Browsing by Subject "North Atlantic"
Results Per Page
Sort Options
Item Open Access North Atlantic Mesoscale Eddy Detection and Marine Species Distribution(2010-04-30T20:28:45Z) Hsu, Ango Chen-TienAn eddy detection workflow was developed and applied to reference series of delayed time maps of sea level anomaly (Ref DT-MSLA) published by Aviso Altimetry, France in the North Atlantic region between 30-55° N and 30-80° W. The eddy detection parameters, maximum/minimum Okubo-Weiss parameter to assign to the eddy core/ring, minimum eddy core area, minimum eddy core area to perimeter ratio, and minimum eddy duration, were set to -0.2/0.2 standard deviation, 5 cells (6869 km2), 0.4, and 10 images (70 days), respectively. Using these parameters, 635 anticyclonic eddies and 930 cyclonic eddies were detected between October 14, 1992 and December 31, 2005. The eddy structure of the 62103 pelagic longline fishery catch records in The Logbook System maintained by Southeast Fisheries Science Center was sampled. One-way ANOVA and t-test were conducted to compare the mean catch-per-unit-effort (CPUE) of bluefin tuna (Thunnus thynnus), yellowfin tuna (Thunnus albacores), bigeye tuna (Thunnus obesus), and swordfish (Xiphius gladius) at different eddy structures. For bluefin tuna and bigeye tuna, the mean CPUE is higher in the eddy area than in the non-eddy area. For yellowfin tuna and swordfish, the mean CPUE is higher in the non-eddy area than in the eddy area. For all three species of tuna, the mean CPUE is higher in the anticyclonic eddies than in the cyclonic eddies. For swordfish, the mean CPUE is higher in the cyclonic eddies than in the anticyclonic eddies. The results suggest different eddy structures make different habitats for large marine predators and eddy activities contribute to marine species distribution.Item Open Access Ocean Heat Transport from the Subtropical Gyre to the Subpolar Gyre in the North Atlantic(2018) Foukal, Nicholas PeterThe North Atlantic Ocean transports on the order of 1 petawatt of heat poleward from the Equator toward the high-latitude Arctic via an exchange of warm, upper-layer water for cold bottom water, reducing the Equator-to-pole temperature gradient and providing an opportunity for regional climate predictability at seasonal to decadal time scales. As more high-quality observations and numerical models of the ocean have become available in the past decade, oceanographers have realized that the variability in this oceanic meridional heat transport is not coherent across latitudes, with a prominent break in the meridional coherence between the subtropical gyre and the subpolar gyre. In this dissertation, I demonstrate the progress I have made on understanding how heat is conveyed from the subtropical gyre to the subpolar gyre in the North Atlantic at these critical inter-gyre latitudes (35°N-50°N).
I use a suite of data including databases of in situ measurements of oceanic temperature and salinity, satellite observations of sea-surface temperature and height, and ocean model output of ocean current velocities, temperature and salinity. I conclude that a majority of the inter-gyre heat transport in the North Atlantic can be explained by variability in the strength of the sub-surface transport between the gyres. In this work, I also test whether pathways for propagating sea-surface temperature anomalies exist between the gyres as has been previously suggested and find that there is no evidence for this pathway in more modern satellite measurements. In addition, I show that variability in the sub-surface pathway cannot be explained by dynamics in the size and strength of the subpolar gyre as has long been assumed. Finally, I do a detailed analysis within two ocean circulation models and conclude that the oceanic heat fluxes are as important or more important as the surface atmospheric forcing to the temperature variability in the northeastern Atlantic, even at high frequencies. I then find that the origin of these oceanic heat fluxes stem from variability in the upper limb of the overturning circulation at inter-gyre latitudes. These results impact how we might expect to track ocean heat fluxes and thus where to look for climate predictability in the North Atlantic sector.
Item Open Access On the Horizontal Advection and Biogeochemical Impacts of North Atlantic Mode Waters and Boundary Currents(2007-07-26) Palter, Jaime BethUsing a combination of hydrographic data and the trajectories and profiles of isobaric floats, this dissertation evaluates the connections between remote regions in the North Atlantic. First, I establish that the production and advection of the North Atlantic Subtropical Mode Water (STMW) introduces spatial and temporal variability in the subsurface nutrient reservoir of the subtropical gyre. As the mode water is formed, its nutrients are depleted by biological utilization. When the depleted water mass is exported to the gyre, it injects a wedge of low-nutrient water into the upper layers of the ocean. Contrary to intuition, cold winters that promote deep convective mixing and vigorous mode water formation may diminish downstream primary productivity by altering the subsurface delivery of nutrients. Next, the source of elevated nutrient concentrations in the Gulf Stream is assessed. The historical hydrographic data suggest that imported water advected into the Gulf Stream via the tropics supplies an important source of nutrients to the Gulf Stream. Because the high nutrients are likely imported from the tropics, diapycnal mixing need not be invoked to explain the Gulf Stream's high nutrient concentrations, as had been previously hypothesized. Furthermore, nutrients do not increase along the length of the Stream, as would be expected with strong diapycnal mixing.Finally, profiling float data are used to investigate how the Labrador Sea Water enters the Deep Western Boundary Current, one of the primary pathways by which it exits the subpolar gyre. With the trajectories and profiles of an extensive array of P-ALACE floats I evaluate three processes for their role in the entry of Labrador Sea Water in the Deep Western Boundary Current (DWBC): 1) LSW is formed directly in the DWBC, 2) Eddies flux LSW laterally from the interior Labrador Sea to the DWBC, and 3) A horizontally divergent mean flow advects LSW from the interior to the DWBC. Each of the three processes has the potential to remove heat from the boundary current, and both the formation of LSW directly in the boundary current and the eddy heat flux are possible sources of interannual variability in the exported LSW product.