scholarly journals The Middle Miocene climate as modelled in an atmosphere-ocean-biosphere model

2011 ◽  
Vol 7 (4) ◽  
pp. 1169-1188 ◽  
Author(s):  
M. Krapp ◽  
J. H. Jungclaus
Keyword(s):  
Global Warming ◽  
Temperature Gradient ◽  
Ocean Circulation ◽  
Large Scale ◽  
Middle Miocene ◽  
Ocean Dynamics ◽  
Co2 Levels ◽  
Water Vapour Feedback ◽  
Fossil Records ◽  
Biosphere Model

Abstract. We present simulations with a coupled atmosphere-ocean-biosphere model for the Middle Miocene 15 million years ago. The model is insofar more consistent than previous models because it captures the essential interactions between ocean and atmosphere and between atmosphere and vegetation. The Middle Miocene topography, which alters both large-scale ocean and atmospheric circulations, causes a global warming of 0.7 K compared to present day. Higher than present-day CO2 levels of 480 and 720 ppm cause a global warming of 2.8 and 4.9 K. The associated water vapour feedback enhances the greenhouse effect which leads to a polar amplification of the warming. These results suggest that higher than present-day CO2 levels are necessary to drive the warm Middle Miocene climate, also because the dynamic vegetation model simulates a denser vegetation which is in line with fossil records. However, we do not find a flatter than present-day equator-to-pole temperature gradient as has been suggested by marine and terrestrial proxies. Instead, a compensation between atmospheric and ocean heat transport counteracts the flattening of the temperature gradient. The acclaimed role of the large-scale ocean circulation in redistributing heat cannot be supported by our results. Including full ocean dynamics, therefore, does not solve the problem of the flat temperature gradient during the Middle Miocene.

scholarly journals The Middle Miocene climate as modelled in an atmosphere-ocean-biosphere model

2011 ◽  
Vol 7 (3) ◽  
pp. 1935-1972 ◽  
Author(s):  
M. Krapp ◽  
J. H. Jungclaus
Keyword(s):  
Global Warming ◽  
Heat Transport ◽  
Large Scale ◽  
Middle Miocene ◽  
Poleward Heat Transport ◽  
Co2 Levels ◽  
Water Vapour Feedback ◽  
Fossil Records ◽  
Biosphere Model ◽  
Atmospheric Heat

Abstract. We present simulations with a coupled ocean-atmosphere-biosphere model for the Middle Miocene 15 million years ago. The Middle Miocene topography, which alters both large-scale ocean and atmospheric circulations, causes a global warming of 0.7 K compared to present-day. Higher than present-day CO2 levels of 480 and 720 ppm cause a global warming of 2.8 and 4.9 K, thereby matching proxy-based Middle Miocene global temperature estimates of 3–6 K warming. Higher CO2 levels and the associated water vapour feedback enhance the greenhouse effect and lead to a polar amplification of the warming. Although oceanic and atmospheric poleward heat transport are individually altered by 10–30 % in the mid and high latitudes, changes of the total heat transport account only for 4–8 %, pointing toward a compensation between oceanic and atmospheric heat transport. Our model reproduces a denser vegetation in agreement with fossil records. These results suggest that higher than present-day CO2 levels are essential to drive the warm Middle Miocene climate.

scholarly journals Simulation of the Arctic—North Atlantic Ocean Circulation with a Two-Equation K-Omega Turbulence Parameterization

2018 ◽  
Vol 6 (3) ◽  
pp. 95 ◽  
Author(s):  
Sergey Moshonkin ◽  
Vladimir Zalesny ◽  
Anatoly Gusev
Keyword(s):  
Prandtl Number ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Large Scale ◽  
Mixed Layer Depth ◽  
Ocean Dynamics ◽  
Dynamics Simulation ◽  
The Arctic ◽  
Buoyancy Frequency ◽  
Physical Factors

The results of large-scale ocean dynamics simulation taking into account the parameterization of vertical turbulent exchange are considered. Numerical experiments were carried out using k − ω turbulence model embedded to the Institute of Numerical Mathematics Ocean general circulation Model (INMOM). Both the circulation and turbulence models are solved using the splitting method with respect to physical processes. We split k − ω equations into the two stages describing transport-diffusion and generation-dissipation processes. At the generation-dissipation stage, the equation for ω does not depend on k. It allows us to solve both turbulence equations analytically that ensure high computational efficiency. The coupled model is used to simulate the hydrophysical fields of the North Atlantic and Arctic Oceans for 1948–2009. The model has a horizontal resolution of 0.25 ∘ and 40 σ -levels along the vertical. The numerical results show the model’s satisfactory performance in simulating large-scale ocean circulation and upper layer dynamics. The sensitivity of the solution to the change in the coefficients entering into the analytical solution of the k − ω model which describe the influence of some physical factors is studied. These factors are the climatic annual mean buoyancy frequency (AMBF) and Prandtl number as a function of the Richardson number. The experiments demonstrate that taking into account the AMBF improves the reproduction of large-scale ocean characteristics. Prandtl number variations improve the upper mixed layer depth simulation.

Atmospheric and Oceanic Origins of Tropical Precipitation Variability

2017 ◽  
Vol 30 (9) ◽  
pp. 3197-3217 ◽  
Author(s):  
Jie He ◽  
Clara Deser ◽  
Brian J. Soden
Keyword(s):  
Time Scales ◽  
Ocean Circulation ◽  
Large Scale ◽  
Southern Oscillation ◽  
Noise Spectrum ◽  
Precipitation Variability ◽  
Ocean Dynamics ◽  
Atmospheric Variability ◽  
Tropical Precipitation ◽  
Summer Hemisphere

The intrinsic atmospheric and ocean-induced tropical precipitation variability is studied using millennial control simulations with various degrees of ocean coupling. A comparison between the coupled simulation and the atmosphere-only simulation with climatological sea surface temperatures (SSTs) shows that a substantial amount of tropical precipitation variability is generated without oceanic influence. This intrinsic atmospheric variability features a red noise spectrum from daily to monthly time scales and a white noise spectrum beyond the monthly time scale. The oceanic impact is inappreciable for submonthly time scales but important at interannual and longer time scales. For time scales longer than a year, it enhances precipitation variability throughout much of the tropical oceans and suppresses it in some subtropical areas, preferentially in the summer hemisphere. The sign of the ocean-induced precipitation variability can be inferred from the local precipitation–SST relationship, which largely reflects the local feedbacks between the two, although nonlocal forcing associated with El Niño–Southern Oscillation also plays a role. The thermodynamic and dynamic nature of the ocean-induced precipitation variability is studied by comparing the fully coupled and slab ocean simulations. For time scales longer than a year, equatorial precipitation variability is almost entirely driven by ocean circulation, except in the Atlantic Ocean. In the rest of the tropics, ocean-induced precipitation variability is dominated by mixed layer thermodynamics. Additional analyses indicate that both dynamic and thermodynamic oceanic processes are important for establishing the leading modes of large-scale tropical precipitation variability. On the other hand, ocean dynamics likely dampens tropical Pacific variability at multidecadal time scales and beyond.

Re-examining inferences from Hadley cell theory on tropical expansion under global warming throughout the seasonal cycle

2020 ◽  
Author(s):  
Spencer Hill ◽  
Jonathan Mitchell ◽  
Simona Bordoni
Keyword(s):  
Global Warming ◽  
Temperature Gradient ◽  
Large Scale ◽  
Climate Models ◽  
Numerical Models ◽  
Global Climate Models ◽  
Hadley Cell ◽  
Critical Examination ◽  
Cell Width ◽  
Hadley Cells

<p>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.</p><p>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.</p>

Coherent changes in large-scale thermal structure and baroclinic life cycle of synoptic eddies in the Northern Hemisphere under global warming

2020 ◽  
Author(s):  
Pei-Chun Hsu ◽  
Huang-Hsiung Hsu
Keyword(s):  
Global Warming ◽  
Life Cycle ◽  
Temperature Gradient ◽  
Northern Hemisphere ◽  
Large Scale ◽  
Thermal Structure ◽  
Mean Flow ◽  
Upper Troposphere ◽  
The Past ◽  
Synoptic Eddies

<p><strong>There is a growing concern that human-induced climate change has been affecting weather systems. However, robust observational evidences that confirm the links between global warming and synoptic phenomena at the global scale are lacking. Here we reveal robust covarying signals between poleward temperature gradient and baroclinic life cycle of synoptic (1-10 days) eddies under global warming. We note that the changes in temperature structure in Northern Hemisphere winter and summer in the past decades are different. In boreal winter, the tropospheric warming has been larger in tropical upper troposphere and around 30°N than for the midlatitude (30-60°N). This inhomogeneous warming resulted in the enhancement of poleward temperature gradient in the subtropical upper troposphere and in the lower midlatitude (30-45°N). We observed correlated increasing trends in the entire baroclinic life cycle of synoptic eddies</strong><strong> — </strong><strong>including eddy fluxes of heat and momentum, and zonal mean jet</strong><strong> — </strong><strong>associated with steepened poleward temperature gradients in these regions in the winter Northern Hemisphere over the past four decades. By contrast, in the summer Northern Hemisphere, the overall tropospheric warming over the mid- to high-latitude land areas has been accompanied by weakly reduced synoptic eddy activities and zonal mean flow. Our findings suggest that if greenhouse gas–induced warming continue to change the atmospheric thermal structure as projected in a warming climate, extratropical synoptic disturbances and large-scale circulations may change accordingly. </strong></p>

scholarly journals The Thermally Driven Ocean Circulation with Realistic Bathymetry

2018 ◽  
Vol 48 (3) ◽  
pp. 647-665 ◽  
Author(s):  
Ada Gjermundsen ◽  
Joseph H. LaCasce ◽  
Liv Denstad
Keyword(s):  
Temperature Gradient ◽  
Surface Temperature ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Large Scale ◽  
Western Boundary ◽  
The North ◽  
Surface Gradient ◽  
Thermally Driven ◽  
Surface Temperature Gradient

AbstractThe global circulation driven solely by relaxation to an idealized surface temperature profile and to interior mixing is examined. Forcing by winds and evaporation/precipitation is excluded. The resulting circulation resembles the observed in many ways, and the overturning is of similar magnitude. The overturning is driven by large-scale upwelling in the interior (which is relatively large, because of the use of a constant mixing coefficient). The compensating downwelling occurs in the northern North Atlantic and in the Ross and Weddell Seas, with an additional, smaller contribution from the northern North Pacific. The latter is weaker because the Bering Strait limits the northward extent of the flow. The downwelling occurs in frictional layers near the boundaries and depends on the lateral shear in the horizontal flow. The shear, in turn, is linked to the imposed surface temperature gradient via thermal wind, and as such, the downwelling can be reduced or eliminated in selected regions by removing the surface gradient. Doing so in the northern North Atlantic causes the (thermally driven) Antarctic Circumpolar Current to intensify, increasing the sinking along Antarctica. Eliminating the surface gradient in the Southern Ocean increases the sinking in the North Atlantic and Pacific. As there is upwelling also in the western boundary currents, the flow must increase even more to accomplish the necessary downwelling. The implications of the results are then considered, particularly with respect to Arctic intensification of global warming, which will reduce the surface temperature gradient.

Global Warming and the Weakening of the Tropical Circulation

2007 ◽  
Vol 20 (17) ◽  
pp. 4316-4340 ◽  
Author(s):  
Gabriel A. Vecchi ◽  
Brian J. Soden
Keyword(s):  
Global Warming ◽  
Indian Ocean ◽  
Ocean Circulation ◽  
El Niño ◽  
Climate Model ◽  
El Nino ◽  
Thermal Structure ◽  
Ocean Dynamics ◽  
Tropical Circulation ◽  
Indian Ocean Dipole Mode

Abstract This study examines the response of the tropical atmospheric and oceanic circulation to increasing greenhouse gases using a coordinated set of twenty-first-century climate model experiments performed for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The strength of the atmospheric overturning circulation decreases as the climate warms in all IPCC AR4 models, in a manner consistent with the thermodynamic scaling arguments of Held and Soden. The weakening occurs preferentially in the zonally asymmetric (i.e., Walker) rather than zonal-mean (i.e., Hadley) component of the tropical circulation and is shown to induce substantial changes to the thermal structure and circulation of the tropical oceans. Evidence suggests that the overall circulation weakens by decreasing the frequency of strong updrafts and increasing the frequency of weak updrafts, although the robustness of this behavior across all models cannot be confirmed because of the lack of data. As the climate warms, changes in both the atmospheric and ocean circulation over the tropical Pacific Ocean resemble “El Niño–like” conditions; however, the mechanisms are shown to be distinct from those of El Niño and are reproduced in both mixed layer and full ocean dynamics coupled climate models. The character of the Indian Ocean response to global warming resembles that of Indian Ocean dipole mode events. The consensus of model results presented here is also consistent with recently detected changes in sea level pressure since the mid–nineteenth century.

scholarly journals 100 Years of Progress in Ocean Observing Systems

2019 ◽  
Vol 59 ◽  
pp. 3.1-3.46 ◽  
Author(s):  
Russ E. Davis ◽  
Lynne D. Talley ◽  
Dean Roemmich ◽  
W. Brechner Owens ◽  
Daniel L. Rudnick ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
Large Scale ◽  
Southern Oscillation ◽  
World Ocean ◽  
Ocean Modeling ◽  
Ocean Dynamics ◽  
Global Ocean ◽  
Tropical Ocean ◽  
New Instrument ◽  
Observing Systems

Abstract The history of over 100 years of observing the ocean is reviewed. The evolution of particular classes of ocean measurements (e.g., shipboard hydrography, moorings, and drifting floats) are summarized along with some of the discoveries and dynamical understanding they made possible. By the 1970s, isolated and “expedition” observational approaches were evolving into experimental campaigns that covered large ocean areas and addressed multiscale phenomena using diverse instrumental suites and associated modeling and analysis teams. The Mid-Ocean Dynamics Experiment (MODE) addressed mesoscale “eddies” and their interaction with larger-scale currents using new ocean modeling and experiment design techniques and a suite of developing observational methods. Following MODE, new instrument networks were established to study processes that dominated ocean behavior in different regions. The Tropical Ocean Global Atmosphere program gathered multiyear time series in the tropical Pacific to understand, and eventually predict, evolution of coupled ocean–atmosphere phenomena like El Niño–Southern Oscillation (ENSO). The World Ocean Circulation Experiment (WOCE) sought to quantify ocean transport throughout the global ocean using temperature, salinity, and other tracer measurements along with fewer direct velocity measurements with floats and moorings. Western and eastern boundary currents attracted comprehensive measurements, and various coastal regions, each with its unique scientific and societally important phenomena, became home to regional observing systems. Today, the trend toward networked observing arrays of many instrument types continues to be a productive way to understand and predict large-scale ocean phenomena.

scholarly journals Observations of Seasonal Upwelling and Downwelling in the Beaufort Sea Mediated by Sea Ice

2018 ◽  
Vol 48 (4) ◽  
pp. 795-805 ◽  
Author(s):  
Gianluca Meneghello ◽  
John Marshall ◽  
Mary-Louise Timmermans ◽  
Jeffery Scott
Keyword(s):  
Ocean Circulation ◽  
Large Scale ◽  
Chukchi Sea ◽  
Ocean Dynamics ◽  
Wind Forcing ◽  
Ekman Pumping ◽  
Beaufort Gyre ◽  
Surface Ice ◽  
Yearly Means ◽  
Upwelling And Downwelling

AbstractWe present observational estimates of Ekman pumping in the Beaufort Gyre region. Averaged over the Canada Basin, the results show a 2003–14 average of 2.3 m yr−1 downward with strong seasonal and interannual variability superimposed: monthly and yearly means range from 30 m yr−1 downward to 10 m yr−1 upward. A clear, seasonal cycle is evident with intense downwelling in autumn and upwelling during the winter months, despite the wind forcing being downwelling favorable year-round. Wintertime upwelling is associated with friction between the large-scale Beaufort Gyre ocean circulation and the surface ice pack and contrasts with previous estimates of yearlong downwelling; as a consequence, the yearly cumulative Ekman pumping over the gyre is significantly reduced. The spatial distribution of Ekman pumping is also modified, with the Beaufort Gyre region showing alternating, moderate upwelling and downwelling, while a more intense, yearlong downwelling averaging 18 m yr−1 is identified in the northern Chukchi Sea region. Implications of the results for understanding Arctic Ocean dynamics and change are discussed.

A first glimpse from the EUREC4A-OA/ATOMIC air-sea experiment

2020 ◽  
Author(s):  
Sabrina Speich ◽  
Johannes Karstensen ◽  
Chris Fairall ◽  
Paquita Zuidema ◽  
Chelle Gentemann ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
Large Scale ◽  
Field Experiments ◽  
Mesoscale Eddy ◽  
Lessons Learned ◽  
Ocean Dynamics ◽  
Trade Wind ◽  
Tropical Region ◽  
Interaction Patterns ◽  
North Brazil

<p>Ocean mesoscale eddies create specific air-sea interaction patterns that can have an integral effect on the large scale atmosphere and ocean dynamics. Recent advances in the state of the art of these processes has been predominately obtained from modeling efforts, but only very few observational studies exist, and there are all located in the extra-tropics. Adding a dedicated ocean mesoscale eddy air-sea interaction experiment to the EUREC4A campaign will enhance the objectives and success of the whole program as it sets a local, oceanic constrain to the atmospheric evolution (as been outlined in the overall EUREC4A design: www.eurec4a.eu; Bony et al. 2017). Temporal variability over a fixed mesoscale (500 km x 500 km) study area allows for sampling varying atmospheric states, on the time-scales of the EUREC4A field study. The oceanographic component that add on EUREC4A consists of four ships and many autonomous platforms (gliders, Saildrones, specific surface drifters, Argo floats) sampling the mesoscale ocean and air-sea exchanges at different edges of the Northwest Atlantic tropical region. This will enable the extended spatial sampling required to characterize ocean variability and gather enough air-sea observations at different locations to assess with accuracy the involved processes and impacts.</p><p>The western tropical Atlantic is an ideal laboratory for the proposed study. It hosts rich ocean mesoscale variability under an atmosphere characterized by a rather steady trade wind regime. In this region, eddies have a diameter of 200 to 300 km and lifetimes of several months up to years. In particular south of Barbados, very energetic and long-lived anticyclonic North Brazil Current rings are commonly found. These eddies are key for the northward transport of properties from the South to the North Atlantic within the Atlantic Meridional Ocean Circulation. Moreover, preliminary studies based on satellite observations have suggested that they play a crucial role in air-sea interactions and on the atmosphere. This region and eddies are the focus of EUREC4A-OA, the French oceanographic component of the larger EUREC4A field experiments.</p><p>In this talk we will present the preliminary lessons learned and results obtained by this unprecedented observing effort.</p>

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