 |
|
||
|
|
CGD Research Catalog
Dr. Frank Bryan
Abstract: Changes in the ventilation rate of the global ocean during the 20th and 21st centuries, as indicated by changes in the distribution of ideal age, are examined in a series of integrations of the Community Climate System Model version 3. The global mean age changes little in the 20th Century relative to pre-industrial conditions, but increases in the 21st Century, by an amount that is independent of the range of climate forcings considered. The increase is primarily due to a decrease in the ventilation rate of Antarctic Bottom Water (AABW), and to a lesser degree, North Atlantic Deep Water (NADW). Changes in a regional volumetric census of age indicate that the changes in AABW are predominantly for waters that are already older than 100 years, so will likely have a moderate direct feedback on oceanic uptake of CO2 and other tracers. On the other hand, the changes in NADW occur most strongly in waters that are a few decades old, so are more likely to have a feedback on the climate system. While the global mean age increases, the age does not increase everywhere in the ocean. Regions newly exposed to strong atmospheric forcing as sea ice retreats experience an increase in convection and decreasing age. Age also decreases over a large volume of the lower thermocline as the rate of upwelling of old deep water decreases with the weakening of the thermohaline circulation. Support: National Science Foundation and Central Research Institute of Electric Power Industry (Japan). Figure. (High resolution figure.) Zonal mean ensemble average ideal age in the Atlantic Ocean averaged from 1980 to 2000 in CCSM3 20th century (20C) integrations(contours, interval of 25 years above 1 km and 50 years below, dashed contours at 5 and 10 years) and change in ensemble mean age between 1870–1890 and 1980–2000 in 20C (colors). b) Zonal mean ensemble average age from 2080 to 2100 in TCC and age change between 20C years 1980–2000 and twentieth century commitment experiment years 2080–2100. c) Zonal mean ensemble average age in SRES scenario A1B from 2080 to 2100 and age change between 20C years 1980–2000 and A1B years 2080–2100.
Abstract: The response of the North Atlantic thermohaline circulation to idealized climate forcing of 1% per year compound increase in CO2 is examined in three configurations of the Community Climate System Model version 3 that differ in their component model resolutions. The strength of the Atlantic overturning circulation declines at a rate of 22% to 26% of the corresponding control experiment maximum overturning per century in response to the increase in CO2. The mean meridional overturning and its variability on decadal timescales in the control experiments, the rate of decrease in the transient forcing experiments, and the rate of recovery in periods of CO2 stabilization all increase with increasing component model resolution. By examining the changes in ocean surface forcing with increasing CO2 in the framework of the water mass transformation function, we show that the decline in the overturning is driven by decreasing density of the subpolar North Atlantic due to increasing surface heat fluxes. While there is an intensification of the hydrologic cycle in response to increasing CO2, the net effect of changes in surface freshwater fluxes on those density classes that are involved in deep water formation is to increase their density, i.e., changes in surface freshwater fluxes act to maintain a stronger overturning circulation. The differences in the control experiment overturning strength and the response to increasing CO2 are well predicted by the corresponding differences in the water mass transformation rate. Reduction of meridional heat transport and enhancement of meridional salt transport from mid- to high-latitudes with increasing CO2, also act to strengthen the overturning circulation. Analysis of the trends in an ideal age tracer provides a direct measure of changes in ocean ventilation timescale in response to increasing CO2. In the subpolar North Atlantic south of the Greenland-Scotland ridge system, there is a significant increase in subsurface ages as open ocean deep convection is diminished and ventilation switches to a predominance of overflow waters. In mid- and low-latitudes there is a decrease in age within and just below the thermocline in response to a decrease in the upwelling of old deep waters. However, when considering ventilation within isopycnal layers, age increases for layers in and below the thermocline due to the deepening of isopycnals in response to global warming. Support: National Science Foundation and Central Research Institute of Electric Power Industry (Japan). Figure. (High resolution figure.) Response of the North Atlantic maximum overturning streamfunction to transient climate forcing for three resolution versions of CCSM3 a.) T31x3 b.) T42x1 c.) T85x1. Control experiment is shown in black, transient experiments in red and cyan, double CO2 stabilization in green and quadruple CO2 stabilization in blue. Note that the ordinate is shifted, but the range is the same in each plot.
Abstract: The fidelity of numerical simulations of the general circulation of the North Atlantic Ocean in basin- to global-scale models have improved considerably in the last several years. This improvement appears to represent a regime shift in the dynamics of the simulated flow as the horizontal grid spacing decreases to around 10 km. Nevertheless, some significant biases in the simulated circulation and substantial uncertainties about the robustness of these results with respect to parameterization choices remain. A growing collection of simulations obtained with the POP primitive equation model allow us to investigate the convergence properties and sensitivity of high resolution numerical simulations of the North Atlantic, with particular attention given to Gulf Stream Separation and the subsequent path of the North Atlantic Current into the Northwest Corner. Increases in resolution and reductions in dissipation both contribute to the improvements in the circulation seen in recent studies. We find that our highest resolution eddy-resolving simulations retain an appreciable sensitivity to the closure scheme. Our most realistic simulations of the Gulf Stream are not obtained at the lowest levels of dissipation, while the simulation of the North Atlantic Current continues to improve as dissipation is reduced to near the numerical stability limit. In consequence, there is a limited range of parameter space where both aspects of the simulated circulation can be brought into agreement with observations. This experience gained with the comparatively affordable regional North Atlantic model is now being used to configure the next generation of ocean climate models. Support: National Science Foundation. Figure. (High resolution figure.) Velocity and potential temperature at 730m during 1998-2000, for two experiments with a 0.1? North Atlantic basin model. a) control. and b) experiment with ¼ of the viscosity as the control. Grey arrows show observed mean velocity near this depth level as reported by Schott et al. (2004).
Abstract: Eleven coupled climate-carbon cycle models used a common protocol to study the coupling between climate change and the carbon cycle. The models were forced by historical emissions and the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A2 anthropogenic emissions of CO2 for the 1850-2100 time period. For each model, two simulations were performed in order to isolate the impact of climate change on the land and ocean carbon cycle, and therefore the climate feedback on the atmospheric CO2 concentration growth rate. There was unanimous agreement among the models that future climate change will reduce the efficiency of the earth system to absorb the anthropogenic carbon perturbation. A larger fraction of anthropogenic CO2 will stay airborne if climate change is accounted for. By the end of the twenty-first century, this additional CO2 varied between 20 and 200 ppm for the two extreme models, the majority of the models lying between 50 and 100 ppm. The higher CO2 levels led to an additional climate warming ranging between 0.1° and 1.5°C. All models simulated a negative sensitivity for both the land and the ocean carbon cycle to future climate. However, there was still a large uncertainty on the magnitude of these sensitivities. Eight models attributed most of the changes to the land, while three attributed it to the ocean. Also, a majority of the models located the reduction of land carbon uptake in the Tropics. However, the attribution of the land sensitivity to changes in net primary productivity versus changes in respiration is still subject to debate; no consensus emerged among the models. Support: National Science Foundation. Figure. (High resolution figure.) (a) Atmospheric CO2 for the coupled simulations (ppm) as simulated by the HadCM3LC (solid black), IPSL-CM2C (solid red), IPSL-CM4-LOOP (solid yellow), CSM-1 (solid green), MPI (solid dark blue), LLNL (solid light blue), FRCGC (solid purple), UMD (dash black), UVic-2.7 (dash red), CLIMBER (dash green), and BERN-CC (dash blue). (b) Atmospheric CO2 difference between the coupled and uncoupled simulations (ppm). (c) Land carbon fluxes for the coupled runs (GtC/yr). (d) Differences between coupled and uncoupled land carbon fluxes (GtC/yr). (e), (f) Same as (c), (d), respectively, for the ocean carbon fluxes.
Abstract: Computational simulation is now an essential methodology of science, along with theory and observation. The ability of scientists to understand and predict planetary climate variability largely depends on the veracity of the climate simulations produced by numerical models of the interacting components of the Earth system. Oceanic and atmospheric models are numerical approximations to continuous forms of the equations governing fluid flow and are “closed" by sub-gridscale parameterizations that represent physical processes on time and space scales that are not resolved by the chosen model grid. In the past two decades the rate at which the world’s fastest computers perform floating point operations (flops) has increased by a factor of 10,000. This increase in computing capability has been exploited in several ways. Longer integrations for applications such as paleoclimate (Dijkstra and Ghil, 2005), and the inclusion of additional processes such as biogeochemical cycles and ecosystem dynamics (Moore et al, 2004) are two such examples. Another example, and the focus of the present study, is to increase the spatial resolution such that a greater fraction of the physical processes are explicitly resolved, and fewer are parameterized. Support: National Science Foundation and Central Research Institute of Electric Power Industry (Japan). Figure. (High resolution figure.) The ratio of mesoscale to total root-mean-square (RMS) sea surface height anomaly (SSHA) from (a) the AVISO blended altimetry (TOPEX/POSEIDON and ERS 1 and 2) product and the (b) global 0.1° POP model for 1997-2001. The mesoscale variability is obtained by band-pass filtering the SSHA between 20 and 150 days. The agreement is generally good except along the pathway of the Agulhas eddies in the South Atlantic where the variability is overestimated relative to the observed results. This bias may be due the representation of bottom topography in the model.
Abstract: In this study we examine the behavior of the thermohaline circulation, as simulated by the Community Climate System Model version 3 (CCSM3), for several centuries following CO2 stabilization for the SRES B1 and A1B scenarios and for an “overshoot" scenario in which CO2 levels temporarily reach the same level as in the A1B scenario before declining to an ultimate stabilization level that is identical to the B1 case. While we find no evidence for irreversible changes of the thermohaline circulation in the overshoot experiment, the interplay of the different timescales of the temperature response of the surface and interior ocean does lead to a number of differences in the long term response of the ocean between it and the B1 stabilization scenario where the same GHG levels are approached by different paths. The stronger initial warming and its slow penetration into the deeper ocean, followed by a transient surface cooling in the overshoot scenario leads to lower static stability, deeper mixing, and a more rapid recovery of the thermohaline circulation than in the B1 stabilization scenario. While the overshoot scenario recovers surface conditions (e.g. SST, sea ice extent) very similar to the B1 scenario shortly after reaching the same GHG levels, the additional accumulation of heat in the interior ocean during the period of higher forcing causes the global mean ocean temperature to remain higher than in the B1 stabilization scenario for at least another several centuries. Support: National Science Foundation and Central Research Institute of Electric Power Industry (Japan). Figure. (High resolution figure.) Temporal variation of maximum North Atlantic meridional overturning (Sv) for SRES B1 and A1B scenarios, their stabilization extensions, and an overshoot scenario in which CO2 is decreased from A1B levels to B1 levels over the period 2150 to 2250; Transport is calculated in 30°N-50°N below 500m depth. The timeseries for the control is smoothed with a 101 year running mean (the shaded area is the standard deviation within the 101 year window). The timeseries for each ensemble member are smoothed with a 51 year running mean.
Abstract: The spatial and temporal properties of zonally coherent jet-like structures found in high resolution ocean models is examined. We focus on the Pacific Ocean. We find the properties of the jets are not very sensitive to the model configuration. Distinct differences are found in the persistence and vertical structure of the jets poleward of 30 N and S compared with those in the tropics. We make a quantitative comparison between the meridional scale of the jets and the Rhines scale. We find a local scaling applies in that the horizontal variations of the meridional scale of the jets is consistent with horizontal variations in the Rhines scale. Support: National Science Foundation and Central Research Institute of Electric Power Industry (Japan). Figure. (High resolution figure.) Zonal component of velocity at 380m depth averaged over 3 years from an integration of the 0.1? POP ocean model illustrating the emergence of zonally banded structures in the time mean flow. Color saturates at -0.06ms-1 (blue) and 0.06ms-1 (red).
Abstract: Analysis of the output of high resolution ocean models reveals the presence of a class of flow structures that is intermediate in scale between the large scale circulation and the geostrophic eddy field. These structures are coherent over considerably long zonal distances and have a relatively small meridional scale of 300–500 kms. They take the form of either quasi–persistent multiple jets, or long crested Rossby waves with high meridional and low zonal wavenumbers. Here we discuss the properties of these features in the Pacific Ocean, possible mechanisms for their formation, and the potential impact on the transport of tracers. Support: National Science Foundation and Central Research Institute of Electric Power Industry (Japan). Figure. (High resolution figure.) Zonal component of velocity at 380m depth averaged over 3 years from an integration of the 0.1? POP ocean model illustrating the emergence of zonally banded structures in the time mean flow. Color saturates at -0.06ms-1 (blue) and 0.06ms-1 (red).
|
|
Bryan,
F. O., G. Danabasoglu, P. R. Gent, and K. Lindsay, 2006: Ocean ventilation
changes during the 21st century in CCSM3. Ocean Modelling, in press.
Bryan, F. O., G. Danabasoglu,
N. Nakshiki, Y. Yoshida, D. -H. Kim, J. Tsutsui, and S. C. Doney, 2006:
Response of the North Atlantic thermohaline circulation and ventilation
to increasing CO2 in CCSM3. Journal of Climate, 19, 2382-2397.
Bryan, F. O., M. W.
Hecht, and R. D. Smith, 2006: Resolution convergence and sensitivity studies
with North Atlantic circulation models. Part I: The western boundary current
system. Ocean Modelling, in press.
Friedlingstein, P.,
P. Cox, R. Betts, L. Bopp, W. von Bloh, V. Brovkin, P. Cadule, S. Doney,
M. Eby, I. Fung, G. Bala, J. John, C. Jones, F. Joos, T. Kato, M. Kawamiya,
W. Knorr, K. Lindsay, H. D. Matthews, T. Raddatz, P. Rayner, C. Reick,
E. Roeckner, K.-G. Schnitzler, R. Schnur, K. Strassmann, A. J. Weaver,
C. Yoshikawa, N. Zeng, 2006: Climate-carbon cycle feedback analysis: Results
from the C4MIP model intercomparison. Journal of Climate, 19, 3337-3353.
McClean, J. L., M. E.
Maltrud, and F.O. Bryan, 2006: Quantitative measures of the fidelity of
eddy-resolving ocean models. Oceanography, 19, 60-73.
Nakashiki, N., D. -H.
Kim, F. O. Bryan, Y. Yoshida, D. Tsumune, K. Maruyama, and H. Kitabata,
2006: Recovery of the thermohaline circulation and sea ice area under
CO2 stabilization and overshoot scenarios. Ocean Modelling, in press.
Richards, K. J., N.
A. Maximenko, F. O. Bryan, and H. Sasaki (2006), Zonal jets in the Pacific
Ocean, Geophysical Research Letters, 33, L03605, doi:10.1029/2005GL024645.
Richards, K. J., H.
Sasaki, and F. O. Bryan, 2006: Jets and waves in the Pacific Ocean. In:
High Resolution Numerical Modelling of the Atmosphere and Ocean. W. Ohfuchi
and K. Hamilton (Eds.) Springer, New York, in press.
