Tom Rippeth

Bangor University · School of Ocean Sciences

Two microstructure profilers, two ships, and four moorings with acoustic Doppler current profilers and conductivity-temperature loggers were used in an intensive effort to map the spatial and temporal variations of vertical mixing in the stagnant deep basin of Gullmar Fjord, Sweden. During three days in the beginning of August 2001 a continuous tim...

Shelf seas are an important global carbon sink. In the seasonal thermocline, the subsurface chlorophyll maximum (SCM) supports almost half of summer shelf production. Using observations from the seasonally stratified Celtic Sea (June 2010), we identify wind-driven inertial oscillations as a mechanism for supplying the SCM with the nitrate needed fo...

In the Southern Ocean, small-scale turbulence causes diapycnal mixing which influences important water mass transformations, in turn impacting large-scale ocean transports such as the Meridional Overturning Circulation (MOC), a key controller of Earth'sclimate. We present direct observations of mixing over the Antarctic continental slope between wa...

The combination of acoustic Doppler current profilers and the structure function methodology provides an attractive approach to making extended time series measurements of oceanic turbulence (the rate of turbulent kinetic energy dissipation ε) from moorings. However, this study shows that for deployments in the upper part of the water column, estim...

The tides are a major source of the kinetic energy supporting turbulent mixing in the global oceans. The prime mechanism for the transfer of tidal energy to turbulent mixing results from the interaction between topography and stratified tidal flow, leading to the generation of freely propagating internal waves at the period of the forcing tide. How...

Seasonally stratified shelf seas are amongst the most biologically productive on the planet. A consequence is that the deeper waters can become oxygen deficient in late summer. Predictions suggest global warming will accelerate this deficiency. Here we integrate turbulence timeseries with vertical profiles of water column properties from a seasonal...

Seasonally stratified shelf seas are amongst the most biologically productive on the planet. A consequence is that the deeper waters can become oxygen deficient in late summer, with predictions that global warming will accelerate this deficiency. Here we integrate novel turbulence timeseries with vertical profiles of water column properties from a...

Plain Language Summary Microscopic algae, growing in the sunlit surface layer of the ocean, provide food for other species and form the basis of the ecosystem. In the Arctic Ocean, their growth is limited by the availability of nutrients. The main source of these nutrients are waters entering from the Atlantic and Pacific Oceans. These nutrient‐ric...

Historically, the Arctic Ocean has been considered an ocean of low variability and weak turbulent mixing. However, the decline in seasonal sea ice cover over the past couple of decades has led to increased coupling between the atmosphere and the ocean, with potential enhancement of turbulent mixing. Here, we review studies that allow identifying en...

The offshore wind energy sector has rapidly expanded over the past two decades, providing a renewable energy solution for coastal nations. Sector development has been led in Europe, but is growing globally. Most developments to date have been in well-mixed, i.e. unstratified, shallow-waters near to shore. Sector growth is, for the first time, pushi...

Observations of turbulent kinetic energy dissipation rate (ϵ) from a range of historical shelf seas data sets are viewed from the perspective of their forcing and dissipation mechanisms: barotropic to baroclinic tidal energy conversion, and pycnocline and bottom boundary layer (BBL) dissipation. The observations are placed in their geographical con...

Turbulent mixing is a key process in the transport of heat, salt and nutrients in the marine environment, with fluxes commonly derived directly from estimates of the turbulent kinetic energy dissipation rate, ϵ. Time series of ϵ estimates are therefore useful in helping to identify and quantify key biogeochemical processes. Estimates of ϵ are typic...

Plain Language Summary In the Arctic Ocean deep basins, only a tiny fraction of the algae that grows in the surface layer sinks down to the sea floor. Most of the particles reaching the sea floor originate from the shallower regions closer to the coast. These particles have already settled on the sea floor once, and originate from rivers or algae t...

In the Arctic Ocean, limited measurements indicate that the strongest mixing below the atmospherically forced surface mixed layer occurs where tidal currents are strong. However, mechanisms of energy conversion from tides to turbulence and the overall contribution of tidally driven mixing to Arctic Ocean state are poorly understood. We present meas...

Plain Language Summary Previous studies demonstrated that in recent years density gradients above the warm and salty intermediate (~150–900 m) water of Atlantic origin in the eastern Arctic Ocean have weakened, allowing stronger upward transport of heat to the bottom of the sea ice. Using mooring observations, we show that this weakening of stratif...

A 15-year duration record of mooring observations from the eastern (>70°E) Eurasian Basin (EB) of the Arctic Ocean is used to show and quantify the recently increased oceanic heat flux from intermediate-depth (∼150-900 m) warm Atlantic Water (AW) to the surface mixed layer (SML) and sea ice. The upward release of AW heat is regulated by the stabili...

Abstract The ocean surface boundary layer is a critical interface across which momentum, heat, and trace gases are exchanged between the oceans and atmosphere. Surface processes (winds, waves, and buoyancy forcing) are known to contribute significantly to fluxes within this layer. Recently, studies have suggested that submesoscale processes, which...

Understanding the processes that control the evolution of the ocean surface boundary layer (OSBL) is a prerequisite for obtaining accurate simulations of air–sea fluxes of heat and trace gases. Observations of the rate of dissipation of turbulent kinetic energy ( ε), temperature, salinity, current structure, and wave field over a period of 9.5 days...

High-resolution satellite images of ocean color and sea surface temperature reveal an abundance of ocean fronts, vortices and filaments at scales below 10 km but measurements of ocean surface dynamics at these scales are rare. There is increasing recognition of the role played by small scale ocean processes in ocean-atmosphere coupling, upper-ocean...

Receding seasonal sea ice extent over the Arctic Ocean is increasing access to what was a largely inaccessible region. At lower latitudes the complex vertical current structure associated with large amplitude, high frequency non-linear internal waves, sometimes referred to as solitons, present a significant challenge to the safe engineering design...

Ocean energy represents a vast and largely untapped resource; power generation from rapid tidal currents in shallow seas represent potentially high yield environments. These environments are particularly challenging for development due to the strong currents and the associated turbulent stresses. Optimum and efficient design of tidal energy convert...

Recent studies show that the glaciers draining both the West Antarctic and the Greenland Ice Sheets are experiencing an accelerated ice loss, highlighting the possibility of large-scale ice sheet retreat and sea level rise in the coming centuries and millennia. These sea level changes would vary spatially and could significantly alter global tides...

Recent seasonal Arctic Ocean sea-ice retreat is a major indicator of polar climate change. The Arctic Ocean is generally quiescent with the interior basins characterised by low levels of turbulent mixing at intermediate depths. In contrast, under conditions of reduced sea-ice cover, there is evidence of energetic internal waves that have been attri...

Inertial Oscillations are a ubiquitous feature of the surface ocean. Here we combine new observations with a numerical model to investigate the role of inertial oscillations in driving deepening of the surface mixed layer in a seasonally stratified sea. Observations of temperature and current structure, from a mooring in the Western Irish Sea, reve...

Storms that affect ocean surface layer dynamics and primary production are a frequent occurrence in the open North Atlantic Ocean. In this study we use an interdisciplinary data set collected in the region to quantify nutrient supply by two pathways associated with a storm event: entrainment of nutrients during a period of high wind forcing and sub...

Storms that affect ocean surface layer dynamics and primary production are a frequent occurrence in the open North Atlantic Ocean. In this study we use an interdisciplinary dataset collected in the region to quantify nutrient supply by two pathways associated with a storm event: entrainment of nutrients during a period of high wind forcing and subs...

The largest oceanic heat input to the Arctic Ocean results from inflowing Atlantic water. This inflowing water is warmer than it has been in the past 2,000 years. Yet the fate of this heat remains uncertain, partly because the water is relatively saline, and thus dense: it therefore enters the Arctic Ocean at intermediate depths and is separated fr...

The structure function method for estimating the dissipation rate of turbulent kinetic energy, previously validated for measurements from seabed fixed mounts, is applied to data from 1.2-MHz acoustic Doppler current profiler (ADCP) instruments operating in pulse–pulse coherent mode and mounted in midwater below a tethered buoy. Movements of the buo...

A key parameter in determining the exchange of CO2 across the ocean-atmosphere interface is the sea surface partial pressure of carbon dioxide (pCO2). Temperate seasonally stratified shelf seas represent a significant sink for atmospheric CO2. Here an analytical model is used to quantify the impact of vertical mixing across the seasonal thermocline...

The seasonal thermocline in shelf-seas represents an important biogeophysical barrier to the vertical flux of nutrients into the photic zone. Episodic weakening of this barrier plays an important role in sustaining the sub-surface chlorophyll maximum in summer and hence impacts the carbon draw-down in the seasonally-stratified zones of the shelf se...

The diapycnal flux of nitrate from the deep water provides a limit on new production in the subsurface chlorophyll maximum (SCM) during summer in stratified shelf seas. Here we estimate the diapycnal nitrate flux into the SCM in the stratified western Irish Sea (SWIS). Sampling took place immediately before neap tides when winds were light, so flux...

Observations are used to evaluate a simple theoretical model for the generation of near-inertial shear spikes at the base of the open ocean mixed layer when the upper ocean displays a two-layer structure. The model predicts that large changes in shear squared can be produced by the alignment of the wind and shear vectors. A climatology of stratific...

Vertical mixing, driven by turbulence in the ocean, underpins many of the critical interactions that allow life on earth to flourish since vertical buoyancy flux maintains global overturning circulation and vertical nutrient fluxes are critical to primary production. Prediction of the ocean system is therefore dependent on accurate simulation of tu...

In this chapter, the fundamentals of turbulence and stratification are developed in the context of estuarine and coastal flows. We start with a theoretical development of the dynamics of turbulence in sheared, stratified flow, including turbulent lengthscales and energetic, the effects of boundaries, and the efficiency of turbulent mixing. The rema...

Liverpool Bay, a region of freshwater influence subject to semi-diurnal and enduring periods of stratification, is home to a long-term coastal observatory. The observatory provides a new array of data which include vertical profiles of velocity from an Acoustic Doppler Current Profiler and a high frequency radar system (which provides measurements...

Vertical mixing in the bottom boundary layer and pycnocline of the Laptev Sea is evaluated from a rapidly sampled 12-h time series ofmicrostructure temperature, conductivity, and shear observations collected under 100% sea ice during October 2008. The bottom boundary turbulent kinetic energy dissipation was observed to be enhanced (∈ ~ 10-4 W m-3)...

New insight into a key process, instrumental in the onset of stratification in tidally driven systems, is presented for the first time using long-term field observations from the Liverpool Bay region of freshwater influence (ROFI). Tidal ellipses respond to changes in stratification, becoming more circular with greater vertical structure, and revea...

Recent observations indicate seasonally stratified temperate and polar shelf seas display a significant summer dissolved inorganic carbon (DIC) deficit, which maintains a major CO2 sink. Here we couple a simple vertical exchange model with the CO2SYS carbon chemistry model to investigate the role of the diapycnal nutrient flux in maintaining the su...

Data from moored stations, sampled in a patch of isolated seasonal stratification in the western Irish Sea in 2002 and 2006, show the presence of large-amplitude internal waves with both semi-diurnal and supertidal frequencies. The barotropic and baroclinic signals were separated, and a semi-diurnal tidal fit was performed on the baroclinic signal....

Wind is shown to have an important influence on water column structure and residual circulation through a straining mechanism in the Liverpool Bay region of freshwater influence. By combining a long-term data set of water column observations collected from a mooring we estimate the horizontal gradients in temperature and salinity that are used to d...

Wind and tidal straining are proposed as key mechanisms influencing the magnitude and timing of the horizontal flux of freshwater across regions of freshwater influence (ROFIs). Evidence for this hypothesis is presented in estimates of the tidally averaged residual current profile, obtained from 5 years of continuous acoustic doppler current profil...

Using a global tidal model it is shown that the supply of tidal energy to the deep ocean was larger during the Last Glacial Maximum (LGM; 18,000-22,000 years BP). The results were used to modify the rate of vertical mixing in a coupled ocean-atmosphere model set up for the present and LGM oceans. The increased levels of mechanical energy during the...

Especially in high Reynolds number, naturally-occurring flows, turbulence is a highly variable process. It is challenging to measure yet it is vital that we do so in order to quantify the internal transports of mass, nutrients, energy and momentum. Isolated turbulence profiles are difficult to interpret; systematic sampling and subsequent averaging...

We report on an intensive campaign in the summer of 2006 to observe turbulent energy dissipation in the vicinity of a tidal mixing front which separates well mixed and seasonally stratified regimes in the western Irish Sea. The rate of turbulent dissipation ε was observed on a section across the front by a combination of vertical profiles with the...

We estimate the diapcynal nitrate flux in the seasonally stratified western Irish Sea using the dissipation method. By comparing the divergence of the reported benthic and diapcynal nitrate fluxes, we are able to explain much of the observed annual summer decline in deep-water nitrate at this location. We then show that the new production, fuelled...

Recent finescale observations of shear and stratification in temperate shelf sea thermoclines show that they are of marginal stability, suggesting that episodes of enhanced shear could potentially lead to shear instability and diapcynal mixing. The bulk shear between the upper and lower boundary layers in seasonally stratified shelf seas shows rema...

Microstructure and hydrographic observations, during September 2007 in the boundary current on the East Siberian continental slope, document upper ocean stratification and along-stream water mass changes. A thin warm surface layer overrides a shallow halocline characterized by a ~40-m thick temperature minimum layer beginning at ~30 m depth. Below...

1] Although shelf seas account for only 7% of the oceanic surface area, recent observations demonstrate that they host significant ocean-atmosphere CO 2 fluxes. A mechanism implicated in driving a significant CO 2 sink in the temperate shelf seas is the Continental Shelf Pump. Here we present an analysis of the impact of sea-level rise, and the con...

The shelf sea seasonal thermocline is a critical interface within the marine environment, separating the euphotic zone from nutrient-rich deep water. Fluxes across the thermocline therefore represent a key biogeochemical pathway. In this paper we quantify the rate of mixing across the seasonal thermocline for a location in the Celtic Sea and invest...

Microstructure and hydrographic observations, during September 2007 in the boundary current on the East Siberian continental slope, document upper ocean stratification and along-stream water mass changes. A thin warm surface layer overrides a shallow halocline characterized by a ~40-m thick temperature minimum layer beginning at ~30 m depth. Below...

The shelf sea seasonal thermocline is a critical interface within the marine environment, separating the euphotic zone from nutrient-rich deep water. Fluxes across the thermocline therefore represent a key biogeochemical pathway. In this paper we quantify the rate of mixing across the seasonal thermocline for a location in the Celtic Sea and invest...

In this review paper, state-of-the-art observational and numerical modeling methods for small scale turbulence and mixing with applications to coastal oceans are presented in one context. Unresolved dynamics and remaining problems of field observations and numerical simulations are reviewed on the basis of the approach that modern process-oriented...

Although shelf seas account for only 7% of the oceanic surface area, they play a key role in the global carbon cycle, linking the terrestrial, atmospheric and oceanic carbon pools. Recent observations suggest that whilst tropical shelf seas act as major sources of atmospheric CO2, temperate and polar shelf seas are significant sinks. Here we provid...

Measurements of the intra-tidal and spring-neap variation in the vertical flux of nitrate into the base of the sub-surface chlorophyll maximum (SCM) were made at the shelf edge of the Celtic Sea, a region with strong internal mixing driven by an internal tide. The neap tide daily mean nitrate flux was 1.3 (0.9-1.8, 95% confidence interval) mmol m22...

We present a new technique for the estimation of profiles of the rate of dissipation of turbulent kinetic energy ($\varepsilon$, TKE) in the marine environment using a standard acoustic Doppler current profiler (ADCP). The technique is based on the structure function method used in radar meteorology. The new method is validated through comparisons...

Measurements of turbulence and suspended particulate matter (SPM) were measured over a 50 h period at a site in high tidal energy, mixed waters of the Irish Sea, NW European shelf. Turbulence parameters included both production (variance method from seabed ADCP) and dissipation (FLY profiler); SPM parameters included mass and volume concentrations...

Although continental shelf seas make up a relatively small fraction (ca 7%) of the world ocean's surface, they are thought to contribute significantly (20-50% of the total) to the open-ocean carbon dioxide storage through processes collectively known as the shelf sea pump. The global significance of these processes is determined by the vertical mix...

Vertical mixing in shelf seas and estuaries is an important process that dominates the transport of water properties (e.g. sediments or pollution) in the marine environment. Acoustic Doppler Current Profilers (ADCPs) have been widely used for both monitoring and research in shelf seas and estuaries. While the potential of ADCPs to measure velocity...

Analysis is presented of detailed observations of turbulence in stratified flow over topography. Key results are: 1) Directly measured dissipative energy losses due to skin friction and an internal hydraulic transition are relatively small, each accounting for approximately 10% of the mean flow energy loss, and 2) Production and dissipation of turb...

A key process in the shelf sea pumping of CO2 to the open ocean is vertical mixing in the summer stratified zones of continental shelf seas. Here, we present measurements of profiles of the rate of dissipation of turbulent kinetic energy ($\varepsilon$), from which vertical mixing is inferred, for several locations in the summer stratified zones of...

A detailed set of observations are presented of the tidal forcing and basin response of Loch Etive, a jet-type fjordic system on the west coast of Scotland. The characteristics of the tidal jet observed during a spring tide are discussed in detail, and with reference to laboratory studies of Baines and Hoinka (1985). Although the system is categori...

Observations of phytoplankton physiology collected using a fast repetition-rate fluorometer (FRRF) in the vicinity of a shelf-sea tidal-mixing front are presented. These data are combined with more traditional C-14-based measurements and observations of environmental parameters, including estimates of turbulent dissipation rates, in order to invest...

Simultaneous measurements of the rates of turbulent energy dissipation (epsilon) and production (P) have been made over a period of 24 hours at a tidally energetic site in the Northern Irish Sea in a water depth of 25 m. Epsilon profiles from 5 m below the surface to 15 cm above the sea bed were obtained using a FLY microstructure pro- filer while...

A detailed set of observations are presented of the tidal forcing and basin response of Loch Etive, a jet-type fjordic system on the west coast of Scotland. The characteristics of the tidal jet observed during a spring tide are discussed in detail, and with reference to laboratory studies. Evidence is presented which shows the breaking of an intern...

Net transport of suspended particulate matter (SPM) is well-known in tidal regions where there is time-velocity asymmetry due to frictional modification of the tide in shallow water. We present here observations which show a new mechanism for net flux of SPM in response to tidal straining in a region of freshwater influence (ROFI). In situ measurem...

Tidal mixing (T.M.) fronts are postulated to be favoured sites for the enhancement of primary production and, hence, important contributors to shelf sea biomass. Surveys of plankton standing crop provide clear evidence of such enhancement along fronts but there is no consensus about which of the several candidate mechanisms is primarily responsible...

An understanding of turbulence is a key goal in the study of shelf sea oceanography since turbulence processes are critical in controlling flow dynamics and vertical exchange of momentum and scalars within the water column. Recently our knowledge of the structure of turbulence in shelf seas and estuaries has been greatly improved as difficulties in...

This paper sets out to test the hypothesis that vertical mixing due to the dissipation of the internal tide accounts for a significant proportion of the total vertical mixing in a fjordic basin during a period of deep water isolation. During July and August 1999 two locations in the Clyde Sea were instrumented with moored RD Instruments Acoustic Do...

The Liverpool Bay Region of Freshwater Influence in the Irish Sea exhibits strong horizontal gradients which interact with the dominant tidal flow. A 25 h series of measurements of the cycle of turbulent dissipation with the FLY dissipation profiler shows a strong asymmetry between ebb and flood which is associated with a cycle of increasing strati...