CL4.7

The global atmospheric circulation determines the local weather and climate. To better understand this circulation and how it may change in a warming world, we separate the atmospheric energy transport by the spatial scale, the quasi-stationary and transient nature, and the latent and dry-static component in the ERA-5 reanalysis and climate-model simulations with EC-Earth. Different to previous studies that distinguish the scale by wave-numbers, here the meso, synoptic and planetary scales are separated at wavelengths below 2000km, between 2-8000km, and above the latter, respectively. The scale (wavelength) of most transient energy transport is around 5000km for all latitudes and is associated with baroclinic, synoptic-scale cyclones. Transient, synoptic-scale waves are the largest contributor to the energy transport at all latitudes outside the tropics, where the meridional overturning circulation is dominant. Planetary-scale waves are both of quasi-stationary and transient character, strongest at latitudes with much orography, and responsible for most of the inter-annual variability of the energy transport. The energy transport associated with mesoscale waves is negligible.

In a warming world, the moisture transport increases everywhere and in all components, however strongest for planetary waves, making dry areas dryer and moist areas moister, and supporting large and long-lasting events that favour floods and droughts. The total energy transport increases at latitudes smaller than 60 degrees, with the main contribution from quasi-stationary, planetary-scale waves, indicating that weather patterns become more persistent. The changing energy transport can be associated both with changing zonal gradients in temperature and with an atmospheric circulation that becomes more effective in transporting energy.

How to cite: Stoll, P., Graversen, R. G., Heiskanen, T. I. H., and Bintanja, R.: Changes in the global atmospheric energy transport separated by spatial scales in a warming world, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5121, https://doi.org/10.5194/egusphere-egu22-5121, 2022.

Energy transport in the atmosphere is accomplished by systems of several length scales, from cyclones to Rossby waves. From recently developed Fourier and wavelet based methods it has been found that the planetary component of the latent heat transport affects the Arctic surface temperatures more than its dry-static counterpart and the synoptic scale component of the latent heat transport.  

However, both the Fourier and wavelet based methods require enormous amounts of data and are time consuming to process. The Fourier and wavelet decompositions are computed  from 6 hourly data, throughout the whole vertical column of the atmosphere. The data required are usually only available from reanalysis archives, or possibly from climate model experiments where a goal is to examine the decomposed energy transport. However, the vast CMIP5 and CMIP6 archives are out of reach for the exact computations of the Fourier and wavelet decompositions. Even if all the data were available in the CMIP archives, it would be a computationally, and storage-wise, intensive task to compute the Fourier and wavelet decompositions for a large selection of the CMIP experiments.

Here we suggest a deep-learning approach to approximate the decomposed energy transport from significantly less data than the original methods. The idea is to train a convolutional neural network (CNN) on ERA5 data, where we have already computed the Fourier decomposition of the energy transport. The CNN is trained on data at 850hPa in the atmosphere on a daily temporal resolution. The required data are only a small fraction of the data required to compute the exact Fourier decompositon of the energy transport. Once the CNN is trained, the model is tested on data from the EC-Earth climate model. For EC-Earth we have an ensemble of model runs where the energy transport is decomposed using the Fourier method, hence the CNN may be evaluated on the EC-Earth dataset.

The CNN based energy transport decomposition matches well with the classically computed energy transport from EC-Earth.The CNN captures the mean meridional transport well, and the projected changes from the 1950s to the 2090s in EC-Earth. Additionally the CNN model captures the day to day variability well, as regressions of temperature on the transport from the CNNcomputations and the classical Fourier decomposition are similar. Further we may investigate how the decomposed energy transport changes in a range of CMIP models and experiments

How to cite: Heiskanen, T. I. H. and Graversen, R.: Wave decomposition of energy transport using deep-learning, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7235, https://doi.org/10.5194/egusphere-egu22-7235, 2022.

Atmospheric spatial and temporal variability are closely related with the former being relatively well assessed compared to the latter. New opportunities for understanding the spatio-temporal variability spectrum are offered by coupled high-resolution climate models. However, the models still suffer from significant systematic errors (biases) calling for an approach that assesses circulation variability in relation to biases. Furthermore, biases in simulated variability are often of remote origin; for example, biases in the Atlantic sea-surface temperature in boreal winter may be responsible for changes in simulated variability over Asia.

We present a novel framework for the multivariate, multi-scale variability evaluation in relation to remote biases. Centennial simulations are carried out using a general circulation model PLASIM and a perfect-model framework. Biases in simulated circulation originate from regional errors in the surface forcing by prescribed sea surface temperature (SST). A reference simulation is forced with the monthly SST from ERA-20C reanalyses from January 1900 to December 2010. Sensitivity simulations are forced with the same SST with addition of regional perturbations that mimic the errors in the surface forcing of the atmosphere and lead to systematic errors in the simulated mean state and temporal variance. The erroneous SST is respectively located in tropical basins of Indian ocean, Western Pacific, Central Pacific, Eastern Pacific, and Atlantic, and in extra-tropical areas of North Pacific and North Atlantic.

The bias is the time-averaged difference between the reference and sensitivity simulations. Using the normal-mode function decomposition, the amplitude and phase of the bias can be related to deficiencies in spatial and temporal variance of the two main dynamical regimes: quasi-geostrophic regime and unbalanced circulation. The results show that biases are mainly established in the zonal-mean state and at planetary scales of balanced flow. In boreal winter, the biases at scales with zonal wavenumber k>0 are typically manifested in the barotropic Rossby wave train across the Northern Hemisphere. The structure of tropical biases is that of unbalanced flow, projecting predominantly on the Kelvin wave and the vertical baroclinic structure. The effects of biases on spatio-temporal variability are further investigated in spectral space.

How to cite: Zhao, Y.-B., Lunkeit, F., and Žagar, N.: Bias teleconnections: atmospheric variability associated with biases in remote regions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7317, https://doi.org/10.5194/egusphere-egu22-7317, 2022.

Baroclinic instability in the mid-latitudes is a significant component of the climate system, as it is associated with the meridional transport of a large amount of energy and momentum. Hence, the ability of climate models to correctly predict the properties of atmospheric circulation in that latitudinal band is a very important requirement. This study aims to estimate the power content of the atmospheric phenomena typical of mid-latitudes, such as baroclinic perturbations, and to understand how seasonal forecasts can be practically used to assess energy transfer in the atmosphere. We compare the Southern Hemisphere mid-latitude winter variability of the long-range forecasting system SEAS5 with the ERA5 reanalysis. Both datasets are produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). The analysis is carried out by computing the Hayashi spectra of the 500-hPa geopotential height field. Both the reanalysis and the seasonal forecast show a series of peaks in the spectral region of eastward-traveling waves, which corresponds to the high frequency-high wavenumber domain. We quantify the amount of energy released from the atmosphere by calculating the Baroclinic Amplitude Index. Results suggest that the seasonal forecasts correctly reflect the variability of the geopotential height power spectra in the Southern Hemisphere, with some minor discrepancies related to the sub-daily variability, which is not correctly discriminated. However, the energy associated with the baroclinic activity is well represented by the seasonal forecast in the Southern Hemisphere, where the orographic effect is negligible compared to the Northern Hemisphere. This work is carried out as part of the European FOCUS-Africa project, which develops innovative and sustainable climate services in the Southern African Development Community (SADC) region.

How to cite: Trentini, L., Dal Gesso, S., Dell'Aquila, A., and Petitta, M.: Spectral analysis of the Southern Hemisphere atmospheric variability to assess the role of baroclinic instability in seasonal forecasts, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11288, https://doi.org/10.5194/egusphere-egu22-11288, 2022.

Anthropogenic climate change in the Southern Hemisphere is driven by two forces, the greenhouse gas emissions and the stratospheric ozone levels. In the past, the combination of increasing greenhouse gas emissions and ozone depletion over Antarctica worked together leading to an increase in sea surface temperatures and a poleward shift of the storm tracks. With the ozone expected to recover by mid-century, however, the greenhouse gas and ozone forces will oppose each other and the changes observed previously will begin to weaken or reverse. The role the greenhouse gases and the ozone recovery play in the Southern Hemisphere climate system are examined using Community Climate System Model, version 4 (CCSM4) coupled ocean eddy-parameterized and eddy-resolving simulations. The greenhouse gas emissions and ozone levels are specified independently to represent the two extremes, peak greenhouse gas emissions and a recovered ozone. In the eddy-parameterized simulations, the ozone recovery signal is found to be stronger on average. In the case of the eddy-resolving simulations, however, the increase in greenhouse gases is stronger especially in eddy-rich regions like western boundary current regions and the Antarctic Circumpolar Current. The volume transport is also calculated for the Southern Hemisphere western boundary currents (Agulhas, Brazil, and East Australian Currents) and the two external forces are found to not play an important role in the mean transports, but the model resolution does. The eddy-parameterizing simulations yield a more accurate transport than the eddy-resolving simulations. The eddy-resolving simulations however, are able to resolve a more accurate eddy field in these highly active regions. The relationship between the sea surface temperatures in the western boundary currents and regional precipitation over nearby South Africa, South America, and Australia is then analyzed in greater detail.

How to cite: Daher, H. and Kirtman, B.: The impact of greenhouse gas and ozone forcing on the Southern Hemisphere climate system, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10915, https://doi.org/10.5194/egusphere-egu22-10915, 2022.

In studying recent climate, changes to atmospheric circulation are often understood as a response to temperature changes. This work instead quantifies the contribution to temperature trends from the atmospheric dynamics, by analysing trends in the ERA5 zonal-mean temperature budget over the satellite era. The results are consistent with several previously highlighted trends in the circulation. In the winter hemisphere, the region of subtropical descent and heating associated with the Hadley cell strengthens on its poleward side, and the deep diabatic heating in the ITCZ intensifies and shifts northward in the northern hemisphere (NH) winter. In keeping with other studies, we find a weakening of the transient eddy heating associated with the NH summer storm tracks. At high northern latitudes, the climatological eddy heating is weakened at low levels; this signal is strongest in NH winter, consistent with the reduced baroclinicity associated with arctic warming. Our work also points towards emerging trends in the transition seasons, SON and MAM, and underlines the importance of circulation changes in understanding trends in atmospheric temperature.

How to cite: Thomas, R., Woollings, T., and Dunstone, N.: Diagnosing the effect of circulation trends on atmospheric temperature, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9166, https://doi.org/10.5194/egusphere-egu22-9166, 2022.

Precipitation changes are notoriously highly variable, and climate models misplace circulation features, making it difficult to evaluate if mechanisms of precipitation change are well reproduced in climate models. Several methods have been developed to detect externally forced precipitation change tracking circulation features rather than specific locations. For example, analysis of monthly ascending and descending regions in reanalysis show the increase of rainfall in ascending regions. Analysis of wet and dry regions in GPCP blended data shows that if the locations of wet and dry regions are tracked from month to month then trends over the past 3-4 decades can be attributed to a combination of human influences and the recovery from drying associated with the Mount Pinatubo eruption in wet regions. In response to volcanic eruptions, wet regions tend to dry and dry regions may get wetter, indicating a reduced moisture transport to the wettest regions of the tropics under strong volcanic forcing. However, this is also impacted by the hemispheric characteristics of the eruptions. 

How to cite: Hegerl, G., Ballinger, A., and Schurer, A.: Towards attributing change in tropical and subtropical precipitation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13547, https://doi.org/10.5194/egusphere-egu22-13547, 2022.

During the boreal summer monsoon, the temperature gradient between land and ocean in the Northern Hemisphere (NH) facilitates large transports of moist air masses towards the land regions, where their convergence causes precipitation. This is associated with an export of net energy (internal, potential, and latent energy) away from the land. On a global scale, there is a tight relationship between the location of the intertropical convergence zone (ITCZ) and the cross-equatorial atmospheric heat transport (AHT) on seasonal, interannual and climate time scales: a more northward cross-equatorial AHT is associated with a displacement of the ITCZ (as defined by precipitation) toward the equator. We further analyse the relationships between cross-equatorial AHT and common streamfunction-based measures of the ITCZ position and width found in the literature. However, it remains unclear whether links between energy transport and the monsoonal precipitation exist at the scale of monsoon regions.

To address this question, we combine data from the European Centre for Medium-Range Weather Forecast (ECMWF) reanalysis ERA5 and Global Precipitation Climatology Project (GPCP-version 2.3) rainfall data. In the annual cycle, the cross-equatorial northward AHT transport peaks in July and the annual net northward cross-equatorial AHT is -0.34 PW (negative sign denotes southward). A regression analysis confirms that the global ITCZ shifts southward when the cross-equatorial AHT is anomalously large, although we demonstrate this mainly happens over the Pacific Ocean. Outside of the Pacific sector, the relationship between cross-equatorial AHT and JJA precipitation is complex. For the West African monsoon region, greater northward cross-equatorial AHT is related to weaker rainfall along the Gulf of Guinea coast, while there is stronger rainfall in the Atlantic Ocean ITCZ. In the Indian sector, anomalous northward AHT is associated with a weak monsoon, marked by strong decreases in precipitation on the Western coast of India and the southern flank of the Himalayas.

In future work, the CMIP6 multi-model dataset will be analysed to examine future projection of AHT and its impact on monsoonal precipitation. The characteristics of the ITCZ will be explored using the same datasets.

How to cite: Awal, M. R., Turner, A., and Ferreira, D.: The relationship between atmospheric heat transport and monsoonal precipitation variability, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7957, https://doi.org/10.5194/egusphere-egu22-7957, 2022.

GCMs robustly project a delay in the timing of the global monsoon onset and tropical precipitation intensification with warming. However, a closer look at the response of different monsoon regions shows less consistency. To better understand how monsoons will respond to a warming climate, with a particular focus on the timing of monsoon onset, we use a hierarchy of climate models, starting from idealized aquaplanet simulations all the way to CMIP6 projections, to identify the robust and uncertain changes and investigate the underlying mechanisms. Our idealized work covers two sets of simulations: 1) aquaplanet runs with a uniform mixed layer depth (MLD) in a wide range of climates, from colder to warmer than the current climate, and 2) simulations with an idealized saturated zonally symmetric continent extending from 10^oN to the North Pole in a similar range of colder to warmer climates. Monsoon onset is determined using a change point detection method on the cumulative moisture flux convergence (MFC) (or net precipitation), which robustly links monsoon onset to changes in the large-scale monsoonal circulation. The idealized uniform MLD aquaplanet simulations show a robust progressive delay of monsoon onset, consistent with results reported in the literature. Analyses of the atmospheric energy budget suggest this delay is due to the increased atmospheric latent heat capacity with warming. Interestingly, this delay is not evident in the simulations with the idealized saturated continent. Mechanisms are explored by analyzing changes in the energetics and dynamics of the tropical circulation and related monsoonal precipitation. CMIP6 projections in different monsoon regions are investigated to determine if mechanisms exposed in the idealized simulations can shed some light on the differing monsoon onset responses in more complex climate models.

How to cite: Bordoni, S. and Hui, K.: Monsoon Onset Response to Warming in Idealized GCM and CMIP6 Simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10666, https://doi.org/10.5194/egusphere-egu22-10666, 2022.