摘要:

We present a review of β spiral techniques which have recently been developed for the determination of absolute velocity profiles from hydrographic observations. A specific technique is then designed and applied to the North Atlantic part of Levitus' (1982) climatological hydrographic atlas with the aim of estimating reference velocities and diffusivities for heat, salt, and potential vorticity. These quantities are determined on a 1° grid from the local gradients of temperature and salinity under the constraints of the thermal wind relations and the conservation of the respective tracers including diapycnic and isopycnic mixing terms. The estimation procedure includes the statistical framework of inverse modeling in the weighting of the constraints by the data noise variances and the determination of the covariances of the model parameters. The resulting circulation pattern bears strong resemblance to the classical view of the North Atlantic circulation as put foreward by Wüst (1935) and Defant (1941). The upper layers are dominated by the Gulf Stream/North Atlantic current system with a broad subtropical gyre recirculation. In the lower layer a western boundary current is fed from Norwegian Sea overflow penetrating the Gibbs fracture zone and partly circulating around the Labrador Sea. As a consequence of the climatological averaging the currents appear in broad shape with much reduced velocities, in particular in the upper layer. The vertical structure reveals an almost horizontal level of no motion pattern much along the concepts of Defant (1941). Diffusion coefficients were determined for an upper layer (depth of mixed layer to 800 m depth) and a lower layer (800 m to 2000 m). The spatial pattern of these coefficients correlates with maps of eddy activity, showing higher values in the strong current regimes and low values within the subtropical and subpolar gyre. Average values in the lower layer of the quiet regions are 10−5m²/s and 10² m²/s for the diapycnal and isopycnal diffusivity, respectively, and 10−1m²/s for the vertical diffusivity of vorticity (which yields 10² m²/s for the lateral diffusivity of potential vorticity). Toward the regions of strong currents and in the upper layer these values roughly increase by an order

ISSN:8755-1209    

DOI:10.1029/RG023i004p00313

年代:1985

数据来源: WILEY

摘要:

The mechanisms that contribute to the generation and damping of large‐scale mid‐latitude sea surface temperature (SST) anomalies are discussed. The SST anomalies reflect primarily the response of the upper ocean to the changes in air‐sea fluxes that are associated with daily weather fluctuations. Heat flux forcing is dominant in the lower middle latitudes, while wind‐driven entrainment may be most effective in the high latitudes; advection by anomalous Ekman current is generally less important, and Ekman pumping is negligible. The SST anomalies decay in part because of entrainment effects associated with mixed‐layer deepening and oceanic mixing and in part because of heat exchanges with the atmosphere. The three approaches commonly used to model the evolution of SST anomalies are reviewed: case studies based on monthly or seasonal anomaly maps of the large‐scale SST and atmospheric anomalies, numerical simulations with one‐dimensional mixed‐layer models, and stochastic forcing models. We stress the similarities in the different approaches and discuss their main advantages and limitations. The response of the atmosphere to mid‐latitude SST anomalies is considered. First, we discuss the poorly known relationship between SST anomalies and diabatic heating. Using a crude assumption for the air‐sea coupling, we consider a two‐layer quasi‐geostrophic channel model and discuss the stationary wave response to SST anomaly forcing and the resulting air‐sea feedback. It is found that the back interaction of the SST anomalies onto the atmosphere causes a weak SST anomaly damping at large scales and a strong one at small scales; the air‐sea coupling should also act as an eastward propagator for the SST anomalies. The response of more realistic linear wave models to prescribed diabatic heating is then reviewed, and it is suggested that realistic mid‐latitude SST anomalies have a weak influence on the atmospheric circulation, corresponding to changes in the geopotential height of 10–30 m at most. This order of magnitude is consistent with the results of general circulation model experiments and with the limited climate predictability associated

ISSN:8755-1209    

DOI:10.1029/RG023i004p00357

年代:1985

数据来源: WILEY

摘要:

The shergottites, nakhlites, and Chassigny (SNC meteorites) are apparently cumulate mafic and ultramafic rocks that crystallized at shallow levels in the crust of their parent body. The mineralogy and chemistry of these meteorites are remarkably like equivalent terrestrial rocks, although their ratios of Fe/(Fe + Mg) and certain incompatible elements and their oxygen isotopic compositions are distinctive. All have crystallization ages of 1.3 b.y. or younger and formed from magmas produced by partial melting of previously fractionated source regions. Isotope systematics suggest that the SNC parent body had a complex and protracted thermal history spanning most of geologic time. Some meteorites have been severely shock metamorphosed, and all were ejected from their parent body at relatively recent times, possibly in several impact events. Late crystallization ages, complex petrogenesis, and possible evidence for a large gravitational field suggest that these meteorites are derived from a large planet. Trapped gases in shergottite shock melts have compositions similar to the composition measured in the Martian atmosphere. Ejection of Martian meteorites may have been accomplished by acceleration of near‐surface spalls or other mechanisms not fully understood. If SNC meteorites are of Martian origin, they provide important information on planetary composition and evolution. The bulk composition and redox state of the Martian mantle, as constrained by shergottite phase equilibria, must be more earthlike than most current models. Planetary thermal models should benefit from data on the abundances of radioactive heat sources, the melting behavior of the mantle, and the timing of planetary differentiation. Calculated depletion of chalcophile elements in source regions indicates a core dominated by sulfides, and paleomagnetic measurements suggest the presence of a weak magnetic field within the last several hundred thousand years. Concentrations of volatile elements indicate that the SNC parent body was not volatile depleted, and trapped atmospheric components measured in shock melts may be useful in understanding planetary degassing. By providing comparisons for spectral reflectance data and Viking soil analyses, these meteorites may also constrain surface mineralogy and weathering mechanism

ISSN:8755-1209    

DOI:10.1029/RG023i004p00391

年代:1985

数据来源: WILEY