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Rossby Wave Changes During the Recent Decades in Mid-Latitude Continents in the Northern Hemisphere
Get PDFIncreasing 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
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An objective global climatology of polar lows based on reanalysis data
Get PDFHere 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
Get PDFPolar 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 the recent Arctic Warming
No full textThe Arctic region attracts considerable scientific interest in these years. Some of the Earth's most pronounced signs of the recent climate change are found here. The summer sea-ice cover is shrinking at an alarming rate. At the same time the region warms faster than the rest of the globe. The sea-ice reduction implies an increase of solar-radiation absorption at the surface leading to warming which is expected to be larger at higher than at lower latitudes. It is therefore often assumed that the sea-ice reduction is a major cause of the observed Arctic temperature amplification. However, results presented in this thesis suggest that the snow and ice-albedo feedbacks are a contributing but not dominating mechanism behind the Arctic amplification. A coupled climate-model experiment with a doubling of the atmospheric CO2 concentration reveals a considerable Arctic surface-air-temperature amplification in a world without surface-albedo feedback. The amplification is only 8 % larger when this feedback is included. Instead the greenhouse effect associated with an increase of humidity and cloud cover over the Arctic seems to play a major role for the amplification. Reanalysis data, which are partly based on observations, show Arctic temperature amplification well above the surface in the troposphere. In the summer season, the amplification has its maximum at ~ 2 km height. These trends cannot be explained by the snow- and ice-albedo feedbacks which are expected to induce the largest amplification near the surface. Instead, a considerable part of the trends aloft can be linked to an increase of the atmospheric energy transport into the Arctic. A major topic of this thesis is the linkage between the mid-latitude circulation and the Arctic warming. It is suggested that the atmospheric meridional energy transport is an efficient indicator of this linkage
The vertical structure of the lower Arctic troposphere analysed from observations and the ERA-40 reanalysis
No full textIn situ atmospheric observations in the central Arctic are few and mostly from near the surface. A majorityare from coastal regions whereas soundings over the Arctic Ocean are rare. This limits our understanding of the Arcticatmosphere, in particular aloft. It has been established that the vertical thermal structure is often stably stratified; this hasbeen termed the ‘Arctic inversion’. It has also been established that near-surface warming in the Arctic has been larger thanthe global mean warming during the last several decades. To estimate climate trends in this data-sparse region, reanalysisdata have often been used. In this paper we analyse the vertical thermal structure of the lower troposphere over the Arctic Ocean, using soundingsfrom the SHEBA project. We find a strong annual cycle with strong surface inversions occurring only during autumnand winter, typically 500–800 metres deep and ∼10 ◦C strong. Summer is dominated by weaker elevated inversions at∼100–400 m, a few hundred metres deep. Interestingly, this latter type of inversion also occurs frequently in winter,almost half the time. These soundings thus indicate that associating Arctic winter only with strong surface inversions isnot entirely correct. We also compare these soundings to the ERA-40 reanalysis data. Systematic biases in ERA-40 in the SHEBA regioninclude a near-surface warm bias, on average ∼0.5–1.0 ◦C, and a slight mid-troposphere cool bias. There is a significantdifference in ERA-40 performance statistics for the SHEBA year comparing with years without soundings for the sameregion. The analysis increment – a measure of the impact of the observations in the assimilation process – confirms this.For example, the assimilation of the SHEBA soundings reduces the near-surface warm bias by about 50%. However, theoverall vertical structure and its annual variation are surprisingly insensitive to the assimilation of the soundings, and arein fact well represented by ERA-40. We speculate that the main improvement in assimilating the SHEBA soundings lies inan improvement in the timing of weather systems whereas their climatological vertical structure is less affected.</p
Validation of a SAR-only wind-vector retrieval against shipborne in situ wind observations in the European Arctic
Get PDFSpace-borne synthetic aperture radar (SAR) observations provide broad coverage of high-resolution snapshots of the sea surface conditions in polar regions. However, their potential has not yet been fully harnessed for meteorological applications. For instance, standard methods for SAR wind-vector retrieval rely on wind direction inputs from numerical weather prediction models, which hampers the high-resolution capabilities of SAR wind retrievals and the use of these in data assimilation. A recently proposed SAR-only wind-vector retrieval method, that uses SAR information more exhaustively than standard methods do, is compared to in situ ship observations and is found to perform similarly to a standard method under average wind conditions at open sea. However, in coastal regions, at high wind speeds, and in complex meteorological conditions this new application outperforms the standard method. It is concluded here that wind fields obtained from the SAR-only wind-vector retrieval are suitable for data assimilation in high-resolution weather prediction models, since they can provide model-independent, high-quality, and high-resolution observational wind information. In addition, a simple interpolation technique is introduced to substitute land in the calibration procedure of the Doppler centroid anomaly for open-ocean SAR scenes
On the impact of net-zero forcing Q-flux change
Get PDFNumerical climate model simulations suggest that global warming is enhanced or hampered by the spatial pattern of the warming itself. This phenomenon is known as the “pattern effect” and has in recent years become the most promising explanation for the change over time of climate sensitivity in climate models. Under historical global warming, different patterns of surface-temperature change have emerged, notably a yet unexplained cooling in the Southern Ocean and the East Pacific. Historical climate model simulations notoriously fail to reproduce this cooling, which may contribute to the deviation of the simulated global-mean warming from the observed record. Here we qualitatively investigate the potential impact of historical and other surface-temperature pattern changes by changing the ocean heat transport convergence (Q-flux) in a slab-ocean model. The Q-flux changes are always implemented such that in the global mean they impose no net forcing. Consistent with earlier studies we find that the impact of a negative Q-flux change in the Southern Ocean has a stronger effect than in other regions because of a feedback loop between sea-surface temperatures (SSTs) and clouds in the Southern Ocean and the stably stratified regions in the tropics. The SST-cloud feedback loop facilitates the expansion of the Antarctic sea ice, indeed taking the model into a Snowball-Earth state. The intensity of this effect is found to be model dependent, especially due to differences in the cloud parametrisation. In experiments with deactivated sea ice the impact of the negative Q-flux change is much weaker
Change in Climate Sensitivity and Its Dependence on the Lapse-Rate Feedback in 4 x CO2 Climate Model Experiments
Get PDFFor information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy.Robust estimates of climate sensitivity are important for decision-making on mitigation of climate change. However, climate sensitivity and its governing processes are still subject to large uncertainty. Recently it has been established that climate sensitivity changes over time in numerical climate model experiments with abrupt quadrupling of the CO2 concentration. Here we conduct an analysis of such experiments from a range of climate models from phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP). Climate feedbacks associated with clouds, lapse rate, Planck radiation, surface albedo, and water vapor and their changes over time are diagnosed based on a radiative kernel method. We find two clearly distinct model groups, one with weak and one with strong lapse-rate feedback change. The Arctic is the region showing the largest differences between these two model groups, with respect to both warming change and individual feedback changes. We retrace this change to the development over time of the Arctic sea ice, which impacts both the surface-albedo and lapse-rate feedbacks. Generally, models that warm quickly, both globally and in the Arctic, also quickly lose their Arctic sea ice and change their total global-mean climate feedback only little, and vice versa. However, it remains unclear if the Arctic changes are a cause or rather a by-product of the total global-mean feedback change. Finally, we find support for the results of previous studies finding that the relative warming in the tropical Indo-Pacific region may control the change of total climate feedback over time
On the Control of Northern Hemispheric Feedbacks by AMOC: Evidence from CMIP and Slab Ocean Modeling
Get PDFThe climate sensitivity of Earth and the radiative climate feedback both change over time as a result of a so-called “pattern effect,” i.e., changing patterns of surface warming. This is suggested by numerical climate model experiments. The Atlantic meridional overturning circulation (AMOC) influences surface warming patterns as it redistributes energy latitudinally. Thus, this ocean circulation may play an important role for climate-feedback change over time. In this study, two groups of members from the abrupt4xCO2 experiment of phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP) are distinguished: one group showing weak feedback change and the other showing strong feedback change over time. It is found that both groups differ significantly in the AMOC response to 4xCO2. Therefore, experiments with a slab ocean model (SOM) with quadrupling of the CO2 concentration are performed in which the AMOC change is mimicked by changing the ocean heat transport. It is found that in the Northern Hemisphere extratropics the CMIP model group differences can be qualitatively reproduced by the SOM experiments, indicating that the AMOC plays an important role in setting the surface warming pattern. However, in the tropics and especially in the Southern Hemisphere other explanations are necessary
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