Tommy D. Dickey

Influence of mesoscale eddies on new production in the Sargasso Sea

It is problematic that geochemical estimates of new production — that fraction of total primary production in surface waters fuelled by externally supplied nutrients — in oligotrophic waters of the open ocean surpass that which can be sustained by the traditionally accepted mechanisms of nutrient supply., In the case of the Sargasso Sea, for example, these mechanisms account for less than half of the annual nutrient requirement indicated by new production estimates based on three independent transient-tracer techniques. Specifically, approximately one-quarter to one-third of the annual nutrient requirement can be supplied by entrainment into the mixed layer during wintertime convection, with minor contributions from mixing in the thermocline, and wind-driven transport (the potentially important role of nitrogen fixation — for which estimates vary by an order of magnitude in this region — is excluded from this budget). Here we present four lines of evidence — eddy-resolving model simulations, high-resolution observations from moored instrumentation, shipboard surveys and satellite data — which suggest that the vertical flux of nutrients induced by the dynamics of mesoscale eddies is sufficient to balance the nutrient budget in the Sargasso Sea.

Mesoscale Eddies Drive Increased Silica Export in the Subtropical Pacific Ocean

Mesoscale eddies may play a critical role in ocean biogeochemistry by increasing nutrient supply, primary production, and efficiency of the biological pump, that is, the ratio of carbon export to primary production in otherwise nutrient-deficient waters. We examined a diatom bloom within a cold-core cyclonic eddy off Hawai`i. Eddy primary production, community biomass, and size composition were markedly enhanced but had little effect on the carbon export ratio. Instead, the system functioned as a selective silica pump. Strong trophic coupling and inefficient organic export may be general characteristics of community perturbation responses in the warm waters of the Pacific Ocean.

A Vector Geometry–Based Eddy Detection Algorithm and Its Application to a High-Resolution Numerical Model Product and High-Frequency Radar Surface Velocities in the Southern California Bight

Automated eddy detection methods are fundamental tools to analyze eddy activity from the large datasets derived from satellite measurements and numerical model simulations. Existing methods are either based on the distribution of physical parameters usually computed from velocity derivatives or on the geometry of velocity streamlines around minima or maxima of sea level anomaly. A new algorithm was developed based exclusively on the geometry of the velocity vectors. Four constraints characterizing the spatial distribution of the velocity vectors around eddy centers were derived from the general features associated with velocity fields in the presence of eddies. The grid points in the domain for which these four constraints are satisfied are detected as eddy centers. Eddy sizes are computed from closed contours of the streamfunction field, and eddy tracks are retrieved by comparing the distribution of eddy centers at successive time steps. The results were validated against manually derived eddy fields. Two parameters in the algorithm can be modified by the users to optimize its performance. The algorithm is applied to both a high-resolution model product and highfrequency radar surface velocity fields in the Southern California Bight.

The emergence of concurrent high-resolution physical and bio-optical measurements in the upper ocean and their applications

Time and space scales of processes relevant to the upper ocean ecosystem span more than 9 orders of magnitude, consequently adequate sampling of the oceanic environment is a major challenge. However, new capabilities for studying upper ocean physical and bio-optical processes have developed rapidly within the past decade with the advancement of sensor technology and data acquisition and storage capabilities. Several oceanic measurement systems have been designed recently to enable the collection of physical and bio-optical data concurrently. Problems concerning variability in primary production by phytoplankton, upper ocean heating, small-scale physical mixing and advection, the dispersion of oceanic discharges, and global scale climate change related to the carbon cycle can now benefit from in situ concurrent physical and bio-optical data sets. The ocean sampling platforms utilized for deployment of these systems have included ships, the stable platform Research Platform FLIP, moorings and drifters. Examples of recent data sets obtained from these platforms are presented, and the merits of each of these sampling modes are described. In addition, the needs for complementary in situ and remotely sensed (for example, from satellites) data sets for both analytical and modeling efforts are summarized. Several research applications of the new technology are highlighted.

Spectral particulate attenuation and particle size distribution in the bottom boundary layer of a continental shelf

Spectral attenuation and absorption coefficients of particulate matter and colocated hydrographic measurements were obtained in the Mid-Atlantic Bight during the fall of 1996 and the spring of 1997 as part of the Coastal Mixing and Optics experiment. Within the bottom boundary layer (BBL) the magnitude of the beam attenuation decreased and its spectral shape became steeper with distance from the bottom. Concurrently, the slope of the particulate size distribution (PSD) was found to increase with distance from the bottom. Changes in the PSD shape and the magnitude of the beam attenuation as functions of distance from the bottom in the BBL are consistent with particle resuspension and settling in the BBL, two processes that are dependent on particle size and density. For particles of similar density, resuspension and settling would result in a flattening of the PSD and an increase in the beam attenuation toward the bottom. In both fall and spring the magnitude of the particle attenuation coefficient correlates with its spectral shape, with a flatter shape associated with higher values of the attenuation. This observation is consistent with idealized optical theory for polydispersed nonabsorbing spheres. According to this theory, changes in the steepness of the particle size distribution (particle concentration as a function of size) will be associated with changes in the steepness of the attenuation spectra as a function of wavelength; a flatter particle size distribution will be associated with a flatter attenuation spectrum. In addition, the observed ranges of the beam attenuation spectral slope and the PSD exponent are found to be consistent with this theory.

New Chemical, Bio-Optical and Physical Observations of Upper Ocean Response to the Passage of a Mesoscale Eddy off Bermuda

A mesoscale eddy advected across the Bermuda Testbed Mooring site over a 30-day period centered on July 14, 1995. Temperature and current measurements along with biogeochemical measurements were used to characterize the biological response of the upper ocean associated with the introduction of nitrate into the euphotic layer due to the doming of isotherms associated with the eddy. Complementary shipboard data showed an anomalous water mass, which extended from a depth of ;50 to 1000 m, manifesting as a cold surface expression and warm anomaly at depth. Although mesoscale eddies are frequently observed in the Sargasso Sea, the present observations are particularly unique because of the high-temporal-resolution measurements of the new instrumentation deployed on the mooring. Analyzers that measure nitrate plus nitrite were placed at depths of 80 and 200 m and bio-optical sensors were located at depths of 20, 35, 45, 71, and 86 m. Peak nitrate values of nearly 3.0 mM at 80 m and chlorophyll a values of 1.4 mg m 23 at 71 m were observed, as well as a 25- to 30-meter shoaling of the 1% light level depth. A Doppler shift from the inertial period (22.8 hours) to 25.2 hours was observed in several time series records due to the movement of the eddy across the mooring. Inertial pumping brought cold, nutrient-rich waters farther into the euphotic zone than would occur solely by isothermal lifting. Silicic acid was depleted to undetectable levels owing to the growth of diatoms within the eddy. The chlorophyll a values associated with the eddy appear to be the largest recorded during the 8 years of the ongoing U.S. JGOFS Bermuda Atlantic Time Series Study (BATS) program.

Decaying turbulence in neutral and stratified fluids

Decaying turbulence in neutral and stratified fluids has been studied experimentally for relatively high mesh Reynolds numbers and long time-histories. The neutral case indicates an initial period decay law, q 2 ∝ t −1 , through non-dimensional time \[ W_gt/M \simeq 800 \] which is considerably longer than previous measurements at the same mesh Reynolds number ( Re = 48260). The stratified experiment resulted in a decay rate virtually identical to that of the neutral case through W g t / M = 275. However the decay rate sharply decreased after this time when the field of turbulence was replaced by internal gravity waves. A critical Richardson number marks the transition from the turbulence to an internal gravity wave domain.

Upper-Ocean Temperature Response to Hurricane Felix as Measured by the Bermuda Testbed Mooring

Hurricane Felix passed over the Bermuda testbed mooring on 15 August 1995, providing a unique opportunity to observe the response of the upper ocean to a hurricane. In the vicinity of Bermuda, Felix was a particularly large hurricane with hurricane-force winds over a diameter of about 300‐400 km and tropical storm‐force winds over a diameter of about 650‐800 km. Felix moved northwestward at about 25 km h21 with the eye passing about 65 km southwest of the mooring on 15 August. Peak winds reached about 135 km h21 at the mooring. Complementary satellite sea surface temperature maps show that a swath of cooler water (by about 3.58‐4.08C) was left in the wake of Felix with the mooring in the center of the wake. Prior to the passage of Felix, the mooring site was undergoing strong heating and stratification. However, this trend was dramatically interrupted by the passage of the hurricane. As Felix passed the mooring, large inertial currents (speeds of 100 cm s 21 at 25 m) were generated within the upper layer. The e-folding decay timescale of the inertial currents was about 9 days. The mixed layer depth was about 15 m before the arrival of Felix and deepened to about 45 m within three days after Felix’s passage; the temperature at 25 m decreased by approximately 3.5 8‐4.08C. Large-amplitude temperature oscillations (;1.58C) near the inertial period (inertial pumping effect) were set up by the hurricane in the seasonal thermocline resulting in vertical displacements of isotherms of approximately 15 m at 60‐70 m. Comparative scale analyses of the upper-ocean responses to Hurricane Felix and Hurricane Gloria (1985) indicate that they have several similarities.

Initial results from the Bermuda Testbed Mooring program

The Bermuda Testbed Mooring (BTM) has been deployed since June 1994 and provides the oceanographic community with a deep-water platform for testing and intercomparing new instruments. The mooring is located about 80 km southeast of Bermuda. Surface instruments collect meteorological and spectral radiometric measurements from a buoy tower. Measurements at depth include: currents, temperature, conductivity, optical properties, and nitrate and trace element concentrations. Data have been sent to shore and to a nearby ship using a new inductive-link telemetry system. The high temporal resolution, long-term data collected from the mooring provide important information concerning episodic and periodic processes ranging in scale from minutes to years. For example, short nitrate pulses and associated biological events have been observed in the mooring data sets, which were not seen in the periodic ship-collected time-series data. Evaluation of undersampling and aliasing effects characteristic of infrequent sampling are also enabled with these data sets. The primary purposes of this report are to describe new systems and to illustrate early data resulting from the BTM program.

Optical determination of particulate abundance and production variations in the oligotrophic ocean

Observed diurnal variations of the beam attenuation coefficient at 660 ran (c) are used to estimate variations of particulate concentration and production in the open ocean. The diurnal cycles of c are found to be statistically siotmificant throughout the euphotic zone (the upper 95 m), but not below this depth. Their amplitudes are nearly uniform with respect to depth with characteristic peak to peak values of -0.0065 m -1. The minimum (maximum) of these daily variations occurs during local sunrise (sunset), indicating a daytime accumulation of particles. Specific particle production rates (r0) calculated with beam transmissometer data alone have maximum noontime values of -0.5 d -1 and are well-correlated with the incident solar radiative flux. The high correlation between the optically determined specific production rates and solar radiation allows r0 to be decomposed into light-dependent and light-independent which are interpreted as particle-specific growth and grazing rates. The resulting specific growth and grazing rates are balanced (i.e. net daffy production is approximately zero) and are consistent with previous biological determinations for the open ocean. Daffy mean specific growth and grazing rates are found to be ~0.15 d -~. The value of the water column integrated community production estimated using the optical data is ~ 160 mg C m -2 d -~, which compares well with both simultaneous and seasonal mean carbon uptake rate measurements. This optical method should prove useful for in situ observations of particulate production in the oligotrophic ocean.