Oxygen variability in the near-surface waters of the northern North Atlantic: Observations and a model

As part of the World Ocean Circulation Experiment a major study was undertaken to determine the absolute circulation of the Subpolar North Atlantic using a large number of acoustically tracked isopycnal floats deployed on the 27.5 σ surface. Fifty floats were equipped with sensors to study dissolved oxygen from a Lagrangian perspective. In this paper we comment on very large variations in oxygen along trajectories of fluid parcels that outcrop in winter and resubduct the following spring. We employ a one-dimensional model to interpret these in terms of biophysical processes at and near the surface. In an attempt to understand the observed variability, we find that a modified form of the Price-Weller-Pinkel mixed layer model using NCEP-derived surface forcing accurately reproduces both the float-observed temperature and the meteorological-based sea-surface temperatures to within 1°C for an entire year, including the timing of the ventilation and restratification observed by the float. The model also employs satellite-derived observations to represent three processes of oxygen exchange: an air-sea gas flux dependent upon wind-driven turbulence, oxygen production in the mixed layer as a result from primary productivity, and oxygen consumption at depth as a result of net community respiration. The model accurately reproduces the observed ~3% supersaturation in the wintertime mixed layer, a level which is supported by the air-sea gas flux. We also find that later in the year, during springtime restratification, the model reproduces the observed decline from 105% to 92% oxygen saturation. The good agreement between observation and model depends upon a one-dimensional balance in the vertical, i.e.the absence of horizontal advective effects. For floats outcropping in an area of horizontal thermal contrast, conspicuous errors in the predicted vertical structure arise, most likely due to horizontal advection or displacement of the float by surface winds, effects which cannot be assessed without additional information. This limitation notwithstanding, the agreement between model and observation indicates the power of Lagrangian techniques for understanding how the properties of surface waters are set and later modified as they subduct into the interior of the ocean

Absolute Transports of Mass and Temperature for the North Atlantic Current– Subpolar Front System

The flow of subtropical waters carried into the northern North Atlantic Ocean by the North Atlantic Current– subpolar front system (NAC–SPF) is an important component of the meridional overturning circulation. These waters become colder and denser as they flow through the subpolar region, both by mixing with the colder subpolar waters and by atmospheric cooling. The relative roles of these two processes remain to be quantified, and the mechanisms driving lateral mixing need to be better understood. To address those questions, a new methodology is developed to estimate the mean absolute transports of mass and heat for the top 1000 dbar in the region of the NAC–SPF for the time period 1993–2000. The transports are obtained by combining historical hydrography with isopycnal RAFOS float data from the area. The mean absolute transport potential field shows an NAC–SPF “pipe,” defined by two bounding transport potential contours. This pipe transports 10.0 ± 3.5 Sv (Sv ≡ 106 m3 s−1) (top 1000 dbar) from the subtropics into the eastern subpolar North Atlantic. In contrast to earlier studies, the northward-flowing NAC follows a distinct meandering path, with no evidence of permanent branches peeling off the current before reaching the “Northwest Corner.” As the current enters the Northwest Corner, it loses its tight structure and maybe splits into two or more branches, which together constitute the eastward flow along the SPF. The eastward flow between the Northwest Corner and the Mid-Atlantic Ridge is not as tightly defined because of the meandering and/or eddy shedding of the branches constituing the SPF. As the flow approaches the Mid-Atlantic Ridge, it converges to cross above the Charlie–Gibbs and Faraday Fracture Zones. The mean absolute temperature transport (top 1000 dbar) by the 10-Sv pipe was estimated across 10 transects crossing the NAC–SPF. Because the mean mass flux is constant in the pipe, variations in the mean temperature transports result from lateral exchange and mixing across the pipe\u27s side walls and from air–sea fluxes across the surface of the pipe. The NAC–SPF current loses 0.18 ± 0.05 PW on its transit through the region, most of the loss occuring upstream of the Northwest Corner. The heat loss is 10 times the corresponding heat lost to the atmosphere. We conclude that cross-frontal exchange induced by the steep meanders of the northward-flowing NAC is the main mechanism by which heat is lost along the current in the region between the “Tail of the Grand Banks” and the Mid-Atlantic Ridge

Ocean circulation causes the largest freshening event for 120 years in eastern subpolar North Atlantic

The Atlantic Ocean overturning circulation is important to the climate system because it carries heat and carbon northward, and from the surface to the deep ocean. The high salinity of the subpolar North Atlantic is a prerequisite for overturning circulation, and strong freshening could herald a slowdown. We show that the eastern subpolar North Atlantic underwent extreme freshening during 2012 to 2016, with a magnitude never seen before in 120 years of measurements. The cause was unusual winter wind patterns driving major changes in ocean circulation, including slowing of the North Atlantic Current and diversion of Arctic freshwater from the western boundary into the eastern basins. We find that wind-driven routing of Arctic-origin freshwater intimately links conditions on the North West Atlantic shelf and slope region with the eastern subpolar basins. This reveals the importance of atmospheric forcing of intra-basin circulation in determining the salinity of the subpolar North Atlantic

A Method for Obtaining the Mean Transports of Ocean Currents by Combining Isopycnal Float Data with Historical Hydrography

This article presents a method for obtaining the mean structure of the temperature, specific volume anomaly, and velocity of an ocean current, using isopycnal float data combined with gravest empirical mode (GEM) fields calculated from historical hydrography. A GEM field is a projection on a geostrophic streamfunction space of hydrographic data, which captures most of the vertical structure associated with frontal regions. This study focuses on the North Atlantic Current–subpolar front (NAC–SPF) current system, but the float–GEM method has broad applicability to baroclinic ocean currents in general. The NAC–SPF current system is of climatic interest, being an important conduit of warm salty waters into the northern North Atlantic. It constitutes the upper limb of the thermohaline circulation of the Atlantic Ocean and plays a crucial role in the moderation of European climate, but uncertainties regarding its transport and corresponding heat fluxes remain, mainly because the structure of the system is not well known. This paper shows how isopycnal floats can be used to obtain such estimates. The performance of the float–GEM method is tested in two ways. First, two synoptic hydrographic sections (one across the NAC and the other across the SPF) are reconstructed from simulated isopycnal float pressure measurements. The baroclinic transports of volume and temperature (relative to 1000 dbar) across the sections are well reproduced by the method: the float–GEM transport estimates have an accuracy of ±20% and a precision of ±15% or less, which result in deviations of less than ±10% from the “real” values. In the second test, horizontal maps of pressure and temperature on the δ = −12.7 × 10−8 m3 kg−1 specific volume anomaly surface (σθ ≈ 27.5 kg m−3) are produced, using RAFOS float data from two experiments that sampled the region from 1993 to 2000. These maps compare well with similar maps constructed in previous studies and establish the consistency of the method. The good performance of the float–GEM method gives confidence in this novel way of using isopycnal floats to obtain information on the structure of the ocean. Combined with the velocity measured by the floats, it has the potential to estimate absolute transports and heat fluxes along the NAC–SPF system

Sustained ocean observations from merchant marine vessels

Merchant marine vessels have provided invaluable information about weather and climate over the seas. In this note, it is shown how these vessels also contribute to ocean research through systematic surveys of upper ocean temperature, salinity and currents. By repeatedly sampling a particular route, one can obtain an accurate picture of the mean state of the ocean and where and how it varies. With a few examples drawn from our own work we show how commercial shipping and cruise vessels, with their unparalleled access to the oceans, could give society far more extensive and valuable information about upper ocean and atmospheric conditions on a regular basis. But for this to happen, a new generation of ocean instrumentation needs to be developed that is optimized for completely automatic and unattended operation on such vessels. It also means working with the merchant marine community to develop guidelines and procedures for future cooperative efforts

Direct measurements of the mean flow and eddy kinetic energy structure of the upper ocean circuletion in the NE Atlantic

The upper ocean circulation in the sub-polar northeast Atlantic has been a challenge to quantify due to strong and variable wind-forcing, and strong and variable deep currents that lead to large uncertainties in the use of the standard dynamical method. Since 1999 we have been operating an acoustical Doppler current profiler on a container vessel that operates between Denmark and Greenland to repeatedly sample upper ocean currents across the northeast Atlantic. Individual transects exhibit a highly energetic mesoscale variability, but ensemble-averaging of the sections reveals a striking organization of the mean field along the Reykjanes Ridge: a distinct southward flow along its eastern slope and two clearly defined peaks with seasonal modulation flowing to the north along its western slope. Higher values of eddy kinetic energy (about 150–600 cm2 s−2) are observed along the transect, O(1.5) greater than surface drifter estimates

An alternative hypothesis for the origin of the mediterranean salt lens observed off the bahamas in the fall of 1976

A hypothesis is presented that the original salt lens, or meddy, observed off the Bahamas in the fall of 1976 may have been formed, not near the Mediterranean outflow, but instead in the vicinity of the northwest corner (51°N, 43°W) of the North Atlantic Current. An eddy was observed near the northwest corner by an isopycnal RAFOS float deployed during the 1993-95 North Atlantic Current Experiment, and had nearly identical temperature/salinity properties as those of the Bahamas lens. Hydrographie evidence of thick homogeneous layers with similar properties near the northwest corner suggest a possible formation mechanism by which surface eddies containing warm and saline waters are cooled and subducted. A plausible scenario is made whereby a northwest corner eddy might be advected southward in the Newfoundland Basin by the flow around the high pressure ridge east of the North Atlantic Current and then enter the recirculation gyre immediately south of the Gulf Stream. Such an eddy could be advected to the site of the Bahamas lens in just three years, perhaps much more quickly than an eddy of Mediterranean origin and without encountering the topographic barrier of the Mid-Atlantic Ridge. This conclusion is ironic because the Bahamas lens is considered the first observation of an eddy of Mediterranean origin, and led to the coining of the term meddy. © 1999 American Meteorological Society

On the size and distribution of rings and coherent vortices in the Sargasso Sea

The container motor vessel CMV Oleander, which operates between New Jersey and Bermuda, crosses the Gulf Stream and Sargasso Sea all year round on a semiweekly schedule. Using an acoustic Doppler current profiler, measurements of upper ocean currents have been made on a regular basis since the fall of 1992. In this paper we examine the database for evidence of axisymmetric coherent vortices including the distribution and intensity of cold core rings. To detect the existence of coherent vortices, the patterns of current vectors averaged between 40 and 80 m depth were fit to an axisymmetric Gaussian vortex model. The parameters of the model were axis location, maximum tangential, or swirl, speed, and radius at which the maximum swirl was measured. We were able to distinguish between the well-known cold core rings (CCRs) pinched from the Gulf Stream, and a population of cyclonic and anticyclonic vortices in the Sargasso Sea. Both the rings and the Sargasso Sea vortices showed radii of 64 ± 18 km, albeit with different swirl speeds. The rings, close to the Gulf Stream, exhibited a typical maximum swirl speed of 0.98 ± 0.40 m s-1 and a center relative vorticity of 0.64 ± 0.35 × 10-4 s-1, almost 80% of the planetary vorticity for the region. The more uniform population of Sargasso Sea vortices contained approximately equal numbers of cyclones and anticyclones, with mean speeds of+0.43 and -0.55 m s-1, and center relative vorticities of+0.24 × 10-4 s-1 and -0.29 × 10-4 s-1, respectively. Copyright 2008 by the American Geophysical Union

Observations of the Faroe Bank Channel overflow using bottom-following RAFOS floats

Overflows do not easily lend themselves to study by Lagrangian floats that remain on a constant isobaric (pressure) or isopycnal (density) surface, since the mixing, entrainment, and descent of an overflow plume result in an increase of the pressure and typically a decrease in the density of the overflow waters. A simple technique to maintain the float\u27s altitude above the bottom was developed, and 12 bottom-following RAFOS floats were deployed at or downstream of the sill in the Faroe Bank Channel in the summer of 2000 from the R.S.S. Discovery. A technical problem resulted in the majority of the floats becoming stuck to the bottom; nevertheless several floats were able to traverse the Iceland Basin and surface near the southeastern slope of Iceland. These floats made a descent from the mouth of the Faroe Bank Channel, only to shoal along the southern slope of the Iceland-Faroe Ridge before descending again when passing through the northwest corner of the Iceland Basin. Typical current speeds through the Basin were 0.20-0.30ms-1, with peaks of 0.40-0.50ms-1. Although the floats that were stuck on the bottom provided no trajectory information, they were able to provide a time-series of bottom or near-bottom temperature. In addition, a crude estimate of the flow regime could be made by interpreting the pressure signals from these stuck floats as a response to strong or weak currents. Floats that were bottom stuck near the mouth of the Channel experienced large fluctuations in temperature (0-5°C) and height of the bottom (and thus presumably speed) on scales from 1 to 4 days. Another float stuck 100 km downstream of the sill underwent temperature and speed excursions on similar time scales, albeit over a smaller range. The behavior of the floats is assumed to be the result of the mesoscale variability of the overflow plume downstream of the Faroe Bank Channel. © 2005 Elsevier Ltd. All rights reserved

Pathways of cross-frontal exchange in the North Atlantic Current

The North Atlantic Current (NAC) forms part of the boundary between the subtropical and subpolar gyres in the North Atlantic Ocean. The current has topographically controlled stationary meanders that appear to grow and decay. A region east of the current in the Newfoundland Basin contains water of mixed subpolar/subtropical properties, suggesting that there is exchange across the NAC. This study considers data from isopycnal RAFOS floats launched in the NAC region from 1993 to 1995. We use the RAFOS data to define the frontal zone as a pressure range where the jet is most likely to be found. This definition requires a latitudinal dependence as the NAC shoals to the north. Floats shallower and deeper than this range are defined to be on the subpolar and subtropical side, respectively. These definitions are used to estimate mixing that occurs between the current and its surroundings and to estimate the relative quantity of exchange of water parcels between the two gyres. Only small quantities of mass exchange from one gyre to the other are found, but there is a distinct asymmetry leading to a mean flux from the subpolar to subtropical sides. We also find that floats spend significant time in the frontal region and are frequently exchanged between fast and slow moving waters, particularly at the meander extrema. Diffusion, while in the jet, leads to eddy cross-frontal exchange which is important for the exchange of properties across the NAC. Copyright 2001 by the American Geophysical Union