Abstract: Thorium-234 (234Th) is a powerful tracer of particle dynamics and the biological pump in the surface ocean; however, variability in carbon: thorium ratios of sinking particles adds substantial uncertainty to estimates of organic carbon export. We coupled a mechanistic thorium sorption and desorption model to a one-dimensional particle sinking model that uses realistic particle settling velocity spectra. The model generates estimates of 238U234Th disequilibrium, particulate organic carbon concentration, and the C:234Th ratio of sinking particles, which are then compared to in situ measurements from quasi-Lagrangian studies conducted on six cruises in the California Current Ecosystem. Broad patterns observed in in situ measurements, including decreasing C:234Th ratios with depth and a strong correlation between sinking C:234Th and the ratio of vertically-integrated particulate organic carbon (POC) to vertically-integrated total water column 234Th, were accurately recovered by models assuming either a power law distribution of sinking speeds or a double log normal distribution of sinking speeds. Simulations suggested that the observed decrease in C:234Th with depth may be driven by preferential remineralization of carbon by particle-attached microbes. However, an alternate model structure featuring complete consumption and/or disaggregation of particles by mesozooplankton (e.g. no preferential remineralization of carbon) was also able to simulate decreasing C:234Th with depth (although the decrease was weaker), driven by 234Th adsorption onto slowly sinking particles. Model results also suggest that during bloom decays C:234Th ratios of sinking particles should be higher than expected (based on contemporaneous water column POC), because high settling velocities minimize carbon remineralization during sinking.

Abstract: Zooplankton are important consumers of sinking particles in the ocean's twilight zone. However, the impact of different taxa depends on their feeding mode. In contrast to typical suspension-feeding zooplankton, flux-feeding taxa preferentially consume rapidly sinking particles that would otherwise penetrate into the deep ocean. To quantify the potential impact of two flux-feeding zooplankton taxa [Aulosphaeridae (Rhizaria), and Limacina helicina (euthecosome pteropod)] and the total suspension-feeding zooplankton community, we measured depth-stratified abundances of these organisms during six cruises in the California Current Ecosystem. Using allometric-scaling relationships, we computed the percentage of carbon flux intercepted by flux feeders and suspension feeders. These estimates were compared to direct measurements of carbon flux attenuation (CFA) made using drifting sediment traps and U-238-Th-234 disequilibrium. We found that CFA in the shallow twilight zone typically ranged from 500 to 1000 m mol organic C flux remineralized per 10-m vertical depth bin. This equated to approximately 6-10% of carbon flux remineralized/10 m. The two flux-feeding taxa considered in this study could account for a substantial proportion of this flux near the base of the euphotic zone. The mean flux attenuation attributable to Aulosphaeridae was 0.69%/10 m (median = 0.21%/10 m, interquartile range = 0.04-0.81%) at their depth of maximum abundance (similar to 100 m), which would equate to similar to 10% of total flux attenuation in this depth range. The maximum flux attenuation attributable to Aulosphaeridae reached 4.2%/10 m when these protists were most abundant. L. helicina, meanwhile, could intercept 0.45-1.6% of carbon flux/10 m, which was slightly greater (on average) than the Aulosphaeridae. In contrast, suspension-feeding zooplankton in the mesopelagic (including copepods, euphausiids, appendicularians, and ostracods) had combined clearance rates of 2-81 L m(-3) day(-1) (mean of 19.6 L m(-3) day(-1)). This implies a substantial impact on slowly sinking particles, but a negligible impact on the presumably rapidly sinking fecal pellets that comprised the majority of the material collected in sediment traps. Our results highlight the need for a greater research focus on the many taxa that potentially act as flux feeders in the oceanic twilight zone.

Abstract: Tidal mixing fronts, which represent boundaries between stratified and tidally mixed waters, are locations of enhanced biological activity. They occur in summer shelf seas when, in the presence of strong tidal currents, mixing due to bottom friction balances buoyancy production due to seasonal heat flux. In this paper we examine the occurrence and fidelity of tidal mixing fronts in shelf seas generated within a global 3-dimensional simulation of the HYbrid Coordinate Ocean Model (HYCOM) that is simultaneously forced by atmospheric fields and the astronomical tidal potential. We perform a first order assessment of shelf sea tides in global HYCOM through comparison of sea surface temperature, sea surface tidal elevations, and tidal currents with observations. HYCOM was tuned to minimize errors in M2 sea surface heights in deep water. Over the global coastal and shelf seas (depths <200&#8239;m) the area-weighted root mean square error of the M2 sea surface amplitude in HYCOM represents 35% of the 50&#8239;cm root mean squared M2 sea surface amplitude when compared to satellite constrained models TPXO8 and FES2014. HYCOM and the altimeter constrained tidal models TPXO8 and FES2014 exhibit similar skill in reproducing barotropic tidal currents estimated from in-situ current meter observations. Through comparison of a global HYCOM simulation with tidal forcing to a global HYCOM simulation with no tides, and also to previous regional studies of tidal mixing fronts in shelf seas, we demonstrate that HYCOM with embedded tides exhibits quite high skill in reproducing known tidal mixing fronts in shelf seas. Our results indicate that the amount of variability in the location of the tidal mixing fronts in HYCOM, estimated using the Simpson-Hunter parameter, is consistent with previous studies when the differences in the net downward heat flux, on a global scale, are taken into account. We also provide evidence of tidal mixing fronts on the North West Australian Shelf for which we have been unable to find references in the existing scientific literature.

Abstract: Approximately 260 individuals from forecasting centers, research laboratories, academia, and industry representing 40 countries met to discuss recent developments in operational oceanography and brainstorm about the future directions of ocean prediction services.

Abstract: The ocean, through its large capacity to store heat, plays a critical role in Earth's climate and climate variability. Warming of the world's oceans since 1955 accounts for approximately 93% of the warming of the Earth system. However, this warming is neither spatially uniform nor temporally constant. Superimposed on the global long-term trend is climate variability on inter-annual to inter-decadal time scales and regional to basin scales. Satellite altimeters and hydrographic observations show that the North Atlantic, including the sub-polar region, has rapidly become warmer and saltier since the early 1990s. An emerging picture is that the most recent 20 years or so of warming in the North Atlantic represents, in part, a transition of the Atlantic multi-decadal variability pattern from a cold to a warm phase. These decadal climate transitions involve changes both laterally in the sub-tropical and sub-polar gyres of the North Atlantic and vertically in the Atlantic Meridional Overturning Circulation (AMOC), a key component of the global heat and freshwater circulation system. This study of the North Atlantic circulation concentrates on a transition region around the Grand Banks of Newfoundland, where the effects of boundary currents and jets, recirculations, and mesoscale eddies (length scales typically less than 100 km) are dominant. Strong interactions occur in this transition region, laterally between the subpolar and subtropical gyres and vertically between the cold and warm limbs of the Atlantic Meridional Circulation (AMOC). There is evidence that this relatively compact region plays a key role in altering and even modulating the AMOC over a much larger scale and thus is important for the long-term, decadal variability of the Atlantic Ocean. Yet, despite many observational field programs, the dynamics and impacts of this region are not well understood. The project will contribute to understanding the variability of the AMOC by addressing the connectivity of the sub-polar and the sub-tropical gyres. The results of this model-data synthesis will be of particular significance to coupled climate models, which are central to understanding and predicting global climate change. The educational/outreach components of this project will be focused on cultivating scientific literacy with regards to ocean climate research in K-12 schools, at the university level, and in the local community through a variety of online resources/interactive tools for educators, the Florida State University Young Scholars program for high school students, and the “Scientists in the Schools” program. Finally, the requested funding will support a junior faculty member and a graduate student who will be trained in ocean modeling, data analysis and interpretation.
Through ongoing major observation programs in the sub-polar and sub-tropical North Atlantic Ocean, oceanographers are making great strides toward a better understanding of the structure and variability of the AMOC within these sub-basins. The work proposed here complements these observations by focusing on key questions pertaining to what controls the circulation in between and how much the sub-polar to sub-tropical connectivity modulates the larger scale AMOC. This project aims to elucidate the physical dynamics that controls circulation in the transition region, especially the relative importance of the eddies and the deep western boundary current, and document the role and impact of the transition region on the larger scale circulation, especially the variability of the AMOC and water properties in the sub-polar and sub-tropical North Atlantic from inter-annual to decadal and longer time scales. The interaction of eddies and time mean circulations represents one of the greatest challenges to prediction of global climate variability, and it can be studied with the fine-grid resolution model included in this project. These objectives will be met by performing a detailed model-data synthesis study, combining numerical results from a suite of high-resolution Atlantic simulations using the HYbrid Coordinate Ocean Model (HYCOM) and existing observations (satellite altimetry, drifters/floats, hydrography, tracers, and mooring arrays). The three-dimensional Atlantic circulation will be quantified by performing analysis of water mass transport and transformation, passive tracers, and potential vorticity and momentum fluxes. It has been demonstrated that the eddy-resolving HYCOM represents the basic circulation features in the transition region and larger scale North Atlantic Ocean, including both time mean structure and temporal variability.