The atmospheric circulation is driven by heat transport from the tropics to the polar regions, implying energy conversions between available potential and kinetic energy through various mechanisms. The processes of energy transformations can be quantitatively investigated in the global climate system through the Lorenz energy cycle formalism. In this study, we examine these variations and the impacts of modes of climate variability on the Lorenz energy cycle by using reanalysis data from the Japanese Meteorological Agency (JRA-55). We show that the atmospheric circulation is overall becoming more energetic and efficient. For instance, we find a statistically significant trend in the eddy available potential energy, especially in the transient eddy available potential energy in the Southern Hemisphere. We find significant trends in the conversion rates between zonal available potential and kinetic energy, consistent with an expansion of the Hadley cell, and in the conversion rates between eddy available potential and kinetic energy, suggesting an increase in mid-latitudinal baroclinic instability. We also show that planetary-scale waves dominate the stationary eddy energy, while synoptic-scale waves dominate the transient eddy energy with a significant increasing trend. Our results suggest that interannual variability of the Lorenz energy cycle is determined by modes of climate variability. We find that significant global and hemispheric energy fluctuations are caused by the El Nino-Southern Oscillation, the Arctic Oscillation, the Southern Annular Mode, and the meridional temperature gradient over the Southern Hemisphere.

How to cite: Ma, Q., Lembo, V., and Franzke, C.: The Lorenz Energy Cycle: Trends and the Impact of Modes of Climate Variability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-360, https://doi.org/10.5194/egusphere-egu21-360, 2020.

The atmospheric circulation response to global warming is an important problem which is theoretically still not well understood. This is a particular problem since climate model simulations provide uncertain, and at times contradictory, projections of future climate. In particular, it is still unclear how a warmer and moister atmosphere will affect the atmospheric circulation and mid-latitude storms. Here we perform a trend analysis of various atmospheric circulation measures and of the budgets of dry and moist static energy transports, which will contribute to our understanding of the role of moisture in circulation changes. Our analysis is based on the JRA-55 reanalysis data covering the period 1958 through 2018 for both winter and summer seasons. We focus our analysis on zonal mean quantities for the full latitudinal circles as well as for the Atlantic and Pacific sectors.

We find significant trends in zonal wind, eddy kinetic energy, Eady growth rate, diabatic heating rates, and specific humidity. The zonal wind changes appear to be in thermal wind balance. We also find that the increase in specific humidity is intensifying the trend in eddy moist static energy transport when compared with eddy dry static energy transport. Since band-pass filtered eddy moist static energy transports are related to storm tracks this suggests that increasing moisture in the atmosphere is contributing to the intensification and meridional shifts of storm tracks. Furthermore, our results suggest that global warming predominantly enhance heat fluxes and to a lesser extend momentum fluxes.

How to cite: Franzke, C. and Harnik, N.: Observed Long-Term Trends of the Atmospheric Circulation and of Dry and Moist Static Energies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-406, https://doi.org/10.5194/egusphere-egu21-406, 2021.

This study aims to evaluate to what extent atmospheric, land and ocean related datasets in the Climate Data Store are suitable for performing studies on the Arctic freshwater cycle and the interaction with the North Atlantic. The Arctic freshwater cycle is analyzed on the mean, seasonal cycle, and the trend of the atmospheric terms, runoff, ocean liquid and sea-ice freshwater storage over Arctic Ocean (AO) and transport through the Fram Strait (FS), Bering Strait (BS), Barents Sea branch (BSB) and Canadian Arctic Archipelago (CAA).

It is found that (1) the annual mean freshwater input to the AO is dominated by the river runoff (38%), inflow through BS (30%), and net precipitation (24%) and the total freshwater export from the AO is dominated by the outflow through the FS (53%) and CAA (34%). Though the net precipitation over ocean, runoff from drainage basin and seawater and sea-ice freshwater transport through the BS are close to other studies, the much lower annual mean ocean freshwater exports from the FS and CAA contribute to the imbalance of the AO freshwater cycle based on ORAS5 reanalysis data. (2) The precipitation and total water column over the ocean and land are largest in summer, while the evaporation is smallest over ocean and largest over land in summer. The total runoff in June is largest and is modulated by the snow melting though the net precipitation is the smallest. AO liquid freshwater storage increases from May to September with a peak value in September. The ocean liquid freshwater imports from the BS and exports from CAA show much larger values in summer, while the sea-ice freshwater exports in summer is strongest for the CAA but weakest for the FS. The weakest sea-ice freshwater export from the FS is consistent with other studies though the values are much smaller. (3) Both the precipitation and evaporation over the AO increased significantly, while over land only the evaporation increased and the net precipitation decreased during both 1979-2018 and 1990-2018. The moisture convergence over land increased significantly during 1979-2018 and the total water volume over the ocean and land has also increased. The annual mean runoff decreased during 1979-2018 and is much improved with a lower trend from ERA5-land outputs than ERA5. The annual mean AO freshwater storage as sea ice decreased, while the annual mean ocean liquid freshwater storage increased during both 1979-2018 and 1990-2018. 

It is indicated that (1) the usage of ERA5 reanalysis data is recommended for the atmospheric freshwater cycle, and ERA5-land data for runoff, while freshwater transport from the FS and CAA are not well represented on ORAS5 reanalysis data. (2) The trends of AO liquid and sea-ice freshwater transport are very sensitive to the chosen period and quite uncertain. Extreme care must be exercised when using ORAS5 data to study the AO freshwater transport. (3) The use of ORAS5 ocean products is not recommended before 1990, as some adjustment seems to occur during the 1979-1990 period. 

How to cite: Lin, X., Massonnet, F., Yang, C., Artale, V., de Toma, V., and Rana, A.: Arctic freshwater cycle and the interaction with the North Atlantic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-436, https://doi.org/10.5194/egusphere-egu21-436, 2021.

The loss of Arctic sea ice as a consequence of global warming is changing the forcing of the atmospheric large-scale circulation.  Areas not covered with sea ice anymore may act as an additional heat source.  Associated changes in Rossby wave propagation can initiate tropospheric and stratospheric pathways of Arctic - Mid-latitude linkages.  These pathways have the potential to impact on the large-scale energy transport into the Arctic.  On the other hand, studies show that the large-scale circulation contributes to Arctic warming by poleward transport of moist static energy. This presentation shows results from research within the Transregional Collaborative Research Center “ArctiC Amplification: Climate Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms (AC)3” funded by the Deutsche Forschungsgemeinschaft.  Using the ERA interim and ERA5 reanalyses the meridional moist static energy transport during high ice and low ice periods is compared.  The investigation discriminates between contributions from planetary and synoptic scale.  Special emphasis is put on the seasonality of the modulations of the large-scale energy transport.

How to cite: Höschel, I., Handorf, D., Jacobi, C., and Quaas, J.: Linkage of Arctic Sea Ice and Energy Transport, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2057, https://doi.org/10.5194/egusphere-egu21-2057, 2021.

The intertropical convergence zone (ITCZ) expresses itself as a band of strong convection around the equator with associated heavy precipitation. The ITCZ migrates annually to the warmer hemisphere, and the extent to which it ventures away from the equator varies from year to year and across the different oceans and continents. These variations drastically affect rainfall and droughts in the equatorial area and beyond.Till now, various approaches have been proposed to quantify the ITCZ, e.g. based on maximum precipitation or energy budgets. However, a robust quantifier of the actual convergence of surface winds around the equator is still lacking. Here, we propose to quantify ITCZ mid position with a fundamental and intuitive definition using surface wind data and wind convergence only. We use surface wind data from ERA5 reanalysis at 0.25 degree grid resolution as a proxy for calculating the ITCZ mid position on a global scale. Given the u and v components of the wind we calculate the convergence of the windfields around the equator between 20° North and 20° South. We define the latitudinal ITCZ mid position as the maximum convergence on each longitude. We then validate our approach by comparing it to the ITCZ location as given by existing ITCZ position proxies. We also look at characteristics of the ITCZ width to learn more about the influence of wind fields on the extent of the ITCZ. Our results reveal the interannual variability and trends in the ITCZ in the last half century. It also highlights the different characteristics of the ITCZ over the different oceans and continents.

How to cite: Elsemüller, L. and Goswami, B.: Quantifying the ITCZ using wind convergence, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8187, https://doi.org/10.5194/egusphere-egu21-8187, 2021.

How to cite: Jalihal, C., Mikolajewicz, U., and Kapsch, M.-L.: Hemispheric shift of the zonal mean ITCZ during the last deglaciation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13976, https://doi.org/10.5194/egusphere-egu21-13976, 2021.

The atmospheric circulation is expected to change in response to anthropogenic CO₂ emissions. Both theory and model simulations of future climate suggest that the tropical overturning will weaken, with a weaker Hadley Circulation ascent, while the stratification of moist static energy (MSE) will strengthen. These two changes have opposite effects on the energy balance of the deep tropics. In the unperturbed system, the equatorward convergence of the mean flow in the lower troposphere (i.e. at low MSE) is compensated by a divergence in the upper troposphere (i.e. at high MSE), resulting in a net lateral export of MSE out of the region of ascent.

The weakening of the circulation in a future warmer climate would weaken the export of MSE while the strengthening of the stratification -- an increase of the MSE contrast between the upper and lower branches -- would reinforce it. However, previous studies suggest that these two effects do not exactly cancel out. A neglected element in this picture is the primary driver of these changes: due to the long-wave trapping by higher CO₂ concentration, the tropical atmosphere will also receive more energy at the top and bottom (an increased Net Energy Input, NEI).

In this study, we attempt to reconcile changes in the circulation, stratification and NEI under climate change. Specifically, we investigate 1) to which extent the effects of circulation and stratification changes on the MSE budget compensate and 2) if inclusion of the NEI changes brings the MSE budget closer to equilibrium.

To address these questions, we compute the Gross Moist Stability in a series of simulations from the Coupled Model Intercomparison Project 5 archive. To test our understanding of the MSE budget, we consider both a future climate scenario (RCP8.5) and the mid-Holocene (6000 A.D). For the future climate, we show that, although there is a rough balance by the circulation and stratification effects, inclusion of the NEI term significantly improves the closure of the MSE budget in the deep tropics. The mid-Holocene case is, however, fundamentally different as both stratification and circulation weaken, reinforcing their effects on the MSE export. In this case, inclusion of the NEI term is critical to establish the MSE balance of the deep tropics.

Both cases underline that a three-term balance (between changes in circulation, MSE stratification and NEI) provides a robust description of the deep tropics MSE budget under climate change.

How to cite: Ferreira, D. and D'Agostino, R.: The energy budget of the tropical band in future climate , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14854, https://doi.org/10.5194/egusphere-egu21-14854, 2021.

The impact of volcanic forcing on tropical precipitation is investigated in a new set of sensitivity experiments within Max Planck Institute Grand Ensemble framework. Five ensembles are created, each containing 100 realizations for an idealized tropical volcanic eruption located at the equator, analogous the Mt. Pinatubo eruption, with emissions covering a range of 2.5 - 40 Tg S. The ensembles provide an excellent database to disentangle the influence of volcanic forcing on regional monsoons and tropical hydroclimate over the wide spectrum of the climate internal variability. Monsoons are generally weaker during the two years after volcanic eruptions and their weakening is a function of emissions: the strongest the volcanic eruption, the weakest are the land monsoons. The extent of rain belt is also affected: the monsoon area is overall narrower than the unperturbed control simulation. While the position of main ascents does not change, the idealised tropical volcanic eruption supports the shrinking of Hadley Cell's ascent and the narrowing of the ITCZ. We investigate this behavior by analysing the changes in Hadley/Walker circulation, net energy input and energy budget to find analogies/differences with global warming.

How to cite: D'Agostino, R. and Timmreck, C.: Volcanic impact on the tropical hydrological cycle , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15234, https://doi.org/10.5194/egusphere-egu21-15234, 2021.