American Geophysical Union (AGU)
0148-0227
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Coupling between the Atlantic cold tongue and the West African monsoon in boreal spring and summer
Tập 116 Số C4
Wind‐driven lateral circulation in a stratified estuary and its effects on the along‐channel flow In the stratified rotating estuary of Chesapeake Bay, the Ekman transport drives a counterclockwise lateral circulation under down‐estuary winds and a clockwise lateral circulation under up‐estuary winds (looking into estuary). The clockwise circulation is about twice as strong as the counterclockwise circulation. Analysis of the streamwise vorticity equation reveals a balance among three terms: titling of the planetary vorticity by vertical shear in the along‐channel current, baroclinic forcing due to sloping isopycnals at cross‐channel sections, and turbulent diffusion. The baroclinic forcing is highly asymmetric between the down‐ and up‐estuary winds. While the counter‐clockwise lateral circulation tilts isopycnals vertically and creates lateral barolinic pressure gradient to oppose the Ekman transport under the down‐estuary wind, the clockwise circulation initially flattens the isopycnals and the baroclinic forcing reinforces the Ekman transport under the up‐estuary wind. The Coriolis acceleration associated with the lateral flows is of the first‐order importance in the along‐channel momentum balance. It has a sign opposite to the stress divergence in the surface layer and the pressure gradient in the bottom layer, thereby reducing the shear in the along‐channel current. Compared with the non‐rotating system, the shear reduction is about 30–40%. Two summary diagrams are constructed to show how the averaged streamwise vorticity and along‐channel current shear vary with the Wedderburn (W ) and Kelvin (Ke ) numbers.
Tập 117 Số C9 - 2012
Perspectives on long‐term variations in hypoxic conditions in western Long Island Sound Western Long Island Sound (wLIS) has experienced a long‐term decline in the July/August summer minima bottom dissolved oxygen concentrations. This decline continues despite New York City having eliminated routine raw discharges of sewage, upgraded sewage treatment to nearly complete secondary, and introduced nitrogen control. It is our conclusion that long‐term changes in physical oceanographic processes are having an impact on the hypoxia problem in wLIS. Specifically, we show that interannual variations in summertime thermal and haline stratification contribute to variations in vertical mixing which controls the ventilation of bottom waters. Analyses of bottom dissolved oxygen and density stratification point directly to the importance of wind‐induced current shear in controlling stratification and vertical mixing; numerical simulations support this result. Interannual variations in both the direction and directional constancy of summertime winds over wLIS are shown to control the ventilation of bottom waters and thereby the seasonal development of hypoxia.
Tập 113 Số C12 - 2008
Lateral circulation and sediment transport driven by axial winds in an idealized, partially mixed estuary A 3D hydrodynamic model (ROMS) is used to investigate lateral circulation in a partially mixed estuary driven by axial wind events and to explore the associated transport of sediments. The channel is straight with a triangular cross section. The model results suggest that driving mechanisms for lateral circulation during axial wind events are different between stratified and unstratified conditions. When the water column is largely unstratified, rotational effects do not drive significant lateral circulation. Instead, differential advection of the axial salinity gradient by wind‐driven axial flow is responsible for regulating the lateral salinity gradients that in turn drive bottom‐divergent/convergent lateral circulation during down/up‐estuary winds. From the subtidal lateral salt balance, it is found that the development of lateral salinity gradient by wind‐induced differential advection is largely counterbalanced by the advection of salt by lateral circulation itself. When the water column is stratified, the lateral flow and salinity structures below the halocline closely resemble those driven by boundary mixing, and rotational effects are important. Lateral sediment flux and the event‐integrated sediment transport are from channel to shoals during down‐estuary winds but reversed for up‐estuary winds. Potential impacts of wind‐generated waves on lateral sediment transport are evaluated with two cases representing event conditions typical of upper Chesapeake Bay. Accounting for wind wave effects results in an order of magnitude increase in lateral sediment fluxes because of greater bottom stresses and sediment resuspension.
Tập 114 Số C12 - 2009
Intermittent ventilation in the hypoxic zone of western Long Island Sound during the summer of 2004 Observations of dissolved oxygen (DO) concentration, salinity, and temperature, during summer of 2004, at three levels on two moorings in the area of western Long Island Sound that is prone to seasonal hypoxia are described. Ship surveys in the area reveal that the DO concentration below the pycnocline decreases at approximately 2.4 mM m−3 d−1 throughout the summer. We show that this is the net result of oscillations in the rate of change of the DO concentration with periods of 3 to 7 days. During intervals of declining DO concentration, the rate of change is consistent with previous estimates of the rate of community respiration. Since there is insufficient light for photosynthesis below the pycnocline, increasing DO concentration (ventilation) must be a consequence of either vertical mixing or horizontal advection from regions of higher concentration. Analysis of the covariation of DO, salinity, and temperature and knowledge of the mean property distributions allow us to associate most (∼80%) of the ventilation intervals with increased vertical mixing. Comparison of DO and wind stress measurements suggest that it is the component in the along‐sound direction that controls the occurrence of ventilation, perhaps through modification of the rate of stratification by the density‐driven circulation. We conclude that the spatial and temporal variability of vertical mixing is crucial to understanding the duration and extent of hypoxia in the Long Island Sound estuary.
Tập 113 Số C9 - 2008
Mechanics of fold‐and‐thrust belts and accretionary wedges The overall mechanics of fold‐and‐thrust belts and accretionary wedges along compressive plate boundaries is considered to be analogous to that of a wedge of soil or snow in front of a moving bulldozer. The material within the wedge deforms until a critical taper is attained, after which it slides stably, continuing to grow at constant taper as additional material is encountered at the toe. The critical taper is the shape for which the wedge is on the verge of failure under horizontal compression everywhere, including the basal decollement. A wedge of less than critical taper will not slide when pushed but will deform internally, steepening its surface slope until the critical taper is attained. Common silicate sediments and rocks in the upper 10–15 km of the crust have pressure‐dependent brittle compressive strengths which can be approximately represented by the empirical Coulomb failure criterion, modified to account for the weakening effects of pore fluid pressure. A simple analytical theory that predicts the critical tapers of subaerial and submarine Coulomb wedges is developed and tested quantitatively in three ways: First, laboratory model experiments with dry sand match the theory. Second, the known surface slope, basal dip, and pore fluid pressures in the active fold‐and‐thrust belt of western Taiwan are used to determine the effective coefficient of internal friction within the wedge, μ = 1.03, consistent with Byerlee's empirical law of sliding friction, μb b and μ we predict Hubbert‐Rubey fluid pressure ratios λ = λb
Tập 88 Số B2 - Trang 1153-1172 - 1983
Mechanics of fold‐and‐thrust belts and accretionary wedges: Cohesive Coulomb Theory A critically tapered fold‐and‐thrust belt or submarine accretionary wedge is one that is on the verge of Coulomb failure everywhere, including its base where frictional sliding along a decollement is assumed to be occurring. Cohesion within a wedge can add significantly to the overall strength near the toe; the effect of this is to decrease the near‐toe taper, leading to a critical topographic profile that is concave upward if the decollement is planar. We obtain an approximate self‐consistent solution for the state of stress within a thin‐skinned cohesive critical Coulomb wedge, and determine the relationship between the wedge taper and its strength and basal friction. The theory is then applied to the presently deforming fold‐and‐thrust belt of western Taiwan. Fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step‐up angles of thrust faults from the basal decollement are used to constrain the Taiwan wedge strength parameters. An attractive assertion fully consistent with all the observations is that the mechanics of fold‐and‐thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory. In particular, if Byerlee's law µb = 0.85 is adopted as the coefficient of sliding friction on the base, we find a coefficient of internal friction µ = 0.9–1.0 in the wedge and a wedge cohesion So = 5–20 MPa. Other solutions having strengths and ambient stresses up to 4 times lower than this can also, however, satisfy the data.
Tập 89 Số B12 - Trang 10087-10101 - 1984
Rotary motions and convection as a means of regulating primary production in warm core rings We believe that two mechanisms are important in regulating primary production and hence the abundance of phytoplankton in warm core rings. The first mechanism, anticyclogenesis, is associated with the rotary motion of rings since the resultant geostrophic forces are believed to be the basis for nutrient enrichment in the high velocity region. Phytoplankton populations in this peripheral region experience a near steady state growth closely coupled with the rotational velocity of the ring. The second mechanism is that phytoplankton populations in the ring center rely on seasonal changes in the depth of the mixed layer due to convection and stabilization. This process regulates both the mean light energy reaching phytoplankton and equalizes the nutrient distribution over the water column. Growth in populations at ring center occurs as pulses, responding to changes in the depth of the mixed layer.
Tập 90 Số C2 - Trang 3237-3248 - 1985
Prediction of phytoplankton growth in response to the frictional decay of a warm‐core ring A modelling study was conducted to examine the question, Is the high phytoplankton biomass which often develops in warm‐core rings of the Gulf Stream a consequence of the circulation associated with the frictional decay of the ring? A time‐dependent, two‐dimensional (r, z, t ) model of plankton dynamics in a hypothetical ring similar in features to warm‐core ring 82B generates a lens of high phytoplankton biomass at ring center. Phytoplankton grow on nutrients advected into the euphotic zone as the depressed warm water in the ring's core rebounds and spreads out at the surface. This vertical motion induced as the ring's rate of rotation slows may be an important process maintaining the high production in warm‐core rings.
Tập 91 Số C6 - Trang 7603-7610 - 1986
Frictionally induced circulations and spin down of a warm‐core ring Meridional motion driven by kinematic viscosity v and thermal diffusivity k within a warm‐core ring is examined. An analytic quasi‐geostrophic model is constructed that shows that the vertical mixing of heat or momentum is ineffective in driving secondary motion. Single‐celled meridional flows will occur within the ring whenever the Prandtl number Pr = v h h Pr is greater than 1, the flow is up in the ring core, while Pr less than 1 causes downward motion at ring center. A numerical primitive equation model leads to qualitatively identical results but shows that the secondary flow is not long‐lived when Pr is less than 1, decaying within 2 months, while in the Pr greater than 1 case the flows are still vigorous at the end of the experiment (90 days).
Tập 90 Số C5 - Trang 8917-8927 - 1985