Francesco Nencioli

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.

Three‐dimensional oceanic eddy analysis in the Southern California Bight from a numerical product

With eight islands, complex coastlines and bottom topography, strong wind curls, and frequent upwelling fronts, the Southern California Bight (SCB) is an area with strong eddy activity. By applying an automated eddy detection scheme to a 12 year high-resolution numerical product of the oceanic circulation in the SCB, a three-dimensional eddy data set is developed. It includes information for each eddys location, polarity, intensity, size, boundary, and moving path at nine vertical levels. Through a series of statistical analyses applied to the eddy data set, three-dimensional statistical characteristics of mesoscale and submeoscale eddy variations in the SCB are elucidated; these shed light on how eddies are generated, evolve, and terminate. A significant percentage of eddies is found to be generated around islands and headlands along the coastline, which indicates that islands in the SCB play a vital role in eddy generation. Three types of eddies, based on shape, are identified from the numerical product: bowl, lens, and cone. A dynamic analysis shows that some submesoscale eddies with finite local Rossby numbers tend to be ageostrophic balanced while mesoscale eddies are in geostrophic balance. The present research results are useful for the interpretation of data sets obtained during the interdisciplinary Santa Barbara Channel Radiance in a Dynamic Ocean (RaDyo) field experiment conducted on September 3-25, 2008.

An oceanic cyclonic eddy on the lee side of Lanai Island, Hawai'i

[1]xa0A young cold core cyclonic eddy displaying a significant increase in surface chlorophyll was observed offshore Lanai Island, Hawaii, where the Marine Optical Buoy (MOBY) is located. During one of its deployments, MOBY broke free from its mooring. In the course of its 3-day free drifting period, MOBY followed a cyclonic eddy, which is manifested by satellite remote sensing data, chlorophyll data from Moderate Resolution Imaging Spectroradiometer, and sea surface temperature (SST) from a Geostationary Operational Environmental Satellite. The time series of the SST show that the cold core eddy was in a formative stage. It existed as a stand-alone eddy for about 9 days before it merged with cold water south of Oahu Island. A high-resolution numerical model simulation reproduces similar eddies in terms of location, size, and intensity. An eddy detection algorithm is described and applied to locate and track the modeled eddies. The results demonstrate that mesoscale and submesoscale eddies are frequently generated on and pass through the lee side of Lanai Island and the statistical analysis quantifies the general features of eddies in the area.

Platform effects on optical variability and prediction of underwater visibility.

We present hydrographic and optical data collected concurrently from two different platforms, the R/P FLoating Instrument Platform and the R/V Kilo Moana, located about 2km apart in the Santa Barbara Channel in California. We show that optical variability between the two platforms was due primarily to platform effects, specifically the breakdown of stratification from mixing by the hull of R/P FLIP. Modeled underwater radiance distribution differed by as much as 50% between the two platforms during stratified conditions. We determine that the observed optical variability resulted in up to 57% differences in predicted horizontal visibility of a black target.

Diagnosing cross-shelf transport along an ocean front: an observational case study in the Gulf of Lion

Exchanges between coastal regions and the open ocean are often associated with intermittent and localized processes such as eddies, fronts and filaments. Since these features are difficult to observe, their impact has been predominantly investigated using numerical models and remote sensing. In this study, satellite sea surface temperature maps, Lagrangian surface drifter trajectories, and ship-based surveys of currents and hydrography from the Latex10 campaign are used to quantify cross-shelf exchanges associated with a temperature front in the western Gulf of Lion. Satellite imagery and thermosalinograph sections provide the characterization of the various water masses associated with the front. Lagrangian drifter trajectories are used to identify the main transport structures and to quantify the velocity components associated with near-inertial oscillations. These are removed from the instantaneous ADCP observations with which the cross-shelf exchanges are then computed. The results indicate an average outflow of 0.074 ±0.013 Sv and an inflow of 0.021 ±0.006 Sv. Integrated over the two-week lifetime of the front, such outflow induced a total export of ∼ 90u2009±u200914 km3 of water, indicating that 3 to 4 of such events are sufficient to completely renew the surface waters of the Gulf of Lion. The total import was ∼ 25u2009±u20097 km3, suggesting larger inflows at depth or in the eastern part of the gulf to maintain its volume balance. These in-situ estimates represent a key term of comparison for the further development of numerical model- and satellite-based studies of cross-shelf exchanges associated with this type of processes. This article is protected by copyright. All rights reserved.