Simple General Circulation Model Research Articles

Changes in the strength of the Brewer‐Dobson circulation in a simple AGCM

A simple general circulation model is used to explore the response of the Brewer‐Dobson circulation to warming of the tropical troposphere typically associated with greenhouse gas warming in climate models. The strength of the modeled Brewer‐Dobson circulation increases in response to tropospheric tropical warming. The tropical warming causes the midlatitude baroclinicity to increase, which leads to an increase in the wave activity at synoptic and planetary scales. Increased wave propagation from the troposphere causes stronger EP flux convergence in the stratosphere. The EP flux convergence drives a stronger Brewer‐Dobson circulation via downward control arguments. These results are compared to other recent GCM studies.

Seasonal Forecasting with a Simple General Circulation Model: Predictive Skill in the AO and PNA

Abstract A primitive equation dry atmospheric model is used to perform ensemble seasonal predictions. The predictions are done for 51 winter seasons [December–January–February (DJF)] from 1948 to 1998. Ensembles of 24 forecasts are produced, with initial conditions of 1 December plus small perturbations. The model uses a forcing field that is calculated empirically from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalyses. The forcing used to forecast a given winter is the sum of its winter climatological forcing plus an anomaly. The anomalous forcing is obtained as that of the month prior to the start of the forecast (November), which is also calculated from NCEP data. The predictions are thus made without using any information about the season to be predicted. The ensemble-mean predictions for the 51 winters are verified against the NCEP–NCAR reanalyses. Comparisons are made with the results obtained with a full GCM. It is found that the skill of the simple GCM is comparable in many ways to that of the full GCM. The skill in predicting the amplitude of the main patterns of Northern Hemisphere mean-seasonal variability, the Arctic Oscillation (AO) and the Pacific–North American (PNA) pattern is also discussed. The simple GCM has skill not only in predicting the PNA pattern during winters with strong ENSO forcing, but it also has skill in predicting the AO in winters without appreciable ENSO forcing.

On the effect of thermal forcing on the global atmospheric angular momentum and the general circulation

The effect of latitudinal differential heating on the atmospheric general circulation is studied using a simple general circulation model driven by different heating rates. It is found that an increase in differential heating leads to a strengthening of the general circulation as characterized by an increase in global available potential energy, kinetic energy and atmospheric angular momentum. The strengthening of the circulation results in three circulation regimes characterized by different eddy activities. One is a zonally symmetric Hadley regime with no eddy activity while the other two are Rossby regimes dominated by eddies with intermediate and low wave numbers, respectively. Relative to other global indices, the global relative atmospheric angular momentum is superior in detecting the transition of circulation regimes. The regime changes in mid- and high-latitudes resemble the response of the atmosphere to large changes in Earth’s rotation rate.

A Simplified General Circulation Model for a Baroclinic Ocean with Topography. Part I: Theory, Waves, and Wind-Driven Circulations

A new type of ocean general circulation model with simplified physics is described and tested for various simple wind-driven circulation problems. The model consists of the vorticity balance of the depth-averaged flow and a hierarchy of equations for “vertical moments” of density and baroclinic velocity. The first vertical density moment is the (vertically integrated) potential energy, which is used to describe the predominant link between the barotropic and the baroclinic oceanic flow in the presence of sloping topography. Tendency equations for the vertical moments of density and baroclinic velocity and an appropriate truncation of the coupled hierarchy of moments are derived that, together with the barotropic vorticity balance, yield a closed set of equations describing the barotropic–baroclinic interaction (BARBI) model of the oceanic circulation. Idealized companion experiments with a numerical implementation of the BARBI model and a primitive equation model indicate that wave propagation properties and baroclinic adjustments are correctly represented in BARBI in midlatitudes as well as in equatorial latitudes. Furthermore, a set of experiments with a realistic application to the Atlantic/Southern Ocean system reproduces important aspects that have been previously reported by studies of gyre circulations and circumpolar currents using full primitive equation models

Quasi‐decadal variability in paleoclimate records: Sunspot cycles or intrinsic oscillations?

Paleoclimatic proxies for interannual variability reveal power within the 7–15 year quasi‐decadal (QD) band in Earth's history. Despite the fact that the best‐established sources of QD variability in modern climate are internal, this QD variability in paleoclimatic records is commonly attributed to the ∼11 year “sunspot cycle” of solar variability. Intrinsic QD variability has received little attention in paleoclimatology. We apply two different modeling frameworks to assess the degree to which QD variability might have occurred in the past and question whether existing paleoclimate records, no matter how well‐resolved, distinguish between internally driven and solar‐driven variability. We show that even a simple atmospheric general circulation model (GCM), driven by constant solar radiation, produces climate oscillations with a period of ∼11 years. Then, motivated by numerous studies arguing for the presence of sunspot cycles in Eocene (∼50 Ma) paleoclimate records, we demonstrate that a fully coupled GCM simulation with constant solar forcing reproduces the full spectrum of reconstructed Eocene variability. In the examples we consider, there is no compelling reason to invoke solar cycles as a cause of interpreted variability. We stress the importance of identifying internal modes of variability from paleoclimate records to increase understanding of the climate system.

Comment on “Tropospheric response to stratospheric perturbations in a relatively simple general circulation model” by Lorenzo M. Polvani and Paul J. Kushner

Comment on “Tropospheric response to stratospheric perturbations in a relatively simple general circulation model” by Lorenzo M. Polvani and Paul J. Kushner

Atmosphere–sea‐ice low‐frequency variability with a simple model of the southern hemisphere

AbstractA simple primitive‐equation general circulation model (SGCM) is used as a representation of an ‘aqua‐planet’ (with no orography) to model constant southern‐hemisphere‐winter conditions. The statistical significance of the decadal and quasi‐biennial variability is first assessed in a long‐time SGCM run by means of a frequency‐domain statistical decomposition. The modelled long‐wave atmospheric pattern is then associated with the observed southern‐hemisphere ‘annular mode’, as suggested by previous studies using the same SGCM.A simple linear relationship between the low‐level wind and the ice velocity is then introduced into the SGCM to investigate the feedbacks between the sea‐ice cover and the baroclinic waves. The zonal mean state reached by the sea‐ice cover in a long coupled run (a few hundred years) exhibits characteristics similar to the observations. Investigation of the low‐frequency variability in the coupled simulation leads to the following results: (1) statistically significant variability is observed on interdecadal, decadal, interannual and quasi‐biennial time‐scales; (2) interdecadal, decadal and quasi‐biennial signals are associated with the atmospheric annular mode that forces changes in the northward extent of the sea‐ice cover at all Southern Ocean longitudes; (3) the interannual signal (3–6 year period) exhibits spatio‐temporal patterns and propagation speed similar to the well‐known ‘Antarctic circumpolar wave’. Copyright © 2003 Royal Meteorological Society.

Tropospheric response to stratospheric perturbations in a relatively simple general circulation model

The sensitivity of the tropospheric extratropical circulation to thermal perturbations of the polar stratosphere is examined in a dry primitive equation general circulation model with zonally symmetric forcing and boundary conditions. For sufficiently strong cooling of the polar winter stratosphere, the winter‐hemisphere tropospheric jet shifts polewards and strengthens markedly at the surface; this is accompanied by a drop in surface pressure at high latitudes in the same hemisphere. In addition, this extratropical tropospheric response is found to be very similar to the model's leading pattern of internal variability. These results are tested for robustness at several horizontal and vertical resolutions, and the same tropospheric response is observed at all but the lowest resolution tested. The behavior of this relatively simple model is broadly consistent with recent observational and modeling studies of trends in extratropical atmospheric variability.

Symmetric Stress Tensor Formulation of Horizontal Momentum Diffusion in Global Models of Atmospheric Circulation

Abstract In climate and weather forecast models, small-scale turbulence in the free atmosphere is usually parameterized by horizontal diffusion of horizontal momentum. This study proposes a formulation that is based on a symmetric stress tensor. The advantage over conventional methods is twofold. First, the Eulerian law of angular momentum conservation is fulfilled. Second, a self-consistent formulation of the momentum and thermodynamic equations of motion becomes possible due to incorporation of the local frictional heating rate, that is, the proper dissipation. The importance of these issues is demonstrated by numerical experiments performed with a simple general circulation model. For example, the new scheme precisely accounts for the irreversible increase of total potential energy during the decay of a baroclinic life cycle. Also the stress generated by horizontal momentum diffusion is found to be significant in the angular momentum budget of multiple life cycle experiments.

Wave–Mean Flow Interaction Associated with a QBO-like Oscillation Simulated in a Simplified GCM

Abstract The interaction between convectively excited waves and the mean zonal wind in the equatorial lower stratosphere is investigated with a simplified general circulation model (GCM). The model has T42 truncation, and the vertical resolution is about 700 m in the stratosphere. Although it is an “aquaplanet” model with uniform sea surface temperature, cumulus convection in low latitudes has realistic hierarchical structures with reasonable space–time spectral distributions. The model produced an oscillation having quite similar features to the equatorial quasi-biennial oscillation (QBO), although the period is 400 days. Waves in the equatorial lower stratosphere of the model are excited mainly by the cumulus convection in low latitudes. The energy of these waves is a little larger than that observed in the real atmosphere. The dominant waves are gravity waves having an equivalent depth of about 200 m and those of 40–100 m. About half of the transport and deposition of zonal momentum contributing to the...

The feedback of midlatitude waves onto the Hadley cell in a simple general circulation model

The feedback of midlatitude waves onto the Hadley cell in a simple general circulation model

The feedback of midlatitude waves onto the Hadley cell in a simple general circulation model

We present self-consistent comparisons of axissymmetric and 3-dimensional simulations of the tropospheric circulation under idealized physical conditions. These reveal a feedback of transient eddies onto the Hadley circulation which, first, strongly depends on equatorial heating and, second, is in case of realistic heating quite different from prescribed eddy forcing. A quantitative estimation for the eddy-induced mixing of vorticity into the poleward flow of the Hadley cell is given. The proposed relation is in accordance with observations. It is derived from the computational result that eddy momentum flux convergence is of the same order as the equatorward flux of relative vorticity generated by the Hadley circulation. Evaluation of the local budgets of sensible heat gives rise to a clear picture of how the poleward heat transports due to Hadley circulation and transient eddies interlock. This mechanism is found to be essential for an interpretation of the eddy feedback.

Martian atmospheric data assimilation with a simplified general circulation model: orbiter and lander networks

A meteorological data assimilation scheme for the martian atmosphere has been implemented and tested, based on techniques used in the current operational scheme for weather forecasting at the U.K. Meteorological Office. The scheme has been interfaced with a range of simple models and with the martian GCM currently under simultaneous development at Laboratoire de Météorologie Dynamique du CNRS in Paris and at Oxford. As well as the interpretation of data from any future spacecraft, the assimilation scheme may be used for comparisons between different models, for model validation using earlier martian data, and for data impact studies to assist in planning new missions. Despite proposed new missions to Mars, observations of the atmosphere of Mars in the near future are still likely to be very sparse compared to those of the Earth (perhaps comprising a single orbiter and a few surface stations at any one time) and the scheme has been adapted with this in mind. Twin model experiments are conducted in which simulated observations are generated from a second model started from different initial conditions. Such experiments reveal the importance of surface pressure measurements (in combination with an accurate topographic map, such as will be available from laser altimetry) in the determination of the amplitude of large-scale atmospheric waves. It is shown that atmospheric temperature profiles from a remote-sensing instrument on a polar orbiting satellite combined with simultaneous surface pressure observations at a limited number of sites, as planned for the InterMarsNet mission, is a useful scenario for data assimilation.

Interdecadal Variability in a Hybrid Coupled Ocean-Atmosphere Model

Abstract A hybrid coupled ocean–atmosphere model is used to investigate low-frequency variability in the climate system. The model's atmospheric component is a Budyko-Sellers-North, two-dimensional energy-balance model; the oceanic component is a simplified general circulation model. The coupled model is confined to an idealized, rectangular North Atlantic basin. In the present model version, the ocean density depends exclusively on temperature. An interdecadal oscillation with a period of 40–50 years is found in the hybrid coupled model when model parameters are within the climatological range, even though density does not depend on salinity. This interdecadal oscillation is characterized by a pair of vortices of opposite signs, that grow and decay in quadrature with each other in the ocean's upper layer; their centers follow each other anticlockwise through the northwestern quadrant of the model domain. The interdecadal oscillation's physical mechanism resembles that of the interdecadal oscillation anal...

Gravity wave drag in three‐dimensional atmospheric models of Mars

Gravity waves are disturbances whose restoring force is buoyancy. In the atmosphere they are made possible by the variation of potential temperature with height. They can be excited by various mechanisms including flow over orography. The momentum carried by these disturbances can exert significant drag forces on the mean flow in the terrestrial mesosphere. One parameterization of this effect, the Lindzen gravity wave drag scheme, is inserted into two simplified general circulation models (GCMs) of the Martian atmosphere, and its effects are described. The influence of large‐amplitude, planetary‐scale topography on wave breaking altitudes is discussed. We show that above 60 km in altitude, gravity wave drag dominates the momentum balance of the Martian atmosphere on the largest scales. This indirectly alters the thermal state of the atmosphere through the thermal wind relation. Model winds at this level are consistent with other, simpler studies, as well as with Earth‐based spectroscopic observations. Below 60 km, the effects of planetary wave‐mean flow interaction, as well as heating associated with Hadley circulation descent, are significant in determining the wind strength. Eddy diffusion coefficients derived from the Lindzen parameterization above 50 km are consistent with values implied from chemical modeling of the Martian middle atmosphere, as well as with simpler models of gravity wave drag.

Choked Flows from the Pacific to the Indian Ocean

Abstract The author considers two oceanic basins separated by a meridional wall. The wall contains a gap that is initially blocked by a gate; westward winds are allowed to blow over the two-layered oceans creating western boundary currents and a sea level difference between the basins. The conceptual gate is then removed and the resulting nonlinear flow is computed. The analytical calculations are based on a simple wind-driven general circulation model and a nonlinear integrated momentum constraint. Two classes of nonlinear solutions are constructed. One corresponds to a situation where the flow through the gap originates from the right-hand side (looking upstream) of the inner Pacific basin and the other to a situation where the flow originates from the left-hand side. It is suggested that the actual Indonesian Throughflow is composed of both of these classes of flows; that is, the throughflow corresponds to an exchange via two adjacent gaps. Computations suggest that approximately 6 Sv (Sv ≡ 106 m3 s−1)...

Low-Frequency Aspects of the Large-Scale Circulation and West Coast United States Temperature/Precipitation Fluctuations in a Simplified General Circulation Model

Abstract Behavior of regional precipitation and temperature over the West Coast of the United States was examined in a long perpetual winter simulation from a simplified global general circulation model. The model, a simplified version of the U.S. National Weather Service global operational forecast model, was run over a series of 568 winters, complete with geopotential, precipitation, and near-surface temperature. In spite of the fixed climatologica boundary conditions, the simulated winter-mean precipitation and temperature anomalies have a fairly realistic low-frequency regional variability. Both synoptic-scale events and seasonal average behavior are produced quite realistically by the model. Like observations, the regional surface variations can be related to the large-scale low-frequency circulation. Four regional temperature/precipitation extreme—namely, warm/dry, cool/wet, cool/dry, and warm/wet—can be identified from the simulated winter-mean time series over the West Coast. Associated with these...

The Influence of Tropical heating Displacements on the Extratropical Climate

The hypothesis is advanced that a latitudinal shift in the tropical convective heating pattern can significantly alter temperatures in the extratropics. Results of a simplified general circulation model (GCM) show that the shift of a prescribed tropical heating toward the summer pole, on time scales longer than a few weeks, leads to a more intense cross-equatorial 'winter' Hadley circulation, enhanced upper-level tropical easterlies, and a slightly stronger subtropical winter jet, accompanied by warming at the winter middle and high latitudes as a result of increased dynamical heating. The indications are that there is a robust connection between the net dynamic heating in the extratropics and the implied changes in the subtropical wind shear resulting from adjustments in the Hadley circulation associated with convective heating displacements in the tropics. The implications are that (1) the low-frequency temporal variability in the Hadley circulation may play an important role in modulating wave transport in the winter extratropics, (2) the global climate may be sensitive to those processes that control deep cumulus convection in the tropics, and (3) systematic temperature biases in GCMs may be reduced by improving the tropical rainfall simulation.

Dynamically Stratified Monte Carlo Forecasting

A new method for performing Monte Carlo forecasts is introduced. The method, called dynamic stratification, selects initial perturbations based on a stratification of the error distribution. A simple implementation is presented in which the error distribution used for the stratification is estimated from a linear model derived from a large ensemble of 12-h forecasts with the full dynamical model. The stratification thus obtained is used to choose a small subsample of initial states with which to perform the dynamical Monte Carlo forecasts. Several test cases are studied using a simple two-level general circulation model (GCM) with uncertain initial conditions. It is found that the method provides substantial reductions in the sampling error of the forecast mean and variance when compared to the more traditional approach of choosing the initial perturbations at random. The degree of improvement, however, is sensitive to the nature of the initial error distribution and to the base state. In practice the method may be viable only if the computational burden involved in obtaining an adequate estimate of the error distribution is shared with the data-assimilation procedure.

On the physics of droughts. II. Analysis and simulation of the interaction of atmospheric and hydrologic processes during droughts

In this study a simple general circulation model (GCM) is developed to investigate the interaction of atmospheric and hydrologic processes during droughts. Only the vertically and latitudinally averaged mean temperature and mean water vapor content over the strip of Earth between 30 and 50°N latitudes are considered as atmospheric state variables; only two hydrologic state variables, the water storage (through the water balance equation) and ground temperature, are considered for describing the behavior of the hydrologic system. The ocean temperature is specified as a function of season only. The coupling between the atmosphere and the Earth's surface occurs through exchanges of thermal energy (radiation, sensible and latent heat fluxes) and of water (evapotranspiration and rain), which are parameterized functions of the four state variables of the atmospheric-hydrologic interactive system. Using this GCM, the mechanism leading to drought conditions is investigated. It is shown to be a non-linear positive feedback mechanism in the sense that it feeds on itself. Imposing a temperature wave over a geographical region (western USA in this study) for a short period of the order of few months induces geophysical conditions in that region which tend to reduce the hydrologic water storage for a period of time of the order of years (called ‘drought growth period’). These conditions consist initially of increased cloudiness over the region, which reduces the net radiative flux of energy available at the surface. This in turn reduces the evapotranspiration which is needed to achieve condensation conditions in the atmosphere. Consequently, a reduction in rainfall occurs. After the end of the drought growth period, the hydrologic system recovers the water storage mainly owing to decreased evapotranspiration (related to reduced water storage) while rainfall resumes its normal levels. The time duration necessary to return to the climatological average conditions of water storage (called ‘drought recovery period’) is proportional to the peak drought severity during the growth period.