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1.
A three-dimensional mesoscale numerical model is used to investigate mesoscale circulation over a Gulf Stream filament. Two numerical experiments are performed with different initial uniform ambient wind speeds (U=0.1 m s–1, 3.5 m s–1 and 7 m s–1) for a typical winter day. It is found that for both low and moderate winds, a closed mesoscale circulation forms over the Gulf Stream filament. When the Gulf Stream filament was removed, the model did not predict a mesoscale circulation. The modeled circulation over the filament is in agreement with the observations, suggesting that the atmospheric circulations over the filaments may be an important mechanism in the U.S. East Coast cyclogenesis.  相似文献   

2.
The long-term variability of the non-tidal circulation in Southampton Water, a partially mixed estuary, was investigated using 71-day acoustic Doppler current profiler (ADCP) time series. The data show evidence that the spring–neap tidal variability of the turbulent mixing modulates the strength of the non-tidal residual circulation, with subtidal neap tide surface flows reaching 0.12 m s–1 compared to <0.05 m s–1 at spring tides. The amplitude of the neap-tide events in this non-tidal circulation is shown to be related to a critical value of the tidal currents, illustrating the strong dependence on tidal mixing. The results suggest that the dominant mechanism for generating these neap-tide circulation events is the baroclinic forcing of the horizontal density gradient, rather than barotropic forcing associated with ebb-induced periodic stratification. While tidal turbulence is thought to be the dominant control on this gravitational circulation, there is evidence of the additional effect of wind-driven mixing, including the effects of wind fetch and possibly wave development with along-estuary winds being more efficient at mixing the estuary than across-estuary winds. Rapid changes in atmospheric pressure also coincided with fluctuations in the gravitational circulation. The observed subtidal flows are shown to be capable of rapidly flushing buoyant material out of the estuary and into the coastal sea at neap tides.Responsible Editor: Iris Grabemann  相似文献   

3.
Gravitational effect of water circulation in the northwest Yunnan   总被引：1，自引：0，他引：1  
ＧｒａｖｉｔａｔｉｏｎａｌｅｆｆｅｃｔｏｆｗａｔｅｒｃｉｒｃｕｌａｔｉｏｎｉｎｔｈｅｎｏｒｔｈｗｅｓｔＹｕｎｎａｎＭｉｎ－ＹｕＪＩＡ；（贾民育）Ｓｈａｏ－ＡｎＳＵＮ；（孙少安）Ａｉ－ＭｉｎＸＩＡＮＧ；（项爱民）ａｎｄＤｏｎｇ－ＺｈｉＬＩＵ（刘冬至）（...  相似文献   

4.
A two-? ocean reservoir model is employed to examine the combined effects of vertical ocean circulation, organic matter extraction and cosmic ray production on the14C/12C in the atmosphere, ocean and sediment. In this model, dissolution of deep-sea calcium carbonate sediment is assumed to respond to the supply of particulate carbonate from the surface ocean and to the dissolved carbonate-ion concentration of the deep-sea.If the vertical ocean circulation decreased by 50% during the maximum rate of deglaciation, the atmospheric14C/12C would have increased concurrently by 10% relative to the pre-bomb present. Further, if cosmic ray production of14C was 50% greater than present at about 7800 years B.P. as suggested by archeomagnetic measurements, a double maximum of 10% occurs. The first is at about 10.5 ka and the second at 6.5 ka. This result is similar to the variation of14C/12C over the last 10,000 years calculated from the14C dates measured by Stuiver on the varved Lake of the Clouds. The result is not sensibly altered if 1017 moles of organic carbon is extracted from the ocean during sea-level rise.Reduction of the thermohaline ocean circulation by 50% over a one- to two-thousand-year interval would not be sensibly detected in the dating of deep-sea sediment. If Broecker's organic nutrient extraction occurs in conjunction with reduction of vertical circulation, a decrease in the atmospheric PCO2 will precede the eventual rise.  相似文献   

5.
Deep circulation driven by strong vertical mixing in the Timor Basin   总被引：1，自引：1，他引：0  
The importance of deep mixing in driving the deep part of the overturning circulation has been a long debated question at the global scale. Our observations provide an illustration of this process at the Timor Basin scale of ～1000 km. Long-term averaged moored velocity data at the Timor western sill suggest that a deep circulation is present in the Timor Basin. An inflow transport of ～0.15 Sv is observed between 1600 m and the bottom at 1890 m. Since the basin is closed on its eastern side below 1250 m depth, a return flow must be generated above 1600 m with a ～0.15 Sv outflow. The vertical turbulent diffusivity is inferred from a heat and transport balance at the basin scale and from Thorpe scale analysis. Basin averaged vertical diffusivity is as large as 1 × 10?3 m2 s?1. Observations are compared with regional low-resolution numerical simulations, and the deep observed circulation is only recovered when a strong vertical diffusivity resulting from the parameterization of internal tidal mixing is considered. Furthermore, the deep vertical mixing appears to be strongly dependent on the choice of the internal tide mixing parameterization and also on the prescribed value of the mixing efficiency.  相似文献   

6.
The ocean circulation on Australia's Northern Shelf is dominated by the Monsoon and influenced by large-scale interannual variability. These driving forces exert an ocean circulation that influences the deep Timor Sea Passage of the Indonesian Throughflow, the circulation on the Timor and Arafura Shelves and, further downstream, the Leeuwin Current. Seasonal maxima of northeastward (southwestward) volume transports on the shelf are almost symmetric and exceed 106 m3/s in February (June). The associated seasonal cycle of vertical upwelling from June to August south of 8.5°S and between 124°E and 137.5°E exceeds 1.5×106 m3/s across 40 m depth. During El Niño events, combined anomalies from the seasonal means of high regional wind stresses and low inter-ocean pressure gradients double the northeastward volume transport on the North Australian Shelf to 1.5×106 m3/s which accounts for 20% of the total depth-integrated transport across 124°E and reduce the total transport of the Indonesian Throughflow. Variability of heat content on the shelf is largely determined by Pacific and Indian Ocean equatorial wind stress anomalies with some contribution from local wind stress forcing.  相似文献   

7.
Global ocean circulation models usually lack an adequate consideration of high-latitude processes due to a limited model domain or insufficient resolution. Without the processes in key areas of the global thermohaline circulation, the characteristics and flow of deep and bottom waters cannot be modeled realistically. In this study, a high-resolution (~20 km) ocean model focused on the Weddell Sea sector of the Southern Ocean is combined with a low-resolution (2° × 2°) global ocean model applying the state estimation technique. Temperature, salinity, and velocity data on two Weddell Sea sections from the regional model are used as constraints for the large-scale model in addition to satellite altimetry and sea-surface temperatures. The differences between the model with additional constraints and without document that the Weddell Sea circulation exerts significant influence on the course of the Antarctic Circumpolar Current with consequences for Southern Ocean water mass characteristics and the spreading of deep and bottom waters in the South Atlantic. Furthermore, a warming trend in the period 1993–2001 was found in the Weddell Sea and adjacent basins in agreement with float measurements in the upper Southern Ocean. Teleconnections to the North Atlantic are suggested but need further studies to demonstrate their statistical significance.  相似文献   

8.
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9.
An 8-m continuous sediment core, approximately 250-ky-old at the bottom, from Academician Ridge in Lake Baikal, has been analyzed for the stable isotopes of carbon, nitrogen and sulfur, in order to study the paleoclimatic and paleobiological changes that occurred in the Eurasian continental interior. These isotopic changes are closely related to changes in vertical lake-water circulation between glacial and interglacial periods. Sedimentary organic carbon in cool periods is more enriched in 13C (−23.8‰ on average) than that in warm periods (−27.0‰ on average). The 13C-enrichment of organic carbon suggests a decrease of land-derived organic matter influx to the lake, less precipitation, and loss of terrestrial vegetation around Lake Baikal in cool periods. Pyrite in high total sulfur/total organic carbon (TS/TOC) layers shows strong depletion in 34S (−20.8‰ to −32.4‰) during climate transitions from glacial to interglacial periods at the beginning of oxygen isotope stages (OIS) 1, 5 and 7. The 34S-depleted pyrite indicates augmentation of dissimilatory sulfate reduction by sulfate reducing bacteria (SRB) at the sediment-water interface. Enhancement of aqueous sulfate concentrations and limitation of oxygen circulation to the surface sediments might also occur in the climate transition periods. The δ15N values of total nitrogen increase abruptly by ∼2‰ just after the δ34S negative peaks, which may result from low nutrient concentrations in the euphotic zone associated with water circulation changes in Lake Baikal.  相似文献   

10.
The correlation between geomagnetic activity and the sunspot number in the 11-year solar cycle exhibits long-term variations due to the varying time lag between the sunspot-related and non-sunspot related geomagnetic activity, and the varying relative amplitude of the respective geomagnetic activity peaks. As the sunspot-related and non-sunspot related geomagnetic activity peaks are caused by different solar agents, related to the solar toroidal and poloidal fields, respectively, we use their variations to derive the parameters of the solar dynamo transforming the poloidal field into toroidal field and back. We find that in the last 12 cycles the solar surface meridional circulation varied between 5 and 20 m/s (averaged over latitude and over the sunspot cycle), the deep circulation varied between 2.5 and 5.5 m/s, and the diffusivity in the whole of the convection zone was ～108 m2/s. In the last 12 cycles solar dynamo has been operating in moderately diffusion dominated regime in the bulk of the convection zone. This means that a part of the poloidal field generated at the surface is advected by the meridional circulation all the way to the poles, down to the tachocline and equatorward to sunspot latitudes, while another part is diffused directly to the tachocline at midlatitudes, “short-circuiting” the meridional circulation. The sunspot maximum is the superposition of the two surges of toroidal field generated by these two parts of the poloidal field, which is the explanation of the double peaks and the Gnevyshev gap in sunspot maximum. Near the tachocline, dynamo has been operating in diffusion dominated regime in which diffusion is more important than advection, so with increasing speed of the deep circulation the time for diffusive decay of the poloidal field decreases, and more toroidal field is generated leading to a higher sunspot maximum. During the Maunder minimum the dynamo was operating in advection dominated regime near the tachocline, with the transition from diffusion dominated to advection dominated regime caused by a sharp drop in the surface meridional circulation which is in general the most important factor modulating the amplitude of the sunspot cycle.  相似文献   

11.
Surf zone drifters and a current meter were used to study the nearshore circulation patterns in the lee of groynes at Cottesloe Beach and City Beach in Western Australia. The circulation patterns revealed that a persistent re-circulation cell was present in the lee of the groyne which was driven by changes in wave set-up resulting from lower wave heights in the lee of the groyne. The re-circulation consisted of a longshore current directed towards the groyne which was deflected offshore due to groyne resulting in a rip current along the groyne face. The offshore-flowing rip current and the incoming waves converged at the offshore extent of this circulation cell, with the deflection of the rip current parallel to the shoreline and then completing the recirculation through an onshore component. The Eulerian measurements revealed that 55% of the currents on the lee side of the groyne were directed offshore and that these currents had a maximum speed of 2 m s?1. Spectral analysis of the wave heights and the currents revealed several corresponding peaks in the measured spectral densities with timescales between 12 s and 50 min. Numerical simulations of an idealised beach with a shore-normal groyne were conducted using a circulation model driven by waves, and confirmed the formation of a persistent eddy in the lee of the groyne. Sensitivity studies indicated that the incident wave angle, wave period, and especially the wave height controlled the circulation. The eddy vorticity, a measure of an eddy's strength, increased roughly proportional to an increase in the incident wave energy flux.  相似文献   

12.
There is a paucity of field data to describe the transition in nearshore circulation between alongshore, meandering and rip current systems. A combination of in‐situ current meters and surf zone drifters are used to characterize the nearshore circulation over a transverse bar and rip morphology at Pensacola Beach, Florida in the presence of relatively low energy oblique waves. Current speeds vary in response to the relative wave height ratio (Hs/h), which defines the degree and extent of breaking over the shoal. In the absence of wave breaking the nearshore circulation was dominated by an alongshore current driven by the oblique waves. As waves begin to break across the shoal (0.2<Hs/ h<0.5) the nearshore circulation is characterized by a meandering alongshore current. As conditions became more dissipative (Hs/h>0.5), the meandering current is replaced by an unsteady rip circulation that moves offshore between the shoals before turning alongshore in the direction of wave advance outside the surf zone. The increase in wave dissipation is associated with an increase in very low frequency (VLF) variations in the current speed across the shoal and in the rip channel that caused the circulation to oscillate between an offshore and an alongshore flow. The unsteady nature of the nearshore circulation is responsible for 55% of all surf zone exits under these more dissipative conditions. In contrast, only 29% of the drifters released from the shoal exited the surf zone and bypassed the adjacent shoal with the alongshore‐meandering current. While the currents had a low velocity (maximum of ~0.4 m s‐1) and would not pose a significant hazard to the average swimmer, the results of this study suggest that the transverse bar and rip morphology is sufficient to create an alongshore variation in wave dissipation that forces alongshore meandering and low‐energy rip circulation systems under oblique wave forcing. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

13.
The question of whether or not global warming has paused since more than ten years ago, namely "warming hiatus", has attracted the attention of climate science community including the IPCC. Some authors have attributed the "warming hiatus" to the internal changes in the climate system, i.e., the recombination of ocean-atmosphere circulations. Therefore, it is necessary to propose higher requirements on reconstructing circulation background of climate change for the past millennium. However, the analyses of changes in atmospheric circulation over the last millennium as well as the conclusions of related regional climate patterns are so widely different and contradictory, bringing uncertainties to our understanding of regional even global climate change to a great extent. On the other hand, in the last 10 years the high-precision U/Th-dated stalagmite oxygen isotope ratio(δ~(18)O) sequences provided an accurate chronological frame for the paleoclimate study of the middle and late Pleistocene, in which all authors from China took the Chinese stalagmite δ~(18)O as the summer monsoon index without exception. However, this point of view misleads the climate scientists into thinking that the stalagmite δ~(18)O can be as the proxy of precipitation amount. Nevertheless, it is well known that all of these records have a lot in common in the low frequency trend. However, most sequences cannot be calibrated by instrumental precipitation records, and thus the uncertainty of the climate research framework of China and even of the world has increased. Therefore, it is imperative for climatology to clarify the origin of contradiction and to reduce the uncertainty as early as possible. On the basis of analyzing the significance of stalagmite δ~(18)O in the monsoon regions of China, the author tries to propose a new circulation proxy in this paper: integrating the Chinese stalagmite oxygen isotope sequence to reconstruct the tropical Pacific sea surface temperature gradient, i.e., the large-scale ENSO-like state over the past millennium. Furthermore, the author speculates that it was warm in the modern times and the Medieval Period, but the circulation recombination was different in both periods. And this inference could be supported by the longer record since Last Glacial Maximum. In other words, the attribution analysis of the identical low-frequency trends of Chinese stalagmite δ~(18)O on a large scale shows that the ENSO-like state controls the climate change in the monsoon regions of China at different time scales(from interannual to century or even longer time scales). Wherein the important connection of circulations is the western Pacific subtropical high(WPSH), that is to say, besides the interannual and decadal time scales, the WPSH would possess the circulation mode on longer timescales. For example, we may discuss the change of the WPSH in the whole Holocene epoch, i.e., the half precession period. These discussions could make sense to the study of not only the paleoclimate but also the modern climate.  相似文献   

14.
Linear mesospheric circulation models can quite realistically simulate mid-latitude mesospheric circulation and structure. In doing so they provide estimates for dissipation (Rayleigh friction 2×10–6s–1) and vertical mixing (30 m2s–1) appropriate near the mesopause.  相似文献   

15.
Stable isotopes in precipitation are useful tracers to strengthen understanding of climate change and hydrological processes. In this study, the moisture sources of 190 precipitation events in Beijing were analysed using the Hybrid Single‐particle Lagrangian Integrated Trajectory model, based on which we studied the relation between variations in precipitation δ18O and dynamics in moisture sources and atmospheric circulation in seasonal and interannual timescales. Categorization of 7 groups of moisture sources was performed, among which oceanic moisture sources presented lower δ18O in precipitation than continental moisture sources. The results show that seasonal variations of precipitation δ18O were caused by changes of moisture sources. In summer, moisture from proximal oceans dominated vapour transport to Beijing due to increasing monsoon strength and resulted in a relatively small variation of precipitation δ18O. At the interannual timescale, the variations of δ18O in summer precipitation were related to dynamics in oceanic moistures, showing depleted values when the contribution of oceanic moistures, especially the proportion of long‐distance oceanic moisture, was high. Further analysis indicated that changes of oceanic moisture sources were controlled by the strength of summer monsoons. These findings address the complexity of moisture sources in midlatitude monsoon areas and suggest that isotopic signals in precipitation have the potential to deduce changes in moisture sources and atmospheric circulation and can therefore serve for palaeoclimate reconstruction.  相似文献   

16.
Recent observations have quantified the auroral wind O+ outflow in response to magnetospheric inputs to the ionosphere, notably Poynting energy flux and precipitating electron density. For moderate to high activity periods, ionospheric O+ is observed to become a significant or dominant component of plasma pressure in the inner plasma sheet and ring current regions. Using a global circulation model of magnetospheric fields and its imposed ionospheric boundary conditions, we evaluate the global ionospheric plasma response to local magnetospheric conditions imposed by the simulation and evaluate magnetospheric circulation of solar wind H+, polar wind H+, and auroral wind O+. We launch and track the motions of millions of test particles in the global fields, launched at randomly distributed positions and times. Each particle is launched with a flux weighting and perpendicular and parallel energies randomly selected from defined thermal ranges appropriate to the launch point. One sequence is driven by a two-hour period of southward interplanetary magnetic field for average solar wind intensity. A second is driven by a 2-h period of enhanced solar wind dynamic pressure for average interplanetary field. We find that the simulated ionospheric O+ becomes a significant plasma pressure component in the inner plasma sheet and outer ring current region, particularly when the solar wind is intense or its magnetic field is southward directed. We infer that the reported empirical scalings of auroral wind O+ outflows are consistent with a substantial pressure contribution to the inner plasma sheet and plasma source surrounding the ring current. This result violates the common assumption that the ionospheric load is entirely confined to the F layer, and shows that the ionosphere is often an important dynamic element throughout the magnetosphere during moderate to large solar wind disturbances.  相似文献   

17.
The mean seasonal hydrography and circulation of the Western Iberian Margin (WIM) are studied by means of a high-resolution configuration of the Regional Oceanic Modeling System. A comparison of 5-year model averages for January and July with climatological datasets shows a general good agreement in the reproduction of the mean water mass properties and hydrographic distribution. We find that there is a prevailing tendency for slope poleward flow at about 80–100 km offshore at all latitudes from the surface to 1,500 m with strong vertical coupling. This northward flow, which is mainly along slope and amounts up to 8–10 cm s?1, exhibits several mean flow recirculation regions on its way and evidences of an offshore pathway of poleward flow. Transports at different zonal sections further confirm the poleward flow tendency with two peaks of poleward transport in summer (3–10 Sv) and winter (2–7 Sv). The transport time series emphasize the seasonal character of the alongshore circulation and the interannual intrinsic variability of the circulation, since the forcing fields are climatological. As a conceptual essay with the purpose of assessing the Mediterranean Water flow influence on the WIM mean circulation, a second model configuration is setup, where the Mediterranean outflow into the study domain is removed. We find that there is an attenuation of the mesoscale field, but the slope poleward flow intensifies and remains as a mean dynamical feature closer to the upper slope.  相似文献   

18.
Western Port, Victoria, Australia is a tide-dominated embayment with an unusual and complex shape. Bottom currents and circulation and their effects on sediment processes were examined using instrumented tripods to measure currents, tides and wave activity, and to estimate sediment transport at nine locations in the bay. Overall bottom water movement patterns were determined by use of seabed drifters. The characteristics of the bay reflect a small catchment and low freshwater and sediment input. A complex system of channels is flanked by extensive intertidal areas. Tidal range varies up to more than 3 m, generating bottom currents up to 70cm s?1. Flow directions generally conform to channel alignments but major deviations are important. Net circulation in the bay is clockwise around the large central island. The ratio of tidal range to half-tidal period (ΔH/ΔT) when compared with measured currents gave a method of prediction of the annual frequency distribution of maximum bottom current velocities. Determination of threshold current velocities enabled prediction of annual frequency of bedload movement (generally 50–100 per cent of tide cycles). Bedload mass transport for all observed tide cycles was calculated, and estimates of annual mass transport capacity (between 102 and 106 g cm?1 a?1( were obtained from a relationship between predicted tidal conditions and mass transport. Seabed drifters delineated the major bottom water movements in the bay and adjacent Bass Strait, and also detailed circulation patterns. This linked the data from the tripod stations, in particular patterns of ebb- and flood-dominance. A coherent picture of the processes operating in Western Port is presented by integrating these studies with corroborative studies of sediment distribution and morphology, hydrochemistry, and mathematical modelling. Some morphological characteristics related to tide-dominance are discussed.  相似文献   

19.
The principles of a dating method based on the cosmic-ray-produced radioisotope39Ar are given. Technical requirements such as background and standard gas samples and gas proportional counting systems are described.With samples extracted from Greenland ice it can be demonstrated that39Ar ages agree with ages obtained by other methods. First results on ocean water samples show that with this isotope valuable information on ocean mixing and circulation can be expected.39Ar results on groundwater samples disagree for some aquifers with conventional14C ages; possible explanations are discussed, especially underground production of39Ar.  相似文献   

20.
Two methods for estimating the true formation temperature from well logs are compared. One method requires knowledge of the circulation time, whereas the other requires an estimate of the thermal diffusivity of the contents of the well. Both methods require three or more successive bottom-hole temperature measurements. Data from 157 wells have been analyzed, and the calculated formation temperatures from the two methods agree well. By an analysis of best matching of the two methods, it is found that 0.35 × 10-6 m2 s-1 is a good estimate for the thermal diffusivity of the well contents.  相似文献   

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