Rossby wave propagation in a barotropic atmosphere

The ideas of ray tracing from geometrical optics and wave propagation in a slowly varying medium are applied to Rossby waves propagating in a barotropic atmosphere.The propagation of low-frequency Rossby waves in a zonally symmetric basic state is compared with that for stationary waves presented by Hoskins and Karoly (1981). These ideas are then used to study the propagation of Rossby waves in a basic state with zonally varying middle latitude or low latitude jets. Conditions which allow cross-equatorial wave propagation are presented. For a zonally varying middle latitude jet, there is weak wave convergence in regions of decreasing jet speed, However, this is not sufficient to explain the enhanced wave amplitude found in numerical-model experiments using a zonally varying basic state.     

Meridional component of oceanic Rossby wave propagation

A three-dimensional spectral analysis of Topex altimeter data reveals a large meridional component k_y of the wavevector k for baroclinic Rossby waves of all timescales. Its existence necessitates some refinements in our estimates of certain basic properties of the Rossby wave field. In particular, by taking into account an actual off-zonal direction of k (often exceeding 70°), one finds that the wavelength, phase speed, and group velocity of mid-latitude Rossby waves (with periods less than 2 years) are much smaller than they appear to be on the assumption of a purely zonal wavenumber vector. Because of a shorter wavelength (yielding kL as high as 0.6, where L is the Rossby radius of deformation), these waves are essentially dispersive. Their group velocity vector may depart from zonal by more than 30°. An important intrinsic feature of the wave spectrum confirmed by our analysis is a broad-band distribution with respect to k_y. Some of the dynamical implications of the large k_y/k_x ratio are discussed.     

Atlantic watermass and circulation response to persistent freshwater forcing in two coupled general circulation models

The sensitivity of the Atlantic circulation and watermasses to biases in the convergence of moisture into the basin is examined in this study using two different general circulation models. For a persistent positive moisture flux into the tropical Atlantic, the average salinity and temperature in the basin is reduced, mainly below mid-depths and in high latitudes. A transient reduction in the Atlantic overturning strength occurs in this case, with a recovery timescale of 1–2 centuries. In contrast, a similar amount of freshwater directed into the Subpolar North Atlantic results in a persistent reduction in overturning and an increase in basin heat and salt content. In the unperturbed pre-industrial simulations, the Atlantic is unambiguously warmer and saltier than historical observations below mid-depths and in the Nordic Seas. The models’ tropical freshwater flux sensitivities project strongly onto the spatial pattern of this bias, suggesting a common atmospheric deficiency. The integrated Atlantic plus Arctic surface freshwater flux in these models is between ?0.5 and ?0.6 Sv, compared with an observational estimate of ?0.28 Sv. Our results suggest that shortcomings in the models’ ability to reproduce realistic bulk watermass properties are due to an overestimation of the inter-basin moisture export from the tropical Atlantic.     

Stationary Rossby wave propagation in a shear flow along a reflective boundary

Summary This study investigates the impact of lateral boundary conditions on the propagation and dispersion of locally excited Rossby waves in a zonally periodic, barotropic, quasigeostrophic channel model on the β-plane. We use basic flows with either a linear meridional shear or a jet-like profile. On the southern boundary of the channel we impose either a rigid wall or a radiation condition, whereas the northern sidewall is permeable for Rossby waves. We compare the numerical solutions found for a reflecting southern boundary in a weakly dissipative flow to the solutions obtained from a WKB-analysis for the corresponding unforced nondissipative situation. Furthermore, we compare the generalized Eliassen-Palm flux vectors to the ray paths of Rossby wave packets, obtained from WKB ray tracing. In particular, we focus our investigation on the two-dimensional structure of trapped modal waves and wavetrains in a simple linear numerical model. Summarizing our results, we find that along the reflective wall, trapped modal wave structures as well as reflected wavetrains occur with characteristics (e.g., wavenumbers, turning latitudes) similar to the ones computed using asymptotic methods. In a linear sheared flow wave packets are trapped for all zonal wave numbers in contrast to a jet-like mean flow which has a selective effect on the waves; i.e., a turning latitudes phenomenon between the coast and the flow maximum occurs for short waves, while long waves can propagate freely across the zonal mean flow. This comes out clearly when studying the stream lines of the Eliassen-Palm flux vectors of the numerical model simulations. Furthermore, due to the reflected wave activity, the dispersion of Rossby waves is influenced by the southern boundary condition not only in the vicinity of the border but also in regions away from the boundary. These results appear to be important on the one hand for the existence of trapped Rossby waves in large-scale oceanic shear flows along a zonally oriented coast. And, on the other hand for large-scale boundary waves in conceptional atmospheric channel models which can lead to unwanted resonance effects. Received July 18, 2000/Revised June 9, 2001     

Predictability of SST forced climate signals in two atmospheric general circulation models

 The predictability of atmospheric responses to global sea surface temperature (SST) anomalies is evaluated using ensemble simulations of two general circulation models (GCMs): the GENESIS version 1.5 (GEN) and the ECMWF cycle 36 (ECM). The integrations incorporate observed SST variations but start from different initial land and atmospheric states. Five GEN 1980–1992 and six ECM 1980–1988 realizations are compared with observations to distinguish predictable SST forced climate signals from internal variability. To facilitate the study, correlation analysis and significance evaluation techniques are developed on the basis of time series permutations. It is found that the annual mean global area with realistic signals is variable dependent and ranges from 3 to 20% in GEN and 6 to 28% in ECM. More than 95% of these signal areas occur between 35 °S–35 °N. Due to the existence of model biases, robust responses, which are independent of initial condition, are identified over broader areas. Both GCMs demonstrate that the sensitivity to initial conditions decreases and the predictability of SST forced responses increases, in order, from 850 hPa zonal wind, outgoing longwave radiation, 200 hPa zonal wind, sea-level pressure to 500 hPa height. The predictable signals are concentrated in the tropical and subtropical Pacific Ocean and are identified with typical El Ni?o/ Southern Oscillation phenomena that occur in response to SST and diabatic heating anomalies over the equatorial central Pacific. ECM is less sensitive to initial conditions and better predicts SST forced climate changes. This results from (1) a more realistic basic climatology, especially of the upper-level wind circulation, that produces more realistic interactions between the mean flow, stationary waves and tropical forcing; (2) a more vigorous hydrologic cycle that amplifies the tropical forcing signals, which can exceed internal variability and be more efficiently transported from the forcing region. Differences between the models and observations are identified. For GEN during El Ni?o, the convection does not carry energy to a sufficiently high altitude, while the spread of the tropospheric warming along the equator is slower and the anomaly magnitude smaller than observed. This impacts model ability to simulate realistic responses over Eurasia and the Indian Ocean. Similar biases exist in the ECM responses. In addition, the relationships between upper and lower tropospheric wind responses to SST forcing are not well reproduced by either model. The identification of these model biases leads to the conclusion that improvements in convective heat and momentum transport parametrizations and basic climate simulations could substantially increase predictive skill. Received: 25 April 1996 / Accepted: 9 December 1996     

Teleconnection patterns and Rossby wave propagation associated to generalized frosts over southern South America

Based on previous observational studies of the mean atmospheric circulation leading to generalized frosts (GF) in central Southern South America, it is possible to establish a hypothesis that specific large scale patterns are associated to the frequency of occurrence of these events through the propagation of Rossby waves remotely excited. This hypothesis is tested here through a teleconnection analysis for austral winters which present an extreme frequency of occurrence of GF in southeastern South America, particularly over the Wet Pampa area in Argentina. Rossby wave propagation regions are identified for two basic states given by the composition of winters with maximum and minimum frequency of GF occurrence, during the 1961–1990 period. The stationary wavenumber K _s indicates the regions where the Rossby wave propagation is permitted and those where it will be inhibited (K _s = 0), highlighting the importance of the jets as waveguides. Nevertheless, differences exist between both basic states analyzed. These differences indicate that the locations for wave generation and its later evolution are conditioned by the basic state. Results are validated through a baroclinic model, which simulates the Rossby wave patterns responsible for the teleconnection. Numerical experiments confirm that the principal wave activity takes place inside the subtropical and polar jets. In particular, for the basic state with maximum frequency of GF occurrence, the wave trains propagating inside the subtropical and polar waveguides merge just before entering the continent, as shown by the observations prior to the occurrence of GF events. This configuration favors the development of an intense south wind anomaly with large meridional extension which results in the intensification of anticyclonic circulation in southern South America. A conceptual model is presented to summarise all these results.     

Latitudinal dependence of signal-to-noise patterns from two general circulation models with CO2 forcing

 Detection of an enhanced greenhouse effect on climate depends on recognition of a signal of change amidst the combined noise of climatic variability and uncertainty in the nature of the signal (functional response to changing CO₂). Using two different GCMs (one with a coupled dynamic upper ocean) and an ensemble of 20 equilibrium experiments with CO₂ ranging from 100 to 3500 ppm, we find that that two measures of signal-to-noise (S/N) for the response of surface temperature to CO₂ forcing are larger over tropical and subtropical oceans than over low-latitude landmasses and larger than at higher latitudes generally. One S/N measure has the noise based solely on inherent model variability, while the other S/N measure includes both this variability and a measure of the uncertainty in the functional nature of the signal. Although the experiments were not for transient forcing and sulphate aerosols and other potentially important forcings (e.g., ozone or solar variability) were not considered, the results suggest that the effects of enhanced greenhouse climate may be detected more readily in surface temperatures from low-latitude oceanic regions than from global or zonal temperature averages. Received: 27 June 1995/Accepted: 28 October 1996     

The seasonal cycle in coupled ocean-atmosphere general circulation models

We examine the seasonal cycle of near-surface air temperature simulated by 17 coupled ocean-atmosphere general circulation models participating in the Coupled Model Intercomparison Project (CMIP). Nine of the models use ad hoc “flux adjustment” at the ocean surface to bring model simulations close to observations of the present-day climate. We group flux-adjusted and non-flux-adjusted models separately and examine the behavior of each class. When averaged over all of the flux-adjusted model simulations, near-surface air temperature falls within 2?K of observed values over the oceans. The corresponding average over non-flux-adjusted models shows errors up to ～6?K in extensive ocean areas. Flux adjustments are not directly applied over land, and near-surface land temperature errors are substantial in the average over flux-adjusted models, which systematically underestimates (by ～5?K) temperature in areas of elevated terrain. The corresponding average over non-flux-adjusted models forms a similar error pattern (with somewhat increased amplitude) over land. We use the temperature difference between July and January to measure seasonal cycle amplitude. Zonal means of this quantity from the individual flux-adjusted models form a fairly tight cluster (all within ～30% of the mean) centered on the observed values. The non-flux-adjusted models perform nearly as well at most latitudes. In Southern Ocean mid-latitudes, however, the non-flux-adjusted models overestimate the magnitude of January-minus-July temperature differences by ～5?K due to an overestimate of summer (January) near-surface temperature. This error is common to five of the eight non-flux-adjusted models. Also, over Northern Hemisphere mid-latitude land areas, zonal mean differences between July and January temperatures simulated by the non-flux-adjusted models show a greater spread (positive and negative) about observed values than results from the flux-adjusted models. Elsewhere, differences between the two classes of models are less obvious. At no latitude is the zonal mean difference between averages over the two classes of models greater than the standard deviation over models. The ability of coupled GCMs to simulate a reasonable seasonal cycle is a necessary condition for confidence in their prediction of long-term climatic changes (such as global warming), but it is not a sufficient condition unless the seasonal cycle and long-term changes involve similar climatic processes. To test this possible connection, we compare seasonal cycle amplitude with equilibrium warming under doubled atmospheric carbon dioxide for the models in our data base. A small but positive correlation exists between these two quantities. This result is predicted by a simple conceptual model of the climate system, and it is consistent with other modeling experience, which indicates that the seasonal cycle depends only weakly on climate sensitivity.     

Tropical precipitation regimes and mechanisms of regime transitions: contrasting two aquaplanet general circulation models

The atmospheric general circulation models ARPEGE-climate and LMDz are used in an aquaplanet configuration to study the response of a zonally symmetric atmosphere to a range of sea surface temperature (SST) forcing. We impose zonally-symmetric SST distributions that are also symmetric about the equator, with varying off-equatorial SST gradients. In both models, we obtain the characteristic inter-tropical convergence zone (ITCZ) splitting that separates two regimes of equilibrium (in terms of precipitations): one with one ITCZ over the equator for large SST gradients in the tropics, and one with a double ITCZ for small tropical SST gradients. Transition between these regimes is mainly driven by changes in the low-level convergence that are forced by the SST gradients. Model-dependent, dry and moist feedbacks intervene to reinforce or weaken the effect of the SST forcing. In ARPEGE, dry advective processes reinforce the SST forcing, while a competition between sensible heat flux and convective cooling provides a complex feedback on the SST forcing in the LMDz. It is suggested that these feedbacks influence the location of the transition in the parameter range.     

Development of global coupled ocean-atmosphere general circulation models

It has long been believed that a climate model capable of realistically simulating many features of global climate, variability, and climate change must interactively represent the major components of the dynamically coupled climate system, particularly the atmosphere, ocean, and cryosphere. This effort traditionally has been constrained by computing power, our understanding of the observed system, and climate modeling capability. With the advent of supercomputers, improved understanding of global climate processes, and computationally efficient general circulation climate models, we have witnessed a rapid increase in the simulation of global climate by coupling together various representations of atmosphere, ocean, and sea ice. Beginning in the late 1960s and continuing through the early 1980s, general circulation models (GCMs) of the atmosphere, ocean, and sea ice were coupled and run asynchronously to produce credible simulations of the global climate. Systematic errors in these component models later led some modeling groups to use flux correction or flux adjustment, whereby either one or several of the variables at the air-sea interface are adjusted to bring the simulations in closer agreement with observations. Further advances in computing power and climate modeling techniques in the past few years have allowed global coupled ocean-atmosphere GCMs to be run synchronously (i.e., atmosphere and ocean communicate at least once each model day). Computing constraints, combined with the need for multidecadal climate integrations, still only allow relatively coarse-grid ocean GCMs to be coupled to correspondingly coarse-grid atmospheric models (on the order of 500 km × 500 km). However, results from this current generation of global, coupled GCMs have revealed interesting characteristics associated with ocean dynamics and global climate in experiments with gradual increases of carbon dioxide. Another somewhat surprising aspect of the global-coupled GCM simulations is the appearance of some features associated with the El Niño-Southern Oscillation. Along with concurrent efforts with other types of limited-domain, dynamical coupled models, this has led to the realization that inherent unstable coupled modes exist in the climate system that are the unique product of the interactive coupling of the atmosphere and the ocean. All of these efforts are leading to the next generation of coupled ocean-atmosphere GCMs. These models will run on even faster and larger-memory computers and will have higher-resolution atmosphere and ocean components, more accurate sea-ice formulations, improved cloud-radiation schemes, and increasingly realistic land-surface processes.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. DümenilThe National Center for Atmospheric Research is sponsored by the National Science Foundation     

On the direction of Rossby wave breaking in blocking

The direction of Rossby wave breaking at the onset of large-scale atmospheric blocking events is shown to relate closely to its position relative to the location of the climatological storm tracks. Using ERA-Interim reanalysis data from October 1989 to March 2009 and a dynamically-based blocking index, Rossby wave breaking is shown to occur preferentially cyclonically to the north, and anticyclonically to the south of the average storm tracks location. Therefore the results support existing theory on the relation between Rossby wave breaking direction and barotropic shear of the background wind. The further away from the storm tracks the breaking occurs, the stronger this preference in breaking direction. Regional differences can also be explained. For the European region on average 70?% of the detected blocking took place after anticyclonic Rossby wave breaking event that occurred on average 6° south of the climatological storm tracks position. Over Western Pacific wave breaking prior to blocking occurs predominantly cyclonically and on average 6° north of the storm tracks. Differences in blocking duration and intensity are found to be within estimated error margins at most longitudes, except for the Atlantic-Europe sector where the blocking events following anticyclonic blocking are also the strongest.     

Global Coupled Ocean-Atmosphere General Circulation Models in LASG／IAP

Coupled ocean-atmospheric general circulation models are the only tools to quantitatively simulate the climate system. Since the end of the 1980s, a group of scientists in the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences (CAS), have been working to develop a global OGCM and a global coupled ocean-atmosphere general circulation model (CGCM). From the original flux anomalycoupling model developed in the beginning of the 1990s to the latest directly-coupling model, LASG scientists have developed four global coupled GCMs. This study summarizes the development history of these models and describes the third and fourth coupled GCMs and selected applications. Strengths and weaknesses of these models are highlighted.     

On the origin of equatorial Atlantic biases in coupled general circulation models

Many coupled ocean–atmosphere general circulation models (GCMs) suffer serious biases in the tropical Atlantic including a southward shift of the intertropical convergence zone (ITCZ) in the annual mean, a westerly bias in equatorial surface winds, and a failure to reproduce the eastern equatorial cold tongue in boreal summer. The present study examines an ensemble of coupled GCMs and their uncoupled atmospheric component to identify common sources of error. It is found that the westerly wind bias also exists in the atmospheric GCMs forced with observed sea surface temperature, but only in boreal spring. During this time sea-level pressure is anomalously high (low) in the western (eastern) equatorial Atlantic, which appears to be related to deficient (excessive) precipitation over tropical South America (Africa). In coupled simulations, this westerly bias leads to a deepening of the thermocline in the east, which prevents the equatorial cold tongue from developing in boreal summer. Thus reducing atmospheric model errors during boreal spring may lead to improved coupled simulations of tropical Atlantic climate.     

Simple general atmospheric circulation and climate models with memory

This article examines some general atmospheric circulation and climate models in the context of the notion of “memory”. Two kinds of memories are defined： statistical memory and deterministic memory. The former is defined through the autocorrelation characteristic of the process if it is random （chaotic）, while for the latter, a special memory function is introduced. Three of the numerous existing models are selected as examples. For each of the models, asymptotic （at t →∞） expressions are derived. In this way, the transients are filtered out and that which remains concerns the final behaviour of the models.     

The deep water stratification of ocean general circulation models

Abstract

A central problem in climate and ocean modelling is the accurate simulation of the climatological state of the oceanic density field. A constant vertical diffusivity for heat and salt is frequently employed in ocean general circulation models (OGCMs) and it is usually assigned a value designed to optimize the depth of the pycnocline. One undesired consequence of this choice is a poor representation of the deep water, which is usually insufficiently stratified. In contrast to the uniform diffusivity of many models, some observational studies suggest that the vertical diffusivity is not constant but increases with depth, possibly in inverse proportion to the local buoyancy frequency. Numerical experiments with an OGCM are presented that demonstrate that allowing the vertical diffusivity to increase below the pycnocline substantially increases the stratification of the abyssal water mass of these models without significantly affecting the pycnocline depth, and hence may lead to a better representation of the vertical density structure.     

Stability dependent parameterisations of vertical mixing in ocean general circulation models

Summary Parameterisations of mixing induced through shear instability, internal wave breaking, and double diffusion are investigated in simulations of ocean climate using a global ocean general circulation model (OGCM). Focus is placed on the sensitivity of the large scale circulation, water mass formation and transport of heat as measures of the model's ability to represent current climate. The model resolution is typical of OGCMs being coupled to atmospheric. GCMs in climate models and the parameterisations investigated are all computationally inexpensive enough to allow for integrations on long time scales. Under the assumption of constant vertical eddy coefficients (the control case), the model climatology displays acceptable values of North Atlantic Deep Water formation, Antarctic Circumpolar Current (ACC) transport, and Indonesian through-flow but an excessively deep and diffuse pycnocline structure with weak stratification in the deep ocean. It is found that various circulation and water mass properties are sensitive to the choice of parameterisation of vertical mixing and that determining a scheme which works satisfactorily over all regions (tropical, mid-latitude, and polar) of the domain is not straightforward. Parameterisations of internal wave breaking or upper ocean shear instability lead to some improvements in the model water mass formation. ACC and poleward heat transport when compared to the control case whereas parameterisations of double diffusive processes did not. Based on these and other results, various recommendations are made for mixing parameterisations in ocean climate models.With 8 Figures