The nearshore current field, defined here as the residual horizontal flow after averaging over the incident wave period, exhibits variability at a range of time and space scales. Some of the variable currents are low frequency gravity wave motions. However, variable, rotational (in the sense of possessing vertical vorticity) flow can also exist as part of the overall nearshore current field. A field and numerical modelling investigation of these variable rotational currents has been carried out. Drifters, which were developed for surfzone use, enabled measurement of the nearshore current structure; the design and testing of these new instruments is described. Two sets of field measurements, using the new drifters and Eulerian instruments were carried out for conditions with swell perpendicular to a plane beach and in strong longshore currents. In the perpendicular swell conditions, an interesting and well-defined feature of the measured trajectories was the development of transient rip currents. Discrete vortices were also observed. In the longshore current case, trajectories with the longshore current displacement removed had complex meandering paths. Lagrangian data were used to make estimates of length scales and dispersion, both of which provide strong evidence that the current field cannot be due to low frequency gravity waves alone. Under the assumption of equipartition of kinetic and potential energy for low frequency gravity waves, Eulerian measurements of velocities and pressure show significant energy due to non-divergent, rotational flow in both the perpendicular swell and longshore current case. A numerical model that can simulate horizontal flow with a directionally spread, random wave field incident on a plane beach was implemented. The model developed transient rip currents that are qualitatively very similar to those seen in the drifter trajectories from the field. The number and intensity of rip currents in the model depended on the beach slope and incident wave spectra. The energy content and cross-shore flux (and hence transport of material) of the rotational current flow component in the simulated flow fields is comparable to that due to low frequency gravity waves. The modelling also provided some evidence that there may be universal characteristics of the rotational currents. The field results and modelling show that variable rotational currents are ubiquitous in the field even when longshore currents and hence shear waves are not present. The term “infragravity turbulence” is suggested to describe the general class of nearshore hydrodynamics not directly associated with shear waves, which is largely disorganised, but contains well defined features such as transient rips currents and large scale horizontal vortices. The results have important implications in the understanding of the transport of material, including sediment, biological material, pollution, and sometimes bathers, in the nearshore zone.

Thesis (Ph.D.)--Flinders University, School of Chemistry, Physics and Earth Sciences. / Typescript bound. Includes bibliographical references: (leaves167-173) Also available online.

Thesis (M.S.) -- Mississippi State University. Department of Computational Engineering. / Title from title screen. Includes bibliographical references.

Thesis (Ph. D.)--Purdue University, 1998. / Vita. Includes bibliographical references (leaves 135-141).

Thesis (M.S.)--Naval Postgraduate School, 1984. / "March 1984." "N0001484 WR24001"--P. 1. Includes bibliographical references (leaves 144-148).

Identifying the extent to which coral reef species are connected by dispersal is a fundamental challenge for developing marine conservation strategies. Many coral reef species are relatively sedentary as adults, yet have a pelagic larval phase where larvae can potentially be widely dispersed by ocean currents. This thesis focuses on the role of ocean currents in driving spatially explicit patterns of population connectivity among ecologically and commercially important coral reef species by combining research tools from population genetics, oceanography, and biophysical modeling. Despite the substantial differences among the life histories of each coral reef species in this thesis, some similarities in connectivity patterns were found among all species. The results of the kinship and genetic outlier analyses consistently found high levels of connectivity among distant populations separated by hundreds to thousands of kilometers. Despite the high levels of connectivity among distant populations, there was substantial variation in gene flow among the populations of each species. The findings of this thesis highlight the importance of international cooperation for the sustainable management of ecologically and commercially important coral reef species in the Caribbean. In conclusion, the findings of this thesis suggest that marine conservation strategies should conservatively plan for uncertainty, particularly since the many of ecological and physical drivers of connectivity among coral reef species in the Caribbean remain uncertain.

Studies were carried out to determine the circulation of water in the northern basin of Howe Sound, a small fjord on the mainland coast of British Columbia, and to determine the extent of the influence of the winds, the tide and river runoff on the circulation. In one experiment, surface-layer drogues were tracked by radar for four periods each of approximately three days duration. Data sere recorded photographically, then digitized for computer processing. Cubic spline interpolation was used to produce positions, velocities and accelerations at one minute intervals along every drogue track. The interpolated data were averaged in a suitable manner to produce pseudo-Eulerian estimates of velocity. Near the head of the fjord, both wind and tide appeared to cause temporal fluctuations in the surface current of magnitude similar to the expected mean flow due to the river. The river was the cause of spatial inhomogeneity in the flow, but did not appear to be a significant source of temporal variations. Farther down the inlet, wind forcing was the dominant cause of temporal variations in the surface-layer flow of about five times the magnitude of the expected mean river-driven flow. At no distance along the fjord was the velocity observed to be laterally uniform. Lateral gradients of long-channel velocity were strong at the inlet head and decreased away from it, indicating that the fresh water from the river was slowly mixing across the inlet. Another experiment using drogues at three depths in the upper 6 m of the water indicated that the velocity structure was not uniform, either laterally or with depth. Analysis was done on data from six current meters moored in the northern basin of Howe Sound. The mean currents from these meters showed a surface-layer outflow and a return inflow in the waters just below. A mean down-inlet current was seen at 150 m, 80 m below sill depth. Spectra of the currents showed dominant peaks at diurnal and semi-diurnal periods. The wind was coherent with the currents at 3m for periods longer than 10 hours. Below this depth, no consistent relationship was seen. In the diurnal band, the currents were strongest at the surface, indicative of forcing from the surface by the wind. In the semi-diurnal band, the currents were strongest at 10m depth. Both bands also showed a phase variation with depth indicative of a baroclinic structure. These results were compared with some models for surface-layer behaviour. The first model assumed that the wind momentum input was distributed uniformly throughout the surface layer and that the layer was not frictionally coupled to the deeper waters. Drag coefficients calculated from the wind stress and drogue acceleration gave values of 1 to 2x10-3, similar to values measured in other ways. This model was only valid for the first few hours after the onset of the wind. Another model, developed by Farmer (1972), analysed the behaviour of the surface layer of a semi-infinite canal under the influence of a steady wind stress. It predicted correctly the length of time of wind dominance of the flow, the magnitude of the velocity change and the magnitude of the acceleration of the water. A baroclinic tidal model in a two-layer fjord, adapted from Rattray (1960), correctly predicted the phase of the surface-layer currents near the head of a fjord with respect to the height of the tide. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

Although it has long been known that cold dense waters from the Nordic Seas overflow the Wyville Thomson Ridge, the water masses' subsequent pathways and fate have been uncertain. This study conclusively places Wyville Thomson Ridge Overflow Water (WTOW) as an important water mass in the eastern subpolar North Atlantic for the first time. Using a variety of chemical tracer s (chlorofluorocarbons, oxygen, nutrients and aluminium) in conjunction with temperature and salinity, WTOW is traced southwards into the northern and central Rockall Trough as well as into the channels between the western banks. The overflow water has a clear temperature, salinity and chlorofluorocarbon (CFC-11 and CFC-12) signature. Additionally, levels of aluminium are elevated in WTOW suggesting that this element is potentially a useful and novel water mass tracer. The lower oxygen layer complicates the use of dissolved oxygen and nitrate as tracers in the mid water column. However, higher and lower concentrations respectively in the western trough reveal the presence of WTOW in this area. The overflow water does not appear to have a silicate or phosphate signature. Two branches of WTOW exist in the Rockall Trough: a slow-moving indistinct intermediate branch (600-1200 m) f ound in both the east and west of the basin; and a coherent deep branch (> 1200 m) that flows southward along the western banks of the trough. As well as having a large spatial footprint within the Rockall Trough, intermediate a nd deep WTOW are temporally persistent being present 65-75 % of the time between 1975 and 2008. The signature of WTOW at intermediate depths is absent from the Ellett Line record in the mid-1980s and early-1990s. As deep WTOW is still observed during these periods flow over the Wyville Thomson Ridge cannot have ceased. Instead, it is proposed that the strength of the Subpolar Gyre is an important driver in the temporal distribution of intermediate WTOW within the Rockall Trough. When the gyre is strong, such as in the mid-1980s and early-1990s, the mid water column is dominated by waters originating from the west which block the southward flow of intermediate WTOW. In contrast, when the gyre is weak, such as in the late-1990s and 2000s, subpolar waters lie further west enabling intermediate waters within the Rockall Trough to be dominated by the southern orig inating Mediterranean Overflow Water and the northern water mass of WTOW.

The Arctic is a hotspot of global change. For example, changes caused by global warming are both amplified and are seen more rapidly in the Arctic (e.g., Serreze & Francis, 2006; Bekryaev et al., 2010; Serreze & Barry, 2011; Overland et al. 2015; Macdonald et al., 2015). Thus, the Arctic is an indicator of the state of the planet. Among the strongest changes that have been observed in the Arctic Ocean are changes in circulation regimes, hydrographic properties and freshwater content and composition. These changes have the potential of global impact through interaction with the deep-water formation regions of the North Atlantic Ocean, a major source of deep and bottom water in the global ocean. Although significant progress in understanding the signals of change in the Arctic Ocean and their causes has been made during the past decades there are still some fundamental questions unanswered. They include the stability of the circulation of the upper waters and changes in the freshwater budget and how these changes are connected to changes in the composition of the freshwater lens that covers the Arctic Ocean. In this thesis, we address these two topics using measurements of isotopes obtained during over three decades of Arctic Ocean section work.This dissertation is composed by three parts and its structure mimics the layered vertical structure of the Arctic Ocean water column. Chapter 1 is dedicated to the Atlantic waters, Chapter 2 to the halocline waters, and Chapter 3 to the freshwater sources and their distribution and variability in the surface layer. In the first two chapters, we present transient tracer (³H/³He) and hydrographic data from over 25 years of Arctic oceanographic campaign ranging from 1987 to 2013 to evaluate flow rates and circulation pathways in the Upper Halocline Water (UHW), Lower Halocline Water (LHW), and Atlantic Layer on a pan-Arctic scale. In agreement with previously established circulation schemes, tracer data show that the flow paths in the LHW and the Atlantic layer are typically topographically steered with the presence of a cyclonic boundary current along the continental shelf and separate circulation branches tracking major bathymetric features, such as the Lomonosov Ridge. Tracer data suggest that the general circulation of UHW is decoupled from the cyclonic regime observed in the deeper layer, and strongly influenced by surface stress forcing, such as the anticyclonic Beaufort Gyre. Within the limits of our method, tracer data show that the mean flow paths and spreading velocities have been more or less constant over the past three decades despite dramatic shifts in the Arctic system heat and freshwater balances from anthropogenic climate change over imposed to a high natural variability. The third and final chapter discusses the water-mass composition and the distribution of freshwater sources in Canadian Basin, the western section of the Arctic Ocean. Results are produced by performing a water-mass decomposition on the water samples collected during the 2015 Arctic GEOTRACES (GN01) oceanographic expedition. Stable isotope measurements (H₂¹⁸O/H₂¹⁶O and DHO/H₂O ratios) are used in combination with salinity and nutrients data to calculate the water-mass components for the upper 500 m Arctic Ocean (mixed layer through Atlantic Water layer). The sources of liquid freshwater into the Arctic Ocean include Pacific water, sea ice meltwater, river discharge and net precipitation. The topmost 50 meters of Canadian Basin contain the large fraction of freshwater from sea ice meltwater and meteoric water. Pacific water dominated the freshwater budget along the 2015 GN01 transects from 100 to 250 m. These depths are also characterized by a strong brine rejection signal, reflecting an enhanced annual sea ice cycle with more ice refreezing and melting each year, and an overall loss of multiyear ice. The 2015 results are compared with the overlapping 1994 and 2005 Arctic Ocean Sections (AOS94 and AOS05) and discussed in the context of regional and temporal variability of liquid freshwater and its components distribution. Our findings show significant increases in the Canadian Basin total liquid freshwater reservoir both compared to the 1994 and 2005 transects confirming a freshwater accumulation in the Canadian Basin already established by numerous observations and modeling studies (Gilles et al., 2012; Carmack et al., 2016; Proshutinsky et al. 2019; Solomon et al., 2021). The total freshwater reservoir increased by ca. 12,500 km³ from 1994 to 2015, of which ca. 5,000 km3 are within the Beaufort Gyre. Meteoric and Pacific freshwater components were the largest sources of the observed freshwater accumulation in the upper 500m of the western Arctic Ocean. An intensified Ekman transport in the Beaufort Gyre and increased availability of freshwater for accumulation are the two primary drivers for freshwater accumulation in the Canadian Basin. Within the limits of our analysis, it is not possible to quantitatively estimate the relative importance of the each forcing nor to resolve the seasonal to year‐to‐year variability. Our tracer-based analysis suggests that there is a significant variability in the freshwater components and UHL distribution while the major features of the circulation patterns and spreading velocities of the AW and the LHW have remained largely stable over the past decades. Future research should address whether in a fast changing Arctic, the dynamics of the surface layer will expand to the halocline and Atlantic layer substantially destabilizing the current Arctic Ocean water column with potentially dramatic consequences.

Large-scale circulation in the northern North Atlantic plays a crucial role in the global climate by influencing ocean storage of atmospheric heat and carbon. Temperature and salinity changes in this region can have important consequences on ocean circulation due to density stratification at sites of deep water formation. Such influences can involve feedback mechanisms related to the Atlantic Meridional Overturning Circulation, which has been shown to influence the hydrography of the northern North Atlantic on decadal timescales. Current expectations are that through increasing sea-ice melting, river discharge, an intensifying hydrological cycle and glacial melt anomalies, future climate change could disrupt North Atlantic circulation patterns with cascading effects on carbon cycling and global climate. These interactions were investigated through circulation changes associated with salinity and freshwater variability, as well as variability in temperature and heat content. Recent changes in phytoplankton concentration and biological productivity in the Labrador Sea were also examined as part of this study. Spatial and temporal patterns of salinity in the North Atlantic were examined with the help of objective analysis and reanalysis salinity products using Argo observations of the recent decade (2005 to 2015). An overall freshening trend was evident, but with clear regional differences, particularly between the western subpolar gyre and the central North Atlantic. In general, the western subpolar region exhibited high interannual variability in surface salinity compared to the central North Atlantic. The western subpolar region also revealed a seasonal pattern of salinity fluctuation related to sea ice retreat and accretion, but with some years (i.e., 2008, 2012 and 2015) showing unusually large and negative salinity anomalies which were not present in the central or eastern North Atlantic. To understand the dominant factors influencing salinity and freshwater in the northern North Atlantic, budgets for liquid freshwater content over the northern North Atlantic were derived using a state-of-the-art ocean state estimate (ECCOv4). Here the subpolar North Atlantic (between $\sim$45\oN and the Greenland Scotland ridge) is distinguished from the Nordic Seas (north of the Greenland Scotland ridge). In a separate investigation ECCOv4 was used to describe global ocean heat budgets at varying spatial and temporal resolutions. This analysis showed that anomalies in temperature tendency are driven by atmospheric forcing at short time scales, while advection is the principle term at long time scales. ECCOv4 budget analysis was then used to investigate mechanisms behind interannual freshwater content variability in the northern North Atlantic over the time period 1992-2015. From the mid-1990s to the mid-2000s warming and salinification occurred in the subpolar North Atlantic. Consistent with the upper layer analysis with Argo-observations, ECCOv4 confirmed an overall freshening since about 2005. This freshening occurs simultaneously with an overall cooling in the subpolar North Atlantic. Advective convergence has been identified as the dominant driver of liquid freshwater content and ocean heat content variability in the subpolar North Atlantic, with liquid freshwater and heat content being anti-correlated. Consistent with the global heat analysis in ECCOv4, our results revealed that forcing is only important for establishing anomalies over shorter time scales (i.e., seasonal to interannual), but advective convergence becomes more important at longer (i.e., decadal) scales. Advection is the dominant term due to changes across the southern boundary on the decadal time scale, while exchanges with the Arctic Ocean have minor impact. Changes in freshwater and heat content in the subpolar North Atlantic due to advection occur through anomalies in the circulation itself, and not by the advection of anomalies in either liquid freshwater or heat content. In contrast to the subpolar North Atlantic, in the Nordic Seas interannual changes in liquid freshwater content are predominantly driven by forcing due to sea ice melting, which is in turn strongly correlated with Arctic sea ice export through Fram Strait. The overall concurrent warming and salinification followed by cooling and freshening in the subpolar North Atlantic suggests a relationship with changes in northward heat and salt transport through the Atlantic Meridional Overturning Circulation. This is consistent with decadal variability in deep convection in the Labrador Sea. It is evident that another consequence of changes in the Labrador Sea deep convection is the potential effects on nutrient availability and thus biological productivity. The Labrador Sea has become more productive in recent years, with mean chlorophyll-a concentrations closely correlated with silicate concentrations in the upper waters, which in turn are strongly correlated with wintertime convection depth. Thus annual production in the Labrador Sea appears to be influenced by the extent of deep winter mixing, thereby linking the Atlantic Meridional Overturning Circulation and deep convection to nutrient availability and ocean productivity in the subpolar North Atlantic.