Phytoplankton patches at fronts: A model of formation and response to wind events

A subsurface patch of chlorophyll is a feature common to many fronts. The dynamics underlying the patch formation are not well understood; however, there appears to be a strong link between the biological and physical dynamics. Here we test the hypothesis that patch formation is a result of wind-forced motions of the front. Frontal responses to transient wind events include acceleration of the surface layers, changes in mixed-layer depth, and excitation of nonlinear oscillations at the front. Such dynamics can affect nutrient fluxes across the pycnocline and the coupling between trophic levels. To test this hypothesis, we developed a two-dimensional coupled mixed-layer/primitive-equation/ecosystem model, which we forced with transient wind events. We explored a range of initial conditions, including the cross-frontal nutrient gradient, the sinking of phytoplankton, the depth of the euphotic zone, and the depth of the front. The model showed the subsurface chlorophyll patch to be dependent on all these factors. The cross-frontal scale of the patch was found to be a historical artifact of past wind events, and so was not strongly related to the scale of the front (given by the Rossby radius of deformation). Winds aligned with the frontal jet weakened the cross-frontal density gradient, causing a flux of nutrients into the warmer waters of the front, and eroding the chlorophyll patch through deep vertical mixing. Winds aligned against the frontal jet enhanced the cross-frontal density gradient and isolated the subsurface patch from vertical mixing. The transient forcing of the fronts led to relatively high f-ratios in the subsurface patches; this enhanced biomass would be invisible to most satellite sensors

A model for the vertical flux of nitrogen in the upper ocean: Simulating the alteration of isotopic ratios

An idealized, one-dimensional, constant diffusivity mathematical model for the study of the vertical flux of nitrogen in the upper-ocean is presented. We attempt to simulate observed patterns in vertical profiles for the natural abundance of 15N in particulate organic nitrogen (PON) and the concentrations of PON and NO3−. The concentration of phytoplankton nitrogen (II) increased as a result of either increasing the upward flux of NO3−(N) or by increasing the residence time of II. A minimum in the δ15N of phytoplankton nitrogen (δ2) appeared near a maximum in II at the inflection point of the N profile. Increasing the residence time or the vertical eddy diffusivity, reduced the amplitude of the δ2 profile. The model was able to produce reasonably good simulations of observed profiles from two warm-core rings, Rings 82-E and 82-H, using the most appropriate values for the light extinction coefficient and the residence time of PON. These results lend general support to current views regarding the nature and significance of the vertical fluxes of nitrogen in the upper-ocean and hypotheses presented previously concerning the factors which affect the δ15N of PON

Dynamics of the Coastal Transition Zone Jet 2. Nonlinear finite amplitude behavior

The finite amplitude nonlinear behavior of the coastal transition zone (CTZ) jet, assumed to be governed by quasi-geostrophic dynamics, is studied utilizing numerical experiments in an idealized geometry. Finite difference solutions to initial value problems are obtained for a stratified, six-layer fluid in a periodic, flat bottom, f plane channel. The initial flow field in all experiments includes a uniform along-channel jet with horizontal and vertical structure obtained from CTZ hydrographic and current measurements made in May 1987. The maximum velocity in the jet is about 0.5 m s-1, the vertical shear is such that the velocities below 500 m depth are small, and the jet width is about 60 km. The Rossby radius for the first baroclinic mode is 24.6 km. An analysis of the linear stability of the jet flow field in part 1 shows that the jet is linearly unstable to disturbances with along-jet wavelengths greater than 50 km. The largest growth rate is found for a wavelength around 260 km. The objectives here are to find the characteristics of the finite amplitude nonlinear jet instabilities and to examine local dynamical balances for signatures of instability processes that would help with the physical interpretation of results from limited-area CTZ data assimilation models. Experiments are run with periodic channels of different length L(x), where the initial flow includes the basic jet and a small perturbation in the form of the most unstable linear mode for a wavelength equal L(x). In the basic experiment (BC) L(x) = 250 km. One experiment (LC) is run in a long channel L(x) = 1280 km with initial disturbances forced by application of a weak wind stress curl for three days. For times less than about 70 days, experiments BC and LC show the growth and development of finite amplitude meanders in the CTZ jet with spatial variations similar to those observed in spring 1987. During this early time period, the amplitudes of the meanders grow at rates of 1-4 km d-1, increasing with meander amplitude, and the meanders propagate in the direction of the jet upper layer flow at phase velocities of 5-3 km d-1, decreasing with meander amplitude. Initially, at small amplitudes the instability involves comparable contributions from barotropic and baroclinic processes in agreement with linear theory, but for large amplitude meanders the baroclinic energy conversion processes dominate. The vertical velocity field develops a characteristic spatial structure in relation to the jet meanders as do terms in the kinetic energy balance corresponding to the time rate of change of kinetic energy following fluid particles and to the rate of conversion from potential energy. The spatial structure of the latter field may provide a useful indication for jet baroclinic instability processes in limited-area models. At later times in experiment BC (days 90-110), the meander growth is limited by a pinching-off process that results in the formation of detached eddies on both sides of the jet. This process is characterized by a large relative increase of the kinetic energy in the lower layers and in the depth-independent component of the flow. Experiments with L(x) = 180 and 150 km give qualitatively different behavior than that found in experiments BC and LC. The meander growth is bounded, and the jet flow field exhibits time-dependent variations in the volume integrated kinetic and potential energies. For large time the flow may asymptotically approach a nearly steady state or states with irregular or nearly periodic oscillations in the integrated kinetic and potential energies

Dynamics of the Coastal Transition Zone Jet 1. Linear stability analysis

The linear stability of a coastal transition zone (CTZ) jet is analyzed using a six-layer quasi-geostrophic model with observed basic state velocity profiles. The velocity profiles are obtained from objectively analyzed hydrographic and acoustic doppler data from the 1987 CTZ pilot experiment. Along-jet perturbation wave-lengths of 260-265 km are found to be the most unstable, with e-folding growth periods of 7-11 days and along-jet phase speeds of 4-8 km/d downstream. Energy transformation terms and energy budgets are discussed. Both barotropic and baroclinic instability processes are important.Copyrighted by American Geophysical Union

Dynamics of the Coastal Transition Zone though data assimilation studies

The dynamics of the coastal transition zone off Northern California during late May and early June 1987 art examined through assimilation modeling studies. A regional baroclinic quasi-geostrophic model is driven by the data through initial and boundary conditions. These initial and boundary conditions are specified by objective analysis of hydrographic and acoustic Doppler current profiler data. The data assimilation is accomplished by varying the objective analysis parameters, numerical parameters, and subgrid-scale parameters until the final solution of the model is in best agreement with the analysis of the data. The solution which best agrees with the data is regarded as a four dimensional field estimate of the coastal transition zone how. An aspect of this study that is new to data assimilation modeling of mesoscale eddy fields is the use of acoustic Doppler current profiler data. These data prove to be very important to accurate description of the oceanic flow field as indicated by comparison with float trajectories. The complete data seL provides an opportunity to study the ability of dynamical constraints to improve field estimates when acting upon a subset of the data (hydrography). Data assimilation modeling generates field estimates that are substantially better than those based upon the hydrographic data alone as indicated by comparison with the acoustic Doppler current profiler based analyses. The kinematics and energetics of this constrained (quasi-geostrophic) field estimate are examined. Despite the relatively small region (108 by 324 km) land short period of the field estimate (21 days), interesting processes are identified. A meandering barotropically unstable part of the jet is identified using the results of related idealized numerical studies and stability analyses. Similarly, this jet may be undergoing a simultaneous larger scale mixed instability. Another part of the jet interacts with an eddy and meanders in a much different manner. Characteristics of the energy balances are identified and compared with the results of linear analysis and of nonlinear studies utilizing idealized models of the observed jet as described in this issue by Pierce et al. and Allen et al. respectively.Copyrighted by American Geophysical Union