Erik W. Kolstad

NORCE, Bergen, Norway · Climate dept.

The North Atlantic Oscillation (NAO) is the main driver of weather variability in parts of Eurasia, Greenland, North America and North Africa on a range of timescales. Successful extended-range NAO predictions would equate to improved predictions of precipitation and temperature in these regions. It has become clear that the NAO is influenced by th...

On average, about two tropical storms or cyclones enter or form in the Mozambique Channel between the African mainland and Madagascar each year. Their impact can be devastating. The tropical cyclone Idai, which hit land in Mozambique in 2019, was one of the deadliest storms on record in the Southern Hemisphere. Previous studies have found that the...

Plain Language Summary In East Africa, the rainfall is usually concentrated in specific periods of the year. One of these periods occurs between October and December and is called the ‘short rains’. Failure of the short rains can have disastrous effects on farmers and pastoralists, as there exist little infrastructure for irrigation in the region....

The East African ‘short rains’ in October–December (OND) exhibit large interannual variability. Drought and flooding are not unusual, and long-range rainfall forecasts can guide planning and preparedness for such events. Although seasonal forecasts based on dynamical models are making inroads, statistical models based on sea surface temperature (SS...

The Arctic stratospheric polar vortex is an important driver of winter weather and climate variability and predictability in North America and Eurasia, with a downward influence that on average projects onto the North Atlantic Oscillation (NAO). While tropospheric circulation anomalies accompanying anomalous vortex states display substantial case‐b...

Extreme weather events have devastating impacts on human health, economic activities, ecosys tems, and infrastructure. It is therefore crucial to anticipate extremes and their impacts to allow for preparedness and emergency measures. There is indeed potential for probabilistic subseasonal prediction on timescales of several weeks for many extreme e...

The East African ‘short rains’ in October–December (OND) exhibit large interannual variability. Drought and flooding are not unusual, and long-range rainfall forecasts can guide planning and preparedness. Although seasonal forecasts based on dynamical models are making inroads, statistical models based on sea surface temperature (SST) precursors ar...

The horizontal resolution in Numerical Weather Prediction models can have a large impact on the quality of polar low forecasts. However, there are few studies on the effect of the horizontal grid spacings that are currently in operational use at the European Centre for Medium‐Range Weather Forecasts (ECMWF). Here, we evaluate the 10‐m wind speed fo...

Plain Language Summary European winter weather is heavily influenced by the North Atlantic Oscillation (NAO). The so‐called positive NAO brings mild and wet conditions to northern Europe in winter, and the negative NAO tends to be cold and dry. Scientists attempt to forecast the NAO in advance by one of two ways: using complex weather forecast mode...

Global climate models have circulation biases that the community aims to reduce, for instance through high‐resolution dynamical downscaling. We used WRF to downscale both ERA‐Interim and a bias‐corrected version of the Norwegian climate model NorESM1‐M on a high‐resolution grid. By varying the domain size, we investigated the influence of the drivi...

A coordinated atmosphere-ocean research project, centered on a rare wintertime field campaign to the Iceland and Greenland Seas, seeks to determine the location and causes of dense water formation by cold-air outbreaks. The Iceland Greenland Seas Project (IGP) is a coordinated atmosphere-ocean research program investigating climate processes in the...

Climate change yields both challenges and opportunities. In both cases, costly adaptations and transformations are necessary and desirable, and these must be based on realistic and relevant climate information. However, it is often difficult for climate scientists to communicate this information to decision-makers and stakeholders, and it can be eq...

Climate change yields both challenges and opportunities. In both cases, costly adaptations and transformations are necessary and desirable, and these must be based on realistic and relevant climate information. However, it is often difficult for climate scientists to communicate this information to decision-makers and stakeholders, and it can be eq...

Heavy precipitation and associated flooding are major concerns for western Norway under both present climate conditions and projected future scenarios. In winter, these events are mostly caused by North Atlantic low pressure systems. However, the storm track in this region is systematically biased in most global climate models. Further, these model...

Arctic sea ice extent and sea surface temperature (SST) anomalies have been shown to be skillful predictors of weather anomalies in the midlatitudes on the seasonal time scale. In particular, below-normal sea ice extent in the Barents Sea in fall has sometimes preceded cold winters in parts of Eurasia. Here we explore the potential for predicting s...

Skillful predictions of continental climate would be of great practical benefit for society and stakeholders. It nevertheless remains fundamentally unresolved to what extent climate is predictable, for what features, at what time scales, and by which mechanisms. Here we identify the dominant time scales and sources of European surface air temperatu...

Dynamical subseasonal-to-seasonal (S2S) weather forecasting has made strides in recent years, thanks partly to better initialization and representation of physical variables in models. For instance, realistic initializations of snow and soil moisture in models yield enhanced temperature predictability on S2S time scales. Snow depth and soil moistur...

Marine cold air outbreaks (MCAOs) are large-scale events in which cold air masses are advected over open ocean. It is well-known that these events are linked to the formation of polar lows and other mesoscale phenomena associated with high wind speeds, and that they therefore in some cases represent a hazard to maritime activities. However, it is s...

Polar lows are intense, small-scale cyclones in the high latitudes. Typically, polar lows are initiated through baroclinic processes, but they sometimes evolve into a post-baroclinic mature stage where air–sea interaction becomes more important. In this stage some polar lows have developed hurricane-like cloud structures, and idealized axisymmetric...

Floods due to intense rainfall are a major hazard to both people and infrastructure in western Norway. Here steep orography enhances precipitation and the complex terrain channels the runoff into narrow valleys and small rivers. In this study we investigate a major rainfall and flooding event in October 2014. We compare high-resolution numerical si...

Polar lows are intense mesoscale cyclones that occur at high latitudes in both hemispheres during winter. Their sometimes evidently convective nature, fuelled 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 deve...

Recent periods of extreme weather in Europe, such as in the cold winter of 2009–2010, have caused widespread impacts and were remarkable due to their persistence. It is therefore of great interest to improve our ability to forecast such events. Weather forecasts at midlatitudes generally show low skill beyond 5–10 days, but long-range forecast skil...

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 cyclo...

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 su...

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 c...

The 3-week campaign that took place during the period 25 February-17 March 2008, with a base of operations at Andøya on the Atlantic coast of northern Norway, at 69°N, 16°E, some 280 km north of the Arctic circle, is presented. The main measurement platform of the field campaign was the German research aircraft DLR Falcon, equipped with three measu...

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 location...

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 forma...

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 positiv...

Climate change is expected to have large impacts on health at low latitudes where droughts and malnutrition, diarrhea, and malaria are projected to increase. 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 th...

The strength and location of the Northern Hemisphere (NH) stratospheric polar vortex are linked to weather developments in the troposphere. Previous studies have shown that during the first 60 days after the onset of anomalous weak stratospheric vortex regimes in winter, the large-scale tropospheric circulation is nudged towards the negative phase...

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 temperatu...

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 Sout...

Klimaendringer er vår tids store spørsmål. hva er KLIMA gir en engasjert og bredt anlagt introduksjon til dette sammensatte og livsviktige temaet. Forfatterne deler klimasystemet opp i oversiktlige biter, og gir leseren svar. Klimaet påvirkes av vinder som kan snu på sekundet til istider som dukker opp med 100 000 års mellomrom. Nettopp slike sams...

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 hazar...

1] 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 À1 in the Denmark Strait. Five distinct wind speed nadirs...

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 s...

ABSTRACTA 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 a...