Mesoscale Eddy Field Research Articles

Establishment of momentum balance by form stress in a wind-driven channel

We examine the establishment of form stress in the spinup of a rotating isopycnal wind-driven channel model, with reference to the Antarctic Circumpolar Current. Initially, the force balance resembles Ekman layer transport, where zonal surface winds are balanced by the Coriolis forces in the upper layers, while the bottom layers consist of an opposing meridional flow balanced by topographic form stress. As the meridional transport increases, the isopycnal slopes and zonal transport also increase and the flow becomes baroclinically unstable, leading to the development of interfacial form stresses due to small-scale baroclinic eddies. This form stress alters the force balance of the layers, causing the system to geostrophically readjust to new states with progressively lower meridional transports. This trend continues until the meridional transport stops and the current is balanced solely by winds and form stress. The final state is a turbulent flow consisting of a several meandering jets that are maintained by the mesoscale eddy field. Our results resemble recent observations of the Antarctic Circumpolar Current and illustrate the contribution of mesoscale eddies to the force balance of the Southern Ocean.

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 regions.

Mesoscale Eddy–Internal Wave Coupling. Part II: Energetics and Results from PolyMode

Abstract The issue of internal wave–mesoscale eddy interactions is revisited. Previous observational work identified the mesoscale eddy field as a possible source of internal wave energy. Characterization of the coupling as a viscous process provides a smaller horizontal transfer coefficient than previously obtained, with vh ≅ 50 m2 s−1 in contrast to νh ≅ 200–400 m2 s−1, and a vertical transfer coefficient bounded away from zero, with νυ + ( f 2/N 2)Kh ≅ 2.5 ± 0.3 × 10−3 m2 s−1 in contrast to νυ + ( f 2/N 2)Kh = 0 ± 2 × 10−2 m2 s−1. Current meter data from the Local Dynamics Experiment of the PolyMode field program indicate mesoscale eddy–internal wave coupling through horizontal interactions (i) is a significant sink of eddy energy and (ii) plays an O(1) role in the energy budget of the internal wave field.

Interannual Variability of the North Pacific Subtropical Countercurrent and Its Associated Mesoscale Eddy Field

Abstract Interannual changes in the mesoscale eddy field along the Subtropical Countercurrent (STCC) band of 18°–25°N in the western North Pacific Ocean are investigated with 16 yr of satellite altimeter data. Enhanced eddy activities were observed in 1996–98 and 2003–08, whereas the eddy activities were below average in 1993–95 and 1999–2002. Analysis of repeat hydrographic data along 137°E reveals that the vertical shear between the surface eastward-flowing STCC and the subsurface westward-flowing North Equatorial Current (NEC) was larger in the eddy-rich years than in the eddy-weak years. By adopting a 2½-layer reduced-gravity model, it is shown that the increased eddy kinetic energy level in 1996–98 and 2003–08 is because of enhanced baroclinic instability resulting from the larger vertical shear in the STCC–NEC’s background flow. The cause for the STCC–NEC’s interannually varying vertical shear can be sought in the forcing by surface Ekman temperature gradient convergence within the STCC band. Rather than El Niño–Southern Oscillation signals as previously hypothesized, interannual changes in this Ekman forcing field, and hence the STCC–NEC’s vertical shear, are more related to the negative western Pacific index signals.

New production stimulated by high‐frequency winds in a turbulent mesoscale eddy field

Using an idealized model of an oligotrophic open‐ocean region characterized by intense sub‐mesoscale turbulence, we show that the presence of energetic near‐inertial motions, forced by high‐frequency winds, triggers transient nutrient inputs in the surface mixed‐layer, stimulating new production. We also show that this production increase is larger than the increase due to the Ekman transport resulting from a slow‐evolving wind forcing. The nutrient supplies are due to the interaction between near‐inertial motions and the sub‐mesoscale frontogenetic dynamics that reinforces both the vertical advection and vertical diffusion, especially within sub‐mesoscales features. The net result is an uplift of new production from the subsurface to the mixed‐layer. A direct consequence is that the sub‐mesoscale filamentary patterns of phytoplankton should become much more observable from space in the presence of high‐frequency winds.

The Role of Eddies in the Southern Ocean Temperature Response to the Southern Annular Mode

Abstract The role of eddies in modulating the Southern Ocean response to the southern annular mode (SAM) is examined, using an ocean model run at multiple resolutions from coarse to eddy resolving. The high-resolution versions of the model show an increase in eddy kinetic energy that peaks 2–3 yr after a positive anomaly in the SAM index. Previous work has shown that the instantaneous temperature response to the SAM is characterized by predominant cooling south of 45°S and warming to the north. At all resolutions the model captures this temperature response. This response is also evident in the coarse-resolution implementation of the model with no eddy mixing parameterization, showing that eddies do not play an important role in the instantaneous response. On the longer time scales, an intensification of the mesoscale eddy field occurs, which causes enhanced poleward heat flux and drives warming south of the oceanic Polar Front. This warming is of greater magnitude and occurs for a longer period than the initial cooling response. The results demonstrate that this warming is surface intensified and strongest in the mixed layer. Non-eddy-resolving models are unable to capture the delayed eddy-driven temperature response to the SAM. The authors therefore question the ability of coarse-resolution models, such as those commonly used in climate simulations, to accurately represent the full impacts of the SAM on the Southern Ocean.

Low‐frequency variability in the Gulf of Alaska from coarse and eddy‐permitting ocean models

An eddy‐permitting ocean model of the northeast Pacific is used to examine the ocean adjustment to changing wind forcing in the Gulf of Alaska (GOA) at interannual‐to‐decadal timescales. It is found that the adjustment of the ocean model in the presence of mesoscale eddies is similar to that obtained with coarse‐resolution models. Local Ekman pumping plays a key role in forcing pycnocline depth variability and, to a lesser degree, sea surface height (SSH) variability in the center of the Alaska gyre and in some areas of the eastern and northern GOA. Westward Rossby wave propagation is evident in the SSH field along some latitudes but is less noticeable in the pycnocline depth field. Differences between SSH and pycnocline depth are also found when considering their relationship with the local forcing and leading modes of climate variability in the northeast Pacific. In the central GOA pycnocline depth variations are more clearly related to changes in the local Ekman pumping than SSH. While SSH is marginally correlated with both Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO) indices, the pycnocline depth evolution is primarily related to NPGO variability. The intensity of the mesoscale eddy field increases with increasing circulation strength. The eddy field is generally more energetic after the 1976–1977 climate regime shift, when the gyre circulation intensified. In the western basin, where eddies primarily originate from intrinsic instabilities of the flow, variations in eddy kinetic energy are statistically significant correlated with the PDO index, indicating that eddy statistics may be inferred, to some degree, from the characteristics of the large‐scale flow.

Observing Oceanic Submesoscale Processes From Space

Observations made by satellite altimeters since the 1980s have provided progressively improved views of the global ocean mesoscale eddy field, which contains most of the kinetic energy of the ocean circulation. Along with these improved views, ocean models have progressed from coarseresolution, highly dissipative mesh grids to higher resolutions where mesoscale eddies dominate the model simulations. We are now able to produce simulations of the present state of the ocean that compare increasingly well with observations. However, the skill of these models in making long‐range predictions of the ocean is still very limited, because the models lack a physically based representation of the submesoscales, i.e., scales of 1100 kilometers, that are important for turbulent transport and energy dissipation. Ocean models running at sufficient resolutions to address submesoscale dynamics have just recently begun to emerge [e.g., Capet et al., 2008], but we need global observations at these scales to guide the model development.

The mesoscale variability in the Caribbean Sea. Part II: Energy sources

The processes which drive the production and the growth of the strong mesoscale eddy field in the Caribbean Sea are examined using a general circulation model. Diagnostics of the simulations suggest that: (1) The mean currents in the Caribbean Sea are intrinsically unstable. The nature of the instability and its strength vary spatially due to strong differences of current structure among basins. (2) The greatest and most energetic eddies of the Caribbean Sea originate in the Venezuela Basin by mixed barotropic-baroclinic instability of an intense jet, formed with waters mostly from the surface return flow of the Meridional Overturning Circulation and the North Equatorial Current which converge and accelerate through the Grenada Passage. The vertical shear of this inflow is enhanced by an eastward undercurrent, which flows along the south American Coast between 100 and 250 m depth. The shallow eddies (less than 200 m depth) formed in the vicinity of the Grenada Passage get rapidly deeper (down to 1000 m depth) and stronger by their interaction with the deep interior flow of the Subtropical Gyre, which enters through passages north of St. Lucia. These main eastern Caribbean inflows merge and form the southern Caribbean Current, whose baroclinic instability is responsible for the westward growth and strengthening of these eddies from the Venezuela to the Colombia Basin. (3) Eddies of lesser strength are produced in other regions of the Caribbean Sea. Their generation and growth is also linked with instability of the local currents. First, cyclones are formed in the cyclonic shear of the northern Caribbean Current, but appear to be rapidly dissipated or absorbed by the large anticyclones coming from the southern Caribbean. Second, eddies in the Cayman Sea, which impact the Yucatan region, are locally produced and enhanced by barotropic instability of the deep Cayman Current. (4) The role of the North Brazil Current (NBC) rings is mostly to act as a finite perturbation for the instability of the mean flow. Their presence near the Lesser Antilles is ubiquitous and they appear to be linked with most of the Caribbean eddies. There are some evidences that the frequency at which they form near the Grenada Passage is influenced by the frequency at which the NBC rings impinge the Lesser Antilles. But large Caribbean eddies also form without a close influence of any ring, and comparison between simulations shows that mean eddy kinetic energy and eddy population in the Caribbean Sea are not substantially different in absence or presence of NBC rings: their presence is not a necessary condition for the generation and growth of the Caribbean eddies.

Propagation of Wind Energy into the Deep Ocean through a Fully Turbulent Mesoscale Eddy Field

Abstract The authors analyze the 3D propagation of wind-forced near-inertial motions in a fully turbulent mesoscale eddy field with a primitive equation numerical model. Although the wind stress is uniform, the near-inertial motion field quickly becomes spatially heterogeneous, involving horizontal scales much smaller than the eddy scales. Analysis confirms that refraction by the eddy relative vorticity is the main mechanism responsible for the horizontal distortion of the near-inertial motions, which subsequently triggers their vertical propagation. An important result is the appearance of two maxima of near-inertial vertical velocity (both with rms values reaching 40 m day−1): one at a depth of 100 m and another unexpected one much below the main thermocline around 1700 m. The shallow maximum, captured by the highest vertical normal modes, involves near-inertial motions with a spatial heterogeneity close to the eddy vorticity gradient field. These characteristics match analytical results obtained with Young and Ben Jelloul’s approach. The deep maximum, captured by the lowest vertical normal modes, involves superinertial motions with a frequency of twice the inertial frequency and much smaller horizontal scales. Because of these characteristics, not anticipated by previous analytical studies, these superinertial motions may represent an energy source for small-scale mixing through a mechanism not taken into account in the present study: the parametric subharmonic instability (PSI). This reveals a pathway by which wind energy may have a significant impact on small-scale mixing in the deep interior. Further studies that explicitly take into account PSI are needed to estimate this potential impact.

A Resonance Mechanism Leading to Wind-Forced Motions with a 2f Frequency

Abstract This study revisits the mechanisms that spatially reorganize wind-forced inertial motions embedded in an oceanic mesoscale eddy field. Inertial motions are known to be affected by the eddy relative vorticity, being expelled from cyclonic structures and trapped within anticyclonic ones. Using shallow water equations (involving a single baroclinic mode), the authors show that nonlinear wave–wave interactions excite, through a resonance mechanism, motions with a 2f frequency (with f being the inertial frequency) and a specific length scale. In a more general situation involving several baroclinic modes, this resonance is found to be reinforced and principally efficient for the lowest baroclinic modes. Such characteristics make the energy of the wind-forced motions potentially available to small-scale mixing through parametric subharmonic instability (not taken into account in the present study).

Diapycnal mixing by meso-scale eddies

The mean available potential energy released by baroclinic instability into the meso-scale eddy field has to be dissipated in some way and Tandon and Garrett [Tandon, A., Garrett, C., 1996. On a recent parameterization of mesoscale eddies. J. Phys. Oceanogr. 26 (3), 406–416] suggested that this dissipation could ultimately involve irreversible mixing of buoyancy by molecular processes at the small-scale end of the turbulence cascade. We revisit this idea and argue that the presence of dissipation within the thermocline automatically requires that a component of the eddy flux associated with meso-scale eddies must be associated with irreversible mixing of buoyancy within the thermocline. We offer a parameterisation of the implied diapycnal diffusivity based on (i) the dissipation rate for eddy kinetic energy given by the meso-scale eddy closure of Eden and Greatbatch [Eden, C., Greatbatch, R.J., 2008. Towards a meso-scale eddy closure. Ocean Modell. 20, 223–239.] and (ii) a fixed mixing efficiency. The implied eddy-induced diapycnal diffusivity ( κ) is implemented in a coarse resolution model of the North Atlantic. In contrast to the vertical diffusivity given by a standard vertical mixing scheme, large lateral inhomogeneities can be found for κ in the interior of the ocean. In general, κ is large, i.e. up to o(10) cm 2/s, near the western boundaries and almost vanishing in the interior of the ocean.

On Quadratic Bottom Drag, Geostrophic Turbulence, and Oceanic Mesoscale Eddies

Abstract Many investigators have idealized the oceanic mesoscale eddy field with numerical simulations of geostrophic turbulence forced by a horizontally homogeneous, baroclinically unstable mean flow. To date such studies have employed linear bottom Ekman friction (hereinafter, linear drag). This paper presents simulations of two-layer baroclinically unstable geostrophic turbulence damped by quadratic bottom drag, which is generally thought to be more realistic. The goals of the paper are 1) to describe the behavior of quadratically damped turbulence as drag strength changes, using previously reported behaviors of linearly damped turbulence as a point of comparison, and 2) to compare the eddy energies, baroclinicities, and horizontal scales in both quadratic and linear drag simulations with observations and to discuss the constraints these comparisons place on the form and strength of bottom drag in the ocean. In both quadratic and linear drag simulations, large barotropic eddies develop with weak damping, large equivalent barotropic eddies develop with strong damping, and the comparison in goal 2 above is closest when the nondimensional friction strength parameter is of order 1. Typical values of the quadratic drag coefficient (cd ∼ 0.0025) and of boundary layer depths (Hb ∼ 50 m) imply that the quadratic friction strength parameter cdLd/Hb, where Ld is the deformation radius, may indeed be of order 1 in the ocean. Model eddies are realistic over a wider range of friction strengths when drag is quadratic, because of a reduced sensitivity to friction strength in that case. The quadratic parameter is independent of the mean shear, in contrast to the linear parameter. Plots of eddy length scales, computed from satellite altimeter data, versus mean shear and versus rough estimates of the friction strength parameters suggest that both linear and quadratic bottom drag may be active in the ocean. Topographic wave drag contains terms that are linear in the bottom flow, thus providing some justification for the use of linear bottom drag in models.

A numerical study of the eddying characteristics of the Leeuwin Current System

The Regional Ocean Modelling System (ROMS v2.1) was used to investigate the physical processes that sustained the mesoscale eddy fields. The model was able to mimic the spatial, temporal, and migratory scales of the Leeuwin Current (LC) System's mean flow and eddying characteristics. Importantly, eddy generation sites were consistent with those reported previously observed from satellite imagery, and two main regions were identified: site 1, adjacent to the Abrolhos Islands’ western edge (latitude 29°S); and site 2, adjacent to the shelf break (latitude 27.8°S). The description of the eddying dynamics suggested the eddy field evolution followed a similar pathway at both sites. An offshore flow impinged on the continental shelf and was redirected southward, intensifying the LC's southward alongshore flow on the shelf. This southward redirection formed a current loop, which closed and formed a distinct clockwise eddy. The eddy remained near stationary and attached to the bottom topography, but it redirected LC and continental shelf waters offshore to dominate the local dynamics. The resulting offshore meander grew laterally, shallowed, and closed to form an anticlockwise eddy to the original clockwise eddy's south, forming a characteristic LC eddy pair (dipole). The model demonstrated that the LC and Leeuwin Undercurrent coupling played an important role in the onset of eddies at both sites. An energy diagnostic scheme was used, and the dominant instability process linked to the developing anticlockwise eddy at site 1 was mixed mode barotropic and baroclinic instability. The baroclinic instability source was the available potential energy stored within the mean lateral density gradient between the eddy dipole. The LC's meandering southward flow interacting with the undercurrent's northward subsurface flow generated horizontal shear, which was the source of barotropic instability. The dominant instability process at site 2 was baroclinic in origin. At both sites, the baroclinic mechanism followed a series of phases coinciding with the physical description of the evolving eddies.

The physical oceanographic conditions along the Mid-Atlantic Ridge north of the Azores in June–July 2004

Abstract During the MAR-ECO expedition in June and July 2004, 39 deep CTD stations were occupied with uneven spacing along the Mid-Atlantic Ridge (MAR) between 41°N and 61°N. The CTD stations coincided with the biological sampling stations and thus gave a detailed vertical description of the water properties at these locations. Many different water masses were identified. However, using the predominant water mass in the upper 500 m, four different hydrographic regions were identified. Stations in the vicinity of the Reykjanes Ridge north of 57°N were dominated by Modified North Atlantic Water (MNAW). Stations south of 56°30′N and north of the Sub-Polar Front (SPF) were dominated by Sub-Arctic Intermediate Water (SAIW). CTD data combined with along-track continuous surface temperature and salinity measurements, satellite sea-level anomaly data and sea-surface temperature (SST) and along-track current data (ADCP data) showed that the position of the SPF was at about 52°N, at the southern edge of the Charlie–Gibbs Fracture Zone (CGFZ). South of the SPF a Frontal Region with both North Atlantic Central Water (NACW) and SAIW present was observed, and south of about 50°N the predominant water mass in the upper 500 m was NACW. A ship-mounted RDI 75 kHz ADCP was operated during the cruise and yielded high quality data during the first leg. The currents showed large variations over short distances and were strongly affected by topography. The ADCP data was be used to describe the mesoscale eddy field along the MAR and to detect three jets in the North Atlantic Current (NAC). Data from a current meter mooring left for 10 months after the cruise at a seamount just to the south of CGFZ showed the dominance of the tidal currents in the area. Typical spring tidal currents were about 0.15 m s−1 and the mean flow an order of magnitude smaller, about 0.03 m s−1. A short-term mooring on a seamount farther south showed tidal currents of 0.3 m s−1. Binned 8-day, 4-km SST satellite imagery were collected and processed for 2004. The SST data were analysed, and showed characteristic frontal structures in the MAR-ECO region.

Aegean Surface Circulation from a Satellite-Tracked Drifter Array

Abstract A pilot experiment using an array of 45 drifters to explore the circulation in the north and central Aegean Sea is described. The global positioning system drifters with holey-sock drogues provide positions every hour with data recovery through the Argos system. The drifters were launched in four separate deployments over a 1-yr period. The resulting trajectories confirm the existence of a current around the rim of the basin consistent with a buoyancy plume created by the outflow of Black Sea waters through the Dardanelles (Strait of Çanakkale in Turkish). The degree to which this is augmented by an Ekman response to the dominant northerly winds is not obvious in the dataset owing to mesoscale dynamics that obscure the existence of any westward Ekman flow. The mesoscale eddy field involves anticylonic eddies in the current around the rim of the basin consistent with eddies with low-salinity-water cores. Cyclones are also seen, with the most prominent forming over deep regions in the basin topography. The array also documents the interaction of the currents with the straits through the Sporades and Cyclades island groups. These interactions are complicated by the nature of the mesoscale flow and in some trajectories suggest a Bernouilli acceleration in straits; in others the flow through the island groups appears to be more diffusive and involves deceleration and eddy motions. The rapid sampling by the drifters reveals an extremely nonlinear submesoscale eddy field in the basin with length scales less than 4 km and Rossby numbers of order 1. A better understanding of the dynamics of these features is of importance for understanding the circulation of the basin.

Spreading of near‐inertial energy in a 1/12° model of the North Atlantic Ocean

Near‐inertial energy in the ocean is thought to be redistributed by β‐dispersion, whereby near‐inertial waves generated at the surface by wind forcing propagate downward and equatorward. In this letter, we examine the spreading of near‐inertial energy in a realistic 1/12° model of the North Atlantic driven by synoptically varying wind forcing. We find that (1) near‐inertial energy is strongly influenced by the mesoscale eddy field and appears to be locally drained to the deep ocean, largely by the chimney effect associated with anticyclonic eddies, and (2) the interior of the subtropical gyre shows very low levels of near‐inertial energy, contrary to expectations based on the β‐dispersion effect.

Oceanography, primary production and dissolved inorganic nitrogen uptake in two Leeuwin Current eddies

Abstract The Leeuwin Current (LC) is an unusual poleward-flowing eastern boundary current that carries warm, low-salinity water southward along the coast of Western Australia. LC dynamics include the formation of a dynamic mesoscale eddy field whose biological dynamics have not been studied. Satellite altimetry indicates that the eddies studied in the 2003 field programme were dynamically typical of LC eddies, but the warm-core (WC) eddy was relatively large and long-lived. The WC eddy contained relatively elevated chlorophyll a concentrations thought to originate, at least in part, from the continental shelf/shelf break region and to have been incorporated during eddy formation. Primary production per unit volume in the WC eddy was ∼2× higher than in the cold-core (CC) eddy due to an historical accumulation of chlorophyll a over the period since eddy formation (5–6 months), though chlorophyll a-specific daily production was volumetrically ∼50% greater in the CC eddy. In the WC eddy, nitrate uptake rates were 4× greater than in the CC eddy, despite the fact that vertical diffusive fluxes of nitrate into the WC eddy were probably only 50% of those in the CC eddy. We therefore hypothesize that other nitrate sources were important, possibly including isopycnal mixing and/or lateral transport into the eddy from surrounding waters. In addition, a deep mixed layer favoured a large (>5 μm) diatom population within the centre of the WC eddy while the CC eddy was persistently stratified, with a shallower mean mixed-layer depth (∼100 m vs. ∼200 m for the WC eddy) and a well developed deep chlorophyll a maximum at ∼100 m composed of 30% prochlorophytes (not capable of taking up nitrate). Both factors probably contributed to higher f-ratios in the WC and in the >5 μm phytoplankton relative to total phytoplankton.

The Scripps Coupled Ocean–Atmosphere Regional (SCOAR) Model, with Applications in the Eastern Pacific Sector

AbstractA regional coupled ocean–atmosphere model is introduced. It is designed to admit the air–sea feedbacks arising in the presence of an oceanic mesoscale eddy field. It consists of the Regional Ocean Modeling System (ROMS) and the Regional Spectral Model (RSM). Large-scale forcing is provided by NCEP/DOE reanalysis fields, which have physics consistent with the RSM. Coupling allows the sea surface temperature (SST) to influence the stability of the atmospheric boundary layer and, hence, the surface wind stress and heat flux fields. The system is denominated the Scripps Coupled Ocean–Atmosphere Regional (SCOAR) Model.The model is tested in three scenarios in the eastern Pacific Ocean sector: tropical instability waves of the eastern tropical Pacific, mesoscale eddies and fronts of the California Current System, and gap winds of the Central American coast. Recent observational evidence suggests air–sea interactions involving the oceanic mesoscale in these three regions. Evolving SST fronts are shown to drive an unambiguous response of the atmospheric boundary layer in the coupled model. This results in significant model anomalies of wind stress curl, wind stress divergence, surface heat flux, and precipitation that resemble the observations and substantiate the importance of ocean–atmosphere feedbacks involving the oceanic mesoscale.

Observability of the Irminger Sea circulation using variational data assimilation

This paper explores the prospects for constraining circulation and stratification estimates in the Irminger Sea and Denmark Strait using variational data assimilation. A regional circulation model with 9 km horizontal grid spacing, seven vertical levels and open boundaries is used. The circulation it simulates is realistic in several important respects and includes a chaotic mesoscale eddy field. Pulsation of the outflow of dense water through the Denmark Strait is also reasonably realistic, although entrainment and mixing of this water downstream is crudely represented. Identical twin assimilation experiments are performed to address the observability of the system with the existing in situ and satellite observing network in the absence of systematic errors. That is, the constraints exerted on the circulation and stratification by the data assimilation system are quantified. Synthetic data come from a reasonably realistic observing network that includes in situ hydrographic and current measurements, and remotely sensed surface temperature and sea level. Over a 15-day assimilation period in summer, the system fits all the observations to the relevant noise level, largely by correcting the initial conditions. Only modest changes in the time-dependent surface forcing and negligible changes to the open boundary conditions are needed. The system can accurately estimate sea surface height and sea surface temperature, correcting errors in the locations of surface fronts and eddies. The mid-depth salinity is almost completely unconstrained, however, whereas the velocity fields are somewhat constrained by the assimilation, especially by the remote-sensing data. Assimilation of satellite data enables us to accurately track the phase and amplitude of the overflow variability at the exit of Denmark Strait in the identical twin experiments. At other model outflow transects the assimilated solution is contaminated with spurious inertial oscillations. This is true even at a section offshore of Angmagssalik where a current-meter mooring array exists (in our experiments and in the real ocean). Nevertheless, improvement of the model dense outflow near the moorings is significant and propagates several hundred kilometres upstream and downstream. Copyright © 2006 Royal Meteorological Society