Greater Stratification Research Articles

Response of the meridional overturning circulation to variable buoyancy forcing in a double hemisphere basin

We consider how a highly idealized double-hemisphere basin responds to a zonally constant restoring surface temperature profile that oscillates in time, with periods ranging from 0.5 to 32,000 years. In both hemispheres, the forcing is similar but can be either in phase or out of phase. The set-up is such that the Northern Hemisphere always produces the densest waters. The model’s meridional overturning circulation (MOC) exhibits a strong response in both hemispheres on decadal to multi-millennial timescales. The amplitude of the oscillations reaches up to 140% of the steady-state maximum MOC and exhibits resonance-like behaviour, with a maximum at centennial to millennial forcing periods. When the forcing is in phase between the Northern and Southern Hemispheres, there is a marked decrease in the amplitude of the MOC response as the forcing period is increased beyond the resonance period. In this case the resonance-like behaviour is identical to the one we found earlier in a single-hemisphere model and occurs for the same reasons. When the forcing is out of phase between the Northern and Southern Hemispheres, the amplitude of the MOC response is substantially greater for long forcing periods (millennial and longer), particularly in the Southern Hemisphere. This increased MOC amplitude occurs because for an out of phase forcing, either the northern or the southern deep water source is always active, leading to generally colder bottom waters and thus greater stratification in the opposite hemisphere. This increased stratification in turn stabilises the water column and thus reduces the strength of the weaker overturning cell. The interaction of the two hemispheres leads to response timescales of the deep ocean at half the forcing period. Our results suggest a possible explanation for the half-precessional time scale observed in the deep Atlantic Ocean palaeo-temperature record.

Primary productivity stimation during autumm-winter in Bahía de La Paz, B.C.S., Mexico.

We analyzed the variability of primary productivity in Bahía de La Paz during November 1997 and 2000, January 1998 and February 2001. Water was identified in the Gulf of California in higher proportion; Surface Water and Water Subsuperficial Equatorial Subtropical. In November 1997 we saw the average maximum temperature (~ 27.62 ° C) in February and the lowest value (~ 19.16 ° C). The index of Simpson (Ö) indicated that during November 1997 (~ 286.6 J-3 m) column of water with greater stratification, while in November 1998 (~ 60.4 m J-3) and February 2001 (~ 94.5 J m-3) was mixed. In November 1997 there were low values of nitrate (~ 1.52 µM), nitrites (~ 0.04 µM) and chlorophyll a (Cla ~ 0.44 mg m-3). In February2001, there were larger values of nitrate (~ 7.46 µM), nitrites (~ 0.76 µM) and chlorophyll a (Cla ~ 1.07 mgm-3). The primary productivity and integrated surface presented their highest averages in November 2000 (C ~ 5.09 mg m-3 h-1 and 75.54. mg m-2 h-1) and minimum in November 1997 (C ~ 2.93 mg m-3 h-1 and ~ 55.29 mg m-2 h-1), respectively. According to previous research, reported productivity values are characteristic of the cold season of the bay with exception of November 1997, which showed influence of El Niño.

Variations of turbulent flow with river discharge in the Altamaha River Estuary, Georgia

Turbulent flow characteristics under two significantly different river discharge periods were studied in the Altamaha River Estuary, Georgia, using a variety of moored instrumentation, combined with detailed water column profiling from an anchored vessel. Estimates of the Reynolds stress, shear production (P), and dissipation rate (ɛ) were derived and compared for the two contrasting river conditions which essentially characterized the estuary as weakly stratified during low discharge (2001) and partially mixed during high discharge (2003). Wave effects were removed from the measurement of turbulent kinetic energy (TKE) using a linear filtration method and a −5/3 slope was fit for an indirect measurement of ɛ. We suggest two possible mechanisms for observed flood/ebb asymmetries in the shear production of energy: wave‐induced bottom roughness change or the competing effects of the barotropic and baroclinic pressure gradients. For 2001 the buoyancy flux was estimated by calculating the dissipation of temperature variance from rapidly sampled temperature time series over two different tidal cycles. The mixing efficiency Γ approached 0.25 but was on average 0.05. The mixing rate depended on the flow and stratification with highest values of 200 cm2/s during maximum flow and weakest stratification and lowest values of 0.1 cm2/s during low flow and greater stratification. A balance of production and dissipation of energy was achieved only during ebb tide in 2001, implying that turbulent transport of TKE maybe a consideration since buoyancy dissipation is too small for 2001 and would only enhance the imbalance further in 2003 because of strong stratification that exists during the flood and ebb tide.

The role of Southern Ocean mixing and upwelling in glacial-interglacial atmospheric CO2 change

Decreased ventilation of the Southern Ocean in glacial time is implicated in most explanations of lower glacial atmospheric CO2. Today, the deep (>2000 m) ocean south of the Polar Front is rapidly ventilated from below, with the interaction of deep currents with topography driving high mixing rates well up into the water column. We show from a buoyancy budget that mixing rates are high in all the deepwaters of the Southern Ocean. Between the surface and∼2000m depth, water is upwelled by a residual meridional overturning that is directly linked to buoyancy fluxes through the ocean surface. Combined with the rapid deep mixing, this upwelling serves to return deep water to the surface on a short time scale.We propose two new mechanisms by which, in glacial time, the deep Southern Ocean may have been more isolated from the surface. Firstly, the deep ocean appears to have been more stratified because of denser bottom water resulting from intense sea ice formation near Antarctica. The greater stratification would have slowed the deep mixing. Secondly, subzero atmospheric temperatures may have meant that the present-day buoyancy flux from the atmosphere to the ocean surface was reduced or reversed. This in turn would have reduced or eliminated the upwelling (contrary to a common assumption, upwelling is not solely a function of the wind stress but is coupled to the air–sea buoyancy flux too). The observed very close link between Antarctic temperatures and atmospheric CO2 could then be explained as a natural consequence of the connection between the air–sea buoyancy flux and upwelling in the Southern Ocean, if slower ventilation of the Southern Ocean led to lower atmospheric CO2. Here we use a box model, similar to those of previous authors, to show that weaker mixing and reduced upwelling in the Southern Ocean can explain the low glacial atmospheric CO2 in such a formulation.

Ciclo anual de estratificación y circulación en el Embalse Arenal, Costa Rica

Yearly cycle of stratification and circulation at the Arenal Reservoir, Costa Rica. The behavior of the water column of Arenal Reservoir, a tropical reservoir in Costa Rica, is analyzed based on temperature and dissolved oxygen data gathered on a monthly to bimonthly basis between 1986 and 1992. The depth profiles of temperature at Arenal Reservoir show the typical asymmetry of reservoirs, with a greater stratification in its deeper part near the dam, even though the main outflow is located at its head at the northwest end instead of the dam. A comparison of sampling stations at both ends show a deeper circulation pattern at the northern end. This coincides with other reports of a tilted structure of the thermal isopleth pattern in front of the water outlet. The dissolved oxygen showed some degree of anoxia at the bottom during part of the year, mainly from May until October. This was caused by the decomposition of the organic matter accumulated at the bottom at the time, mainly as remains of the vegetation from the former lagoon that was not removed before flooding. The stratification is clearly seasonal, occurring during the rainy season (April to October), specially in the deeper zones of the reservoir. Rev. Biol. Trop. 54 (Suppl. 1): 257-263. Epub 2006 Sept. 30.

Vouchers for Basic Education in Developing Economies: An Accountability Perspective

Advocates argue that voucher programs can correct the incentive problems of education systems in developing economies. An accountability perspective, based on a principal-agent framework, was developed to clarify the arguments for and against education vouchers. An assessment of findings on voucher programs in industrial countries and a review of voucher or quasi-voucher experiences in Bangladesh, Chile, Colombia, Cote d'Ivoire, and the Czech Republic support the usefulness of the analytic framework. The assessment concludes that the policy relevance of voucher programs for developing economies remains uncertain. Major voucher initiatives have been attempted only in countries with a well-developed institutional infrastructure. Some studies find favorable benefits for at least some population groups, but others find limited effects and evidence of increasing social stratification in schools. Whether vouchers lead to better outcomes or greater stratification appears related to specific contexts, institutional variables, and program designs.

Subtidal variability of transverse flow off a cape

Time series of velocity profiles at two Chesapeake Bay entrance sites were used to characterize the subtidal variability of transverse flows off a cape. A shallow sampling site was located near Cape Henry over 6 m of water and separated from a deep site, 20 m deep, by a distance of ∼4 km. The velocity profiles showed that wind-induced subtidal variations in general masked curvature effects (centrifugal accelerations) that may produce secondary circulation associated with tidal flow around a cape. Such secondary circulation, consisting of flow away from the cape at surface and toward the cape at depth, was observed only during periods of weak winds. Most of the time, transverse flows were unidirectional throughout the water column and moved in opposite directions at the two sites examined. This caused convergence of transverse flow between the two sites under the influence of northerly winds and divergence of transverse flow with southwesterly winds. In addition to unidirectional and curvature-induced secondary flows, other modes of subtidal variability consisted of (1) two-layered responses with surface flow toward the cape, and (2) three-layered responses. These two- and three-layered structures were observed more frequently at the deep site, where greater stratification is expected, than at the shallow site.

Features of the Observed Annual Ocean–Atmosphere Flux Variability on the West Florida Shelf

The annual cycle of sea surface temperature and ocean–atmosphere fluxes on the west Florida shelf is described using in situ measurements and climatology. Seasonal reversals in water temperature tendency occur when the net surface heat flux changes sign in boreal spring and fall. Synoptic-scale variability is also important. Momentum and heat flux variations result in successive water column stratification and destratification events, particularly at shallower depths during spring. Fall is characterized by destratification of the water column and a series of steplike decreases in the temperature. These are in response to both tropical storms and extratropical fronts. Tropical storms are responsible for the largest momentum fluxes, but not necessarily for the largest surface heat fluxes. A one-dimensional analysis of the temperature equation suggests that surface heat flux is primarily responsible for the spring and fall seasonal ocean temperature changes, but that synoptic-scale variability is also controlled by the ocean circulation dynamics. During summer, the situation is reversed and the major influence on water temperature is ocean dynamics, with the heat flux contributing to the synoptic-scale variability. There is also evidence of interannual variability: the wintertime temperatures get increasingly colder from 1998 to 2000, and the greatest stratification and coldest subsurface temperatures occur in 1998. NCEP–NCAR reanalysis fields do not reproduce the high spatial flux variability observed in situ or with satellite measurements. Reconciling these differences and their impacts on the climate variability of this region provides challenges to coupled ocean–atmosphere models and their supporting observing systems.

The Role of Information and Institutional Arrangements in Stemming the Stratifying Effects of School Choice

The design and implementation of school choice programs have emerged as critical issues in debates about how to reform American schools. At the core of these debates are issues of equity and fairness. For example, does the introduction of school choice lead to greater stratification along racial and socio-economic dimensions? Do all parents have equal access to the choice programs available to them? In this study I investigate such questions by first reviewing previous research that examines how school choice affects the equality of educational opportunities and then developing a framework and a set of testable hypotheses that provide a better understanding of how institutional, demographic, and contextual factors affect potential inequities in those who participate in school choice. In applying this framework to evaluate the public school choice programs of two inner-city New York districts, I find that institutional arrangements and information play a critical role in the distribution of educational opportunity. In East Harlem’s District 4, the gap in participation levels among advantaged and disadvantaged parents has been bridged not only by the maturity of the choice program and the institutional structures that are in place to disseminate and facilitate the flow of information, but also because the district culture and norms reinforce choice behavior among parents.

On Coastal Trapped Waves: Analysis and Numerical Calculation by Inverse Iteration

Abstract Waves of sub-inertial frequency in a continuously stratified ocean and trapped over a continental shelf and slope are considered. They form one infinite discrete sequence of modes with frequencies decreasing to zero. The mode frequencies increase with stratification. All modes progress with the coast on their right in the Northern Hemisphere. In three formal asymptotic limits the waves adopt special forms: (1) large longshore wavenumber [Rhines (1970) bottom–trapped waves]; (2) small stratification [barotropic continental shelf waves]; and (3) large stratification [baroclinic (internal) Kelvin-like waves]. These results are illustrated by numerical calculations using the method of inverse iteration, which avoids time integration. Further calculations demonstrate the strong influence of the depth and density profiles on the wave forms. In particular, a realistic context (i.e., a gently sloping shelf bounded by a steeper continental slope, together with greater stratification near to the surface) a...

STRATIFICATION AND DEFORMATION OF ARBACIA PUNCTULATA EGGS CENTRIFUGED IN CAFFEINE SOLUTIONS

1. Caffeine does not change the existing viscosity state of the egg.2. Egg fragmentation, under centrifugation, decreases with increased caffeine concentration.3. The "apparent" effect of greater stratification of the granules in Arbacia eggs centrifuged in caffeine does not occur if the centrifugal force to which the eggs are subjected is sufficient to produce sedimentation but insufficient to cause deformation.4. Evidence indicates that the delay of deformation in the caffeinized eggs, centrifuged at 10,000 x g or less, may be due to the action of caffeine (trimethylated purine) upon the total tension forces at the surface areas of both unfertilized and fertilized Arbacia eggs.