Oceanic density/pressure gradients and slope currents

Eastern boundary currents are some of the most energetic features of the global ocean, contributing significantly to meridional mass, heat and salt transports. We take a new look at the form of an oceanic slope current in equilibrium with oceanic density gradients. We depth-integrate the linearised x and y momentum and continuity equations, assume an equilibrium force balance in the along-slope direction (no along-slope variation in the along-slope flow) and zero cross-slope flow at a coastal boundary. We relate the bottom stress to a bottom velocity via a simple boundary friction law (the precise details are easily modified), and then derive an expression for the slope current velocity by integrating upwards including thermal wind shear. This provides an expression for the slope current as a function of depth and of cross-slope coordinate, dependent on the oceanic density field and surface and bottom stresses. This new expression for the slope current allows for more general forms of oceanic density fields than have been treated previously. Wind stress is also now considered. The emphasis here is on understanding the simplified equilibrium force balance rather than the evolution towards that balance. There is a direct relationship between the slope current strength, friction and along-slope forcing (e.g. wind); also between the total along-slope forcing and bottom Ekman transport, illustrating that “slippery” bottom boundaries in literature are a direct consequence of unrealistically assuming zero along-slope pressure gradient. We demonstrate the utility of the new expression by comparison with a high resolution hydrodynamic numerical model

Storms modify baroclinic energy fluxes in a seasonally stratified shelf sea: inertial-tidal interaction

Observations made near the Celtic Sea shelf edge are used to investigate the interaction between wind-generated near-inertial oscillations and the semidiurnal internal tide. Linear, baroclinic energy fluxes within the near-inertial (f) and semidiurnal (M2) wave bands are calculated from measurements of velocity and density structure at two moorings located 40 km from the internal tidal generation zone. Over the 2 week deployment period, the semidiurnal tide drove 28–48 W m−1 of energy directly on-shelf. Little spring-neap variability could be detected. Horizontal near-inertial energy fluxes were an order of magnitude weaker, but nonlinear interaction between the vertical shear of inertial oscillations and the vertical velocity associated with the semidiurnal internal tide led to a 25–43% increase in positive on-shelf energy flux. The phase relationship between f and M2 determines whether this nonlinear interaction enhances or dampens the linear tidal component of the flux, and introduces a 2 day counter-clockwise beating to the energy transport. Two very clear contrasting regimes of (a) tidally and (b) inertially driven shear and energy flux are captured in the observations

Structure and Transport of the North Atlantic Current in the Eastern Subpolar Gyre From Sustained Glider Observations

Repeat glider sections obtained during 2014–2016, as part of the Overturning in the Subpolar North Atlantic Program, are used to quantify the circulation and transport of North Atlantic Current (NAC) branches over the Rockall Plateau. Using 16 glider sections collected along 58°N and between 21°W and 15°W, absolute geostrophic velocities are calculated, and subsequently the horizontal and vertical structure of the transport are characterized. The annual mean northward transport (± standard deviation) is 5.1 ± 3.2 Sv over the Rockall Plateau. During summer (May to October), the mean northward transport is stronger and reaches 6.7 ± 2.6 Sv. This accounts for 43% of the total NAC transport of upper‐ocean waters (σO<27.55 kg/m3) estimated by Sarafanov et al. (2012, https://doi.org/10.1029/2011JC007572) along 59.5°N, between the Reykjanes Ridge and Scotland. Two quasi‐permanent northward flowing branches of the NAC are identified: (i) the Hatton Bank Jet (6.3 ± 2.1 Sv) over the eastern flank of the Iceland Basin (20.5°W to 18.5°W) and (ii) the Rockall Bank Jet (1.5 ± 0.7 Sv) over the eastern flank of the Hatton‐Rockall Basin (16°W to 15°W). Transport associated with the Rockall Bank Jet is mostly depth independent during summer, while 30% of the Hatton Bank Jet transport is due to vertical geostrophic shear. Uncertainties are estimated for each individual glider section using a Monte Carlo approach, and mean uncertainties of the absolute transport are less than 0.5 Sv. Although comparisons with altimetry‐based estimates indicate similar large‐scale circulation patterns, altimetry data do not resolve small mesoscale current bands in the Hatton‐Rockall Basin which are strongly needed for the right transport estimates

Are any coastal internal tides predictable?

Author Posting. © The Oceanography Society, 2012. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 25, No. 2 (2012): 80-95, doi:10.5670/oceanog.2012.44.Surface tides are the heartbeat of the ocean. Because they are controlled by Earth's motion relative to other astronomical objects in our solar system, surface tides act like clockwork and generate highly deterministic ebb and flow familiar to all mariners. In contrast, baroclinic motions at tidal frequencies are much more stochastic, owing to complexities in how these internal motions are generated and propagate. Here, we present analysis of current records from continental margins worldwide to illustrate that coastal internal tides are largely unpredictable. This conclusion has numerous implications for coastal processes, as across-shelf exchange and vertical mixing are, in many cases, strongly influenced by the internal wave field.This work was supported through grants from the US Office of Naval Research and NSF (Nash, Shroyer, Musgrave, Levine, and Duda), by NERC (grant NE/IO30224/1 FASTNEt; Inall), and by AIMS/CSIRO (Kelly and Jones)

Subpolar North Atlantic western boundary density anomalies and the Meridional Overturning Circulation

Changes in the Atlantic Meridional Overturning Circulation, which have the potential to drive societally-important climate impacts, have traditionally been linked to the strength of deep water formation in the subpolar North Atlantic. Yet there is neither clear observational evidence nor agreement among models about how changes in deep water formation influence overturning. Here, we use data from a trans-basin mooring array (OSNAP—Overturning in the Subpolar North Atlantic Program) to show that winter convection during 2014–2018 in the interior basin had minimal impact on density changes in the deep western boundary currents in the subpolar basins. Contrary to previous modeling studies, we find no discernable relationship between western boundary changes and subpolar overturning variability over the observational time scales. Our results require a reconsideration of the notion of deep western boundary changes representing overturning characteristics, with implications for constraining the source of overturning variability within and downstream of the subpolar region

Are Any Coastal Internal Tides Predictable?

Surface tides are the heartbeat of the ocean. Because they are controlled by Earth's motion relative to other astronomical objects in our solar system, surface tides act like clockwork and generate highly deterministic ebb and flow familiar to all mariners. In contrast, baroclinic motions at tidal frequencies are much more stochastic, owing to complexities in how these internal motions are generated and propagate. Here, we present analysis of current records from continental margins worldwide to illustrate that coastal internal tides are largely unpredictable. This conclusion has numerous implications for coastal processes, as across-shelf exchange and vertical mixing are, in many cases, strongly influenced by the internal wave field

Climate change impacts on ocean circulation relevant to the UK and Ireland

What is happening • Observations of the Atlantic Meridional Overturning Circulation or Gulf Stream System since the 1980s have shown a strengthening in the 1990s and a weakening in the 2000s, with no clear overall trend. • Shifts in North-east Atlantic circulation, leading to a greater influence of warmer subtropical-origin waters which can impact marine ecosystems and economically important fish species such as mackerel. The changing subpolar ocean circulation is also having impacts on the food supply for deep-sea ecosystems. • The subpolar gyre recorded its freshest values on record in the 2010s. Ongoing freshwater build-up in the rapidly changing Arctic Ocean may exacerbate this freshening. What could happen • Projections from climate models consistently project a weakening of the Atlantic Meridional Overturning Circulation due to anthropogenic climate change. • Warming of Atlantic waters is expected to reduce the depth of mixed layers and limit nutrient supply to surface layers

Overturning in the Subpolar North Atlantic Program: A New International Ocean Observing System

For decades oceanographers have understood the Atlantic meridional overturning circulation (AMOC) to be primarily driven by changes in the production of deep-water formation in the subpolar and subarctic North Atlantic. Indeed, current Intergovernmental Panel on Climate Change (IPCC) projections of an AMOC slowdown in the twenty-first century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep-water formation. The motivation for understanding this linkage is compelling, since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic Program (OSNAP), to provide a continuous record of the transbasin fluxes of heat, mass, and freshwater, and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the Rapid Climate Change–Meridional Overturning Circulation and Heatflux Array (RAPID–MOCHA) at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014, and the first OSNAP data products are expected in the fall of 2017

Exceptional atmospheric conditions in June 2023 generated a northwest European marine heatwave which contributed to breaking land temperature records

The Northwest European shelf experienced unprecedented surface temperature anomalies in June 2023 (anomalies up to 5 °C locally, north of Ireland). Here, we show the shelf average underwent its longest recorded category II marine heatwave (16 days). With state-of-the-art observation and modelling capabilities, we show the marine heatwave developed quickly due to strong atmospheric forcing (high level of sunshine, weak winds, tropical air) and weak wave activity under anticyclonic weather regimes. Once formed, this shallow marine heatwave fed back on the weather: over the sea it reduced cloud cover and over land it contributed to breaking June mean temperature records and to enhanced convective rainfall through stronger, warmer and moister sea breezes. This marine heatwave was intensified by the last 20-year warming trend in sea surface temperatures. Such sea surface temperatures are projected to become commonplace by the middle of the century under a high greenhouse gas emission scenario

The impact of glacier geometry on meltwater plume structure and submarine melt in Greenland fjords

Meltwater from the Greenland Ice Sheet often drains subglacially into fjords, driving upwelling plumes at glacier termini. Ocean models and observations of submarine termini suggest that plumes enhance melt and undercutting, leading to calving and potential glacier destabilization. Here we systematically evaluate how simulated plume structure and submarine melt during summer months depends on realistic ranges of subglacial discharge, glacier depth, and ocean stratification from 12 Greenland fjords. Our results show that grounding line depth is a strong control on plume-induced submarine melt: deep glaciers produce warm, salty subsurface plumes that undercut termini, and shallow glaciers produce cold, fresh surface-trapped plumes that can overcut termini. Due to sustained upwelling velocities, plumes in cold, shallow fjords can induce equivalent depth-averaged melt rates compared to warm, deep fjords. These results detail a direct ocean-ice feedback that can affect the Greenland Ice Sheet