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Frank O. Bryan

Abstract

The National Center for Atmospheric Research’s Climate System Model is a comprehensive model of the physical climate system. A 300-yr integration of the model has been carried out without flux correction. The solution shows very little drift in the surface temperature distribution, sea-ice extent, or atmospheric circulation. The lack of drift in the surface climate is attributed to relatively good agreement in the estimates of meridional heat transport in the uncoupled ocean model and that implied by the uncoupled atmospheric model. On the other hand, there is significant drift in the temperature and salinity distributions of the deep ocean. The ocean loses heat at an area-averaged rate of 0.35 W m−2, the upper ocean becomes fresher, and the deep ocean becomes colder and saltier than in the uncoupled ocean model equilibrium or in observations. The cause of this drift is an unreasonably large meridional transport of freshwater in the sea ice model, resulting in the production of excessively cold and salty Antarctic Bottom Water. There is also significant drift in the Arctic basin, with the complete erosion of the surface halocline early in the coupled integration.

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Scott C. Doney
,
William G. Large
, and
Frank O. Bryan

Abstract

The global distributions of the air–sea fluxes of heat and freshwater and water mass transformation rates from a control integration of the coupled National Center for Atmospheric Research (NCAR) Climate System Model (CSM) are compared with similar fields from an uncoupled ocean model equilibrium spinup and a new surface climatology. The climatology and uncoupled model use the same bulk-flux forcing scheme and are forced with National Centers for Environmental Predicition (formerly the National Meteorological Center) atmospheric reanalysis data and satellite-based cloud cover, solar flux, and precipitation estimates. The climatological fluxes for the open ocean are adjusted to give a global net balance and are in broad general agreement with standard ship-based estimates. An exception is the ice-free Southern Ocean, where the net heat and evaporative fluxes appear to be too weak but where the observational coverage underlying the reanalyis is quite poor. Major differences are observed between the climatology and the NCAR CSM coupled solution, namely, enhanced tropical and subtropic solar insolation, stronger energy and hydrologic cycles, and excessive high-latitude ice formation/melt producing a several-fold increase in Arctic and Antarctic deep water formation through brine rejection. The anomalous fluxes and corresponding water-mass transformations are closely tied to the coupled ocean model drift, characterized by a reorganization of the vertical salinity distribution. Some error features in the heat flux and sea surface temperature fields are common to both the coupled and uncoupled solutions, primarily in the western boundary currents and the Antarctic circumpolar current, and are thus likely due to the poor representation of the circulation field in the coarse-resolution NCAR ocean model. Other problems particular to the uncoupled spinup are related to the bulk-flux forcing scheme, an example being excess freshwater deposition in the western boundary currents arising from the inclusion of a weak open ocean surface salinity restoring term. The effective thermal restoring coefficent, which relates the change in nonsolar surface heat flux to sea surface temperature changes, is on average 14.6 W m−2 K−1 for the coupled solution or about a third of the range from the bulk flux forcing scheme, 40–60 W m−2 K−1.

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William R. Holland
,
Julianna C. Chow
, and
Frank O. Bryan

Abstract

The National Center for Atmospheric Research (NCAR) Ocean Model has been developed for use in NCAR’s Climate System Modeling project, a comprehensive development of a coupled ocean–atmosphere–sea ice–land surface model of the global climate system. As part of this development, new parameterizations of diffusive mixing by unresolved processes have been implemented for the tracer equations in the model. Because the strength of the mixing depends upon the density structure under these parameterizations, it is possible that local explicit mixing may be quite small in selected locations, in contrast to the constant diffusivity model generally used. When a spatially centered advection scheme is used in the standard model configuration, local overshooting of tracer values occurs, leading to unphysical maxima and minima in the fields. While the immediate problem is a local Gibbs’s phenomenon, there is the possibility that such local tracer anomalies might propagate by advection and diffusion far from the source, causing inaccuracies in the tracer fields globally.

Because of these issues, a third-order upwind scheme was implemented for the advection of tracers. Numerical experiments show that this scheme is computationally efficient compared to alternatives (such as the flux-corrected transport scheme) and that it works well with other aspects of the model, such as acceleration (important for spinup efficiency) and the new mixing parameterizations. The scheme mimimizes overshooting effects while keeping the dissipative aspect of the advective operator reasonably small. The net effect is to produce solutions in which the large-scale fields are affected very little while local extrema are nearly (but not completely) removed, leading to physically much more realistic tracer patterns.

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Frank O. Bryan
,
Peter R. Gent
, and
Robert Tomas

Abstract

Present-day control and 1% yr−1 increasing carbon dioxide runs have been made using two versions of the Community Climate System Model, version 3.5. One uses the standard versions of the ocean and sea ice components where the horizontal resolution is 1° and the effects of mesoscale eddies are parameterized, and the second uses a resolution of 1/10° where the eddies are resolved. This is the first time the parameterization has been tested in a climate change run compared to an eddy-resolving run. The comparison is made not straightforward by the fact that the two control run climates are not the same, especially in their sea ice distributions. The focus is on the Antarctic Circumpolar Current region, where the effects of eddies are of leading order. The conclusions are that many of the differences in the two carbon dioxide transient forcing runs can be explained by the different control run sea ice distributions around Antarctica, but there are some quantitative differences in the meridional overturning circulation, poleward heat transport, and zonally averaged heat uptake when the eddies are parameterized rather than resolved.

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Stuart P. Bishop
,
R. Justin Small
,
Frank O. Bryan
, and
Robert A. Tomas

Abstract

It has traditionally been thought that midlatitude sea surface temperature (SST) variability is predominantly driven by variations in air–sea surface heat fluxes (SHFs) associated with synoptic weather variability. Here it is shown that in regions marked by the highest climatological SST gradients and SHF loss to the atmosphere, the variability in SST and SHF at monthly and longer time scales is driven by internal ocean processes, termed here “oceanic weather.” This is shown within the context of an energy balance model of coupled air–sea interaction that includes both stochastic forcing for the atmosphere and ocean. The functional form of the lagged correlation between SST and SHF allows us to discriminate between variability that is driven by atmospheric versus oceanic weather. Observations show that the lagged functional relationship of SST–SHF and SST tendency–SHF correlation is indicative of ocean-driven SST variability in the western boundary currents (WBCs) and the Antarctic Circumpolar Current (ACC). By applying spatial and temporal smoothing, thereby dampening the signature SST anomalies generated by eddy stirring, it is shown that the oceanic influence on SST variability increases with time scale but decreases with increasing spatial scale. The scale at which SST variability in the WBCs and the ACC transitions from ocean to atmosphere driven occurs at scales less than 500 km. This transition scale highlights the need to resolve mesoscale eddies in coupled climate models to adequately simulate the variability of air–sea interaction. Away from strong SST fronts the lagged functional relationships are indicative of the traditional paradigm of atmospherically driven SST variability.

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Emily R. Newsom
,
Cecilia M. Bitz
,
Frank O. Bryan
,
Ryan Abernathey
, and
Peter R. Gent

Abstract

The dynamics of the lower cell of the meridional overturning circulation (MOC) in the Southern Ocean are compared in two versions of a global climate model: one with high-resolution (0.1°) ocean and sea ice and the other a lower-resolution (1.0°) counterpart. In the high-resolution version, the lower cell circulation is stronger and extends farther northward into the abyssal ocean. Using the water-mass-transformation framework, it is shown that the differences in the lower cell circulation between resolutions are explained by greater rates of surface water-mass transformation within the higher-resolution Antarctic sea ice pack and by differences in diapycnal-mixing-induced transformation in the abyssal ocean.

While both surface and interior transformation processes work in tandem to sustain the lower cell in the control climate, the circulation is far more sensitive to changes in surface transformation in response to atmospheric warming from raising carbon dioxide levels. The substantial reduction in overturning is primarily attributed to reduced surface heat loss. At high resolution, the circulation slows more dramatically, with an anomaly that reaches deeper into the abyssal ocean and alters the distribution of Southern Ocean warming. The resolution dependence of associated heat uptake is particularly pronounced in the abyssal ocean (below 4000 m), where the higher-resolution version of the model warms 4.5 times more than its lower-resolution counterpart.

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Saulo M. Soares
,
Kelvin J. Richards
,
Frank O. Bryan
, and
Kunio Yoneyama

Abstract

Scale interactions in the coupled ocean and atmosphere of the tropics play a crucial role in shaping the climate state and its spatial and temporal variability. The mechanisms driving the seasonal cycles of mixed layer (ML) temperature and salinity in the tropical south Indian Ocean (TSIO) are revisited and quantified using model and observations to form a basis on which to assess the cycle’s impact on shorter and longer time scale variability in the region. Budgets of ML heat for the western, central, and eastern TSIO in both model and observations indicate that seasonality in ML temperature is driven by surface heat fluxes in all regions; ocean processes, however, are essential to explain east–west differences in the cycle. In contrast, the salt budgets show that ML salinity in the west and central regions of the TSIO is driven by horizontal advection, with salinity increasing during austral winter mainly due to meridional advection, and freshening during spring–summer due to zonal advection; in the east, no single mechanism appears to dominate ML salinity seasonality. The ML seasonal cycle across the entire region is very much influenced by the basin-scale adjustment that occurs in response to monsoon winds in the eastern side of the basin. Zonal advection, as part of the adjustment process, is the key mechanism responsible for bringing fresher/colder waters from the east to the central and western TSIO during austral spring, leading to a lag in the coldest ML temperatures in the east relative to the west/central TSIO, and effectively coupling the eastern and western TSIO beyond simply Rossby wave dynamics.

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Claus W. Böning
,
William R. Holland
,
Frank O. Bryan
,
Gokhan Danabasoglu
, and
James C. Mcwilliams

Abstract

Many models of the large-scale thermohaline circulation in the ocean exhibit strong zonally integrated upwelling in the midlatitude North Atlantic that significantly decreases the amount of deep water that is carried from the formation regions in the subpolar North Atlantic toward low latitudes and across the equator. In an analysis of results from the Community Modeling Effort using a suite of models with different horizontal resolution, wind and thermohaline forcing, and mixing parameters, it is shown that the upwelling is always concentrated in the western boundary layer between roughly 30° and 40°N. The vertical transport across 1000 m appears to be controlled by local dynamics and strongly depends on the horizontal resolution and mixing parameters of the model. It is suggested that in models with a realistic deep-water formation rate in the subpolar North Atlantic, the excessive upwelling can be considered as the prime reason for the typically too low meridional overturning rates and northward heat transports in the subtropical North Atlantic. A new isopycnal advection and mixing parameterization of tracer transports by mesoscale eddies yield substantial improvements in these integral measures of the circulation.

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R. Justin Small
,
Frank O. Bryan
,
Stuart P. Bishop
,
Sarah Larson
, and
Robert A. Tomas

Abstract

A key question in climate modeling is to what extent sea surface temperature and upper-ocean heat content are driven passively by air–sea heat fluxes, as opposed to forcing by ocean dynamics. This paper investigates the question using a climate model at different resolutions, and observations, for monthly variability. At the grid scale in a high-resolution climate model with resolved mesoscale ocean eddies, ocean dynamics (i.e., ocean heat flux convergence) dominates upper 50 m heat content variability over most of the globe. For deeper depths of integration to 400 m, the heat content variability at the grid scale is almost totally controlled by ocean heat flux convergence. However, a strong dependence on spatial scale is found—for the upper 50 m of ocean, after smoothing the data to around 7°, air–sea heat fluxes, augmented by Ekman heat transports, dominate. For deeper depths of integration to 400 m, the transition scale becomes larger and is above 10° in western boundary currents. Comparison of climate model results with observations show that the small-scale influence of ocean intrinsic variability is well captured by the high-resolution model but is missing from a comparable model with parameterized ocean-eddy effects. In the deep tropics, ocean dynamics dominates in all cases and all scales. In the subtropical gyres at large scales, air–sea heat fluxes play the biggest role. In the midlatitudes, at large scales >10°, atmosphere-driven air–sea heat fluxes and Ekman heat transport variability are the dominant processes except in the western boundary currents for the 400 m heat content.

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R. Justin Small
,
Frank O. Bryan
,
Stuart P. Bishop
, and
Robert A. Tomas

Abstract

A traditional view is that the ocean outside of the tropics responds passively to atmosphere forcing, which implies that air–sea heat fluxes are mainly driven by atmosphere variability. This paper tests this viewpoint using state-of-the-art air–sea turbulent heat flux observational analyses and a climate model run at different resolutions. It is found that in midlatitude ocean frontal zones the variability of air–sea heat fluxes is not predominantly driven by the atmosphere variations but instead is forced by sea surface temperature (SST) variations arising from intrinsic oceanic variability. Meanwhile in most of the tropics and subtropics wind is the dominant driver of heat flux variability, and atmosphere humidity is mainly important in higher latitudes. The predominance of ocean forcing of heat fluxes found in frontal regions occurs on scales of around 700 km or less. Spatially smoothing the data to larger scales results in the traditional atmosphere-driving case, while filtering to retain only small scales of 5° or less leads to ocean forcing of heat fluxes over most of the globe. All observational analyses examined (1° OAFlux; 0.25° J-OFURO3; 0.25° SeaFlux) show this general behavior. A standard resolution (1°) climate model fails to reproduce the midlatitude, small-scale ocean forcing of heat flux: refining the ocean grid to resolve eddies (0.1°) gives a more realistic representation of ocean forcing but the variability of both SST and of heat flux is too high compared to observational analyses.

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