The Atlantic Meridional Overturning Circulation (AMOC) is a key mechanism of heat, freshwater, and carbon redistribution in the climate system. The precept that the AMOC has changed abruptly in the past, notably during and at the end of the last ice age, and that it is "very likely" to weaken in the coming century due to anthropogenic climate change is a key motivation for sustained observations of the AMOC. This paper reviews the methodology and technology used to observe the AMOC and assesses these ideas and systems for accuracy, shortcomings, potential improvements, and sustainability. We review hydrographic techniques and look at how these traditional techniques can meet modern requirements. Transport mooring arrays (TMAs) provide the "gold standard" for sustained AMOC observing, utilizing dynamic height, current meter, and other instrumentation and techniques to produce continuous observations of the AMOC. We consider the principle of these systems and how they can be sustained and improved into the future. Techniques utilizing indirect measurements, such as satellite altimetry, coupled with in situ measurements, such as the Argo float array, are also discussed. Existing technologies that perhaps have not been fully exploited for estimating AMOC are reviewed and considered for this purpose. Technology is constantly evolving, and we look to the future of technology and how it can be deployed for sustained and expanded AMOC measurements. Finally, all of these methodologies and technologies are considered with a view to a sustained and sustainable future for AMOC observation.

Since late fall 1992, CMV Oleander III has been measuring upper ocean currents during its weekly trips between Bermuda and Port Elizabeth, New Jersey, by means of an acoustic Doppler current profiler installed in its hull. The overarching objective of this effort has been to monitor transport in the Gulf Stream and surrounding waters. With 25 years of observation in hand, we note that the Gulf Stream exhibits significant year-to-year variations but no evident long-term trend in transport. We show how these data have enabled studies of oceanic variability over a very wide range of scales, from a few kilometers to the full 1,000 km length of its route. We report that the large interannual variations in temperature on the continental shelf are negatively correlated with flow from the Labrador Sea, but that variability in the strength of this flow cannot account for a longer-term warming trend observed on the shelf. Acoustic backscatter data offer a rich trove of information on biomass activities over a wide range of spatial and temporal scales. A peek at the future illustrates how the new and newly equipped Oleander will be able to profile currents to greater depths and thereby contribute to monitoring the strength of the meridional overturning circulation.

The coastal ocean model FVCOM is applied to quantify the changes in circulation, flushing, and exposure time in Great South Bay, New York, after Superstorm Sandy breached the barrier island in 2012. Since then, the lagoon system is connected to the Atlantic via five instead of four inlets. The model simulations are run on two high-resolution unstructured grids, one for the pre-breach configuration, one including the new inlet, with tidal-only forcing, and summer and winter forcing conditions. Despite its small cross-sectional size, the breach has a relatively large net inflow that leads to a strengthening of the along-bay through-flow in Great South Bay (GSB); the tidally driven volume transport in central GSB quadrupled. The seasonal forcing scenarios show that the southwesterly sea breeze in summer slows down the tidally driven flow, while the forcing conditions in winter are highly variable, and the circulation is dependent on wind direction and offshore sea level. Changes in flushing and exposure time associated with the modified transport patterns are evaluated using a Eulerian passive tracer technique. Results show that the new inlet produced a significant decrease in flushing time (approximately 35% reduction under summer wind conditions and 20% reduction under winter wind conditions). Maps of exposure time reflect the local changes in circulation and flushing.

Expendable bathythermographs (XBT) to profile upper-ocean temperatures from vessels in motion have been in use for some 50 years now. Developed originally for navy use, they were soon adapted by oceanographers to map out upper-ocean thermal structure and its space-time variability from both research vessels and merchant marine vessels in regular traffic. These activities continue today. This paper describes a new technology-the Autonomous Expendable Instrument System (AXIS)-that has been developed to provide the capability to deploy XBT probes on a predefined schedule, or adaptively in response to specific events without the presence of an observer on board. AXIS is a completely self-contained system that can hold up to 12 expendable probes [XBTs, XCTDs, expendable sound velocimeter (XSV)] in any combination. A single-board Linux computer keeps track of what probes are available, takes commands from ashore via Iridium satellite on what deployment schedule to follow, and records and forwards the probe data immediately with a time stamp and the GPS position. This paper provides a brief overview of its operation, capabilities, and some examples of how it is improving coverage along two lines in the Atlantic.

Using vessel-mounted acoustic Doppler current profiler data from four different routes between Scotland, Iceland and Greenland, we map out the mean flow of water in the top 400 m of the northeastern North Atlantic. The poleward transport east of the Reykjanes Ridge (RR) decreases from ~8.5 to 10 Sv (1 Sverdrup=106 m3 s−1) at 59.5°N to 61°N to 6 Sv crossing the Iceland–Faroes–Scotland Ridge. The two longest ~1200 km transport integrals have 1.4–0.94 Sv uncertainty, respectively. The overall decrease in transport can in large measure be accounted for by a ~1.5 Sv flow across the RR into the Irminger Sea north of 59.5°N and by a ~0.5 Sv overflow of dense water along the Iceland–Faroes Ridge. A remaining 0.5 Sv flux divergence is at the edge of detectability, but if real could be accounted for through wintertime convection to >400 m and densification of upper ocean water. The topography of the Iceland Basin and the banks west of Scotland play a fundamental role in controlling flow pathways towards and past Iceland, the Faroes and Scotland. Most water flows north unimpeded through the Iceland Basin, some in the centre of the basin along the Maury Channel, and some along Hatton Bank, turning east along the northern slopes of George Bligh Bank, Lousy Bank and Bill Bailey's Bank, whereupon the flow splits with ~3 Sv turning northwest towards the Iceland–Faroes Ridge and the remainder continuing east towards and north of the Wyville-Thomson Ridge (WTR) to the Scotland slope thereby increasing the Slope Current transport from ~1.5 Sv south of the WTR to 3.5 Sv in the Faroes–Shetland Channel.

This paper describes storm surge simulations made for Sandy (2012) for the Metropolitan New York (NYC) area using the Advanced Circulation (ADCIRC) model forced by the Weather Research and Forecasting (WRF) model. The atmospheric forecast uncertainty was quantified using 11-members from an atmospheric Ensemble Kalman Filter (EnKF) system. A control WRF member re-initialized every 24 h demonstrated the capability of the WRF-ADCIRC models to realistically simulate the 2.83 m surge and 4.40 m storm tide (surge + astronomical tide) above mean lower low water (MLLW) for NYC. Starting about four days before landfall, an ensemble of model runs based on the 11 “best” meteorological predictions illustrate how modest changes in the track (20–100 km) and winds (3–5 m s−1) of Sandy approaching the New Jersey coast and NYC can lead to relatively large (0.50–1.50 m) storm surge variations. The ensemble also illustrates the extreme importance of the timing of landfall relative to local high tide. The observed coastal flooding was not the worst case for this particular event. Had Sandy made landfall at differing times, locations and stages of the tide, peak water levels could have been up to 0.5 m higher than experienced.

Atlantic Waters flowing northward into the Nordic Seas are important for their role as an early indicator of changes to deepwater formation. As such, this requires a fundamental understanding of the pathways and volume fluxes through the primary passageways from the Atlantic into the Nordic Seas. A mean annual volume transport of 6.10.3 Sv was observed flowing in above the sigma(t)=27.8 isopycnal (a proxy for the lower limit of Atlantic Water depth), through the Faroe Shetland Channel (FSC) and over the Iceland Faroes Ridge (IFR) from March 2008 to June 2012, using repeat velocity sections obtained from a vessel mounted Acoustic Doppler Current Profiler (ADCP). A new vessel route has expanded the spatial coverage of FSC observations and reveals a difference in average inflow transport, which most likely results from an interannual variation in the total transport through the FSC, which in turn is tied to a weakening of the southerly flow over the western slope of the channel. This interannual variability has increased the mean transport through the FSC from 0.9 Sv observed over the first 2 years of this program by Rossby and Flagg (2012) to a 4.5 year mean of 1.70.2 Sv, which emphasizes the importance of knowing the flow along the Faroese shelf. Interannual fluctuations in transport observed over the IFR are related to the width of the inflow over the Faroese half of the ridge.

• Proxies for the Gulf Stream position are computed using data from the Oleander Project, the Canadian Marine Environmental Data Service (MEDS) and the Radar Altimeter Database System (RADS). The Gulf Stream transport is constructed from the 20 year Oleander time series data and the Florida Current transport is estimated from the Western Boundary Time Series Project. The NAO index is from the NOAA climate prediction center. • For comparison purposes, all time series will have the same time step. To deal with the different issues in data quality and spatial resolution with the ship-based data, we construct annual averages, where Jan-Dec is the averaging interval, with 6 month steps.

During the summer, distinctive, bottom-trapped, cold water mass called the Cold Pool resides over the mid to outer continental shelf in the Middle Atlantic Bight (MAB); strongly influencing parts of the ecosystem including important fisheries. Since 2003, repeated ocean glider temperature and salinity (TS) water property measurements along a seaward transect from the coast of New Jersey have helped to define the important variability in the cross-shelf structure of the Cold Pool there. To develop forecast capability, we need to better understand the relevant processes that control Cold Pool seasonal evolution and variability. To do this, we are now beginning to integrate ocean glider TS measurements with data assimilation models for the purpose of generating prototype Cold Pool forecast maps; with important support from the Mid-Atlantic Regional Association Coastal Ocean Observing System (MARACOOS). Here we report recent progress in having models assimilate zig-zag, along-shelf glider measurements of the Cold Pool. This year's 2012 measurements are revealing a shelf and Cold Pool water, which appear to be as much as 1°C warmer than in previous years.

The wavenumber spectra for velocity and temperature in the Gulf Stream region are calculated from a decade (1994-2004) of shipboard acoustic Doppler current profiler (ADCP) measurements taken as part of the Oleander Project. The velocity and temperature spectra have comparable magnitude, in terms of the kinetic and potential energy. and both indicate a k(-3) slope in the mesoscales. In contrast, the corresponding velocity spectrum determined from satellite altimetry sea surface heights yields a significantly higher energy level and a k(-2) slope. The discrepancy between altimeter-derived and directly measured velocity spectra suggests that altimetric velocity probably is contaminated by noise in sea surface height measurement. Also, the k(-3) slope, which appears to be in agreement with two-dimensional quasigeostrophic turbulence theory, does not support the contemporary surface quasigeostrophic theory. These results highlight large gaps in the current understanding of the nature of surface geostrophic turbulence.

Western Arctic Shelf-Basin Interactions (SBI) process experiment cruises were conducted during spring and summer in 2002 and 2004. A comparison of the 2004 data with the results from 2002 reveals several similarities but also some distinct differences. Similarities included the following: (1) Dissolved inorganic nitrogen (DIN) (ammonium+nitrate+nitrite) limited phytoplankton growth in both years, suggesting that the fixed-N transport through Bering Strait is a major control on biological productivity. (2) The head of Barrow Canyon was a region of enhanced biological production. (3) Plume-like nutrient maxima and N** minima (a signal of sedimentary denitrification) extending from the shelf into the interior were common except at our easternmost section where the nearshore end of these features intersected the slope. (4) Particularly during summer, oxygen supersaturations were common in or just above the shallow nitracline. (5) Surface waters at our deepest stations were already depleted in nitrate, ammonium and urea during our springtime observations. A major difference between the 2 years was the greater influence of warm, relatively low-nutrient Alaska Coastal Water (ACW) during 2004 entering the region via Bering Strait. This increased inflow of ACW may have reduced photic zone nutrient concentrations. The differences in water temperature and nutrients were most pronounced in the upper similar to 100 db, and the increased influence of warm water in 2004 relative to 2002 was most evident in our East Barrow (EB) section. Although the EB data were collected on essentially the same year-days (29 July-4 August 2002 vs. 29 July-6 August 2004), the surface layers were up to 5 degrees warmer in 2004.While the stronger inflow of ACW in 2004 may have reduced the autochthonous nutrient supply, rates of primary production, bacterial production, and particulate organic carbon export were higher in 2004. This conundrum might be explained by differences in the availability of light. Although, springtime ice thicknesses were greater in 2004 than in 2002, snow cover was significantly less and may have more than compensated for the modest differences in ice thickness vis a vis light penetration. In addition, there was a rapid and extensive retreat of the ice cover in summer 2004. Increased light penetration in 2004 may have allowed phytoplankton to increase utilization of nutrients in the shallow nitracline. In addition, more light combined with warmer temperatures could enhance that fraction of primary production supported by nutrient recycling. Enhanced subsurface primary production during summer 2004 is suggested not only by the results of incubation experiments but by more extreme dissolved oxygen supersaturations in the vicinity of the nitracline. We cannot, however, ignore aliasing that might arise from somewhat different station distributions and timing. It is also possible that the rapid ice retreat and warmer temperatures lead to an acceleration in the seasonal progression of biological processes such that the summer 2004 SBI Process Cruise (HLY 04-03) experiment was observing a state that might have existed a few weeks after completion of the 2002 summer cruise (HLY 02-03). Despite these complications, there is little doubt that biological conditions at the ensemble of hydrographic stations occupied in 2004 during the SBI Process Cruises differed significantly from those at the stations occupied in 2002. (c) 2008 Published by Elsevier Ltd.

The flow patterns of the Gulf Stream warm core rings and surrounding shelf break and slope water are constructed from shipboard acoustic Doppler current profiler (ADCP) transects of the Oleander Project. For each warm ring, consecutive ADCP transects are colocated to a common ring center and are mapped into a stream function. Methods to locate ring centers from the ADCP transect and advanced very high resolution radiometer image are developed and tested. Two warm rings in 1999, which have relatively complete data coverage, are examined to study the ring-induced warm and cold streamers. For cold streamers, the estimated volume flux, based on more than 10 independent ADCP transects, is at least 1 x 10(6) m(3) s(-1). This result agrees well with the previous estimate in the Warm Core Ring Experiment. For warm streamers, the result is new. On the basis of a large number of ADCP transects, the estimated volume flux is about 2.5 x 10(6) m(3) s(-1). By pulling large transports associated with the cold/warm streamers, the warm rings likely play a fundamental role in the water exchange in the Slope Sea.

Since 1992, upper ocean ADCP current data between New York and Bermuda have been gathered from the container ship Oleander to identify long‐term changes in the shelf, slope, Gulf Stream and Sargasso Sea. Temperature and surface salinity data have been been collected along this route since 1978 by NOAA/NMFRC. The first ten years of ADCP data from which the effects of warm ring have been removed are used to describe processes within the shelfbreak frontal sub‐region. The Eulerian mean velocity structure shows an along‐isobath shelfbreak jet with maximum speeds of O(0.15 m s−1) offshore of which is a ∼30 km wide relatively quiescent region. There is also an offshore slope current 40 to 50 km wide extending vertically to 300 m, with similar velocities as those found in the shelfbreak jet. The mean shelfbreak jet transport is 0.4 Sv while the slope current adds another 2.5 Sv. Maximum shelfbreak transport occurs in the fall and winter while the slope current reaches its maximum during the spring. In stream coordinates, the shelfbreak jet has maximum speeds of 0.35 m s−1, a width of ∼30 km and a vertical decay scale of ∼50 m. The maximum Rossby number within the jet, defined by ∣dU/dy∣max/f, is about 0.2. Significant interannual fluctuations occur in upper ocean temperature, salinity and currents, some of which appear related to changes in the NAO index. Seasonal changes in the slope current appear to be related to seasonal changes in the wind stress curl over the slope sea.

[i] A field program detected direct crossover of Scotian Shelf Water (SSC) from Browns to Georges Bank during winter/spring 1999 using (1) moored measurements, (2) drogued drifters, and (3) satellite imagery. Statistics of the 30-day trajectories indicate that (18, 55) ± 17% of the drifters crossed the (100, 200) m isobaths on Georges Bank. Transit times ranged from 2 to 26 days. Four of the drifters crossed onto the Northeast Peak (NEP) coinciding with the detection of Scotian Shelf Water at the central NEP mooring. These events are deemed significant because they serve to deliver particles from the Scotian Shelf directly to the gadoid spawning grounds on Georges Bank. Depths of the SSC layers lie between 15 and 50 m, and residence times for the Scotian Shelf Water on the NEP are estimated at 3 to 4 weeks. Canonical correlation analysis of the NEP temperature and salinity records suggest that 10% of the correlation structure among the variables is related to SSCs, while 70% is associated with the annual cycle. A search for SSC driving mechanisms reveals that (1) interannual variability of the annual freshwater discharge from the Gulf of St. Lawrence is not a factor, (2) Ekmanlike response of near-surface currents to southeast wind stress plays a role but is not the dominant factor, and (3) mesoscale baroclinic features penetrating Northeast Channel from offshore are the most likely cause. Furthermore, the incidence of SSCs may be related to offshore fronts whose proximity is positively correlated with the North Atlantic Oscillation index.

Distributions of primary production and chlorophyll size structure were examined in relationship to hydrographic properties during March and July 1996 in continental shelf waters off Cape Hatteras, North Carolina. Chlorophyll concentrations ranged highest (>10mgm−3) during March. Peak values were associated with subsurface maxima near the shelf edge, which were dominated by larger (>8μm) phytoplankton. In July, chlorophyll concentrations were generally lower with higher proportions associated with the <8μm size fraction. Primary production was determined using a wavelength-resolved photosynthesis–irradiance model. Model performance was evaluated by comparison of modeled and measured estimates of optical properties and simulated in situ primary production. Regional patterns of primary production determined using the model revealed high water-column-integrated primary production during March (>2.0g Cm−2d−1) in association with the shelf–slope front. In contrast, highest values in July (>3.0g Cm−2d−1) were associated with an intrusion of high-salinity, high-nutrient water into the southern portion of the study region. Average shelf-wide productivities were 1.3g Cm−2d−1 in March and 2.1g Cm−2d−1 in July. Contemporaneous Advanced Very High-Resolution Radiometer sea-surface temperature imagery revealed that regions of high productivity were proximal to Gulf Stream circulation features. In March, cross-shelf sections of hydrographic properties and nutrients provided evidence for onshore displacement of the shelf–slope front, apparently related to the combined effects of the strong winds prior to the cruise and the offshore presence of energetic Gulf Stream processes. We speculate that variable forcing, related to the dynamics of the Gulf Stream and wind, resulted in significant frontal displacement and frontal velocity fluctuations leading to enhanced mixing and injection of nutrients across the frontal boundary. In July, Gulf Stream circulation, in conjunction with upwelling favorable conditions, resulted in a subsurface intrusion of high-nutrient water onto the shelf analogous to similar phenomena observed in the South Atlantic Bight. Localized regions of enhanced productivity appear to be quantitatively important in the overall carbon/nitrogen budget of the southern Mid-Atlantic Bight shelf.

Sea-level anomalies (SLA) derived from the TOPEX/Poseidon (T/P) altimetry and inferred geostrophic currents within the northern Arabian Sea were examined for the period from March 1993 through November 1996. The primary objective of this study was to confirm and extend our understanding of the upper-ocean mesoscale variability observed in the shipboard acoustic Doppler current profiler (ADCP) data collected during the US JGOFS/ONR Arabian Sea Expedition (September 1994–January 1996). The accuracy of the T/P altimetry data (∼3cm rms) results in an uncertainty in the altimeter-derived velocities comparable to the ADCP measurement error. Thus the T/P data provide a reasonable method for extending studies of the mesoscale dynamics for the region. Comparison of the T/P-derived geostrophic velocities with concurrent ADCP data showed good correlation, with an r2 between 0.7 and 0.9 and rms differences of 10cms−1. The T/P data confirm both the overall spatial and seasonal current patterns observed by the ADCP. The monsoonally averaged rms sea-level anomalies indicate a high degree of intraseasonal variation due to the generation of squirts, jets and eddies all along the coast, the variability of which increases in both intensity and areal extent during the Southwest Monsoon. The SLA data indicate a much reduced degree of variability over the shelf. The SLA-derived eddy kinetic energy (EKE) fields are consistent with those derived earlier from the ADCP data in both distribution and magnitude. There is a large increase in EKE to the west and southwest and to a lesser extent to the south and a large area of relatively reduced eddy activity over much of the eastern and northern Arabian Sea. The area of reduced eddy activity coincides with the location of the most intense portions of the oxygen minimum zone found in the northern Arabian Sea. The spatial scales of the eddies responsible for the EKE distribution over the study area range between 200 and 500km in the nearshore region, decreasing to 100–200km offshore. While there is significant energy variability in annual and semi-annual time scales, a substantial portion of the energy is found between 50 and 120 days, and the relative importance of this frequency band increases offshore. Spectra indicate a distinct break in the frequency content of the eddy field at about 15°N, with little energy at less than annual periods south of this latitude.