CGD's Dr. Frank Bryan

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.

Figure caption: 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).

Support: NSF, Central Research Institute of Electric Power Industry (Japan), MEXT/JAMSTEC/Earth Simulator Center (computing).

 

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Bryan, F.O., N. Nakashiki, Y. Yoshida, and K. Maruyama (2007) Response of the thermohaline circulation during different pathways toward greenhouse gas stabilization. In: Ocean Circulation: Mechanisms and Impacts, Geophysical Monograph Series, Volume 173, 351-364. A. Schmittner, J. Chiang and S. Hemming (Eds). AGU, Washington, D.C.

Abstract

A series of experiments conducted with the Community Climate System Model version 3 is analyzed to investigate the response of the meridional overturning circulation in the Atlantic Ocean following greenhouse gas stabilization in the 22nd through mid-25th centuries. In addition, several "overshoot" experiments that approach the same stabilization level by temporarily exceeding their ultimate greenhouse gas concentrations are considered. In all cases, the index of maximum North Atlantic overturning declines during the 21st century then begins to recover once greenhouse gas stabilization is achieved. However, the vertical structure of the overturning continues to evolve, becoming shallower through the end of the integrations in the mid-25th century. In the overshoot experiments, the overturning circulation recovers a strength slightly higher than, and vertical structure similar to, that in an experiment where the same stabilization level is approached monotonically. The relationship between changes in the overturning strength and changes in the pressure distribution over the upper 1000m are explored.

Figure caption: Maximum of meridional overturning streamfunction in the North Atlantic Ocean below 500m for the 20th century and the SRES B1, A1B, and A2 scenarios, with stabilzation extensions beyond 2100. The A1B-B1, and A2-B1 curves refer to overshoot scenarios in which the ultimate stabilization level is reached after temporarily exceeding it. Each curve is subject to a 21-year running mean filter. All eight ensemble members for the 20th century are shown. Only those ensemble members integrated to 2450 are shown for years beyond 2000. Vertical dotted lines indicate transition points between transient and stabilization phases of the various scenario.

Support: NSF, Central Research Institute of Electric Power Industry (Japan), MEXT/JAMSTEC/Earth Simulator Center (computing).

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Weese, S.R. and F.O. Bryan, 2006: Climate impacts of systematic errors in the simulation of the path of the North Atlantic Current. Geophs. Res. Lett. 33, L19708, doi:10.1029/2006GL027669.

Abstract

Experiments employing an adjustment of the pressure field in the ocean component of a coupled climate system model are undertaken in both ocean-only and coupled experiments to assess the climatic impacts of reducing the systematic errors in the North Atlantic Current. This conservative and adiabatic process substantially decreases North Atlantic Ocean SST biases and locally reverses the associated surface heat flux balance in both model configurations. Ice concentrations in the Labrador Sea increase as the oceanic surface heat fluxes are displaced by the adjustment. Downstream, in the Nordic Seas, the subsurface ocean responds favorably to this adjustment, as the vertical profiles of potential temperature and salinity converge towards the observations. Atmospheric stationary wave patterns show a modest improvement, with a slight weakening of the excessively deep Icelandic low. Further unresolved errors in the coupled model framework potentially contribute to the continued presence of biases in the North Atlantic.

Figure caption: Annual mean surface heat flux (black contours interval of 40 W/m², positive values into the ocean, dashed) superimposed on annual mean SST error (°C) in color for (a) OGCM control experiment and (b) semi-prognostic OGCM experiment.

Support: NSF, Central Research Institute of Electric Power Industry (Japan).

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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, 16(3-4), 141-159.

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.

Figure caption: 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 1/4 of the viscosity as the control. Grey arrows show observed mean velocity near this depth level as reported by Schott et al. (2004).

Support: NSF, DOE (computing).

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Nakashiki, N., D.-H. Kim, F.O. Bryan, Y. Yoshida, D. Tsumune, K. Maruyama, H. Kitabata 2006: Recovery of the thermohaline circulation and sea ice area under CO₂ stabilization and overshoot scenarios. Ocean Modelling, 15, 200-217.

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.

Figure caption: 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.

Support: NSF, Central Research Institute of Electric Power Industry (Japan), MEXT/JAMSTEC/Earth Simulator Center (computing).

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Bryan, F.O., G. Danabasoglu, P.R. Gent, K. Lindsay, 2006: Ocean ventilation changes during the 21st century in CCSM3. Ocean Modelling, 15, 141-156.

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.

Figure caption: 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.

Support: NSF, Central Research Institute of Electric Power Industry (Japan), MEXT/JAMSTEC/Earth Simulator Center (computing).