ORCID Profile
0000-0002-6405-5276
Current Organisations
University of Oxford
,
GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel
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Publisher: American Meteorological Society
Date: 27-02-2014
Abstract: The zonal and meridional components of the atmospheric general circulation are used to define a global thermodynamic streamfunction in dry static energy versus latent heat coordinates. Diabatic motions in the tropical circulations and fluxes driven by midlatitude eddies are found to form a single, global thermodynamic cycle. Calculations based on the Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) dataset indicate that the cycle has a peak transport of 428 Sv (Sv ≡ 109 kg s−1). The thermodynamic cycle encapsulates a globally interconnected heat and water cycle comprising ascent of moist air where latent heat is converted into dry static energy, radiative cooling where dry air loses dry static energy, and a moistening branch where air is warmed and moistened. It approximately follows a tropical moist adiabat and is bounded by the Clausius–Clapeyron relationship for near-surface air. The variability of the atmospheric general circulation is related to ENSO events using reanalysis data from recent years (1979–2009) and historical simulations from the EC-Earth Consortium (EC-Earth) coupled climate model (1850–2005). The thermodynamic cycle in both EC-Earth and ERA-Interim widens and weakens with positive ENSO phases and narrows and strengthens during negative ENSO phases with a high correlation coefficient. Weakening in litude suggests a weakening of the large-scale circulation, while widening suggests an increase in mean tropical near-surface moist static energy.
Publisher: Copernicus GmbH
Date: 06-07-2023
DOI: 10.5194/EMS2023-258
Abstract: The occurrence of European heat events has increased during the last two decades. European heat events are responsible for social, economic and environmental damage and are projected to increase in magnitude, frequency and duration under global warming, strengthening the interest about the contribution of different mechanisms.& Using the ERA5 reanalysis product, we go beyond case studies relating European heat events with cold North Atlantic sea surface temperatures (SSTs), and perform a systematic approach with a composite analysis to investigate the link between North Atlantic SSTs in a domain south of Greenland and the 2m air temperature (T2m) over central Europe. Composites of different North Atlantic SST states show that events with a negative tendency of North Atlantic SSTs are often followed by positive European T2m anomalies during summers when the North Atlantic SSTs are persistently low for several months. Enhanced lower--tropospheric baroclinicity in the vicinity of a warm conveyor belt over the North Atlantic is followed during these events by a slantwise ascent and an enhanced upper--tropospheric waveguide, promoting a downstream development of an European ridge.& Convolutional neural network (CNN) models, one for each of the three different stages in a warm conveyor belt (inflow, ascent and outflow), validate the presence of ascending airstreams in the composite of cold North Atlantic SSTs with a negative tendency.& & A wave number decomposition suggests a combination of a wave number 3 pattern and regionally confined Rossby wave activity contributing to a trough--ridge pattern in the North Atlantic--European sector.& A composite of European heat events further confirms the lagged statistical relationship between cold North Atlantic SSTs with a negative tendency and positive European T2m anomalies. A negative tendency of North Atlantic SSTs precedes 15 of 18 European heat events, and cold North Atlantic SST conditions are present during 14 of 18 European heat events.
Publisher: Elsevier BV
Date: 2018
Publisher: Springer Science and Business Media LLC
Date: 12-07-2023
DOI: 10.1038/S43247-023-00912-4
Abstract: The globally averaged sea-surface temperature (SST) has steadily increased in the last four decades, consistent with the rising atmospheric greenhouse gas concentrations. Parts of the tropical Pacific exhibited less warming than the global average or even cooling, which is not captured by state-of-the-art climate models and the reasons are poorly understood. Here we show that the last four decades featured a strengthening atmospheric circulation and stronger trade winds over the tropical Pacific, which counteracted externally-forced SST warming. Climate models do not simulate the trends in the atmospheric circulation irrespective of whether an external forcing is applied or not and model bias is the likely reason. This study raises questions about model-based tropical Pacific climate change projections and emphasizes the need to enhance understanding of tropical Pacific climate dynamics and response to external forcing in order to project with confidence future climate changes in the tropical Pacific sector and beyond.
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-9199
Abstract: We explored the sensitivity of the atmosphere general circulation model OpenIFS to horizontal resolution and time step. We conducted a series of experiments at different horizontal resolutions (i.e., 100, 50, and 25 km) while maintaining the same time step (i.e., 15 minutes), and using different time steps (i.e., 60, 30 and 15 minutes) at 100 km horizontal resolution.& We find that the zonal wind bias over the Southern Ocean has significantly reduces at high horizontal resolution (i.e., 25 km), and that this improvement is evident too when using a coarse resolution model with smaller time step (i.e., 15 min and 100 km horizontal resolution). There is also evidence of improvements in the mid-latitude westerly jet in the Northern Hemisphere too, which is also sensitive to both model time step and horizontal resolution. We have also found that the biases in wave speed and wave litude reduce when we shorten the model time step or increase the model horizontal resolution. Therefore, it is clear that the improvement in the highest horizontal resolution (i.e., 25 km) simulation is a combination of both the enhanced horizontal resolution and shorter time step. We speculate that the improvement in the surface zonal wind bias in the coarse resolution with shorter time step (i.e., 15 min and 100 km horizontal resolution) simulation is mostly due to shallow convection that is intensified at shorter time step. In addition, we have also noticed improvements in the surface-air temperature when a high resolution and a smaller time step however, the precipitation bias is independent of the model& #8217 s horizontal resolution and time step.We propose based on OpenIFS that by reducing the time step in a coarse resolution atmospheric model (at least in OpenIFS), one can alleviate the surface-wind biases in the extratropics that is important for e.g., climate modeling in the Southern Ocean sector.
Publisher: American Meteorological Society
Date: 2017
Abstract: The thermohaline circulation of the ocean is compared to the hydrothermal circulation of the atmosphere. The oceanic thermohaline circulation is expressed in potential temperature–absolute salinity space and comprises a tropical cell, a conveyor belt cell, and a polar cell, whereas the atmospheric hydrothermal circulation is expressed in potential temperature–specific humidity space and unifies the tropical Hadley and Walker cells as well as the midlatitude eddies into a single, global circulation. The oceanic thermohaline streamfunction makes it possible to analyze and quantify the entire World Ocean conversion rate between cold–warm and fresh–saline waters in one single representation. Its atmospheric analog, the hydrothermal streamfunction, instead captures the conversion rate between cold–warm and dry–humid air in one single representation. It is shown that the ocean thermohaline and the atmospheric hydrothermal cells are connected by the exchange of heat and freshwater through the sea surface. The two circulations are compared on the same diagram by scaling the axes such that the latent heat energy required to move an air parcel on the moisture axis is equivalent to that needed to move a water parcel on the salinity axis. Such a comparison leads the authors to propose that the Clausius–Clapeyron relationship guides both the moist branch of the atmospheric hydrothermal circulation and the warming branches of the tropical and conveyor belt cells of the oceanic thermohaline circulation.
Location: No location found
Location: United Kingdom of Great Britain and Northern Ireland
Location: Sweden
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Joakim Kjellsson.