Mesocyclone activity over the North-East Atlantic. Part 1: vortex distribution and variability

Harold, Julie M.; Bigg, Grant R.; Turner, John

doi:10.1002/(sici)1097-0088(199909)19:11<1187::aid-joc419>3.0.co;2-q

Cited by 51 publications

(34 citation statements)

References 22 publications

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“…The location of each polar low is defined as the location at which its track is detected for the first time. Highest absolute frequency and frequency per maritime area are found in the region south and east of Iceland in accordance with investigations of Harold et al [1999] or Condron et al [2006]. Although Bracegirdle and Gray [2008] mainly focus their investigations on the North Atlantic around Norway, they also detect a maximum of possible polar lows in the Iceland-Greenland region.…”

Section: Trends and Variabilitysupporting

confidence: 81%

A long‐term climatology of North Atlantic polar lows

Zahn¹,

Storch²

2008

Geophysical Research Letters

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[1] A dynamical downscaling of the 6-hourly 1948 -2006 NCEP/NCAR re-analyses for the subarctic region of the North Atlantic has been used to derive a climatology of polar lows by means of a tracking algorithm based on the simulated bandpass filtered MSLP-fields. This climatology is consistent with the limited observational evidence in terms of frequency and spatial distribution. The climatology exhibits strong year-to-year variability but weak decadal variability and a negligible trend. A Canonical Correlation Analysis describes the conditioning of the formation of polar lows by characteristic meridional seasonal mean flow regimes, which favor cold air outbreaks and upper air troughs.

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Section: Trends and Variabilitysupporting

confidence: 81%

A long‐term climatology of North Atlantic polar lows

Zahn¹,

Storch²

2008

Geophysical Research Letters

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“…10d-f) for ERA-40 and seasonal integrations at two different horizontal resolutions (T L 95 versus T L 255). All three datasets show that most cyclones being generated in the Irminger Sea do not propagate much farther than Iceland; that is, they are quasi-stationary (see also Harold et al 1999). The more migratory systems, albeit less frequent, tend to propagate toward the Nordic Seas and northern Europe.…”

Section: Sensitivity Of Extratropical Cyclones Near Greenland To Horimentioning

confidence: 93%

Greenland’s Pressure Drag and the Atlantic Storm Track

Jung

Rhines

2007

Journal of the Atmospheric Sciences

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Some effects of Greenland on the Northern Hemisphere wintertime circulation are discussed. Inviscid pressure drag on Greenland's slopes, calculated from reanalysis data, is related to circulation patterns. Greenland lies north of the core of the tropospheric westerly winds. Yet strong standing waves, which extend well into the stratosphere, produce a trough/ridge system with jet stream lying close to Greenland, mean Icelandic low in its wake, and storm track that interacts strongly with its topography. In the lower troposphere, dynamic height anomalies associated with strongly easterly pressure drag on the atmosphere are quite localized in space and relatively short-lived compared to upper levels, yet they involve a hemispheric-scale dislocation of the stratospheric polar vortex. It is a two-scale problem, however; the high-pass time-filtered part of the height field, responsible for 73% of the pressure drag, is quite different, and expresses propagating cyclonic development in the Atlantic storm track. Eliassen-Palm flux (EP flux) analysis shows that the atmospheric response is (counterintuitively) an acceleration of the westerly winds. The hemispheric influence is consistent with the model results of Junge et al. suggesting that Greenland affects the stationary waves in winter.This discussion shows that Greenland is not a simple "stirring rod" in the westerly circulation, yet involvement of Greenland's topography with the shape, form, and intensity of the storm track is strong. Interaction of traveling storms, the jet stream, and the orographic wake frequently leads to increase of the lateral scale such that cyclonic system expands to the size of Greenland itself (ϳ2500 km). Using the global ECMWF general circulation model, the authors explore the effect of model resolution on these circulations. Statistically, in two case studies, and in higher-resolution global models at T L 255 to T L 799 resolution, intense tip jet, hydraulic downslope jet, and gravity wave radiation appear in strong flow events, in accord with the work of Doyle and Shapiro. Three-dimensional particle trajectories and vorticity maps show the nature and intensity of the summit-gap flow. Cyclonic systems in the lee of Greenland are strongly affected by the downslope jet. Penetration of the Arctic Basin by cyclonic systems arises from this source region, and the amplitude of the pressure drag is enhanced at high resolution. At the higher resolutions, storm-track analysis verifies the splitting of the storm track by Greenland with a substantial minority of storms moving northward through Baffin Bay. Finally, analysis of 20 winters of 40-yr ECMWF Re-Analysis (ERA-40) reforecasts shows little evidence that negative pressure-drag events are followed by anomalously large forecast errors over Europe, throughout the forecast. Forecast skill for the pressure drag is surprisingly good, with a correlation of 0.65 at 144 h.

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“…The majority of the PLs formed between Spitsbergen and the Norwegian mainland or over the Barents Sea and subsequently moved southwards. The high frequency of PLs over the Norwegian and Barents Seas is well known, but other regions of the Northern Hemisphere (NH), such as the Sea of Japan, the Sea of Okhotsk and the region between Iceland and Canada, including the Labrador Sea, have also been found to be prone to MCAOs and PLs (Harold et al, 1999;Kolstad, 2006;Blechschmidt, 2008;Bracegirdle and Gray, 2008;Zahn and von Storch, 2008;Kolstad et al, 2009). PLs have also been identified in the Southern Hemisphere (SH) (Carleton and Carpenter, 1990), and recent work indicates that MCAOs near the Antarctic sea ice edge can be as strong as in the NH (Bracegirdle and Kolstad, 2010).…”

Section: Introductionmentioning

confidence: 99%

A global climatology of favourable conditions for polar lows

Kolstad

2011

Quart J Royal Meteoro Soc

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Polar lows (PLs) are small-scale and intense low-pressure systems that form at high latitudes in both hemispheres. Due to their limited spatial scale and brief lifetimes, weather and climate models are often unable to resolve these systems. One way to overcome this problem is to define a suitable proxy for PLs, with which the likelihood of PL formation can be assessed even in coarse-resolution datasets. This study draws on previous studies and an empirical database of 63 PLs to quantify the respective influences of low-level static stability and upper-level forcing on PL formation, as both of these factors are known to favour PL development. Little redundancy between the two parameters is found. After defining threshold values for the two parameters, climatological properties of favourable conditions for PLs are computed for the North Atlantic, the North-West Pacific and the Southern Hemisphere. The low-level static stability, which is strongly modified during marine cold-air outbreaks, puts important constraints on where PLs can form, while the upper-level forcing determines whether or not they will form. As a result of the climatologically lower tropopause in the Labrador Sea region, favourable conditions for PLs occur more often there than in the Nordic Seas, which has long been believed to be the main PL region in the Northern Hemisphere (NH). In the Southern Hemisphere, favourable conditions for PLs occur substantially less often than in the NH. The PL index defined here is suitable for other climatological studies and PL forecasting.

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Mesocyclone activity over the North-East Atlantic. Part 1: vortex distribution and variability

Cited by 51 publications

References 22 publications

A long‐term climatology of North Atlantic polar lows

A long‐term climatology of North Atlantic polar lows

Greenland’s Pressure Drag and the Atlantic Storm Track

A global climatology of favourable conditions for polar lows

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