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Kevin E. Trenberth, William G. Large, and Jerry G. Olson

Abstract

Computations of the surface wind stress and pseudostress over the global oceans have been made using surface winds from the European Centre for Medium Range Weather Forecasts for 7 years. The drag coefficient is a function of wind speed and atmospheric stability, and the air density is computed for each observation. Assuming a constant density, the effective drag coefficient required to convert the pseudostress into a stress has been computed for each month of the year using several methods. Because the drag coefficient varies from day-to-day and with the seasons, the effective drag coefficient cannot be uniquely defined and is a useful concept if only the very gross characteristics of the field are of interest and errors of the order of 10% are tolerable. Even then, the spatial and seasonal variations in CD must be taken into amount, and occasionally the wind stress may be greatly in error.

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Gokhan Danabasoglu, James C. McWilliams, and William G. Large

Abstract

The approach to equilibrium of a coarse-resolution, seasonally forced, global oceanic general circulation model is investigated, considering the affects of a widely used acceleration technique that distorts the dynamics by using unequal time steps in the governing equations. A measure of the equilibration time for any solution property is defined as the time it takes to go 90% of the way from its present-value to its equilibrium value. This measure becomes approximately time invariant only after sufficiently long integration. It indicates that the total kinetic energy and most mass transport rates attain equilibrium within about 90 and 40 calender years, respectively. The upper-ocean potential temperature and salinity equilibrium times are about 480 and 380 calender years, following 150- and 20-year initial adjustments, respectively. In the abyssal ocean, potential temperature and salinity equilibration take about 4500 and 3900 calender years, respectively. These longer equilibration times are due to the slow diffusion of tracers both along and across the isopycnal surfaces in stably stratified regions, and these times vary with the associated diffusivities. An analysis of synchronous (i.e., not accelerated) integrations shows that there is a complex interplay between convective, advective, and diffusive timescales. Because of the distortion by acceleration of the seasonal cycle, the solutions display some significant adjustments upon switching to synchronous integration. However, the proper seasonal cycle is recovered within five years. Provided that a sufficient equilibrium state has been achieved with acceleration, the model must be integrated synchronously for only about 15 years thereafter to closely approach synchronous equilibrium.

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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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Stephen G. Yeager, Christine A. Shields, William G. Large, and James J. Hack

Abstract

The low-resolution fully coupled configuration of the Community Climate System Model version 3 (CCSM3) is described and evaluated. In this most economical configuration, an ocean at nominal 3° resolution is coupled to an atmosphere model at T31 resolution. There are climate biases associated with the relatively coarse grids, yet the coupled solution remains comparable to higher-resolution CCSM3 results. There are marked improvements in the new solution compared to the low-resolution configuration of CCSM2. In particular, the CCSM3 simulation maintains a robust meridional overturning circulation in the ocean, and it generates more realistic El Niño variability. The improved ocean solution was achieved with no increase in computational cost by redistributing deep ocean and midlatitude resolution into the upper ocean and the key water formation regions of the North Atlantic, respectively. Given its significantly lower resource demands compared to higher resolutions, this configuration shows promise for studies of paleoclimate and other applications requiring long, equilibrated solutions.

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John W. Weatherly, Bruce P. Briegleb, William G. Large, and James A. Maslanik

Abstract

The Climate System Model (CSM) consists of atmosphere, ocean, land, and sea-ice components linked by a flux coupler, which computes fluxes of energy and momentum between components. The sea-ice component consists of a thermodynamic formulation for ice, snow, and leads within the ice pack, and ice dynamics using the cavitating-fluid ice rheology, which allows for the compressive strength of ice but ignores shear viscosity.

The results of a 300-yr climate simulation are presented, with the focus on sea ice and the atmospheric forcing over sea ice in the polar regions. The atmospheric model results are compared to analyses from the European Centre for Medium-Range Weather Forecasts and other observational sources. The sea-ice concentrations and velocities are compared to satellite observational data.

The atmospheric sea level pressure (SLP) in CSM exhibits a high in the central Arctic displaced poleward from the observed Beaufort high. The Southern Hemisphere SLP over sea ice is generally 5 mb lower than observed. Air temperatures over sea ice in both hemispheres exhibit cold biases of 2–4 K. The precipitation-minus-evaporation fields in both hemispheres are greatly improved over those from earlier versions of the atmospheric GCM.

The simulated ice-covered area is close to observations in the Southern Hemisphere but too large in the Northern Hemisphere. The ice concentration fields show that the ice cover is too extensive in the North Pacific and subarctic North Atlantic Oceans. The interannual variability of the ice area is similar to observations in both hemispheres. The ice thickness pattern in the Antarctic is realistic but generally too thin away from the continent. The maximum thickness in the Arctic occurs against the Bering Strait, not against the Canadian Archipelago as observed. The ice velocities are stronger than observed in both hemispheres, with a consistently greater turning angle (to the left) in the Southern Hemisphere. They produce a northward ice transport in the Southern Hemisphere that is 3–4 times the satellite-derived value. Sensitivity tests with the sea-ice component show that both the pattern of wind forcing in CSM and the air-ice drag parameter used contribute to the biases in thickness, drift speeds, and transport. Plans for further development of the ice model to incorporate a viscous-plastic ice rheology are presented.

In spite of the biases of the sea-ice simulation, the 300-yr climate simulation exhibits only a small degree of drift in the surface climate without the use of flux adjustment. This suggests a robust stability in the simulated climate in the presence of significant variability.

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R. Justin Small, Enrique Curchitser, Katherine Hedstrom, Brian Kauffman, and William G. Large

Abstract

Of all the major coastal upwelling systems in the world’s oceans, the Benguela, located off southwest Africa, is the one that climate models find hardest to simulate well. This paper investigates the sensitivity of upwelling processes, and of sea surface temperature (SST), in this region to resolution of the climate model and to the offshore wind structure. The Community Climate System Model (version 4) is used here, together with the Regional Ocean Modeling System. The main result is that a realistic wind stress curl at the eastern boundary, and a high-resolution ocean model, are required to well simulate the Benguela upwelling system. When the wind stress curl is too broad (as with a 1° atmosphere model or coarser), a Sverdrup balance prevails at the eastern boundary, implying southward ocean transport extending as far as 30°S and warm advection. Higher atmosphere resolution, up to 0.5°, does bring the atmospheric jet closer to the coast, but there can be too strong a wind stress curl. The most realistic representation of the upwelling system is found by adjusting the 0.5° atmosphere model wind structure near the coast toward observations, while using an eddy-resolving ocean model. A similar adjustment applied to a 1° ocean model did not show such improvement. Finally, the remote equatorial Atlantic response to restoring SST in a broad region offshore of Benguela is substantial; however, there is not a large response to correcting SST in the narrow coastal upwelling zone alone.

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Peter R. Gent, Frank O. Bryan, Gokhan Danabasoglu, Scott C. Doney, William R. Holland, William G. Large, and James C. McWilliams

Abstract

This paper describes the global ocean component of the NCAR Climate System Model. New parameterizations of the effects of mesoscale eddies and of the upper-ocean boundary layer are included. Numerical improvements include a third-order upwind advection scheme and elimination of the artificial North Pole island in the original MOM 1.1 code. Updated forcing fields are used to drive the ocean-alone solution, which is integrated long enough so that it is in equilibrium. The ocean transports and potential temperature and salinity distributions are compared with observations. The solution sensitivity to the freshwater forcing distribution is highlighted, and the sensitivity to resolution is also briefly discussed.

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Susan C. Bates, Baylor Fox-Kemper, Steven R. Jayne, William G. Large, Samantha Stevenson, and Stephen G. Yeager

Abstract

Air–sea fluxes from the Community Climate System Model version 4 (CCSM4) are compared with the Coordinated Ocean-Ice Reference Experiment (CORE) dataset to assess present-day mean biases, variability errors, and late twentieth-century trend differences. CCSM4 is improved over the previous version, CCSM3, in both air–sea heat and freshwater fluxes in some regions; however, a large increase in net shortwave radiation into the ocean may contribute to an enhanced hydrological cycle. The authors provide a new baseline for assessment of flux variance at annual and interannual frequency bands in future model versions and contribute a new metric for assessing the coupling between the atmospheric and oceanic planetary boundary layer (PBL) schemes of any climate model. Maps of the ratio of CCSM4 variance to CORE reveal that variance on annual time scales has larger error than on interannual time scales and that different processes cause errors in mean, annual, and interannual frequency bands. Air temperature and specific humidity in the CCSM4 atmospheric boundary layer (ABL) follow the sea surface conditions much more closely than is found in CORE. Sensible and latent heat fluxes are less of a negative feedback to sea surface temperature warming in the CCSM4 than in the CORE data with the model’s PBL allowing for more heating of the ocean’s surface.

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Gokhan Danabasoglu, William G. Large, Joseph J. Tribbia, Peter R. Gent, Bruce P. Briegleb, and James C. McWilliams

Abstract

New features that may affect the behavior of the upper ocean in the Community Climate System Model version 3 (CCSM3) are described. In particular, the addition of an idealized diurnal cycle of solar forcing where the daily mean solar radiation received in each daily coupling interval is distributed over 12 daylight hours is evaluated. The motivation for this simple diurnal cycle is to improve the behavior of the upper ocean, relative to the constant forcing over each day of previous CCSM versions. Both 1- and 3-h coupling intervals are also considered as possible alternatives that explicitly resolve the diurnal cycle of solar forcing. The most prominent and robust effects of all these diurnal cycles are found in the tropical oceans, especially in the Pacific. Here, the mean equatorial sea surface temperature (SST) is warmed by as much as 1°C, in better agreement with observations, and the mean boundary layer depth is reduced. Simple rectification of the diurnal cycle explains about half of the shallowing, but less than 0.1°C of the warming. The atmospheric response to prescribed warm SST anomalies of about 1°C displays a very different heat flux signature. The implication, yet to be verified, is that large-scale air–sea coupling is a prime mechanism for amplifying the rectified, daily averaged SST signals seen by the atmosphere. Although the use of upper-layer temperature for SST in CCSM3 underestimates the diurnal cycle of SST, many of the essential characteristics of diurnal cycling within the equatorial ocean are reproduced, including boundary layer depth, currents, and the parameterized vertical heat and momentum fluxes associated with deep-cycle turbulence. The conclusion is that the implementation of an idealized diurnal cycle of solar forcing may make more frequent ocean coupling and its computational complications unnecessary as improvements to the air–sea coupling in CCSM3 continue. A caveat here is that more frequent ocean coupling tends to reduce the long-term cooling trends typical of CCSM3 by heating already too warm ocean depths, but longer integrations are needed to determine robust features. A clear result is that the absence of diurnal solar forcing of the ocean has several undesirable consequences in CCSM3, including too large ENSO variability, much too cold Pacific equatorial SST, and no deep-cycle turbulence.

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Gokhan Danabasoglu, Susan C. Bates, Bruce P. Briegleb, Steven R. Jayne, Markus Jochum, William G. Large, Synte Peacock, and Steve G. Yeager

Abstract

The ocean component of the Community Climate System Model version 4 (CCSM4) is described, and its solutions from the twentieth-century (20C) simulations are documented in comparison with observations and those of CCSM3. The improvements to the ocean model physical processes include new parameterizations to represent previously missing physics and modifications of existing parameterizations to incorporate recent new developments. In comparison with CCSM3, the new solutions show some significant improvements that can be attributed to these model changes. These include a better equatorial current structure, a sharper thermocline, and elimination of the cold bias of the equatorial cold tongue all in the Pacific Ocean; reduced sea surface temperature (SST) and salinity biases along the North Atlantic Current path; and much smaller potential temperature and salinity biases in the near-surface Pacific Ocean. Other improvements include a global-mean SST that is more consistent with the present-day observations due to a different spinup procedure from that used in CCSM3. Despite these improvements, many of the biases present in CCSM3 still exist in CCSM4. A major concern continues to be the substantial heat content loss in the ocean during the preindustrial control simulation from which the 20C cases start. This heat loss largely reflects the top of the atmospheric model heat loss rate in the coupled system, and it essentially determines the abyssal ocean potential temperature biases in the 20C simulations. There is also a deep salty bias in all basins. As a result of this latter bias in the deep North Atlantic, the parameterized overflow waters cannot penetrate much deeper than in CCSM3.

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