Tamay M. Özgökmen

Improving Oceanic Overflow Representation in Climate Models: The Gravity Current Entrainment Climate Process Team

Abstract Oceanic overflows are bottom-trapped density currents originating in semienclosed basins, such as the Nordic seas, or on continental shelves, such as the Antarctic shelf. Overflows are the source of most of the abyssal waters, and therefore play an important role in the large-scale ocean circulation, forming a component of the sinking branch of the thermohaline circulation. As they descend the continental slope, overflows mix vigorously with the surrounding oceanic waters, changing their density and transport significantly. These mixing processes occur on spatial scales well below the resolution of ocean climate models, with the result that deep waters and deep western boundary currents are simulated poorly. The Gravity Current Entrainment Climate Process Team was established by the U.S. Climate Variability and Prediction (CLIVAR) Program to accelerate the development and implementation of improved representations of overflows within large-scale climate models, bringing together climate model dev...

Assimilation of drifter observations for the reconstruction of the Eulerian circulation field

[1] In light of the increasing number of drifting buoys in the ocean and recent advances in the realism of ocean general circulation models toward oceanic forecasting, the problem of assimilation of Lagrangian observations data in Eulerian models is investigated. A new and general rigorous approach is developed based on optimal interpolation (OI) methods, which takes into account directly the Lagrangian nature of the observations. An idealized version of this general formulation is tested in the framework of identical twin experiments using a reduced gravity, quasi-geostrophic model. An extensive study is conducted to quantify the effectiveness of Lagrangian data assimilation as a function of the number of drifters, the frequency of assimilation, and the uncertainties associated with the forcing functions driving the ocean model. The performance of the Lagrangian assimilation technique is also compared to that of conventional methods of assimilating drifters as moving current meters, and assimilation of Eulerian data, such as fixed-point velocities. Overall, the results are very favorable for the assimilation of Lagrangian observations to improve the Eulerian velocity field in ocean models. The results of our assimilation twin experiments imply an optimal sampling frequency for oceanic Lagrangian instruments in the range of 20–50% of the Lagrangian integral timescale of the flow field. INDEX TERMS: 4255 Oceanography: General: Numerical modeling; 4263 Oceanography: General: Ocean prediction; 3337 Meteorology and Atmospheric Dynamics: Numerical modeling and data assimilation; KEYWORDS: Lagrangian assimilation, drifters, Lagrangian velocity, Eulerian velocity, numerical model

Lagrangian analysis and prediction of coastal and ocean dynamics

Preface 1. Evolution of Lagrangian methods in oceanography T. Rossby 2. Measuring surface currents with Surface Velocity Program drifters: the instrument, its data, and some recent results R. Lumpkin and M. Pazos 3. Favourite trajectories A. S. Bower, H. Furey, S. Grodsky, J. Carton, L. R. Centurioni, P. P. Niiler, Y. Kim, D.-K. Lee, V.A. Sheremet, N. Garfield, C. A. Collins, T. A. Rago, R. Paquette, V. Kourafalou, E. Williams, T. Lee, M. Lankhorst, W. Zenk, A. J. Mariano, E. H. Ryan, P.-N. Poulain, H. Valdimarsson and S.-A. Malmberg 4. Particle motion in a sea of eddies C. Pasquero, A. Bracco, A. Provenzale and J. B. Weiss 5. Inertial particle dynamics on the rotating Earth N. Paldor 6. Predictability of Lagrangian motion in the upper ocean L. I. Piterbarg, T. M. Ozgokmen, A. Griffa and A. J. Mariano 7. Lagrangian data assimilation in ocean general circulation models A. Molcard, T. M. Ozgokmen, A. Griffa, L. I. Piterbarg and T. M. Chin 8. Dynamic consistency and Lagrangian data in oceanography: mapping, assimilation and optimization schemes T. M. Chin, K. Ide, C. K. R. T. Jones, L. Kuznetsov and A. J. Mariano 9. Observing turbulence regimes and Lagrangian dispersal properties in the ocean V. Rupolo 10. Lagrangian biophysical dynamics D. B. Olson 11. Plankton: Lagrangian inhabitants of the sea G. L. Hitchcock and R. K. Cowen 12. A Lagrangian stochastic model for the dynamics of a stage structured population. Application to a copepod population G. Buffoni, M. G. Mazzocchi and S. Pasquali 13. Lagrangian analysis and prediction of coastal and ocean dynamics (LAPCOD) A. J. Mariano and E. H. Ryan.

On the Connection between the Mediterranean Outflow and the Azores Current

As the salty and dense Mediteranean overflow exits the Strait of Gibraltar and descends rapidly in the Gulf of Cadiz, it entrains the fresher overlying subtropical Atlantic Water. A minimal model is put forth in this study to show that the entrainment process associated with the Mediterranean outflow in the Gulf of Cadiz can impact the upper-ocean circulation in the subtropical North Atlantic Ocean and can be a fundamental factor in the establishment of the Azores Current. Two key simplifications are applied in the interest of producing an economical model that captures the dominant effects. The first is to recognize that in a vertically asymmetric twolayer system, a relatively shallow upper layer can be dynamically approximated as a single-layer reduced-gravity controlled barotropic system, and the second is to apply quasigeostrophic dynamics such that the volume flux divergence effect associated with the entrainment is represented as a source of potential vorticity. Two sets of computations are presented within the 1‰-layer framework. A primitive-equation-based computation, which includes the divergent flow effects, is first compared with the equivalent quasigeostrophic formulation. The upper-ocean cyclonic eddy generated by the loss of mass over a localized area elongates westward under the influence of the b effect until the flow encounters the western boundary. In the steady state, the circulation pattern consists of bidirectional zonal flows with a limited meridional extent: eastward to the south of the sink and westward to the north of the sink. The localized sink drives a horizontal circulation in the interior ocean whose strength is approximately an order of magnitude greater than the sink’s strength. It is demonstrated that the induced circulation in the far field from a localized sink is insensitive to the neglect of the divergent flow component. A set of parameter sensitivity experiments is then undertaken with the quasigeostrophic model for an idealized midlatitude circulation, driven both by wind forcing and ‘‘thermohaline’’ flow through the open southern and northern boundaries. When a sink near the eastern boundary is superimposed on the idealized midlatitude circulation, it is shown to alter significantly the upper-ocean flow and induce an eastward zonal current, which resembles the Azores Current in location and transport. This mechanism also generates a westward current to the north of the sink location, which could be associated with the Azores Countercurrent. An extensive series of sensitivity experiments is conducted to determine the response of this current system to changes in the boundary layer processes, sink strength, sink distribution, model resolution, and wind forcing. The magnitude of the current transports is found to be sensitive to the sink intensity and to its distance from the coastline.

On the Predictability of Lagrangian Trajectories in the Ocean

The predictability of particle trajectories in oceanic flows is investigated in the context of a primitive equation, idealized, double-gyre ocean model. This study is motivated not only by the fact that this is an important conceptual problem but also by practical applications, such as searching for objects lost at sea, and ecological problems, such as the spreading of pollutants or fish larvae. The original aspect of this study is the use of Lagrangian drifter data to improve the accuracy of predicted trajectories. The prediction is performed by assimilating velocity data from the surrounding drifters into a Gauss‐Markov model for particle motion. The assimilation is carried out using a simplified Kalman filter. The performance of the prediction scheme is quantified as a function of a number of factors: 1) dynamically different flow regimes, such as interior gyre, western boundary current, and midlatitude jet regions; 2) density of drifter data used in assimilation; and 3) uncertainties in the knowledge of the mean flow field and the initial conditions. The data density is quantified by the number of data per degrees of freedom NR, defined as the number of drifters within the typical Eulerian space scale from the prediction particle. The simulations indicate that the actual World Ocean Circulation Experiment sampling (1 particle/[5 83 58 ]o rNR K 1) does not improve particle prediction, but predictions improve significantly when NR k 1. For instance, a coverage of 1 particle/ [1 83 18 ]o rNR ; O(1) is already able to reduce the errors of about one-third or one-half. If the sampling resolution is increased to 1 particle/[0.5 83 0.58] or 1 particle/[0.25 83 0.258 ]o rNR k 1, reasonably accurate predictions (rms errors of less than 50 km) can be obtained for periods ranging from one week (western boundary current and midlatitude jet regions) to three months (interior gyre region). Even when the mean flow field and initial turbulent velocities are not known accurately, the information derived from the surrounding drifter data is shown to compensate when NR . 1. Theoretical error estimates are derived that are based on the main statistical parameters of the flow field. Theoretical formulas show good agreement with the numerical results, and hence, they may serve as useful a priori estimates of Lagrangian prediction error for practical applications.

Seasonality of the submesoscale dynamics in the Gulf Stream region

Frontogenesis and frontal instabilities in the mixed layer are known to be important processes in the formation of submesoscale features. We study the seasonality of such processes in the Gulf Stream (GS) region. To approach this problem, a realistic simulation with the Hybrid Coordinate Ocean Model is integrated for 18 months at two horizontal resolutions: a high-resolution (1/48°) simulation able to resolve part of the submesoscale regime and the full range of mesoscale dynamics, and a coarser resolution (1/12°) case, in which submesoscales are not resolved. Results provide an insight into submesoscale dynamics in the complex GS region. A clear seasonal cycle is observed, with submesoscale features mostly present during winter. The submesoscale field is quantitatively characterized in terms of deviation from geostrophy and 2D dynamics. The limiting and controlling factor in the occurrence of submesoscales appears to be the depth of the mixed layer, which controls the reservoir of available potential energy available at the mesoscale fronts that are present most of the year. Atmospheric forcings are the main energy source behind submesoscale formation, but mostly indirectly through mixed layer deepening. The mixed layer instability scaling suggested in the (Fox-Kemper et al., J Phys Oceanogr 38:1145–1165, 2008) parametrization appears to hold, indicating that the parametrization is appropriate even in this complex and mesoscale dominated area.

Three-Dimensional Turbulent Bottom Density Currents from a High-Order Nonhydrostatic Spectral Element Model

Overflows are bottom gravity currents that supply dense water masses generated in high-latitude and marginal seas into the general circulation. Oceanic observations have revealed that mixing of overflows with ambient water masses takes place over small spatial and time scales. Studies with ocean general circulation models indicate that the strength of the thermohaline circulation is strongly sensitive to representation of overflows in these models. In light of these results, overflow-induced mixing emerges as one of the prominent oceanic processes. In this study, as a continuation of an effort to develop appropriate process models for overflows, nonhydrostatic 3D simulations of bottom gravity are carried out that would complement analysis of dedicated observations and large-scale ocean modeling. A parallel high-order spectral-element Navier‐Stokes solver is used as the basis of the simulations. Numerical experiments are conducted in an idealized setting focusing on the startup phase of a dense water mass released at the top of a sloping wedge. Results from 3D experiments are compared with results from 2D experiments and laboratory experiments, based on propagation speed of the density front, growth rate of the characteristic head at the leading edge, turbulent overturning length scales, and entrainment parameters. Results from 3D experiments are found to be in general agreement with those from laboratory tank experiments. In 2D simulations, the propagation speed is approximately 20% slower than that of the 3D experiments and the head growth rate is 3 times as large, Thorpe scales are 1.3‐1.5 times as large, and the entrainment parameter is up to 2 times as large as those in the 3D experiments. The differences between 2D and 3D simulations are entirely due to internal factors associated with the truncation of the Navier‐Stokes equations for 2D approximation.

Impact of Wind Forcing, Bottom Topography, and Inertia on Midlatitude Jet Separation in a Quasigeostrophic Model

Abstract The persistence of unrealistic Gulf Stream separation in numerical models of the ocean has prompted many theories about possible mechanisms that influence the separation of a western boundary current from the coast. In this paper, the joint effects of (a) coastline orientation, (b) bottom topography, and (c) inertia on the midlatitude jet separation are explored in a wind-driven two-layer quasigeostrophic model. It is shown that topographic effects are of importance in high eddy activity regions and that eddy–topography interactions strongly influence the separation process. In order for the western boundary current to separate from the coastline and cross the f/h contours associated with the continental rise, eddy fluctuations need to be weak at the separation point. This can be achieved either by introducing a positive wind stress curl in the northern part of the domain or by increasing the inertia of the western boundary current. In both cases, the separation is facilitated by low eddy activit...

Predictability of Lagrangian particle trajectories: Effects of smoothing of the underlying Eulerian flow

The increasing realism of ocean circulation models is leading to an increasing use of Eulerian models as a basis to compute transport properties and to predict the fate of Lagrangian quantities. There exists, however, a signie cant gap between the spatial scales of model resolution and that of forces acting on Lagrangian particles. These scales may contain high vorticity coherent structures that are not resolved due to computational issues and/or missing dynamics and are typically suppressed by smoothing operators. In this study, the impact of smoothing of the Eulerian e elds on the predictability of Lagrangian particles is e rst investigated by conducting twin experiments that involve release of clusters of synthetic Lagrangian particles into “ true” (unmodie ed) and “ model” (smoothed) Eulerian e elds, which are generated by a QG model with a e ow e eld consisting of many turbulent coherent structures. The Lagrangian errors induced by Eulerian smoothing errors are quantie ed by using two metrics, the difference between the centers of mass (CM) of particle clusters, r, and the difference between scattering of particles around the center of mass, s. The results show that the smoothing has a strong effect on the CM behavior, while the scatter around it is only partially affected. The QG results are then compared to results obtained from a multi-particle Lagrangian Stochastic Model (LSM) which parameterizes turbulent e ow using main e ow characteristics such as mean e ow, velocity variance and Lagrangian time scale. In addition to numerical results, theoretical results based on the LSM are also considered, providing asymptotics of r, s and predictability time. It is shown that both numerical and theoretical LSM results for the center of mass error ( r) provide a good qualitative description, and a quantitatively satisfactory estimate of results from QG experiments. The scatter error ( s) results, on the other hand, are only qualitatively reproduced by the LSM.

Lagrangian Analysis and Predictability of Coastal and Ocean Dynamics 2000

Abstract The first Lagrangian Analysis and Predictability of Coastal and Ocean Dynamics (LAPCOD) meeting took place in Ischia, Italy, 2–6 October 2000. The material presented at LAPCOD 2000 indicated both a maturing of Lagrangian-based observing systems and the development of new analysis and assimilation techniques for Lagrangian data. This summary presents a review of the state-of-the-art technology in Lagrangian exploration of oceanic and coastal waters that was presented at the meeting.