pp. 2539-2558 ◽  
Author(s):  
David James Brayshaw ◽  
Brian Hoskins ◽  
Michael Blackburn
Keyword(s):  
North Atlantic ◽  
Rocky Mountains ◽  
Large Scale ◽  
Storm Track ◽  
Storm Tracks ◽  
Cold Pool ◽  
North American Continent ◽  
The North ◽  
The North Atlantic ◽  
The Impact

Abstract Understanding and predicting changes in storm tracks over longer time scales is a challenging problem, particularly in the North Atlantic. This is due in part to the complex range of forcings (land–sea contrast, orography, sea surface temperatures, etc.) that combine to produce the structure of the storm track. The impact of land–sea contrast and midlatitude orography on the North Atlantic storm track is investigated through a hierarchy of GCM simulations using idealized and “semirealistic” boundary conditions in a high-resolution version of the Hadley Centre atmosphere model (HadAM3). This framework captures the large-scale essence of features such as the North and South American continents, Eurasia, and the Rocky Mountains, enabling the results to be applied more directly to realistic modeling situations than was possible with previous idealized studies. The physical processes by which the forcing mechanisms impact the large-scale flow and the midlatitude storm tracks are discussed. The characteristics of the North American continent are found to be very important in generating the structure of the North Atlantic storm track. In particular, the southwest–northeast tilt in the upper tropospheric jet produced by southward deflection of the westerly flow incident on the Rocky Mountains leads to enhanced storm development along an axis close to that of the continent’s eastern coastline. The approximately triangular shape of North America also enables a cold pool of air to develop in the northeast, intensifying the surface temperature contrast across the eastern coastline, consistent with further enhancements of baroclinicity and storm growth along the same axis.

scholarly journals Impact of a Reduced Arctic Sea Ice Cover on Ocean and Atmospheric Properties

2012 ◽  
Vol 25 (1) ◽  
pp. 307-319 ◽  
Author(s):  
Jan Sedláček ◽  
Reto Knutti ◽  
Olivia Martius ◽  
Urs Beyerle
Keyword(s):  
Sea Ice ◽  
North Atlantic ◽  
Arctic Basin ◽  
Ice Cover ◽  
Arctic Sea Ice ◽  
Storm Tracks ◽  
The Arctic ◽  
The North ◽  
Arctic Sea ◽  
The North Atlantic

Abstract The Arctic sea ice cover declined over the last few decades and reached a record minimum in 2007, with a slight recovery thereafter. Inspired by this the authors investigate the response of atmospheric and oceanic properties to a 1-yr period of reduced sea ice cover. Two ensembles of equilibrium and transient simulations are produced with the Community Climate System Model. A sea ice change is induced through an albedo change of 1 yr. The sea ice area and thickness recover in both ensembles after 3 and 5 yr, respectively. The sea ice anomaly leads to changes in ocean temperature and salinity to a depth of about 200 m in the Arctic Basin. Further, the salinity and temperature changes in the surface layer trigger a “Great Salinity Anomaly” in the North Atlantic that takes roughly 8 yr to travel across the North Atlantic back to high latitudes. In the atmosphere the changes induced by the sea ice anomaly do not last as long as in the ocean. The response in the transient and equilibrium simulations, while similar overall, differs in specific regional and temporal details. The surface air temperature increases over the Arctic Basin and the anomaly extends through the whole atmospheric column, changing the geopotential height fields and thus the storm tracks. The patterns of warming and thus the position of the geopotential height changes vary in the two ensembles. While the equilibrium simulation shifts the storm tracks to the south over the eastern North Atlantic and Europe, the transient simulation shifts the storm tracks south over the western North Atlantic and North America. The authors propose that the overall reduction in sea ice cover is important for producing ocean anomalies; however, for atmospheric anomalies the regional location of the sea ice anomalies is more important. While observed trends in Arctic sea ice are large and exceed those simulated by comprehensive climate models, there is little evidence based on this particular model that the seasonal loss of sea ice (e.g., as occurred in 2007) would constitute a threshold after which the Arctic would exhibit nonlinear, irreversible, or strongly accelerated sea ice loss. Caution should be exerted when extrapolating short-term trends to future sea ice behavior.

Trends and characteristics of winter storms in CMIP6

2021 ◽  
Author(s):  
Mareike Schuster ◽  
Uwe Ulbrich
Keyword(s):  
Climate Change ◽  
North Atlantic ◽  
Life Time ◽  
Storm Tracks ◽  
The Arctic ◽  
Cmip5 Models ◽  
Atlantic Basin ◽  
North Atlantic Basin ◽  
The North ◽  
The North Atlantic

<p>Windstorms are considered the most devastating natural peril in many regions around the globe. For insurance associations in Europe for example, the damages generated by windstorms make up to about 90% of the claims in the category of natural hazards. The interannual variability of windstorms can be quite strong and thus research has recently focused on this topic.</p><p>However, storm risk and its changes under anthropogenically induced climate change are so far rather little discussed in literature. Thus, there are still large uncertainties about how climate change will affect the extratropical circulation. CMIP5 models showed at times opposing signals regarding number and strength of windstorm generating cyclones and storm tracks. In more detail, the latest IPCC AR5 states that substantial uncertainty and low confidence remains in projecting changes in NH storm tracks, especially for the North Atlantic basin.</p><p>With the lately released CMIP6 simulations, providing model output of increased spatial and temporal resolution, there is potential for new insights and enhanced confidence regarding future trends of storminess.</p><p>In our study, we assess characteristics and trends of windstorm diagnostics in an ensemble of the latest CMIP6 climate scenario simulations, with a focus to the North Atlantic basin and winterstorms affecting Europe.</p><p>In the CMIP6 model ensemble the trends of winter windstorm frequencies appear to be overall weaker in an anthropogenically changed climate than in the preceding CMIP5 scenarios; yet, first results indicate that they are somewhat more consistent amongst models. All CMIP6 models exhibit a windstorm frequency increase locally confined over the Arctic, while in the mid and high latitudes a wide-ranging decrease of windstorm activity is simulated. In our study we will also assess what this entails for characteristics like life time, intensity and size.</p>

scholarly journals Tropical and Extratropical Responses of the North Atlantic Atmospheric Circulation to a Sustained Weakening of the MOC

2009 ◽  
Vol 22 (11) ◽  
pp. 3146-3155 ◽  
Author(s):  
David J. Brayshaw ◽  
Tim Woollings ◽  
Michael Vellinga
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Western Europe ◽  
Storm Track ◽  
Meridional Overturning Circulation ◽  
Boreal Winter ◽  
Mean Flow ◽  
Near Surface ◽  
The North ◽  
The North Atlantic

Abstract The tropospheric response to a forced shutdown of the North Atlantic Ocean’s meridional overturning circulation (MOC) is investigated in a coupled ocean–atmosphere GCM [the third climate configuration of the Met Office Unified Model (HadCM3)]. The strength of the boreal winter North Atlantic storm track is significantly increased and penetrates much farther into western Europe. The changes in the storm track are shown to be consistent with the changes in near-surface baroclinicity, which can be linked to changes in surface temperature gradients near regions of sea ice formation and in the open ocean. Changes in the SST of the tropical Atlantic are linked to a strengthening of the subtropical jet to the north, which, combined with the enhanced storm track, leads to a pronounced split in the jet structure over Europe. EOF analysis and stationary box indices methods are used to analyze changes to the North Atlantic Oscillation (NAO). There is no consistent signal of a change in the variability of the NAO, and while the changes in the mean flow project onto the positive NAO phase, they are significantly different from it. However, there is a clear eastward shift of the NAO pattern in the shutdown run, and this potentially has implications for ocean circulation and for the interpretation of proxy paleoclimate records.

scholarly journals Changes in Northern Hemisphere Winter Storm Tracks under the Background of Arctic Amplification

2017 ◽  
Vol 30 (10) ◽  
pp. 3705-3724 ◽  
Author(s):  
Jiabao Wang ◽  
Hye-Mi Kim ◽  
Edmund K. M. Chang
Keyword(s):  
North America ◽  
North Atlantic ◽  
North Pacific ◽  
Storm Track ◽  
Storm Tracks ◽  
Arctic Amplification ◽  
Northeastern North America ◽  
The North ◽  
The North Atlantic ◽  
The North Pacific

Abstract An interdecadal weakening in the North Atlantic storm track (NAST) and a poleward shift of the North Pacific storm track (NPST) are found during October–March for the period 1979–2015. A significant warming of surface air temperature (Ts) over northeastern North America and a La Niña–like change in the North Pacific under the background of Arctic amplification are found to be the contributors to the observed changes in the NAST and the NPST, respectively, via modulation of local baroclinicity. The interdecadal change in baroclinic energy conversion is consistent with changes in storm tracks with an energy loss from eddies to mean flow over the North Atlantic and an energy gain over the North Pacific. The analysis of simulations from the Community Earth System Model Large Ensemble project, although with some biases in storm-track and Ts simulations, supports the observed relationship between the NAST and Ts over northeastern North America, as well as the link between the NPST and El Niño–Southern Oscillation. The near-future projections of Ts and storm tracks are characterized by a warmer planet under the influence of increasing greenhouse gases and a significant weakening of both the NAST and the NPST. The potential role of the NAST in redistributing changes in Ts over the surrounding regions is also examined. The anomalous equatorward moisture flux associated with the weakening trend of the NAST would enhance the warming over its upstream region and hinder the warming over its downstream region via modulation of the downward infrared radiation.

scholarly journals Impact of ocean resolution on storms in the North Atlantic region

2021 ◽  
Author(s):  
Johanna Knauf ◽  
Joakim Kjellsson ◽  
Annika Reintges
Keyword(s):  
Heat Flux ◽  
North Atlantic ◽  
Storm Track ◽  
Life Time ◽  
Horizontal Resolution ◽  
Storm Tracks ◽  
Cold Bias ◽  
The North ◽  
Storm Tracking ◽  
The North Atlantic

<p>We study the impact of ocean horizontal resolution on storm tracks over the North Atlantic Ocean using the FOCI-OpenIFS climate model and the TRACK storm-tracking algorithm. We find that increasing ocean resolution from 1/2° to 1/10° reduces a cold bias over the North Atlantic which leads to a northward shift of the storm tracks, in particular in winter and spring seasons.<span> </span></p><p>Most climate models with non-eddying oceans, i.e. horizontal resolutions of 100 km or higher, suffer from a cold SST bias in the North Atlantic. Refining the horizontal resolution from 1/2° to 1/10° allows for a distinct Gulf Stream extension and better representation of the major current systems which reduces this cold bias. The associated warming of the ocean surface with increasing resolution also warms the troposphere and leads to a northward shift in the tropospheric eddy-driven jet. Overall, the increased ocean resolution thus improves the ocean circulation as well as the atmospheric circulation.<span> </span></p><p>We use two metrics to evaluate the storm track activity in the simulations. We calculate 2-8 day bandpass-filtered mean sea-level pressure (MSLP) and eddy heat flux (v’T’) which is an Eulerian metric that shows variability of low- and high-pressure systems as well as their associated heat flux, but says nothing about the genesis, lysis or life time of individual storms. We also use the TRACK storm-tracking algorithm with 12-hourly MSLP data to produce trajectories of individual storms, which allows us to study individual storms.<span> </span></p><p>The Eulerian approach using MSLP variance and eddy heat fluxes clearly shows a northward shift of the storm tracks as the ocean resolution is increased. Overall, the northward shift leads to reduced biases compared to ERA-Interim reanalysis. Storm-track trajectories show higher storm track and storm genesis densities around 60°N with the higher ocean resolution. Interestingly, a higher ocean resolution also results in longer life time of storms. We speculate that this is due to enhanced air-sea interactions where cyclones are fed more energy from the eddy-resolving ocean than from the non-eddying ocean.</p>

scholarly journals Using Bayesian Networks to investigate the influence of subseasonal Arctic variability on midlatitude North Atlantic circulation

2020 ◽  
pp. 1-46
Author(s):  
Nathanael Harwood ◽  
Richard Hall ◽  
Giorgia Di Capua ◽  
Andrew Russell ◽  
Allan Tucker
Keyword(s):  
Bayesian Networks ◽  
North Atlantic ◽  
Hidden Variables ◽  
Polar Vortex ◽  
The Arctic ◽  
Stratospheric Polar Vortex ◽  
Arctic Warming ◽  
The North ◽  
The North Atlantic

AbstractRecent enhanced warming and sea ice depletion in the Arctic have been put forward as potential drivers of severe weather in the midlatitudes. Evidence of a link between Arctic warming and midlatitude atmospheric circulation is growing, but the role of Arctic processes relative to other drivers remains unknown. Arctic-midlatitude connections in the North Atlantic region are particularly complex but important due to the frequent occurrence of severe winters in recent decades. Here, Dynamic Bayesian Networks with hidden variables are introduced to the field to assess their suitability for teleconnection analyses. Climate networks are constructed to analyse North Atlantic circulation variability at 5-day to monthly timescales during the winter months of the years 1981-2018. The inclusion of a number of Arctic, midlatitude and tropical variables allows for an investigation into the relative role of Arctic influence compared to internal atmospheric variability and other remote drivers.A robust covariability between regions of amplified Arctic warming and two definitions of midlatitude circulation is found to occur entirely within winter at submonthly timescales. Hidden variables incorporated in networks represent two distinct modes of stratospheric polar vortex variability, capturing a periodic shift between average conditions and slower anomalous flow. The influence of the Barents-Kara Seas region on the North Atlantic Oscillation is found to be the strongest link at 5- and 10-day averages, whilst the stratospheric polar vortex strongly influences jet variability on monthly timescales.

scholarly journals Further Investigation of the Impact of Idealized Continents and SST Distributions on the Northern Hemisphere Storm Tracks

2012 ◽  
Vol 69 (3) ◽  
pp. 840-856 ◽  
Author(s):  
Jérôme Saulière ◽  
David James Brayshaw ◽  
Brian Hoskins ◽  
Michael Blackburn
Keyword(s):  
Boundary Conditions ◽  
North Atlantic ◽  
North Pacific ◽  
Rossby Waves ◽  
Storm Track ◽  
Storm Tracks ◽  
The North ◽  
The Pacific ◽  
The North Atlantic ◽  
The Impact

Abstract Building on previous studies of the basic ingredients of the North Atlantic storm track (examining land–sea contrast, orography, and SST), this article investigates the impact of Eurasian topography and Pacific SST anomalies on North Pacific and Atlantic storm tracks through a hierarchy of atmospheric GCM simulations using idealized boundary conditions in the Hadley Centre HadGAM1 atmospheric circulation model. The Himalaya–Tibet mountain complex is found to play a crucial role in shaping the North Pacific storm track. The northward deflection of the westerly flow around northern Tibet generates an extensive pool of very cold air in the northeastern tip of the Asian continent, which strengthens the meridional temperature gradient and favors baroclinic growth in the western Pacific. The Kuroshio SST front is also instrumental in strengthening the Pacific storm track through its impact on near-surface baroclinicity, while the warm waters around Indonesia tend to weaken it through the impact on baroclinicity of stationary Rossby waves propagating poleward from the convective heating regions. Three mechanisms by which the Atlantic storm track may be affected by changes in the boundary conditions upstream of the Rockies are discussed. In the model configuration used here, stationary Rossby waves emanating from Tibet appear to weaken the North Atlantic storm track substantially, whereas those generated over the cold waters off Peru appear to strengthen it. Changes in eddy-driven surface winds over the Pacific generally appear to modify the flow over the Rocky Mountains, leading to consistent modifications in the Atlantic storm track. The evidence for each of these mechanisms is, however, ultimately equivocal in these simulations.

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