Meridional circulation in the tropical North Atlantic

A transatlantic CTD/ADCP section nominally located at 11°N was carried out in March 1989. In this paper relative geostrophic velocities are computed from these data via the thermal wind balance, with reference level choices based primarly on water mass distributions. A brief overview of the meridional circulation of the upper waters resulting from these analysis techniques is presented. Schematic circulation patterns of the NADW and AAW are also presented. In both the western and eastern basins these waters are characterized by cyclonic recirculation gyres. A paricularly notable result of the deep western basin analysis is the negligible net flow of middle NADW. Although the horizontal circulation patterns described in this study agree well with results from many previous studies, the meridional overturning cell and net heat flux are considerably lower, while the net freshwater flux is slightly higher than previous estimates. These discrepancies may be attbuted to: (1) differences in methodologies, (2) the increased resolution of this section, and (3) temporal (including decadal, synoptic, and most importantly, seasonal) variability.Funding was provided by the National Science Foundation through Grant Nos. OCE-8716314 and OCE-9101636 and the Office of Naval Research through the American Society for Engineering Education

A data assimilative marine ecosystem model of the central equatorial Pacific: Numerical twin experiments

A five-component, data assimilative marine ecosystem model is developed for the high-nutrient low-chlorophyll region of the central equatorial Pacific (0N, 140W). Identical twin experiments, in which model-generated synthetic \u27data\u27 are assimilated into the model, are employed to determine the feasibility of improving simulation skill by assimilating in situ cruise data (plankton, nutrients and primary production) and remotely-sensed ocean color data. Simple data assimilative schemes such as data insertion or nudging may be insufficient for lower trophic level marine ecosystem models, since they require long time-series of daily to weekly plankton and nutrient data as well as adequate knowledge of the governing ecosystem parameters. In contrast, the variational adjoint technique, which minimizes model-data misfits by optimizing tunable ecosystem parameters, holds much promise for assimilating biological data into marine ecosystem models. Using sampling strategies typical of those employed during the U.S. Joint Global Flux Study (JGOFS) equatorial Pacific process study and the remotely-sensed ocean color data available from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), parameters that characterize processes such as growth, grazing, mortality, and recycling can be estimated. Simulation skill is improved even if synthetic data associated with 40% random noise are assimilated; however, the presence of biases of 10-20% proves to be more detrimental to the assimilation results. Although increasing the length of the assimilated time series improves simulation skill if random errors are present in the data, simulation skill may deteriorate as more biased data are assimilated. As biological data sets, including in situ, satellite and acoustic sources, continue to grow, data assimilative biological-physical models will play an increasingly crucial role in large interdisciplinary oceanographic observational programs

Deep circulation in the tropical North Atlantic

A transatlantic CTD/ADCP (Conductivity, Temperature, Depth/Acoustic Doppler Current Profiler) section along 11N, taken in March 1989, has been used to compute geostrophic velocities; geostrophic transport is required to balance in situ values of the Ekman and shallow boundary current transports. The horizontal flow structure is described for eight layers, with particular emphasis on deep and bottom waters (four layers below = 4.7°C). In the shallow layers, total North Brazil Current (NBC) transport agrees with other observations previously made in the month of March, while net northward flow of these layers across the western basin is also consistent with recent observations to the north. For each of the four deep layers, circulation patterns are illustrated by means of schematic cartoons. Each of these layers flows southward in the Deep Western Boundary Current, which has a magnitude of 26.5 Sv. Roughly half of this flow returns northward to the west of the Mid-Atlantic Ridge, confirming the existence of a hypothesized cyclonic recirculation gyre in the western basin of the tropical Atlantic. To varying degrees the deep and bottom waters also circulate cyclonically in the eastern basin, with net northward flow across this basin. Partly as a result of the unusual appearance of the North Equatorial Countercurrent in March 1989, the in situ values of the meridional overturning cell (5.2 Sv), heat flux (3.0 × 1014 W), and freshwater flux (−0.65 Sv) computed from the 11N section depart significantly from estimates of these quantities in the literature. By forcing the 11N geostrophic velocities to balance annual average Ekman and NBC transports, annual average values of these fluxes (12 Sv; 11 × 1014 W; −0.6 Sv) are obtained, and are shown to agree well with historical estimates

Well-posedness of boundary layer equations for time-dependent flow of non-Newtonian fluids

We consider the flow of an upper convected Maxwell fluid in the limit of high Weissenberg and Reynolds number. In this limit, the no-slip condition cannot be imposed on the solutions. We derive equations for the resulting boundary layer and prove the well-posedness of these equations. A transformation to Lagrangian coordinates is crucial in the argument

Combining observations and numerical model results to improve estimates of hypoxic volume within the Chesapeake Bay, USA

© The Author(s), 2013. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Journal of Geophysical Research: Oceans 118 (2013): 4924–4944, doi:10.1002/jgrc.20331.The overall size of the “dead zone” within the main stem of the Chesapeake Bay and its tidal tributaries is quantified by the hypoxic volume (HV), the volume of water with dissolved oxygen (DO) less than 2 mg/L. To improve estimates of HV, DO was subsampled from the output of 3-D model hindcasts at times/locations matching the set of 2004–2005 stations monitored by the Chesapeake Bay Program. The resulting station profiles were interpolated to produce bay-wide estimates of HV in a manner consistent with nonsynoptic, cruise-based estimates. Interpolations of the same stations sampled synoptically, as well as multiple other combinations of station profiles, were examined in order to quantify uncertainties associated with interpolating HV from observed profiles. The potential uncertainty in summer HV estimates resulting from profiles being collected over 2 weeks rather than synoptically averaged ∼5 km3. This is larger than that due to sampling at discrete stations and interpolating/extrapolating to the entire Chesapeake Bay (2.4 km3). As a result, sampling fewer, selected stations over a shorter time period is likely to reduce uncertainties associated with interpolating HV from observed profiles. A function was derived that when applied to a subset of 13 stations, significantly improved estimates of HV. Finally, multiple metrics for quantifying bay-wide hypoxia were examined, and cumulative hypoxic volume was determined to be particularly useful, as a result of its insensitivity to temporal errors and climate change. A final product of this analysis is a nearly three-decade time series of improved estimates of HV for Chesapeake Bay.Funding for this study was provided by the IOOS COMT Program through NOAA grants NA10NOS0120063 and NA11NOS0120141. Additional funding was provided by NSF grant OCE-1061564

The importance of organic content to fractal floc properties in estuarine surface waters, insights from video, LISST, and pump sampling: Supporting data

The linked folders and associated data files contain the observations utilized in Fall, K.A., Friedrichs, C.T., Massey, G.M., Bowers, D.G., and Smith, S.J. (2021). The importance of organic content to fractal floc properties in estuarine surface waters: Insights from video, LISST, and pump sampling. JGR Oceans. The file “Description of Data Files.pdf” outlines the content of the ten data folders, each of which is associated with a data set collected on an individual one-day cruise in the York River estuary

Identification of suspended resilient pellets in particles tracked by a Particle Image Camera System (PICS) in a muddy estuary

The Particle Imaging Camera System (PICS) was designed to allow for the measurement of the settling velocity of individual particles in situ by using the smaller particles (\u3c density \u3c 1800kg/m3 ). This classification system, while adequate for suspended dredge plumes, needs to be revisited when the PICS is used in a muddy estuary, such as the York River Estuary, Virginia. Figure 1B shows the settling velocities of particles tracked within a video captured 2.5m from the surface in the Clay Bank region of the York River, plotted against their equivalent spherical diameters. While most of the particles are classified as flocs, as indicated by the blue dots in Figure 1C and the peak in the relative number of particles in Figure 1E, there is still a large number of particles classified as “bed aggregates” (red dots). This number of higher density particles may be unexpected, as this video was captured 4.25m over a “muddy bed” in a natural system with a flood current of 40cm/s. However, biologically compacted mud in the form of resilient pellets (see Figure 2) may be the answer. Bed sediments from five sediment cruises during this study period (Aug 2012 – Nov 2014) were found to be comprised of 86-96% mud (Figure 3A). However, 9-14% of the mud was packaged as resilient pellets (Figure 3B). Sediment captured 38cm above the bed by traps deployed on tripods were found to have 92-98% mud, with 4-14% of the mud packaged as resilient pellets (Figures 3A and B). Pellets isolated from the Apr to Jul 2014 trap were sampled with the PICS to determine the distribution of settling velocities (Ws), particle densities, and the ratio of the long and short axis of the particles. This will be used to identify the pellets in PICS videos captured during the five 6h anchor stations (black lines in Figure 3) where three depths were sampled each hour

Spring-neap variation in fecal pellet properties within surficial sediment of the York River stuary, Virginia

Fecal pellet abundance was measured within the upper seabed of the York River Estuary as part of a larger study investigating relationships between fine sediment aggregates and bed erodibility. Sedimentalogical surveys were conducted twice a month during the spring and summer of 2011 to coincide with spring or neap tidal cycles. Particle size distributions were determined by sieving the sediment using three methods: 1) typical grain size analysis, 2) gentle agitation with seawater, 3) gentle agitation with deionized water. Each method used four sieves (150, 90, 63 and 45 microns) to constrain the size abundance of the particles. The study found that resilient fecal pellets comprised up to ~30% of the total sediment within the top centimeter of the seabed, and abundance was not directly related to spring-neap tidal cycles. There was a tendency, however, for larger pellets to persist around neap tide, perhaps because stronger currents at spring tide were more likely to break apart the largest pellets. Also, a greater mass of pellets was preserved when seawater rather than deionized water was used during sieving