Observed basin-scale response of the North Atlantic Meridional Overturning Circulation to wind stress forcing

The 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 years. These time series are derived from four mooring arrays located on the western boundary of the North Atlantic: the RAPID WAVE array (42.5°N), the Woods Hole Oceanographic Institution Line W array (39°N), the RAPID MOC/MOCHA array (26.5°N), and the 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 drives the MOC coherently at four latitudes, on the timescales available for this study. The dominant mode of covariance is interpreted as rapid barotropic oceanic adjustments to wind stress forcing, eventually forming two counter-rotating Ekman overturning cells centered on the tropics and subtropical gyre. A second mode of covariance appears related to patterns of wind stress and wind stress curl associated with the North Atlantic Oscillation, spinning anomalous horizontal circulations which likely interact with topography to form overturning cells

A Dataset of Hourly Sea Surface Temperature From Drifting Buoys

A dataset of sea surface temperature (SST) estimates is generated from the temperature observations of surface drifting buoys of NOAA's Global Drifter Program. Estimates of SST at regular hourly time steps along drifter trajectories are obtained by fitting to observations a mathematical model representing simultaneously SST diurnal variability with three harmonics of the daily frequency, and SST low-frequency variability with a first degree polynomial. Subsequent estimates of non-diurnal SST, diurnal SST anomalies, and total SST as their sum, are provided with their respective standard uncertainties. This Lagrangian SST dataset has been developed to match the existing hourly dataset of position and velocity from the Global Drifter Program

Can Drake Passage Observations Match Ekman's Classic Theory?

Ekman's theory of the wind-driven ocean surface boundary layer assumes a constant eddy viscosity and predicts that the current rotates with depth at the same rate as it decays in amplitude. Despite its wide acceptance, Ekman current spirals are difficult to observe. This is primarily because the spirals are small signals that are easily masked by ocean variability and cannot readily be separated from the geostrophic component. This study presents a method for estimating ageostrophic currents from shipboard acoustic Doppler current profiler data in Drake Passage and finds that observations are consistent with Ekman's theory. By taking into account the sampling distributions of wind stress and ageostrophic velocity, the authors find eddy viscosity values in the range of 0.08–0.12 m2 s−1 that reconcile observations with the classic theory in Drake Passage. The eddy viscosity value that most frequently reconciles observations with the classic theory is 0.094 m2 s−1, corresponding to an Ekman depth scale of 39 m

Towards two decades of Atlantic Ocean mass and heat transports at 26.5° N

Continuous measurements of the Atlantic meridional overturning circulation (AMOC) and meridional ocean heat transport at 26.5° N began in April 2004 and are currently available through December 2020. Approximately 90% of the total meridional heat transport (MHT) at 26.5° N is carried by the zonally averaged overturning circulation, and an even larger fraction of the heat transport variability (approx. 95%) is explained by the variability of the zonally averaged overturning. A physically based separation of the heat transport into large-scale AMOC, gyre and shallow wind-driven overturning components remains challenging and requires new investigations and approaches. We review the major interannual changes in the AMOC and MHT that have occurred over the nearly two decades of available observations and their documented impacts on North Atlantic heat content. Changes in the flow-weighted temperature of the Florida Current (Gulf Stream) over the past two decades are now taken into account in the estimates of MHT, and have led to an increased heat transport relative to the AMOC strength in recent years. Estimates of the MHT at 26.5° N from coupled models and various surface flux datasets still tend to show low biases relative to the observations, but indirect estimates based on residual methods (top of atmosphere net radiative flux minus atmospheric energy divergence) have shown recent promise in reproducing the heat transport and its interannual variability. This article is part of a discussion meeting issue ‘Atlantic overturning: new observations and challenges’

Atlantic Meridional Overturning Circulation: Observed Transport and Variability

The Atlantic Meridional Overturning Circulation (AMOC) extends from the Southern Ocean to the northern North Atlantic, transporting heat northwards throughout the South and North Atlantic, and sinking carbon and nutrients into the deep ocean. Climate models indicate that changes to the AMOC both herald and drive climate shifts. Intensive trans-basin AMOC observational systems have been put in place to continuously monitor meridional volume transport variability, and in some cases, heat, freshwater and carbon transport. These observational programs have been used to diagnose the magnitude and origins of transport variability, and to investigate impacts of variability on essential climate variables such as sea surface temperature, ocean heat content and coastal sea level. AMOC observing approaches vary between the different systems, ranging from trans-basin arrays (OSNAP, RAPID 26°N, 11°S, SAMBA 34.5°S) to arrays concentrating on western boundaries (e.g., RAPID WAVE, MOVE 16°N). In this paper, we outline the different approaches (aims, strengths and limitations) and summarize the key results to date. We also discuss alternate approaches for capturing AMOC variability including direct estimates (e.g., using sea level, bottom pressure, and hydrography from autonomous profiling floats), indirect estimates applying budgetary approaches, state estimates or ocean reanalyses, and proxies. Based on the existing observations and their results, and the potential of new observational and formal synthesis approaches, we make suggestions as to how to evaluate a comprehensive, future-proof observational network of the AMOC to deepen our understanding of the AMOC and its role in global climate

Surface drifter pair spreading in the North Atlantic

This study examines spreading of surface drifter pairs deployed as part of the CLIVAR Mode Water Dynamic Experiment (CLIMODE) project in the Gulf Stream region. The spreading is resolved at hourly resolution and quantified by relative dispersion and finite-scale Lyapunov exponents. At scales from 1-3 km to 300-500 km, the dispersion follows Richardson's law, indicating stirring by eddies comparable in scale to the pair separation distance. At larger scales, the spreading becomes a random walk described by a constant diffusivity. The behavior from 1-3 km to the local deformation radius is inconsistent with the enstrophy cascade of 2-D quasigeostrophic turbulence. To test various hypotheses for this result, drifter pair spreading is examined for pairs that were not launched together, pairs deployed in the eastern subtropical North Atlantic, and CLIMODE pairs subsampled to daily temporal resolution. Our results indicate the presence of significant energy at the submesoscale in the Gulf Stream region which flattens the wave number spectrum and dominates surface stirring at this scale range. Results in the less energetic subtropical eastern Atlantic are more equivoca

Spectral characterization of Ekman velocities in the Southern Ocean based on surface drifter trajectories

Velocity time series from surface drifter data are exploited in a novel way to study the Southern Ocean surface circulation response to wind forcing. The ageostrophic component of the drifter velocities at 15 m is approximated by subtracting altimeter-derived geostrophic velocities from the drifter velocities. The resultant ageostrophic velocity time series are studied in the frequency domain jointly with contemporaneous time series of local wind stress from atmospheric reanalysis data. Rotary spectral analysis indicates that both wind stresses and ocean velocities are predominantly anticyclonic. Cross-spectral analysis shows that the upper ocean responds preferentially to anticyclonic winds not only at the inertial frequency but also at subinertial frequencies. The phase of the cross-spectra which is interpreted as a geometric angle indicates that the component of velocity that is coherent with the wind stress is to the left of the wind at subinertial frequencies and to the right at supra-inertial frequencies, and is seen as evidence of Ekman-type currents. A first order closure of the oceanic vertical turbulence, where the oceanic stress is equal to a viscosity coefficient K times the velocity vertical shear, is used to interpret the cross-spectrum. In this framework, the real part of the cross-spectrum of the wind stress and ocean surface ageostrophic velocity is shown to be a measure of the wind energy input rate to the Ekman layer. This energy input is therefore estimated across the Southern Ocean. The observed transfer function, which is the cross-spectrum divided by the auto-spectrum of the wind stress, is compared to the theoretical transfer functions arising from 10 different Ekman-type boundary layer models. These models differ in the dependence of K on the vertical coordinate and in the bottom boundary condition. The most dynamically consistent model has a vertical viscosity that is finite at the surface and increases linearly to the bottom of the boundary layer depth. Results of the comparison to models provide in situ seasonal estimates of zonally averaged near-surface viscosities and boundary layer depths across the Southern Ocea

Spectral characterization of Ekman velocities in the Southern Ocean based on surface drifter trajectories

Velocity time series from surface drifter data are exploited in a novel way to study the Southern Ocean surface circulation response to wind forcing. The ageostrophic component of the drifter velocities at 15 m is approximated by subtracting altimeter-derived geostrophic velocities from the drifter velocities. The resultant ageostrophic velocity time series are studied in the frequency domain jointly with contemporaneous time series of local wind stress from atmospheric reanalysis data. Rotary spectral analysis indicates that both wind stresses and ocean velocities are predominantly anticyclonic. Cross-spectral analysis shows that the upper ocean responds preferentially to anticyclonic winds not only at the inertial frequency but also at subinertial frequencies. The phase of the cross-spectra which is interpreted as a geometric angle indicates that the component of velocity that is coherent with the wind stress is to the left of the wind at subinertial frequencies and to the right at supra-inertial frequencies, and is seen as evidence of Ekman-type currents. A first order closure of the oceanic vertical turbulence, where the oceanic stress is equal to a viscosity coefficient K times the velocity vertical shear, is used to interpret the cross-spectrum. In this framework, the real part of the cross-spectrum of the wind stress and ocean surface ageostrophic velocity is shown to be a measure of the wind energy input rate to the Ekman layer. This energy input is therefore estimated across the Southern Ocean. The observed transfer function, which is the cross-spectrum divided by the auto-spectrum of the wind stress, is compared to the theoretical transfer functions arising from 10 different Ekman-type boundary layer models. These models differ in the dependence of K on the vertical coordinate and in the bottom boundary condition. The most dynamically consistent model has a vertical viscosity that is finite at the surface and increases linearly to the bottom of the boundary layer depth. Results of the comparison to models provide in situ seasonal estimates of zonally averaged near-surface viscosities and boundary layer depths across the Southern Ocea

Modification of inertial oscillations by the mesoscale eddy field

The modification of near-surface near-inertial oscillations (NIOs) by the geostrophic vorticity is studied globally from an observational standpoint. Surface drifter are used to estimate NIO characteristics. Despite its spatial resolution limits, altimetry is used to estimate the geostrophic vorticity. Three characteristics of NIOs are considered: the relative frequency shift with respect to the local inertial frequency; the near-inertial variance; and the inverse excess bandwidth, which is interpreted as a decay time scale. The geostrophic mesoscale flow shifts the frequency of NIOs by approximately half its vorticity. Equatorward of 30°N and S, this effect is added to a global pattern of blue shift of NIOs. While the global pattern of near-inertial variance is interpretable in terms of wind forcing, it is also observed that the geostrophic vorticity organizes the near-inertial variance; it is maximum for near zero values of the Laplacian of the vorticity and decreases for nonzero values, albeit not as much for positive as for negative values. Because the Laplacian of vorticity and vorticity are anticorrelated in the altimeter data set, overall, more near-inertial variance is found in anticyclonic vorticity regions than in cyclonic regions. While this is compatible with anticyclones trapping NIOs, the organization of near-inertial variance by the Laplacian of vorticity is also in very good agreement with previous theoretical and numerical predictions. The inverse bandwidth is a decreasing function of the gradient of vorticity, which acts like the gradient of planetary vorticity to increase the decay of NIOs from the ocean surface. Because the altimetry data set captures the largest vorticity gradients in energetic mesoscale regions, it is also observed that NIOs decay faster in large geostrophic eddy kinetic energy region