Article

Climatic change, orography and geostrophic motion

Authors:
To read the full-text of this research, you can request a copy directly from the author.

Abstract

The paleoclimatological role of continental scale orographic effects is examined from a dynamical standpoint. Evidence for the so-called 'relief hypotheses' of climatic change is summarised, and juxtaposed with the geological evidence that suggests that major large-scale glaciations are only possible if large continents occupy a polar position for a sufficient period of time. Quasi-geostrophic dynamical models are then used to discuss the effect of a longitudinally symmetric polar land mass upon the general circulation of the atmosphere. In particular studies are made of the effect of this terrain upon baroclinic instability and the poleward propagation of quasi-geostrophic eddies. It is shown that the polar land mass can inhibit or curtail the poleward transfer of sensible heat by the baroclinically generated eddies unless there is a compensating increase in the overall meridional baroclinicity of the atmosphere. It is surmised that the isolated repercussive effect of the dynamical mechanisms considered is to induce a relative cooling of the polar regions. However it is stressed that the direct application of the results to the climatic change problem remains an academic exercise.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the author.

Article
It is shown that a topographically-induced instability can exist for a laterally sheared flow aligned along the contours of a two-dimensional ridge. The nature of the destabilization mechanism is explored within a quasi-geostrophic, f -plane framework for adiabatic flow of an incompressible stratified fluid occupying the half-space z > 0. For a basic state of uniform barotropic shear with (or without) the superposition of an uniform baroclinic component, the instability is shown to be engendered by the terrain shape, sustained by the lateral shear, and inhibited by the baroclinicity. A phenomenological interpretation in terms of the interaction of two “vortex-like” wave-trains located on the adjacent slopes of the terrain accounts neatly for the salient features of the instability. It is shown that there is a similar instability associated with the hybrid configuration of an enhanced baroclinic zone set alongside a terrain slope. Consideration is also given to the limitations of the diagnosed instability and to the possibility of its manifestation in various geophysical systems. In particular, it is suggested that some of the observed cyclogenetic activity along the eastern seaboard of north America and around the periphery of Antarctica could be related to terrain-induced instability effects. DOI: 10.1034/j.1600-0870.1991.t01-4-00007.x
Article
This paper documents significant long-term changes in the tropospheric circulation and progression of the seasonal cycle over the South Pole and relates aspects of the changes to the enhanced springtime stratospheric ozone depletion of recent decades. During the winter-to-summer transition over Antarctica, the formation of the thermal tropopause above 150 hPa is now delayed by a month compared to the early 1960s. A similar delay occurs in the breakdown of the winter polar stratospheric vortex, a prerequisite to the formation of the strong summertime tropopause near 300 hPa. A delay in tropopause formation implies delays in those aspects of the transition from winter to summer in the dynamics of the troposphere that are sensitive to its upper boundary condition, including the contraction of the Rossby radius of deformation during the shallow troposphere phase of summer. Consistent with this expectation, the normal springtime decline of baroclinic-eddy activity in the lower troposphere now occurs later by a month. Similar delays occur in the time of an early summer minimum in total cloud fraction that coincides with the time of the maximum stratification between 150 and 100 hPa. In addition, the analyses reveal (1) an increase of 15 to 20% in spring and summer cloud fraction over the last four decades; (2) a separation of spring and summer cloudiness into two transport regimes with a distinct intervening minimum in cloud fraction; (3) a decadal modulation of springtime cloudiness that is coincident with decadal shifts in the directional mode of 300-hPa winds following solar minima; (4) a long-term increase in total inversion strength, inversion depth, and 650- to 300-hPa thickness; and (5) a long-term decrease in wintertime surface wind speed, surface temperature, and near-surface lapse rate. The study also introduces a covariance technique that exploits the control by continental-scale orography of synoptic and surface flow regimes, increasing the value of lower tropospheric observations for the study of climate change over Antarctica. Finally, a cautionary note arises from the apparent effect of the Weddell Polynyas and the reduction of the maximum extent of sea ice of the middle to late 1970s, with increased tropospheric and lower stratospheric temperatures and an earlier time of formation of the tropopause. This observation suggests that the occurrence of decadal and subdecadal variability in the ocean and the atmosphere of the Southern Hemisphere may obscure the detection of long-term trends in the circulation over Antarctica.
Article
Summary The existence of a major zonal mountain belt may be an important climatic agent during glacial periods. The combined effect of a large-scale ice field and a zonal mountain chain south of it, might contribute to the relative warming of the polar regions as the result of the increased poleward heat transport due to enhanced wave activity.
Article
Abstract Results of an analysis of the distribution and movement of extratropical cyclones around Antarctica for the period September, 1973, through May, 1975, are presented. Variations in seasonal cyclone frequencies, their preferred longitudinal positions, and the spatial and temporal fluctuations in latitude and pressure are shown, and these indicate good agreement with the known features of Southern Hemisphere climatology. The greatest numbers of cyclones occur during spring and fall, and their most common locations are near coastal embayments. A semiannual oscillation is present in the frequencies, mean latitudes, and mean pressures of the cyclones, although to varying degrees. Longitudinal variations in the intensity of the oscillation are also evident. The belt of maximum cyclone activity exhibits a distinct half-yearly cycle in latitudinal position, although the trough is located some 50 north of the position determined from mean monthly pressures. Harmonic analysis of the spatial variation of this belt and the simultaneous position of the sea-ice margin derived from microwave imagery suggests that there is little correlation between sea ice and cyclone tracks on a hemispheric scale. Close relationships are apparent for short time periods or in specific regions but will require more intensive study before causal mechanisms can be suggested. The asymmetry of the continent, rather than the extent of sea ice, is probably the principal factor involved in determining the longitudinal variations in the positions and tracks of cyclones around Antarctica on a hemispheric scale.
Article
Full-text available
Seasonal ice cover off Canada's east coast was examined in relation to synoptic scale atmospheric events. Ice concentration information derived from Nimbus-7 scanning multichannel microwave radiometer (SMMR) measurements of surface brightness temperatures, supplemented by AES composite ice charts, provided timely coverage of the study area during the 1971/72, 1980/81 and 1984/85 ice seasons, 1971/72 and 1984/85 seasons with high ice concentrations and 1980/81 a season with low concentrations. Atmospheric variability was monitored using the 850 hPa height at three upper air stations along the western edge of the study region. Additional information was drawn from storm track records, providing an indication of surface variability. Properties of specific storm events were obtained from the storm track data, permitting evaluation of the ice response to passing synoptic disturbances. Results of this investigation indicate that, (1) passive microwave-derived ice information can be used to monitor high frequency variability in the marginal ice, (2) despite short time series, spectral relationships between ice concentration variability and 850 hPa pressure height indicate a strong association between the two at synoptic frequencies zone, and (3) variability in ice cover extent and concentration can be related to the passage of individual synoptic events.
Article
Full-text available
Auf der Grundlage der jüngsten Fortschritte in vielen Zweigen der Geophysik — Glaziologie, Meteorologie, Ozeanographie, Paläomagnetismus und Tektonophysik — wird ein synthetisches, rein geophysikalisches Modell der Klimaentwicklung im Tertiär und Pleistozän (mit Ausblicken auf das Permokarbon) entwickelt. Hierbei wird besonderes Gewicht auf die Abschätzung des Wärmehaushaltes der Ozeane gelegt; extraterrestrische Faktoren liefern höchstens einen sekundären Beitrag. Als Unterlage für eine weitere Diskussion wird eine knappe Zusammenstellung der wesentlichen Gesichtspunkte in Form von 10 Sätzen (Kapitel 6) gegeben.
Article
Full-text available
This paper describes the method of incorporating into the NCAR global circulation model the dynamic effect of mountains, the prediction of cloudiness for radiation calculations, and the calculation of ground surface temperature using a heat balance equation. Other aspects of the physics of the model and the finite-difference schemes are very similar to those discussed by the authors in 1967 and 1970. For the simulation of seasonal climate we specify two parameters: the sun's declination and the distribution of ocean surface temperatures. Since the prediction of cloudiness is parameterized in terms of the relative humidity and the vertical motion fields, solar and atmospheric radiation processes interact closely with the dynamics of the atmosphere through variations in the fields of cloudiness, temperature and water vapor. Coupling between radiation and dynamics helps to maintain stronger baroclinic activity in middle latitudes. Although a hydrologic cycle is included in the model atmosphere and the ground surface temperature is computed, a hydrologic cycle in the ground is not taken into account. Instead, it is assumed that the latent heat transport from the ground to the atmosphere and the soil heat transport below the surface are both functions of the sensible heat transport between the ground and the atmosphere.Experiments are conducted to simulate January climate with and without the earth's orography. In both experiments the domain of continents, the January mean ocean surface temperatures, and the sun's declination for mid-January are unchanged during the time integrations. The model has a spherical horizontal mesh spacing of 5° in both longitude and latitude and six vertical layers at 3-km height increments. The time step is 6 min and both cares are integrated up to 80 days starting from an isothermal atmosphere at rest. The results of the 41-70 days of the time integration are analyzed for various diagnostic studies. Synoptic comparisons of the two experiments are made for selective meteorological variables to discuss the relative importance of the thermal and orographic influences upon the large-scale motions of the atmosphere. Detailed studies are made on the balance of momentum, water vapor, heat and energy. The present experiments indicate that the six-layer and 5° mesh model can simulate successfully a January climate and that the earth's orography plays a minor role over the thermal effect of continentality in determining the major features in the transport mechanism of momentum, water vapor, heat and energy in terms of the zonal mean state. However, for the regional aspects of general circulation the effects of orography are significant.
Chapter
Continents occupied much of the region around the north pole during the glaciations that occurred in the late Paleozoic and late Cenozoic eras. The geologic evidence is thus at least permissive that polar continentality is a necessary condition for glaciation. However, continents also occupied the polar regions during most of the Paleozoic, Mesozoic and Cenozoic eras at times when glaciation did not occur, so that clearly polar continentality is not a sufficient condition for glaciation. Polar wandering has been linked to the onset of glaciation by several theories which suggest that glaciation began as the rotation axis migrated toward potential glaciation generating regions such as Antarctica or the Arctic Ocean. These theories are not in agreement with paleomagnetic evidence, which indicates that the rotation axis has been within the present polar region for an interval at least 10 times longer than the interval of glaciation. Similarly, rates of continental drift are too small to permit changes in the configuration of the continents to have played an important role in the onset of Quaternary glaciation. The possibility remains that vertical movements of land masses, many of which occurred at the end of the Pliocene, may have played a significant role in the initiation of Quaternary glaciation.
Article
The temperature contrast across middle southern latitudes is larger in summer than in winter. This is because of the decrease in annual temperature range with increasing latitude between approximately 35 degrees S and 50 degrees S. Two factors are responsible for the decreasing range. One is the higher cloudiness in 50 degrees S than in 35 degrees S, which reduces the possible, higher annual range of short-wave radiation reaching the surface in 50 degrees S than in 35 degrees S to a lower actual range in 50 degrees S. The other is that the incoming radiation is distributed over a deeper ocean layer in 50 degrees S than in 35 degrees S, primarily owing to more vigorous wind stirring.
Article
In order to identify the effects of mountains upon the general circulation of the atmosphere, a set of numerical experiments is performed by use of a general circulation model developed at the Geophysical Fluid Dynamics Laboratory of NOAA. The numerical time integrations of the model are performed with and without the effects of mountains. By comparing the structure of the model atmospheres that emerged from these two numerical experiments, it is possible to discuss the role of mountains in maintaining the stationary and transient disturbances in the atmosphere. The model adopted for this study has a global computational domain and covers both the troposphere and stratosphere. The results of the analysis reveal many important effects of mountains. For example, the probability of cyclogenesis in the model atmosphere increases significantly on the lee side of major mountain ranges where the core of the westerly jet is located. Also, mountains affect the hydrologic processes in the model atmosphere by modifying the field of three-dimensional advection of moisture, and alter the global distribution of precipitation very significantly. In general, the distribution of the model with mountains is less zonal and more realistic than that of the model without mountains.
Article
A theory is presented for the finite-amplitude behavior of unstable baroclinic waves in a quasi-geostrophic two-layer model. It is shown that in the absence of dissipation the equilibrated finite-amplitude state exhibits an oscillation, both of the mean flow and the baroclinic wave. On the other hand, if sufficient dissipation is present, the final state is a wave whose amplitude is steady and the approach to that state is non-oscillatory.
Article
The importance of bottom topography in the linear baroclinic instability of zonal flows on the β plane is examined by using analytical calculations and a quasigeostrophic eddy-resolving numerical model. The particular focus is on the effects of a zonal topographic slope, comparedwith the effects of ameridional slope.A zonal slope always destabilizes background zonal flows that are otherwise stable in the absence of topography regardless of the slope magnitude, whereas the meridional slopes stabilize/destabilize zonal flows only through changing the lower-level background potential vorticity gradient beyond a known critical value. Growth rates, phase speeds, and vertical structure of the growing solutions strongly depend on the slope magnitude. In the numerical simulations configured with an isolatedmeridional ridge, unstablemodes develop on both sides of the ridge and propagate eastward of the ridge, in agreement with analytical results.
Article
When applied to the earth, the model yields the values 〈∂θ/∂z〉≈+2K km⁻¹, 〈∂θ/∂y〉≈−0.4K (100 km)⁻¹, Ri≈30. These values are much more realistic than can he obtained from the traditional assumption of radiative-convective equilibrium, although the static stability is still about one-half the observed value because of neglected fluxes. When applied to Mars the model yields the values 〈∂θ/∂z〉≈+2K km⁻¹, 〈∂θ/∂y〉≈−1.2K (100 km)⁻¹, Ri≈10. which are in good agreement with the Mariner observations and with the numerical results of Leovy and Mintz. The destabilization of the Martian atmosphere compared to the earth's (i.e., the smaller value of Ri) is due to the much shorter radiative relaxation time on Mars, which makes the radiative fluxes more efficient at destabilizing the atmosphere. When applied to Jupiter the model yields the values 〈∂θ/∂z〉≈10⁻⁴K km⁻¹, 〈∂θ/∂y〉≈−2K (10,000 km)⁻¹. The destabilization of Jupiter's atmosphere compared to Mars’ and the earth's is caused by the small horizontal temperature gradients, which in turn are due to both the large scale of Jupiter and the presence of an internal heat source. These small gradients lead to relatively weak large-scale motions, so that the dynamical fluxes are less efficient at stabilizing the atmosphere. The value of Ri found for Jupiter is subject to large error because of its sensitivity to the values of the external parameters. It may lie anywhere in the range O<Ri<20. Consequently, Jupiter's dynamical regime cannot be specified with certainty, but the results do show that Jupiter's mean temperature structure will in any case be very near radiative-convective equilibrium.
Article
The stability with respect to quasi-geostrophic disturbances of atmospheric and oceanic currents containing both horizontal and vertical shear is investigated for both a continuously stratified and a twolayer model.Certain necessary conditions for instability are derived. The potential vorticity gradient of the basic flow must be both positive and negative for instability to occur in the two-layer model. In the continuous model the condition for instability states that the potential vorticity gradient must either change sign or be balanced by surface terms incorporating the surface potential temperature gradient and topographical variations. For a large class of flows, the product of the zonal current and the potential vorticity gradient must be somewhere positive for instability to occur, and the maximum of this product bounds the growth rate. The square of the growth rate is bounded for an arbitrary flow field by the maximum value of the available mean kinetic and potential energy densities.The phase speed of unstable waves is within limits specified by the mean velocity field, the planetary vorticity gradient, and topographical variations across the stream. It is also shown that a wide class of flows in the two-layer model must be stable for sufficiently high zonal wave number.
Article
Late Cenozoic glaciation as a whole can be attributed to the present near-polar position of North America. The continent had undergone no extensive previous glaciation for about years, nor had it been located near a pole during that interval. The specific onset of Pleistocene glaciation can, perhaps, be assigned to the time of a hypothetical (but geologically probable) tectonic event, in the Bering Strait area, which reduced influx of warm Pacific water into the Arctic basin, thereby converting the Arctic Ocean into a large "continental lake." This history has been greatly favored by the polar position of Antarctica. Glacial oscillation on the continental scale is thought to be due to a meteorological phenomenon: an ice sheet migrates toward its source, and hence (in North America) essentially toward the south. As such an icecap rim migrates, it grows higher, until ultimately it starves its own starting or "kernel" area, while the southern ridge has extended itself into latitudes which are too warm and altitudes where moisture is insufficient to maintain an icecap. The meteorological model, taken by itself, would not permit long interglacials. Isostasy, which moves the land surface up and down (in response to loading and unloading) over a vertical range of perhaps as much as 1.0 km., provides the necessary time lag. The meteorological model can operate effectively only on "wide" continents such as North America and Europe. Greenland and Antarctic ice sheets are, therefore, safe from the dangers of migrating into warmer areas, and hence will not melt until over-all glacial conditions have been terminated. Field evidence suggests that complete melting did not take place during any of the Pleistocene interglacials. The combined model can be used to predict that sea level willl not return, in the near future, to its "normal" position, but rather that, in an interval on the order of years, it will begin to drop again as the earth enters a new cold epoch. The precise timing will depend on the rate at which isostatic rebound re-elevates pertinent parts of North America and Europe to the critical position, and perhaps also on the timing of an ''extreme value" sequence of heavy-snowfall years. In the light of the rate at which North America has approached its polar position over the last years, the over-all history of Pleistocene glaciation can be expected to last for approximately years, interrupted by various interglacials as the ice sheets starve themselves.
Article
The possibility that a significant part of the energy of the planetary-wave disturbances of the troposphere may propagate into the upper atmosphere is investigated. The propagation is analogous to the transmission of electromagnetic radiation in heterogeneous media. It is found that the effective index of refraction for the planetary waves depends primarily on the distribution of the mean zonal wind with height. Energy is trapped (reflected) in regions where the zonal winds are easterly or are large and westerly. As a consequence, the summer circumpolar anticyclone and the winter circumpolar cyclone in the upper stratosphere and mesosphere are little influenced by lower atmosphere motions. Energy may escape into the mesosphere near the equinoxes, when the upper-atmosphere zonal flow reverses. At these times tunneling of the energy through a reflecting barrier is also possible. Most of the time, however, there appears to be little mechanical coupling on a planetary scale between the upper and lower atmospheres. Tropospheric sources of wave disturbances in the zonal flow are baroclinic instability and the forcing action of zonally asymmetric heating and topography. The transmissivity of the upper atmosphere increases with wavelength and is greater for the forced perturbations than for the unstable tropospheric waves, whose lengths must be smaller than the critical length for instability. The analysis indicates that baroclinically unstable wave disturbances originating in the troposphere probably do not propagate energy vertically at all. When energy is propagated to great heights, nonlinear vertical eddy transports of heat and momentum associated with the vertically propagating waves should modify the basic zonal flow. However, when the wave disturbance is a small stationary perturbation on a zonal flow that varies vertically but not horizontally, the second-order effect of the eddies on the zonal flow is zero.
Article
The transfer of heat and momentum by motion on the scale of cyclone waves and long waves is deduced from a knowledge of their mechanics. The law of horizontal transfer of entropy is independent of notions of mixing but is similar to that for non-isotropic diffusion. The law accurately represents the annual variation of heat-flux in the Northern Hemisphere. Perturbation theory suggests realistic spatial variations for the transfer and provides a rational basis for calculating transfer coefficients which can then be used to find the flux of other conservative quantities. Most importantly the Equatorward flux of potential vorticity is well defined and is shown to determine the horizontal flux of momentum uv. The mean surface winds, and the mean meridional circulation (which is seen to be frictionally driven) are then determined. A model of the general circulation is defined by these properties, and this is integrated analytically for a simplified case and compared with motion observed in the Gulf Stream, the mesosphere, and dishpan experiments as well as the troposphere.
Article
Thesis (Ph.D.)--University of Chicago, Department of Meteorology. Includes bibliographical references.
Article
The effects of a sloping lower boundary on the quasi-geostrophic baroclinic instability model of Eady are considered. Although the analytical model is too simplified for direct application to Martian conditions, the results should be useful in the interpretation of observations and numerical experiments. For slope orientations of most interest on Mars, the wavelength, growth rate and heat fluxes associated with the most unstable waves are decreased. As a result, the radiative-dynamical equilibrium state is closer to radiative equilibrium than one would calculate for a model with a horizontal lower boundary.
Sloping Convection in a Container with Sloping and Walls
  • P J Mason
  • P. J. Mason
The Energy Balance of the Global Atmosphere. In: The Global Circulation of the Atmosphere
  • R E Newell
  • D G Vincent
  • T G Dopplick
  • D Ferruza
  • J W Kidson
The Pulse of the Earth. The Hague: Wijhoff
  • J H F Umgrove
  • J. H. F. Umgrove
Ice-Age Meteorology. Ice-Age Theory in Encyc
  • R W Fairbridge
The Energy Balance of the Global Atmosphere
  • R E Newell
  • D G Vincent
  • T G Dopplick
  • D Ferruza
  • J W Kidson
  • R. E. Newell