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L. D. Danny Harvey

Abstract

A two-dimensional (latitude–depth) ocean–climate model is used to assess the impact of calculating diffusive heat and salinity fluxes along and across isopycnal surfaces rather than in a vertical–horizontal coordinate system. Differences between the two model versions are small and are overwhelmed by uncertainties in the appropriate value of the diapycnal/vertical diffusion coefficient Kv. Isopycnal diffusion is important only if convection is not explicitly included and Kv is fixed rather than parameterized as N −1 (where N is the Brunt–Väisälä frequency). When convection is present, the switch to diffusion in isopycnal coordinates causes large changes in the convective heat flux that are largely offset by diffusion along isopycnal surfaces, with little change in the diapycnal heat flux. The effective vertical diffusion coefficient due to combined mixing across and along isopycnal surfaces is negative in some regions due to upward heat diffusion along sloping isopycnal surfaces combined with temperature decreasing downward. As Kv is varied, the model exhibits a range of qualitatively different behavior in response to heating due to a greenhouse gas increase. The qualitative behavior is unaffected by the use of isopycnal rather than vertical–horizontal diffusion. In some sensitivity tests the isopycnal diffusion coefficient is parameterized such that KiN 3/2/f. Since this causes Ki to decrease as isopycnal slope increases, its use further reduces the differences between isopycnal and nonisopycnal model versions. Differences in the transient surface temperature response of the two model versions to external forcing changes are small to negligible.

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L. D. Danny Harvey

Abstract

A sea ice model for use in zonally averaged energy balance climate models is presented which includes the following processes: surface melting, basal freezing and melting, lateral melting from ice-flee water or growth of new ice in leads, snowfall and the formation of white ice, ice advection, and a parameterized ice and snow thickness distribution which represents the effects of small-scale dynamics. The ice growth equations of Hibler are solved analytically, thereby permitting a gradual increase in zonal ice fraction in fall and winter. Both lateral and vertical melting lead to a continuous decrease of ice fraction during ice decay.

The correlation between ice thickness and ice thickness sensitivity to the upward heat flux at the ice base is of opposite sign seasonally and latitudinally. The parameterized feedback between ice thickness and the minimum permitted lead fraction is found to be very important to the ice simulation, and is a process which needs to be studied using higher resolution, dynamic-thermodynamic sea ice models. The interaction between lateral melting and advection is crucial to the simulated rapid retreat of Southern Hemisphere ice area in spring. With uniform snow on ice, the introduction of an ice-thickness distribution increases mean annual ice thickness by up to 20%, but simultaneously introducing an ice and snow thickness distribution such that the ratio of snow to ice thickness is constant for each ice thickness category leads to increase of mean ice thickness of up to 90%. The effect on mean annual sea ice thickness of the parameterized surface albedo temperature dependence tends to increase with increasing latitude, even though the length of the melt season and incident solar radiation decrease with latitude. Model sensitivity to variation of time-step length from 1 to 6 days is insignificant.

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L. D. Danny Harvey

Abstract

An energy balance climate model with sea ice and seasonal land snow cover is used to investigate the climatic response to Milankovitch orbital variations. Model response to a change of obliquity is in phase in the Northern and Southern hemispheres and governed by summer insulation changes, whereas model response to a change of longitude of perihelion is 125° out of phase in the two hemispheres and more closely related to spring insulation. Temperature sensitivity is greater in the Northern Hemisphere (NH) for both orbital changes and temperature response in both hemispheres is 3–5 times greater for a change of obliquity from 22° to 25° than for a change of perihelion from 270° to 90° with an eccentricity of 0.04, even at low latitudes.

High latitude tundra fraction is parameterized in terms of July land air temperature and low latitude forest and grassland fraction are parameterized in terms of summer land-sea temperature difference. Both vegetation feedbacks make a significant contribution to the local model response when orbital parameters are changed from those of 125 kyr BP to 114 kyr BP. The rate of formation of North Atlantic Deep Water (NADW) is parameterized in terms of summer to winter cooling of the ice-free mixed layer at 60°N and is assumed to be associated with the upward heat flux Fb to the base of the mixed lava at high latitudes in the Southern Hemisphere (SH). The feedback between NH inflation and SH Fb, gives a high latitude SH temperature response 50% or more larger than the high latitude NH temperature response, and could explain the tendency for SH temperature changes to lead NH temperature changes as peak NH temperature response would occur later, after the buildup of ice sheets. Land July temperatures fall by 6°C or more poleward of 60°N for this orbital change, but the adjacent mixed layer remains relatively warm, favoring the development of NH ice sheets. For all the experiments performed here, low latitude warming occurs in response to a change from 125 kyr BP to 114 kyr BP orbital parameters; low latitude cooling seems to be dependent on buildup of NH ice sheets.

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L. D. Danny Harvey

Abstract

The asynchronous coupling schemes used in the seasonal, coupled atmosphere–ocean general circulation models (A/O GCMs) of Manabe et al. 1979 and Washington et al. 1980 are tested in the seasonal, coupled atmosphere–ocean model of Harvey and Schneider. In these schemes ocean temperatures are held fixed during atmospheric integrations and, conversely, atmospheric temperatures are held fixed during ocean integrations. The alternative atmospheric boundary conditions investigated using a simple mean annual model in the first part of this study are also tested. The occurrence of a seasonal cycle poses a number of problems for the accurate simulation of the synchronous transient response using asynchronous coupling methods. Unlike the mean annual model, it is necessary to integrate the atmosphere at 1east one year at a time in order to recalibrate the entire seasonal array of atmospheric variables before each multiyear ocean model integration. However, with such long atmospheric integration times it is no longer reasonable to hold ocean temperatures fixed during atmospheric integrations. This problem leads to the definition of a periodically-synchronous coupling mode, whereby atmosphere–ocean integrations of length τa years alternate with asynchronous ocean integrations of length τ0 years. A periodically-synchronous mode using a second-order Taylor series extrapolated atmosphere-mixed layer temperature difference during asynchronous ocean integrations is tested for a step-function increase of the solar constant, sinusoidal solar constant variations with periods of 100, 200, and 500 years, and a time dependent CO2 increase. This method is found to closely reproduce the synchronously coupled results. For the CO2 increase, using τa = 5 years and τ0 = 20 years gives temperature errors of only a few percent while reducing potential problems associated with stochastic variability during atmosphere GCM integrations.

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L. D. Danny Harvey

Abstract

An energy balance climate model (EBCM) is presented having 1) a seasonal cycle; 2) surface-air, land-sea, and latitudinal resolution; 3) simulation of sea ice in terms of a number of explicit physical processes and in such a way that the sea ice fraction in any given zone changes continuously during the course of the seasonal cycle; 4) simulation of a continuously varying land-snow fraction in terms of explicit physical processes; and 5) a detailed treatment of surface and planetary albedo. A semianalytic solution is used which permits use of 6 day time steps, with very little dependence of the simulated climate on the choice of time step length for time steps of 1 to 6 days.

Model sensitivity to internal parameter changes is investigated. The temperature response to a doubling of the drag coefficients for the vertical fluxes of latent and sensible heat is complex, and involves radiative constraints, the effect of stronger coupling to the large thermal inertia of the mixed layer, and ice and snow feedbacks. The model response to changes in the drag coefficient depends, in part, on the functional form of the parameterization of infrared emission to space, even when this parameter change has no direct or feedback effect on radiation. The effect on meridional heat fluxes of doubling the meridional diffusion coefficients is largely governed by radiative constraints; an important implication for EBCMs is that one cannot use model-simulated meridional heat fluxes to tune the diffusion coefficients or heat flux parameterization.

The most important elements of the surface albedo parameterization for climate, in decreasing order of importance, are 1) the temperature dependence of ice and snow albedo, 2) the effect of partial vegetational masking of land snowcover, and 3) surface albedo zenith angle dependencies, with the latter having a negligible effect on planetary albedo. Removing the temperature dependence of ice and snow albedo leads to a doubling of both sea ice extent and thickness in the Northern Hemisphere, with smaller changes in the Southern Hemisphere. When both direct and diffuse beam surface albedos are altered, changes in planetary albedo are 60%–70% and 20%–25% the changes in surface albedo for clear and cloudy skin respectively. Because observed cloudiness tends to be large at high latitudes, zonally averaged planetary albedo changes are about 30% the size of surface albedo changes resulting from ice and snow feedbacks. The effect on temperature of partial masking of land snowcover by forests is found to be significantly smaller than obtained by others.

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L. D. Danny Harvey and Robert K. Kaufmann

Abstract

An energy balance climate model with latitudinal, surface–air, and land–sea resolution is coupled to a two-dimensional (latitude–depth) ocean model and used to simulate changes in surface and surface air temperature since 1765. The climate model sensitivity can be prescribed by adjusting the parameterization of infrared radiation to space, and sensitivities corresponding to an equilibrium, global average warming to a CO2 doubling (ΔT ) of 1.0° to 5.0°C are used here. The model is driven with various combinations of greenhouse gas (GHG), fossil fuel aerosol, biomass aerosol, solar, and volcanic forcings. The fossil fuel aerosol forcing is concentrated in the NH, while the biomass aerosol forcing is centered near the equator. The variation in the global mean air temperature, and in the NH minus SH temperature, is examined over the period 1856–2000, in order to simultaneously constrain both climate sensitivity and aerosol forcing. The model performance, compared to observations, is evaluated using three statistical measures. It is possible to identify a group of experiments that performs better than other experiments, but it cannot be claimed that any member of the group is better than any other member in a statistically rigorous manner. The different statistical measures and temperature variables (global mean, NH − SH, NH, or SH temperature) give slightly different groups of “more accurate” experiments.

Based on the statistical measures and examination of the time series of model-simulated global mean and NH − SH temperature variation, the following conclusions can be drawn: (i) The most likely ΔT is around 2°C, which is at the lower end of the range of 2.1°–4.8°C obtained by recent general circulation models; (ii) the fossil fuel aerosol forcing is unlikely to have exceeded −1.0 W m−2 in the global mean by 1990; and (iii) the net biomass plus soil dust aerosol forcing is unlikely to have exceeded −0.5 W m−2 in the global mean by 1990. As an independent check of these conclusions, it was found that the simulated change of ocean heat content (over the 0–3000-m depth interval, during the period 1948–98) agrees well with the observed change in ocean heat content for climate sensitivity and aerosol forcing combinations that produce a good simulation of the observed temperature change during this time period, thereby validating the model uptake of heat by the oceans.

Although the preferred ΔT is 2°C in this study, it is possible to choose fossil and biomass aerosol forcing combinations (within the ranges given above) that produce comparable simulations of global mean and NH − SH temperature variation after the 1880s for any ΔT in the range 1.0°–5.0°C. However, and in common with other models, this model simulates much too large a drop in temperature during the 1880s (in response to the eruption of Mount Krakatau in 1883). As ΔT ranges from 1.0° to 5.0°C, the simulated drop ranges from about 0.3° to about 0.7°C, compared to an observed change of about 0.2°C. On this basis, a lower ΔT is preferred.

Inasmuch as the model response to the 1991 eruption of Mount Pinatubo accords well with observations, especially for intermediate and high sensitivities, it may be that the estimated radiative forcing due to the eruption of Krakatau is too large or that there was a short-term negative feedback, dependent on conditions just before this eruption, which reduced the effective radiative forcing. If half the base case forcing is assumed for Krakatau only, the temperature decrease during the 1880s ranges from 0.2°C for ΔT = 1°C (matching observations) to 0.3°C for ΔT = 5°C (modestly in excess of observations). Thus, the volcanic radiative forcing during the 1880s, and the quality of the historical and proxy temperature records around this time, are critical data in discriminating between different climate sensitivities, inasmuch as a smaller volcanic forcing might permit ΔT at the high end of the 1°–5°C range.

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Adam R. Cornwell and L. D. Danny Harvey

Abstract

Atmosphere–ocean general circulation models (AOGCMs) employ very different land surface schemes (LSSs) and, as a result, their predictions of land surface quantities are often difficult to compare. Some of the disagreement in quantities such as soil moisture is likely due to differences in the atmospheric component; however, previous intercomparison studies have determined that different LSSs can produce very different results even when supplied with identical atmospheric forcing.

A simple off-line LSS is presented that can reproduce the soil moisture simulations of various AOGCMs, based on their modeled temperature and precipitation. The scheme makes use of the well-established Thornthwaite method for estimating potential evapotranspiration combined with a variation of the Manabe “bucket” model. The model can be tuned to reproduce the control climate soil moisture of an AOGCM by adjusting the ease with which runoff and evapotranspiration continue as the moisture level in the bucket goes down. This produces a set of parameter values that provides a good fit to each of several AOGCM control climates. In addition, the parameter values can be set to imitate the LSS from one AOGCM while the model is forced with atmospheric data from another, thus providing an estimate of the magnitude of variation caused by the differences in land surface parameterization and by differences in atmospheric forcing. In general, the authors find that differences in LSSs account for about half of the difference in soil moisture as simulated by different AOGCMs, and the differences in atmospheric forcing account for the other half of the difference. However, the LSS can be more important than differences in atmospheric forcing in some regions (such as the United States) and less important in others (such as East Africa).

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