Displays

A recent assessment of the coupled atmosphere-ocean-sea-ice energy budget of the Arctic using largely independent observational data sources demonstrated a high level of consistency of yearly means and annual cycles of lateral and vertical energy fluxes and storage terms. Moreover, contemporary Arctic regional energy imbalance has been found to be of similar magnitude (~1Wm^-2) as Earth’s global energy imbalance. This suggests that Arctic amplification is predominantly a surface phenomenon and its imprint on the vertically integrated energy budget is small. Nevertheless, the annual cycle of the observed Arctic energy budget has amplified over the past two decades, with marked changes in seasonal patterns of energy fluxes and storage. This contribution draws on satellite observations, mooring-derived oceanic fluxes, data from the fifth European Re-Analysis (ERA5), and state-of-the-art ocean reanalyses to examine recent changes in Arctic heat accumulation as well as trends and variability in seasonal energy budgets. Implications for seasonal energy transports from mid-latitudes towards the Arctic will be discussed as well.

How to cite: Mayer, M., Haimberger, L., Mayer, J., Tsubouchi, T., Tietsche, S., and Zuo, H.: Trends and variations in heat uptake of the Arctic climate system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10729, https://doi.org/10.5194/egusphere-egu2020-10729, 2020.

We present evidence of compensation between the atmosphere and ocean's meridional energy transport variations, also known as Bjerknes compensation. Motivated by previous studies with mostly numerical climate models, we analyze compensation using a range of atmosphere and ocean reanalysis datasets. We show that Bjerknes compensation is present at almost all latitudes from 40 degrees North to 70 degrees North in the Northern Hemisphere from interannual to decadal time scales. In contrast to results from some numerical climate models, which attribute the compensation to variations of eddy energy transports in the atmosphere in response to changes of ocean heat transport and sea ice at multi-decadal time scales, we find a response of the zonal mean of poleward energy transport to ocean heat transport variability that leads to compensation. This is apparent in a meridional shift of the Ferrel Cell at midlatitudes at decadal time scales in winter. This shift in the cell itself is driven by changes in the eddy momentum flux and related baroclinicity. The oceanic response to atmospheric heat transport variations associated by the shift is primarily wind driven. In summer, there is hardly compensation and the proposed mechanism is not at work. Interestingly, these results are robust among all reanalysis datasets and can provide a benchmark for climate modelling studies.

How to cite: Hazeleger, W., Liu, Y., and Attema, J.: Evidence for atmosphere-ocean meridional energy transport compensation in the past decades, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4104, https://doi.org/10.5194/egusphere-egu2020-4104, 2020.

How much Earth warms in response to radiative forcing is determined by the net radiative feedback, which quantifies how much more energy is radiated to space for a given increase in surface temperature.  Estimates from present day observations of temperature and earth's energetic imbalance yield a strongly negative radiative feedback, or, equivalently, a very low climate sensitivity, which lies outside the range of climate sensitivity in coupled climate models. This discrepancy in radiative feedbacks can be linked to discrepancies between models and observations in the pattern of historical sea-surface temperature (SST) anomalies driving tropical atmospheric circulation and radiative damping.  Indeed, we find that an atmospheric model (CAM5) forced with observed SSTs yields a net feedback that is consistent with observational estimates, but up to three times more negative than that from the same period (2000-2017) in historical simulations where the same atmospheric model is coupled to a dynamical ocean model (CESM1). 

To understand the role natural variability can play in this discrepancy, we compare the radiative feedbacks generated by the observed pattern of SSTs to those within the CESM1 large ensemble over the same period. The large ensemble produces a wide range of feedbacks due to internal variability alone. Yet, global radiative feedbacks (cloud feedbacks in particular) generated by observed warming patterns are far outside the range of natural variability in the large ensemble. Using both a Green's function approach, as well as a simple metric based on the East-West tropical pacific gradient, we show that none of the control simulations of CMIP5 climate models can generate sufficiently large natural variability to explain the discrepancy between models and observations. We conclude that the discrepancy in SST patterns, and the resulting discrepancy in radiative feedbacks, is caused by an deficiency in models' ability to simulate either natural variabilty or the forced response over the recent historical period. We will also show preliminary analysis from CMIP6 simulations.

How to cite: Proistosescu, C., Dong, Y., Stuecker, M., Armour, K., Wills, R., and Parsons, L.: Discrepancy in radiative feedbacks between models and observations tied to models inability to reproduce historical surface temperature patterns over the tropical Indo-Pacific., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12353, https://doi.org/10.5194/egusphere-egu2020-12353, 2020.

Simulations of global warming in numerical models ranging from full-complexity atmosphere-ocean global climate models (GCMs) to highly idealized, dry, atmospheric GCMs almost invariably feature poleward expansion of the annual-mean Hadley cell extent.  The attendant widening of the subtropical dry zones underlying the Hadley cell descending branches makes understanding this response of the large-scale circulation to climate change of paramount societal and ecological importance.  Two theories, one that neglects the role of large-scale eddy process and one that does not, yield similar but ultimately distinct dependencies of the Hadley cell width on planetary parameters, including those such as the equator-to-pole temperature gradient that also robustly change under global warming.  A common approach, therefore, is to use the responses of these parameters diagnosed from GCM simulations to make arguments about their influence on the Hadley cell widening.  This talk offers a critical examination of that approach.

The approach's key flaw is that the quantities such as the equator-to-pole temperature gradient that appear in the theoretical scalings refer to their values in the *absence* of any large-scale overturning circulation, Hadley cells or eddies, i.e. in the hypothetical state of latitude-by-latitude radiative convective equilibrium (RCE).  This RCE state is what "forces" the Hadley cells, and once the Hadley cells emerge they modify (among others) the equator-to-pole temperature gradient.  Using these theories to understand the Hadley cell response to increased CO2 therefore requires analyzing the responses of the hypothetical RCE state to the increased CO2, which we do via single column model simulations.  In addition, we present a new scaling for the Hadley cell extent applicable to the solsticial seasons that, unlike the existing scalings, does not depend sensitively on the presence or absence of large-scale eddies, which we use in conjunction with solsticial RCE simulations to clarify arguments regarding tropical expansion over the course of the annual cycle in addition to the annual mean.  The implications for these refined theoretical arguments on results from prior studies and on constraining future Hadley cell expansion are discussed.

How to cite: Hill, S., Mitchell, J., and Bordoni, S.: Re-examining inferences from Hadley cell theory on tropical expansion under global warming throughout the seasonal cycle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11539, https://doi.org/10.5194/egusphere-egu2020-11539, 2020.

How to cite: Iipponen, J. and Donner, L.: Cloud-Radiative Impacts On Tropical Circulation Change in GFDL AM4.1, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11851, https://doi.org/10.5194/egusphere-egu2020-11851, 2020.

Global warming is expected to change the intensity distribution of daily tropical precipitation, with an increased frequency of heavy precipitation and reduced frequency of light precipitation. In general, this is likely to increase the risk of flooding, while also increasing the risk of long dry periods. However, on regional scales circulation change plays a major role in modulating this precipitation distribution change in climate model projections, so related climate change impacts will also be regionally dependent.

We propose a simple physical framework based on the dry static energy budget which explains regional daily precipitation distribution change in terms of changes in two physical drivers: large-scale circulation and time-mean convective inhibition (CIN). In this framework, increased CIN under global warming tends to reduce the frequency of convection, leading to a greater ‘recharge’ of instability between convective events, and consequently greater ‘discharge’ of latent heating (precipitation) during each event. Large-scale circulation regulates the speed of this recharge of instability via dry static energy flux convergence or divergence, and its change under warming is very regionally dependent. Changes in regional time-mean tropical precipitation are closely related to changes in large-scale circulation, so this framework also provides a physical link between changes in time-mean precipitation and changes in the daily intensity distribution of precipitation in each tropical region. 

How to cite: Chadwick, R., Pendergrass, A., Berthou, S., Alves, L., and Moise, A.: Linking regional changes in the intensity distribution of daily tropical precipitation to changes in large-scale circulation and convective inhibition, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7761, https://doi.org/10.5194/egusphere-egu2020-7761, 2020.

In the paleo literature, the emphasis has been on the role of insolation in driving monsoons on orbital timescales, but not on the role of feedbacks internal to the climate system. Here, using the energetics framework, we have underscored the effect of water vapor on the Indian summer monsoon over the last 22,000 years in transient climate simulation, called the TraCE-21K. We show that water vapor amplifies the impact of variations in insolation during cold climates like the Last Glacial Maximum. Insolation affects water vapor through its impact on sea surface temperature. During warmer periods like the Holocene, insolation drives monsoon through its influence on the net energy at the top of the atmosphere. Cloud radiative feedbacks are prominent during these periods. Thus, there are two pathways through which insolation drives monsoons. These pathways can be delineated quantitatively using the energetics. We show further that simultaneous variations in greenhouse gases and ice sheets enhance the effect of water vapor on monsoons. Hence, the sensitivity of monsoon to local summer insolation is different during different periods. Our results suggest that feedbacks play a crucial role in the evolution of Indian monsoon on orbital timescales.

How to cite: Jalihal, C., Srinivasan, J., and Chakraborty, A.: The role of water vapor and cloud feedback on the evolution of the Indian summer monsoon over the last 22,000 years, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-336, https://doi.org/10.5194/egusphere-egu2020-336, 2020.

The mid-Holocene (MH) was a warmer period, similar to the end of the 21st century climate under high emission realizations. The Indus valley civilization believed to be flourished under the expense of enhanced south Asian summer monsoon precipitation associated with the northward migration of the Inter Tropical Convergence Zone (ITCZ) during the mid-Holocene (MH). However, such an enhanced precipitation is not visible over the northwest India and Pakistan belt in future projection. The role of dynamical and various teleconnection factors behind the enhanced MH precipitation over the Indus valley region is still elusive due to the limitation of course resolution modelling efforts available so far as part of the various phases of Paleoclimate Modelling Intercomparison Projects (PMIP).  To overcome this limitation, we have designed high resolution Paleo-climate simulations using a state-of-the-art variable resolution global climate model (LMDZ: Laboratoire Meteorologie Dynamique and Z stand for zoom) which configured with a 35 km spatial resolution over the South Asian region. We conducted various sensitivity experiments to understand the role of dynamics and teleconnection in enhancing monsoon precipitation over the Indus valley in addition to the MH orbital conditions. Boundary conditions from the PMIP-3, CMIP5 and HadISST datasets utilized for various sensitive experiments. High resolution, clearly demonstrates value addition in simulating the enhanced MH precipitation over Northwest India and adjoining Indus basin associated with the northward migration of the ITCZ and shift in the ascending branch of Hadley cell. We explored the role of various oceanic and atmospheric factors responsible for this enhanced Indus valley precipitation through linearized moisture budget analysis and comparing the relative strength and position of Hadley cell. By further decomposing the thermodynamic and dynamic term into their advection and divergence component, we could demonstrate the role of moisture convergence due to the strengthened atmospheric circulation through the oceanic teleconnection, which additionally  plays a crucial role in enhanced MH precipitation comparing to the dynamical factors. Idealized simulation with the end of 21^st century warm condition with the MH orbital forcing and various teleconnection patterns affirms that the thermodynamically induced future precipitation and circulation changes, may not be adequate to make a profound shift in the northern limit of the ITCZ towards its MH locale rather producing enhanced precipitation over the north Indian ocean and localized extreme precipitation over Indian landmass.

Keywords: Indus Valley civilization, Mid-Holocene, Monsoons, Teleconnection, ITCZ and Hadley circulation

How to cite: Mudra B, L., Sabin, T. P., and Krishnan, R.: South Asian summer monsoon in warm epochs of Mid-Holocene and end of 21st century; new insights from high resolution simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1098, https://doi.org/10.5194/egusphere-egu2020-1098, 2020.

A commonly found property of warmer climates on Earth is the tendency, principally through polar amplification, towards more equable conditions with reduced meridional temperature contrasts. Numerical climate models historically have had quite some difficulty in reproducing meridional temperature gradients as low as those suggested by various proxies.
It seems self-evident that an equable climate is governed by enhanced meridional fluxes of heat, to sustain mild high latitude temperatures while keeping low latitudes from becoming exceedingly warm. However, a common hypothesis, borrowed from turbulence theory, is that the meridional heat flux is proportional to the meridional temperature gradient. A more equable climate should, therefore, exhibit reduced rather than enhanced meridional heat fluxes, posing a physical paradox.

Here, we use a unique set of long and well equilibrated (~25.000 years combined) climate simulations for various past periods using the CESM1. In terms of complexity and resolution, this climate model is comparable to the CMIP5 suite. Comparing the modelled climates of the Eocene (~40Ma), Oligocene (~30Ma), Pliocene (3Ma), pre-industrial era and present-day equilibrium climate confirms the hypothesis that warmer and more equable states overall feature weaker meridional heat fluxes (Figure 1).
This effectively shows that meridional heat fluxes on a global scale are a result of, rather than the driver of the climate state. It is, therefore, the regional radiative balance that determines the temperature distribution and by extension the meridional heat flux. Still, the different components of that flux (atmospheric vs oceanic; sensible vs latent) are crucial in shaping the climate and these are strongly dependent on the background state. Meanwhile, a strongly divergent behaviour is seen in response to an imposed RCP8.5 future scenario which drives the model far from equilibrium. In this presentation, we will address why all of the cases follow a similar slope (with a different reference) in the considered para-meter space, the roles of related heat flux components, and the processes responsible.

Using appropriate boundary conditions, sufficient resolution and an adequate level of equilibration, the model is able to reproduce the warmer and more equable climates of the past. This gives confidence that the physics determining the modelled climate states under a widely varying external forcing are sound and should help us understand meridional temperature gradients in a future warmer climate.

How to cite: Baatsen, M., von der Heydt, A., Kliphuis, M., van Westen, R., Oldeman, A., van Delden, A., and Dijkstra, H.: Equable climates: a meridional flux paradox?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1739, https://doi.org/10.5194/egusphere-egu2020-1739, 2020.

            One of the most robust aspects of the atmospheric circulation response to increasing greenhouse gases is the poleward shift in the subsiding branches of the Hadley circulation, potentially pushing subtropical dry zones poleward toward midlatitudes.  Numerous lines of observational evidence suggest that this tropical expansion may have already begun.  Yet, the degree to which the observed tropical widening is anthropogenically forced has remained a topic of great debate, as previous studies have attributed the recent circulation trends to some combination of increasing greenhouse gases, stratospheric ozone depletion, anthropogenic aerosols, and natural variability.  During the past few years, two international working groups have synthesized recent findings about the magnitude and causes of the observed tropical widening, primarily using output from global climate models that participated in phase 5 of the Coupled Model Intercomparison Project (CMIP5).  In this presentation, we update those findings using the recently released CMIP6 global climate models.

            Over recent decades, the poleward expansion of the Hadley circulation estimated from modern reanalyses is relatively modest (< 0.5 degrees latitude per decade).  The reanalysis trends have similar magnitudes in the annual mean in the Northern Hemisphere (NH) and Southern Hemisphere (SH), but both CMIP5 and CMIP6 models suggest that increasing greenhouse gases should drive 2–3 times larger circulation shifts in the SH.  The reanalysis trends fall within the bounds of the models’ simulations of the late 20^th century and early 21^st century, although prescribing observed coupled atmosphere-ocean variability allows the models to better capture the observed trends in the NH.  We find two notable differences between CMIP5 and CMIP6 models.  First, both CMIP5 and CMIP6 models contract the NH summertime Hadley circulation equatorward (particularly over the Pacific sector) in response to increasing greenhouse gases, but this contraction is larger in CMIP6 models due to their higher average climate sensitivity.  Second, in recent decades, the poleward shift of the NH annual-mean Hadley cell edge is slightly larger in the historical runs of CMIP6 models.  Increasing greenhouse gases drive similar trends in CMIP5 and CMIP6 models, so CMIP6 models imply a stronger role for other forcings (such as aerosols) in recent circulation trends than CMIP5 models.

How to cite: Grise, K. and Davis, S.: An updated view of Hadley cell expansion from CMIP6 models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3944, https://doi.org/10.5194/egusphere-egu2020-3944, 2020.

The present-day hydrological cycle in southeastern Brazil depends on the intensity of the South American Summer Monsoon (SASM) with strong monsoonal precipitation during austral summer (DJF) and weak precipitation during austral winter (JJA). On glacial-interglacial timescales, monsoonal intensity was mainly controlled by precession-forced changes in insolation.

Relatively little is known to date about the spatial distribution of precipitation in the hinterland and coastal areas of SE Brazil and the resulting variability of the fluvial discharge on glacial-interglacial timescales. The Doce River basin is situated at the northern boundary of the present-day South Atlantic Convergence Zone (SACZ), wherefore its run-off and suspension load respond sensitive to changes in both summer monsoon and coastal winter precipitation. The soil and rock distribution of the basin allows for the study of the relative proportions of terrigenous up- and lowland sources among the transported fluvial sediments.

We studied the mineralogical composition and crystallinity of the non-carbonate fine fraction from late Marine Isotope Stage (MIS) 6 to MIS 5 (150-70 ka) in a marine sediment core obtained in the proximity of the Doce river mouth (20° S, 38° W, 2 km water depth). The main non-carbonate mineral content comprises quartz, albite, illite, kaolinite and gibbsite. The relative abundances of the mineral assemblage show distinct changes relative to changes in summer insolation as well as across the MIS 6-5 transition. Thereby, the mineral assemblage shows a distinct end-member pattern, with high contents of illite (80 % 2M-polytype) and high illite crystallinity as a proxy for stronger physical erosion of the parent rocks in the steep upland, and high contents of kaolinite and gibbsite as proxy for intense tropical soil erosion in the lowlands.

During MIS 5, the insolation dependent cyclicity seen in the mineral assemblage shows high illite/kaolinite ratios when austral summer insolation is high and low illite/kaolinite ratios in low insolation phases. This pattern is not visible in late MIS 6, when very low illite/kaolinite ratios are present during high austral summer insolation.

We consider the spatial changes in erosion intensity to be caused by variations in the regional precipitation pattern. Thereby, pronounced upland erosion is caused by severe precipitation and discharge events during a strong SASM. A relatively increased lowland erosion indicates both increased austral winter precipitation due to stronger trade wind forcing and a weaker monsoonal system in the upper discharge area. The lack of a strong insolation-control on illite/kaolinite ratios during MIS 6 is interpreted as an overall weakening of the SASM system during glacial periods, when austral winter precipitation exerted a stronger control on the hydrological budget of the Doce River.

How to cite: Arndt, I., Voigt, S., Petschick, R., Bahr, A., Hou, A., and Raddatz, J.: Effects of tropical precipitation variability on the composition of fluvial sediments from SE Brazil in glacial and interglacial times (MIS 6-5), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4578, https://doi.org/10.5194/egusphere-egu2020-4578, 2020.

Far-IR usually refers to the portion of the electromagnetic spectrum with a wavelength longer than 15 microns. For terrestrial atmosphere, water vapor pure rotational absorption is the most important gaseous absorption in the far -IR and ice clouds have a scattering peak around 25 microns. The far -IR consists of ~50% of the infrared energy emitted by our planet to space and, thus, plays a critical role in the earth's radiation budget and related changes in response to future climate change. 

Due to the overwhelming role played by the water vapor absorption in the far-IR, the traditional wisdom that assumes blackbody surface and non-scattering cloud in the longwave radiation scheme are well justified for the tropics and mid-latitude. However, such approximations widely adopted by virtually all the climate models break down in the high-latitude due to small water vapor abundance. As a consequence, the surface-atmosphere longwave coupling is manifested in the high latitudes, with the most prominent impact in the polar winter. Using the NCAR CESM and DoE E3SM models, we quantitatively show the statistically significant and seasonally dependent impact of such longwave coupling on simulated polar climate and surface energy budget. The effect of surface spectral emissivity and longwave scattering is linearly additive to each other, and the dominant contribution is from the far-IR region. Our results show that the longwave scattering and surface spectral emissivity are both necessities for the faithful simulation of polar climate. Climate models should include both of them, which are missing in virtually all the current models.

Accurate and spectrally resolved measurements in the far -IR have been technically challenging. Though the outgoing mid -IR spectra have been routinely observed from space with high accuracy and dense sampling pattern, as of today, we still have had no global spectrally resolved far-IR measurements from space. The last spectrally resolved measurements from space for 15 -25 microns were made a half-century ago in 1970 -1971. Motivated by recent studies, both NASA and ESA have selected missions dedicated to the far-IR radiation measurements, namely PREFIRE by NASA and FORUM by ESA. Both missions will provide us with critically needed observations for characterizing the surface-atmosphere longwave coupling, primarily through retrieved surface spectral emissivity and cloud properties in the far-IR dirty window (16.7-29 microns). We show here some initial results for relevant retrieval algorithm developments and expected uncertainties for the surface spectral emissivity and cloud properties retrieved from such far-IR measurements. 

How to cite: Huang, X., Chen, Y.-H., Yang, P., Kuo, C.-P., and Chen, X.: High-latitude surface-atmosphere radiative coupling in the far-IR: missing physics in climate models and opportunities in future observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5364, https://doi.org/10.5194/egusphere-egu2020-5364, 2020.

The Tibetan Plateau (TP), known as the “World Roof”, has significant influences on hydrological and atmospheric circulation at both regional and global scale. As the Sanjiangyuan Region (SJY) supplies water resources to the adjacent river basin and the TP could exert strong thermal forcing on the atmosphere over Asian monsoon region, adequate understand of the climate change over this region and its underlying mechanisms is of great importance. Based on gridded data provided by China Meteorological Administration (CMA), a continuous warming trend higher than that over elsewhere in China has been observed over the TP during 1985-2014, especially in the cold season (0.69 K/decade) and over the SJY (1.0 K/decade). On the basis of ERA interim reanalysis datasets, this paper analyzed the factors facilitating this warming trend in the SJY from the perspective of energy transport. At first, the local processes involved were investigated by calculating partial temperature changes using the surface energy budget equation. Then the horizontal convection of heat was quantified by summing the heat flux across the boundaries of the SJY. Finally, a Lagrangian heat source diagnostic method was developed to identify the major heat source. As the results indicating, among all the local heat sources, the enhanced downward longwave radiation reflected to surface air and the increasing upward longwave radiation emitted by warmer land surface were responsible for the pronounced surface air warming. However, the changes in surface sensible and latent heat fluxes had a reduced warming effect on the surface air. As for the non-local horizontal heat sources, rising horizontal heat flux from the south, west and east boundaries into the SJY contributed to the higher surface temperature of the SJY. In winter season, the heat flows stemmed from the South Himalayan vein into the SJY played a dominant role. Moreover, the higher the temperature over the SJY was, the more inclined this heat source was to Nepal.

How to cite: Tian, Y. and Zhong, D.: Causes of the 1985-2014 Surface Warming over the Sanjiangyuan Region of the Tibetan Plateau from the Perspective of Energy Transport, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6744, https://doi.org/10.5194/egusphere-egu2020-6744, 2020.

Eastern Brazil belongs to the ecologically most vulnerable regions on Earth due to its extreme intra- and inter-annual variability in precipitation amount. In order to constrain the driving forces behind this strong natural fluctuations we investigated a high-resolution sediment core taken off the Jequitinhonha river mouth in central E Brazil to reconstruct Holocene river run-off and moisture availability in the river’s catchment. Modern day climate in the hinterland of the Jequitinhonha is influenced by the South American Summer Monsoon (SASM), in particular by the manifestation of the South Atlantic Convergence Zone (SACZ) during austral summer. Variations in the position and strength of the SACZ will have immediate impact on the moisture balance over the continent and hence influence sediment and water delivery. Our multi-proxy records, comprising XRF core-scanning, grain size, mineralogical (XRD), as well as organic biomarker analyses indicate abrupt centennial scale variations between dry and wet conditions throughout the past ~5 kyrs. Our results document a gradual weakening of the SASM over the past ~2,7 kyrs driven by changes in the intertropical heat distribution. This long-term trend is superposed by centennial to millennial-scale spatial shifts in moisture distribution that result from migrations of the SACZ. The combination of both processes caused increasingly pronounced aridity spells in eastern South America over the past 2 kyrs. As the spatial fluctuations were triggered by freshwater anomalies in the North Atlantic, we surmise that enhanced meltwater input into the North Atlantic due to future global warming might severely increase the risk for mega-droughts in tropical South America.

How to cite: Bahr, A., Kaboth-Bahr, S., Jaeschke, A., Chiessi, C., Cruz, F., Rethemeyer, J., Schefuß, E., Geppert, P., Spadano Albuquerque, A. L., Pross, J., and Friedrich, O.: Growing multicentennial-scale precipitation variability in Eastern Brazil during the late Holocene, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7134, https://doi.org/10.5194/egusphere-egu2020-7134, 2020.

Numerous observational studies have found that the Hadley cells have expanded poleward in both the Northern and Southern Hemispheres, and model results suggest that such expansion is likely to continue throughout the 21st century as a result of global warming.  This has led to concerns about future impacts of Hadley cell expansion, including a poleward shift of the subtropical dry zone.  However, climatic changes associated with Hadley cell expansion are zonally asymmetric—especially in the Northern Hemisphere—suggesting that a more regional focus may be necessary.  In this study, we consider the influence of the Northern Hemisphere subtropical highs, and contrast this with the influence of Hadley cell expansion. 

Specifically, we consider the North Pacific and North Atlantic subtropical highs and define, for each high, three indices representing longitude, latitude, and strength.  We find that 21st century trends in variables as diverse as precipitation, sea-level pressure, winds, and ocean upwelling in eastern boundary currents are all driven more by the trends of these subtropical high indices than by the expansion of the Hadley cell.  We conclude that 21st century trends in subtropical high positions and strengths are crucial to understanding the future of Northern Hemisphere climate.  Further work will be needed to determine the dynamical drivers of these subtropical high trends.  

How to cite: Schmidt, D., Grise, K., Amaya, D., and Miller, A.: Impacts of Shifting Subtropical Highs on North American Climate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10083, https://doi.org/10.5194/egusphere-egu2020-10083, 2020.

Here we consider the areas located below the descending branch of the Hadley cell and characterized by arid or semiarid climate in the northern hemisphere. There are controversial suggestions that they are shifting northwards and this trend will continue during the 21st century.  We investigate the time-of-emergence (ToE) of corresponding climate change signals using  the Max Planck Institute Grand Ensemble (MPI-GE), a model experiment which allows by design to disentangle the role of external forcing from the internal climate variability on climate signals. Here we show that the ToE and the regions where it will occur are strongly dependent on the variable that is adopted for the analysis. For most  variables, ToE of regional subtropical expansion would occur only after the end of the 21st century and only in few irregularly distributed areas. Therefore, in spite of the consensus among projections on the future boreal subtropical broadening, the strong role played by the internal climate variability prevents to consider the observed trends in reanalyses over last decades a robust signal of anthropogenic forcing.

How to cite: D'Agostino, R., Scambiati, A. L., Jungclaus, J., and Lionello, P.: The poleward shift of the Subtropics in the Northern Hemisphere winter: time of emergence of the climate change signals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10145, https://doi.org/10.5194/egusphere-egu2020-10145, 2020.

The upper Indus River basin is characterised by heavy precipitation falling near the foothills of the major mountain ranges, during two wet seasons: winter and summer. Winter precipitation is known to be related to the passing of upper-level synoptic systems embedded in the subtropical westerly jet called Western Disturbances. Here, we investigate the precipitation variability in relation to the Western Disturbances at the synoptic scale, using ERA5 reanalysis data. We take advantage of the results of a previous study that showed that the precipitation is mostly triggered by the forced uplift of a low-level moisture-rich southerly flow across the ranges. We show that the low-level southerly wind triggering the precipitation is produced by the interaction of a Western Disturbance with a baroclinic front located between the Iranian plateau and the Arabian Sea. Ahead of the Western Disturbance, low-level winds draw moisture from the extreme north of the Arabian Sea, the Persian Gulf, and to a lower extent, the Red Sea. At the rear, moisture is depleted by the advection of continental dry air in the Indus River basin. However, the balance between moisture drawing and depletion depends on the characteristics of the Western Disturbance, leading to differences in precipitation intensity. We found the jet position and western Russia blockings to play a role in this. These findings offer clues to understand the longer-term precipitation variability in the area.

How to cite: Baudouin, J.-P., Herzog, M., and Petrie, C. A.: The relationship of Western Disturbances to precipitation in the upper Indus River basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11499, https://doi.org/10.5194/egusphere-egu2020-11499, 2020.

Comprehensive general circulation models (GCMs) in the CMIP3/CMIP5 archive project a delay in the timing of monsoon onset, as the climate warms in response to greenhouse gas (GHG) concentration increases. It has been argued that surface latent heat flux, and its differing response to GHG perturbations over land and over ocean, plays an important role in the redistribution of rainfall from early to late in the warm season in monsoon regions. However, similar phase delays in tropical precipitation have been shown to occur even in warming aquaplanet simulations forced by sea surface temperature perturbations. An alternative explanation invokes energetic arguments, in which elevated latent energy demand in the hemisphere warming up seasonally, as dictated by the Clausius–Clapeyron relation, drives a shift of tropical rainfall towards the opposite hemisphere, manifesting itself as a seasonal delay in the onset of the rainy season. 

In this study, we explore mechanisms of delayed monsoon onset with warming in aquaplanet simulations with an idealized GCM spanning a wide range of climates. In earlier work, we have in fact shown how monsoons with rapid circulation and precipitation changes at the beginning of the warm season can be simulated even without any land-sea contrast, provided that the lower boundary has sufficiently low thermal inertia. As the climate is warmed, we find that the onset of the monsoon is progressively delayed to later pentads in the summer season, in agreement with results from the comprehensive GCMs. However, the end of the monsoon season varies less strongly with climate, resulting in a progressive shortening of the overall monsoon season as the climate is warmed. The atmospheric energy balance is examined to separate possible influences of changes in surface fluxes, atmospheric energy storage and gross moist stability on the circulation's seasonality. Radiative-convective equilibrium experiments with the same GCM are also examined, to explore if and to what extent the delayed monsoon onset can indeed result from increases in the effective heat capacity of the atmospheric column with warming, through changes in its latent energy component. 

How to cite: Bordoni, S. and Hui, K.: Mechanisms of delayed monsoon onset with warming, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12518, https://doi.org/10.5194/egusphere-egu2020-12518, 2020.

While ongoing global warming is largely the result of reduced outgoing longwave radiation (OLR), climate feedbacks associated with changes in atmospheric water vapor and surface albedo are expected to enhance the absorption of shortwave radiation (ASR) and to sustain global warming on centennial time scales beyond the OLR modulations. These feedbacks as well as positive cloud feedbacks reduce the reflected shortwave (SW) flux at the top-of-atmosphere (TOA) and are a result of scattering and absorbing processes that differ by their near-infrared (NIR) and visible (VIS) contributions. Since direct measurements of broadband NIR (~0.7-5 mm) and VIS (~0.2-0.7 mm) radiation flux do not exist, we utilize UKESM1 simulations to study SW, NIR, and VIS climate feedbacks under preindustrial and abrupt-4xCO₂ climate forcing.

Besides its global long-term behavior, the spatial variability and key physical controls of ASR are not well characterized either. A prominent example is the unexplained hemispheric symmetry in planetary albedo that is consistently missed by current global climate models yielding unrealistic precipitation and circulation patterns. Although energetically equivalent, the observed hemispheric albedos differ spectrally, reflecting the uneven distribution of clouds and land masses. We use the same UKESM1 simulations to contrast inter-hemispheric differences in SW, NIR and VIS, and their relation to changes in clouds, the gaseous atmosphere and surface properties to shed light on processes relevant to the present-day symmetry, model biases, and potential future changes.

How to cite: Hakuba, M. Z., Bodas-Salcedo, A., and Stephens, G.: Long-term variability of shortwave absorption under abrupt-4xCO2 climate forcing and its visible and near-IR contributions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12529, https://doi.org/10.5194/egusphere-egu2020-12529, 2020.

The arrival of sargassum in a massive way generates adverse environmental, social and economic impacts. Little is known about its origin and trajectory, as well as the atmospheric and oceanic conditions under which it arrives at the Mexican coasts of the Caribbean. This poster presents a diagnosis of the seasonal, annual and interannual variability of atmospheric circulations in the Atlantic and Caribbean Sea, identifying the atmospheric conditions under which sargassum arrived on the Mexican coasts. 30 years of surface wind data from CFSR (Climate Forecast System Reanalysis) of NCAR on the Atlantic and Caribbean were analyzed, dividing the area into six areas, for each one its seasonal, annual and interannual variability was estimated, as well as its extreme values from 1989 to 2018, focusing the study on both the Caribbean Sea and the Atlantic coast of Brazil.

Once the mean, extreme winds (10th and 90th percentiles) and their correlation with the NAO (North Atlantic Oscillation) were diagnosed interannually, particular years of the recent period were analyzed: from 2010 to 2019 incorporating the wind convergence as a physical process associated with the accumulation of sargassum, surface pressure and sea surface temperature (SST) and also correlating it with the NAO index.

The results show that the atmospheric conditions for transporting sargassum along the Mexican coasts of the Caribbean are more favorable in summer than in winter, besides it, the higher extremes (90th percentile) in the Caribbean favor the transport of sargassum both in winter and in summer. However, "connectivity" with other regions (Central Atlantic) makes summer more favorable, but winter is potentially viable. The atmospheric conditions of recent extreme years are discussed: 2013 (without the arrival of sargassum), medium: 2015 and extreme 2018 (with abundant sargassum) for both summer and winter.

How to cite: Salinas, J. A., Maya, M. E., and Hernández, C.: Atmospheric circulation associated with the arrival of sargassum in the Caribbean Sea., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19224, https://doi.org/10.5194/egusphere-egu2020-19224, 2020.

Recent studies on monsoon dynamics have emphasized that monsoon changes are due to local anomalous Net Energy Input (NEI) in the atmospheric column, rather than being associated with land-sea temperature contrasts as in the classical large-scale sea-breeze view of monsoons. In the energy framework, a positive NEI (convergence of energy) must be balanced by a lateral export of moist static energy, which, if achieved by an overturning cell, is associated with moisture import and net precipitation.

This suggests a strong link between monsoonal precipitations and NEI, providing a pathway to understand uncertainties in predictions of past and future monsoon precipitation.

To investigate this, we exploit the CMIP5 and PMIP3 archives (9 models), comparing simulations of Mid-Holocene (~6000 years ago) and future (end of 21^st century, RCP8.5 scenario) climates to pre-industrial (PI) control climate.

Precipitation responses to past and future forcing in monsoon regions exhibit a wide spread which is, as expected, significantly (and positively) correlated with NEI changes. Yet, the latter explain at best 40% of the spread in the precipitation response. In fact, the correlation between NEI and precipitation changes hides a more complex picture.

We show that changes in atmospheric stratification and differences in the control climate contribute to the uncertainties, with varying degrees depending on regions and climates: while the southern hemisphere monsoons are linked to changes in both stratification and NEI, the northern hemisphere monsoons are more strongly associated with stratification changes. Meanwhile, changes in the mid-Holocene are more dominated by NEI changes than in the future climate when stratification changes are larger.

How to cite: Ferreira, D. and D'Agostino, R.: Attribution of uncertainties in predictions of monsoon precipitation using an energy framework, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19233, https://doi.org/10.5194/egusphere-egu2020-19233, 2020.

We derive internally consistent, monthly to interannual, energy and water budgets, with uncertainties, for all the main continents and ocean basins over 2001-2011 based principally on satellite data. An inverse model is used following the Thomas et al (2019) climatology study and the NASA energy and water cycle study (NEWS), L’Ecuyer et al. (2015), Rodell et al. (2015).
Input data include CERES and Cloud_CCI AATSR (radiation), FluxCOM (land turbulent heat fluxes), JOFURO3 (ocean turbulent heat fluxes), GPCP2.3 (Precipitation), GRACE (total water storage), ERA5 (atmospheric water storage), GRUNv1 (land runoff), and we compare these with alternative products to assess component uncertainties. The different components are then brought together and adjusted within respective uncertainties to achieve balanced energy and water budgets.
Preliminary results focus on seasonal and interannual variability over land. Seasonal modifications to the water budget over Eurasia and N America include a delay in spring runoff (and reduced evapotranspiration over Eurasia) as GRACE data indicates retention of water mass over land. Evapotranspiration adjustments to FluxCOM are strongly seasonal and also result in bringing the land seasonal energy budget closer to the DEEPC Liu et al (2015) results demonstrating the value of coupling the energy and water cycles.
Strong correlated interannual variability in African precipitation, runoff and GRACE derived water storage is found, and we assess the relative consistency of different data products, particularly for precipitation, where multiple datasets are available and uncertainties are large. Consistent African precipitation variability is found in the TAMSAT data, which further supports the water cycle change scheme around year 2006 over Africa. Clear ENSO signals are seen, particularly over South America in 2010 and Australia in 2010-11, with correlated variability in rainfall, runoff and water storage distributions. 
Optimisation is sensitive to the uncertainty of each energy and water budget component expressed in their spatial and temporal error covariances.  We introduce spatial error covariance for turbulent heat fluxes between major ocean basins as well as temporal error covariances for all components expressing the expectation of time mean bias adjustments. The results show improved net surface energy flux pattern with larger heat loss over North Atlantic and Arctic Ocean and more heat uptake for other basins and an intensified water cycle, with increased precipitation, evapotranspiration and runoff and stronger ocean-land water transports. 

How to cite: Dong, B., Haines, K., Thomas, C., Liu, C., and Allan, R.: Global surface energy and water cycle variability 2001-2011 from satellite data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19771, https://doi.org/10.5194/egusphere-egu2020-19771, 2020.

Documenting and understanding centennial scale hydroclimatic variability in Australia is significant both to global climate science and to regional efforts to predict and manage water resources. In particular, multidecadal to centennial periods of low rainfall – ‘megadroughts’ – have been observed in semi-arid climates worldwide, however they are poorly constrained in Australia. Here, we bring together multiple, sub-decadally resolved records of hydrological change inferred from lake sediments in western Victoria, Australia. Our analyses incorporate new elemental (ITRAX µXRF) and stable isotope (oxygen, carbon isotopes) geochemical data from West Basin and Lake Surprise, both augmented by high quality radiometric chronologies based on radiocarbon, ²¹⁰Pb and ^239/240Pu analyses. Collectively, the records document a transition towards a more arid and variable climate since the mid-late Holocene, which is comparable to reports of an intensification of the El Nino Southern Oscillation (ENSO) through this period. Furthermore, during the last 2000 years, the records exhibit marked periods of reduced effective moisture which contrast with records of Australian hydroclimate inferred from distal archives, as well those predicted by climate model hindcasts. Our analyses indicate that megadroughts are a natural phenomenon in south-eastern Australia, requiring greater attention in efforts to predict and mitigate future climatic change.

How to cite: Tyler, J., Barr, C., Tibby, J., Dhar, A., Andrew, C., Dean, C., Gadd, P., Zawadzki, A., Child, D., and Jacobsen, G.: Holocene ‘megadroughts’ in south-eastern Australia: deciphering regional patterns from lake sediment archives, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21027, https://doi.org/10.5194/egusphere-egu2020-21027, 2020.

The increase in the Earth's energy balance, due to the surplus of anthropogenic greenhouse gases, has created a warmer earth-based climate regime, widening the excess tropic zone to the poles. The characteristics and rhythms of this climate become unusual, with extreme weather conditions: more frequent heat and cold waves, strong gusts of wind, more severe droughts, and more frequent floods. This is the terrestrial "New Climate".

This situation is characterized by a new thermal distribution: above the ocean, the situation is more in surplus energetic budget, and the land - atmosphere is negative. Warm thermal advection easily reach the Pole, as well as cold advection push deep into Western Mediterranean.

This "New Ground Energy Balance" establishes an atmospheric circulation with an waving character throughout the year, including in winter, characterized by intense energy exchanges latitudinal very active between the surplus and deficit areas on the one hand, and the atmosphere, the ocean and the continent of the other.

The new thermal distribution reorganizes the geography of atmospheric pressure: the ocean energy concentration is transmitted directly to the atmosphere, and the excess torque is pushed northward. The Azores anticyclone is strengthened and is a global lock by the Atlantic ridge at Greenland, which imposes on the jet stream a positive ripple, very strongly marked poleward, bringing cosmic cold advection of polar air masses winter over from Europe to Western Mediterranean. Hence the enormous meridian heat exchanges north-south-north. This is the "New" Meridian Atmospheric Circulation (MAC).

This situation increases the potential evaporation of the atmosphere and provides a new geographical distribution of Moisture: the excess water vapor is easily converted by cold advection to heavy rains that cause floods or snow storms.

Thus, the "New Energy Balance" creates a "New" Meridian-dominated Atmospheric Circulation, which induces excess atmospheric water vapor due to the increase in temperature. Since the hydro-atmospheric capacity has increased, the return to the ground is abundant: It is the "New Water Cycle", which accompanies the “New Energy Balance” of the Earth.

How to cite: Karrouk, M.-S.: New Energy Balance, New Atmospheric Circulation and New Water Cycle in Western Mediterranean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-371, https://doi.org/10.5194/egusphere-egu2020-371, 2020.