On the Currents and Transports Connected With the Atlantic Meridional Overturning Circulation in the Subpolar North Atlantic

Results from an interannually forced, 0.08 degrees eddy-resolving simulation based on the Hybrid Coordinate Ocean Model, in conjunction with a small but well-determined transport database, are used to investigate the currents and transports associated with the Atlantic meridional overturning circulation (AMOC) in the subpolar North Atlantic (SPNA). The model results yield a consistent warming in the western SPNA since the early 1990s, along with mean transports similar to those observed for the trans-basin AMOC across the World Ocean Circulation Experiment hydrographic section AR19 (16.4 Sv) and boundary currents at the exit of the Labrador Sea near 53 degrees N (39.0 Sv) and east of the Grand Banks near 43 degrees N (15.9 Sv). Over a 34 year integration, the model-determined AMOC across the AR19 section and the western boundary current near 53 degrees N both exhibit no systematic trend but some long-term (interannual and longer) variabilities, including a decadal transport variation of 3-4 Sv from relatively high in the 1990s to low in the 2000s. The decadal variability of the model boundary current transport near 53 degrees N lags the observed winter time North Atlantic Oscillation index by about 2 years and leads the model AMOC across the AR19 section by about 1 year. The model results also show that the long-term variabilities are low compared to those on shorter time scales. Thus, rapid sampling of the current over long time intervals is required to filter out high-frequency variabilities in order to determine the lower frequency variabilities of interest. Citation: Xu, X., H. E. Hurlburt, W. J. Schmitz Jr., R. Zantopp, J. Fischer, and P. J. Hogan (2013), On the currents and transports connected with the atlantic meridional overturning circulation in the subpolar North Atlantic, J. Geophys. Res. Oceans, 118, 502-516, doi:10.1002/jgrc.20065

Intraseasonal variability of the North Brazil Undercurrent forced by remote winds

Intraseasonal signals with periods of 2 to 3 weeks in near-surface alongshore current measurements are detected from four moorings (K1 - K4) deployed from 2000 to 2004 at the 11{degree sign}S section close to the Brazilian coast as part of the German CLIVAR Tropical Atlantic Variability Project. This section crosses the path of the North Brazil Undercurrent, the most powerful western boundary current in the South Atlantic Ocean. We investigate the origin of this intraseasonal variability of the North Brazil Undercurrentby relating the oceanic oscillation of the alongshore currents to its atmospheric counterpart, the meridional wind stress. On average, the results indicate a well-defined lagged (10 days) correlation (~0.6) structure between meridional wind stress and alongshore currents. The oceanic region with the highest cross-correlations is identified as a relatively narrow band along the Brazilian coast, from 22{degree sign}-36{degree sign}S and 40{degree sign}-50{degree sign}W, bounded in the north by an eastward change in coastline orientation. The cross-wavelet transform establishes the common power between the time series of meridional wind stress and alongshore currents, predominantly during austral winter and spring. These signals propagate equatorward with an alongshore speed of 285{plus minus}63 km day-1, consistent with Coastal Trapped Wave theory

Estimating ocean transports with dynamic height moorings: an application in the Atlantic deep western boundary current

Efficient monitoring of large-scale current systems for climate research requires the development of new techniques to estimate ocean transports. Here, a methodology for continuous estimation of dynamic height profiles and geostrophic currents from moored temperature sensors is presented. The technique is applied to moorings deployed in the Atlantic Deep Western Boundary Current at 26.5°N, off Abaco, the Bahamas (WOCE ACM-1 array). Relative geostrophic currents are referenced using bottom pressure sensors and available shipboard direct velocity (lowered-ADCP) sections over the period of the deployment, to obtain a time series of absolute volume transport. Comparison with direct velocity measurements from a complete array of current meters shows good agreement for the mean transport and its variability on time scales longer than 10 days, but larger variability in the current meter derived transport at time scales shorter than 10 days. A rigorous error analysis assesses the contributions of various error sources in the geostrophic as well as direct transport estimates. Low-frequency drift of the bottom pressure sensors is found to be the largest error source in the geostrophic transport estimates and recommendations for improvement of the technique and related measurement technologies are made

Intra-seasonal variability of the DWBC in the western subpolar North Atlantic

Highlights: •A joint analysis of deep current meter records in the western North Atlantic. •Intra-seasonal variability dominates the deep boundary current. •Topographic waves near 10 d periods trapped over steep topography. •Basin centers are showing longer periods (50 d) caused by the eddy field. •Observed variability characteristics compared to high resolution model simulation

Mesoscale variations of biogeochemical properties in the Sargasso Sea

A mesoscale resolution biogeochemical survey was carried out in the vicinity of the U.S. Joint Global Ocean Flux Study Bermuda Atlantic Time-series Study (BATS) site during the summer of 1996. Real-time nowcasting and forecasting of the flow field facilitated adaptive sampling of several eddy features in the area. Variations in upper ocean nutrient and pigment distributions were largely controlled by vertical isopycnal displacements associated with the mesoscale field. Shoaling density surfaces tended to introduce cold, nutrient-rich water into the euphotic zone, while deepening isopycnals displaced nutrient-depleted water downward. Chlorophyll concentration was generally enhanced in the former case and reduced in the latter. Eddy-induced upwelling at the base of the euphotic zone was affected by features of two different types captured in this survey: (1) a typical mid-ocean cyclone in which doming of the main thermocline raised the near-surface stratification upward and (2) a mode water eddy composed of a thick lens of 18°C water, which pushed up the seasonal thermocline and depressed the main thermocline. Model hindcasts using all available data provide a four-dimensional context in which to interpret temporal trends at the BATS site and two other locations during the 2 weeks subsequent to the survey. Observed changes in near-surface structure at the BATS site included shoaling isopycnals, increased nutrient availability at the base of the euphotic zone, and enhanced chlorophyll concentration within the euphotic zone. These trends are explicable in terms of a newly formed cyclone that impinged upon the site during this time period. These observations reveal that eddy upwelling has a demonstrable impact on the way in which the nitrate-density relationship changes with depth from the aphotic zone into the euphotic zone. A similar transition is present in the BATS record, suggesting that eddy-driven upwelling events are present in the time series of upper ocean biogeochemical properties. The variability in main thermocline temperature and nitrate in this synoptic spatial survey spans the range observed in these quantities in the 10-year time series available at BATS to date (1988–1998)

Break-up of the Atlantic Deep Western Boundary Current into eddies at 8°S

The existence in the ocean of deep western boundary currents, which connect the high-latitude regions where deep water is formed with upwelling regions as part of the global ocean circulation, was postulated more than 40 years ago1. These ocean currents have been found adjacent to the continental slopes of all ocean basins, and have core depths between 1,500 and 4,000 m. In the Atlantic Ocean, the deep western boundary current is estimated to carry (10–40) times 106 m3 s-1 of water2, 3, 4, 5, transporting North Atlantic Deep Water—from the overflow regions between Greenland and Scotland and from the Labrador Sea—into the South Atlantic and the Antarctic circumpolar current. Here we present direct velocity and water mass observations obtained in the period 2000 to 2003, as well as results from a numerical ocean circulation model, showing that the Atlantic deep western boundary current breaks up at 8° S. Southward of this latitude, the transport of North Atlantic Deep Water into the South Atlantic Ocean is accomplished by migrating eddies, rather than by a continuous flow. Our model simulation indicates that the deep western boundary current breaks up into eddies at the present intensity of meridional overturning circulation. For weaker overturning, continuation as a stable, laminar boundary flow seems possible

Distinct sources of interannual subtropical and subpolar Atlantic overturning variability

The Atlantic meridional overturning circulation (AMOC) is pivotal for regional and global climate due to its key role in the uptake and redistribution of heat and carbon. Establishing the causes of historical variability in AMOC strength on different timescales can tell us how the circulation may respond to natural and anthropogenic changes at the ocean surface. However, understanding observed AMOC variability is challenging because the circulation is influenced by multiple factors that co-vary and whose overlapping impacts persist for years. Here we reconstruct and unambiguously attribute intermonthly and interannual AMOC variability at two observational arrays to the recent history of surface wind stress, temperature and salinity. We use a state-of-the-art technique that computes space- and time-varying sensitivity patterns of the AMOC strength with respect to multiple surface properties from a numerical ocean circulation model constrained by observations. While, on interannual timescales, AMOC variability at 26° N is overwhelmingly dominated by a linear response to local wind stress, overturning variability at subpolar latitudes is generated by the combined effects of wind stress and surface buoyancy anomalies. Our analysis provides a quantitative attribution of subpolar AMOC variability to temperature, salinity and wind anomalies at the ocean surface