331 research outputs found
Recommended from our members
An objective global climatology of polar lows based on reanalysis data
No full textHere an objective global climatology of polar lows has been developed. In order to obtain objective detection criteria the efficacy of several parameters for separating polar lows from other cyclones has been investigated. This parameter efficacy has been compared for polar lows subjectively identified by experts and for all kind of extra-tropical cyclones. The comparison is based on the ERA-Interim reanalysis from 1979 - 2016 and the higher resolution Arctic System Reanalysis from 2000 - 2012. The parameters found to be the most effective at separating polar lows from all other kinds of synoptic and meso-scale cyclones were the difference between the mean sea-level pressure of the low and its surroundings, the difference in the potential temperature between the sea surface and the 500 hPa level, and the tropopause wind poleward of the system. Other parameters often used for distinguishing, such as the 10m wind speed and the temperature difference between the sea surface and the 700 hPa level were found to be less effective. Investigation of the climatologies reveals that PLs occur in all maritime basins at high latitudes, but with high density in the vicinity of the sea-ice edge and coastal zones. The regions showing the highest degree of polar-low activity are the Denmark Strait and the Nordic Seas. Especially the most intense polar lows occur in these two regions. In the North Atlantic and Pacific the main polar-low season ranges from November to March. In the Southern Hemisphere polar lows are mainly detected between 50 - 65'S from April to October, indicating that this hemisphere compared to its northern counterpart has a two months longer, but less intense, polar-low season. No significant hemispheric long-term trends are observed, although some regions, such as the Denmark Strait and the Nordic Sea experience significant downward and upward trends in polar lows, respectively, over the last decades. For intense polar lows a significant decaying trend has been observed for the northern hemisphere
On the linkage between atmospheric circulation changes and Arctic climate change
No full textPolar amplification is a prominent feature of recent and projected climate change. The Arctic region shows some of the strongest signs of climate change, including sea-ice retreat and temperatures increasing at twice the rate averaged over the northern hemisphere. A major concern for humanity is the sea-level rise associated with the melting of the ice-sheets and glaciers due to climate change. The atmospheric circulation transports an amount of energy into to the Arctic equivalent that received by the Arctic from the Sun. Thus, the atmospheric energy transport is an important subject to study in the light of Arctic climate change. The atmospheric energy transport may be decomposed into contributions by planetary-scale waves such as Rossby waves and small-scale waves such as cyclones. The energy transport contributions by the different length-scale separated systems are shown to affect the Arctic differently. The meridional energy transport is separated into length-scale contributions using a Fourier-series-based approach. Here we evaluate this approach by comparing it to a novel wavelet-based length-scale decomposition, developed as a part of this project. Further a machine-learning-based length-scale decomposition approximator is developed. The approximator may be applied to climate model output to investigate future changes in the length-scale decomposed energy transport. From the comparisons it is apparent that both the Fourier and wavelet-based length-scale decompositions are skilled approaches, which produce physically meaningful decompositions. Additionally, the Fourier-based decomposition is further developed to yield a length-scale decomposition on a latitude-longitude grid. Once evaluated the Fourier and wavelet-based decompositions are applied to investigate the effects of recent climate change on the atmospheric energy transport, and how these changes affect the Arctic and the Greenland ice-sheet. Through these studies it is conspicuous that shifts of energy transport between length-scale components has occurred during the last decades, and that these shifts have contributed to Greenland ice-sheet melt and Arctic warming
On sea-ice forecasting
No full textAccurate sea-ice prediction is essential for safe operations in the Arctic and potentially also for weather forecast at high-latitudes. The increasing number of sea-ice related satellite observations in the Arctic can be used to improve the model predictions through data assimilation. For sea ice, sea-ice concentration (SIC) observations have been available for many years. Observational information of SIC can be used to constrain the sea-ice extent in models. In addition to SIC, other sea-ice related observations such as sea-ice thickness (SIT) and snow depth have recently become available. The assimilation of these observations is expected to have a substantial impact on the sea-ice forecast.
In this thesis, the main goal is to enhance the sea-ice model forecast accuracy by improving the initial model state on which the forecast is based. Primarily, the assimilation of sea-ice-related observations that are previously little used in sea-ice data assimilation is investigated. This includes the assimilation of SIT, snow depth and high-resolution SIC observations. A secondary objective of this thesis is to reduce the computational cost of both sea-ice assimilation and modelling. A new direct and computationally cheap method for data assimilation, the Multi-variate nudging (MVN) method, is proposed as an alternative to more complex assimilation methods for sea-ice. In addition, to reduce the computational cost of the sea-ice prediction, two machine-learning methods were applied for sea-ice forecasting, a fully convolutional network and a k nearest neighbours.
It is found that the assimilation of observations other than SIC has the potential to enhance the accuracy of sea-ice models and improve predictions. The proposed new assimilation method, the MVN, proves to be a valid assimilation alternative to the Ensemble Kalman Filter when few observation types are available, and the computational resources are limited. The machine-learning forecasts are found to improve upon persistence and show comparable skills to the dynamical model. Hence there is a potential for machine-learning methods for sea-ice predictions which should be developed further
Warm winds from the Pacific caused extensive Arctic sea-ice melt in summer 2007
No full textDuring summer 2007 the Arctic sea-ice shrank to the lowest extent ever observed. The role of the atmospheric energy transport in this extreme melt event is explored using the state-of-the-art ERA-Interim reanalysis data. We find that in summer 2007 there was an anomalous atmospheric flow of warm and humid air into the region that suffered severe melt. This anomaly was larger than during any other year in the data (1989–2008). Convergence of the atmospheric energy transport over this area led to positive anomalies of the downward longwave radiation and turbulent fluxes. In the region that experienced unusual ice melt, the net anomaly of the surface fluxes provided enough extra energy to melt roughly one meter of ice during the melting season. When the ocean successively became ice-free, the surface-albedo decreased causing additional absorption of shortwave radiation, despite the fact that the downwelling solar radiation was smaller than average. We argue that the positive anomalies of net downward longwave radiation and turbulent fluxes played a key role in initiating the 2007 extreme ice melt, whereas the shortwave-radiation changes acted as an amplifying feedback mechanism in response to the melt
Greenland’s contribution to global sea-level rise by the end of the 21st century
No full textThe Greenland ice sheet holds enough water to raise the global sea level with ~7 m. Over the last few decades, observations manifest a substantial increase of the mass loss of this ice sheet. Both enhanced melting and increase of the dynamical discharge, associated with calving at the outlet-glacier fronts, are contributing to the mass imbalance. Using a dynamical and thermodynamical ice-sheet model, and taking into account speed up of outlet glaciers, we estimate Greenland’s contribution to the 21st century global sea-level rise and the uncertainty of this estimate. Boundary fields of temperature and precipitation extracted from coupled climate-model projections used for the IPCC Fourth Assessment Report, are applied to the icesheet model. We implement a simple parameterization for increased flow of outlet glaciers, which decreases the bias of the modeled present-day surface height. It also allows for taking into account the observed recent increase in dynamical discharge, and it can be used for future projections associated with outlet-glacier speed up. Greenland contributes 0–17 cm to global sea-level rise by the end of the 21st century. This range includes the uncertainties in climate-model projections, the uncertainty associated with scenarios of greenhouse-gas emissions, as well as the uncertainties in future outlet-glacier discharge. In addition, the range takes into account the uncertainty of the ice-sheet model and its boundary fields
Kara-Siberian sea ice anomaly and its impact on the atmosphere
No full textThe behavior of the autumn and winter weather pattern over Europe and Eurasia was investigated during two periods of high and low ice concentration in September over the Kara-Siberian sea. A statistically significant anomalous energy flux from the east Siberian sea to the atmosphere was detected during October of low ice years. Furthermore, an anomalous behavior in the atmospheric circulation was detected during the October of the LIYs. Additionally, statistically significant positive anomaly was observed over northern Europe and central Eurasia during January and February of the years with high ice concentration at the Kara-Siberian sea
An evaluation of the reanalyses ERA-Interim and ERA5 in the Arctic using N-ICE2015 data
No full textThe Arctic climate has changed considerably in the last few decades. Hence, a large fraction of the current studies of the Arctic climate relies on global atmospheric reanalyses, due to the shortage of meteorological observations in the Arctic. However, global climate models have shown to struggle with simulating the current conditions in the Arctic region. Thus, the objective of this thesis is to evaluate how accurate the reanalysis ERA-Interim and ERA5 are in representing the measurements from the Norwegian Young Sea Ice (N-ICE2015) expedition obtained in 2015 North for Svalbard. The observations from N-ICE2015 are a good data set for using in the evaluation. The reason for this is due to the fact that N-ICE2015 provide measurements obtained over the thinner sea ice condition in the Arctic during the winter season, which no other Arctic expedition can provide. Moreover, the ERA5 reanalysis is newest reanalysis produced and few studies have evaluated on ERA5’s performance in the Arctic. In addition, the thesis will focus on how the assimilated N-ICE2015 observations affect the reanalyses ERA-Interim and ERA5. Final objective is to see if new reanalysis ERA5 shows improvements relative to ERA-Interim representing the previous reanalysis generation
The impact of Arctic late summer sea ice variability on mid-latitude autumn and winter weather.
No full textThe influence of Arctic summer sea ice area on autumn and winter climate in the extra-tropic Northern hemisphere is investigated. During the last decades, the Arctic has warmed more than twice the global average rate, a phenomenon called the Arctic Amplification (AA). AA involves a remarkable decrease of the Arctic sea ice cover and a decrease of the latitudinal temperature gradient. Both are supposed to influence the global energy overturning circulation system, including atmospheric mid-latitude waves and the polar cell.
The present study analyzes how the Arctic September sea ice cover influences the temperature and pressure in the Northern hemisphere extra-tropics in the following months and seeks explanations for this phenomenon in the atmospheric circulation.
To obtain good results, the possible connection of year to year variabilities was studied, and underlying external and internal feedbacks were excluded as much as possible.
Three approaches were taken: Firstly, real world data, such as temperature and surface pressure, expressed by ERA-interim from the year 1979 to 2014, were regressed on the September sea ice area. Secondly, the state of the art climate model CESM with the atmospheric model components CAM4 and CAM5 was used to study and compare the same regressions as in ERA, but for preindustrial conditions. Thirdly, to verify the chain of cause and effect, the consequences of three forced low-ice scenarios of the CESM climate model were compared to a control run.
The first two datasets showed that the Arctic sea ice variabilities are not homogenous throughout the Arctic. Two regions, the Beaufort to East Siberian Sea (Be-ES) region and the Barents-Kara Sea (Ba-Ka) region, with rather independent sea ice area anomaly time series, were observed. Therefore, the climatological response in the two regions was studied independently.
The CESM model runs with forced sea ice conditions give some insight of the winter responses to summer sea ice reduction. In autumn the open water in the Arctic act as heat sources, leading to anomalously warm local temperatures. The local warming leads to extending air and anomalously low surface pressure in the Arctic. This reduces the strength of the polar cell and can lead to a strengthening of the Siberian High in winter.
In ERA-interim and CESM CAM4 & 5 the negative September sea ice anomalies persist into the late autumn for Be-ES and through the whole winter for Ba-Ka.
Autumn responses to the sea ice anomalies are similar between the three datasets for the two regions, while the winter responses contradict each other.
Negative September sea ice anomalies in the Be-ES region seem to induce an East Arctic high pressure in autumn, bringing cold conditions to North Siberia.
Ba-Ka region sea ice anomalies induce a low pressure over its region in autumn, a pattern opposing the one induced by the Be-ES region.
The ERA-interim winter response to the Ba-Ka sea ice anomaly shows an anomalously high pressure over West Russia, bringing cold conditions to Central and East Eurasia. This pattern is refound in literature, but is opposed by the CESM model runs with CAM4 and CAM5
Rossby Wave Changes During the Recent Decades in Mid-Latitude Continents in the Northern Hemisphere
No full textIncreasing frequency and severity of extreme weather events throughout the recent years is a highly concerning topic. Extreme and unusual weather conditions, such as heat waves, floods and cold spells are causing a major concern for humanity. Agricultural changes, damages in infrastructure and the loss of human lives are some of the extreme weather event consequences, with the most drastic consequences experienced by the poor and least adaptable groups of society. Recent studies indicate that the increase of extreme weather events can be linked to the effects of global warming, and projections indicate that the frequency and severity of these events is expected to continue increasing in the nearest future. Extreme weather events in mid-latitude continents in the Northern Hemisphere are considered to be linked to atmospheric circulation changes, that are induced by the decreasing meridional temperature gradient due to the effects of Arctic amplification. Atmospheric Rossby waves, in addition to baroclinic cyclones, are considered to be the main factors driving the atmospheric mid-latitude circulation, and recent studies suggest that changes in phase velocity and amplitude of Rossby waves is a indirect consequence of climate change. Here, an attempt to analyse the changes in atmospheric wave amplitudes throughout the recent decades is presented, concentrating on the amplitude tendencies regarding the most extreme amplitude anomalies. This is accomplished by applying Fourier decomposition on a geopotential height field, splitting it into planetary- and synoptic-scaled waves, and further analysing the amplitude changes and tendencies regarding the planetary-scaled Rossby waves. Throughout this studies, no certain amplitude tendencies could be confirmed when regarding all of the planetary waves together, however potential linearly increasing amplitude tendencies could be noted when performing individual Rossby wave analysis
- …
