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Nadia Pinardi and Allan R. Robinson

Abstract

We present the study of a series of very energetic events, which occur in the 360 days of the Polymode Synoptic Dynamics Experiment dataset. The method consists of the assimilation of data by a quasi-geostrophic open boundary model so that dynamically adjusted fields are produced. They are used to study local dynamical vorticity and energy balances during 20 to 30 day benchmark forecast experiments.

The data and forecasts show the presence of strong jets at the thermocline levels (100–1400 meters), which intensify via a process of baroclinic conversion of available gravitational energy into kinetic energy. The formation, together with strengthening, of these jets is explained in terms of amplifying short-scale baroclinic waves growing along the sheared borders of larger eddies. Associated with the local steepening or the frontal areas, there is a cyclone development processes that results from the growth of these short baroclinic waves. Thus. the area at the southern boundary of the Gulf Stream recirculation gyre is found to be a region of in situ baroclinic energy conversions: the time scale of the process is 30 days and the short length-scale waves amplify in a region of the order of a hundred kilometers.

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Glenn R. Flierl and Allan R. Robinson

Abstract

A thin-jet model predicts the location of the axis of a strong current such as the Gulf Stream by using the vertical and cross-stream integrated vorticity balance, under the assumption that the meandering scales are large compared to the width of the jet. We demonstrate that such an integral provides a matching condition upon the barotropic component of the wave or eddy fields which, on either side of the jet, have north–south scales on the order of the meander wavelength. For steady meanders, these exterior fields do not influence the path and our model reproduces the dynamics of Robinson and Niiler, but for the transient case the determination of the jet axis motion and of the external field is a coupled problem.

When the disturbances in the axis position are time-dependent but are very small, the exterior wave problem can be linearized and the matching conditions can be applied at the mean position of the jet. We can therefore derive a dispersion relation for the meandering motion, allowing us to compute the phase speed and growth rates for the meanders in terms of the wavenumber and two integral properties of the stream: the mass and momentum transports. This dispersion relation predicts instability for wave shorter than a critical scale.

We also derive via standard four-dimensional instability theory a long wave approximation to the dispersion relation for perturbations of a quasi-geostrophic jet with both horizontal and vertical shears. The result is identical to that from the thin-jet theory for an interesting class of perturbations which we therefore identity as meandering modes. Thus thin-jet theory has been calibrated by reduction to both finite amplitude steady meandering and infinitesimal instability cases. For the understanding of large amplitude, time-dependent motions of the Gulf Stream and their role in the general circulation, the thin jet theory offers a semi-analytical approach for process studies.

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Glenn R. Flierl and Allan R. Robinson

Abstract

Estimates of the repeatability and accuracy of XBT measurements were made using XBT data taken during MODE. The XBT observations of the depth of isotherms had systematic errors of up to 15 decibars (by comparison to simultaneous CTD drops) as well as random errors on the order of 8 declare. Maps of these observations show small-scale thermal structure which would imply sizeable increases in geostrophic shears.

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Xiang San Liang and Allan R. Robinson

Abstract

The multiscale nonlinear interactive Iceland–Faeroe frontal (IFF) variability during 14–22 August 1993 is investigated for complex dynamics with the localized multiscale energy and vorticity analysis (MS-EVA). In terms of multiscale window transform, the cold meandering intrusion observed in the IFF experiment is represented on a mesoscale window. The resulting mesoscale energetics for the deep layer show an isolated center of transfer of potential energy from the large-scale window into the mesoscale window in the study domain. This large-to-mesoscale potential energy transfer, or BC for short, is a baroclinic instability indicator by the MS-EVA-based stability theory. Signatures on other energetics maps and the reconstructed mesoscale structures all support this baroclinic instability. On the BC map, the transfer hotspot originally resides near the western boundary. It travels along the front into the interior domain in a form of convective instability and then, on 19 August, changes into another instability that is absolute in character. Correspondingly, disturbances switch from a spatial growing pattern into a time growing mode, culminating on 21 August, the day just before the intrusion matures. The whole process lasts for about five days, limited within a small horizontal region and beneath a depth of approximately 150 m. By interaction analysis, the energy locally gained from this process goes to the submesoscale window as well, but most of it remains in the mesoscale window, serving to fuel the growth of the meandering intrusion.

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Artur Hecht, Nadia Pinardi, and Allan R. Robinson

Abstract

Hydrographic measurements in the southeastern Levantine basin are analyzed, and the climatological water masses of the region and their seasonal variations are identified. We observe the formation of the salty and warm Levantine Surface Water Layer (LSW); we characterize the subsurface Atlantic Water Layer (AW); and we describe the properties of the thermocline waters, called Levantine Intermediate Waters (LIW). The baroclinic dynamical modes are computed for the climatological stratification parameters. The empirical orthogonal function (EOF) analysis of the vertical shear profiles shows that considerable energy is contained in the second EOF at the thermocline and deep levels. Maps of the baroclinic streamfunction field referred to 700 meters are displayed: 16 instantaneous flow field realizations show an intense mesoscale eddy file never revealed before in the region. The space scales of the eddies are about 100 km and a smaller scale (60–70 km) variability is also evident. The eddies are present, e.g., stationary for over a season, and there are periods in which only a single eddy center is present embedded in an almost quiescent flow. The velocities in the strong jets at the border of the eddies are of the order of 20–40 cm −1 at the upper thermocline levels. The water mass analysis of this eddy field shows that the AW and LIW salinity properties are distributed in filaments and patches: the maximum salinity cores of LIW are trapped in the anticyclones found in the region. An event of salinity ventilation (down to 200 m) is described that seems to involve the homogenization of the salinity properties but not convective mixing of the density structure. The traditional picture of the basin currents is compared with the mesoscale flow analyzed here, and we speculate upon possible mechanisms of water mass transport.

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X. San Liang and Allan R. Robinson

Abstract

The nonlinear multiscale dynamics of the Monterey Bay circulation during the Second Autonomous Ocean Sampling Network (AOSN-II) Experiment (August 2003) is investigated in an attempt to understand the complex processes underlying the highly variable ocean environment of the California coastal region. Using a recently developed methodology, the localized multiscale energy and vorticity analysis (MS-EVA) and the MS-EVA-based finite-amplitude hydrodynamic instability theory, the processes are reconstructed on three mutually exclusive time subspaces: a large-scale window, a mesoscale window, and a submesoscale window. The ocean is found to be most energetic in the upper layers, and the temporal mesoscale structures are mainly trapped above 200 m. Through exploring the nonlinear window–window interactions, it is found that the dynamics underlying the complex surface circulation is characterized by a well-organized, self-sustained bimodal instability structure: a Bay mode and a Point Sur mode, which are located near Monterey Bay and west of Point Sur, respectively. Both modes are of mixed types, but they are distinctly different in dynamics. The former is established when the wind relaxes, while the latter is directly driven by the wind. Either way, the wind instills energy into the ocean, which is stored within the large-scale window and then released to fuel temporal mesoscale processes. Upon wind relaxation, the generated mesoscale structures propagate northward along the coastline, in a form with dispersion properties similar to that of a free thermocline-trapped coastal-trapped wave. Between these two modes, a secondary instability is identified in the surface layer during 15–21 August, transferring energy to the temporal submesoscale window. Also studied is the deep-layer flow, which is unstable all the time throughout the experiment within the Bay and north of the deep canyon. It is observed that the deep temporal mesoscale flow within the Bay may derive its energy from the submesoscale window as well as from the large-scale window. This study provides a real ocean example of how secondary upwelling can be driven by winds through nonlinear instability and how winds may excite the ocean via an avenue distinctly different from the classical paradigms.

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Ralph F. Milliff and Allan R. Robinson

Abstract

A coastal quasigeostrophic (QG) methodology is extended in an exploration of the mesoscale variability of the Rhodes Gyre region of the Eastern Mediterranean Sea, given multiscale data taken as part of the POEM program. Standard objective analysis procedures are modified to include a coastal constraint that prohibits geostrophic flow normal to the coastline and imposes a characteristic nearshore variability in the coastal portion of the domain. Objectively analyzed fields are dynamically adjusted in short QG model integrations and then interpreted for the structure and characteristics of the subbasin scale flow system and the mesoscale. Two-week simulations are then performed to explore the stability of the subbasin scale flow, the dynamical balances, and the variabilities and interactions of the mesoscale. A matrix of simulation experiments is described to test the sensitivities to two variations in initial conditions, and three parameterizations of the bottom topography. An energy and vorticity analysis (EVA) is used to study balance of terms and dynamical processes in the central simulation.

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Leonard J. Walstad and Allan R. Robinson

Abstract

A regional quasi-geostrophic model has been used to hindcast and forecast the POLYMODE data set. After briefly discussing hindcast methodology, the hindcast fields are compared with the analyzed data set Periods of significant difference of hindcast from analysis are identified and investigated. We find that these differences may be largely attributed to inaccuracies in the analysed fields. The inaccuracies are due to a lack of data. When the data set fails to adequately describe the ocean, the hindcast may be more accurate than the analyzed fields. Model studies also demonstrate that hindcast quality improves after being degraded by a period of poor boundary conditions, topographic forcing is relatively important in improving the accuracy of the hindcasts, and idealized numerical resolution studies are applicable to the assimilation of oceanic data sets.

Methods for forecasting are examined and intercompared for several periods during the POLYMODE experiment. Forecast accuracy is found to be highest when statistical techniques are used to forecast the boundary conditions and the interior evolves as determined by dynamics. Away from boundary condition induced errors, the dynamical model is able to maintain a high level of correlation between the forecast and analyzed fields for 20 days. Also, the accuracy may be affected by the position of the data relative to the forecast domain. The implications for sampling strategies an discussed.

Thew results are important to ocean scientists on several fronts. In studying mesoscale processes, a continuous time series of fields may be important for analysis of the kinematics and dynamics. When conducting a field measurement program, knowledge of evolving mesoscale fields may aid in the positioning of sensors. These topics are briefly discussed and future plans described.

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Michael A. Spall and Allan R. Robinson

Abstract

Primitive equation and quasi-geostrophic eddy resolving, open ocean models are used for hindcast studies in the Gulf Stream meander and ring formation region. A feature model approach is used to initialize the models, based on one month of observations during November to December 1984. Flat bottom and topographic calculations are carried out using an initial Gulf Stream velocity profile based on the Pegasus dataset. All of the major events observed in the upper thermocline are reproduced by both numerical models. The addition of bottom topography is shown to significantly alter the character of the deep velocity fields. Large, basin scale circulations found near the bottom in both flat bottom calculations were replaced by energetic jets and eddies associated with the dominant spatial scales of the bottom topography. Use of the quasi-geostrophic model to dynamically adjust the initial conditions for the primitive equation model is shown to reduce the growth of large scale meanders on time scales of one month. A local primitive equation energy and vorticity analysis (PRE-EVA) routine is used to determine the dominant processes of simulated warm and cold ring formation events. The warm ring formation is achieved by differential horizontal advection of a developed meander system. The cold ring formation involves geostrophic and ageostrophic horizontal advection, vertical advection, and baroclinic conversion. Ageostrophic horizontal and vertical advections and stronger baroclinic conversion are believed to be responsible for the more realistic structure of the rings produced by the primitive equation model.

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Pierre De Mey and Allan R. Robinson

Abstract

Techniques for the synoptic analysis, vertical inference, dynamical adjustment, and forecast of altimetric and deeper in situ data are presented as a first step towards the design of continuous assimilation schemes in limited-area oceanic domains. A year-long time series of streamfunction maps, denoted Mark 2, drawn in the POLYMODE area of the western North Atlantic is used as a benchmark for various tests and simulations. An original projection/extension scheme using empirical modes of density and/or pressure anomalies is used to obtain a first guess of the three-dimensional structure of the currents, starting from surface topography only. In the Mark 2 domain, this technique works well, since the first empirical mode is surface intensified and largely dominant. An alternative approach is to incorporate deeper data, e.g., float trajectories below the main thermocline. The first-guess currents are specified as initial and boundary condition of the Harvard 6-level quasi-geostrophic open ocean model. When surface data only are assimilated, the model makes the deep currents converge dramatically towards Mark 2. The fastest adjustment occurs in 9 days and involves nonlinear mode-mode interactions. The sensitivity of the assimilation scheme to different dynamical regimes, to bottom topography, and to the modal assumptions of the empirical extension is investigated. It is found the results are extremely robust. The presence of bottom topography further increases the rate of convergence of the deep levels. Finally, we use a simple orbital model to generate realistic altimeter track sequences. Mark 2 is sampled, and linear optimal estimation is applied to restore the surface topography. The deep adjustment still occurs. It is also found to be rather insensitive to the choice of sampling strategy and to the horizontal correlation scale.

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