scholarly journals Elements of the dynamical response to climate change over the Mediterranean

2020 ◽  
pp. 1-34
Author(s):  
Alexandre Tuel ◽  
Paul A. O’Gorman ◽  
Elfatih A. B. Eltahir
Keyword(s):  
Large Scale ◽  
Vertical Structure ◽  
Static Stability ◽  
Hadley Cell ◽  
Dynamical Response ◽  
Near Surface ◽  
Low Level ◽  
External Mode ◽  
The Mediterranean ◽  
Circulation Changes

AbstractFuture climate simulations indicate that the Mediterranean Basin will experience large low-level circulation changes during winter, characterized by a strong anomalous ridge that drives a regional precipitation decline. Previous research highlighted how shifts in stationary wave structure and the atmospheric response to reduced warming of the Mediterranean Sea compared to land could explain the development of this anomalous pressure high. Here, we expand on these results and provide new arguments for why and how the Mediterranean is projected to experience large circulation changes during winter. First, we find that zonal asymmetries in the vertical structure of stationary waves are important to explain the enhanced circulation response in the region, and that these asymmetries are related through the external mode to the vertical structure of the mean zonal wind. Second, in winter, the Mediterranean is located just to the north of the Hadley cell edge and consequently relatively free of large-scale descent; together with low near-surface static stability above the sea, this allows the weaker warming trend above the sea to propagate to the low troposphere and trigger a major circulation response. During summer, however, remotely-forced descent and strong static stability prevent the cooling anomaly from expanding upwards. Most of the inter-model scatter in the projected low-level circulation response is related to the spread in upper-tropospheric dynamical trends. Importantly, because climate models exhibit too much vertical coherence over the Mediterranean, our results suggest they overestimate the sensitivity of Mediterranean near-surface circulation to large-scale dynamical changes.

Ocean Salinity changes in the global ocean under global warming conditions Part 1: Mechanisms in a strong warming scenario

2021 ◽  
pp. 1-56
Author(s):  
Anju Sathyanarayanan ◽  
Armin Köhl ◽  
Detlef Stammer
Keyword(s):  
Large Scale ◽  
Water Cycle ◽  
Spatial Scales ◽  
Sea Surface Salinity ◽  
Global Ocean ◽  
Dynamical Response ◽  
Freshwater Flux ◽  
Near Surface ◽  
Salinity Changes ◽  
Circulation Changes

AbstractWe investigate mechanisms underlying salinity changes projected to occur under strong representative concentration pathway (RCP) 8.5 forcing conditions. The study is based on output of the Max Planck Institute Earth System Model Mixed Resolution (MPI-ESM-MR) run with an ocean resolution of 0.4°. In comparison to the present-day oceanic conditions, sea surface salinity (SSS) increases towards the end of the 21st century in the tropical and the subtropical Atlantic. In contrast, a basin-wide surface freshening can be observed in the Pacific and Indian Oceans. The RCP8.5 scenario of the MPI-ESM-MR with a global surface warming of ~2.3°C marks a water cycle amplification of 19 %, which is equivalent to ~8%°C−1 and thus close to the water cycle amplification predicted according to the Clausius–Clapeyron (CC) relationship (~7%°C−1). Large scale global SSS changes are driven by adjustments of surface freshwater fluxes. On smaller spatial scales, it is predominantly advection related to circulation changes that affects near-surface SSS. With respect to subsurface salinity, it is changes in surface freshwater flux that drive their changes over the upper 500 m of the subtropical Pacific and Indian oceans by forcing changes in water mass formation (spice signal). In the subtropical Atlantic Ocean, in contrast, the dynamical response associated with wind stress, circulation changes and associated heaving of isopycnals is equally important in driving subsurface salinity changes over the upper 1000 m.

scholarly journals On the fine vertical structure of the low troposphere over the coastal margins of East Antarctica

2019 ◽  
Author(s):  
Étienne Vignon ◽  
Olivier Traullé ◽  
Alexis Berne
Keyword(s):  
Pressure Gradient ◽  
Vertical Structure ◽  
East Antarctica ◽  
Radiosonde Data ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Near Surface ◽  
Ice Shelves ◽  
Low Level ◽  
Humidity Profiles

Abstract. Eight years of high-resolution radiosonde data at nine Antarctic stations are analysed to provide the first large scale characterization of the fine scale vertical structure of the low troposphere up to 3 km of altitude over the coastal margins of East Antarctica. Radiosonde data show a large spatial variability of wind, temperature and humidity profiles, with different features between stations in katabatic regions (e.g., Dumont d'Urville and Mawson stations), stations over two ice shelves (Neumayer and Halley stations) and regions with complex orography (e.g., Mc Murdo). At Dumont d'Urville, Mawson and Davis stations, the yearly median wind speed profiles exhibit a clear low-level katabatic jet. During precipitation events, the low-level flow generally remains of continental origin and its speed is even reinforced due to the increase in the continent- ocean pressure gradient. Meanwhile, the relative humidity profiles show a dry low troposphere, suggesting the occurence of low-level sublimation of precipitation in katabatic regions but such a phenomenon does not appreciably occur over the ice-shelves near Halley and Neumayer. Although ERA-Interim and ERA5 reanalyses assimilate radiosoundings at most stations considered here, substantial – and sometimes large – low-level wind and humidity biases are revealed but ERA5 shows overall better performances. A free simulation with the regional model Polar WRF (at a 35-km resolution) over the entire continent shows too strong and too shallow near-surface jets in katabatic regions especially in winter. This may be a consequence of an understimated coastal cold air bump and associated sea-continent pressure gradient force due to the coarse 35 km resolution of the Polar WRF simulation. Beyond documenting the vertical structure of the low troposphere over coastal East-Antarctica, this study gives insights into the reliability and accuracy of two major reanalysis products in this region on the Earth and it raises the difficulty of modeling the low-level flow over the margins of the ice sheet with a state-of-the-art climate model.

scholarly journals The Large-Scale Dynamical Response of Clouds to Aerosol Forcing

2017 ◽  
Vol 30 (21) ◽  
pp. 8783-8794 ◽  
Author(s):  
Brian Soden ◽  
Eui-Seok Chung
Keyword(s):  
Atmospheric Circulation ◽  
Radiative Forcing ◽  
Large Scale ◽  
Energy Transport ◽  
Hadley Cell ◽  
Dynamical Response ◽  
Aerosol Radiative Forcing ◽  
Aerosol Forcing ◽  
Meridional Displacement ◽  
Cloud Response

Radiative kernels are used to quantify the instantaneous radiative forcing of aerosols and the aerosol-mediated cloud response in coupled ocean–atmosphere model simulations under both historical and future emission scenarios. The method is evaluated using matching pairs of historical climate change experiments with and without aerosol forcing and accurately captures the spatial pattern and global-mean effects of aerosol forcing. It is shown that aerosol-driven changes in the atmospheric circulation induce additional cloud changes. Thus, the total aerosol-mediated cloud response consists of both local microphysical changes and nonlocal dynamical changes that are driven by hemispheric asymmetries in aerosol forcing. By comparing coupled and fixed sea surface temperature (SST) simulations with identical aerosol forcing, the relative contributions of these two components are isolated, exploiting the ability of prescribed SSTs to also suppress changes in the atmospheric circulation. The radiative impact of the dynamical cloud changes is found to be comparable in magnitude to that of the microphysical cloud changes and acts to further amplify the interhemispheric asymmetry of the aerosol radiative forcing. The dynamical cloud response is closely linked to the meridional displacement of the Hadley cell, which, in turn, is driven by changes in the cross-equatorial energy transport. In this way, the dynamical cloud changes act as a positive feedback on the meridional displacement of the Hadley cell, roughly doubling the projected changes in cross-equatorial energy transport compared to that from the microphysical changes alone.

scholarly journals Mediterranean Summer Climate and the Importance of Middle East Topography*

2015 ◽  
Vol 28 (5) ◽  
pp. 1977-1996 ◽  
Author(s):  
Isla R. Simpson ◽  
Richard Seager ◽  
Tiffany A. Shaw ◽  
Mingfang Ting
Keyword(s):  
Middle East ◽  
Large Scale ◽  
Stationary Wave ◽  
Mountain Range ◽  
North African ◽  
Incident Flow ◽  
Low Level ◽  
Community Atmosphere Model ◽  
Moisture Deficit ◽  
The Mediterranean

Abstract In summer, the atmospheric circulation over the Mediterranean is characterized by localized intense subsidence and low-level northerlies over the central to eastern portion of the basin. Here, simulations with the Community Atmosphere Model, version 5 are used to investigate the influence of the elevated terrain of North Africa and the Middle East on this summertime circulation. This builds on previous work that recognized a role for North African topography in localizing the Mediterranean subsidence. By flattening the two regions of elevated terrain in the model, it is demonstrated that, while they both conspire to produce about 30% of the summertime subsidence, contrary to previous work, the mountains of the Middle East dominate in this topographic contribution by far. This topography, consisting primarily of the Zagros mountain range, alters the circulation throughout the depth of the troposphere over the Mediterranean and farther east. The model results suggest that about 20% of the Mediterranean summertime moisture deficit can be attributed to this mountain-induced circulation. This topography, therefore, plays an important role in the climate of the Mediterranean and the large-scale circulation over the rest of Eurasia during the summer. Further stationary wave modeling reveals that the mountain influence is produced via mechanical forcing of the flow. The greatest influence of the topography occurs when the low-level incident flow is easterly, as happens during the summer, primarily because of the presence of condensational heating over Asia. During other seasons, when the low-level incident flow is westerly, the influence of Middle East topography on the Mediterranean is negligible.

scholarly journals Atmospheric Dynamics Feedback: Concept, Simulations, and Climate Implications

2018 ◽  
Vol 31 (8) ◽  
pp. 3249-3264 ◽  
Author(s):  
Michael P. Byrne ◽  
Tapio Schneider
Keyword(s):  
Climate Change ◽  
Atmospheric Circulation ◽  
Radiative Forcing ◽  
Large Scale ◽  
Climate Models ◽  
Atmospheric Dynamics ◽  
Future Climate Change ◽  
Coupled Models ◽  
Near Surface ◽  
Circulation Changes

AbstractThe regional climate response to radiative forcing is largely controlled by changes in the atmospheric circulation. It has been suggested that global climate sensitivity also depends on the circulation response, an effect called the “atmospheric dynamics feedback.” Using a technique to isolate the influence of changes in atmospheric circulation on top-of-the-atmosphere radiation, the authors calculate the atmospheric dynamics feedback in coupled climate models. Large-scale circulation changes contribute substantially to all-sky and cloud feedbacks in the tropics but are relatively less important at higher latitudes. Globally averaged, the atmospheric dynamics feedback is positive and amplifies the near-surface temperature response to climate change by an average of 8% in simulations with coupled models. A constraint related to the atmospheric mass budget results in the dynamics feedback being small on large scales relative to feedbacks associated with thermodynamic processes. Idealized-forcing simulations suggest that circulation changes at high latitudes are potentially more effective at influencing global temperature than circulation changes at low latitudes, and the implications for past and future climate change are discussed.

Large-scale drivers of the Mediterranean climate change hot-spot

2021 ◽  
Author(s):  
Roman Brogli ◽  
Silje Lund Sørland ◽  
Nico Kröner ◽  
Christoph Schär
Keyword(s):  
Climate Change ◽  
Large Scale ◽  
Hot Spot ◽  
Regional Climate ◽  
Lapse Rate ◽  
Summer Climate ◽  
Climate Simulations ◽  
Summer Warming ◽  
The Mediterranean ◽  
Circulation Changes

<div> <p><span>It has long been recognized that the Mediterranean is a ‘hot-spot’ of climate change. The model-projected year-round precipitation decline and amplified summer warming are among the leading causes of the vulnerability of the Mediterranean to greenhouse gas-driven warming. We investigate large-scale drivers influencing both the Mediterranean drying and summer warming in regional climate simulations. To isolate the influence of multiple large-scale drivers, we sequentially add the respective drivers from global models to regional climate model simulations. Additionally, we confirm the robustness of our results across multiple ensembles of global and regional climate simulations.</span></p> </div><div> <p><span>We will present in detail how changes in the atmospheric stratification are key in causing the amplified Mediterranean summer warming. Together with the land-ocean warming contrast, stratification changes also drive the summer precipitation decline. Summer circulation changes generally have a surprisingly small influence on the changing Mediterranean summer climate. In contrast, changes in the circulation are the primary driver for the projected winter precipitation decline. Since land-ocean contrast and stratification changes are more robust in global climate simulations than circulation changes, we argue that the uncertainty associated with the projected climate change patterns should be considered smaller in summer than in winter.</span></p> </div><div> <p><span>References:</span></p> </div><div> <p><span>Brogli, R., S. L. Sørland, N. Kröner, and C. Schär, 2019: Causes of future Mediterranean precipitation decline depend on the season. Environmental Research Letters, 14, 114017, doi:10.1088/1748-9326/ab4438.</span></p> </div><div> <p><span>Brogli, R., N. Kröner, S. L. Sørland, D. Lüthi and C. Schär, 2019: The Role of Hadley Circulation and Lapse-Rate Changes for the Future European Summer Climate. Journal of Climate, 32, 385-404, doi:10.1175/JCLI-D-18-0431.1</span></p> </div>

scholarly journals The Role of Hadley Circulation and Lapse-Rate Changes for the Future European Summer Climate

2018 ◽  
Vol 32 (2) ◽  
pp. 385-404 ◽  
Author(s):  
Roman Brogli ◽  
Nico Kröner ◽  
Silje Lund Sørland ◽  
Daniel Lüthi ◽  
Christoph Schär
Keyword(s):  
Hadley Circulation ◽  
Global Climate Models ◽  
Hadley Cell ◽  
Lapse Rate ◽  
Climate Change Signal ◽  
Climate Projections ◽  
Minor Importance ◽  
Near Surface ◽  
Summer Warming ◽  
The Mediterranean

Abstract By the end of the century, climate projections for southern Europe exhibit an enhanced near-surface summer warming in response to greenhouse gas emissions, which is known as the Mediterranean amplification. Possible causes for this amplified warming signal include a poleward Hadley cell expansion as well as tropospheric lapse-rate changes. In this work, regional climate model (RCM) simulations driven by three different global climate models (GCMs) are performed, representing the RCP8.5 emission scenario. For every downscaled GCM, the climate change signal over Europe is separated into five contributions by modifying the lateral boundary conditions of the RCM. This simulation strategy is related to the pseudo–global warming method. The results show that a poleward expansion of the Hadley cell is of minor importance for the Mediterranean amplification. During summer, the simulated Hadley circulation is weak, and projections show no distinct expansion in the European sector. The north–south contrast in lapse-rate changes is suggested as the most important factor causing the Mediterranean amplification. Lapse-rate changes are projected throughout Europe, but are weaker over the Mediterranean than over northern Europe (around 0.15 vs 0.3 K km−1 by the end of the century). The weaker lapse-rate changes result in a strong near-surface summer warming over the Mediterranean, since the upper-tropospheric warming is of similar magnitude throughout Europe. The differing lapse-rate changes can be understood as a thermodynamic response to lower-tropospheric humidity contrasts.

Scale sensitivity of the Gill circulation, Part I: Equatorial case

2021 ◽  
Author(s):  
Beatriz Reboredo ◽  
Gilles Bellon
Keyword(s):  
Large Scale ◽  
Diabatic Heating ◽  
Energy Transport ◽  
Maximum Velocity ◽  
Spatial Extent ◽  
Small Scale ◽  
Dynamical Response ◽  
Overturning Circulation ◽  
Low Level ◽  
Westerly Jet

Abstract We investigate the steady dynamical response of the atmosphere on the equatorial β-plane to a steady, localized, mid-tropospheric heating source at the equator. Expanding Gill (1980)’s seminal work, we vary the latitudinal and longitudinal scales of the diabatic heating pattern while keeping its total amount fixed. We focus on characteristics of the response which would be particularly important if the circulation interacted with the hydrologic and energy cycles: the overturning circulation and the low-level wind. In the limit of very small scale in either the longitudinal or latitudinal direction, the vertical energy transport balances the diabatic heating and this sets the intensity of the overturning circulation. In this limit, a fast low-level westerly jet is located around the center of diabatic heating. With increasing longitudinal or latitudinal scale of the diabatic heating, the intensity of the overturning circulation decreases and the low-level westerly jet decreases in maximum velocity and spatial extent relative to the spatial extent of this heating. The associated low-level eastward mass transport decreases only with increasing longitudinal scale. These results suggest that moisture-convergence feedbacks will favor small-scale equatorial convective disturbances while surface-heat-flux feedbacks would favor small-scale disturbances in mean westerlies and large-scale disturbances in mean easterlies. Part II investigates the case of off-equatorial heating.

scholarly journals Warm Organized Rain Systems over the Tropical Eastern Pacific

2016 ◽  
Vol 29 (9) ◽  
pp. 3403-3422 ◽  
Author(s):  
Baohua Chen ◽  
Chuntao Liu
Keyword(s):  
Large Scale ◽  
Tropical Rainfall Measuring Mission ◽  
Mean Value ◽  
Eastern Pacific ◽  
Tropical Eastern Pacific ◽  
Rain Area ◽  
Near Surface ◽  
Thermodynamic Structure ◽  
Low Level ◽  
Upper Level

Abstract This study uses 16-yr Tropical Rainfall Measuring Mission (TRMM) radar precipitation feature (RPF) data to characterize warm rain systems in the tropics with large horizontal extensions, referred to as warm organized rain systems. The systems are selected by specifying the RPFs with minimum infrared brightness temperature warmer than 0°C and rain area greater than 500 km2. ERA-Interim atmospheric fields and SST from NOAA are analyzed to highlight the environmental characteristics of warm organized rain systems. Warm organized systems occur over specific oceanic regions, including the eastern Pacific ITCZ, the eastern part of the SPCZ, and coastal regions. In contrast with ubiquitous warm isolated RPFs, warm organized systems have greater near-surface radar reflectivity. The rainfall amounts generated by warm organized systems are greater in winter than in summer. Composite analyses indicate that warm organized RPFs prefer to coexist with a dry midtroposphere associated with a strong upper-level descent, an enhanced near-surface moisture convergence, and a strong low-level large-scale ascent. The shallow meridional circulation in the eastern Pacific is significantly stronger for warm organized RPFs compared to the circulation for warm isolated RPFs. Warm organized systems over the tropical eastern Pacific occur at warm SSTs with mean value of about 27°C and a strong SST meridional gradient. The warm organized RPFs in the tropical eastern Pacific are found to be at the southern edge of deep ITCZ cores. This is probably related to the meridional asymmetrical thermodynamic structure over the eastern Pacific ITCZ with a higher low-level humidity to the south. Similar favorable large-scale environments for the warm organized RPFs are also found over the SPCZ and other regions.

scholarly journals High and Dry: New Observations of Tropospheric and Cloud Properties above the Greenland Ice Sheet

2013 ◽  
Vol 94 (2) ◽  
pp. 169-186 ◽  
Author(s):  
Matthew D. Shupe ◽  
David D. Turner ◽  
Von P. Walden ◽  
Ralf Bennartz ◽  
Maria P. Cadeddu ◽  
...  
Keyword(s):  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Static Stability ◽  
Energy Budgets ◽  
Unique Region ◽  
Near Surface ◽  
Thermodynamic Structure ◽  
Low Level ◽  
Cloud Properties ◽  
Stratiform Clouds

Cloud and atmospheric properties strongly influence the mass and energy budgets of the Greenland Ice Sheet (GIS). To address critical gaps in the understanding of these systems, a new suite of cloud- and atmosphere-observing instruments has been installed on the central GIS as part of the Integrated Characterization of Energy, Clouds, Atmospheric State, and Precipitation at Summit (ICECAPS) project. During the first 20 months in operation, this complementary suite of active and passive ground-based sensors and radiosondes has provided new and unique perspectives on important cloud–atmosphere properties. High atop the GIS, the atmosphere is extremely dry and cold with strong near-surface static stability predominating throughout the year, particularly in winter. This low-level thermodynamic structure, coupled with frequent moisture inversions, conveys the importance of advection for local cloud and precipitation formation. Cloud liquid water is observed in all months of the year, even the particularly cold and dry winter, while annual cycle observations indicate that the largest atmospheric moisture amounts, cloud water contents, and snowfall occur in summer and under southwesterly flow. Many of the basic structural properties of clouds observed at Summit, Greenland, particularly for low-level stratiform clouds, are similar to their counterparts in other Arctic regions. The ICECAPS observations and accompanying analyses will be used to improve the understanding of key cloud–atmosphere processes and the manner in which they interact with the GIS. Furthermore, they will facilitate model evaluation and development in this data-sparse but environmentally unique region.

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