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CLIMATE AND THE OCEAN CIRCULATION

II. THE ATMOSPHERIC CIRCULATION AND THE EFFECT OF HEAT TRANSFER BY OCEAN CURRENTS

SYUKURO MANABE

Abstract

A general circulation model of the joint ocean-atmosphere system is constructed by combining an ocean model and an atmospheric model. The quantities exchanged between the oceanic part and the atmospheric part of the joint model are momentum, heat, and water. Integration of the atmospheric part yields the surface wind stress, net radiation, sensible heat flux, rates of rainfall and snowfall, rates of evaporation and sublimation, and rates of runoff and iceberg formation, all of which constitute the upper boundary conditions for the oceanic part of the model. From the oceanic part, the thickness of ice and the distribution of sea-surface temperature, which constitute the lower boundary conditions for the atmospheric part of the model, are computed.

An approach toward a quasi-equilibrium state of the joint ocean-atmosphere system is attempted by numerical time integration of the joint model. Since the thermal relaxation time of the oceanic part of the model is much longer than that of the atmospheric part, a special technique for economizing the computation time is developed. Although a state of quasi-equilibrium is not reached satisfactorily, the time variation of the atmospheric “climate” is extremely slow toward the end of the time integration. A detailed analysis of the final solution at the end of the integration is carried out.

According to this analysis, the distributions of various heat balance components such as radiation flux and the turbulent flux of sensible and latent heat compare favorably with the corresponding distributions in the actual atmosphere estimated by Budyko and London.

By comparing the final state of the joint model atmosphere with the quasi-equilibrium state of the previous atmosphere without an active ocean, it is possible to identify the effect of an ocean circulation on the general circulation of the atmosphere. For example, the poleward transport of heat by an ocean circulation reduces the meridional gradient of atmospheric temperature and vertical wind shear in the troposphere. This reduction of vertical wind shear lowers the level of baroclinic instability and causes a general decrease in the magnitude of eddy kinetic energy in the atmosphere. The air mass modification by the energy exchange between the model ocean and atmosphere creates a favorable place for the development of cyclones off the east coast of the continent in high latitudes.

In the Tropics, the upwelling of relatively cold water at the Equator suppresses the intensity of rainfall in the oceanic region and increases it in the continental region. This increase significantly alters the hydrology of the tropical continent. In middle and subtropical latitudes, the advection of warm water by the subtropical gyre increases the flux of sensible and latent heat from the ocean to the atmosphere along the east coast of the continent and increases the intensity of precipitation in the coastal region. The subtropical desert of the joint model is more or less confined to the western half of the continent. In high latitudes, the advection of warm water by the subarctic gyre off the west coast of the continent increases the energy exchange and precipitation there. Most of these modifications contribute to make the hydrology of the joint model highly realistic despite the idealization of the land-sea configuration.

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CLIMATE AND THE OCEAN CIRCULATION

I. THE ATMOSPHERIC CIRCULATION AND THE HYDROLOGY OF THE EARTH'S SURFACE

SYUKURO MANABE

Abstract

The effect of the hydrology of the earth's surface is incorporated into a numerical model of the general circulation of the atmosphere developed at the Geophysical Fluid Dynamics Laboratory of the Environmental Science Services Administration (ESSA). The primitive equation of motion is used for this study. The nine levels of the model are distributed so as to resolve the surface boundary layer and stratosphere. The depletion of solar radiation and the transfer of the terrestrial radiation are computed taking into consideration cloud and atmospheric absorbers such as water vapor, carbon dioxide, and ozone. The scheme treating the hydrology of our model involves the prediction of water vapor in the atmosphere and the prediction of soil moisture and snow cover. In order to represent the mositure-holding capacity of soil, the continent is assumed to be covered by boxes, which can store limited amounts of water. The ocean surface is idealized to be a completely wet surface without any heat capacity. The temperature of the earth's surface is determined in such a way that it satisfies the condition of heat balance. To facilitate the analysis and the interpretation of the results, a simple and idealized distribution of the ocean and the continental region is chosen for this study. The numerical integrations are performed for the annual mean distribution of solar insolation.

In general, the qualitative features of hydrologic and thermodynamic regimes at the earth's surface are successfully simulated. Particularly, the horizontal distribution of rainfall is in excellent qualitative agreement with the observations. For example, the typical subtropical desert, the break of the subtropical dry belt along the east coast of the continent, and the equatorial rain belt emerged as the result of numerical time integration. Some features of the spatial distributions of heat and water balance components at the earth's surface also agree well with those obtained by Budyko for the actual atmosphere.

Owing to the lack of seasonal variation of solar insolation and lack of poleward transport of heat by ocean currents in the model, excessive snow cover develops at higher latitudes. Accordingly, the temperature in the polar region is much lower than the annual mean temperature observed in the actual atmosphere.

This investigation constitutes a preliminary study preceding the numerical integration of the general circulation model of joint ocean-atmosphere interaction, in which the transport of heat by ocean currents plays an important role.

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SYUKURO MANABE and JOSEPH SMAGORINSKY

Abstract

The thermal and dynamical structure of the tropical atmosphere which emerged from the numerical integration of our general circulation model with a simple hydrologic cycle is analyzed in detail.

According to the results of our analysis, the lapse rate of zonal mean temperature in the model Tropics is super-moist-adiabatic in the lower troposphere, and is sub-moist-adiabatic above the 400-mb. level in qualitative agreement with the observed features in the actual Tropics. The flow field in the model Tropics also displays interesting features. For example, a zone of strong convergence and a belt of heavy rain develops around the equator. Synoptic-scale disturbances such as weak tropical cyclones and shear lines with strong convergence develop and are reminiscent of disturbances in the actual tropical atmosphere. The humid towers, which result from moist convective adjustment and condensation, develop in the central core of the regions of strong upward motion, sometimes reaching the level of the tropical tropopause and thus heating the upper tropical troposphere. This heating compensates for the cooling due to radiation and the meridional circulation.

According to the analysis of the energy budget of the model Tropics, the release of eddy available potential energy, which is mainly generated by the heat of condensation, constitutes the major source of eddy kinetic energy of disturbances prevailing in the model Tropics.

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Alex Hall and Syukuro Manabe

Abstract

To understand the role of water vapor feedback in unperturbed surface temperature variability, a version of the Geophysical Fluid Dynamics Laboratory coupled ocean–atmosphere model is integrated for 1000 yr in two configurations, one with water vapor feedback and one without. For all spatial scales, the model with water vapor feedback has more low-frequency (timescale ≥ 2 yr) surface temperature variability than the one without. Thus water vapor feedback is positive in the context of the model’s unperturbed variability. In addition, water vapor feedback is more effective the longer the timescale of the surface temperature anomaly and the larger its spatial scale.

To understand the role of water vapor feedback in global warming, two 500-yr integrations were also performed in which CO2 was doubled in both model configurations. The final surface global warming in the model with water vapor feedback is 3.38°C, while in the one without it is only 1.05°C. However, the model’s water vapor feedback has a larger impact on surface warming in response to a doubling of CO2 than it does on internally generated, low-frequency, global-mean surface temperature anomalies. Water vapor feedback’s strength therefore depends on the type of temperature anomaly it affects. The authors found that the degree to which a surface temperature anomaly penetrates into the troposphere is a critical factor in determining the effectiveness of its associated water vapor feedback. The more the anomaly penetrates, the stronger the feedback. It is also shown that the apparent impact of water vapor feedback is altered by other feedback mechanisms, such as albedo and cloud feedback. The sensitivity of the results to this fact is examined.

Finally, the authors compare the local and global-mean surface temperature time series from both unperturbed variability experiments to the observed record. The experiment without water vapor feedback does not have enough global-scale variability to reproduce the magnitude of the variability in the observed global-mean record, whether or not one removes the warming trend observed over the past century. In contrast, the amount of variability in the experiment with water vapor feedback is comparable to that of the global-mean record, provided the observed warming trend is removed. Thus, the authors are unable to simulate the observed levels of variability without water vapor feedback.

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Thomas Delworth and Syukuro Manabe

Abstract

The influence of land surface processes on near-surface atmospheric variability on seasonal and interannual time scales is studied using output from two integrations of a general circulation model. In the first experiment of 50 years duration, soil moisture is predicted, thereby taking into consideration interactions between the surface moisture budget and the atmosphere. In the second experiment, of 25 years duration, the seasonal cycle of soil moisture is prescribed at each grid point based upon the results of the first integration, thereby suppressing thew interactions. The same seasonal cycle of soil moisture is prescribed for each year of the second integration. Differences in atmospheric variability between the two integrations are due to interactions between the surface moisture budget and the atmosphere.

Analyses of monthly data indicate that the surface moisture budget interacts with the atmosphere in such a way as to lengthen the time scales of fluctuations of near-surface relative humidity and temperature, as well as to increase the total variability of the atmosphere. During summer months at middle latitudes, the persistence of near-surface relative humidity, as measured by correlations of monthly mean relative humidity between successive months, increases from near zero in the experiment with prescribed soil moisture to as large as 0.6 in the experiment with interactive soil moisture, which corresponds to an e-folding time of approximately two months. The standard deviation of monthly mean relative humidity during summer is substantially larger in the experiment with interactive soil moisture than in the experiment with prescribed soil moisture. Surface air temperature exhibits similar changes, but of smaller magnitude.

Soil wetness influence the atmosphere by altering the partitioning of the outgoing energy flux at the surface into latent and sensible heat components. Fluctuations of soil moisture result in large variations in these fluxes, and thus significant variations in near surface relative humidity and temperature. Because anomalies of monthly mean soil moisture are characterized by seasonal and interannual time scales, they create persistent anomalous fluxes of latent and sensible heat, thereby increasing the persistence of near-surface atmospheric relative humidity and temperature.

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Syukuro Manabe and Kirk Bryan

Abstract

No abstract available.

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SYUKURO MANABE and FRITZ MÖLLER

Abstract

In order to incorporate the effect of radiation into the numerical experiment of the general circulation of the atmosphere, a simplified scheme for computing the radiative temperature change is constructed. The effects included are long wave radiation by water vapor, carbon dioxide, and ozone and the absorption of solar radiation by these three gases. The absorptivities of these gases are determined based upon the recent results of laboratory experiments and those of theoretical computations. The effects of clouds are not included.

By use of this scheme the radiative equilibrium temperature is computed for various latitudes and seasons as asymptotic solutions of an initial value problem. To a certain degree the radiative equilibrium solutions reveal some of the typical characteristics of stratospheric temperature and tropopause height variations.

Radiative heat budgets of the atmosphere are also computed and compared with the results of the computations of radiative equilibrium. This comparison is helpful for understanding the role of radiative processes and also suggests the kinds of effect we should expect from other thermal processes in the atmosphere.

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Syukuro Manabe and Douglas G. Hahn

Abstract

A spectral atmospheric circulation model is time-integrated for approximately 18 years. The model has a global computational domain and realistic geography and topography. The model undergoes an annual cycle as daily values of seasonally varying insolation and sea surface temperature are prescribed without any interannual variation. It has a relatively low computational resolution with 15 spectral components retained in both zonal and meridional directions. Analysis of the results from the last 15 years of the time integration indicates that, in middle and high latitudes, the model approximately reproduces the observed geographical distribution of the variability (i.e., standard deviation) of daily, monthly and yearly mean surface pressure and temperature.

In the tropics, the model tends to underestimate the variability of surface pressure, particularly at longer time scales. This result suggests the importance of processes with long time scales such as ocean–atmosphere interaction, in maintaining the variability of the atmosphere in low latitudes.

It is shown that global mean values of standard deviation of daily, 5-daily, 10-daily, monthly, seasonal and annual mean surface pressure of the model atmosphere may be approximately fitted by a corresponding set of standard deviations of a red noise time series with a decay time scale of slightly longer than four days. However, it appears that the temporal variation of surface pressure also includes minor contributions from disturbances with much loner decay time scales.

In general, the model tends to underestimate the persistence (or decay time scale) of atmospheric disturbances. However, it reproduces some of the features of the observed geographical distribution of decay time scale of the surface pressure fluctuations in middle and high latitudes.

The observed standard deviation of annual, hemispheric mean surface air temperature also is compared with model results. Although a clearcut evaluation of model performance is somewhat hampered by observational uncertainty, it appears that the model's value amounts to a substantial fraction of the corresponding standard deviation derived from observational studies.

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Syukuro Manabe and Richard T. Wetherald

Abstract

Radiative convective equilibrium of the atmosphere with a given distribution of relative humidity is computed as the asymptotic state of an initial value problem.

The results show that it takes almost twice as long to reach the state of radiative convective equilibrium for the atmosphere with a given distribution of relative humidity than for the atmosphere with a given distribution of absolute humidity.

Also, the surface equilibrium temperature of the former is almost twice as sensitive to change of various factors such as solar constant, CO2 content, O3 content, and cloudiness, than that of the latter, due to the adjustment of water vapor content to the temperature variation of the atmosphere.

According to our estimate, a doubling of the CO2 content in the atmosphere has the effect of raising the temperature of the atmosphere (whose relative humidity is fixed) by about 2C. Our model does not have the extreme sensitivity of atmospheric temperature to changes of CO2 content which was adduced by Möller.

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Syukuro Manabe and Ronald J. Stouffer

Abstract

To speculate on the future change of climate over several centuries, three 500-year integrations of a coupled ocean-atmosphere model were performed. In addition to the standard integration in which the atmospheric concentration of carbon dioxide remains unchanged, two integrations are conducted. In one integration, the C02 concentration increases by 1% yr−1 (compounded) until it reaches four times the initial value at the 140th year and remains unchanged thereafter. In another integration, the C02 concentration also increases at the rate of 1% yr−1 until it reaches twice the initial value at the 70th year and remains unchanged thereafter.

One of the most notable features of the C02-quadrupling integration is the gradual disappearance of thermohaline circulations in most of the model oceans during the first 250-year period, leaving behind wind-driven cells. For example, thermohaline circulation nearly vanishes in the North Atlantic during the fist 200 years of the integration. In the Weddell and Ross seas, thermohaline circulation becomes weaker and shallower, thereby reducing the rate of bottom water formation and weakening the northward flow of bottom water in the Pacific and Atlantic oceans. The weakening or near disappearance of thermohaline circulation described above is attributable mainly to the capping of the model oceans by relatively fresh water in high latitudes where the excess of precipitation over evaporation increases markedly due to the enhanced poleward moisture transport in the warmer model troposphere.

In the C02-doubling integration, the thermohaline circulation weakens by a factor of more than 2 in the North Atlantic during the first 150 years but almost recovers its original intensity by the 500th year. The increase and downward penetration of positive heat and temperature anomaly in low and middle latitudes of the North Atlantic helps to increase the density contrast between the sinking and rising regions, contributing to this slow recovery. The recovery is aided by the gradual increase in surface salinity that accompanies the intensification of the thermohaline circulation.

During the 500-year period of the doubling and quadrupling experiments, the global mean surface air temperature increases by about 3.5°C and 7°C, respectively. The rise of sea level due to the thermal expansion of sea water is about 1 and 1.8 m, respectively, and could be much larger if the contribution of meltwater from continental ice sheets were included. It is speculated that the two experiments described above provide a probable range of future climate change.

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