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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Baroclinic Residual Circulation and Mass Transport Due to Internal Tides

Baroclinic (BC) tidal residual circulation due to internal tides is investigated around islands over a shallow ridge using a numerical ocean model. Internal tides enhance vertical mixing over shallow slopes, leading to horizontal density gradients that drive BC residual circulation along the main thermocline. For a strongly stratified summer case, the vertical diffusivity estimated by the Mellor and Yamada turbulence closure model exceeds 1 × 10 -2 m 2 s -1 , and the velocity of BC residual circulations reaches 0.2 m s -1 . The magnitude of BC residual circulation is larger than that of barotropic residual tidal circulation, implying that BC residual circulation due to internal tides plays an important role in coastal ocean circulation. Furthermore, BC residual flow accounts for an equal percentage (5%) of the total tidal kinetic energy as the barotropic residual flow under summer stratification conditions. Results from a coupled sediment resuspension and transport model show the growth of intermediate nepheloid layers formed by strong bottom shear stress and BC residual flow. The residual component contributes largely (23% of the total in summer) to the total suspended sediment flux. Seasonal variability is explored, with weaker winter stratification leading to reduced mixing, and thus weaker BC residual circulation and sediment flux. Lastly, the magnitude of the BC residual circulation is also shown to be proportional to the square of the tidal amplitude.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2022

Export of Ice Sheet Meltwater from Upernavik Fjord, West Greenland

Meltwater from Greenland is an important freshwater source for the North Atlantic Ocean, released into the ocean at the head of fjords in the form of runoff, submarine melt, and icebergs. The meltwater release gives rise to complex in-fjord transformations that result in its dilution through mixing with other water masses. The transformed waters, which contain the meltwater, are exported from the fjords as a new water mass Glacially Modified Water (GMW). Here we use summer hydrographic data collected from 2013 to 2019 in Upernavik, a major glacial fjord in northwest Greenland, to describe the water masses that flow into the fjord from the shelf and the exported GMWs. Using an optimum multi-parameter technique across multiple years we then show that GMW is composed of 57.8% ± 8.1% Atlantic Water (AW), 41.0% ± 8.3% Polar Water (PW), 1.0% ± 0.1% subglacial discharge, and 0.2% ± 0.2% submarine meltwater. We show that the GMW fractional composition cannot be described by buoyant plume theory alone since it includes lateral mixing within the upper layers of the fjord not accounted for by buoyant plume dynamics. Consistent with its composition, we find that changes in GMW properties reflect changes in the AW and PW source waters. Using the obtained dilution ratios, this study suggests that the exchange across the fjord mouth during summer is on the order of 50 mSv (1 Sv ≡ 10 6 m 3 s –1 ) (compared to a freshwater input of 0.5 mSv). This study provides a first-order parameterization for the exchange at the mouth of glacial fjords for large-scale ocean models.

58 GEOSCIENCES↗
NASA NTRS1985

A theory for El Nino and the Southern Oscillation

A coupled atmosphere-ocean model is presented for El Nino and the Southern Oscillation that reproduces its major features, including its recurrence at irregular intervals. The interannual El Nino-Southern Oscillation cycle is maintained by deterministic interactions in the tropical Pacific region. Ocean dynamics alter sea-surface temperature, changing the atmospheric heating; the resulting changes in surface wind alter the ocean dynamics. Annually varying mean conditions largely determine the spatial pattern and temporal evolution of El Nino events.

Cane, M. A.↗
NASA NTRS2000

Southern Ocean Response to NADW Changes

The possibility of North Atlantic Deep Water (NADW) changes in both past and future climates has raised the issue of how the Southern Ocean would respond. Recent experiments with the GISS coupled atmosphere-ocean model have shown that a "bipolar see-saw" between NADW production and Antarctic Bottom Water (AABW) production in the Weddell Sea can occur in conjunction with freshening of the North Atlantic. However, this effect operates not through a slow ocean response but via a rapid atmospheric mechanism. As NADW reduces, colder temperatures in the North Atlantic, and Northern Hemisphere in general, are associated with higher surface pressure (increased atmospheric mass). Reduced mass in the Southern Hemisphere occurs in response, with lower pressure over the South Pole (an EOF #1 effect, the "high phase" of the Antarctic Oscillation).The lower pressure is associated with stronger west winds that generate an intensified Antarctic Circumpolar Current (ACC), which leads to longitudinal heat divergence in the South Atlantic (and heat convergence in the Southern Indian Ocean). Colder temperatures in the Weddell Sea region lead to sea ice growth, increased salinity and surface water density, and greater Weddell Sea Bottom Water production. Increased poleward transport of heat occurs in the South Atlantic in conjunction with increased bottom water production, but its convergence at high latitudes is not sufficient to offset the longitudinal heat divergence due to the intensified ACC. The colder temperatures at high latitudes in the South Atlantic increase the latitudinal temperature gradient, baroclinic instability, eddy energy and eddy poleward transport of momentum, helping to maintain the lower pressure over the pole in an interactive manner. The heat flux convergence in the Indian Ocean provides a warming tendency in that region, and overall global production of AABW remains unchanged. These results have implications for the interpretation of the ice core records of the last deglaciation, but may also be relevant for changes during the Holocene and perhaps even in response to increased CO2 forcing,

Rind, David↗
NASA NTRS2014

Petrology and Geochemistry of Unbrecciated Harzburgitic Diogenite MIL 07001: A Window Into Vestan Geological Evolution

There is a strong case that asteroid 4 Vesta is the parent of the howardite, eucrite and diogenite (HED) meteorites. Models developed for the geological evolution of Vesta can satisfy the compositions of basaltic eucrites that dominate in the upper crust. The bulk compositional characteristics of diogenites - cumulate harzburgites and orthopyroxenites from the lower crust - do not fit into global magma ocean models that can describe the compositions of basaltic and cumulate eucrites. Recent more detailed formation models do make provision for a more complicated origin for diogenites, but this model has yet to be completely vetted. Compositional studies of bulk samples has led to the hypothesis that many diogenites were formed late by interaction of their parent melts with a eucritic crust, but those observations may alternatively be explained by subsolidus equilibration of trace elements between orthopyroxene and plagioclase and Ca-phosphate in the rocks. Differences in radiogenic Mg-26 content between diogenites and eucrites favors early formation of the former, not later formation. Understanding the origin of diogenites is crucial for understanding the petrologic evolution of Vesta. We have been doing coordinated studies of a suite of diogenites including petrologic investigations, bulk rock major and trace element studies, and in situ trace element analyses of orthopyroxene. Here we will focus on an especially unusual, and potentially key, diogenite, MIL 07001.

Mittlefehldt, D. W.↗
NASA NTRSDate not supplied

High-Resolution Global Ocean Simulation in the SWOT ERA: Introducing ECCO Llc4320v2

The Surface Water and Ocean Topography (SWOT) mission is opening up new opportunities to evaluate and drive improvement in km-scale global ocean and atmosphere-ocean models via its simultaneous swath observations of sea-surface height, wind speed, and significant wave height down to 2 km scales globally. Motivated by this, we have initiated a collaborative effort at NASA Ames to run a new global ocean-sea-ice simulation in MITgcm at a nominal 1/48-deg resolution of the year 2023 that enables direct comparisons with SWOT observations. New features relative to a prior simulation at a similar resolution of the year 2012 include much more realistic tidal forcing, thermodynamic coupling in ice-shelf cavities, high-frequency hourly atmospheric forcing, much improved river runoff, double the vertical resolution in the deep ocean (173 vs 90 levels), updated bathymetry including improvements in finescale bathymetry derived from SWOT, and updated vertical mixing scheme parameters that improve the equatorial Pacific circulation.

ECCO↗
NASA NTRS2017

GEOS Atmospheric Model: Challenges at Exascale

The Goddard Earth Observing System (GEOS) model at NASA's Global Modeling and Assimilation Office (GMAO) is used to simulate the multi-scale variability of the Earth's weather and climate, and is used primarily to assimilate conventional and satellite-based observations for weather forecasting and reanalysis. In addition, assimilations coupled to an ocean model are used for longer-term forecasting (e.g., El Nino) on seasonal to interannual times-scales. The GMAO's research activities, including system development, focus on numerous time and space scales, as detailed on the GMAO website, where they are tabbed under five major themes: Weather Analysis and Prediction; Seasonal-Decadal Analysis and Prediction; Reanalysis; Global Mesoscale Modeling, and Observing System Science. A brief description of the GEOS systems can also be found at the GMAO website. GEOS executes as a collection of earth system components connected through the Earth System Modeling Framework (ESMF). The ESMF layer is supplemented with the MAPL (Modeling, Analysis, and Prediction Layer) software toolkit developed at the GMAO, which facilitates the organization of the computational components into a hierarchical architecture. GEOS systems run in parallel using a horizontal decomposition of the Earth's sphere into processing elements (PEs). Communication between PEs is primarily through a message passing framework, using the message passing interface (MPI), and through explicit use of node-level shared memory access via the SHMEM (Symmetric Hierarchical Memory access) protocol. Production GEOS weather prediction systems currently run at 12.5-kilometer horizontal resolution with 72 vertical levels decomposed into PEs associated with 5,400 MPI processes. Research GEOS systems run at resolutions as fine as 1.5 kilometers globally using as many as 30,000 MPI processes. Looking forward, these systems can be expected to see a 2 times increase in horizontal resolution every two to three years, as well as less frequent increases in vertical resolution. Coupling these resolution changes with increases in complexity, the computational demands on the GEOS production and research systems should easily increase 100-fold over the next five years. Currently, our 12.5 kilometer weather prediction system narrowly meets the time-to-solution demands of a near-real-time production system. Work is now in progress to take advantage of a hybrid MPI-OpenMP parallelism strategy, in an attempt to achieve a modest two-fold speed-up to accommodate an immediate demand due to increased scientific complexity and an increase in vertical resolution. Pursuing demands that require a 10- to 100-fold increases or more, however, would require a detailed exploration of the computational profile of GEOS, as well as targeted solutions using more advanced high-performance computing technologies. Increased computing demands of 100-fold will be required within five years based on anticipated changes in the GEOS production systems, increases of 1000-fold can be anticipated over the next ten years.

ESMF↗
NASA NTRS1995

Comparison of tropical pacific temperature and current simulations with two vertical mixing schemes embedded in an ocean general circulation model and reference to observations

The Pacanowski-Philander (PP) and Mellor-Yamada (MY) parameterization models of vertical mixing by turbulent processes were embedded in the Geophysical Fluid Dynamics Laboratory high-resolution ocean general circulation model of the tropical Pacific Ocean. All other facets of the numerical simulations were the same. Simulations were made for the 1987-1988 period. At the equator the MY simulation produced near-surface temperatures more uniform with depth, a deeper thermocline, a deeper core speed of the Equatorial Undercurrent, and a South Equatorial Current with greater vertical thickness compared with that computed with the PP method. Along 140 deg W, between 5 deg N and 10 deg N, both simulations were the same. Moored buoy current and temperature observations had been recorded by the Pacific Marine Environmental Laboratory at three sites (165 deg E, 140 deg W, 110 deg W) along the equator and at three sites (5 deg N, 7 deg N, 9 deg N) along 140 deg W. Simulated temperatures were lower than those observed in the near-surface layer and higher than those observed in the thermocline. Temperature simulations were in better agreement with observations compared to current simulations. At the equator, PP current and temperature simulations were more representative of the observations than MY simulations.

Halpern, David↗
NASA NTRS1994

Geosat-derived sea level and surface current anomalies in the equatorial Pacific during the 1986-1989 El Nino and La Nina

Equatorial wave dynamics are essential in most oceanic models for reproducing aspects of the El Nino-Southern Oscillation (ENSO) phenomenon. Observational evidence of first baroclinic equatorial Kelvin and first symmetric meridional Rossby waves is found in Geosat-derived sea level anomalies (SLA) and surface zonal current anomalies (ZCA) in the equatorial Pacific ocean during the 1986-1987 El Nino and the ensuing 1988-1989 La Nina. This was made possible after extensive quality control and specific processing of the recently improved Geosat geophysical data records pertaining to the 17-day Exact Repeat Mission. In particular, the processing was made so that the Geosat-derived ZCA best fit near-surface zonal currents from three equatorial moorings at 165 deg E, 140 deg W, and 110 deg W. The Geosat-derived SLA and ZCA are decomposed into first baroclinic equatorial Kelvin and gravest Rossby modes. The emphasis is then put on the chronology of the ZCA at the equator where the currents are the most energetic and where Kelvin and first symmetric Rossby waves explain most of the variance in ZCA, in similar proportion. The 1986-1987 El Nino is mostly characterized by a strong downwelling Kelvin wave in December 1986, a series of downwelling Kelvin waves in March-October 1987, and a strong upwelling Rossby wave in March-September 1987. These waves are consistent with wind forcing, and all give rise to notable eastward ZCA for almost an entire year. During El Nino-La Nina transition period there is the occurrence of two downwelling Rossby waves originating from the eastern Pacific at times of favorable wind forcing. These Rossby waves switch the basin-wide ZCA from eastward to westward, terminating the warm event. Then, the 1988-1989 La Nina shows the generation of a series of upwelling Kelvin waves which are clearly identified in March/April and September/October 1988. These upwelling Kelvin waves are apparently related to the enhencement of the trade wind in the western half of the basin. These waves maintain the basin-wide westward ZCA for more than a year, although the ZCA is slightly reduced ba front of upwelling Rossby waves in phase with the normal seasonal cycle. The Kelvin wave forcing and sign of kelvin wave contribution versus Rossby wave contribution to ZCA in the western Pacific are discussed, and we conclude that neither pleads in favor of the delayed action oscillator mechanism.

Delcroix, Thierry↗
NASA NTRSDate not supplied

Global Climate Impacts of Greenland and Antarctic Glacial Melt: A Comparative Study

A 10-member ensemble simulation using the NASA GISS-E2-1-G climate model shows a clear bifurcation in the Atlantic Meridional Overturning Circulation (AMOC) strength under the SSP2-4.5 future scenario, leading to 8 stable AMOC-on and 2 stable AMOC-off climate states after 400 years of integration, despite identical forcing of each ensemble member. Ocean models have previously shown a tipping point where sufficient forcing (e.g. by freshwater inputs) can shut down the AMOC. A variety of fully coupled models have demonstrated this, either through hosing experiments or increased precipitation and warming at high latitudes due to increased concentrations of greenhouse gases. In the GISS simulations, there are no external freshwater perturbations. The bifurcation arises freely in the coupled system and is the result of stochastic variability associated with sea-ice transport and melting in the Irminger Sea following a slowing of the GHG concentration increase at the end of the twenty-first century. We believe this is the first time that a CMIP-class model has shown such a bifurcation across an initial condition ensemble.

GISS ModelE2.1↗
DOE OSTI2021

FVCOM-ICM4.0

FVCOM-ICM is a water quality model that simulates the influence of nutrients and carbon pollution on biogeochemical oceanographic processes such as phytoplankton primary productivity, nutrient consumption, mineralization, decay and settling of particulate organic matter and computes resulting dissolved oxygen (DO) concentrations and pH levels. An “offline” approach of coupling Finite Volume Coastal Ocean Model (FVCOM) hydrodynamic solutions to CE-QUAL-ICM biogeochemical model was used. FVCOM and CE-QUAL-ICM are public domain Fortran Codes that were developed independently by US Army Corps of Engineers and University of Massachusetts. PNNL developed FVCOM-ICM that has the ability to conduct water quality calculations using a previously computed hydrodynamic solution. The biogeochemical calculations using CE-QUAL-ICM kinetics are conducted over the same finite-volume mesh and in the FVCOM framework.

Khangaonkar, Tarang↗
NASA NTRS1995

On a generating mechanism for Yanai waves and the 25-day oscillation

A spectral Chebyshev-collocation method applied to the linear, 1.5 layer reduced-gravity ocean model equations is used to study the dynamics of Yanai (or mixed Rossby-gravity) wave packets. These are of interest because of the observations of equatorial instability waves (which have the characteristics of Yanai waves) and their role in the momentum and heat budgets in the tropics. A series of experiments is performed to investigate the generation of the waves by simple cross-equatorial wind stress forcings in various configurations and the influence of a western boundary on the waves. They may be generated in the interior ocean as well as from a western boundary. The observations from all the oceans indicate that the waves have a preferential period and wavelength of around 25 days and 1000 km respectively. These properties are also seen in the model results and a plausible explanation is provided as being due to the dispersive properties of Yanai waves.

Kelly, Brian G.↗
NASA NTRS2012

Sensitivity of Ocean Reflectance Inversion Models for Identifying and Discriminating Between Phytoplankton Functional Groups

The daily, synoptic images provided by satellite ocean color instruments provide viable data streams for observing changes in the biogeochemistrY of marine ecosystems. Ocean reflectance inversion models (ORMs) provide a common mechanism for inverting the "color" of the water observed a satellite into marine inherent optical properties (lOPs) through a combination of empiricism and radiative transfer theory. lOPs, namely the spectral absorption and scattering characteristics of ocean water and its dissolved and particulate constituents, describe the contents of the upper ocean, information critical for furthering scientific understanding of biogeochemical oceanic processes. Many recent studies inferred marine particle sizes and discriminated between phytoplankton functional groups using remotely-sensed lOPs. While all demonstrated the viability of their approaches, few described the vertical distributions of the water column constituents under consideration and, thus, failed to report the biophysical conditions under which their model performed (e.g., the depth and thickness of the phytoplankton bloom(s)). We developed an ORM to remotely identifY Noctiluca miliaris and other phytoplankton functional types using satellite ocean color data records collected in the northern Arabian Sea. Here, we present results from analyses designed to evaluate the applicability and sensitivity of the ORM to varied biophysical conditions. Specifically, we: (1) synthesized a series of vertical profiles of spectral inherent optical properties that represent a wide variety of bio-optical conditions for the northern Arabian Sea under aN Miliaris bloom; (2) generated spectral remote-sensing reflectances from these profiles using Hydrolight; and, (3) applied the ORM to the synthesized reflectances to estimate the relative concentrations of diatoms and N Miliaris for each example. By comparing the estimates from the inversion model to those from synthesized vertical profiles, we were able to identifY those bio-optical conditions under which the inversion model performs both well and poorly.

Werdell, P. Jeremy↗
NASA NTRS1991

Net diffusivity in ocean general circulation models with nonuniform grids

The numerical vertical diffusivity K(num), embedded in a numerical ocean general circulation model with nonuniform vertical grid, is estimated. It is shown that in a downwelling region, K(num) is negative for a grid with grid size increasing with depth. When the grid size increment, or the downward vertical velocity, is large, K(num) may exceed the vertical diffusivity specified and may result in a negative effective vertical diffusivity. Therefore care needs to be taken to specify the vertical diffusivity in a numerical model with nonuniform grid, and a lower bound is generally imposed in order to avoid an unphysical negative value. Some possible effects of the negative effective diffusivity are discussed.

Yin, F. L.↗
DOE OSTI2021

Coupled Ocean Wave-Atmosphere Models for Offshore Wind Energy Applications

Coupling ocean wave models to mesoscale atmospheric models is necessary to represent the effect of waves on wind turbine hub-height winds. In this report we provide a review of the most widely used ocean waves models and the phased-averaged spectral wave modeling paradigm that they are based on. Methodologies used to couple these wave models to mesoscale atmospheric models are described along with details of existing coupled modeling systems. We summarize impacts on offshore wind resource assessments that have been investigated with such coupled modeling systems to-date. Specifically, coupling in the North and Baltic Seas was shown to have a small negative effect on the offshore wind energy resource at weak-to-moderate wind speeds, with little impact at higher wind speeds. Finally, limitations of these existing coupled modeling systems and impact assessments are discussed, including examples of and the potential for improved parameterizations of the air-sea fluxes, and the use of fine-scaled simulations using phased-resolved wave models.

17 WIND ENERGY↗
NASA NTRS2005

Effects of High-frequency Wind Sampling on Simulated Mixed Layer Depth and Upper Ocean Temperature

Effects of high-frequency wind sampling on a near-global ocean model are studied by forcing the model with a 12 hourly averaged wind product and its 24 hourly subsamples in separate experiments. The differences in mixed layer depth and sea surface temperature resulting from these experiments are examined, and the underlying physical processes are investigated. The 24 hourly subsampling not only reduces the high-frequency variability of the wind but also affects the annual mean wind because of aliasing. While the former effect largely impacts mid- to high-latitude oceans, the latter primarily affects tropical and coastal oceans. At mid- to high-latitude regions the subsampled wind results in a shallower mixed layer and higher sea surface temperature because of reduced vertical mixing associated with weaker high-frequency wind. In tropical and coastal regions, however, the change in upper ocean structure due to the wind subsampling is primarily caused by the difference in advection resulting from aliased annual mean wind, which varies with the subsampling time. The results of the study indicate a need for more frequent sampling of satellite wind measurement and have implications for data assimilation in terms of identifying the nature of model errors.

mixed layered depth↗
NASA NTRS2021

Diagnosing Seasonal to Multi-decadal Phytoplankton Group Dynamics in a Highly Productive Coastal Ecosystem

The Santa Barbara Channel, CA (SBC) is a biodiverse marine ecosystem fueled largely by phytoplankton productivity, and the composition of the phytoplankton community influences the magnitude and fates of this productivity. Here, we create a 22-year monthly time series of phytoplankton biomarker pigment concentrations in the SBC by combining 12 years of high performance liquid chromatography phytoplankton pigment concentrations with bio-optical models and 10 additional years of bio-optical observations. The bio-optical models skillfully predict biomarker pigment concentrations representative of five distinct phytoplankton groups (PGs; diatoms, dinoflagellates, chlorophytes, prymnesiophytes, and picophytoplankton) and resolve seasonal responses to the annual upwelling-relaxation cycle for all PGs except the dinoflagellates. Our observations indicate that nanophytoplankton groups respond most rapidly to seasonal upwelling, followed by diatoms, and then by picophytoplankton as the water column stratifies in the summer. A Regional Ocean Modeling System (ROMS) solution is used to relate advection of different source waters to the observed PG dynamics. The ROMS simulation results suggest that, on seasonal time scales, pronounced cross-SBC differences in PG seasonality are related to cross-SBC differences in source waters. El Niño Southern Oscillation events drive interannual variability in the upwelling response of most PGs. On decadal time scales, dinoflagellate blooms are associated with the warm phase of the North Pacific Gyre Oscillation and anomalous advection of Southern California Bight source waters into the SBC. Taken together, our results provide a novel view of phytoplankton community succession in response to seasonal upwelling by considering the dynamics of pico- and nano-phytoplankton and suggest that regional surface ocean advection plays a substantial role in driving phytoplankton composition in the SBC.

Phytoplankton↗