Meridional Mass Research Articles

The influence of Antarctica on the momentum budget of the southern extratropics

AbstractThe antarctic plateau acts as a strong heat sink for the global climate, cooling the atmosphere and radiating energy to space. A cold dense atmospheric boundary layer is formed. Strong surface winds are formed as the boundary layer drains off the plateau. These drainage winds and the eddy fluxes necessary to maintain them are analysed in a general circulation model (GCM). The drainage flow is well represented in the GCM. The associated mean meridional circulation is analysed in isentropic coordinates. The momentum budget over Antarctica reveals a balance between the Eliassen‐Palm flux convergence and the Coriolis torque exerted by the mean meridional mass flux. Both vertical and horizontal components of the Eliassen‐Palm flux contribute, the vertical component being the greater.

The influence of Antarctica on the momentum budget of the southern extratropics

The antarctic plateau acts as a strong heat sink for the global climate, cooling the atmosphere and radiating energy to space. A cold dense atmospheric boundary layer is formed. Strong surface winds are formed as the boundary layer drains off the plateau. These drainage winds and the eddy fluxes necessary to maintain them are analysed in a general circulation model (GCM). The drainage flow is well represented in the GCM. The associated mean meridional circulation is analysed in isentropic coordinates. The momentum budget over Antarctica reveals a balance between the Eliassen-Palm flux convergence and the Coriolis torque exerted by the mean meridional mass flux. Both vertical and horizontal components of the Eliassen-Palm flux contribute, the vertical component being the greater.

Ocean Response to Surface Heat Anomalies

An ocean general circulation model (OGCM) is used to study the response of ocean heat and mass transport to positive and negative heat flux anomalies at the ocean surface. As expected, tropical and low-latitude mixed layers respond rapidly (e-folding time about 50-70 years) to external forcing, while the response of the high-latitude mixed layer, especially the Southern Ocean and northern North Atlantic, is very slow (e-folding time greater than 300 yr). The overall response is faster for negative than positive heat flux anomaly at the surface. The meridional heat transport changes by 15% in the first 50 yr in the southern high latitudes. Surprisingly, for the next 400-500 yr the change is very small. The analysis shows that the meridional mass transport intensifies in response to a negative surface heat flux anomaly but weakens in response to a positive heat flux anomaly. For example, at model year 100 the North Atlantic Deep Water (NADW) is reduced from about 18 Sv to about 10 Sv for the positive heat flux experiment but increased to about 26 Sv for the negative heat flux experiment.

A primitive equation ocean circulation model using a general vertical coordinate transformation: 2. Application to an overflow problem

The version of the ocean general circulation model of the Geophysical Fluid Dynamics Laboratory with a generalized vertical coordinate transformation introduced in part 1 is used to study the overflow of a zonal ridge in a coarse‐resolution grid typical for climate applications. The configuration roughly resembles the Greenland‐Scotland Ridge with respect to scale, latitude, and height of the ridge. Reasonable qualitative agreement with observed conditions over the Iceland‐Faroe section has been achieved, especially when a deeper passage was included at the eastern end of the idealized model ridge. Upwelling of deep water occurs at the eastern boundary, where friction and, more importantly, diapycnal diffusion accomplish the necessary changes in potential vorticity. The overflow consists of intermediate water when the renewal time for the deep water is made much larger than that for intermediate water. The meridional mass transport is most sensitive to geometric factors like the height of the ridge. Frictional and diapycnal processes are crucial for the potential vorticity conversion at the ridge and have a strong effect on the water mass distribution. Bottom friction increases the near‐bottom flow across f/H lines, resulting in a large amount of northern basin deep water south of the ridge. This effect can be represented only when the frictional bottom boundary layer is adequately resolved by the numerical grid.

Thermal advection and stratification effects on surface winds and the low level meridional mass transport

Statistical tests are performed on the Seasat scatterometer observations to examine if and to what degree thermal advection and stratification effects manifest themselves in these remotely sensed measurements of mean wind and wind stress over the ocean. On the basis of a two layer baroclinic boundary layer model which is presented, it is shown that the thermal advection and stratification of the entire boundary layer as well as the geostrophic forcing influence the modeled near surface wind and wind stress profiles. Evidence of diurnal variation in the stratification under barotropic conditions is found in the data, with the daytime marine boundary layer being more convective than its nighttime counterpart. The temporal and spaciai sampling pattern of the satellite makes it impossible to recover the full diurnal cycle, however. The observed effects of the thermal advection are shown to be statistically significant during the day (and presumed more convective) hours, causing a systematic increase in the poleward transport of mass and heat. The statistical results are in a qualitative agreement with the model simulations and cannot be reproduced in randomized control tests.

Dependence of Antarctic surface mass balance on temperature, elevation, and distance to open ocean

The latest compilations of surface mass balance, mean annual surface temperature, and elevation for the Antarctic ice sheet, derived from data sets of approximately 1500, 700, and 105 points, respectively, have been used to obtain areally integrated means for 24 ice drainage systems and 329 grid point values covering the whole ice sheet. Monthly summaries of remotely sensed sea ice data for 1973–1976 have been used to obtain mean annual distance to open ocean. Linear and second‐order regression analyses of surface balance on (1) temperature for the entire ice sheet, (2) elevation for the conterminous grounded ice, and (3) distance to the open ocean for the ice shelves (including ice rises and attached islands) have been made using both system means and grid point values. These analyses show correlation coefficients of between 0.63 and 0.81, and they provide bases for descriptive models of the present Antarctic ice sheet, as well as for predictive models of the response of the ice sheet to air temperature changes and variations in meridional mass and energy transfers. Extrapolation to other ice sheets, past and present, may be possible but should be made cautiously. Linear models are recommended for paleoclimatic reconstructions of ice sheets in upper mid‐latitudes, and second‐order models are recommended for those in high latitudes. System means are shown to be reliable for these purposes. Incidental results are new estimates of the mean annual surface temperature for the whole ice sheet (−36°C) and mean surface elevation for the conterminous grounded ice (2290 m).

Sensitivity of a multilayered oceanic general circulation model to the sea surface thermal boundary condition

Experiments with a low resolution, multilayered model of the oceanic general circulation with wind and thermal forcing were carried out in order to identify processes controlling important aspects of the general circulation. Emphasis was placed on the sensitivity of the model's solution to the sea surface thermal boundary condition. For long restoring time scale, wind‐driven circulation dominated the solution, resembling the circulation in the North Pacific. For short restoring time scale, strong thermohaline circulation driven by deep water formed at high latitude dominated the solution, resembling the circulation in the North Atlantic. Accordingly, other aspects of the circulation, such as the baroclinic structure of the currents and the meridional mass flux partition, changed with the restoring time scale.

A comparison of various normal-mode initialization schemes and the inclusion of diabatic processes

The field changes produced by various normal mode initialization schemes are compared. Dynamically balanced data are obtained from a climate simulation using a forecast model. The data are first degraded by a linear initialization, and then a nonlinear initialization is used in an attempt to recover the original fields. The nonlinear initializations include both diabatic and adiabatic schemes, applied to various sets of modes defined using both equivalent depths and natural periods as criteria. Results indicate that diabatic initialization improves specification of the meridional mass streamfunction (i.e., the zonal wavenumber zero Hadley circulation) compared with adiabatic initialization. However, for zonally varying fields, the difference is less dramatic. An exception occurs when shallow modes are initialized, in which case near-surface fields are better specified using diabatic initialization. Errors in the tropical divergent wind field are typically half their true value, for either diabatic or adiabatic initializations. For vertical modes 1–2, errors in specification of mode coefficients are typically less than 20% of their true r.m.s. values. For shallower modes whose natural periods are less than one day, these errors are greater than 40%, but for those longer period modes, errors are greater than 80%. The prognostic equation for a single mode coefficient is examined. Results suggest that, for medium depth, large horizontal scale tropical modes, a significant error source is the balance condition itself. For these modes, the time scales of their diabatic forcing, observed behavior, and natural (linear) behavior are all similar. In this case, the tendency term in the equations is not negligible. DOI: 10.1111/j.1600-0870.1988.tb00327.x

Similarity analysis of the Martian polar caps

The similarity hypothesis is that the seasonal pattern of deviation from mean CO2 mass in each cap is of the same shape, with an intercap phase difference of half a Martian year. We thereby decompose the annual pressure variation into separate components of CO2 exchange with each cap. Sublimation at one cap and condensation at the other tend to occur simultaneously. The annual fluctuation of CO2 mass in the two caps thus greatly exceeds previous estimates based on treating each cap in isolation. Inferred behavior is generally consistent with optical data on cap recession. The seasonal pattern of meridional mass flux of CO2 is estimated.

Two‐dimensional tracer transport: Derivation of residual mean circulation and eddy transport tensor from a 3‐D model data set

Two‐dimensional distributions for long‐lived species, N2O, CFCl3, and CF2Cl2, have been calculated by using a 2‐D model extending from the ground to 70 km. The model utilizes a residual mean meridional circulation and a set of eddy diffusion coefficients. Both these fields have been obtained from the output of the MIT‐GIT three‐dimensional general circulation model of the stratosphere. The calculation of the residual mean circulation takes into account consistent fields of temperature and diabatic heating, meridional temperature advection, and vertical eddy fluxes. The diffusion tensor is obtained following Holton (1981) and utilizes the eddy field that is an output of the 3‐D model. The chemical source term is treated by introducing an additional tensor, following the suggestion by Tung (1982). This approach has the advantage of using the same matrix for all the chemical compounds. Values obtained for the residual mean meridional mass flux are in good agreement with similar results. The trace gas distributions obtained show a fairly good agreement in the equatorial regions but overpredict the concentration in the mid‐latitude stratosphere. This is a common modeling problem, especially with fluorocarbons, and is attributed, in our case, to values of the Kyy and Kzz components that are too large in the lower stratosphere. This particular result is probably due to the heating parameterization adopted in the original general circulation model.

Regional Circulation Characteristics Associated with a Cold Surge Event over East Asia during Winter MONEX

Abstract A case study is presented of the cold surge over East Asia during 9–13 December 1978; using nine vertical levels of Winter MONEX data. The surge event is manifested by a rapid meridional mass flow in the lower troposphere from the midlatitudes (∼30°N) to the equator within three days. During the period studied, the lower tropospheric circulation dramatically changed in extent and intensity. With the onset of the surge event, the main divergent maximum began shifting from the South China Sea to southeast China. At the same time, the upper tropospheric circulation correspondingly changed in a reversed order from that of the lower troposphere, and a direct vertical coupling between flows in the low and high troposphere was observed. The time-averaged meridional mass circulation between 100 and 126°E reveals a two-cell structure; the southern cell is located between the northern South China Sea and the equator, and the northern cell between midlatitudes and the northern South China Sea. Analysis of s...

Pacific and Indian Ocean Upper-Layer Salinity Budget

Abstract The freshwater balance in the upper layer of the Pacific and Indian Oceans is investigated by means of mass and salinity conservation arguments in simple advective box models. The model uses estimates of atmospheric freshwater input to the ocean and upwelling of deep water into the upper layer at a rate required to balance North Atlantic deep water formation, proportioned by the areas of each ocean. The salinity of the upper layer outflow relative to observed salinity is too low for the Pacific and too high for the Indian Oman. Either the upwelling rates am 5 to 20 times higher than estimated or the freshwater input is grossly exaggerated. The problem is alleviated by taking account of the Pacific-Indian tropical link within the Indonesian Passages of the Southeast Asian Seas. The role of the Pacific-Indian Ocean equatorial connection (through the Southeast Asian Seas) is tested by dividing the Pacific Ocean basin into three zones. Meridional mass transports between zones are estimated from the m...

Fluid motion induced by travelling thermal waves in a rotating fluid

Abstract The mean motion induced by thermal waves travelling along the surfaces of two parallel planes bounding a layer of homogeneous rotating fluid is studied. Under the Boussinesq and long waves approximations, the fluctuating and the mean velocity components are obtained. The asymptotic expressions and numerical calculations of the zonal and meridional mass flux across vertical planes show that the direction of the net mass flux changes for certain combinations of the frequencies of the moving heat source and the inertial oscillations.

Diffusion, meridional circulation, and mass loss in Fm-Am stars

view Abstract Citations (159) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Diffusion, meridional circulation, and mass loss in Fm-Am stars Michaud, G. ; Tarasick, D. ; Charland, Y. ; Pelletier, C. Abstract The gravitational settling of helium is calculated in the presence of meridional circulation. The concentration of Fm-Am stars in binary systems with P between 2.5 and 100 days is then explained without any arbitrary parameter. Together with the other properties of Fm-Am stars that are explained without arbitrary parameter, this constitutes a strong argument in favor of the diffusion model for those objects. Calculations of helium settling and of the diffusion of heavy elements in the presence of mass loss are also carried out to show that a mass loss rate of 10 to the -15th - 10 to the -14th solar masses per yr is likely in Fm-Am stars. Some weak turbulence may subsist below the hydrogen convection zone. The effects of meridional circulation, mass loss, turbulence, and mixing length illustrate how the appearance of abundance anomalies is determined by the hydrodynamics of the outer stellar envelope. The Fm-Am stars offer an important contact between observations and hydrodynamical calculations. If the mass loss rate is about 10 to the -13th solar masses per yr, stars may spend more of their life as Lambda Bootis stars than as Fm-Am stars. Publication: The Astrophysical Journal Pub Date: June 1983 DOI: 10.1086/161034 Bibcode: 1983ApJ...269..239M Keywords: A Stars; Binary Stars; Metallic Stars; Peculiar Stars; Stellar Mass Ejection; Stellar Rotation; Abundance; Calcium; Gaseous Diffusion; Helium; Meridional Flow; Stellar Models; Astrophysics full text sources ADS |

Kinematic and thermal structures of two surges of flow in the Northern Mozambique Channel Area

AbstractTwo atmospheric surges near the northern Mozambique Channel were explored by NCAR's Electra aircraft on 17 June 1977 and 3 July 1977 during MONSOON–77 field experiment. The surges were probed from Nairobi eastwards to the coast and then south from 1°S to 8°S along 41°E. Offshore the former day was characterized by deep south‐south‐easterlies and south‐easterlies, squalls, and rain and the latter day by shallow south‐south‐easterlies overlain by south‐westerlies and fair weather cumulus.Vertical structures of u, v, T and q for both days are discussed. Early on the disturbed day southerlies reached their maximum over land at 880 mb on a cross‐section approximately along 2°S. By noon diurnal heating reduced the overland winds, resulting in an apparent ocean‐ward shift of the jet axis. On the undisturbed day the southerly maximum was over the ocean near the coast.On both days two southerly maxima were found on longitudinal coross‐sections along 41°E: an elevated and stronger maximum near the equator; a depressed and weaker one near 8°S. On the disturbed day the equatorial maximum was located in the south‐south‐easterlies and on the undisturbed day was sandwiched between south‐south‐easterlies in the low levels and south‐south‐westerlies at 770 mb. The southern maximum was from south‐south‐east on both days.During these two periods meridional mass fluxes from two stations, Port Elizabeth and Dar‐es‐Salaam, were computed. They showed that the latter station lags the former by as much as four days; however, its meridional mass flux is double the former.

A Martian General Circulation Experiment with Large Topography

A three-layer general circulation model of the Martian atmosphere is described, and the assumptions governing the model are discussed. The simulated, zonally averaged circulation is found to have only limited sensitivity to differences between this model and an earlier general circulation model; this circulation compares reasonably well with observations. It is also found that the meridional mass flow produced by the seasonal condensation of CO2 in the winter polar region has a major influence on the circulation; owing to the weak influence of atmospheric heat transport, however, the mass flow is governed almost entirely by radiation. Quasi-barotropic stationary waves, which are forced kinematically by the topography and which resemble topographically forced terrestrial planetary waves, are generated by the model in the winter hemisphere region of strong eastward flow, while baroclinic stationary waves are thermally forced by topography in the tropics and summer subtropics. It is also concluded that transient baroclinically unstable waves, of somewhat lower dominant wavenumber than those found on the earth, are generated in winter midlatitudes; their amplitudes, wavenumbers, and phase speeds closely agree with what has been deduced from the Viking lander observations.

A Prograde Jet Driven by Rossby Waves

Abstract Waves are forced by moving sources and sinks of mass on a beta-plane with Ekman friction. If the motion is retrograde and not too fast, these are Rossby waves with a nonzero Reynolds stress u′v′¯. This stress forces a meridional mass flux which builds up a pressure gradient to allow a compensating mass flux in the Ekman layer. In a rotating system, this meridional pressure gradient requires a zonal flow u¯(y)=−∂/∂y(u′v′¯)/E½Ω,which is also the momentum accumulated by the Reynolds stress in one spindown time. An example shows how excitation of Rossby waves brings momentum into the source of energy, causing a prograde jet there, with retrograde currents on each side. A laboratory experiment confirms the theory, which is proposed as an explanation of the observed “negative viscosity” of the jet stream in the atmosphere and the Gulf Stream in the ocean.

A relation between the T‐S curve and global heat and atmospheric water transports

From historical data on the distribution of temperature and salinity on vertical sections across the oceans over complete latitude circles it is possible to test the compatibility of certain meteorologically determined meridional fluxes in the ocean of sensible heat and fresh water as functions of latitude. It is also possible to check the proportion of this flux that each ocean basin carries separately. The information from both these sources leads to estimates of the magnitudes and distribution of the meridional oceanic mass flux over the salinity‐temperature plane and yields some indication of the probable physical processes operating (that is isopycnal mixing versus large‐scale advective currents).

On the Dynamics of the East African Jet. I: Simulation of Mean Conditions for July

The East African jet, also popularly called the Somali jet, is viewed as a western boundary current of the East African highlands. Inertial and Coriolis forces. bottom friction and orography are believed important in the jet dynamics. A barotropic, primitive equation model on an equatorial beta plane is used to test this hypothesis. The flow is driven by a mass source term representing the subsidence in the southern branch of the monsoon Hadley cell. Steady, zonally symmetric solutions indicate that the combination of inertial forces, surface friction and weak subsidence can provide an adequate description of the southeast trades over the South Indian Ocean. It is deduced that, in order for the easterly flow to change into westerlies south of the equator, convergence of the flow must occur at the transition latitude, and the meridional mass flux must vanish. A two-dimensional numerical model successfully simulates most of the major large-scale features of the climatological low-level flow over the South Indian Ocean and cast coast of Africa during the northern summer. It is shown that while the broad outer flank of the jet is inertially controlled, with bottom friction playing a secondary role, the narrow inner flank is the result of orographically enhanced bottom friction. The mountain backbone of Madagascar is demonstrated to be essential to the development of a relative wind speed maximum at the northern tip of the island and of an upstream ridge-downstream trough pressure distribution over the island. The sensitivity of the model jet to variations in the upstream forcing and in the friction parameterization is also examined.

“Cumulus Friction”: Estimated Influence on the Tropical Mean Meridional Circulation

A simple diagnostic study is performed in order to examine the influence of parameterized westerly momentum exchange by cumulus convection on the observed Hadley circulation. Observed precipitation and 200 mb winds are used to diagnose the mean 100–200 mb tropical cumulus friction using the parameterization proposed by Schneider and Lindzen (1976). By using seasonal data on a latitude-longitude grid, we estimate not only the annual (standing) zonally symmetric cumulus friction but the seasonal (transient) and zonally asymmetric contributions as well. In general, the standing zonally symmetric contribution dominates the total cumulus friction, but other components are significant at certain latitudes. The meridional mass flux induced by cumulus friction is estimated through the use of an approximate u-momentum equation. The diagnosed meridional mass transports are about an order of magnitude smaller than those which are observed by direct measurements. Therefore it appears unlikely that cumulus friction plays a dominant role in maintaining the tropical mean meridional circulation.