The heat flow of Europa     

Javier Ruiz 《Icarus》2005,177(2):438-446

The heat flow from Europa has profound implications for ice shell thickness and structure, as well as for the existence of an internal ocean, which is strongly suggested by magnetic data. The brittle-ductile transition depth and the effective elastic thickness of the lithosphere are here used to perform heat flow estimations for Europa. Results give preferred heat flow values (for a typical geological strain rate of ¹⁰⁻¹⁵ s⁻¹) of 70-110 mW m⁻² for a brittle-ductile transition 2 km deep (the usually accepted upper limit for the brittle-ductile transition depth in the ice shell of Europa), 24-35 mW m⁻² for an effective elastic thickness of 2.9 km supporting a plateau near the Cilix impact crater, and >130 mW m⁻² for effective elastic thicknesses of ?0.4 km proposed for the lithosphere loaded by ridges and domes. These values are clearly higher than those produced by radiogenic heating, thus implying an important role for tidal heating. The ?19-25 km thick ice shell proposed from the analysis of size and depth of impact structures suggests a heat flow of ?30-45 mW m⁻² reaching the ice shell base, which in turn would imply an important contribution to the heat flow from tidal heating within the ice shell. Tidally heated convection in the ice shell could be capable to supply ∼100 mW m⁻² for superplastic flow, and, at the Cilix crater region, ∼35-50 mW m⁻² for dislocation creep, which suggests local variations in the dominant flow mechanism for convection. The very high heat flows maybe related to ridges and domes could be originated by preferential heating at special settings.  相似文献   

Temporal and spatial variability of fecal indicator bacteria in the surf zone off Huntington Beach, CA     

Rosenfeld LK  McGee CD  Robertson GL  Noble MA  Jones BH 《Marine environmental research》2006,61(5):471-493

Fecal indicator bacteria concentrations measured in the surf zone off Huntington Beach, CA from July 1998-December 2001 were analyzed with respect to their spatial patterns along 23 km of beach, and temporal variability on time scales from hourly to fortnightly. The majority of samples had bacterial concentrations less than, or equal to, the minimum detection limit, but a small percentage exceeded the California recreational water standards. Areas where coliform bacteria exceeded standards were more prevalent north of the Santa Ana River, whereas enterococci exceedances covered a broad area both north and south of the river. Higher concentrations of bacteria were associated with spring tides. No temporal correspondence was found between these bacterial events and either the timing of cold water pulses near shore due to internal tides, or the presence of southerly swell in the surface wave field. All three fecal indicator bacteria exhibited a diel cycle, but enterococci rebounded to high nighttime values almost as soon as the sun went down, whereas coliform levels were highest near the nighttime low tide, which was also the lower low tide.  相似文献   

Spin states and climates of eccentric exoplanets     

Anthony R. Dobrovolskis 《Icarus》2007,192(1):1-23

The known extrasolar planets exhibit a wide range of orbital eccentricities e. This has a profound influence on their rotations and climates. Because of tides in their interiors, mostly solid exoplanets are expected eventually to despin to a state of spin-orbit resonance, where the orbital period is some integer or half-integer times the rotation period. The most important of these resonances is the synchronous state, where the planet's spin period exactly equals its orbital period (like Earth's Moon, and indeed most of the regular satellites in the Solar System). Such planets seem doomed to roast on one side and freeze on the other. However, synchronous planets rock back and forth by an angle of ∼2Arcsine with respect to the sub-stellar point. For e=0.055 (as for the Moon), this optical libration amounts to only ∼6°; but for a synchronous planet with e=0.50, for example, it would rise to ∼59°. This greatly expands the temperate “twilight zone” near the terminator and considerably improves the planet's prospects for habitability. For e?0.72389, the optical libration exceeds 90°; for such planets, the sector of permanent night vanishes, while the sunniest region splits in two. Furthermore, the synchronous state is not the only possible spin resonance. For example, Mercury (with e≈0.206) has an orbital period exactly 1.5 times its rotation period. A terrestrial exoplanet with e=0.40, say, is liable to have an orbital period of 2.0, 2.5, or 3.0 times its spin period. The corresponding insolation patterns are generally complicated, and all different from the synchronous state. Yet these non-synchronous resonances also protect certain longitudes from the worst extremes of temperature and solar radiation, and improve the planet's habitability, compared to non-resonant rotation. These results also have implications for the direct detectability of extrasolar planets, and the interpretation of their thermal emissions.  相似文献   

A Hamiltonian theory for an elastic earth: Canonical variables and kinetic energy     

Juan Getino  Jose M. Ferrándiz 《Celestial Mechanics and Dynamical Astronomy》1990,49(3):303-326

This paper is the first of a set of four, in which we shall develop the first part of a project dedicated to elaborating a Hamiltonian theory for the rotational motion of a deformable Earth. Here we study only the perturbation due to the deformation of the elastic mantle by tidal body force. In the present paper, we define two canonical systems of variables—we give these variables the names of elastic variables of Euler and Andoyer respectively. Next, using them, we obtain the canonical expression of rotational kinetic energy, which is valid for any Earth model satisfying hypotheses as general as those established in Section 2.  相似文献   

A numerical study on the influence of geometry on the formation of sandbanks     

Noel Carbajal  Stephanie Piney  Jorge Gómez Rivera 《Ocean Dynamics》2005,55(5-6):559-568

Applying a two-dimensional, non-linear hydrodynamic numerical model in combination with a semiempirical equation for bedload sediment transport, the influence of geometry on the formation of sandbanks is investigated. In the first experiment, the formation of sandbanks in an ideal rectangular basin, resembling the Taylor’s problem, was calculated. Sandbanks occur in a small area at the closed boundary. Similar experiments were carried out for a range of wavelengths of the incident Kelvin wave. The results reveal that large wavelengths favor the generation of sandbanks. In subsequent calculations, the basin was modified by introducing new geographical features like bays and peninsulas. The numerical experiments show that geometry is a fundamental factor to determine the position where groups of sandbanks appear. The results suggest that in regions where the Kelvin wave is diffracted, the formation of sandbanks occurs. An experiment, in which we applied an ideal geometric configuration representing that of the Southern Bight of the North Sea, generated sandbank patterns resembling those observed in the region.  相似文献   

The effect of flow at Maud Rise on the sea-ice cover – numerical experiments     

Aike Beckmann  Ralph Timmermann  Adriene F. Pereira  Christian Mohn 《Ocean Dynamics》2001,52(1):11-25

 The role of seamounts in the formation and evolution of sea ice is investigated in a series of numerical experiments with a coupled sea ice–ocean model. Bottom topography, stratification and forcing are configured for the Maud Rise region in the Weddell Sea. The specific flow regime that develops at the seamount as the combined response to steady and tidal forcing consists of free and trapped waves and a vortex cap, which is caused by mean flow and tidal flow rectification. The enhanced variability through tidal motion in particular modifies the mixed layer above the seamount enough to delay and reduce sea-ice formation throughout the winter. The induced sea-ice anomaly spreads and moves westward and affects an area of several 100 000 km². Process studies reveal the complex interaction between wind, steady and periodic ocean currents: all three are required in the process of generation of the sea ice and mixed layer anomalies (mainly through tidal flow), their detachment from the topography (caused by steady oceanic flow) and the westward translation of the sea-ice anomaly (driven by the time-mean wind).  相似文献   

On the effects of wind and tides on the hydrodynamics of a shallow mediterranean estuary     

Clifford J. Hearn  Barbara J. Robson 《Continental Shelf Research》2002,22(18-19)

A study is made of the effect of wind and tides on the hydrodynamics of the shallow inner basins of mediterranean estuaries. The paper includes a case study of Harvey Estuary in southwestern Australia where salinity and temperature data exist for 11 years during the 1980s and 1990s when that estuary experienced massive annual blue-green algal blooms. An analysis is made of salt exchange through the channels that join estuarine basins of this class to either the ocean or, as in the case of Harvey Estuary, to another shallow estuarine basin. A detailed three-dimensional numerical model is also implemented for the basin of Harvey Estuary. It is concluded that exchange through the channel is dominated by the (mainly diurnal) tides, despite the general micro-tidal nature of this class of estuary, although the efficiency of this process is found to be controlled by the length of the channel. Wind set-up in the basin also produces channel exchange and for Harvey Estuary this is about 20% of the exchange due to tides. Baroclinic flow through the channel is also capable of producing significant exchange but this is suppressed by the tidal currents in the channel except immediately after riverflow. Salt transport along the basins of this class of estuary is mainly driven by the longitudinal density gradient and the strength of this process is controlled by vertical mixing from the wind. However, there is also significant salt transport from wind-induced advection, the effect of which changes seasonally with the direction of the salt gradient.  相似文献   

Suppressing bias and drift of coastal circulation models through the assimilation of seasonal climatologies of temperature and salinity     

Keith R. Thompson  Kyoko OhashiJinyu Sheng  Josko BobanovicJie Ou 《Continental Shelf Research》2007

Recently Thompson et al. (2006. A simple method for reducing seasonal bias and drift in eddy resolving ocean models. Ocean Modelling 13, 109–125.) proposed a new method for suppressing the bias and drift of ocean circulation models. The basic idea is to nudge the model toward gridded climatologies of observed temperature and salinity in prescribed frequency–wavenumber bands; outside of these bands the model's dynamics are not directly affected by the nudging and the model state can evolve prognostically. Given the restriction of the nudging to certain frequency–wavenumber bands, the method is termed spectral nudging. The frequency–wavenumber bands are chosen to capture the information in the climatology and thus are centered on the climatological frequencies of zero, one cycle per year and its harmonics, and also low wavenumbers (reflecting the smooth nature of gridded climatologies). The new method is applied in this study to a fully nonlinear, 3D baroclinic circulation model of the continental shelves and inland seas of Atlantic Canada and the northeast US. It is shown that the scheme can suppress drift and bias in a nine month integration (February–October, 2002) while still allowing realistic evolution of tides, surges and wind and tide-driven coastal upwelling. It is also shown that density stratification can affect significantly tidal elevations in some regions. The implications for ocean hindcasting and short-term forecasting are discussed.  相似文献   

Comparison between measured and calculated tidal ellipses in the German Bight     

Noel Carbajal  Thomas Pohlmann 《Ocean Dynamics》2004,54(5):520-530

Using a three-dimensional non-linear shelf model, the elliptical properties (ellipticity, inclination of the ellipse, major and minor semi-axis and phase) of the M₂ tide in the German Bight were calculated and compared with CODAR measurements. A series of barotropic and baroclinic calculations were carried out to investigate the influence of geometry, stratification and particularly inputs of freshwater on these parameters. The elliptical properties undergo stronger changes in zones of influence of embayments and in the deepening of the old Elbe Valley. Friction effects in the shallow areas are responsible for robust vertical variations of the ellipticity. The island of Helgoland induces wakes on its western and eastern sides. The discharge of freshwater of the rivers Elbe, Weser and Ems induced in general negative ellipticity. Although primarily determined by geography, baroclinic effects significantly modified the inclination of the ellipses. The calculated ellipses pattern of anticlockwise and clockwise tidal current rotation agrees quite well with CODAR measurements. The elliptical properties give a general idea of the interaction of tidal waves with coastal geometries.Responsible Editor: Hans Burchard  相似文献   

On the comparison of tidal gravity parameters with tidal models in central Europe     

《Journal of Geodynamics》2014

Two accurately calibrated superconducting gravimeters (SGs) provide high quality tidal gravity records in three central European stations: C025 in Vienna and at Conrad observatory (A) and OSG050 in Pecný (CZ). To correct the tidal gravity factors from ocean loading effects we compared the load vectors from different ocean tides models (OTMs) computed with different software: OLFG/OLMP by the Free Ocean Tides Loading Provider (FLP), ICET and NLOADF. Even with the recent OTMs the mass conservation is critical but the methods used to correct the mass imbalance agree within 0.1 nm/s². Although the different software agrees, FLP probably provides more accurate computations as this software has been optimised. For our final computation we used the mean load vector computed by FLP for 8 OTMs (CSR4, NAO99, GOT00, TPX07, FES04, DTU10, EOT11a and HAMTIDE). The corrected tidal factors of the 3 stations agree better than 0.04% in amplitude and 0.02° in phase. Considering the weighted mean of the three stations we get for O1 δ_c = 1.1535 ± 0.0001, for K1 δ_c = 1.1352 ± 0.0003 and for M2 δ_c = 1.1621 ± 0.0003. These values confirm previous ones obtained with 16 European stations. The theoretical body tides model DDW99/NH provides the best agreement for M2 (1.1620) and MATH01/NH for O1 (1.1540) and K1 (1.1350). The largest discrepancy is for O1 (0.05%). The corrected phase α_c does not differ significantly from zero except for K1 and S2. The calibrations of the two SG's are consistent within 0.025% and agree with Strasbourg results within 0.05%.  相似文献