Showing papers by "Geophysical Fluid Dynamics Laboratory published in 1999"

Remote Sea Surface Temperature Variations during ENSO: Evidence for a Tropical Atmospheric Bridge

Abstract: In an El Nino event, positive SST anomalies usually appear in remote ocean basins such as the South China Sea, the Indian Ocean, and the tropical North Atlantic approximately 3 to 6 months after SST anomalies peak in the tropical Pacific. Ship data from 1952 to 1992 and satellite data from the 1980s both demonstrate that changes in atmospheric circulation accompanying El Nino induce changes in cloud cover and evaporation which, in turn, increase the net heat flux entering these remote oceans. It is postulated that this increased heat flux is responsible for the surface warming of these oceans. Specifically, over the eastern Indian Ocean and South China Sea, enhanced subsidence during El Nino reduces cloud cover and increases the solar radiation absorbed by the ocean, thereby leading to enhanced SSTs. In the tropical North Atlantic, a weakening of the trade winds during El Nino reduces surface evaporation and increases SSTs. These relationships fit the concept of an “atmospheric bridge” that conne...

A Monte Carlo Implementation of the Nonlinear Filtering Problem to Produce Ensemble Assimilations and Forecasts

Abstract: Knowledge of the probability distribution of initial conditions is central to almost all practical studies of predictability and to improvements in stochastic prediction of the atmosphere. Traditionally, data assimilation for atmospheric predictability or prediction experiments has attempted to find a single “best” estimate of the initial state. Additional information about the initial condition probability distribution is then obtained primarily through heuristic techniques that attempt to generate representative perturbations around the best estimate. However, a classical theory for generating an estimate of the complete probability distribution of an initial state given a set of observations exists. This nonlinear filtering theory can be applied to unify the data assimilation and ensemble generation problem and to produce superior estimates of the probability distribution of the initial state of the atmosphere (or ocean) on regional or global scales. A Monte Carlo implementation of the fully n...

Monsoon changes for 6000 years ago: Results of 18 simulations from the Paleoclimate Modeling Intercomparison Project (PMIP)

Abstract: Amplification of the northern hemisphere seasonal cycle of insolation during the mid-Holocene causes a northward shift of the main regions of monsoon precipitation over Africa and India in all 18 simulations conducted for the Paleoclimate Modeling Intercomparison Project (PMIP). Differences among simulations are related to differences in model formulation. Despite qualitative agreement with paleoecological estimates of biome shifts, the magnitude of the monsoon increases over northern Africa are underestimated by all the models.

Validation and Sensitivities of Frontal Clouds Simulated by the ECMWF Model

Abstract: Clouds simulated by the European Centre for Medium-Range Weather Forecasts (ECMWF) model are composited to derive the typical organization of clouds surrounding a midlatitude baroclinic system. Comparison of this composite of about 200 cyclones with that based on satellite data reveals that the ECMWF model quite accurately simulates the general positioning of clouds relative to a low pressure center. However, the optical depths of the model’s high/low clouds are too small/large relative to the satellite observations, and the model lacks the midlevel topped clouds observed to the west of the surface cold front. Sensitivity studies with the ECMWF model reveal that the error in high-cloud optical depths is more sensitive to the assumptions applied to the ice microphysics than to the inclusion of cloud advection or a change of horizontal resolution from 0.5625° to 1.69° lat. This reflects the fact that in the ECMWF model gravitational settling is the most rapid process controlling the abundance of ic...

Tropospheric Aerosol Climate Forcing in Clear-Sky Satellite Observations over the Oceans

Abstract: Tropospheric aerosols affect the radiative forcing of Earth's climate, but their variable concentrations complicate an understanding of their global influence. Model-based estimates of aerosol distributions helped reveal spatial patterns indicative of the presence of tropospheric aerosols in the satellite-observed clear-sky solar radiation budget over the world's oceans. The results show that, although geographical signatures due to both natural and anthropogenic aerosols are manifest in the satellite observations, the naturally occurring sea-salt is the leading aerosol contributor to the global-mean clear-sky radiation balance over oceans.

A Conceptual Framework for Predictability Studies

Abstract: A conceptual framework is presented for a unified treatment of issues arising in a variety of predictability studies. The predictive power (PP), a predictability measure based on information‐theoretical principles, lies at the center of this framework. The PP is invariant under linear coordinate transformations and applies to multivariate predictions irrespective of assumptions about the probability distribution of prediction errors. For univariate Gaussian predictions, the PP reduces to conventional predictability measures that are based upon the ratio of the rms error of a model prediction over the rms error of the climatological mean prediction. Since climatic variability on intraseasonal to interdecadal timescales follows an approximately Gaussian distribution, the emphasis of this paper is on multivariate Gaussian random variables. Predictable and unpredictable components of multivariate Gaussian systems can be distinguished by predictable component analysis, a procedure derived from discriminant analysis: seeking components with large PP leads to an eigenvalue problem, whose solution yields uncorrelated components that are ordered by PP from largest to smallest. In a discussion of the application of the PP and the predictable component analysis in different types of predictability studies, studies are considered that use either ensemble integrations of numerical models or autoregressive models fitted to observed or simulated data. An investigation of simulated multidecadal variability of the North Atlantic illustrates the proposed methodology. Reanalyzing an ensemble of integrations of the Geophysical Fluid Dynamics Laboratory coupled general circulation model confirms and refines earlier findings. With an autoregressive model fitted to a single integration of the same model, it is demonstrated that similar conclusions can be reached without resorting to computationally costly ensemble integrations.

Response of a Coupled Ocean–Atmosphere Model to Increasing Atmospheric Carbon Dioxide: Sensitivity to the Rate of Increase

Abstract: The influence of differing rates of increase of the atmospheric CO2 concentration on the climatic response is investigated using a coupled ocean–atmosphere model. Five transient integrations are performed each using a different constant exponential rate of CO2 increase ranging from 4% yr−1 to 0.25% yr−1. By the time of CO2 doubling, the surface air temperature response in all the transient integrations is locally more than 50% and globally more than 35% of the equilibrium response. The land–sea contrast in the warming, which is evident in the equilibrium results, is larger in all the transient experiments. The land–sea difference in the response increases with the rate of increase in atmospheric CO2 concentration. The thermohaline circulation (THC) weakens in response to increasing atmospheric CO2 concentration in all the transient integrations, confirming earlier work. The results also indicate that the slower the rate of increase, the larger the weakening of the THC by the time of doubling. Two...

The Surface Branch of the Zonally Averaged Mass Transport Circulation in the Troposphere

Abstract: The near-surface branch of the overturning mass transport circulation in the troposphere, containing the equatorward flow, is examined in isentropic and geometric coordinates. A discussion of the zonal momentum balance within isentropic layers shows that the equatorward flow at a given latitude is confined to isentropic layers that typically intersect the surface at that latitude. As a consequence of mass transport within the surface mixed layer, much of the equatorward flow occurs in layers with potential temperatures below the mean surface potential temperature. In the conventional transformed Eulerian mean formulation for geometric coordinates, the surface branch of the overturning circulation is represented in an unrealistic manner: streamlines of the residual circulation do not close above the surface. A modified residual circulation is introduced that is free from this defect and has the additional advantage that its computation, unlike that of the conventional residual circulation, does not require division by the static stability, which may approach zero in the planetary boundary layer. It is then argued that cold air advection by the residual circulation is responsible for the formation of surface inversions at all latitudes in idealized GCMs with weak thermal damping. Also included is a discussion of how a general circulation theory for the troposphere must be built upon a theory for the near-surface meridional mass fluxes.

A Linear Stochastic Model of a GCM’s Midlatitude Storm Tracks

Abstract: A linear stochastic model is used to simulate the midlatitude storm tracks produced by an atmospheric GCM. A series of six perpetual insolation/SST GCM experiments are first performed for each month. These experiments capture the ‘‘midwinter suppression’’ of the Pacific storm track in a particularly clean way. The stochastic model is constructed by linearizing the GCM about its January climatology and finding damping and stirring parameters that best reproduce that model’s eddy statistics. The model is tested by examining its ability to simulate other GCM integrations when the basic state is changed to the mean flow of those models, while keeping the stirring and damping unchanged. The stochastic model shows an impressive ability to simulate a variety of eddy statistics. It captures the midwinter suppression of the Pacific storm track qualitatively and is also capable of simulating storm track .

The rôle of thermohaline circulation in climate

Abstract: This article discusses the role of the THC in climate, based upon the results of several numerical experiments which use a coupled ocean-atmosphere model developed at the Geophysical Fluid Dynamics Laboratory of NOAA, USA. The first part of the article explores the mechanism which is responsible for the abrupt climate change such as the Younger Dryas event using the coupled model. In response to the freshwater discharge into high north Atlantic latitudes over a period of 500 years, the THC in the Atlantic Ocean weakens, reducing surface air temperature over the northern north Atlantic Ocean, the Scandinavian Peninsula, and the circumpolar ocean and Antarctic Continent of the southern hemisphere. Upon the termination of the water discharge at the 500th year, the THC begins to intensify, regaining its original intensity in a few hundred years. In addition, the sudden onset and the termination of the discharge of freshwater induces the multidecadal fluctuation in the intensity of the THC, which generates the almost abrupt change of climate. It is noted that similar but much weaker oscillation of the THC is also evident in the control integration of the coupled model without freshwater forcing. The irregular oscillation of the THC mentioned above appears to be related to the fluctuation of the Subarctic Gyre and associated east Greenland current, yielding the evolution of the surface salinity anomaly which resembles that of “great salinity anomaly”. The second part of this article describes the response of a coupled ocean-atmosphere model to the doubling and quadrupling of atmospheric carbon dioxide over centuries time-scale. In one integration, the CO 2 concentration increases by 1%/year (compounded) until it reaches 4 × the initial value at the 140th year and remains unchanged thereafter. In another integration, the CO 2 concentration also incleases at the rate of 1%/year until it reaches 2 × the initial value at the 70th year and remains unchanged thereafter. One of the most notable features of the CO 2 -quadrupling integration is the gradual disappearance of thermohaline circulation 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 by the 250 years of the integration and remains very weak until the 900th year. However, it begins to restore the original intensity by the 1600th year. In the CO 2 -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 weakening of the THC moderates temporarily the greenhouse warming over the north Atlantic Ocean and its vicinity. In both numerical experiments described above, the initial weakening of the THC results from the capping of oceanic surface by relatively fresh, low-density water, which surpresses the convective cooling of water in the sinking region of the THC. DOI: 10.1034/j.1600-0889.1999.00008.x

The Role of Water Vapor Feedback in Unperturbed Climate Variability and Global Warming

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 3.388C, while in the one without it is only 1.058C. 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.

Increased hurricane intensities with CO2-induced warming as simulated using the GFDL hurricane prediction system

Abstract: The impact of CO2-induced global warming on the intensities of strong hurricanes is investigated using the GFDL regional high-resolution hurricane prediction system. The large-scale initial conditions and boundary conditions for the regional model experiments, including SSTs, are derived from control and transient CO2 increase experiments with the GFDL R30-resolution global coupled climate model. In a case study approach, 51 northwest Pacific storm cases derived from the global model under present-day climate conditions are simulated with the regional model, along with 51 storm cases for high CO2 conditions. For each case, the regional model is integrated forward for five days without ocean coupling. The high CO2 storms, with SSTs warmer by about 2.2 °C on average and higher environmental convective available potential energy (CAPE), are more intense than the control storms by about 3–7 m/s (5%–11%) for surface wind speed and 7 to 24 hPa for central surface pressure. The simulated intensity increases are statistically significant according to most of the statistical tests conducted and are robust to changes in storm initialization methods. Near-storm precipitation is 28% greater in the high CO2 sample. In terms of storm tracks, the high CO2 sample is quite similar to the control. The mean radius of hurricane force winds is 2 to 3% greater for the composite high CO2 storm than for the control, and the high CO2 storms penetrate slightly higher into the upper troposphere. More idealized experiments were also performed in which an initial storm disturbance was embedded in highly simplified flow fields using time mean temperature and moisture conditions from the global climate model. These idealized experiments support the case study results and suggest that, in terms of thermodynamic influences, the results for the NW Pacific basin are qualitatively applicable to other tropical storm basins.

Middle Atmosphere Simulated with High Vertical and Horizontal Resolution Versions of a GCM: Improvements in the Cold Pole Bias and Generation of a QBO-like Oscillation in the Tropics

Abstract: The large-scale circulation in the Geophysical Fluid Dynamics Laboratory “SKYHI” troposphere–stratosphere–mesosphere finite-difference general circulation model is examined as a function of vertical and horizontal resolution. The experiments examined include one with horizontal grid spacing of ∼35 km and another with ∼100 km horizontal grid spacing but very high vertical resolution (160 levels between the ground and about 85 km). The simulation of the middle-atmospheric zonal-mean winds and temperatures in the extratropics is found to be very sensitive to horizontal resolution. For example, in the early Southern Hemisphere winter the South Pole near 1 mb in the model is colder than observed, but the bias is reduced with improved horizontal resolution (from ∼70°C in a version with ∼300 km grid spacing to less than 10°C in the ∼35 km version). The extratropical simulation is found to be only slightly affected by enhancements of the vertical resolution. By contrast, the tropical middle-atmospheric s...

Detectability of summer dryness caused by greenhouse warming

Abstract: This study investigates the temporal and spatial variation of soil moisture associated with global warming as simulated by long-term integrations of a coupled ocean-atmosphere model conducted earlier. Starting from year 1765, integrations of the coupled model for 300 years were performed for three scenarios: increasing greenhouse gases only, increasing sulfate-aerosol loading only and the combination of both radiative forcings. The integration with the combined radiative forcings reproduces approximately the observed increases of global mean surface air temperature during the 20th century. Analysis of this integration indicates that both summer dryness and winter wetness occur in middle-to-high latitudes of North America and southern Europe. These features were identified in earlier studies. However, in the southern part of North America where the percentage reduction of soil moisture during summer is quite large, soil moisture is decreased for nearly the entire annual cycle in response to greenhouse warming. A similar observation applies to other semi-arid regions in subtropical to middle latitudes such as central Asia and the area surrounding the Mediterranean Sea. On the other hand, annual mean runoff is greatly increased in high latitudes because of increased poleward transport of moisture in the warmer model atmosphere. An analysis of the central North American and southern European regions indicates that the time when the change of soil moisture exceeds one standard deviation about the control integration occurs considerably later than that of surface air temperature for a given experiment because the ratio of forced change to natural variability is much smaller for soil moisture compared with temperature. The corresponding lag time for runoff change is even greater than that of either precipitation or soil moisture for the same reason. Also according to the above criterion, the inclusion of the effect of sulfate aerosols in the greenhouse warming experiment delays the noticeable change of soil moisture by several decades. It appears that observed surface air temperature is a better indicator of greenhouse warming than hydrologic quantities such as precipitation, runoff and soil moisture. Therefore, we are unlikely to notice definitive CO2-induced continental summer dryness until several decades into the 21st century.

The Role of Clouds, Water Vapor, Circulation, and Boundary Layer Structure in the Sensitivity of the Tropical Climate

Abstract: The physical mechanisms that affect the tropical sea surface temperature (SST) are investigated using a two-box equilibrium model of the Tropics. One box represents the convecting, warm SST, high humidity region of the Tropics, and the other box represents the subsidence region with low humidity, boundary layer clouds, and cooler SST. The two regions communicate by energy and moisture fluxes that are proportional to the strength of the overturning circulation that couples the two regions. The boundary layer properties in the subsiding region are predicted with a mixing line model. Humidity above the inversion in the subsiding region is predicted from moisture conservation. The humidity above the inversion in the subsiding region increases rapidly with temperature, but this has less effect on the sensitivity than expected, because the inversion lowers as the humidity above the inversion is increased. Some of the increased greenhouse effect of the free troposphere can be offset by decreased greenho...

The influence of transient surface fluxes on North Atlantic overturning in a coupled GCM Climate Change Experiment

Abstract: The mechanism by which the model-simulated North Atlantic thermohaline circulation (THC) weakens in response to increasing greenhouse gas (GHG) forcing is investigated through the use of a set of five multi-century experiments. Using a coarse resolution version of the GFDL coupled climate model, the role of various surface fluxes in weakening the THC is assessed. Changes in net surface freshwater fluxes (precipitation, evaporation, and runoff from land) are found to be the dominant cause for the model's THC weakening. Surface heat flux changes brought about by rising GHG levels also contribute to THC weakening, but are of secondary importance. Wind stress variations have negligible impact on the THC's strength in the transient GHG experiment.

The role of vertically varying cloud fraction in the parametrization of microphysical processes in the ECMWF model

Abstract: Summary Global Circulation Models (GCMs) have generally treated only the radiative impacts of vertically varying cloud fraction by use of a cloud overlap assumption. In this study, the microphysical impacts of vertically varying cloud fraction are addressed by developing a sub-grid scale precipitation model which resolves the vertical variation of cloud fraction. This sub-grid model subdivides the grid boxes into homogeneous columns which are either completely clear or cloudy. By comparing the columnaveraged microphysical quantities from the sub-grid scale precipitation model to the parametrization in the ECMWF model, the ability of the ECMWF model to account for the sub-grid nature of cloud and precipitation microphysics is assessed. It is found that the ECMWF model overestimates precipitation evaporation in the tropical mid-troposphere. This results from (a) an incorrect parametrization of the area of the grid box covered by precipitation, and (b) the inadequacy of assuming a single value for the precipitation rate in the grid box. In addition to assessing the ability of the ECMWF model to parametrize the sub-grid nature of cloud microphysics, the subgrid precipitation model is used to show that the cloud overlap assumption has a large impact on the evaporation of precipitation. In light of the current trend towards more sophisticated cloud and precipitation parametrization in GCMs, more attention should be paid to the impact of vertical cloud fraction variations on the parametrized microphysics.

The macroturbulence of the troposphere

Abstract: Eddy length scales, eddy velocity scales, and the amplitude of eddy fluxes in the mid-latitude troposphere are discussed, primarily from the qualitative perspective provided by studies of quasi-geostrophic turbulence. The utility of a diffusive picture for the near surface poleward flux of heat is emphasized, as is the extent to which a full closure theory for the troposphere, including the interior potential vorticity fluxes, must revolve around this theory for the heat flux. A central problem in general circulation theory is then to determine which factors control the horizontal diffusivity near the surface. The baroclinic eddy production problem has distinctive features that make it stand out from other inhomogeneous turbulence problems such as Benard convection and laboratory shear flows, the crucial point being that there can be scale separation between the eddies and the scale of the mean flow inhomogeneity in the direction of the relevant transport. This scale separation makes diffusive closures more compelling. In addition, it allows one to compute diffusivities from models of homogeneous turbulence. DOI: 10.1034/j.1600-0889.1999.00006.x

The Hadley circulation : assessing NCEP/NCAR reanalysis and sparse in-situ estimates

Abstract: We present a comparison of the zonal mean meridional circulations derived from monthly in situ data (i.e. radiosondes and ship reports) and from the NCEP/NCAR reanalysis product. To facilitate the interpretation of the results, a third estimate of the mean meridional circulation is produced by subsampling the reanalysis at the locations where radiosonde and surface ship data are available for the in situ calculation. This third estimate, known as the subsampled estimate, is compared to the complete reanalysis estimate to assess biases in conventional, in situ estimates of the Hadley circulation associated with the sparseness of the data sources (i.e., radiosonde network). The subsampled estimate is also compared to the in situ estimate to assess the biases introduced into the reanalysis product by the numerical model, initialization process and/or indirect data sources such as satellite retrievals. The comparisons suggest that a number of qualitative differences between the in situ and reanalysis estimates are mainly associated with the sparse sampling and simplified interpolation schemes associated with in situ estimates. These differences include: (1) a southern Hadley cell that consistently extends up to 200 hPa in the reanalysis, whereas the bulk of the circulation for the in situ and subsampled estimates tends to be confined to the lower half of the troposphere, (2) more well-defined and consistent poleward limits of the Hadley cells in the reanalysis compared to the in-situ and subsampled estimates, and (3) considerably less variability in magnitude and latitudinal extent of the Ferrel cells and southern polar cell exhibited in the reanalysis estimate compared to the in situ and subsampled estimates. Quantitative comparison shows that the subsampled estimate, relative to the reanalysis estimate, produces a stronger northern Hadley cell (∼20%), a weaker southern Hadley cell (∼20–60%), and weaker Ferrel cells in both hemispheres. These differences stem from poorly measured oceanic regions which necessitate significant interpolation over broad regions. Moreover, they help to pinpoint specific shortcomings in the present and previous in situ estimates of the Hadley circulation. Comparisons between the subsampled and in situ estimates suggest that the subsampled estimate produces a slightly stronger Hadley circulation in both hemispheres, with the relative differences in some seasons as large as 20–30%. 6These differences suggest that the mean meridional circulation associated with the NCEP/NCAR reanalysis is more energetic than observations suggest. Examination of ENSO-related changes to the Hadley circulation suggest that the in situ and subsampled estimates significantly overestimate the effects of ENSO on the Hadley circulation due to the reliance on sparsely distributed data. While all three estimates capture the large-scale region of low-level equatorial convergence near the dateline that occurs during El Nino, the in situ and subsampled estimates fail to effectively reproduce the large-scale areas of equatorial mass divergence to the west and east of this convergence area, leading to an overestimate of the effects of ENSO on the zonal mean circulation.

Three-Dimensional Cloud-System Modeling of GATE Convection

Abstract: Deep convection and its associated mesoscale circulations are modeled using a three-dimensional elastic model with bulk microphysics and interactive radiation for a composite easterly wave from the Global Atmospheric Research Program Atlantic Tropical Experiment. The energy and moisture budgets, large-scale heat sources and moisture sinks, microphysics, and radiation are examined. The modeled cloud system undergoes a life cycle dominated by deep convection in its early stages, followed by an upper-tropospheric mesoscale circulation. The large-scale heat sources and moisture sinks associated with the convective system agree broadly with diagnoses from field observations. The modeled upper-tropospheric moisture exceeds observed values. Strong radiative cooling at the top of the mesoscale circulation can produce overturning there. Qualitative features of observed changes in large-scale convective available potential energy and convective inhibition are found in the model integrations, although quantitative magnitudes can differ, especially for convective inhibition. Radiation exerts a strong influence on the microphysical properties of the cloud system. The three-dimensional integrations exhibit considerably less sporadic temporal behavior than corresponding two-dimensional integrations. While the third dimension is less important over timescales longer than the duration of a phase of an easterly wave in the lower and middle troposphere, it enables stronger interactions between radiation and dynamics in the upper-tropospheric mesoscale circulation over a substantial fraction of the life cycle of the convective system.

Changes in heat index associated with CO2-induced global warming

Abstract: Changes in Heat Index (a combined measure of temperature and humidity) associated with global warming are evaluated based on the output from four extended integrations of the GFDL coupled ocean-atmosphere climate model. The four integrations are: a control with constant levels of atmospheric carbon dioxide (CO2), a second integration in which an estimate of the combined radiative forcing of greenhouse gases and sulfate aerosols over the period 1765–2065 is used to force the model, and a third (fourth) integration in which atmospheric CO2$ increases at the rate of 1% per year to double (quadruple) its initial value, and is held constant thereafter. While the spatial patterns of the changes in Heat Index are largely determined by the changes in surface air temperature, increases in atmospheric moisture can substantially amplify the changes in Heat Index over regions which are warm and humid in the Control integration. The regions most prone to this effect include humid regions of the Tropics and summer hemisphere extra-tropics, including the southeastern United States, India, southeast Asia and northern Australia.

Simulation of the κ−5/3 mesoscale spectral regime in the GFDL SKYHI General Circulation Model

Abstract: Data from very high horizontal resolution simulations with the Geophysical Fluid Dynamics Laboratory SKYHI general circulation model are used to calculate the kinetic energy spectrum as a function of horizontal wavenumber, k, in the upper troposphere. The spectrum shows the familiar ∼ -3 slope at scales longer than ∼ 1000 km, in agreement with previous general circulation model and observational studies. At shorter scales, the spectrum becomes shallower with a slope ∼ -5/3, also in agreement with available observations. The -5/3 slope spans about a decade of the resolved scales and this result represents the first successful simulation of such a broad range of the mesoscale regime by a global model. Partitioning of the flow between rotational and divergent components shows that the rotational part dominates at large scales and that there is approximate equipartition between rotational and divergent parts at mesoscales. Analysis of the parameterized kinetic energy dissipation shows that vertical diffusion dominates horizontal diffusion for a wide range of wavenumbers extending well into the k -5/3 regime.

An ultimate limiting nutrient

Abstract: A long debate has centred on what nutrient in the oceans is the main limitation on phytoplankton growth Is one nutrient clearly more important than others, and, if so, on what timescales? The answer to emerge from newly published model calculations is that, over long periods, phosphorus, in the form of dissolved phosphate, is the ‘ultimate limiting nutrient’

Global mean surface air temperature and North Atlantic overturning in a suite of coupled GCM climate change experiments

Abstract: The effects of model initial conditions and the starting time of transient radiative forcings on global mean surface air temperature (SAT) and the North Atlantic thermohaline circulation (THC) are studied in a set of coupled climate GCM experiments. Nine climate change scenario experiments, in which the effective levels of greenhouse gases and tropospheric sulfate aerosols vary in time, are initialized from various points in a long control model run. The time at which the transition from constant to transient radiative forcing takes place is varied in the scenario runs, occurring at points representing either year 1766, 1866 or 1916. The sensitivity of projected 21st century global mean SATs and the THC to the choice of radiative forcing transition point is small, and is similar in magnitude to the variability arising from variations in the coupled GCM's initial three-dimensional state.

Observed and GCM-Simulated Westward-Propagating, Planetary-Scale Fluctuations with Approximately Three-Week Periods

Abstract: The structural characteristics and vorticity dynamics of westward-traveling patterns (WTP) in the troposphere are examined using the National Centers for Environmental Prediction‐National Center for Atmospheric Research (NCEP‐NCAR) reanalyses based on observations for the 1973‐95 period, as well as the output from a 100-yr integration of a general circulation model (GCM) with a rhomboidal truncation at 30 wavenumbers and 14 vertical levels. An identical set of diagnostic tools, including progressive/retrogressive variance analysis, crossspectra, and complex empirical orthogonal functions (EOFs), are applied to the reanalysis and GCM datasets for 300-mb height. These diagnoses all indicate that the WTP are most prominent during the cold season in the high-latitude zone extending westward from northwestern Canada to northeastern Siberia, with a typical period of ;22 days. Outstanding episodes are identified on the basis of the temporal coefficients of the leading complex EOF. Composite charts of the anomalous 300-mb height, sea level pressure, and 850-mb temperature fields at various phases of these events are constructed. The typical circulation changes accompanying the passage of the WTP are similar to those associated with well-known regional weather phenomena such as amplified pressure ridges over Alaska, cold air outbreaks over western North America and east Asia, and heavy snowfall over the Pacific Northwest. The occurrence of the WTP over the North Pacific is also characterized by notable changes in the spatial distribution and intensity of synoptic scale activity. The contributions of relative vorticity advection, planetary vorticity advection (the ‘‘ b effect’’), and horizontal divergence to the vorticity tendency in various phases of the composite wave at 300, 500, and 850 mb are investigated. In the mid- and upper troposphere, the vorticity dynamics of the WTP is similar to that of free external Rossby waves, with the b effect (which leads to westward propagation) being the dominant term, whereas the eastward advection of relative vorticity is less important due to the weak mean zonal flow in the Alaska‐Siberia sector. Most of the essential characteristics of the observed WTP deduced from the NCEP‐NCAR reanalyses are well reproduced by the GCM. The realism with which this phenomenon can be simulated in a model environment offers considerable promise for using the GCM as a tool for studying the impact of WTP on intraseasonal atmospheric variability in extended model experiments, and for assessing the dependence of the locality and activity level of the WTP on various states of the ambient circulation.

Potential vorticity thickness fluxes and wave-mean flow interaction

Abstract: The use of eddy flux of thickness between density surfaces has become a familiar starting point in oceanographic studies of adiabatic eddy effects on the mean density distribution. In this study, a dynamical analogy with the density thickness flux approach is explored to reexamine the theory of nonzonal wave‐mean flow interaction in two-dimensional horizontal flows. By analogy with the density thickness flux, the flux of thickness between potential vorticity (PV) surfaces is used as a starting point for a residual circulation formulation for nonzonal mean flows. Mean equations for barotropic PV dynamics are derived in which a modified mean velocity with an eddy-induced component advects a modified mean PV that also has an eddy-induced component. For smallamplitude eddies, the results are analogous to recent results of McDougall and McIntosh derived for stratified flow. The dynamical implications of this approach are then examined. The modified mean PV equation provides a decomposition of the eddy forcing of the mean flow into contributions from wave transience, wave dissipation, and wave-induced mass redistribution between PV contours. If the mean flow is along the mean PV contours, the contribution from wave-induced mass redistribution is ‘‘workless’’ in Plumb’s sense that it is equivalent to an eddy-induced stress that is perpendicular to the mean flow. This contribution is also associated with the convergence along the mean streamlines of a modified PV flux that is equal to the difference between the PV flux and the rotational PV flux term identified by Illari and Marshall. The cross-stream component of the modified PV flux is related to wave transience and dissipation.

Blocking and Frontogenesis by Two-Dimensional Terrain in Baroclinic Flow. Part I: Numerical Experiments

Abstract: The shallow atmospheric fronts that develop in the early winter along the east coast of North America have been attributed, in various modeling and observational studies, to the land‐sea contrasts in both surface heating and friction. However, typical synoptic conditions are such that these ‘‘coastal’’ fronts could also be a type of upstream influence by the Appalachian Mountain chain. Generalized models have suggested that relatively cold air can become trapped on the windward side of a mountain range during episodes of warm advection without a local contribution from differential surface fluxes. Such a process was proposed decades ago in a study of observations along the coast of Norway. Could coastal frontogenesis be primarily a consequence of a mountain circulation acting on the large-scale temperature gradient? A two-dimensional, terrain-following numerical model is used to find conditions under which orography may be sufficient to cause blocking and upstream frontogenesis in a baroclinic environment. The idealized basic flow is taken to have constant vertical shear parallel to a topographic ridge and a constant perpendicular wind that advects warm or cold temperatures toward the ridge. Land‐sea contrasts are omitted. In the observed cases, the mountain is ‘‘narrow’’ in the sense that the Rossby number is large. This by itself increases the barrier effect, but the experiments show that large-scale warm advection is still crucial for blocking. For realistic choices of ambient static stability and baroclinicity, the flow can be blocked by a range like the northern Appalachians if the undisturbed incident wind speed is around 10 m s21. Cold advection weakens the barrier effect. The long-term behavior of the front in strongly blocked cases is described and compared to observations. Because of the background rotation and large-scale temperature advection, blocked solutions cannot become steady in the assumed environment. However, the interface between blocked and unblocked fluid can settle into a balanced configuration in some cases. A simple argument suggests that, in the absence of dissipation, the frontal slope should be similar to that of the ambient ‘‘absolute momentum’’ surfaces.

Blocking and Frontogenesis by Two-Dimensional Terrain in Baroclinic Flow. Part II: Analysis of Flow Stagnation Mechanisms

Abstract: Numerical solutions presented in a companion paper show that two-dimensional mesoscale terrain becomes a much stronger barrier to a continuously stratified flow when the flow contains warm advection. Here it is shown that this baroclinic enhancement is a strictly nonlinear phenomenon. The linear analysis indicates a weakening of the upstream response in warm advection. However, a weakly nonlinear analysis shows that baroclinicity facilitates blocking in warm advection by strengthening the nonlinearity in the cross-mountain momentum equation in such a way as to amplify the vertical shear on the windward flank of the ridge. This is enough to send the flow past the blocking threshold even when conditions over the mountain are too linear to produce wave breaking. A more intuitive mechanism whereby the upstream static stability is increased by the nonlinearity in the temperature equation is found to be much less important.