Matthias Lankhorst

Observation of decadal change in the Atlantic meridional overturning circulation using 10 years of continuous transport data

The meridional overturning circulation (MOC) represents the main mechanism for the oceanic northward heat transport in the Atlantic, and fluctuations of this circulation are believed to have major impacts on northern hemisphere climate. While numerical ocean and climate models and paleo-records show large variability in this circulation, the use of direct observations of the MOC for detecting climate-timescale changes has proven difficult so far. This report presents the first observational record of MOC measurements that is continuous and sufficiently long to exhibit decadal-scale changes, here a decrease by 20% over the observational period (Jan. 2000–June 2009) and large interannual changes in the flow and its vertical structure. Data are from a mooring array at 16°N (Meridional Overturning Variability Experiment, MOVE). The observed change agrees with the amplitude of multi-decadal natural fluctuations seen in numerical ocean and climate models. Knowledge of the existence and phasing of such internal cycles provides multi-decadal climate predictability. Recently, some numerical model simulations have produced results that show a weakening of the MOC since the 1990s and observational confirmation of this now is a high priority.

Zonal intermediate currents in the equatorial Atlantic Ocean

Acoustic float data collected near 800 m depth, are used to map zonal mean currents within the Antarctic Intermediate Water (AAIW) tongue in the equatorial Atlantic. Alternating zonal jets of 2° latitudinal width are revealed between 6°S and 6°N. Displacements from profiling floats drifting near 1000 m depth, also reveal similar zonal jets at the base of the AAIW layer. The strongest jets (15 cm s−1 peak) are found at 4°S, 2°S, 0°, 2°N and 4°N. They are coherent longitudinally over order of 3000 km and, poleward of 1°S and 1°N, generally coherent vertically between 800 m and 1000 m. Large seasonal fluctuations exist at both levels: within 1° of equator, AAIW at 800 m flows westward (8 cm s−1 mean) in boreal summer and fall but eastward (3 cm s−1 mean) in winter, whereas the flow at 1000 m is eastward in late fall and winter.

Lagrangian Observations of the Middepth and Deep Velocity Fields of the Northeastern Atlantic Ocean

The circulation of the northeastern Atlantic Ocean at intermediate depths is characterized by watermass transformation processes that involve Iceland–Scotland Overflow Water (ISOW) from the northeast, Labrador Sea Water (LSW) from the west, and Mediterranean Water from the south. Field observations were carried out with 89 eddy-resolving floats (RAFOS and MARVOR types). The data coverage achieved is remarkably high and enables a comprehensive study of the eastern basins between Iceland and the Azores. The trajectories show typical pathways of the water masses involved and the role that the complex bottom topography plays in defining them. The ISOW paths tend to lean against the slopes of the Reykjanes Ridge and Rockall Plateau. Westward escapes through multiple gaps in the ridge are possible, superimposed on a sustained southward flow in the eastern basin along the Mid-Atlantic Ridge. LSW pathways leading to the eastern basins are subject to high variability in flow direction and eddy activity. In addition to a selection of characteristic trajectories, maps of the horizontal distributions of Lagrangian eddy kinetic energy and integral time scales are presented. These reveal distinct areas of intensified mixing in the Iceland Basin, as well as the sharp contrast between the subpolar and subtropical dynamics. A self-contained eddy detection scheme is applied to obtain statistics on individual eddy properties and their abundance. It is suggested that much of the intensified mixing can be related to cyclonic activity, particularly in the subpolar region.

A Scheme to Identify Loops from Trajectories of Oceanic Surface Drifters: An Application in the Kuroshio Extension Region

When a drifter is trapped in an eddy, it makes either a cycloidal or a looping trajectory. The former case takes place when the translating speed is larger than the eddy spinning speed. When the background mean velocity is removed, drifter trajectories make loops. Thus, eddies can be detected from a drifter trajectory by identifying looping segments. In this paper, an automated scheme is developed to identify looping segments from Lagrangian trajectories,based on a geometric definition of a loop, that is, a closingcurve with its starting point overlapped by its ending point. The scheme is to find the first returning point, if it exists, along a trajectory of a surface drifter with a few other criteria. To further increase the chance that detected loops are eddies, it is considered that a loop identifies an eddy only when the loop’s spinning period is longer than the local inertial period and shorter than the seasonal scale, and that at least two consecutive loops with the same polarity that stay sufficiently close are found. Five parameters that characterize an eddy are estimated by the scheme: location (eddy center), time (starting and ending time), period, polarity, and intensity. As an example, the scheme is applied to surface drifters in the Kuroshio Extension region. Results indicate that numbers of eddies are symmetrically distributed for cyclonic and anticyclonic eddies, mean eddy sizes are 40‐ 50 km, and eddy abundance is the highest along the Kuroshio path with more cyclonic eddies along its southern flank.

A Self-Contained Identification Scheme for Eddies in Drifter and Float Trajectories

It is becoming increasingly recognized that the eddy field plays an important—possibly dominating—role for oceanic motions in many aspects (e.g., transport of properties and risk assessment in the case of extreme events). This motivates the study of individual eddy events. In the Lagrangian coordinate system, vorticity possibly associated with eddies appears in two forms: as shear vorticity between neighboring particles, and as curvature of the trajectory of a single particle. Typical field experiments in physical oceanography using surface drifters or subsurface floats do not reach data densities high enough to produce enough encounters of drifters to calculate shear vorticity between them. However, curvature in individual tracks is easily observed. This study presents a methodology that extracts segments from within a trajectory that are “looping,” which will be interpreted as a drifter being caught in an eddy. The method makes use of autoregressive processes, a simple type of stochastic processes, which easily enables a fit to the nonperfectly shaped trajectory data usually expected from field experiments. These processes also deliver frequency and persistence of the detected eddies by a very simple calculation, which makes the methodology highly suited for automatized scanning of larger datasets.

ASIRI : an ocean–atmosphere initiative for Bay of Bengal

AbstractAir–Sea Interactions in the Northern Indian Ocean (ASIRI) is an international research effort (2013–17) aimed at understanding and quantifying coupled atmosphere–ocean dynamics of the Bay of Bengal (BoB) with relevance to Indian Ocean monsoons. Working collaboratively, more than 20 research institutions are acquiring field observations coupled with operational and high-resolution models to address scientific issues that have stymied the monsoon predictability. ASIRI combines new and mature observational technologies to resolve submesoscale to regional-scale currents and hydrophysical fields. These data reveal BoB’s sharp frontal features, submesoscale variability, low-salinity lenses and filaments, and shallow mixed layers, with relatively weak turbulent mixing. Observed physical features include energetic high-frequency internal waves in the southern BoB, energetic mesoscale and submesoscale features including an intrathermocline eddy in the central BoB, and a high-resolution view of the exchange...

Observed Basin-Scale Response of the North Atlantic Meridional Overturning Circulation to Wind Stress Forcing

AbstractThe response of the North Atlantic meridional overturning circulation (MOC) to wind stress forcing is investigated from an observational standpoint, using four time series of overturning transports below and relative to 1000 m, overlapping by 3.6 yr. These time series are derived from four mooring arrays located on the western boundary of the North Atlantic: the RAPID Western Atlantic Variability Experiment (WAVE) array (42.5°N), the Woods Hole Oceanographic Institution Line W array (39°N), RAPID–MOC/MOCHA (26.5°N), and the Meridional Overturning Variability Experiment (MOVE) array (16°N). Using modal decompositions of the analytic cross-correlation between transports and wind stress, the basin-scale wind stress is shown to significantly drive the MOC coherently at four latitudes, on the time scales available for this study. The dominant mode of covariance is interpreted as rapid barotropic oceanic adjustments to wind stress forcing, eventually forming two counterrotating Ekman overturning cells c...

Meridional overturning circulation and heat transport observations in the Atlantic Ocean

Special supplement to the Bulletin of the American Meteorological Society vol.94, No. 8, August 2013

Meridional overturning circulation observations in the North Atlantic Ocean

Special supplement to the Bulletin of the American Meteorological Society vol.94, No. 8, August 2013

The global component of the US Ocean Observatories Initiative and the global OceanSITES project

The NSF-funded Ocean Observatories Initiative (OOI) will establish four comprehensive and interactive open-ocean observatories for sustained timeseries observations, two of which are located in the Pacific Basin. At each site they will include four moorings and three gliders, with a wide range of multidisciplinary sensors. These installations will represent infrastructure for the ocean research community and enable both process experiments and climate-timescale investigations. In addition to free data access and mechanisms to manipulate the assets and sensors of the observatories, there will be expansion capacity for user-provided instruments. This presentation will explain the plans and capabilities for these open-ocean observatories. The OOI sites are embedded in the global and international timeseries project OceanSITES, which is an established component of the global ocean observing system. Many countries are contributing to this network. OceanSITES provides coordination, integration into a global system, and a data management framework which enables the accessibility and dissemination of the data to the community. There are presently about 100 sites with 200 platforms globally in OceanSITES. The status and benefits of the project will be explained. New sites are continually being added and both the research and operational community are encouraged to add their open-ocean sustained timeseries sites to OceanSITES.