Near-surface recirculation over Georges Bank

Satellite-tracked drifters with drogues centered near-surface (5 m) and below the seasonal thermocline (50 m) were launched during late winter and spring of 1988 and 1989 in the northern Great South Channel in the western Gulf of Maine to investigate the regional circulation as part of the South Channel Ocean Productivity Experiment (SCOPEX). Many of the near-surface drifters became entrained in the clockwise gyre over Georges Bank, and eight drifters made a total of 16 complete circuits around the bank during the stratified season. The average recirculation period of these eight drifters was 48 days, and the average drifter speed around the bank was 12 cm s⁻¹. There is no clear evidence from the drifter data that the strength of the clockwise gyre over the bank increased with time during the stratified season. On average, these drifters (i) followed a relatively narrow path around the bank, except over the eastern end of the bank where three preferred paths were observed, (ii) moved fastest over the northern and southern flanks of the bank, (iii) did not enter a core area of 3500 km² centered at 41°17′N, 68°00′W, approximately 30 km southwest of the topographic center of the bank, and (iv) stopped circling the bank by the end of November, due in part to strong wind events that appeared to drive drifters off the bank. Curiously, none of the near-surface drifters moved from the southern flank of Georges Bank onto the New England shelf as might be expected from continuity of flow along the outer shelf; instead, the drifters that circled the bank tended to move off the bank along its southern flank. None of the drifters with drogues centered at 50 m appeared to recirculate around Georges Bank.     

Cross-Frontal Water Exchange on Georges Bank: Some Results from an U.S. GLOBEC/Georges Bank Program Model Study

This paper reviews recent progress on modeling cross-frontal water exchange on Georges Bank undertaken as part of the U.S. Global Ecosystem Northwest Atlantic/Georges Bank Study (U.S. GLOBEC/Georges Bank Program). A simple conceptual model is described first, followed by a discussion of four physical mechanisms associated with (1) strong nonlinear interaction, (2) asymmetric tidal mixing, (3) varying wind forcing, and (4) chaotic mixing. Some critical issues in modeling studies of fronts are also addressed. A new unstructured grid, finite-volume coastal ocean ecosystem model is introduced. This model combines the best of the finite-difference method for the simplest discrete computational efficiency and the finite-element method for geometric flexibility. Because the finite-volume method discretizes the integral form of the governing equations, this approach provides a better representation for the conservation laws of mass and momentum are satisfied, which is particularly important in the frontal regions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Balancing end-to-end budgets of the Georges Bank ecosystem

Oceanographic regimes on the continental shelf display a great range in the time scales of physical exchange, biochemical processes and trophic transfers. The close surface-to-seabed physical coupling at intermediate scales of weeks to months means that the open ocean simplification to a purely pelagic food web is inadequate. Top-down trophic depictions, starting from the fish populations, are insufficient to constrain a system involving extensive nutrient recycling at lower trophic levels and subject to physical forcing as well as fishing. These pelagic-benthic interactions are found on all continental shelves but are particularly important on the relatively shallow Georges Bank in the northwest Atlantic. We have generated budgets for the lower food web for three physical regimes (Well-mixed, Transitional and Stratified) and for three seasons (Spring, Summer and Fall/Winter). The calculations show that vertical mixing and lateral exchange between the three regimes are important for zooplankton production as well as for nutrient input. Benthic suspension feeders are an additional critical pathway for transfers to higher trophic levels. Estimates of production by mesozooplankton, benthic suspension feeders and deposit feeders, derived primarily from data collected during the GLOBEC years of 1995-1999, provide input to an upper food web. Diets of commercial fish populations are used to calculate food requirements in three fish categories, planktivores, benthivores and piscivores, for four decades, 1963-2002, between which there were major changes in the fish communities. Comparisons of inputs from the lower web with fish energetic requirements for plankton and benthos indicate that we obtained reasonable agreement for the last three decades, 1973-2002. However, for the first decade, the fish food requirements were significantly less than the inputs. This decade, 1963-1972, corresponds to a period characterized by a strong Labrador Current and lower nitrate levels at the shelf-edge, demonstrating how strong bottom-up physical forcing may determine overall fish yields.     

Characteristics of resuspended sediment from Georges Bank collected with a sediment trap

A sediment trap was deployed 3 m from the bottom at a water depth of 62 m on the southern flank of Georges Bank (41°02·2′N, 67°33·5′W) from 30 September 1978 to 10 March 1979 to qualitatively determine the size of sediments resuspended from the bottom by winter storms and to determine if seasonal changes in the phytoplankton could be observed in the trapped sediment.Bulk X-ray analyses of the trapped sediment showed layers of distinctly different textures preserved in the collection vessel. The median grain size of sampled layers ranged from 2·7 to 6·5 φ (fine sand to silt), but all layers contained a pronounced mode in the 3 φ (fine sand) range. Nine layers containing relatively large amounts of sand were present. The sand content was 75% in the coarest layers and about 32% in the fine layers. The median grain size of bottom sediments at the deployment site was considerably coarser than the trap samples, although the dominant grain size was also 3 φ.Average bottom-current speeds during the deployment period were about 30 cm s^?1 with a range of 10 to 50 cm s^?1. Bottom stress, computed from the observed currents and waves, suggest that 11 storms caused sufficient stress to resuspend 3 φ-sized sediments, in good agreement with the nine layers of relatively coarse sediments collected in the trap. Surface waves had to be included in the calculation of bottom stress because the bottom currents alone were insufficient to cause the resuspension of 3 φ-sized sediment.The trapped sediments contain numerous diatoms and coccoliths that are typical of late fall and winter assemblages. No clear seasonal difference in the flora was noted among sampled layers, probably due to the large influx of resuspended material and a reduced primary flux during this period. An undescribed species of Thalassiosira (G. Fryxell, personal communication), and siliceous scales of unknown systematic position were observed at all levels.     

The role of frontal currents in larval fish transport on Georges Bank

The hydrography and distributions of cod larvae on Georges Bank were surveyed during two research cruises in April and May 1993 in order to relate larval drift between cruises to the vernal intensification of the frontal component of the residual circulation. We observed the transport of two patches of cod larvae. One patch, which had maximum larval cod densities of 45 larvae 100 m⁻³ in April, appeared to have been advected south about 75 km between surveys, while the other, which had maximum larval cod densities of 20 larvae 100 m⁻³ in April, appeared to have been advected north-northeast about 25 km. Maximum larval densities in each patch sampled during the second cruise in May were 15 and 18 larvae 100 m⁻³, respectively, and mean growth in total length for larvae in the two patches was approximately 5.5 mm and 4.5 mm, respectively, between the two cruises. During the April cruise there was a large volume of anomalous cold, fresh water, of Scotian Shelf origin, which occupied much of the eastern third of Georges Bank. During May, relatively cold, fresh water appeared in a band from the Northeast Peak along the Southern Flank, between Georges Bank water on the top of the Bank, and upper Slope Water offshore. The distribution of cold, fresh water suggests its participation in the general clockwise circulation around the Bank. The transport of cod larvae comprising the first patch appeared to become organized within, and move along, the frontal boundary established by the Scotian Shelf-like water mass, while larvae in the second patch, which we assumed to have moved to the north, may have been transported northward in an on-Bank flow of warmer and saltier upper Slope Water, which may have originated from a Gulf Stream Ring. Based upon observed transport of the first patch of larvae in relation to the frontal boundary, we present a conceptual model of frontal mixing currents on Georges Bank, where current velocities may reach 5 cm s⁻¹ at the depth of the pycnocline. We suggest that this frontal component of the residual circulation, which is in addition to that resulting from tidal rectification, may be important in the transport of fish larvae, and that interannual variability in the degree of intrusion of extrinsic water masses may contribute to variable larval cod drift patterns, to variable larval cod retention on the Bank, and ultimately, to variable larval fish recruitment to the early juvenile stage.     

The distribution and abundance of pelagic gammarid amphipods on Georges Bank and Nantucket Shoals

In this study, long-term, broad-scale zooplankton survey data were used to estimate the temporal and spatial distribution and abundance of gammarid amphipods present in the water column on Georges Bank. Delta-mean abundances computed from 10 years of data showed that gammarid amphipod abundances peaked in summer and again in fall. The amphipods were also most numerous in water less than 50 m deep. The statistical tests employed revealed no conclusive evidence for diurnal vertical migration. Interannual delta-mean abundances fluctuated approximately 5-fold between 1977 and 1986, ranging from 217 to 1181 amphipods 100 m⁻³. Peak amphipod biomass occurred in July and was estimated to be 2.8 kcal m⁻². Using production-to-biomass ratios from the literature, mean annual production of amphipods in these samples was estimated to be between 1.6 and 9.8 kcal m⁻². Production in shallow areas was especially high, 22 kcal m⁻² year⁻¹.     

Distribution and vertical dynamics of planktonic communities at Sofala Bank, Mozambique

Coastal ecosystem processes are largely influenced by the interaction of different factors operating at various temporal and spatial scales, specifically those responsible for primary production patterns that modulate zooplankton and subsequent trophic levels. Hydrological processes, such as tidal cycles and coastal currents, nutrients availability, phytoplankton groups (studied through algal pigment signatures analysed by HPLC), and zooplankton abundance and distribution were investigated at the Sofala Bank (Mozambique), with special emphasis on their horizontal distribution and vertical dynamics (48 h). Horizontal distribution has shown inshore–offshore gradients in all analysed parameters, as well as inshore waters intrusion probably related to Zambezi River delta runoff. Tidal currents were responsible for major hydrological vertical variations and for horizontal and vertical advection of phytoplankton biomass in the surface and deepest layers, respectively. Nutrient concentrations were typical from oligotrophic regions, and nutrient ratios were strongly influenced by depleted nitrate + nitrite concentrations, indicating low estuarine discharges typical from the dry season. The very low N:P ratio obtained suggests strong nitrogen limitation to phytoplankton communities, supporting the low phytoplankton abundance observed. Both phytoplankton pigments and zooplankton were found mainly near the bottom (40 m depth), despite the latter displayed vertical migrations triggered by light variations. Phytoplankton community was dominated by microflagellates, specifically prymnesiophyceans, and behaved as a whole, except Cyanobacteria that displayed vertical distribution movements different from other phytoplankton groups, being mainly concentrated at mid-water column depths (10–20 m). This investigation enhances physico-chemical phenomena and their importance determining the planktonic communities vertical dynamics at Sofala Bank, a tropical coastal ecosystem of the Western Indian Ocean where planktonic dynamics are still poorly described and understood.     

Intrusion layering and vertical diffusion in the ocean owing to tidal mixing at a sloping bottom

The empirical and analytical relationships between the vertical scale of intrusions, which are formed owing to the layering of the mixed boundary layer at a sloping bottom, and the determining parameters are obtained on the basis of field measurements in the coastal zone and energy considerations. An estimate of the vertical diffusion coefficient is given which allows us to determine the role of tidal mixing at the rigid boundaries as a mixing factor for the entire world's oceans.Translated by Mikhail M. Trufanov.     

Parameterization of vertical mixing in the Weddell Sea

A series of vertical mixing schemes implemented in a circumpolar coupled ice–ocean model of the BRIOS family is validated against observations of hydrography and sea ice coverage in the Weddell Sea. Assessed parameterizations include the Richardson number-dependent Pacanowski–Philander scheme, the Mellor–Yamada turbulent closure scheme, the K-profile parameterization, a bulk mixed layer model and the ocean penetrative plume scheme (OPPS). Combinations of the Pacanowski–Philander parameterization or the OPPS with a simple diagnostic model depending on the Monin–Obukhov length yield particularly good results. In contrast, experiments using a constant diffusivity and the traditional convective adjustment cannot reproduce the observations. An underestimation of wind-driven mixing in summer leads to an accumulation of salt in the winter water layer, inducing deep convection in the central Weddell Sea and a homogenization of the water column. Large upward heat fluxes in these simulations lead to the formation of unrealistic, large polynyas in the central Weddell Sea after only a few years of integration. Furthermore, spurious open-ocean convection affects the basin-scale circulation and leads to a significant overestimation of meridional overturning rates. We conclude that an adequate parameterization of both wind-induced mixing and buoyancy-driven convection is crucial for realistic simulations of processes in seasonally ice-covered seas.     

Evaluation of various vertical mixing parameterizations in a tropical Pacific Ocean GCM

Coupled general circulation models (GCMs) have had weak El Niño/Southern Oscillation variability that has been attributed to a diffuse thermocline in the modeled equatorial Pacific Ocean. Consequently, there have been many attempts to improve the thermocline by developing new or improved ocean vertical mixing schemes. This paper investigates the influence of gradient Richardson Number-based vertical mixing scheme profiles in a tropical Pacific Ocean GCM. It has been common for vertical mixing schemes to be assessed in tropical Pacific Ocean models that have a limited latitudinal domain bounded by zonal walls with sponge layers. However, recent work has shown that warm surface water can accumulate in these models and stop them from achieving the observed sharp equatorial thermocline. The present model employs a parameterized wall heat transport scheme that prevents warm surface water from accumulating. Thus we are able assess the influence of vertical mixing profiles in an ocean model that does not allow warm surface water to accumulate and influence the thermocline.In this paper we evaluate the equatorial performance of three different Richardson number (Ri)-based vertical mixing profiles: an integer power (IP) profile based on the observations of Peters, Gregg and Toole; a form of the Pacanowski and Philander profile modified to have low background mixing; and the Max Planck Institute profile. With the accumulation of warm surface water prevented, each of these profiles is able to achieve a sharp thermocline. When compared with observations, the IP profile achieves a better upwelling velocity distribution. We also examine the influence on equatorial performance of very high mixing coefficients at low Richardson number, and of low background mixing coefficients.     

海洋模式中垂直混合参数化方案介绍

介绍了海洋垂直混合过程参数化方案的发展,以及不同参数化方案在海洋模式中的应用情况。首先,介绍不同垂直混合参数化方案的物理问题、理论依据、数学表达和特征,并对不同参数化方案进行了比较。其次,针对中尺度涡、亚中尺度涡以及波浪、潮流混合参数化的最新研究进展进行了总结并对垂直混合参数化的未来发展提出了一些建议。