Luis Zamudio

Surface-Generated Mesoscale Eddies Transport Deep-Sea Products from Hydrothermal Vents

Deep-reaching eddies transport heat and material hundreds of kilometers from the northern East Pacific Rise. Atmospheric forcing, which is known to have a strong influence on surface ocean dynamics and production, is typically not considered in studies of the deep sea. Our observations and models demonstrate an unexpected influence of surface-generated mesoscale eddies in the transport of hydrothermal vent efflux and of vent larvae away from the northern East Pacific Rise. Transport by these deep-reaching eddies provides a mechanism for spreading the hydrothermal chemical and heat flux into the deep-ocean interior and for dispersing propagules hundreds of kilometers between isolated and ephemeral communities. Because the eddies interacting with the East Pacific Rise are formed seasonally and are sensitive to phenomena such as El Niño, they have the potential to introduce seasonal to interannual atmospheric variations into the deep sea.

ENSO and eddies on the southwest coast of Mexico

TOPEX/POSEIDON and ERS-2 (T/ERS) sea surface height altimeter observations and the Naval Research Laboratory Layered Ocean Model (NLOM) are used to study the circulation along the southwest coast of Mexico. The results of this research indicate that strong El Nino/Southern Oscillation (ENSO) warm phase Kelvin waves (KW) destabilize the upper ocean circulation. The effect of ENSO appears as three distinct stages. First, a coastal jet characterized by strong vertical shear flow develops. Second, the shear flow strengthens, increasing its horizontal dimension and the amplitude of its oscillations. Finally, the jet becomes unstable and breaks into anticyclonic eddies, which separate from the coast and drift southwestward. The genesis and strengthening of the jet is due to the simultaneous occurrence of the poleward-flowing currents along the southwest coast of Mexico and the poleward circulation associated with ENSO downwelling KW.

Noninertial flow in NSCAT observations of Tehuantepec winds

NASA scatterometer (NSCAT) observations provide high temporal and spatial resolution wind fields, which are used to examine gap flow through the Chivela Pass and the influence of Hurricane Marco. For approximately 1 week in November 1996, Hurricane Marco caused the flow through the gap to change from its usual flow pattern: the winds turned to the left rather than the right. Previous studies of this gap flow have used monthly averages of sparse ship observations, European Centre for Medium-Range Weather Forecasts fields, mesoscale models, or proxies such as cloud motion or sea surface temperatures (SSTs). NSCAT provided unprecedented accuracy (rms differences less than 1.5 m s−1 and 15°) and resolution (daily and 1° × 1° in this study). The observations show that the gap flow often turns to the right (dominated by the Coriolis force); however, unusual events can cause highly noninertial flow for several days. The SSTs respond slowly to changes in the wind pattern, and they are a poor proxy for short-term wind patterns. The NSCAT winds, gridded with a new methodology presented herein, have the spatial and temporal resolution required to show the evolution of the gap outflow in the presence of a hurricane. The wind fields are used to generate parcel pseudotrajectories, which show that Marco caused Tehuantepec winds to turn to the left and pass through the Gulf of Papagayo into the Caribbean Sea. Gridding techniques are required to fill gaps in fields of NSCAT observations. NSCAT coverage of the (ice free) global oceans is approximately 77% in 1 day and 90% in 2 days. The new gridding technique temporally averages the winds in a manner that leaves very little evidence of the pattern of satellite tracks in the wind fields and little evidence of wind field curl and divergence. Daily fields of 1° × 1° resolution are generated. The temporal sampling characteristics of the wind fields are shown to be nonhomogeneous, with the distribution of characteristic sampling times peaking at 1 day and usually within the range of 0.75 to 1.75 days.

Dynamical Evaluation of Ocean Models Using the Gulf Stream as an Example

The Gulf Stream is the focus of an effort aimed at dynamical understanding and evaluation of current systems simulated by eddy-resolving Ocean General Circulation Models (OGCMs), including examples with and without data assimilation and results from four OGCMs (HYCOM, MICOM, NEMO, and POP), the first two including Lagrangian isopycnal coordinates in the vertical and the last two using fixed depths. The Gulf Stream has been challenging to simulate and understand. While different non-assimilative models have at times simulated a realistic Gulf Stream pathway, the simulations are very sensitive to small changes, such as subgrid-scale parameterizations and parameter values. Thus it is difficult to obtain consistent results and serious flaws are often simulated upstream and downstream of Gulf Stream separation from the coast at Cape Hatteras. In realistic simulations, steering by a key abyssal current and a Gulf Stream feedback mechanism constrain the latitude of the Gulf Stream near 68.5°W. Additionally, the Gulf Stream follows a constant absolute vorticity (CAV) trajectory from Cape Hatteras to ~70°W, but without the latitudinal constraint near 68.5°W, the pathway typically develops a northern or southern bias. A shallow bias in the southward abyssal flow of the Atlantic Meridional Overturning Circulation (AMOC) creates a serious problem in many simulations because it results in abyssal currents along isobaths too shallow to feed into the key abyssal current or other abyssal currents that provide a similar pathway constraint. Pathways with a southern bias are driven by a combination of abyssal currents crossing under the Gulf Stream near the separation point and the increased opportunity for strong flow instabilities along the more southern route. The associated eddy-driven mean abyssal currents constrain the mean pathway to the east. Due to sloping topography, flow instabilities are inhibited along the more northern routes west of ~69°W, especially for pathways with a northern bias. The northern bias occurs when the abyssal current steering constraint needed for a realistic pathway is missing or too weak and the simulation succumbs to the demands of linear dynamics for an overshoot pathway. Both the wind forcing and the upper ocean branch of the AMOC contribute to those demands. Simulations with a northern pathway bias were all forced by a wind product particularly conducive to that result and they have a strong or typical AMOC transport with a shallow bias in the southward flow. Simulations forced by the same wind product (or other wind products) that have a weak AMOC with a shallow bias in the southward limb exhibit Gulf Stream pathways with a southern bias. Data assimilation has a very positive impact on the model dynamics by increasing the strength of a previously weak AMOC and by increasing the depth range of the deep southward branch. The increased depth range of the southward branch generates more realistic abyssal currents along the continental slope. This result in combination with vortex stretching and compression generated by the data-assimilative approximation to meanders in the Gulf Stream and related eddies in the upper ocean yield a model response that simulates the Gulf Stream-relevant abyssal current features seen in historical in situ observations, including the key abyssal current near 68.5°W, a current not observed in the assimilated data set or corresponding simulations without data assimilation. In addition, the model maintains these abyssal currents in a mean of 48 14-day forecasts, but does not maintain the strength of the Gulf Stream east of the western boundary.

Toward An Internal Gravity Wave Spectrum In Global Ocean Models

Abstract : High-resolution global ocean models forced by atmospheric fields and tides are beginning to display realistic internal gravity wave spectra, especially as model resolution increases. This paper examines internal waves in global simulations with 0.08 Degrees and 0.04 Degrees (approximately 8 and 4 km) horizontal resolutions, respectively. Frequency spectra of internal wave horizontal kinetic energy in the North Pacific lie closer to observations in the 0.04 Degrees simulation than in the 0.08 Degrees simulation. The horizontal wave number and frequency (K- omega) kinetic energy spectra contain peaks in the semidiurnal tidal band and near-inertial band, along with a broadband frequency continuum aligned along the linear dispersion relations of low-vertical-mode internal waves. Spectral kinetic energy transfers describe the rate at which nonlinear mechanisms remove or supply kinetic energy in specific K-omega ranges. Energy is transferred out of low-mode inertial and semidiurnal internal waves into a broad continuum of higher-frequency and higher-wave number internal waves.

On the effect of the alongshore pressure gradient on numerical simulations over the Northern California Continental Shelf

Chen and Wangs (1990) two-dimensional shelf circulation model has been extended to include the alongshore (the dimension absent in the model) pressure gradient. The alongshore pressure gradient, calculated independently from a linear coastal-trapped wave model, is included as an external “forcing”. The extended model has been used to simulate the temperature and velocity fields observed during the Coastal Ocean Dynamics Experiment 2 (CODE 2) in the spring and summer of 1982 on the Northern California Continental Shelf. The results were compared to the CODE 2 observations and to the results of the purely two-dimensional and coastal trapped wave models. Results using the extended model show some improvement in the cross-shore and alongshore velocity fluctuations. The differences between the models are explained in terms of the opposition between the wind stress and the pressure gradient in the alongshore direction, which reduces the magnitude of the alongshore velocity. The reduced magnitude of the alongshore flow decreases the alongshore bottom stress which, in turn, reduces the offshore transport in the bottom boundary layer.

Semidiurnal Internal Tide Energy Fluxes and Their Variability in a Global Ocean Model and Moored Observations

We examine the temporal means and variability of the semidiurnal internal tide energy fluxes in 1/25° global simulations of the Hybrid Coordinate Ocean Model (HYCOM) and in a global archive of 79 historical moorings. Low-frequency flows, a major cause of internal tide variability, have comparable kinetic energies at the mooring sites in model and observations. The computed root-mean-square (RMS) variability of the energy flux is large in both model and observations and correlates positively with the time-averaged flux magnitude. Outside of strong generation regions, the normalized RMS variability (the RMS variability divided by the mean) is nearly independent of the flux magnitudes in the model, and of order 23% or more in both the model and observations. The spatially averaged flux magnitudes in observations and the simulation agree to within a factor of about 1.4 and 2.4 for vertical mode-1 and mode-2, respectively. The difference in energy flux computed from the full-depth model output versus model output subsampled at mooring instrument depths is small. The global historical archive is supplemented with six high-vertical resolution moorings from the Internal Waves Across the Pacific (IWAP) experiment. The model fluxes agree more closely with the high-resolution IWAP fluxes than with the historical mooring fluxes. The high variability in internal tide energy fluxes implies that internal tide fluxes computed from short observational records should be regarded as realizations of a highly variable field, not as “means” that are indicative of conditions at the measurement sites over all time.

Frequency content of sea surface height variability from internal gravity waves to mesoscale eddies

High horizontal-resolution (1=12:5 and 1=25 ) 41-layer global simulations of the HYbrid Coordinate Ocean Model (HYCOM), forced by both atmospheric fields and the astronomical tidal potential, are used to construct global maps of sea surface height (SSH) variability. The HYCOM output is separated into steric and nonsteric and into subtidal, diurnal, semidiurnal, and supertidal frequency bands. The model SSH output is compared to two data sets that offer some geographical coverage and that also cover a wide range of frequencies—a set of 351 tide gauges that measure full SSH and a set of 14 in situ vertical profilers from which steric SSH can be calculated. Three of the global maps are of interest in planning for the upcoming Surface Water and Ocean Topography (SWOT) two-dimensional swath altimeter mission: (1) maps of the total and (2) nonstationary internal tidal signal (the latter calculated after removing the stationary internal tidal signal via harmonic analysis), with an average variance of 1:05 and 0:43 cm2, respectively, for the semidiurnal band, and (3) a map of the steric supertidal contributions, which are dominated by the internal gravity wave continuum, with an average variance of 0:15 cm2. Stationary internal tides (which are predictable), nonstationary internal tides (which will be harder to predict), and nontidal internal gravity waves (which will be very difficult to predict) may all be important sources of high-frequency ‘‘noise’’ that could mask lower frequency phenomena in SSH measurements made by the SWOT mission.

Impact of the Madden–Julian Oscillation on the Indonesian Throughflow in the Makassar Strait during the CINDY/DYNAMO Field Campaign

AbstractPrevious studies indicate that equatorial zonal winds in the Indian Ocean can significantly influence the Indonesian Throughflow (ITF). During the Cooperative Indian Ocean Experiment on Intraseasonal Variability (CINDY)/Dynamics of the Madden–Julian Oscillation (DYNAMO) field campaign, two strong MJO events were observed within a month without a clear suppressed phase between them, and these events generated exceptionally strong ocean responses. Strong eastward currents along the equator in the Indian Ocean lasted more than one month from late November 2011 to early January 2012. The influence of these unique MJO events during the field campaign on ITF variability is investigated using a high-resolution (1/25°) global ocean general circulation model, the Hybrid Coordinate Ocean Model (HYCOM). The strong westerlies associated with these MJO events, which exceed 10 m s−1, generate strong equatorial eastward jets and downwelling near the eastern boundary. The equatorial jets are realistically simulat...

The impact of high-frequency current variability on dispersion off the eastern Antarctic Peninsula

We present observations of high-frequency current variability on the continental shelf and the slope of the Antarctic Peninsula using Lagrangian surface drifters deployed as part of the Antarctic Drifter Experiment: Links to Isobaths and Ecosystems (ADELIE) project. Here we focus on high-frequency processes such as tides and inertial oscillations that are typically smoothed out of large-scale spatially averaged, and/or temporally averaged, observed current fields. We investigate the role that this class of motion plays in the transport of physical or biogeochemical properties. Lateral displacements on the shelf and slope are found to be larger than displacements in deeper waters where tidal currents are negligible. We apply this result in a parameterization of the lateral dispersion during an off-line drifter modeling study. The outcome is an improvement on the modeling of Lagrangian drifting particles compared with a standard random walk scheme.