Miami Isopycnic Coordinate Ocean Model Research Articles

The Vertical Structure of Large-Scale Unsteady Currents

AbstractA linear model based on the quasigeostrophic equations is constructed in order to predict the vertical structure of Rossby waves and, more broadly, of anomalies resolved by altimeter data, roughly with periods longer than 20 days and with wavelengths larger than 100 km. The subsurface field is reconstructed from sea surface height and climatological stratification. The solution is calculated in periodic rectangular regions with a 3D discrete Fourier transform. The effect of the mean flow on Rossby waves is neglected, which the authors believe is a reasonable approximation for low latitudes. The method used has been tested with an idealized double-gyre simulation [performed with the Miami Isopycnal Coordinate Ocean Model (MICOM)]. The linear model is able to give reasonable predictions of subsurface currents at low latitudes (below approximately 30°) and for relatively weak mean flow. However, the predictions degrade with stronger mean flows and higher latitudes. The subsurface velocities calculated with this model using AVISO altimetric data and velocities from current meters have also been compared. Results show that the model gives reasonably accurate results away from the top and bottom boundaries, side boundaries, and far from western boundary currents. This study found, for the regions where the model is valid, an energy partition of the traditional modes of approximately 68% in the barotropic mode and 25% in the first baroclinic mode. Only 20% of the observed kinetic energy can be attributed to free Rossby waves of long periods that propagate energy to the west.

North Atlantic ventilation using chlorofluorocarbons and idealised-tracer simulations

ABSTRACTThe simulated chlorofluorocarbon CFC-11 and 29 geographically defined CFC-11 tracers, as well as 29 geographically defined idealised tracers, are used to quantify the regional contribution to the ventilation of the North Atlantic Ocean in a global version of the Miami Isopycnal Coordinate Ocean Model (MICOM) driven by the daily NCEP/NCAR forcing. Age tracers attached to 29 idealised tracers are also used to estimate the timescales for the water masses’ transports. Our results show that the simulated overturning circulation matches the available observed data for both intensity and variability, and the simulated distribution of CFC-11 concentration in the subtropical North Atlantic Ocean is in good agreement with the observations, particularly above 800 m in depth. We found that the sandwich-like distribution of CFC-11 concentration in the subtropical North Atlantic in both the observations and simulations is mainly caused by subduction from the western and eastern subpolar North Atlantic, but the contribution of the former (56.0%) is almost four times larger than that of the latter (15.7%). We demonstrated that the ocean dynamics, instead of the source function, determine the annual and inter-annual variability in both dynamically active tracer (such as water temperature and salinity) and passive tracer (such as CFC-11 and idealised tracer) concentrations in the deep North Atlantic. The ‘apparent age’ distribution shows that the surface water in the western subpolar North Atlantic takes about 12 yr to reach the Nordic Seas and takes 20 yr from the Nordic Seas to the subtropical deep North Atlantic. The transit-time derived by ‘optimum time lag’ approach shows a 9.3–13.6 yr lag for the signals propagating from the western subpolar North Atlantic to the subtropical North Atlantic, which is generally consistent with that of about 10 yr derived from the ‘apparent age’. The study suggests that geographically defined tracers can be used as an efficient tool to investigate the source and spreading pathway of water, and to forecast the spreading and spreading time of environmental accidents such as the radioactive waste in the world's oceans.

Sources and pathways of 90Sr in the North Atlantic–Arctic region: present day and global warming

The spatial and temporal distributions of the anthropogenic radionuclides 137Cs and 90Sr, originating from nuclear bomb testing, the Sellafield reprocessing plant in the Irish Sea (UK), and from the Ob and Yenisey river discharges to the Arctic Ocean, have been simulated using the global version of the Miami Isopycnic Coordinate Ocean Model (MICOM). The physical model is forced with daily atmospheric re-analysis fields for the period of 1948–1999. Comparison of the temporal evolution of the observed and the simulated concentrations of 90Sr has been performed in the Kara Sea. The relative contributions of the different sources on the temporal and spatial distributions of the surface 90Sr are quantified over the simulated period. It follows that the Ob river discharge dominated the surface 90Sr over most of the Arctic Ocean and along the eastern and western coasts of Greenland before 1960. During the period of 1980–1990, the atmospheric fallout and the Ob river discharge were equally important for the 90Sr distribution in the Arctic Ocean. Furthermore, an attempt has been made to explore the possible dispersion of accidental released 90Sr from the Ob and Yenisey rivers under a global warming scenario (2 × CO 2). The difference between the present-day and the global warming scenario runs indicates that more of the released 90Sr from the Ob and Yenisey rivers is confined to the Arctic Ocean in the global warming run, particularly in the near coastal, non-European part of the Arctic Ocean.

A two-layer approximation to the Brazil Current–Intermediate Western Boundary Current System between 20°S and 28°S

It has been shown that the vertical structure of the Brazil Current (BC)–Intermediate Western Boundary Current (IWBC) System is dominated by the first baroclinic mode at 22°S–23°S. In this work, we employed the Miami Isopycnic Coordinate Ocean Model to investigate whether the rich mesoscale activity of this current system, between 20°S and 28°S, is reproduced by a two-layer approximation of its vertical structure. The model results showed cyclonic and anticyclonic meanders propagating southwestward along the current axis, resembling the dynamical pattern of Rossby waves superposed on a mean flow. Analysis of the upper layer zonal velocity component, using a space-time diagram, revealed a dominant wavelength of about 450 km and phase velocity of about 0.20 ms −1 southwestward. The results also showed that the eddy-like structures slowly grew in amplitude as they moved downstream. Despite the simplified design of the numerical experiments conducted here, these results compared favorably with observations and seem to indicate that weakly unstable long baroclinic waves are responsible for most of the variability observed in the BC–IWBC system.

A multi-model study of the restratification phase in an idealized convection basin

The representation of baroclinic instability in numerical models depends strongly upon the model physics and significant differences may be found depending on the vertical discretization of the governing dynamical equations. This dependency is explored in the context of the restratification of an idealized convective basin with no external forcing. A comparison is made between an isopycnic model including a mixed layer (the Miami Isopycnic Coordinate Ocean Model, MICOM), its adiabatic version (MICOM-ADIAB) in which the mixed layer physics are removed and the convective layer is described by a deep adiabatic layer outcropping at the surface instead of a thick dense mixed layer, and a z-coordinate model (OPA model). In the absence of a buoyancy source at the surface, the mixed layer geometry in MICOM prevents almost any retreat of this layer. As a result, lateral heat exchanges in the upper layers are limited while mass transfers across the outer boundary of the deep convective mixed layer result in an unrealistic outward spreading of this layer. Such a widespread deep mixed layer maintains a low level of baroclinic instability, and therefore limits lateral heat exchanges in the upper layers over most of the model domain. The behavior of the adiabatic isopycnic model and z-coordinate model is by far more satisfactory although contrasted features can be observed between the two simulations. In MICOM-ADIAB, the more baroclinic dynamics introduce a stronger contrast between the surface and the dense waters in the eddy kinetic energy and heat flux distributions. Better preservation of the density contrasts around the dense water patch maintains more persistent baroclinic instability, essentially associated with the process of dense water spreading. The OPA simulation shows a faster growth of the eddy kinetic energy in the early stages of the restratification which is attributed to more efficient baroclinic instability and leads to the most rapid buoyancy restoring in the convective area among the three simulations. Dense water spreading and warm surface capping occur on fairly similar time scales in MICOM-ADIAB although the former is more persistent that the latter. In this model, heat is mainly transported by anticyclonic eddies in the dense layer while both cyclonic and anticyclonic eddies are involved in the upper layers. In OPA, heat is mainly brought into the convective zone through the export of cold water trapped in cyclonic eddies with a strong barotropic structure. Probably the most interesting difference between the z-coordinate and the adiabatic isopycnic model is found in the temperature distribution ultimately produced by the restratification process. OPA generates a spurious volume of intermediate water which is not seen in MICOM-ADIAB where the volume of the dense water is preserved.

Kuroshio in the Luzon Strait

Three acoustic Doppler current profilers (ADCPs) were deployed in the central Luzon Strait to monitor current velocity. The profilers were deployed from 1997 to 1999, with the duration of deployment varying by location. The observed current velocities indicated that the Kuroshio flowed consistently into the South China Sea. Further information provided by composite shipboard ADCP data showed that the Kuroshio intruded into the Luzon Strait through the deepest channels (∼20.5°N). Most of the intruded component made a loop and flowed out of the northern Luzon Strait, but a branch of the Kuroshio intruded into the South China Sea. While the current velocity obtained from mooring data did not show seasonal variation, significant intraseasonal variation ranging from several to 100 days was found. The seasonal reversal of monsoonal winds (i.e., northeasterly in winter and southwesterly in summer) did not cause noticeable variation in current velocity. The Miami Isopycnic Coordinate Ocean Model, together with monthly wind data, provided by the European Centre for Medium‐Range Weather Forecasts was used to interpret the observed current velocity. Validation of the model results by the ADCP data showed that the model can explain the seasonal and interannual variations in the observed velocity fields. For example, the model reproduced the branch of the Kuroshio that consistently intruded into the South China Sea. The modeled velocity showed slight seasonal variation in the central Luzon Strait, although the large interannual signal could have hindered the seasonal timescale fluctuation. The interaction between the Kuroshio and South China Sea cyclonic flow caused variation in current velocities in both the Luzon Strait and the northern sector of the South China Sea. The model results also indicated that the Kuroshio axis bent clockwise within the Luzon Strait and flowed into and out of the South China Sea through the central and northern Luzon Strait, respectively. This pattern is similar with that shown by composite shipboard ADCP data. In summer, the Kuroshio looped west of the Luzon Strait, with a small intruding branch confined to the northwestern South China Sea. In winter, the intruding branch of the Kuroshio extended west and into the interior of the South China Sea. The net zonal transport of inflow and outflow across the Luzon Strait calculated from the model results was primarily westward, with the maximum varying from 4.8 to 6.5 Sv. Eastward transport was found occasionally in summer in some years, with a maximum from 0.3 to 2.2 Sv. While the Kuroshio intruded consistently into the South China Sea, transport out of the South China Sea was also observed. In summer, the current on the northern South China Sea shelf break contributed to the outflow. Variation in zonal transport was caused by variation in the sea surface height (SSH) west of northern Luzon caused by wind stress curl. Ekman transport, driven by monsoonal winds, and the strength of the Kuroshio off the Luzon Strait had little impact on the intrusion, which varied with fluctuation of the SSH west of northern Luzon.

Influence of the Meridional Overturning Circulation on Tropical Atlantic Climate and Variability

Abstract The influence of the meridional overturning circulation on tropical Atlantic climate and variability has been investigated using the atmosphere–ocean coupled model Speedy-MICOM (Miami Isopycnic Coordinate Ocean Model). In the ocean model MICOM the strength of the meridional overturning cell can be regulated by specifying the lateral boundary conditions. In case of a collapse of the basinwide meridional overturning cell the SST response in the Atlantic is characterized by a dipole with a cooling in the North Atlantic and a warming in the tropical and South Atlantic. The cooling in the North Atlantic is due to the decrease in the strength of the western boundary currents, which reduces the northward advection of heat. The warming in the tropical Atlantic is caused by a reduced ventilation of water originating from the South Atlantic. This effect is most prominent in the eastern tropical Atlantic during boreal summer when the mixed layer attains its minimum depth. As a consequence the seasonal cycle as well as the interannual variability in SST is reduced. The characteristics of the cold tongue mode are changed: the variability in the eastern equatorial region is strongly reduced and the largest variability is now in the Benguela, Angola region. Because of the deepening of the equatorial thermocline, variations in the thermocline depth in the eastern tropical Atlantic no longer significantly affect the mixed layer temperature. The gradient mode remains unaltered. The warming of the tropical Atlantic enhances and shifts the Hadley circulation. Together with the cooling in the North Atlantic, this increases the strength of the subtropical jet and the baroclinicity over the North Atlantic.

The mean flow and variability of the Gulf stream-slopewater system from MICOM

The slopewater region is influenced by surface wind-driven, deep buoyancy-driven and shelf currents, whose complex interactions can affect both the northward heat transport and southward return flows. The mean flow and variability of the Gulf Stream-slopewater system are studied using four year outputs of Miami Isopycnic Coordinate Ocean Model (MICOM) realistic high-resolution simulation of the North Atlantic circulation. Special attention is focused on the eastward Slopewater Jet (SJ), a surface current characterized by a mean path coinciding with the strong outcropping temperature front in the slopewater. The water mass, path and transport of the SJ in MICOM are found to be in reasonable agreement with the existing observations. The modeled SJ is associated in part with a GS’ secondary branch, induced by a Taylor column effect of the New England Seamount Chain (NESC) on the upper GS. This upper-ocean-topographic coupling results in a spatial GS bifurcation, and advection of GS waters into the slopewater region shortly downstream of the NESC. An EOF analysis of the pycnocline depth confirms this tendency, as the first mode displays a qualitative dependence of the GS fan-shape streamline dispersion on the strength and intersecting latitude of the incident GS. Additionally, the model displays a strong influence of the Deep Western Boundary Current (DWBC) on the path of the SJ, by acting as a potential vorticity barrier. Important interactions between the two currents are suggested by the statistical EOF of the slopewater column, as in observations. Downstream of the NESC, the SJ transport variability is seasonal in MICOM, due to the north-south annual oscillation of the GS path and mergings with anticyclonic eddies. However, the variability of the SJ velocity profile is dominated (49% eigenvalue) by lateral translations of the current, at a 9-month timescale characteristic of GS meander-intensity variability. South of the Grand Banks, the transport variations of both the SJ and Labrador Current (LC) are captured by the first mode of the upper-slope water (37% of the variability), with predominant timescales corresponding to the upstream variability of the GS, and seasonality of the LC. Both modes are also in reasonable agreement with observations.

Loop Current Ring Shedding: The Formation of Cyclones and the Effect of Topography

Abstract The formation of cyclones in the vicinity of the Loop Current ring during the separation stage is analyzed in the frame of a high-resolution ECMWF daily wind-forced Miami Isopycnic Coordinate Ocean Model (MICOM) simulation. Mesoscale cyclones, observed in sea surface height maps in the vicinity of the Loop Current in a necking-down position, are found to contribute to the separation of the ring from the Loop Current as they grow between the Loop Current and the ring in the MICOM simulation. To understand the origin of the cyclones, the instability of the Loop Current idealized as an isolated vortex is studied. After noticing the cyclonic vorticity belt around the Loop Current, and based on the vertical distribution of potential vorticity in a Loop Current ring in the MICOM simulation, the linear stability of a shielded vortex is studied in the quasigeostrophic formalism. To simulate the effects of the planetary vorticity gradient and topography on the Loop Current, the nonlinear states of the idealized Loop Current are analyzed using the adiabatic MICOM code. Results from the analysis of the MICOM simulation show that cyclones are the products of a vortex rim instability. The modal analysis of the Loop Current instability reveals that mode 4 is the fastest-growing mode and that baroclinic (barotropic) instability is intensified in the deep (surface) layers. These results are confirmed by the analytical study of the idealized Loop Current. The nonlinear state shows that a Loop Current–like vortex is indeed a pentapole on an f plane. On the β plane, the northern cyclone is separated from the anticyclone by the β effect and both drift westward. When the topography of the Gulf of Mexico is taken into account—namely, the Campeche Bank, the southward slope north of the Loop Current, and the Florida shelf east of the Loop Current—several effects are observed: 1) the northern corner of the Campeche Bank erodes the Loop Current ring and its cyclones and interacts with the vortex’s most unstable mode, 2) the northern southward slope scatters the northern cyclone while the anticyclone remains coherent and propagates to the west, and 3) realistic westward propagation speeds are obtained in the presence of the northern Campeche shelf, which acts as a mirror effect on the Loop Current ring, as opposed to the Florida shelf, which tends to block the ring.

Investigation of variability of the North Atlantic Subtropical Mode Water using profiling float data and numerical model output

The basic characteristics of the North Atlantic Subtropical Mode Waters (STMW) are well documented in the literature from one-time hydrographic sections or from long-term measurements at one location. From July 1997 to March 1998, 71 vertical profiling floats were deployed in the Western Subtropical North Atlantic region to provide a broader spatial and temporal coverage of STMW. In this study, the STMW properties are estimated from a total of 5352 float temperature profiles in an area covering 30°W–80°W and 20°N–45°N, from January 1998 to December 2000. These STMW properties are compared to those derived from numerical simulation output using the Miami Isopycnic Coordinate Ocean Model (MICOM). The model is configured to the North Atlantic Ocean with a 1-deg horizontal resolution and 20 vertical layers. The model temperature profiles are collocated with the observed float profiles for a direct comparison. The values of the mean STMW temperature derived from both the float data and model output are roughly equal (18.1 °C for floats and 18.0 °C for the model). The model is found to capture well the observed annual cycle of the STMW volume. Given good agreement between float and model profiles, we use a longer MICOM simulation to investigate decadal variability of the STMW renewal rate in terms of the annual subduction rate and its relationship to the large-scale atmospheric circulation pattern changes measured by the North Atlantic Oscillation index. Good correlation is found between the model annual subduction rate anomaly at the Panulirus Station and the NAO winter index anomaly on decadal time scales with NAO leading by 2–3 years.

Dominant Modes of Variability in the South Atlantic: A Study with a Hierarchy of Ocean–Atmosphere Models

Abstract Using an atmosphere model of intermediate complexity and a hierarchy of ocean models, the dominant modes of interannual and decadal variability in the South Atlantic Ocean are studied. The atmosphere Simplified Parameterizations Primitive Equation Dynamics (SPEEDY) model has T30L7 resolution. The physical package consists of a set of simplified physical parameterization schemes, based on the same principles adopted in the schemes of state-of-the-art AGCMs. It is at least an order of magnitude faster, whereas the quality of the simulated climate compares well with those models. The hierarchy of ocean models consists of simple mixed layer models with an increasing number of physical processes involved such as Ekman transport, wind-induced mixing, and wind-driven barotropic transport. Finally, the atmosphere model is coupled to a regional version of the Miami Isopycnal Coordinate Ocean Model (MICOM) covering the South Atlantic with a horizontal resolution of 1° and 16 vertical layers. The coupled modes of mean sea level pressure and sea surface temperature simulated by SPEEDY–MICOM strongly resemble the modes as analyzed from the NCEP–NCAR reanalysis, indicating that this model configuration possesses the required physical mechanisms for generating these modes of variability. Using the ocean model hierarchy the authors were able to show that turbulent heat fluxes, Ekman transport, and wind-induced mixing contribute to the generation of the dominant modes of coupled SST variability. The different roles of these terms in generating these modes are analyzed. Variations in the wind-driven barotropic transport mainly seem to affect the SST variability in the Brazil–Malvinas confluence zone. The spectra of the mixed layer models appeared to be too red in comparison with the fully coupled SPEEDY–MICOM model due to the too strong coupling between SST and surface air temperatures (SATs), resulting from the inability to advect and subduct SST anomalies by the mixed layer models. In SPEEDY–MICOM anomalies in the southeastern corner of the South Atlantic are subducted and advected toward the north Brazilian coast on a time scale of about 6 yr.

The North Atlantic Subpolar Gyre in Four High-Resolution Models

Abstract The authors present the first quantitative comparison between new velocity datasets and high-resolution models in the North Atlantic subpolar gyre [1/10° Parallel Ocean Program model (POPNA10), Miami Isopycnic Coordinate Ocean Model (MICOM), ⅙° Atlantic model (ATL6), and Family of Linked Atlantic Ocean Model Experiments (FLAME)]. At the surface, the model velocities agree generally well with World Ocean Circulation Experiment (WOCE) drifter data. Two noticeable exceptions are the weakness of the East Greenland coastal current in models and the presence in the surface layers of a strong southwestward East Reykjanes Ridge Current. At depths, the most prominent feature of the circulation is the boundary current following the continental slope. In this narrow flow, it is found that gridded float datasets cannot be used for a quantitative comparison with models. The models have very different patterns of deep convection, and it is suggested that this could be related to the differences in their barotropic transport at Cape Farewell. Models show a large drift in watermass properties with a salinization of the Labrador Sea Water. The authors believe that the main cause is related to horizontal transports of salt because models with different forcing and vertical mixing share the same salinization problem. A remarkable feature of the model solutions is the large westward transport over Reykjanes Ridge [10 Sv (Sv ≡ 106 m3 s−1) or more].

On an adaptive filter for altimetric data assimilation and its application to a primitive equation model, MICOM

In a reduced-order adaptive filtering approach, we demonstrate a possibility to overcome two major difficulties in estimating oceanic circulation: a very high dimension of the system state and uncertainties in specification of the model error statistics. This approach is based essentially on the assumption that a particular parametrized gain matrix has been selected and the tuning parameters are adjusted by minimizing the mean prediction error. In the present paper we apply a reduced-order adaptive filter for solving the problem of assimilating altimetric sea surface height into a primitive equation model: the Miami isopycnic coordinate ocean model (MICOM). A gain structure is described which is proven to be very efficient in the twin experiments. The assimilation algorithm to be employed in the identical twin experiment is a reduced-order filter whose reduced state consists of the layer thickness. The velocity update is calculated from the geostrophic hypothesis. The gain structure for the non-adaptive filter is obtained on the basis of three principal hypotheses: (H1) analysis error for the system output is cancelled in the case of noise-free observations (as is done naturally in a standard Kalman filter for noise-free observation); (H2) conservation of linear potential vorticity; (H3) no correction for the velocity at the bottom layer. The initial values of the parameters in the gain will be selected in such a way that the filter behaves exactly as the Cooper’Haines filter (CHF) at the first data update step. It is shown that the adaptive filter, which relaxes one or several of the above hypotheses, is capable of producing the better estimates for the ocean state (layer thickness and velocity) compared to that produced by the CHF in all layers, surface or subsurface. Numerical experiments demonstrate the excellent capacity of the adaptive filter to extract useful information from surface observations for inferring the oceanic circulation in the MICOM.

Tracer-derived transit time of the waters in the eastern Nordic Seas

The spatial-temporal distribution of the anthropogenic radionuclide 137Cs originating from nuclear bomb testing and the Sellafield reprocessing plant in the Irish Sea is simulated using a global version of the Miami Isopycnic Coordinate Ocean Model (MICOM). The physical model has a horizontal resolution of about 40 km, and it is forced with daily atmospheric reanalysed fields for the period 1948 to 2002. A comparison of the temporal evolution of observed and simulated concentrations of 137Cs has been conducted for the regions east of Scotland, west of central Norway and at the entrance of the Barents Sea. It follows that the annual mean 137Cs concentration signal from the Sellafield discharge reaches the southwestern Barents Sea region in 5 yr, in accordance with available observations. To more accurately assess the age of the caesium-tagged water masses, three additional experiments were performed with an Eulerian age coupled to the evolution of the tracer concentration. It is demonstrated that the transit time depends on the duration of the tracer release history, on the dynamical ocean state and on whether the transit time is derived from the time difference between the release history and the concentration evolution downstream of the release, or as an explicit Eulerian age tracer. The Eulerian age tracer gives a transit time for a Sellafield-like discharge to reach the Barents Sea of 56 months in the 1970s, 52 months in the 1960s and 51 months in the 1980s, and that this difference is strongly governed by the ocean dynamics in the Faeroe—Scotland region.

The effect of diapycnal mixing on the ventilation and CFC-11 uptake in the Southern Ocean

The Miami Isopycnic Coordinate Ocean Model (MICOM) is used to investigate the effect of diapycnal mixing on the oceanic uptake of CFC-11 and the ventilation of the surface waters in the Southern Ocean (south of 45°S). Three model experiments are performed: one with a diapycnal mixing coefficientK d (m2 s−1) of 2 × 10−7/N (Expt. 1), one withK d = 0 (Expt. 2), and one withK d = 5 × 10−8/N (Expt. 3),N (s−1) is the Brunt-Väisälä frequency. The model simulations indicate that the observed vertical distribution of CFC-11 along 88°W (prime meridian at 0°E) in the Southern Ocean is caused by local ventilation of the surface waters and westward-directed (eastward-directed) isopycnic transport and mixing from deeply ventilated waters in the Weddell Sea region. It is found that at the end of 1997, the simulated net ocean uptake of CFC-11 in Expt. 2 is 25% below that of Expt. 1. The decreased uptake of CFC-11 in the Southern Ocean accounts for 80% of this difference. Furthermore, Expts. 2 and 3 yield far more realistic vertical distributions of the ventilated CFC-waters than Expt. 1. The experiments clearly highlight the sensitivity of the Southern Ocean surface water ventilation to the distribution and thickness of the simulated mixed layer. It is argued that inclusion of CFCs in coupled climate models could be used as a test-bed for evaluating the decadal-scale ocean uptake of heat and CO2.

Rotational dynamics of turbulence and Tsallis statistics

The Langevin equation and its adaptations can reproduce accurately key aspects of fluid-particle motions in turbulent flows including: ballistic and normal-diffusive transport regimes, Lagrangian velocity autocorrelation functions and integral Lagrangian timescales. More recently it has become apparent that trajectory-rotations are also important dynamical quantities and that non-zero mean rotations are associated with suppressed rates of turbulent dispersion and oscillatory Lagrangian velocity autocorrelation functions. Here, it is shown that rotations of the Lagrangian velocity vector produced by the simplest and most widely used of such models coincide closely with the intense rotations measured in the recent seminal experiment by Zeff et al. [Nature 421 (2003) 146] and are described precisely by Tsallis statistics. Model predictions (Tsallis distributions) for the rotational statistics of the North Atlantic Ocean (region between 35–43 °W and between 36–42 °N) are found to be in close agreement with simulation data produced by the Miami Isopycnic-Coordinate Ocean Model.

Putting models to the data test: a case study using Indian Ocean CFC-11 data

A method is presented for comparing time-averaged model output with snapshot data. The focus is on a comparison between the cholorofluorocarbon CFC-11 simulated in the Parallel Ocean Program (POP) model, and the near-synoptic Indian Ocean World Ocean Circulation Experiment (WOCE) CFC data-set collected in 1995. In the thermocline, the simulated fields fairly closely approximate the observations. We seek to distinguish between regions in which there is room for improvement in the model physics, and regions where the mismatch is the result of comparing time-averaged model fields with snapshot data. CFC variance is estimated using an eddy-permitting Miami Isopycnic Coordinate Ocean Model (MICOM) simulation. The results indicate that the widespread method of visual comparison of vertically-gridded data with model output might lead to misattribution of model deficiencies in regions of high oceanic variability. It is suggested that consideration of error-bars obtained from eddy-permitting model simulations in future model-data comparisons could help avoid such potentially erroneous conclusions.

Thermohaline Structure in the Subarctic North Pacific Simulated in a General Circulation Model*

The Miami Isopycnic Coordinate Ocean Model configured with 18 horizontal resolution and 23 layers is used to examine processes that maintain the mesothermal structure, a subsurface temperature inversion, in the subarctic North Pacific. The model successfully reproduces the mesothermal structure consisting of a shallow temperature minimum and an underlying temperature maximum that are called the dichothermal and mesothermal waters, respectively. The mesothermal water is formed through cross-gyre exchange between the subtropical and subarctic gyres, whereas the dichothermal water originates from cold and low-salinity waters formed in the winter mixed layer. The horizontal distribution of the passive tracer injected into the subsurface layers south of Japan shows that warm and saline water of the Kuroshio in the density range of 26.8‐27.0 su is the source of the mesothermal water. There are three pathways through which the Kuroshio waters enter the subarctic region. First, the Kuroshio waters that cross the gyre boundary in the western boundary region are carried to the Alaskan gyre by the northern part of the North Pacific Current. Second, the Kuroshio waters carried by the southern part of the North Pacific Current enter the Alaskan gyre through a cross-gyre window in the eastern basin. Third, the Kuroshio waters that diffuse along the isopycnal in the Kuroshio‐Oyashio Extension enter the western subarctic gyre. The mesothermal water thus formed in the subarctic region is entrained into the winter mixed layer and returns to the subtropics as surface water by the southward Ekman drift, forming the subpolar cell.

Biophysical isopycnic-coordinate modelling of plankton dynamics in the Arabian Sea

A Nutrient–Phytoplankton–Zooplankton–Detritus (NPZD) ecosystem model is coupled to the Miami Isopycnic-Coordinate Ocean Model (MICOM) to study plankton dynamics in the Arabian Sea. Experiments oriented to testing the NPZD–MICOM coupled model sensitivity to variations in several parameters are performed. Particular attention is paid to the rates of detritus sinking and maximum phytoplankton growth rate. The coarse features of the blooms are captured by all the experiments considered, and agree with earlier models for the biological activity of the region. Intensity, duration, and peaks of the blooms are found to be quite sensitive to the parameter choices. The existence of an offshore deep chlorophyll maximum in the model is found to be closely related to the increase of the detritus sinking rate. The use of temperature-dependent maximum growth rate of phytoplankton and the increase of detritus sinking rate are shown to improve model results compared with in situ and satellite observations. The differences between the present results and those from previous modelling efforts in the region, where few- or multi-layer hydrodynamical models or even fixed-level hydrodynamical models have been employed, are found to be in the same range as the differences among the results from the various sensitivity experiments presented here. This indicates that a small uncertainty in the knowledge of the ecosystem model parameters can be more important in ecosystem modelling than the uncertainty associated to the differences in the vertical coordinate representation of the underlying hydrodynamical model. There are still biological processes, such as denitrification which affect the vertical distribution of nutrient concentrations, that are not included neither in the previous works in the area nor in the present work. This might mask any potential advantages of the present NPZD-MICOM coupled model, especially at mid-depths in the ocean.

Gulf of Mexico circulation within a high‐resolution numerical simulation of the North Atlantic Ocean

The Gulf of Mexico circulation is examined from the results of a high‐resolution (1/12°) North Atlantic simulation using the Miami Isopycnic Coordinate Ocean Model. The motivation for this paper is twofold: first, we validate the model's performance in the Gulf of Mexico by comparing the model fields to past and recent observations, and second, given the good agreement with the observed Gulf of Mexico surface circulation and Loop Current variability, we expand the discussion and analysis of the model circulation to areas that have not been extensively observed/analyzed, such as the vertical structure of the Loop Current and associated eddies, especially the deep circulation below 1500 m. The interval between successive model eddy sheddings is 3 to 15 months, the eddy diameters range between 140 and 500 km, the life span is about 1 year, and the translational speeds are 2–3 km d−1, in good agreement with observations. Areas of high cyclonic eddy occurrence in the model are southwest of Florida, the Loop Current boundary, and the western Campeche Bay area. The cyclonic eddy diameters range between 50 and 375 km, the orbital speeds range between 1 and 55 cm s−1, the translational speeds range between 0.5 and 14 km d−1, and the eddy life spans range between 1 and 3 months. The vertical structure of the temperature and salinity of each modeled eddy, from the moment it is shed until it disintegrates in the western Gulf of Mexico, is in agreement with the few available observations. Below 1500 m, deep cyclonic eddies are associated with the surface Loop Current anticyclones. The eddy variability is consistent with Rossby waves propagating westward, and there is bottom intensification of the flow close to steep topography. Overall, we show that this very high horizontal resolution isopycnic coordinate ocean model, which is able to produce a quite realistic surface circulation for the North and equatorial Atlantic, is also able to reproduce well the smaller‐scale, basin‐wide intricate dynamics such as the Gulf of Mexico variability.