Glider observations of enhanced deep water upwelling at a shelf break canyon: a mechanism for cross-slope carbon and nutrient exchange

Using underwater gliders we have identified canyon driven upwelling across the Celtic Sea shelf-break, in the vicinity of Whittard Canyon. The presence of this upwelling appears to be tied to the direction and strength of the local slope current, which is in itself highly variable. During typical summer time equatorward flow, an unbalanced pressure gradient force and the resulting disruption of geostrophic flow can lead to upwelling along the main axis of two small shelf break canyons. As the slope current reverts to poleward flow, the upwelling stops and the remnants of the upwelled features are mixed into the local shelf water or advected away from the region. The upwelled features are identified by the presence of sub-pycnocline high salinity water on the shelf, and are upwelled from a depth of 300 m on the slope, thus providing a mechanism for the transport of nutrients across the shelf break onto the shelf

The diurnal mixed layer and upper ocean heat budget in the western equatorial Pacific

This paper presents the results of an experiment in the western equatorial Pacific centered on the equator at 165°E which was designed to study the changes to the structure of the upper ocean on timescales of a few days and spatial scales of tens of kilometers. The results show that the response of the upper ocean to atmospheric forcing is very sensitive to the vertical structure of both the temperature and salinity. The diurnal response of the near-surface temperature to daytime heating and nighttime cooling was found to have an amplitude of a few tenths of a degree Celsius, This compares with a horizontal variation of temperature on scales of a few tens of kilometers of a similar magnitude. Even away from the very fresh surface layers typical of the area, salinity is found to play an important role in limiting the depth of nighttime mixing. In this case a subsurface salinity maximum restricts the depth to around 40 m. The nighttime convection is severely limited by either a small change in the surface forcing or the horizontal advection of slightly cooler waters from the east; we are unable to determine which is the dominant mechanism in the present case. The reduced mixing leads to an increase of the diurnal variation of sea surface temperature to over 1°C. The estimated net surface heat flux from the atmosphere to the ocean was found to be not significantly different from zero at 10 W m?2, in agreement with recent measurements. The net surface heat flux during the period of the heat budget experiment, which took place on the equator, was substantially higher at 65 W m?2. Changes of in situ temperature are found to be dominated by advection. The vertical velocity is estimated to be of order 10 m d?1 and to be caused by advection along east-west sloping density surfaces. Changes to the temperature structure of the upper ocean induced by motions with a timescale of a few days (possibly planetary waves) are found to be significantly greater than longer-term wind-induced upwelling or advectio

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

Meridional Heat and Salinity Transports and the Surface Freshwater Exchange Derived from the OSNAP (Overturning in the Subpolar North Atlantic Program) Array between August 2014 and May 2018

Data from the full OSNAP array for the first 4 years between 2014 and 2018 have been used to produce the 30-day mean meridional heat (MHT) and salinity transports (MST), and the derived surface freshwater (FW) exchange time series.Related article: Observation-based estimates of heat and freshwater exchanges from the subtropical North Atlantic to the ArcticAn international effort, Overturning in the Subpolar North Atlantic Program (OSNAP), is a partnership among oceanographers from the US, UK, Germany, the Netherlands, Canada and China whose goal is to measure and understand what drives the subpolar overturning circulation and its associated property exchanges. OSNAP is consisted of more than 53 moorings that stretch from Labrador to Greenland to Scotland, providing a continuous record of the full water column, trans-basin velocity, temperature and salinity in the subpolar North Atlantic. Data from the full OSNAP array for the first 4 years between 2014 and 2018 have been used to produce the 30-day mean meridional heat (MHT) and salinity transports (MST), and the derived surface freshwater (FW) exchange time series.National Science Foundation Award Number 194833

Meridional Overturning Circulation Observed by the OSNAP (Overturning in the Subpolar North Atlantic Program) Array from August 2014 to May 2018

This item replaces a previous published version of the data with persistent identifiers: http://hdl.handle.net/1853/63707 and https://doi.org/10.35090/wa93-m688The project scientists would appreciate it if you would use the data DOI https://doi.org/10.35090/gatech/65537 and add the following acknowledgement to any publication that use this data: “OSNAP data were collected and made freely available by the OSNAP (Overturning in the Subpolar North Atlantic Program) project and all the national programs that contribute to it (www.o-snap.org).”Strength of the Meridional Overturning Circulation across the OSNAP array, defined as the maximum of the streamfunction in density space.An international effort, Overturning in the Subpolar North Atlantic Program (OSNAP), is a partnership among oceanographers from the US, UK, Germany, the Netherlands, Canada and China whose goal is to measure and understand what drives the Atlantic Meridional Overturning Circulation (MOC) and its variability. OSNAP is consisted of more than 53 moorings that stretch from Labrador to Greenland to Scotland, providing a continuous record of the full water column, trans-basin volume transports in the subpolar North Atlantic. The first 4 years of data (August 2014 - May 2018) from the full OSNAP array has been used to produce the 30-day mean estiamtes of the MOC at OSNAP. All data are freely available from www.o-snap.org. item_description: Strength of the Meridional Overturning Circulation across the OSNAP array, defined as the maximum of the streamfunction in density space. The original version of this dataset is available at https://doi.org/10.35090/wa93-m688National Science Foundation Award No. 194833

Meridional Overturning Circulation Observed by the OSNAP (Overturning in the Subpolar North Atlantic Program) Array from August 2014 to May 2018 - Originally published data

Strength of the Meridional Overturning Circulation across the OSNAP array, defined as the maximum of the streamfunction in density space.The project scientists would appreciate it if you would use the data DOI https://doi.org/10.35090/wa93-m688 and add the following acknowledgement to any publication that use this data: “OSNAP data were collected and made freely available by the OSNAP (Overturning in the Subpolar North Atlantic Program) project and all the national programs that contribute to it (www.o-snap.org).”NOTE: This item is replaced by http://hdl.handle.net/1853/65537. An error was detected in this original dataset.An international effort, Overturning in the Subpolar North Atlantic Program (OSNAP), is a partnership among oceanographers from the US, UK, Germany, the Netherlands, Canada and China whose goal is to measure and understand what drives the Atlantic Meridional Overturning Circulation (MOC) and its variability. OSNAP is consisted of more than 53 moorings that stretch from Labrador to Greenland to Scotland, providing a continuous record of the full water column, trans-basin volume transports in the subpolar North Atlantic. The first 4 years of data (August 2014 - May 2018) from the full OSNAP array has been used to produce the 30-day mean estimates of the MOC at OSNAP. All data are freely available from www.o-snap.org. The corrected version of this dataset is available at https://doi.org/10.35090/gatech/65537National Science Foundation Award Number 194833

Overturning in the Subpolar North Atlantic Program : a new international ocean observing system

A new ocean observing system has been launched in the North Atlantic in order to understand the linkage between the meridional overturning circulation and deep water formation.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 IPCC projections of an AMOC slowdown in the 21st 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 (OSNAP), to provide a continuous record of the trans-basin 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/MOCHA array 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