What Can Hydrography Between the New England Slope, Bermuda and Africa Tell us About the Strength of the AMOC Over the Last 90 years?

The Gulf Stream is the only pathway in the subtropical North Atlantic by which warm water flows poleward. This transport of warm water and return of cold water at depth is called the Atlantic Meridional Overturning Circulation (AMOC). The dynamic method is applied to hydrocasts collected since the 1930s to estimate upper-ocean transport (0–1,000 m) between the U.S. Continental Slope and Bermuda and separately to Africa with focus on the longest directly observable timescale. Calculating transport between the Slope and Bermuda eliminates the Gulf Stream\u27s northern and southern recirculation gyres, while calculations between the Slope and Africa remove all other recirculating geostrophic flow. The net Slope-Bermuda upper-ocean transport is estimated to be 41.1 ± 0.4 Sv, decreasing by 2.0 ± 0.8 Sv between 1930 and 2020. The AMOC contribution is 18.4 ± 0.6 Sv, decreasing by 0.4 ± 0.6 Sv between 1930 and 2020

Gulf Stream rings may rival atmospheric iron supply to the North Atlantic subtropical gyre

Substantial amounts of nitrogen fixation occur in the North Atlantic subtropical gyre, due to the activity of cyanobacteria with high iron requirements. Iron is delivered to this region by dust from the Sahara Desert. However, this dust deposition is typically localized and episodic. Therefore, other sources of iron may also be important. Here, we report observations of dissolved iron concentrations in a Gulf Stream cold-core ring, which transported iron-rich water from near the continental slope into the subtropical gyre. We find that iron concentrations were elevated in the ring compared with subtropical waters, reflecting its source waters. Using iron data from these source waters and the identification of ring activity in satellite data, we estimate that cold-core rings provide a net flux of 0.3 ± 0.17 × 108 mol Fe yr−1 across the northwestern gyre edge, on the order of 15% of our median estimates of gyre-wide supply of iron by dust deposition. We suggest that iron supply from cold-core rings is an important source of iron to the northwestern gyre edge. We conclude that mesoscale ocean circulation features may play an important role in subtropical nutrient and carbon cycling

Rapid coastal deoxygenation due to ocean circulation shift in the NW Atlantic.

Global observations show that the ocean lost approximately 2% of its oxygen inventory over the last five decades 1-3, with important implications for marine ecosystems 4, 5. The rate of change varies with northwest Atlantic coastal waters showing a long-term drop 6, 7 that vastly outpaces the global and North Atlantic basin mean deoxygenation rates 5, 8. However, past work has been unable to resolve mechanisms of large-scale climate forcing from local processes. Here, we use hydrographic evidence to show a Labrador Current retreat is playing a key role in the deoxygenation on the northwest Atlantic shelf. A high-resolution global coupled climate-biogeochemistry model 9 reproduces the observed decline of saturation oxygen concentrations in the region, driven by a retreat of the equatorward-flowing Labrador Current and an associated shift toward more oxygen-poor subtropical waters on the shelf. The dynamical changes underlying the shift in shelf water properties are correlated with a slowdown in the simulated Atlantic Meridional Overturning Circulation 10. Our results provide strong evidence that a major, centennial-scale change of the Labrador Current is underway, and highlight the potential for ocean dynamics to impact coastal deoxygenation over the coming century

Labrador Sea Water transport across the Charlie-Gibbs Fracture Zone

Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 125(8), (2020): e2020JC016068, doi:10.1029/2020JC016068.Labrador Sea Water (LSW) is a major component of the deep limb of the Atlantic Meridional Overturning Circulation, yet LSW transport pathways and their variability lack a complete description. A portion of the LSW exported from the subpolar gyre is advected eastward along the North Atlantic Current and must contend with the Mid‐Atlantic Ridge before reaching the eastern basins of the North Atlantic. Here, we analyze observations from a mooring array and satellite altimetry, together with outputs from a hindcast ocean model simulation, to estimate the mean transport of LSW across the Charlie‐Gibbs Fracture Zone (CGFZ), a primary gateway for the eastward transport of the water mass. The LSW transport estimated from the 25‐year altimetry record is 5.3 ± 2.9 Sv, where the error represents the combination of observational variability and the uncertainty in the projection of the surface velocities to the LSW layer. Current velocities modulate the interannual to higher‐frequency variability of the LSW transport at the CGFZ, while the LSW thickness becomes important on longer time scales. The modeled mean LSW transport for 1993–2012 is higher than the estimate from altimetry, at 8.2 ± 4.1 Sv. The modeled LSW thickness decreases substantially at the CGFZ between 1996 and 2009, consistent with an observed decline in LSW volume in the Labrador Sea after 1994. We suggest that satellite altimetry and continuous hydrographic measurements in the central Labrador Sea, supplemented by profiles from Argo floats, could be sufficient to quantify the LSW transport at the CGFZ.A. G. N. appreciates conversations with Kathy Donohue, Tom Rossby and Lisa Beal, which helped to interpret the results. J. B. P. acknowledges support from NSF through Grant OCE‐1947829. The authors thank all colleagues and ship crew involved in the R/V Meteor cruise M‐82/2 and Maria S. Merian cruise MSM‐21/2. The mooring data presented in this paper were funded by NSF through Grant OCE‐0926656.2021-01-0

CLIVAR Mode Water Dynamics Experiment (CLIMODE) fall 2006 R/V Oceanus voyage 434 November 16, 2006–December 3, 2006

CLIMODE (CLIVAR Mode Water Dynamic Experiment) is a research program designed to understand and quantify the processes responsible for the formation and dissipation of North Atlantic subtropical mode water, also called Eighteen Degree Water (EDW). Among these processes, the amount of buoyancy loss at the ocean-atmosphere interface is still uncertain and needs to be accurately quantified. In November 2006, cruise 434 onboard R/V Oceanus traveled in the region of the separated Gulf Stream and its recirculation, where intense oceanic heat loss to the atmosphere in the winter is believed to trigger the formation of EDW. During this cruise, the surface mooring F that was anchored in the core of the Gulf Stream was replaced by a new one, as well as two subsurface moorings C and D located on the southeastern edge of the stream. Surface drifters, ARGO and bobbers RAFOS floats were deployed, CTD profiles and water samples were also carried out. This array of instruments will permit a characterization of EDW with high spatial and temporal resolutions and accurate in-situ measurements of air-sea fluxes in the EDW formation region. The present report documents this cruise, the methods and locations for the deployments of instruments and some evaluation of the measurements from these instruments.Funding was provided by the National Science Foundation under contract No. OCE04-2453

High N2 Fixation in and Near the Gulf Stream Consistent with a Circulation Control on Diazotrophy

The stoichiometry of physical nutrient supply may provide a constraint on the spatial distribution and rate of marine nitrogen (N2) fixation. Yet agreement between the N2 fixation rates inferred from nutrient supply and those directly measured has been lacking. The relative transport of phosphate and nitrate across the Gulf Stream suggests that 3–6 Tg N year−1 must be fixed to maintain steady nutrient stoichiometry in the North Atlantic subtropical gyre. Here we show direct measurements of N2 fixation consistent with these estimates, suggesting elevated N2 fixation in and near the Gulf Stream. At some locations across the Gulf Stream, we measured diazotroph abundances and N2 fixation rates that are 1–3 orders of magnitude greater than previously measured in the central North Atlantic subtropical gyre. In combination, rate measurements and gene abundances suggest that biogeochemical budgets can be a robust predictive tool for N2 fixation hot spots in the global ocean

CLIMODE Subsurface Mooring Report : November 2005 - November 2007

Two years of temperature, salinity, current, and nutrient data were collected on four subsurface moorings as part of the 2 year field component of the CLIMODE experiment. The moorings were located in North Atlantic’s subtropical gyre, south-east of the Gulf Stream. Two moorings, the most heavily instrumented, were close to the Gulf Stream, in the region where cold air outbreaks force large air-sea fluxes and where Eighteen Degree Water outcrops. Two other moorings were located farther south and carried more limited instrumentation. The moorings were initially deployed in November of 2005, turned around in November of 2006 and finally recovered in November of 2007. During the first year, the moorings close to the Gulf Stream suffered considerable blow down, and some of the instruments failed. During the second year, the blow down was greatly reduced and most instruments collected a full year worth of data.Funding was provided by the Division of Ocean Sciences of the National Science Foundation under Grant No. OCE-0424536

Roles of the ocean mesoscale in the horizontal supply of mass, heat, carbon and nutrients to the Northern Hemisphere subtropical gyres

Horizontal transport at the boundaries of the subtropical gyres plays a crucial role in providing the nutrients that fuel gyre primary productivity, the heat that helps restratify the surface mixed layer, and the dissolved inorganic carbon (DIC) that influences air‐sea carbon exchange. Mesoscale eddies may be an important component of these horizontal transports; however, previous studies have not quantified the horizontal tracer transport due to eddies across the subtropical gyre boundaries. Here we assess the physical mechanisms that control the horizontal transport of mass, heat, nutrients and carbon across the North Pacific and North Atlantic subtropical gyre boundaries using the eddy‐rich ocean component of a climate model (GFDL's CM2.6) coupled to a simple biogeochemical model (mini‐BLING). Our results suggest that horizontal transport across the gyre boundaries supplies a substantial amount of mass and tracers to the ventilated layer of both Northern Hemisphere subtropical gyres, with the Kuroshio and Gulf Stream acting as main exchange gateways. Mass, heat, and DIC supply is principally driven by the time‐mean circulation, whereas nutrient transport differs markedly from the other tracers, as nutrients are mainly supplied to both subtropical gyres by down‐gradient eddy mixing across gyre boundaries. A budget analysis further reveals that the horizontal nutrient transport, combining the roles of both mean and eddy components, is responsible for more than three quarters of the total nutrient supply into the subtropical gyres, surpassing a recent estimate based on a coarse resolution model and thus further highlighting the importance of horizontal nutrient transport

Impact of Weddell Sea deep convection on natural and anthropogenic carbon in a climate model

A climate model is used to investigate the influence of Weddell Sea open ocean deep convection on anthropogenic and natural carbon uptake for the period 1860-2100. In a three-member ensemble climate change simulation, convection ceases on average by year 1981, weakening the net oceanic cumulative uptake of atmospheric CO2 by year 2100 (-4.3 Pg C) relative to an ocean that has continued convection. This net weakening results from a decrease in anthropogenic carbon uptake (-10.1 Pg C), partly offset by an increase in natural carbon storage (+5.8 Pg C). Despite representing only 4% of its area, the Weddell Sea is responsible for 22% of the Southern Ocean decrease in total climate-driven carbon uptake and 52% of the decrease in the anthropogenic component of oceanic uptake. Although this is a model-specific result, it illustrates the potential of deep convection to produce an inter-model spread in future projections of ocean carbon uptake

How does Labrador Sea Water enter the deep western boundary current?

Author Posting. © American Meteorological Society, 2008. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 38 (2008): 968-983, doi:10.1175/2007JPO3807.1.Labrador Sea Water (LSW), a dense water mass formed by convection in the subpolar North Atlantic, is an important constituent of the meridional overturning circulation. Understanding how the water mass enters the deep western boundary current (DWBC), one of the primary pathways by which it exits the subpolar gyre, can shed light on the continuity between climate conditions in the formation region and their downstream signal. Using the trajectories of (profiling) autonomous Lagrangian circulation explorer [(P)ALACE] floats, operating between 1996 and 2002, three processes are evaluated for their role in the entry of Labrador Sea Water in the DWBC: 1) LSW is formed directly in the DWBC, 2) eddies flux LSW laterally from the interior Labrador Sea to the DWBC, and 3) a horizontally divergent mean flow advects LSW from the interior to the DWBC. A comparison of the heat flux associated with each of these three mechanisms suggests that all three contribute to the transformation of the boundary current as it transits the Labrador Sea. The formation of LSW directly in the DWBC and the eddy heat flux between the interior Labrador Sea and the DWBC may play leading roles in setting the interannual variability of the exported water mass.We are also grateful to the NSF for their support of this research