Abstract: The ocean is an important sink for anthropogenic CO 2 emissions, but there are only a few measurements which confirm the oceanic CO 2 uptake. Since 1981, partial pressure of CO 2 (pCO 2 ) in the western North Pacific (35°N−3°N, 128°E−155°E) and the overlying air have been measured periodically to clarify the seasonal and long-term trends of the oceanic carbonate system. The partial pressure of CO 2 in surface seawater (pCO 2 sea ) observed every boreal winter during the period from 1984 to 1993 give a growth rate of 1.8 ± 0.6 µ atm yr -1 ( n = 27) north of 15°N and 0.5 ± 0.7 µ atm yr -1 ( n = 23) south of 14°N with an average of 1.2 ± 0.9 µ atm yr -1 ( n = 50). The rate of pCO 2 sea a increase north of 15°N is equal to that of atmospheric CO 2 (1.8 µ atm yr -1 ) during the same period but that south of 14°N is lower. The difference in rate of pCO 2 sea a increase is suggestive of temporal variations in ΔpCO 2 distribution. After removing the long-term trend from the pCO 2 sea data, the seasonal variation of pCO 2 sea in the western North Pacific (132°E−142°E) was evaluated with a linear regression between the pCO 2 sea and sea surface temperature (SST). Generally, a thermodynamic process (temperature effect) plays a predominant role in determining the seasonal variations of pCO 2 sea . South of 14°N, however, a clear interannual variability is significant relative to the seasonal changes if an El Nino event is accompanied by enhanced vertical mixing. The annual air-sea CO 2 flux showed a large influx of CO 2 into the ocean north of 27°N (Kuroshio Counter Current) because of a large negative ΔpCO 2 (− 60 µ atm) and strong wind during the winter season. Toward the south, the annual average air-sea CO 2 flux increased by 9 mmol m -2 day -1 from – 8 mmol m -2 day -1 at 31°N to 1 mmol M -2 day -1 at 5°N. South of 10°N, the ocean acts as a source for atmospheric CO 2 (0.2-0.7 mmol m -2 day -1 ), but this is a considerably weaker source as compared with those of the central and eastern equatorial Pacific. The observed increase of pCO 2 sea and the estimated air/sea CO 2 flux suggest the importance of carbon transport from the mixed layer to the intermediate/deep water in the area of Subtropical Mode Water formation, south of the Kuroshio and east of Japan. DOI: 10.1034/j.1600-0889.47.issue4.2.x

Abstract: A decadal oscillation of the geopotential heights in the lower stratosphere, which is well-correlated with the 11-year solar cycle, is closely linked to changes of the temperature on the same time scale in the middle and upper troposphere, the temperature being higher on an average in the maxima than in the minima of the 11-year solar cycle. It appears likely that these changes in the tropospheric temperature are associated with changes in the Hadley circulation. It is well established that the decadal oscillation in the stratosphere is modulated by the Quasi-Biennial Oscillation in winter. We point out that the QBO also modulates the decadal oscillation at other times of the year, but that the effect of the modulation is then weaker because of the different wind regime in the stratosphere. DOI: 10.1034/j.1600-0870.1995.t01-1-00008.x

Abstract: A three-dimensional numerical ocean model was set up for a domain covering the Nordic Seas to produce dynamically consistent climatological archives of sea level, current, salinity and temperature. The flow relaxation scheme (FRS) was applied as open boundary condition. As a first step the model was run in diagnostic mode until a stationary solution was obtained. The only forcing was provided by gridded climatological fields of salinity and temperature. Compared with available data, this model set up produced a satisfactory circulation in the Nordic Seas. The results were stored in The Diagnostic Archive . In the next step, the obtained stationary solutions were used as boundary values in prognostic simulations to produce The Prognostic Archive . This paper describes a method for handling open boundary conditions in diagnostic simulations. The method requires knowledge of the depth mean current. A rather artificial condition of zero depth mean current or transport was chosen. Because the FRS does not conserve mass or momentum, it is able to redistribute the specified zero transports to nonzero in a realistic way through the relaxation zones. However, if the open boundaries are situated in dynamical active areas, in which contour lines of f/H enter or leave the model domain, the production of nonzero transports in the FRS zones is not sufficient to provide satisfactory boundary conditions. Instead, if already computed sea level and currents from a simulation with a larger model domain are used as external solutions at the open boundaries, the model gives excellent results. To illustrate the performance of the FRS when the temperature and salinity fields also are allowed to evolve, i.e., prognostic simulation, an example is shown from The Prognostic Archive . Here, boundary values were extracted from The Diagnostic Archive. DOI: 10.1034/j.1600-0870.1995.t01-2-00006.x

Abstract: A number of experiments investigating four-dimensional variational data assimilation using the adjoint method are presented. It has been proposed that the method will be able to produce improved initial conditions in data-sparse regions. In order to describe the flow in a region where there is little data, it is necessary for observational information either to be advected into the region, or to cascade downscale from larger scales characterizing the separation between observations. We focus on the latter and examine the method's ability to “fill in” small-scale detail determined dynamically from large-scale data. We choose to examine barotropic β-plane flow since it is one of the simplest geophysical settings involving a wide range of scales. In the limit of small error, predictability studies have shown that exponential error growth occurs along the gradients of two highly-correlated realizations. When the realizations have decorrelated, error statistics saturate at climatological levels. By appealing to the adjoint of the linearized equations, the adjoint method accounts for the former behaviour, but not the latter. When the assimilation period exceeds the validity timescale of the linearization, the assimilated fields show spectra which are spuriously shallow in the small scales, following the basic-state gradients. Moreover, it is essential to note that the validity timescale of the linearization is a function of lengthscale. Therefore, for a given assimilation period there is a scale below which useful initial conditions cannot be obtained. Equivalently, for a given model resolution, there is an assimilation period beyond which the exact initial conditions cannot be recovered. Some speculation on the optimal resolution at which to. perform 4D data assimilation as a function of the assimilation period is offered. DOI: 10.1034/j.1600-0870.1995.00204.x

Abstract: The dynamical behavior of the climate system is investigated through the use of a low-order coupled atmosphere-ocean general circulation model. The goal is to gain some qualitative understanding of how non–linear interactions between the individual system components may affect the climate. Both the atmosphere and ocean models are fully dynamic: the former is defined by 3 ordinary differential equations derived from a truncated Fourier series expansion of the mean and perturbation components of the quasi-geostrophic potential vorticity equation, while the latter is specified by 6 ordinary differential equations representing the time-dependent variations of ocean temperature and salinity in a 3-box model of the North Atlantic. Despite the existence of 2 basic equilibrium ocean model responses to perpetual atmospheric conditions, equilibrium states are never attained in the coupled system within 10000 years of integration; the deep ocean flow continually adjusts to the atmospheric regime changes associated with particular ocean circulations, which leads to new circulations and new atmospheric regimes. Low-frequency quasi-periodic oscillations about a single state of the thermohaline circulation result from an advective-diffusive process, modulated by the correlation of the atmospheric behavior with the phase of the ocean cycle. The climate is strongly effected by interactions with the ocean, leading to distinct atmospheric patterns for different phases in the oscillations, and a conversion of some of the high-frequency atmospheric signal to lower frequencies. This conversion also results in a measurable ocean response at high frequencies. Furthermore, owing to the richness of the atmospheric response to small modifications in the meridional and zonal gradients in diabatic heating, even modest adjustments in the ocean circulation resulting from interactions with the high-frequency atmospheric component can also lead to climate change over relatively short time periods. The results of the model are applied to recent deductions of climate variability in the North Atlantic, obtained from Greenland ice-cores. DOI: 10.1034/j.1600-0870.1995.t01-3-00006.x

Abstract: The impact of wind-generated ocean waves upon the structure and evolution of an idealized cyclone is investigated. High-resolution numerical experiments were performed with a constant surface roughness, roughness length from Charnock's formula, and from a coupled ocean-wave/atmosphere mesoscale model. Results indicate that young ocean waves increase the effective surface roughness, increase the surface stress, significantly decrease the 10-m wind speeds and modulate the heat and moisture transport between the atmosphere and ocean. These effects are maximized along the warm front and to the rear of the cyclone near the southwestern quadrant. As a result of this boundary layer modification, the mesoscale structures associated with the cyclone are perturbed. Regions of young windsea act to locally enhance the low-level frontogenesis, convergence and rainfall. Roughness effects associated with ocean waves modulate the deepening rate during rapid cyclogenesis and enhance the cyclone filling process. Kinetic energy of the entire cyclone system is reduced significantly near the surface and by 3–8% above the boundary layer as a result of enhanced roughness associated with young ocean waves. DOI: 10.1034/j.1600-0870.1995.00119.x

Abstract: This paper introduces a technique to identify and track midtropospheric mobile troughs. Mobile troughs are objectively identified as maxima in the transport of geostrophic curvature vorticity. A 20-year climatology of mobile trough genesis and termination regions is constructed by applying the method to National Meteorological Center 500 mb geopotential height data. The objective tracking results are compared to Sanders' (1988) climatology of mobile troughs on the 552 dam contour. The objective method was shown to be more consistent and accurate than Sanders' subjective method, and therefore, determined to be superior in tracking mid-tropospheric mobile troughs. The mean lifespan of troughs over the 20-year dataset was 5.3 days and the maximum lifespan was 44 days. The geographical shape of the trough genesis and termination regions are associated with the orography. The three preferred genesis regions are north-central North America, the Mediterranean Sea, and southeast Asia. The three preferred termination regions are the extreme eastern Pacific Ocean, the Mediterranean Sea, and southwest Asia. Trough genesis regions tend to be well upstream of major areas of wintertime surface cyclogenesis. DOI: 10.1034/j.1600-0870.1995.00110.x

Abstract: A major problem which is envisaged in the course of man-made climate change is sea-level rise. The global aspect of the thermal expansion of the sea water likely is reasonably well simulated by present day climate models; the variation of sea level, due to variations of the regional atmospheric forcing and of the large-scale oceanic circulation, is not adequately simulated by a global climate model because of insufficient spatial resolution. A method to infer the coastal aspects of sea level change is to use a statistical “downscaling” strategy: a linear statistical model is built upon a multi-year data set of local sea level data and of large-scale oceanic and/or atmospheric data such as sea-surface temperature or sea-level air-pressure. We apply this idea to sea level along the Japanese coast. The sea level is related to regional and North Pacific sea-surface temperature and sea-level air pressure. Two relevant processes are identified. One process is the local wind set-up of water due to regional low-frequency wind anomalies; the other is a planetary scale atmosphere-ocean interaction which takes place in the eastern North Pacific. DOI: 10.1034/j.1600-0870.1995.00008.x

Abstract: The ability of a digital filter technique to control high-frequency gravity wave noise in numerical weather forecasts based on the primitive equations is examined in the context of a global data assimilation system. The method uses a 12-h forward integration of the complete model to generate a time series that is filtered to give a balanced model state valid 6 h into the integration. This state is free of high-frequency noise and serves as a background field for the next analysis. The technique is referred to as digital filter finalization. The technique is first applied to a long model run in order to identify the impact of the chosen cutoff period in the design of the filter on a properly balanced model state. The robustness of the technique to typical imbalances between the mass and wind fields produced by an operational statistical interpolation procedure is also examined. Results of data assimilation experiments performed with the digital filter finalization and with the currently-operational adiabatic nonlinear normal mode initialization scheme are compared. The digital filter finalization technique examined here is shown to be an accurate, consistent and very simple way to remove the undesirable high-frequency noise from a global model forecast. DOI: 10.1034/j.1600-0870.1995.t01-2-00002.x

Abstract: A theoretical and modelling study is undertaken of an airstream of uniform flow ( U ) and stratification ( N ) impinging normally upon a two-dimensional bell-shaped ridge of half-width ( L ) and height ( H ) that is located on an f -plane. The associated Rossby number ( R o = U/fL ) and inverse Froude Number ( F = NH/U ) are taken to be such that the effects of both the earth's rotation and non-linear processes can significantly influence the nature of the flow response. It is shown that within this intermediate meso-α/ β scale sub-domain of ( R o , F ) parameter space there are several distinctive signatures to the flow response, and each signature is associated with the generation of appreciable buoyancy wave energy. As R o decreases across the sub-domain, the source for this wave energy changes continuously from the con-ventional direct orographic forcing of vertically propagating waves (for R o > 1), to that linked with the cusping of a leeward train of low-level near-inertial waves (for R o ∼ 1), and thereafter to the forcing induced by the adjustment that accompanies the scale contraction of balanced flow (for R o < 1). In effect this change highlights the various pathways to regime transition within the intermediate domain. DOI: 10.1034/j.1600-0870.1995.t01-2-00008.x

Abstract: Profiles of the power scattered at vertical incidence by an MST radar have been compared with temperature profiles measured by radiosondes launched nearby. The results are consistent with a standard scattering model for low and medium scattered power, but deviate from the model at high power. Tropopause heights have been derived from the radar data by two methods: the maximum power and maximum power gradient. The two criteria appear to be equally effective as predictors of the tropopause, despite the theoretical expectation that the latter should be superior. Radiosonde tropopauses which were highly indefinite (no marked change in lapse rate between troposphere and stratosphere) were associated with a shallower minimum than normal in the radar power profile in the upper troposphere. This ability to detect highly indefinite tropopauses shows that an MST radar may be used to monitor continuously the structure of the thermal tropopause. DOI: 10.1034/j.1600-0870.1995.00118.x

Abstract: There is considerable evidence that many extra-tropical cyclones are generated by the inter-action between an upper-level potential vorticity anomaly and a low-level baroclinic zone. A variety of satellite data and the ECMWF high-resolution objectively-analyzed fields are used to investigate the origin of these tropopause features and their role in cyclogenesis for a remarkable rapidly developing cyclone that developed in the Labrador Sea on 1 and 2 March 1992. According to the ECMWF analysis, this storm deepened by 42 mb in 24 h, attaining a central pressure of less than 935 mb on 2 March The use of satellite column ozone data, from the TOMS and TOVS instruments is emphasized for the identification of mesoscale tropopause features implicated in cyclogenesis. Both data sets are able to resolve the intrusions of ozone-rich stratospheric air associated with this storm. DOI: 10.1034/j.1600-0870.1995.00124.x

Abstract: A symbiotic relationship between the high- and low-frequency transients in the Southern Hemisphere has been examined by using ECMWF analyses of 500 mb geopotential height and temperature fields over 9 winter seasons. The low-frequency transients organize the high-frequency eddies over the regions where low-frequency westerly anomalies prevail, forming traveling storm tracks that move together with the low-frequency waves. The high-frequency eddies, in turn, reinforce the barotropic component of the low-frequency waves while tending to diminish the baroclinic component of the low-frequency waves. All of these findings give further credence to earlier results for the Northern Hemisphere. As a supplementary to the feedback analyses, the energetics analysis for the winter seasons in both the Northern and the Southern Hemispheres reveals that the forcing of the low-frequency flow by the high-frequency transients in the Southern Hemisphere is much stronger than in the Northern Hemisphere. The contribution of the stationary waves to maintenance of the low-frequency flow through barotropic instability is significantly less in the Southern Hemisphere. DOI: 10.1034/j.1600-0870.1995.t01-2-00004.x

Abstract: A new approach based on the travelling salesman problem and simulated annealing is used to classify weather maps and to associate them with weather regimes. The usual classification methods are able to identify the preferred patterns of large-scale atmospheric flow. They fail, however, in characterizing the boundaries of various regimes which are generally a result of the geometrical constraints imposed by the chosen method. The method is aimed at overcoming this difficulty and helping to define the boundaries between weather regimes in order to study the characteristics of and transitions between these regimes. Weather regimes are derived from a series of 37 winters of 700 hPa geopotential height observations using the algorithm developed by Vautard (1990). The new approach is then applied to assign each observation to its corresponding weather regime. It is shown that the method provides results which agree with the literature and helps to establish recurrence in agreement with observations. We then, speculate on the relationship which may link the boundaries of weather regimes to the transitions between these regimes. DOI: 10.1034/j.1600-0870.1995.00203.x

Abstract: While the formation of upper-level mobile troughs has received little observational attention, a variety of theories have been proposed to explain the genesis and intensification of mobile troughs. For purposes of intercomparison and qualitative application to observed cases, these theories are described in terms of the generation of waves along the tropopause potential vorticity discontinuity. These models are then tested on a case of mobile trough formation which occurred on 19–20 April 1991 and which was poorly forecasted by operational models. The ridge-trough system is found to have been produced primarily by Rossby wave propagation along the tropopause from a newly-developed cutoff cyclone located to the west. Both vertical and horizontal propagation contributed to the development. Another mechanism which appeared to have a positive effect was superposition due to vertical shear and horizontal deformation. A vorticity maximum within the newly-formed trough was amplified and advected southeastward by the developing larger-scale ridge/trough system and later participated in a surface cyclogenesis event. DOI: 10.1034/j.1600-0870.1995.00114.x

Abstract: In the first study of a series of studies of intense cyclones in the active region around Iceland, the explosive synoptic-scale “Greenhouse Low” that hit Iceland on 3 February 1991 is studied. We have investigated this cyclone mainly through numerical simulations with an NWP model (HIRLAM), but also by careful reanalysis of surface observations and satellite pictures. The cyclone formed in an unusually baroclinic air mass just north of the Azores in the early hours of 2 February 1991. A baroclinic wave formed and developed, aided by latent heat release. During this period, we have found evidence of symmetric instability in a deep layer. Between 18 UTC 2 February and 06 UTC 3 February, the central pressure fell by more than 30 hPa, as the cyclone deepening continued. Over the next 6 h, a further deepening of 15 hPa occurred, as an upper level potential vorticity anomaly caught up with and reinforced the cyclone. At this time, hurricane-force winds were observed at numerous sites in the southern and western parts of Iceland, with mean winds up to 57 m s−1. As the cyclone continued its northward movement, a pressure rise of 30.4 hPa in 3 h was recorded at one station. Reruns with the HIRLAM model, using ECMWF analyses from 12 UTC 2 February as initial conditions, captured all the main features of the observed development, helping us to draw the above picture. Corresponding runs from 00 and 06 UTC had considerable errors in both positioning and deepening of the storm. The sensitivity of the simulations to the removal of surface energy fluxes and latent heat of condensation was investigated separately. It was found that over the time span of the simulation, surface energy fluxes had negligible impact on the explosive development. The contribution of latent heat release to the development appears to be about half that of dry baroclinic instability. These findings are discussed in view of related studies in the literature. DOI: 10.1034/j.1600-0870.1995.00111.x

Abstract: The National Oceanic and Atmospheric Administration Forecast Systems Laboratory (FSL) has configured an operational local-domain analysis and forecast system designed to function in the local weather office on an inexpensive computer workstation. The real-time system utilizes the 10-km grid increment analyses from the Local Analysis and Prediction System (LAPS) to initialize the Regional Atmospheric Modeling System (RAMS) mesoscale forecast model. A unique aspect of the LAPS/RAMS operational system is the initialization of the high-resolution model with comparable high-resolution analyses. The aim of the LAPS/RAMS real-time system is to provide local-scale guidance to the local weather office in addition to guidance that is currently available from coarser resolution, large-domain operational products. The operational system configuration and the utilization of three-dimensional visualization to display system output is discussed. All components of the LAPS/RAMS system are designed to run efficiently on a computer workstation affordable to individual weather forecast offices. Model validation results suggest that the LAPS/RAMS system is capable of resolving meso-β-scale, often terrain-forced, flow patterns. The three-dimensional visualization of the model output allows the operational forecaster to rapidly peruse the enormous amounts of data and aids the meteorologist's ability to comprehend and conceptualize mesoscale weather events. Potential improvements to the system are being implemented as computer workstation capabilities increase, which should lead to valuable forecast guidance of meso-γ-scale weather phenomena. DOI: 10.1034/j.1600-0870.1995.00125.x

Abstract: A multivariate model testing procedure is used to intercompare several tropical ocean models, using the evolution of the thermocline depth during the 1982–1984 FOCAL/SEQUAL experiment in the equatorial Atlantic as observational basis. Four models of increasing complexity are considered: Cane's linear multimode model, a nonlinear 2-layer model developed at LODYC, the KNMI GCM and the LODYC GCM. Some of the uncertainties in the atmospheric forcing are taken into account by forcing the models with 3 equally plausible windstress fields whose differences are consistent with the wind measurement and sampling errors, and the drag coefficient indeterminacy. Although the resulting uncertainties in model response are substantial, none of the models is fully consistent with the observations, i.e., within the error bars. The more complex models represent the thermocline depth variations significantly better than the simpler ones, in particular when the comparison is done over broad geographical areas. When the whole intercomparison domain (12°N−12°S) is considered, the LODYC GCM performs better than the KNMI GCM, both for the yearly mean conditions and for the variations around the 3-year mean, while the 2 models have comparable performance in the equatorial waveguide (3°N−3°S). In view of its simplicity, the linear model shows much skill, in particular for simulating the long-term mean. The 2-layer model poorly represents the long-term mean position of the thermocline, but represents well the variations around the 3-year mean, outperforming the linear model in the equatorial waveguide. Along meridional sections, model performance may be more variable, stressing the need for a global approach to model validation. Finally, a coarse investigation of the simulations of the thermocline depth anomalies with respect to the mean seasonal cycle suggests that, because of the small signal-to-noise ratio, most models have only little skill in the 12°N−12°S domain, but more in the equatorial waveguide. The linear model, however, does not seem to have predicative skill for these noisy anomalies. DOI: 10.1034/j.1600-0870.1995.t01-2-00005.x

Abstract: A case of incipient explosive marine cyclogenesis along the east coast of North America from February 1974 is investigated. The cyclone was noteworthy for deepening 36 mb in 15 h as it crossed a region of anomalously warm sea surface temperatures beneath a very favorable flow configuration aloft for explosive development. The precyclogenetic environment featured coastal frontogenesis and cyclonic vorticity production in association with oceanic sensible and latent heat fluxes in the westward-flowing air in advance of the cyclone. The resulting relatively high values of surface vorticity in the coastal baroclinic zone probably helped to contribute to the intensity of the ensuing cyclogenesis, as a peak cyclonic vorticity tendency of ∼ 30 × 10−9 s−2 was computed in the coastal baroclinic zone at the time of most rapid surface intensification. DOI: 10.1034/j.1600-0870.1995.470101.x

Abstract: The structure and evolution of frontal rainbands in relation to conditional symmetric instability (CSI) are presented with a high-resolution, three-dimensional (3-D) simulation of an Eady wave. This work extends an earlier 2-D study of Knight and Hobbs by use of the same mesoscale model and similar initial conditions. The model simulates numerous updraft bands that correspond to the warm-sector (WSF), the surface cold frontal (SCF), the wide cold frontal (WCF) and the post-frontal (PCF) bands as observed. It is shown that the WSF band develops in a convectively near-neutral environment, while the WCF band forms under a convectively stable but slantwisely unstable condition and it interacts closely with the PCF and SCF bands. It is found that the present 3-D results conform in many respects to CSI theory and support the earlier 2-D interpretations of the rainbands. However, some differences exist, which include the development of deep and intense rainbands without downshear tilt, the deviation of the rainbands from the direction of thermal winds, strong low-level jets and intense across-frontal thermal gradients. By introducing the third spatial dimension, the model appears to show better the relationships between these rainbands and their environments, thus providing more realistic representation of the mesoscale structure and evolution of the rainbands. The importance of some physical parameters in determining the formation of various rainbands is examined. It is found that the surface friction, ice microphysics, static stability, baroclinicity and the magnitude of centrifugal forces all have important contributions to the formation, structure and dynamics of the frontal rainbands. DOI: 10.1034/j.1600-0870.1995.00003.x

Abstract: A multivariate model testing procedure is used to intercompare several tropical ocean models, using the evolution of the thermocline depth during the 1982-1984 FOCAL/SEQUAL experiment in the equatorial Atlantic as observational basis. Four models of increasing complexity are considered: Cane's linear multimode model, a nonlinear 2-layer model developed at LODYC, the KNMI GCM and the LODYC GCM. Some of the uncertainties in the atmospheric forcing are taken into account by forcing the models with 3 equally plausible windstress fields whose differences are consistent with the wind measurement and sampling errors, and the drag coefficient indeterminacy. Although the resulting uncertainties in model response are substantial, none of the models is fully consistent with the observations, i.e., within the error bars. The more complex models represent the thermocline depth variations significantly better than the simpler ones, in particular when the comparison is done over broad geographical areas. When the whole intercomparison domain (12°N-12°S) is considered, the LODYC GCM performs better than the KNMI GCM, both for the yearly mean conditions and for the variations around the 3-year mean, while the 2 models have comparable performance in the equatorial waveguide (3°N-3°S). In view of its simplicity, the linear model shows much skill, in particular for simulating the longterm mean. The 2-layer model poorly represents the long-term mean position of the thermocline, but represents well the variations around the 3-year mean, outperforming the linear model in the equatorial waveguide. Along meridional sections, model performance may be more variable, stressing the need for a global approach to model validation. Finally, a coarse investigation of the simulations of the thermocline depth anomalies with respect to the mean seasonal cycle suggests that, because of the small signal-to-noise ratio, most models have only little skill in the 12°N-12°S domain, but more in the equatorial waveguide. The linear model, however, does not seem to have predicative skill for these noisy anomalies

Abstract: Estimates of the number of degrees of freedom (dof) of the 700 hPa Northern Hemisphere extratropical wintertime daily and time average circulation patterns are presented using two statistical methods. In the first method, it is assumed that the distribution of the mean square distances between the circulation patterns and the climate mean follows a χ2 distribution. The number of dof of the best χ2 fit to the empirical data is the most probable number of dof of the circulation data. The other, newly developed hyperspheres method compares the local density properties in the empirical data to density in independently and identically distributed (iid) normal distributions with different number of dof. The best fit, again, determines the most probable number of dof for the observed data. Though the hyperspheres method uses the smallest scales available in the data, just as the Grassberger-Procaccia method (GPM) and other “local” dimension estimation methods, it has distinct features. Most importantly, the GPM assumes that the empirical data is unbounded and distributed uniformly in the multivariate space. In contrast, the hyperspheres method, in accordance with many geophysical applications, assumes a bounded distribution in which density is a strong function of distance from the mean. Additionally, the hyperspheres method directly uses density information, that is neglected by GPM. The advantages of the hyperspheres method as compared to GPM are: (1) no systematic negative bias is present in the dof estimates; (2) the edge effect is eliminated; (3) no scaling is necessary. The results from the two methods overlap, thus indicating that at the 10% significance level the number of dof of the daily circulation data set is 24. This result pertains to a finite dataset in which the definitional requirement for dynamical dimension that the scales measured go to zero is not satisfied. We interpret our dof results as the dimension of a hypothetical subset of the atmospheric circulation that governs the large scale motions. Since the database of this study covers only part of the global atmosphere, the dimension of the whole atmosphere would be larger than our estimate. Our qualitative dimension estimate for the global circulation (60–90) refers to the minimum number of independent variables needed to model the large scale, low frequency variability of the atmosphere. In the present study grid point values served to describe the dynamical system of the atmosphere (spatial embedding, instead of time-delay or temporal embedding.) An argument is made that due to weak coupling between remote processes, estimates with temporal embedding measure only the dimension of a regional subset of the large-scale global circulation DOI: 10.1034/j.1600-0870.1995.t01-3-00005.x

Abstract: A series of dropwindsondes released over the sea during the FRONTS 92 experiment is analysed to reveal the mesoscale structure of a kata-cold front characterised broadly by a dry airstream over-running a warm moist boundary layer. The front had a multiple structure with two main surface fronts and this paper is concerned with the detailed structure of one of the surface cold fronts. Two kinds of circulations are shown to coexist at such a front: vertical convection and a slantwise circulation. The slantwise circulation is a feature not normally resolved by conventional observations of kata-cold fronts: it is shown to be a small-scale version of the rearward-sloping ascent encountered in ana-cold fronts. Much of the slantwise circulation was cloud-free but where it impinged upon the boundary layer it triggered the development of a narrow band of convective cloud, the tops of which extended downstream within air that was overtaking the front. DOI: 10.1034/j.1600-0870.1995.00128.x

Abstract: A nine equation low-order model of the shallow water equations is used as a testbed to compare several initialization strategies and to find points satisfying conditions of “slowness.” Several methods are explored to initialize the low order model: (1) Lorenz's method of successively zeroing tendencies; (2) minimization of the sum of the squares of the tendencies; (3) use of a balance condition. We find that the balance condition produces the smoothest solution on a consistent basis. In addition, Lorenz initialization, the Newton-Kantorovich Theorem, and the Nested Interval Property are used to compute points devoid of gravity waves to order N for low values of the forcing (F1 ≤ 0.1). Several of these points are calculated.

Abstract: Differences between the general circulations calculated in an axisymmetric 2-dimensional model (2D) and a non-axisymmetric 3-dimensional model (3D) are examined, particularly in terms of meridional circulations, angular momentum budget and zonal wind distributions. Circulations of the two models have similar properties in the low-latitudes. The Hadley cell exists in both cases, and their widths and intensities are comparable, though the cell is slightly stronger in the 3D model than in the 2D model especially in the terrestrial standard case. The surface winds in the Hadley regions are easterly, so that angular momentum is pumped from the surface to the atmosphere and transported to the upper layers and to the mid-latitudes. The most distinct difference is in cell shapes: the cell widths are broader at higher altitudes in 2D, while they do not depend on height in 3D. Circulations in the mid-latitudes are essentially different. In 2D, a symmetric multiple cellar structure exists. The cells are systematically moving equatorward, and they transport the angular momentum downward. The time averaged surface zonal winds almost vanish and a strong westerly shear appears in the mid-latitudes. In 3D, on the other hand, zonal mean winds in the mid-latitudes are westerly from the surface to the upper troposphere and the westerly shear is relatively weaker because of the existence of non-axisymmetric modes. This suggests that baroclinic waves in 3D efficiently transport angular momentum downward in comparison to the symmetric cells in 2D. Dependencies on the rotation rates show that the width of the Hadley cell becomes smaller as the rotation rate becomes faster. In the case of a faster rotation rate than the terrestrial rotation rate, the steady easterlies and westerlies alternately exist in the zonal mean surface winds of the mid- and high-latitudes, while a multiple jet structure exists at the tropopause level. DOI: 10.1034/j.1600-0870.1995.00104.x

Abstract: The internal dynamics associated low-frequency variabilities (LFVs) with timescales between 7 days and 31 days, relevant to the potential predictability of monthly means, are investigated. Using observed data and a set of general circulation model produced data, it is shown that the LFV characteristics are distinct for widely different basic flows: much smaller variability for north Pacific cyclonic basic flows than anti-cyclonic basic flows. Their preferred development locations are also distinct. The dynamical processes leading to this large difference is examined in the light of the local barotropic energy conversion between basic flow and low-frequency components. Over the eastern North Pacific where the LFVs are primarily located, the energy conversion decays more from the low-frequency disturbances into the cyclonic basic flows, while much more extraction of LFV energy from the anti-cyclonic basic flows takes place. The internal LFVs considered here are closely related to the climate noise pertaining to the prediction of monthly means. A dynamical link between the phase of El Nino/Southern Oscillation (ENSO) and the potential predictability of monthly means can therefore be established: during the warm phase of ENSO when the mean north Pacific flow is usually cyclonic, the potential predictability of the monthly means can be expected to be higher than during the cool phase, due to the substantially different magnitudes of unpredictable noise being generated through barotropic instability of these different zonally varying basic flows. DOI: 10.1034/j.1600-0870.1995.00102.x

Abstract: A method of analyzing the divergent part of the marine boundary layer flow is presented in the context of two-dimensional statistical interpolation. Instead of assuming geostrophic balance of observed minus background differences, a linearized Ekman balance is assumed. This results in correlation functions that are rotated with respect to the geostrophic correlation functions by a turning angle that is precisely the difference between the actual and geostrophic wind at mean sea level. The turning angle is also related to the degree to which divergent flow due to friction is analysed; the larger the turning angle, the larger the extent of the analysis of divergent flow. Case studies comparing geostrophic and ageostrophic analyses show that changes to the pressure field occur when the turning angle is large and wind observations are sufficiently dense. Ageostrophic analyses are shown to contain divergent analysis increments and consequently, realistic wind analysis increments result. DOI: 10.1034/j.1600-0870.1995.t01-1-00005.x