Publications

Kolstad, E. W. & T. Breiteig & A. A. Scaife: The association between stratospheric weak polar vortex events and cold air outbreaks. Submitted to Quarterly Journal of the Royal Meteorological Society

Abstract
Previous studies have identified an association between near-surface temperature anomalies in the Northern Hemisphere and weak stratospheric polar westerlies. Large regions in northern Asia, Europe and North America have been found to cool in the mature and late stages of stratospheric weak vortex events. A substantial part of the temperature changes are associated with changes to the tropospheric Northern Annular Mode and North Atlantic Oscillation pressure patterns. The apparent coupling between the stratosphere and the troposphere may be of relevance for weather forecasting, but only if the temporal and spatial nature of the coupling is known. Here we show, using 51 winters of re-analysis data, that the tropospheric temperature development relative to stratospheric weak polar vortex events goes through a series of well-defined stages, including geographically distinct cold air outbreaks. At the inception of weak vortex events, a precursor signal in the form of a strong high-pressure anomaly is found over Northwest Europe. At the same time, long-lived and robust cold anomalies appear over Asia and Western Europe. A few weeks later, near the mature stage of weak vortex events, a shorter-lived cold anomaly emerges off the east coast of North America.The probability of cold air outbreaks in different phases of the weak vortex life cycle increases by 40-70 % in four key regions. This shows that the stratospheric polar vortex contains information that can be used to enhance forecasts of cold air outbreaks. 300-year pre-industrial control runs of 11 state-of-the-art coupled climate models corroborate our results.

---

Kolstad, E. W. & T. J. Bracegirdle & I. A. Seierstad: Marine cold-air outbreaks in the North Atlantic: temporal distribution and associations with large-scale atmospheric circulation. Climate Dynamics, published online 19 June, 2008. DOI:10.1007/s00382-008-0431-5

Abstract
The spatial and temporal distributions of marine cold air outbreaks (MCAOs) over the northern North Atlantic have been investigated using re-analysis data for the period from 1958 to 2007. MCAOs are large-scale outbreaks of cold air over a relatively warm ocean surface. Such conditions are known to increase the severity of particular types of hazardous mesoscale weather phenomena. We used a simple index for identifying MCAOs: the vertical potential temperature gradient between the sea surface and 700 hPa. It was found that atmospheric temperature variability is considerably more important than the sea surface temperature variability in governing both the seasonal and the inter-annual variability of MCAOs. Furthermore, a composite analysis revealed that a few well-defined and robust synoptic patterns are evident during MCAOs in winter. Over the Labrador and Irminger Seas the MCAO index was found to have a correlation of 0.70 with the North Atlantic Oscillation (NAO) index, while over the Barents Sea a negative correlation of 0.42 was found.

---

Kolstad, E. W.: A QuikSCAT climatology of ocean surface winds in the Nordic Seas: identification of features and comparison with NCEP/NCAR reanalysis. J. Geophys. Res., 113, D11106. DOI: 10.1029/2007JD008918

Abstract
High-resolution satellite-derived QuikSCAT ocean surface wind data are used to provide an 8-year climatology of average as well as below- and above-average winds in the Nordic Seas. A number of localized wind speed maxima are identified, with average wintertime wind speeds exceeding 14 m/s in the Denmark Strait. Five distinct wind speed nadirs are also discovered, of which the two most pronounced are located in the Greenland Sea. In the second part of the paper, two data sets derived from the NCEP/NCAR reanalysis are found to concur well with QuikSCAT in terms of both low-frequency (monthly means) and high-frequency (daily means) variability. Correlation coefficients between QuikSCAT and reanalysis daily and monthly mean wind speed are around 0.9. Notable and systematic differences are also identified. Root mean square differences between QuikSCAT and reanalysis of daily mean wind speed averaged over four separate regions ranged from 1.11 to 1.81 m/s, and from 0.75 to 1.00 m/s for monthly means. Furthermore, a correspondence between the ocean surface wind speed and five teleconnection indices, of which the North Atlantic Oscillation (NAO) and the Scandinavian pattern are the most important, is found.

---

Kolstad, E. W. & Bracegirdle, T. J. (2008): Marine cold-air outbreaks in the future: an assessment of IPCC AR4 model results for the northern hemisphere. Climate Dynamics, 30, 871-885. DOI: 10.1007/s00382-007-0331-0

Abstract
For many locations around the globe some of the most severe weather is associated with outbreaks of cold air over relatively warm oceans, referred to here as marine cold-air outbreaks (MCAOs). Drawing on empirical evidence, an MCAO indicator is defined here as the difference between the skin potential temperature, which over open ocean is the sea surface potential temperature, and the potential temperature at 700 hPa. Extreme MCAOs are defined as the 95th percentile of this indicator. Climate model data that have been provided as part of the Intergovernmental Panel on Climate Change (IPCC) Assessment Report Four (AR4) were used to assess the models’ projections for the 21st century and their ability to represent the observed climatology of MCAOs. The ensemble average of the models broadly captures the observed spatial distribution of the strength of MCAOs. However, there are some significant differences between the models and observations, which are mainly associated with simulated biases of the underlying sea ice, such as excessive sea-ice extent over the Barents Sea in most of the models. The future changes of the strength of MCAOs vary significantly across the Northern Hemisphere. The largest projected weakening of MCAOs is over the Labrador Sea. Over the Nordic seas the main region of strong MCAOs will move north and weaken slightly as it moves away from the warm tongue of the Gulf Stream in the Norwegian Sea. Over the Sea of Japan there is projected to be only a small weakening of MCAOs. The implications of the results for mesoscale weather systems that are associated with MCAOs, namely polar lows and arctic fronts, are discussed.

---

Kolstad, E. W.: Extreme winds in the Nordic Seas: polar lows and Arctic fronts in a changing climate. PhD thesis, defended 30 March, 2007.

---

Kolstad, E. W.: A new climatology of favourable conditions for reverse-shear polar lows. Tellus A, volume 58, page 344 – May, 2006:

Abstract
A new climatology of conditions that are favourable for development of polar lows in reverse-shear flow is presented. In such flow, the wind at the low-level steering level is in the opposite direction of the thermal wind in the adjoining layers. A framework for identifying such conditions along with weak lower-level static stability from any gridded data is developed by defining simple dynamical constraints on standard atmospheric fields, applied here to 40 years of ERA-40 data. The relevance of the constraints is directly demonstrated using satellite images. There are several areas where such conditions occur with high frequency: the Norwegian Sea (> 15 % of the time during NDJFM), the region to the south of the Denmark Strait (> 10 %) and the Bering Sea and the Sea of Okhotsk (> 5 %). In the Nordic Seas region, the polar low season is longer than in the Pacific because the air temperature stays low throughout March. There are primary peaks in December and January and a secondary peak in March, preceded by a distinct nadir in February. A link between the NAO and reverse-shear conditions is suggested.