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ABOUT

PERSONAL DETAILS
Nygårdsgaten 112, Bergen
erik.kolstad at uni.no
+47 555 82432
+47 411 22457
Welcome to my personal and academic profile
This page is under construction

BIO

ABOUT ME

Hi, and thanks for visiting this page. You're probably here because of my work in climate research; please click on the WORK tab for more info about that. In addition to loving my work in research, I'm also passionate about hiking, skiing, travelling and photography.

CURRENT

I’m a researcher at Uni Research and the Bjerknes Centre for Climate Research at the University of Bergen, Norway.

SUNCURVES

I’m a co-founder of Suncurves, where we compute actual times for sunrise and sunset, accounting for shading from terrain.

PHOTOGRAPHY

If you're interested, please check out my portfolio at 500px.


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RESUME

  • EDUCATION
  • 2003
    2007
    Bergen, Norway

    Ph.D. in meteorology

    University of Bergen

    Thesis: Extreme winds in the Nordic Seas: polar lows and Arctic fronts in a changing climate
  • 1993
    1997
    Bergen, Norway

    M.Sc. in applied mathematics

    University of Bergen

    Thesis: Stability of a tokamak in the presence of a resistive wall
  • 1992
    1993
    Toulouse, France

    Undergraduate studies

    INSA Toulouse

    First year of technical college
  • ACADEMIC AND PROFESSIONAL POSITIONS
  • 2014
    NOW
    Bergen, Norway

    Senior Researcher

    Uni Research Climate and Bjerknes Centre for Climate Research

    My current position
  • 2011
    2014
    Bergen, Norway

    Senior Researcher and Developer

    StormGeo

    I spent three years at StormGeo, a commercial weather company, where I oversaw all operational forecasts and hindcasts using WRF
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PUBLICATIONS

PUBLICATIONS LIST
AUG 2016

Causal Chains for Attributing Temperature Persistence to Physical Mechanisms: Applications to Northern Europe

Quarterly Journal of the Royal Meteorological Society

EW Kolstad, EA Barnes, and SP Sobolowski
Weather persistence from month to month is an important source of predictability on the intraseasonal time scale. Persistence in the lower troposphere is mainly due to dynamical feedbacks between the chaotic and rapidly evolving atmospheric flow and more inert physical variables at or below the surface. Many recent improvements of dynamical seasonal forecasting models have been due to better initialisations and representations of the lower boundary. However, not all of the physical mechanisms that mediate persistence are currently understood, hindering the full use of actual sources of predictability in the earth system. The purpose of this paper is to develop a framework to identify the mechanisms that drive atmospheric persistence, using causal chains and lagged linear regressions. While the focus is on Northern Europe, where the annual cycle of the persistence of near-surface temperature is shown to be caused by fluctuating influences of soil moisture, soil temperature, snow cover, and sea surface temperature. The framework outlined here, however, can be applied to the persistence of any variable in any region. A more complete understanding and mapping of the physical mechanisms for atmospheric persistence will not only improve the dynamical models themselves, but also support hybrid forecasting systems where empirical data are combined with model data to yield more skilful seasonal weather forecasts.

Journal Paper Submitted
JUL 2016

Re-examining the roles of surface heat flux and latent heat release in a “hurricane-like” polar low over the Barents Sea

Journal of Geophysical Research Atmospheres

Polar lows are intense mesoscale cyclones that occur at high latitudes in both hemispheres during winter. Their sometimes evidently convective nature, fueled by strong surface fluxes and with cloud-free centers, have led to some polar lows being referred to as “arctic hurricanes.” Idealized studies have shown that intensification by hurricane development mechanisms is theoretically possible in polar winter atmospheres, but the lack of observations and realistic simulations of actual polar lows have made it difficult to ascertain if this occurs in reality. Here the roles of surface heat fluxes and latent heat release in the development of a Barents Sea polar low, which in its cloud structures showed some similarities to hurricanes, are studied with an ensemble of sensitivity experiments, where latent heating and/or surface fluxes of sensible and latent heat were switched off before the polar low peaked in intensity. To ensure that the polar lows in the sensitivity runs did not track too far away from the actual environmental conditions, a technique known as spectral nudging was applied. This was shown to be crucial for enabling comparisons between the different model runs. The results presented here show that (1) no intensification occurred during the mature, postbaroclinic stage of the simulated polar low; (2) surface heat fluxes, i.e., air-sea interaction, were crucial processes both in order to attain the polar low’s peak intensity during the baroclinic stage and to maintain its strength in the mature stage; and (3) latent heat release played a less important role than surface fluxes in both stages.

Journal Paper
JUN 2016

Sensitivity of an Apparently Hurricane-like Polar Low to Sea Surface Temperature

Quarterly Journal of the Royal Meteorological Society

EW Kolstad and TJ Bracegirdle
We study a polar low which formed in an extreme marine cold air outbreak over the Barents Sea and which had cloud structures with similarities to hurricanes. Using a high-resolution weather model, we artificially modified the sea surface temperatures (SSTs) to assess if the polar low was close to developing into a true hurricane-like system, with air-sea interaction processes leading to intensification in the post-baroclinic, mature phase of its life cycle.

Journal Paper Submitted
JUL 2015

Intraseasonal Persistence of European Surface Temperatures

Journal of Climate

EW Kolstad, SP Sobolowski, and AA Scaife
Recent periods of extreme weather in Europe, such as the cold winter of 2009/10, have caused widespread impacts and were remarkable because of their persistence. It is therefore of great interest to improve the ability to forecast such events. Weather forecasts at midlatitudes generally show low skill beyond 5–10 days, but long-range forecast skill may increase during extended tropospheric blocking episodes or perturbations of the stratospheric polar vortex, which can affect midlatitude weather for several weeks at a time. Here a simple, linear approach is used to identify previously undocumented persistence in northern European summer and winter temperature anomalies in climate model simulations, corroborated by observations and reanalysis data. For instance, temperature anomalies of at least one standard deviation above or below climatology in March were found to be about 20%–120% more likely than normal if the preceding February was anomalous by 0.5–1.5 standard deviations (with the same sign). The corresponding range for April (i.e., persistence over two months) is between 20% and 80%. The persistence is observed irrespective of the data source or driving mechanisms, and the temperature itself is a more skillful predictor of the temperatures one month ahead than the stratospheric polar vortex or the NAO and even than both factors together. The results suggest potential to conditionally improve the skill of long-range forecasts and enhance recent advancements in dynamical seasonal prediction.

Journal Paper
JAN 2015

Extreme small-scale wind episodes over the Barents Sea: When, where and why?

Climate Dynamics

EW Kolstad
The Barents Sea is mostly ice-free during winter and therefore prone to severe weather associated with marine cold air outbreaks, such as polar lows. With the increasing marine activity in the region, it is important to study the climatology and variability of episodes with strong winds, as well as to understand their causes. Explosive marine cyclogenesis is usually caused by a combination of several mechanisms: upper-level forcing, stratospheric dry intrusions, latent heat release, surface energy fluxes, low-level baroclinicity. An additional factor that has been linked to extremely strong surface winds, is low static stability in the lower atmosphere, which allows for downward transfer of high-momentum air. Here the most extreme small-scale wind episodes in a high-resolution (5 km) 35-year hindcast were analyzed, and it was found that they were associated with unusually strong low-level baroclinicity and surface heat fluxes. And crucially, the 12 most severe episodes had stronger cold-air advection than 12 slightly less severe cases, suggesting that marine cold air outbreaks are the most important mechanism for extreme winds on small spatial scales over the Barents Sea. Because weather models are often unable to explicitly forecast small-scale developments in data-sparse regions such as the Barents Sea, these results can be used by forecasters as supplements to forecast model data.

Journal Paper
JUL 2012

A ‘hurricane-like’ polar low fuelled by sensible heat flux: high-resolution numerical simulations

I Føre, JE Kristjánsson, EW Kolstad, TJ Bracegirdle, Ø Sætra, and B Røsting

Quarterly Journal of the Royal Meteorological Society

An unusually deep (961 hPa) hurricane-like polar low over the Barents Sea during 18–21 December 2002 is studied by a series of fine-mesh (3 km) experiments using the Weather Research and Forecasting (WRF) model. The simulated polar low was similar to hurricanes and similar previous case-studies in that it had a clear, calm and warm eye structure surrounded by moist convection organized in spiral cloud bands, and the highest surface wind speeds were found in the eye wall. The proximity to the sea ice and the high surface wind speeds (about 25 m/s) during the deepening stage triggered extremely high surface sensible and latent heat fluxes at the eye wall of about 1200 and 400 W/m2, respectively. As the polar low moved eastward and weakened, maximum surface sensible and latent heat fluxes dropped to about 600 and 300 W/m2, respectively. Two types of sensitivity experiments were designed to analyse the physical properties of the polar low. Firstly, physical processes such as condensational heating and sensible and/or latent heat fluxes were switched off–on throughout the simulation. In the second type, these processes were turned off–on after the polar low had reached its peak intensity, which minimized the deformation of the polar-low environment, making it suitable to study the direct effect of physical processes on the mature vortex. The experiments suggest that the deepening stage of the polar low was dominated by baroclinic growth and that upper-level potential vorticity forcing contributed throughout its life cycle. After the deepening stage, the baroclinicity vanished and the polar low was fuelled by surface sensible heat fluxes while latent heat fluxes played a minor role. Condensational heating was not essential for the energetics of the polar low. Surprisingly, in experiments where condensational heating was turned off throughout the simulation, the polar low intensified.

Journal Paper
JAN 2012

Linking past flood frequencies in Norway to regional atmospheric circulation anomalies

EN Støren, EW Kolstad, and Ø Paasche

Journal of Quaternary Science

Analysis of two continuous, high-resolution palaeo-flood records from southern Norway reveals that the frequency of extreme flood events has changed significantly during the Holocene. During the early and middle Holocene, flood frequency was low; by contrast, it was high over the last 2300 years when the mean flood frequency was about 2.5–3.0 per century. The present regional discharge regime is dominated by spring/summer snowmelt, and our results indicate that the changing flood frequency cannot be explained by local conditions associated with the respective catchments of the two lakes, but rather long-term variations of solid winter precipitation and related snowmelt. Applying available instrumental winter precipitation data and associated sea-level pressure re-analysis data as a modern analogue, we document that atmospheric circulation anomalies, significantly different from the North Atlantic Oscillation (NAO), have some potential in explaining the variability of the two different palaeo-flood records. Centennial-scale patterns in shifting flood frequency might be indicative of shifts in atmospheric circulation and shed light on palaeo-pressure variations in the North Atlantic region, in areas not influenced by the NAO. Major shifts are found at about 2300, 1200 and 200 years ago (cal. a BP).

Journal Paper
NOV 2011

The Norwegian IPY–THORPEX: Polar Lows and Arctic Fronts during the 2008 Andøya Campaign

JE Kristjánsson, I Barstad, T Aspelien, I Føre, Ø Godøy, Ø Hov, E Irvine, T Iversen, EW Kolstad, TE Nordeng, H McInnes, R Randriamampianina, J Reuder, Ø Sætra, M Shapiro, T Spengler, and H Ólafsson

Bulletin of the American Meteorological Society

From a weather forecasting perspective, the Arctic poses particular challenges for mainly two reasons: 1) The observational data are sparse and 2) the weather phenomena responsible for severe weather, such as polar lows, Arctic fronts, and orographic influences on airflow, are poorly resolved and described by the operational numerical weather prediction (NWP) models. The Norwegian International Polar Year (IPY)– The Observing System Research and Predictability Experiment (THORPEX) project (2007–10) sought to significantly improve weather forecasts of these phenomena through a combined modeling and observational effort. The crux of the observational effort was a 3-week international field campaign out of northern Norway in early 2008, combining airborne and surface-based observations. The main platform of the field campaign was the Deutsches Zentrum für Luft- und Raumfahrt (DLR) research aircraft Falcon, equipped with lidar systems for profiling of aerosols, humidity, and wind, in addition to in situ measurements and dropsondes. A total of 12 missions were flown, yielding detailed observations of polar lows, Arctic fronts, and orographic low-level jets near Spitsbergen, the coast of northern Norway, and the east coast of Greenland. The lidar systems enabled exceptionally detailed measurements of orographic jets caused by the orography of Spitsbergen. Two major polar low developments over the Norwegian Sea were captured during the campaign. In the first polar low case, three f lights were carried out, providing a first-ever probing of the full life cycle of a polar low. Targeting observations by the aircraft in sensitive areas led to improvements in predicted track and intensity of the polar low. Here highlights from the field campaign, as well as from ongoing follow-up investigations, are presented. Highlights from the development of a new limitedarea model ensemble prediction system for the Arctic, as well as an exploitation of new satellite data [Infrared Atmospheric Sounding Interferometer (IASI) data], are also included.

Journal Paper
OCT 2011

Observed and simulated precursors of stratospheric polar vortex anomalies in the Northern Hemisphere

EW Kolstad and AJ Charlton-Perez

Climate Dynamics

The Northern Hemisphere stratospheric polar vortex is linked to surface weather. After Stratospheric Sudden Warmings in winter, the tropospheric circulation is often nudged towards the negative phase of the Northern Annular Mode (NAM) and the North Atlantic Oscillation (NAO). A strong stratospheric vortex is often associated with subsequent positive NAM/NAO conditions. For stratosphere–troposphere associations to be useful for forecasting purposes it is crucial that changes to the stratospheric vortex can be understood and predicted. Recent studies have proposed that there exist tropospheric precursors to anomalous vortex events in the stratosphere and that these precursors may be understood by considering the relationship between stationary wave patterns and regional variability. Another important factor is the extent to which the inherent variability of the stratosphere in an atmospheric model influences its ability to simulate stratosphere–troposphere links. Here we examine the lower stratosphere variability in 300-year pre-industrial control integrations from 13 coupled climate models. We show that robust precursors to stratospheric polar vortex anomalies are evident across the multi-model ensemble. The most significant tropospheric component of these precursors consists of a height anomaly dipole across northern Eurasia and large anomalies in upward stationary wave fluxes in the lower stratosphere over the continent. The strength of the stratospheric variability in the models was found to depend on the variability of the upward stationary wave fluxes and the amplitude of the stationary waves.

Journal Paper
OCT 2011

A global climatology of favourable conditions for polar lows

EW Kolstad

Quarterly Journal of the Royal Meteorological Society

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.

Journal Paper
OCT 2011

The Spitsbergen South Cape tip jet

MA Reeve and EW Kolstad

Quarterly Journal of the Royal Meteorological Society

We investigated low-level tip jets generated at the southern tip of the island of Spitsbergen, part of the Svalbard archipelago in the Arctic. Low-level tip jets occur in many locations where airflow converges around obstacles, such as islands. They are often poorly resolved in forecasts or re-analyses, so it is important to document their locations and shed light on why they occur. Tip jets are the result of flow stagnation and flow splitting upstream of an obstacle; both of these processes are dependent on the stability of the air column, wind speed and direction upstream. Jets generated around Sørkapp, the southern cape of Spitsbergen, have been resolved previously in numerical studies, but no climatology exists. In this study, we used the Weather Research and Forecasting model (WRF) to demonstrate the influence of topography on the development of tip jets. We used QuikSCAT satellite-derived surface wind data to identify tip jet events and compile climatologies, and the ERA-Interim data to investigate the prevailing synoptic conditions during jet events and identify the main driving forces. We found that tip jets can occur throughout the year, associated with a negative surface-level pressure anomaly moving in a northeast direction from the Norwegian Sea and towards the Barents Sea. On average, jets occurred just over 8% of days throughout the year. The maximum 60-day running mean of occurrence was around 12% and occurred between February and March. The results showed that negative wind speed and positive static stability anomalies were statistically significant upstream of the island group. These anomalies remain significant when seasons of high, middle and low occurrence were analysed separately. We conclude therefore that wind direction persistence may play an important role in the seasonality of jet occurrences in the study region.

Journal Paper
OCT 2011

Orographic influence of east Greenland on a polar low over the Denmark Strait

JE Kristjánsson, S Thorsteinsson, EW Kolstad, A-M Blechschmidt

Quarterly Journal of the Royal Meteorological Society

We present a numerical study of a polar low which hit western Iceland in January 2007, with heavy snowfall and mean wind speeds exceeding 20 m/s in several locations. The operational models at the time captured the polar low formation rather well, but there was a large spread in their predictions of the subsequent evolution and track of the polar low. The objective of this study is to investigate possible orographic forcing from Greenland as a trigger for the polar low development. In addition to an analysis of surface observations and satellite imagery, sensitivity studies using HIRLAM were carried out with various degradations of Greenland's orography, as well as with modifications to the sea-surface temperature (SST), surface roughness and the data assimilation scheme. Despite the presence of an upper-level trough and weak static stability in all the simulations, the polar low development was found to be very sensitive to the presence of the high mountains of eastern Greenland. Whereas the control run captured well the main features of the polar low, simulations with parts of east Greenland's orography removed gave a southward-displaced polar low which moved rapidly eastward, resulting in substantially underestimated near-surface winds and snowfall amounts. Setting the orographic heights over all of Greenland to zero led to the complete disappearance of the polar low. On the other hand, artificially increasing the SST by 4 K in the Denmark Strait, reducing the orographic roughness or replacing the four-dimensional variational assimilation scheme (4D-Var) by 3D-Var had only a small effect on the polar low. We suggest that hitherto unreported interactions between the high mountains of east Greenland and polar low development over the Denmark Strait may be more important for polar low formation than katabatic flow from valleys in east Greenland that was highlighted in earlier studies.

Journal Paper
MAR 2011

Uncertainties Associated with Quantifying Climate Change Impacts on Human Health: A Case Study for Diarrhea

EW Kolstad and KA Johansson

Environmental Health Perspectives

Background: Climate change is expected to have large impacts on health at low latitudes where droughts and malnutrition, diarrhea, and malaria are projected to increase. Objectives: The main objective of this study was to indicate a method to assess a range of plausible health impacts of climate change while handling uncertainties in a unambiguous manner. We illustrate this method by quantifying the impacts of projected regional warming on diarrhea in this century. Methods: We combined a range of linear regression coefficients to compute projections of future climate change-induced increases in diarrhea using the results from five empirical studies and a 19-member climate model ensemble for which future greenhouse gas emissions were prescribed. Six geographical regions were analyzed. Results: The model ensemble projected temperature increases of up to 4°C over land in the tropics and subtropics by the end of this century. The associated mean projected increases of relative risk of diarrhea in the six study regions were 8–11% (with SDs of 3–5%) by 2010–2039 and 22–29% (SDs of 9–12%) by 2070–2099. Conclusions: Even our most conservative estimates indicate substantial impacts from climate change on the incidence of diarrhea. Nevertheless, our main conclusion is that large uncertainties are associated with future projections of diarrhea and climate change. We believe that these uncertainties can be attributed primarily to the sparsity of empirical climate–health data. Our results therefore highlight the need for empirical data in the cross section between climate and human health.

Journal Paper
APR 2010

The association between stratospheric weak polar vortex events and cold air outbreaks in the Northern Hemisphere

EW Kolstad, T Breiteig, and AA Scaife

Quarterly Journal of the Royal Meteorological Society

Previous studies have identified an association between temperature anomalies in the Northern Hemisphere and the strength of stratospheric polar westerlies. Large regions in northern Asia, Europe and North America have been found to cool during the mature and late stages of weak vortex events in the stratosphere. A substantial part of the temperature changes are associated with changes in the Northern Annular Mode (NAM) and North Atlantic Oscillation (NAO) pressure patterns in the troposphere. 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. Using 51 winters of re-analysis data, we show that the development of the lower-tropospheric temperature relative to stratospheric weak polar vortex events goes through a series of well-defined stages, including the formation of geographically distinct cold air outbreaks. At the inception of weak vortex events, a precursor signal in the form of a strong high-pressure anomaly over northwest Eurasia is associated with long-lived and robust cold anomalies over Asia and Europe. A few weeks later, near the mature stage of the weak vortex events, a shorter-lived cold anomaly emerges off the east coast of North America. The probability of cold air outbreaks increases by more than 50% in one or more of these regions during all phases of the weak vortex events. This shows that the stratospheric polar vortex contains information that can be used to enhance forecasts of cold air outbreaks. As large changes in the frequency of extremes are involved, this process is important for the medium-range and seasonal prediction of extreme cold winter days. Three-hundred-year pre-industrial control simulations by 13 coupled climate models corroborate our results.

Journal Paper
MAR 2010

Climatology and variability of Southern Hemisphere marine cold-air outbreaks

TJ Bracegirdle and EW Kolstad

Tellus A

Marine cold air outbreaks (MCAOs) are events where cold air flows over a relatively warm sea surface. Such outbreaks are associated with severe mesoscale weather systems that are not generally resolved in global climate models, such as polar lows and boundary-layer fronts. Here, an analysis of winter climatology and variability of MCAOs in the Southern Hemisphere (SH) is presented. Near the sea ice edge, north–south fluctuations of the Southern Annular Mode (SAM) index are key, while further north, large-scale wave disturbances are needed to move air masses far enough away from the Antarctic continent to instigate MCAOs. Unlike in the Northern Hemisphere (NH), the spatial patterns of mean and extreme values of the MCAO index differ considerably. Near 60◦S, both mean and extreme values of the index are similar to those found in the main MCAO regions in the NH. Further north, the mean MCAO index is quite high, but the extreme values are much lower than in the NH. We conclude that MCAOs in the SH are as widespread and can be as strong as in the NH, but severe MCAOs near densely populated regions such as the Tasman Sea are less common than in the Nordic Seas and near Japan.

Journal Paper
AUG 2009

Marine cold-air outbreaks in the North Atlantic: temporal distribution and associations with large-scale atmospheric circulation

EW Kolstad, TJ Bracegirdle, and IA Seierstad

Climate Dynamics

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 index, while over the Barents Sea a negative correlation of 0.42 was found.

Journal Paper
JUN 2008

A QuikSCAT climatology of ocean surface winds in the Nordic seas: Identification of features and comparison with the NCEP/NCAR reanalysis

EW Kolstad

Journal of Geophysical Research Atmospheres

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 National Centers for Environmental Protection/National Center for Atmospheric Research (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, but 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 range 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.

Journal Paper
JUN 2008

Marine cold-air outbreaks in the future: an assessment of IPCC AR4 model results for the Northern Hemisphere

EW Kolstad and TJ Bracegirdle

Climate Dynamics

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. Rare 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 twenty-first 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.

Journal Paper
MAY 2006

A new climatology of favourable conditions for reverse-shear polar lows

EW Kolstad

Tellus A

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 yr 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.

Journal Paper