Astronomy

Gravitational eddies across the galaxy?

Gravitational eddies across the galaxy?

I read about natural gravitational eddies that travel in a wave that black holes have. They also have a strong magnetic field. Does those eddies follow magnetic field lines of the rotating black hole? Can a galaxies have their own gravitational eddies like a black hole does?


NASA’s S-MODE Takes to the Air and Sea to Study Ocean Eddies

After being delayed over a year due to the pandemic, a NASA field campaign to study the role of small-scale whirlpools and ocean currents in climate change is taking flight and taking to the seas in May 2021.

Using scientific instruments aboard a self-propelled ocean glider and several airplanes, this first deployment of the Sub-Mesoscale Ocean Dynamics Experiment (S-MODE) mission will deploy its suite of water- and air-borne instruments to ensure that they work together to show what’s happening just below the ocean’s surface. The full-fledged field campaign will begin in October 2021, with the aircraft based out of NASA’s Ames Research Center in Mountain View, California.

“This campaign in May is largely to compare different ways of measuring ocean surface currents so that we can have confidence in those measurements when we get to the pilot in October,” said Tom Farrar, associate scientist at the Woods Hole Oceanographic Institution in Massachusetts and principal investigator for S-MODE.

The S-MODE team hopes to learn more about small-scale movements of ocean water such as eddies. These whirlpools span about 6.2 miles or ten kilometers, slowly moving ocean water in a swirling pattern. Scientists think that these eddies play an important role in moving heat from the surface to the ocean layers below, and vice versa. In addition, the eddies may play a role in the exchange of heat, gases and nutrients between the ocean and Earth’s atmosphere. Understanding these small-scale eddies will help scientists better understand how Earth’s oceans slow down global climate change.


Sub-mesoscale ocean dynamics, like eddies and small currents, are responsible for the swirling pattern of these phytoplankton blooms (shown in green and light blue) in the South Atlantic Ocean on Jan. 5, 2021.
Credits: NASA’s Goddard Space Flight Center Ocean Color, using data from the NOAA-20 satellite and the joint NASA-NOAA Suomi NPP satellite.

A Self-Powered Surfboard, for Science!

The team is using a self-propelled commercial Wave Glider decked out with scientific instruments that can study the ocean from its surface. The most important gadgets aboard are the acoustic Doppler current profilers, which use sonar to measure water speed and gather information about the how fast the currents and eddies are moving, and in which direction. The glider also carries instruments to measure wind speed, air temperature and humidity, water temperature and salinity, and light and infrared radiation from the Sun.

“The wave glider looks like a surfboard with a big venetian blind under it,” said Farrar.

That “venetian blind” is submerged under the water, moving up and down with the ocean’s waves to propel the glider forward at about one mile per hour. In this way, the wave glider will be deployed from La Jolla, California, collecting data as it travels over 62 miles (100 kilometers) out into the ocean offshore of Santa Catalina Island.
Decked out with solar panels and several scientific instruments, the wave glider will propel itself from Santa Catalina Island farther out to sea.

Laurent Grare of the Scripps Institution of Oceanography prepares to recover a Wave Glider during a pre-deployment test. Decked out with solar panels and several scientific instruments, the wave glider will propel itself from Santa Catalina Island farther out to sea.
Credits: Courtesy of Benjamin Greenwood / Woods Hole Oceanographic Institution

The new data will allow the scientists to estimate the exchange of heat and gases between Earth’s atmosphere and the ocean, and consequently better understand global climate change.

“We know the atmosphere is heating up. We know the winds are speeding up. But we don’t really understand where all that energy is going,” said Ernesto Rodriguez, research fellow at NASA’s Jet Propulsion Laboratory in Pasadena, California, and deputy principal investigator for the airborne parts of S-MODE. It’s likely that this energy is going into the ocean, but the details of how that process works are still unknown. The team thinks that small-scale eddies may help move heat from the atmosphere to the deeper layers of the ocean.
Eyes and Scientific Instruments in the Skies

While the Wave Glider continues its slow trek across the ocean’s surface, several airplanes will fly overhead to collect data from a different vantage.

“In an airplane, we can get a snapshot of a large area to see the context of how the bigger- and smaller-scale ocean movements interact,” said Rodriguez.

For example, a ship or wave glider travels slowly along a straight line, taking precise measurements of sea surface temperature at specific times and places. Airplanes move faster and can cover more ground, measuring the sea surface temperature of a large swath of ocean very quickly.

“It’s like taking an infrared image rather than using a thermometer,” explained Farrar.


A flight crew prepares for the B200 King Air Sub-Mesoscale Ocean Dynamics Experiment (S-MODE) at NASA’s Armstrong Flight Research Center in Edwards, California. From left to right are Jeroen Molemaker and Scott “Jelly” Howe.

Two planes will be used in the May test flights: a B200 plane from NASA’s Armstrong Flight Center in Edwards, California and a commercial plane from Twin Otter International. The B200 is carrying an instrument from NASA JPL called DopplerScatt to measure currents and winds near the ocean surface with radar. The Multiscale Observing System of the Ocean Surface (MOSES) instrument from the University of California, Los Angeles is also aboard to collect sea surface temperature data. On the Twin Otter plane is the Modular Aerial Sensing System (MASS) from the Scripps Institution of Oceanography at the University of California, San Diego, which is an instrument capable of measuring the height of waves on the surface of the ocean.


Delphine Hypolite, Multiscale Observing System of the Ocean Surface (MOSES) Operator from University of California Los Angeles, performs pre-flight checks on the MOSES Camera System at NASA’s Armstrong Flight Research Center in Edwards, California.

The fleet will gain a third member for the October experiments: NASA’s Langley Research Center Gulfstream III plane with JPL’s Portable Remote Imaging SpectroMeter (PRISM), an instrument to measure phytoplankton and other biological material in the water. The October deployments will also use a large ship and some autonomous sailing vessels, called Saildrones, in addition to planes and Wave Gliders.

After nearly a year and a half of delays due to the pandemic, the S-MODE team is excited to get their planes in the sky and the gliders in the water. “It was frustrating,” Rodriguez said, “but the science team hasn’t slowed down. The science keeps progressing.”

S-MODE is NASA’s ocean physics Earth Venture Suborbital-3 (EVS-3) mission, funded by the Earth System Science Pathfinder (ESSP) Program Office at NASA’s Langley Research Center in Hampton, Virginia, and managed by the Earth Science Project Office (ESPO) at Ames Research Center.

By Sofie Bates
NASA’s Earth Science News Team
Last Updated: May 19, 2021
Editor: Sofie Bates


Contents

A galaxy is a large gravitational aggregation of stars, dust, gas, and an unknown component termed dark matter. The Milky Way Galaxy [3] is only one of the billions of galaxies in the known universe. Galaxies are classified into spirals, [4] ellipticals, irregular, and peculiar. Sizes can range from only a few thousand stars (dwarf irregulars) to 10 13 stars in giant ellipticals. Elliptical galaxies are spherical or elliptical in appearance. Spiral galaxies range from S0, the lenticular galaxies, to Sb, which have a bar across the nucleus, to Sc galaxies which have strong spiral arms. In total count, ellipticals amount to 13%, S0 to 22%, Sa, b, c galaxies to 61%, irregulars to 3.5%, and peculiars to 0.9%.

At the center of most galaxies is a high concentration of older stars. This portion of a galaxy is called the nuclear bulge. Beyond the nuclear bulge lies a large disc containing young, hot stars, called the disk of the galaxy. There is a morphological separation: Ellipticals are most common in clusters of galaxies, and typically the center of a cluster is occupied by a giant elliptical. Spirals are most common in the field, i.e., not in clusters.

The primordial vorticity theory predicts that the spin vectors of galaxies are distributed primarily perpendicular to the cluster plane. [5] The primordial vorticity is called top-down scenario. Sometimes it is also called turbulence model. In the turbulence scenario, first flattened rotating proto-clusters formed due to cosmic vorticity in the early universe. Subsequent density and pressure fluctuations caused galaxies to form.

The idea that galaxy formation is initiated by primordial turbulence has a long history. Ozernoy (1971, 1978) proposes that galaxies form from high-density regions behind the shocks produced by turbulence. According to the primordial vorticity theory, the presence of large chaotic velocities generates turbulence, which, in turn, produces density and pressure fluctuations.

Density fluctuations on the scale of clusters of galaxies could be gravitationally bound, but galactic mass fluctuations are always unbound. Galaxies form when unbound galactic mass eddies, expanding faster than their bound cluster background. So forming galaxies collide with each other as clusters start to recollapse. These collisions produce shocks and high-density proto-galaxies at the eddy interfaces. As clusters recollapse, the system of galaxies undergoes a violent collective relaxation.

The pancake model was first proposed in the 1970s by Yakob B. Zel'dovich at the Institute of Applied Mathematics in Moscow. [6]

The pancake model predicts that the spin vectors of galaxies tend to lie within the cluster plane. In the pancake scenario, formation of clusters took place first and it was followed by their fragmentation into galaxies due to adiabatic fluctuations. According to the non-linear gravitational instability theory, a growth of small inhomogeneities leads to the formation of thin, dense, and gaseous condensations that are called `pancakes'. These condensations are compressed and heated to high temperatures by shock waves causing them to quickly fragment into gas clouds. The later clumping of these clouds results in the formation of galaxies and their clusters.

Thermal, hydrodynamic, and gravitational instabilities arise during the course of evolution. It leads to the fragmentation of gaseous proto-clusters and, subsequently, clustering of galaxies takes place. The pancake scheme follows three simultaneous processes: first, gas cools and new clouds of cold gas form secondly, these clouds cluster to form galaxies and thirdly, the forming galaxies and, to an extent, single clouds cluster together to form a cluster of galaxies.

According to the hierarchy model, the directions of the spin vectors should be distributed randomly. In hierarchy model, galaxies were first formed and then they obtained their angular momenta by tidal force while they were gathering gravitationally to form a cluster. Those galaxies grow by subsequent merging of proto-galactic condensations or even by merging of already fully formed galaxies. In this scheme, one could imagine that large irregularities like galaxies grew under the influence of gravities from small imperfections in the early universe.

The angular momentum transferred to a developing proto-galaxy by the gravitational interaction of the quadrupole moment of the system with the tidal field of the matter.


Gravitational wave search provides insights into galaxy mergers

The Earth is constantly jostled by low-frequency gravitational waves from supermassive black hole binaries in distant galaxies. Astrophysicists are using pulsars as a galaxy-sized detector to measure the Earth’s motion from these waves. Illustration credit: B. Saxton (NRAO/AUI/NSF). The recent LIGO detection of gravitational waves from merging black holes with tens of solar masses has confirmed that distortions in the fabric of space-time can be observed and measured. Researchers from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) have spent the past decade searching for low-frequency gravitational waves emitted by black hole binaries with masses many millions of times larger than those seen by LIGO.

Analysis of NANOGrav’s nine-year dataset provides very constraining limits on the prevalence of such supermassive black hole binaries throughout the universe. Given scientists’ current understanding of how often galaxies merge, these limits point to fewer detectable supermassive black hole binaries than were previously expected. This result has significant impacts on our understanding of how galaxies and their central black holes co-evolve.

Low-frequency gravitational waves are very difficult to detect, with wavelengths spanning light-years, and originating from black hole binaries in galaxies spread across the sky. The combination of all these giant binary black holes leads to a constant “hum” of gravitational waves that models predict should be detectable at Earth. Astrophysicists call this effect the “stochastic gravitational wave background,” and detecting it requires special analysis techniques.

Pulsars are the cores of massive stars left behind after stars go supernova and emit pulses of radio waves as they spin. The fastest pulsars rotate hundreds of times each second and emit a pulse every few milliseconds. These “millisecond pulsars” (MSPs) are considered nature’s most precise clocks and are ideal for detecting the small signal from gravitational waves. “This measurement is possible because the gravitational wave background imprints a unique signature onto the radio waves seen from a collection of MSPs,” said Justin Ellis, Einstein Fellow at NASA’s Jet Propulsion Laboratory, California Institute of Technology in Pasadena, California, and a co-author on the report just published in The Astrophysical Journal.

Astrophysicists use computer models to predict how often galaxies merge and form supermassive black hole binaries. Those models use several simplifying assumptions about how black hole binaries evolve when they predict the strength of the stochastic gravitational wave background. By using information about galaxy mergers and constraints on the background, the scientists are able to improve their assumptions about black hole binary evolution.

Ellis continues, “After nine years of observing a collection of MSPs, we haven’t detected the stochastic background, but we are beginning to rule out many predictions based on current models of galaxy evolution. We are now at a point where the non-detection of gravitational waves is actually improving our understanding of black hole binary evolution.”

“Pulsar timing arrays like NANOGrav are making novel observations of the evolution and nature of our universe,” says Sarah Burke Spolaor, Jansky Fellow at the National Radio Astronomy Observatory (NRAO) in Socorro, New Mexico, and a co-author on the paper.

According to Spolaor, there are two possible interpretations of this non-detection. “Some supermassive black hole binaries may not be in circular orbits or are significantly interacting with gas or stars. This would drive them to merge faster than simple models have assumed in the past.” An alternate explanation is that many of these binaries inspiral too slowly to ever emit detectable gravitational waves.

NANOGrav is currently monitoring 54 pulsars, using the National Science Foundation’s Green Bank Telescope in West Virginia and Arecibo Radio Observatory in Puerto Rico, the two most sensitive radio telescopes at these frequencies. Their array of pulsars is continually growing as new MSPs are discovered. In addition, the group collaborates with radio astronomers in Europe and Australia as part of the International Pulsar Timing Array, giving them access to many more pulsar observations. Ellis estimates that this increase in sensitivity could lead to a detection in as little as five years.

In addition, this measurement helps constrain the properties of cosmic strings, very dense and thin cosmological objects, which many theorists believe evolved when the universe was just a fraction of a second old. These strings can form loops, which then decay through gravitational wave emission. The most conservative NANOGrav limit on cosmic string tension is the most stringent limit to date, and will continue to improve as NANOGrav continues operating.

“These new results from NANOGrav have the most important astrophysical implications yet,” said Scott Ransom, an astronomer with NRAO in Charlottesville, Virginia, and CIFAR Associate Fellow. “As we improve our detection capabilities, we get closer and closer to that important threshold where the cosmic murmur begins to be heard. At that point, we’ll be able to perform entirely new types of physics experiments on cosmic scales and open up a new window on the universe, just like LIGO just did for high-frequency gravitational waves.”

NANOGrav is a collaboration of over 60 scientists at over a dozen institutions in the United States and Canada whose goal is detecting low-frequency gravitational waves to open a new window on the universe. The group uses radio pulsar timing observations to search for the ripples in the fabric of spacetime.

In 2015, NANOGrav was awarded $14.5 million by the National Science Foundation to create and operate a Physics Frontiers Center. “The Physics Frontier Centers bring people together to address frontier science, and NANOGrav’s work in low-frequency gravitational wave physics is a great example,” said Jean Cottam Allen, the NSF program director who oversees the Physics Frontiers Center program. “We’re delighted with their progress thus far, and we’re excited to see where it will lead.”


NASA's S-MODE takes to the air and sea to study ocean eddies

Sub-mesoscale ocean dynamics, like eddies and small currents, are responsible for the swirling pattern of these phytoplankton blooms (shown in green and light blue) in the South Atlantic Ocean on Jan. 5, 2021. Credit: NASA’s Goddard Space Flight Center Ocean Color, using data from the NOAA-20 satellite and the joint NASA-NOAA Suomi NPP satellite

After being delayed over a year due to the pandemic, a NASA field campaign to study the role of small-scale whirlpools and ocean currents in climate change is taking flight and taking to the seas in May 2021.

Using scientific instruments aboard a self-propelled ocean glider and several airplanes, this first deployment of the Sub-Mesoscale Ocean Dynamics Experiment (S-MODE) mission will deploy its suite of water- and air-borne instruments to ensure that they work together to show what's happening just below the ocean's surface. The full-fledged field campaign will begin in October 2021, with the aircraft based out of NASA's Ames Research Center in Mountain View, California.

"This campaign in May is largely to compare different ways of measuring ocean surface currents so that we can have confidence in those measurements when we get to the pilot in October," said Tom Farrar, associate scientist at the Woods Hole Oceanographic Institution in Massachusetts and principal investigator for S-MODE.

The S-MODE team hopes to learn more about small-scale movements of ocean water such as eddies. These whirlpools span about 6.2 miles or ten kilometers, slowly moving ocean water in a swirling pattern. Scientists think that these eddies play an important role in moving heat from the surface to the ocean layers below, and vice versa. In addition, the eddies may play a role in the exchange of heat, gases and nutrients between the ocean and Earth's atmosphere. Understanding these small-scale eddies will help scientists better understand how Earth's oceans slow down global climate change.

Laurent Grare of the Scripps Institution of Oceanography prepares to recover a Wave Glider during a pre-deployment test. Decked out with solar panels and several scientific instruments, the wave glider will propel itself from Santa Catalina Island farther out to sea. Credit: Benjamin Greenwood / Woods Hole Oceanographic Institution

The team is using a self-propelled commercial wave glider decked out with scientific instruments that can study the ocean from its surface. The most important gadgets aboard are the acoustic Doppler current profilers, which use sonar to measure water speed and gather information about the how fast the currents and eddies are moving, and in which direction. The glider also carries instruments to measure wind speed, air temperature and humidity, water temperature and salinity, and light and infrared radiation from the Sun.

"The wave glider looks like a surfboard with a big venetian blind under it," said Farrar.

That "venetian blind" is submerged under the water, moving up and down with the ocean's waves to propel the glider forward at about one mile per hour. In this way, the wave glider will be deployed from La Jolla, California, collecting data as it travels over 62 miles (100 kilometers) out into the ocean offshore of Santa Catalina Island.

The new data will allow the scientists to estimate the exchange of heat and gases between Earth's atmosphere and the ocean, and consequently better understand global climate change.

"We know the atmosphere is heating up. We know the winds are speeding up. But we don't really understand where all that energy is going," said Ernesto Rodriguez, research fellow at NASA's Jet Propulsion Laboratory in Pasadena, California, and deputy principal investigator for the airborne parts of S-MODE. It's likely that this energy is going into the ocean, but the details of how that process works are still unknown. The team thinks that small-scale eddies may help move heat from the atmosphere to the deeper layers of the ocean.

A flight crew prepares for the B200 King Air Sub-Mesoscale Ocean Dynamics Experiment (S-MODE) at NASA’s Armstrong Flight Research Center in Edwards, California. From left to right are Jeroen Molemaker and Scott “Jelly” Howe. Credit: Lauren Hughes, NASA Armstrong

Eyes and Scientific Instruments in the Skies

While the Wave Glider continues its slow trek across the ocean's surface, several airplanes will fly overhead to collect data from a different vantage.

"In an airplane, we can get a snapshot of a large area to see the context of how the bigger- and smaller-scale ocean movements interact," said Rodriguez.

For example, a ship or wave glider travels slowly along a straight line, taking precise measurements of sea surface temperature at specific times and places. Airplanes move faster and can cover more ground, measuring the sea surface temperature of a large swath of ocean very quickly.

"It's like taking an infrared image rather than using a thermometer," explained Farrar.

Two planes will be used in the May test flights: a B200 plane from NASA's Armstrong Flight Center in Edwards, California and a commercial plane from Twin Otter International. The B200 is carrying an instrument from NASA JPL called DopplerScatt to measure currents and winds near the ocean surface with radar. The Multiscale Observing System of the Ocean Surface (MOSES) instrument from the University of California, Los Angeles is also aboard to collect sea surface temperature data. On the Twin Otter plane is the Modular Aerial Sensing System (MASS) from the Scripps Institution of Oceanography at the University of California, San Diego, which is an instrument capable of measuring the height of waves on the surface of the ocean.

Delphine Hypolite, Multiscale Observing System of the Ocean Surface (MOSES) Operator from University of California Los Angeles, performs pre-flight checks on the MOSES Camera System at NASA's Armstrong Flight Research Center in Edwards, California. Credit: Lauren Hughes, NASA Armstrong

The fleet will gain a third member for the October experiments: NASA's Langley Research Center Gulfstream III plane with JPL's Portable Remote Imaging SpectroMeter (PRISM), an instrument to measure phytoplankton and other biological material in the water. The October deployments will also use a large ship and some autonomous sailing vessels, called Saildrones, in addition to planes and Wave Gliders.

After nearly a year and a half of delays due to the pandemic, the S-MODE team is excited to get their planes in the sky and the gliders in the water. "It was frustrating," Rodriguez said, "but the science team hasn't slowed down. The science keeps progressing."


Gravitational eddies across the galaxy? - Astronomy

Recently, Canadian and American scientists teamed up to collect and analyze data from satellite and ship-borne sensors taken over the region. With these data, they set out to determine the properties and behavior of the eddies and measure their impact on the Gulf of Alaska’s ecosystem. The researchers found that the eddies, particularly those created during El Niño years, can last several years. They found that the eddies migrate slowly through the Gulf, moved about by shifting currents, and replenish nutrient-starved regions with iron and nitrate.

"Our concern over the depletion of fish in this region makes satellite altimeter measurements such as TOPEX/Poseidon data particularly important in understanding the formation and movement of these nutrient-rich eddies, and how they influence salmon growth and other fisheries," says William Crawford of Fisheries and Oceans Canada at the Institute of Ocean Sciences.
Eddies are rotating masses of water in the ocean that typically form along the boundaries of ocean currents. In the Gulf of Alaska, eddies of warm water, filled with nutrients from shallow coastal water, mix with the cold water off the continental shelf. The mixing fertilizes the nutrient-poor water of the gulf, resulting in blooms of phytoplankton (microscopic ocean plants.) This true color image from the Sea-viewing Wide Field-of-view Sensor shows the green spiral of an eddy in bright blue water. Also notice the sediments suspended in the water along the south coast of Alaska. (Image provided by the SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE)

In 1998, he and colleague Frank Whitney began using TOPEX/Poseidon images produced by the University of Colorado to track the large-scale eddies. The satellite data, along with in situ data collected aboard a Canadian Coast Guard Ship gave Crawford and Whitney unique insight into these eddies as a natural mechanism for nourishing the sea.

The data used in this study are available in one or more of NASA's Earth Science Data Centers.


2 Methodology and Data

The development of numerical tools for the automatic identification and tracking of mesoscale eddies has been spurred by the introduction of readily available gridded altimetric sea surface height data sets [Isern-Fontanet et al., 2003 Chelton et al., 2007 Sangrà et al., 2009 Nencioli et al., 2010 Chelton et al., 2011a Halo et al., 2014 ]. Knowledge of eddy properties (position in time and space, and size) permits the specification of so-called eddy-centric coordinates, which can be used to construct eddy composites of other measured variables, obtained either from remote sensing (such as sea surface temperature, turbulent heat fluxes, or chlorophyll) [Chelton et al., 2011b Hausmann and Czaja, 2012 Gaube et al., 2013 Villas-Bôas et al., 2015 ] or from in situ sources (e.g., temperature and salinity from Argo floats) [Chaigneau et al., 2011 Abraham et al., 2013 Frenger et al., 2015 ]. These techniques provide rich opportunities to increase our understanding of physical (and biogeochemical) processes associated with mesoscale structures in the ocean [McGillicuddy, 2016 ].

2.1 Eddy Tracking

Eddy track coordinates and associated properties were obtained using the py-eddy-tracker eddy tracking code [Mason et al., 2014 ]. The eddy tracker used daily 0.25° × 0.25° altimetric all-sat gridded sea level anomaly (SLA) data (DT14) [Capet et al., 2014 ] from AVISO [ 2015 ]. This product is based on observations from a minimum of at least three altimeter missions (four between June 2002 and December 2007), providing enhanced resolution of mesoscale features in comparison with the two-sat reference product [Ducet et al., 2000 Le Traon et al., 2003 Pascual et al., 2006 ]. The DT14 reprocessing eliminates the unknown geoid by removal of the 20 year mean SSH (1993–2012) [Pujol et al., 2016 ].

The tracking domain limits were set to 70°W–30°E, 15°S–65°S for the period 1 January 1996 to 31 December 2013. Eddy properties including position, date, speed-based radius (Ls), effective radius (Le), amplitude (A), swirl speed (U), and EKE were saved at each time step. Le is estimated as the radius of a circle with the same area as the outermost closed SLA contour that defines a single eddy instance Ls is similarly based on a circle that corresponds to the SLA contour inside the eddy with maximum mean U. A is estimated as the absolute difference between SLA at the contour used to define Le and the SLA extrema within that contour. With the exception of the EKE, full descriptions of the estimation of the above properties can be found in Mason et al. [ 2014 ] and Chelton et al. [ 2011a ]. The EKE is estimated as the mean of the EKE within the speed-based contour [e.g., Chaigneau et al., 2008 ]. In the following sections, we refer to the eddy interior and the eddy periphery the former is the region inside Ls, while the latter is the region in between Ls and Le.

2.2 Temperature, Salinity, and Geostrophic Currents From ARMOR3D

We use the ARMOR3D multivariate global ocean state estimation that is based on satellite estimates of the SLA, SST, and mean dynamic topography (MDT), and in situ temperature (T) and salinity (S) vertical profile measurements. The 3-D reconstruction procedure involves three steps. (1) Satellite data (SLA + SST) are projected onto the vertical via a multiple linear regression method where covariances are deduced from historical in situ observations. Then (2) the derived profiles from step (1) are combined with T/S in situ profiles via an optimal interpolation method [Guinehut et al., 2004, 2012 ]. Finally (3), 3-D geostrophic currents and geopotential heights are computed from the combination of ARMOR3D T/S in situ fields with surface altimetric geostrophic currents through the thermal wind equation referenced at the surface [Mulet et al., 2012 ]. The surface altimetric geostrophic currents are computed from SLA and MDT through the geostrophic relation.

The satellite products provide global coverage and high resolution at the surface, but their accuracy decreases with projection into the water column. The in situ profiles are relatively sparse but provide 3-D estimates with higher accuracy. The novelty of the ARMOR3D observation-based product is that it capitalizes on the respective strengths of these two types of data, and thus offers an accurate 3-D product at a resolution close to that of the surface fields. In step (2), the combination with in situ profiles smooths slightly the T/S fields and leads to a horizontal resolution near to 100 km. The same resolution is expected for the geostrophic currents. In the vertical, we note that the ARMORD currents tend to be too barotropic this is a consequence of the reference surface currents not being sufficiently balanced by the hydrographic structure during projection to the vertical in step (3) (S. Mulet, personal communication, 2017).

The ARMOR3D data set used here is the weekly version 3.1 of the 1993–2015 reprocessing. It is computed on a 0.3° horizontal Mercator grid with 33 unevenly spaced layers between the surface and 5500 m depth. Version 3.1 is based on satellite SLAs from the 7 day gridded DT10 product [see Capet et al., 2014 ] that has a grid resolution of 0.3°. Satellite SST is from the daily Reynolds L4 at 0.25° [Reynolds et al., 2007 Reynolds, 2009 ]. Historical in situ profiles of T and S are sourced from Argo floats, CTDs, XBTs, and moorings.

2.3 Computation of Vertical Velocity

(1) (2)

2.4 Eddy Compositing

For any desired variable (e.g., SST, salinity), eddy composites are made by matching eddy observations from the eddy tracker with in time and space. The radial dimensions of each eddy instance are normalized by the eddy radius, allowing interpolation to a Cartesian grid (limits ±4 at intervals Δx = 0.2). These collocated observations are then binned in time and/or space to produce composites of for different subregions or periods. Note that while the compositing grid has dimensions of 8 × 8 normalized radii, we focus on the eddies in the figures in section 3 below by limiting the ranges of the plotting axes to ±2 normalized radii.

For 3-D compositing with ARMOR3D data, the above procedure is repeated at each available vertical level. Anomalies of temperature ( ) and salinity ( ) at 100 m are computed following Gaube et al. [ 2014 ] by, for every eddy instance, taking the difference between the original and a low-pass filtered field obtained from the convolution of a Gaussian kernel with a half width of 6° e.g., for temperature, where the angle brackets denote the smoothed field.

Confidence intervals (95%) applied in section 3 for the sample means of , ζ, and QG-ω at each point in space are calculated using a Student's t distribution following von Storch and Zwiers [ 1999 , section 4.6]. The null hypothesis is for zero population means, i.e., for QG-ω, eddies have no influence on vertical velocity. The standard formula for the confidence interval is , where is the QG-ω sample standard deviation in time is here specified as the percentage point, 2.5% (α = 0.025) for two-tailed QG-ω, of the t-distribution with – 1 degrees of freedom is the estimated number of independent QG-ω observations in the respective sample means. For the weekly is conservatively set to the number of long-lived eddies (lifetimes ≥ 4 weeks) [e.g., Chelton et al., 2011b ].

2.5 Subregional Eddy Compositing

  1. Seafloor topography plays a central role in the BMC circulation [e.g., Venaille et al., 2011 ], hence, each subregion is chosen to contain generally homogeneous topographic characteristics (Figure 1a). In particular, note the extreme steepness of the slope along the SAF in the southern subregions, the steepness along the Patagonian shelf, and the form of the ZD along the boundaries of C3–C5 and S3–S5.
  2. Resolution of the general circulation (Figure 1c), i.e., the MC is resolved in the westernmost column placing the ZD at the border between the central and southern subregions allows a coherent separation of the zonal components of the anticyclonic flow around this feature.
  3. Resolution of regions with, where possible, homogeneous levels of wind stress curl [e.g., Risien and Chelton, 2008 ], kinetic energy (KE) and EKE (Figures 1c and 1b).
  4. Coverage of the fertile open ocean waters of the BMC that extend eastward away from the Patagonian shelf is also considered, although here we do not work with chlorophyll composites. The central and southern rows of the subregional domain are rich in chlorophyll, while the northern row covers the transition toward the oligotrophic gyre to the northeast [e.g., Brandini et al., 2000 Gaube et al., 2014 ].

The northwest subregions N2 and C1 deserve mention as they are situated near to the Patagonian shelf, where uncertainty has been expressed about the reliability of altimeter SLA data for eddy detection [e.g., Pilo et al., 2015 ] however, Strub et al. [ 2015 ] have suggested that caution is warranted only south of 40°S where the SLA may be contaminated by tidal model errors. (Strub et al. [ 2015 ] use the same daily DT14 reprocessed altimeter data as used in this study (see section 2.1), whereas Pilo et al. [ 2015 ]'s results are based on the older weekly DT10 product.) The C2 subregion is found immediately east of the meeting point between the MC and BC and is associated with the presence of two near-permanent stationary eddies (a cyclone and an anticyclone). (The C2 and N2 locations are identified as local sources of first baroclinic mode eddy energy by Zhai et al. [ 2010 ] (see their Figure 3).) Assuming that these eddies may be difficult to identify using SLA data, we ran the eddy tracker using absolute dynamic topography (ADT) data in place of SLA. However, we did not find any appreciable improvement in eddy detection within C2 and so did not continue that approach.

2.6 Supplementary Data and Derived Variables

Annual mean surface eddy kinetic energy (EKE), a bulk measure of mesoscale variability, is shown in Figure 1b. Assuming geostrophy, the EKE per unit mass is computed as where and are the respective sea surface geostrophic velocity anomalies, g is the gravitational acceleration, f the local Coriolis parameter the derivatives are obtained using three-point-stencil finite differencing [Arbic et al., 2012 ] of daily SLA data from AVISO [ 2015 ] over latitude (y) and longitude (x) for the period 1993–2014. Annual mean near-surface currents based on drifters presented in Figure 1c for the BMC are taken from version 2.07 of the 0.5° × 0.5° global drifter climatology described by Lumpkin and Johnson [ 2013 ]. In Figure 5 in section 3.3, daily ADT data from AVISO are used. Topographic data are taken from the Shuttle Radar Topography Mission (SRTM) that is based on the 1′ Smith and Sandwell [ 1997 ] bathymetric data set with higher resolution data used where available (also referred to as SRTM30_PLUS).


Hanson Astronomy Photos

Bright spiral galaxy NGC 3169 appears to be unraveling in this cosmic scene, played out some 70 million light-years away just below bright star Regulus toward the faint constellation Sextans. Its beautiful spiral arms are distorted into sweeping tidal tails as NGC 3169 (left) and neighboring NGC 3166 interact gravitationally, a common fate even for bright galaxies in the local universe. In fact, drawn out stellar arcs and plumes, indications of gravitational interactions, seem rampant in the deep and colorful galaxy group photo. The picture spans 20 arc minutes, or about 400,000 light-years at the group's estimated distance, and includes smaller, dimmer NGC 3165 at the right. NGC 3169 is also known to shine across the spectrum from radio to X-rays, harboring an active galactic nucleus that is likely the site of a supermassive black hole.

Taken from DGRO Rancho Hidalgo Animas, New Mexico
14.5" RCOS F8, Apogee U16M High Cooling
Luminance 320, Red 180, Green 180, Blue 180, HA 300


Satellites reveal ocean currents are getting stronger, with potentially significant implications for climate change

By Navid Constantinou, Adele Morrison, Andrew Kiss, Andy Hogg, Josué Martínez Moreno, and Matthew England
22 April 2021

(The Conversation) – Scientists already know the oceans are rapidly warming and sea levels are rising. But that’s not all. Now, thanks to satellite observations, we have three decades’ worth of data on how the speeds of ocean surface currents are also changing over time.

In research published today in the journal Nature Climate Change we detail our findings on how ocean currents have become more energetic over large parts of the ocean. […]

How eddies have been changing

Using available data from 1993 until 2020, we analysed changes in the strength of eddies across the globe. We found regions already rich in eddies are getting even richer! And on average, eddies are becoming up to 5% more energetic each decade.

One of the regions we found with the biggest change is the Southern Ocean, where a massive 5% increase per decade was detected in eddy activity. The Southern Ocean is known to be a hotspot for ocean heat uptake and carbon storage.

Until recently, scientists could only observe changes in ocean eddies by using either sparse ocean measurements or the limited satellite record. The satellite record has only just become long enough for experts to draw robust conclusions about the likely longer-term trends of eddy behaviour.

Why is this important?

Ocean eddies play a profound role in the climate by regulating the mixing and transport of heat, carbon, biota, and nutrients in the oceans. Thus, our research may have far-reaching implications for future climate.

Scientists have known for decades that eddies in the Southern Ocean affect the overturning circulation of the ocean. As such, changes of the magnitude observed for eddies could impact the rate at which the ocean draws down heat and carbon.

But eddies are often not taken into account in climate predictions of a warming world. Since they are relatively small, they remain practically “invisible” in current models used to project future climate. [more]


Ocean Mesoscale Eddies


Alistair Adcroft
Stephen Griffies
Robert Hallberg

What Are Mesoscale Eddies?

Ocean mesoscale eddies are the “weather” of the ocean, with typical horizontal scales of less than 100 km and timescales on the order of a month. The mesoscale eddy field includes coherent vortices, as well as a rich cascade of other structures such as filaments, squirts and spirals (Fig. 1). The mesoscale field is characterized by temperature and salinity anomalies with associated flow anomalies that are nearly in geostrophic balance. Although only the surface expression of mesoscale eddies is visible in satellite images of sea surface height or temperature, they are in fact three dimensional structures that reach down into the pycnocline. A special class of eddies, known as meddies (Mediterranean eddies), are predominantly sub-surface lenses of salty water that form off the Atlantic coast of Spain/Portugal from the deep Mediterranean outflow.

Why Do Mesoscale Eddies Matter?

Mesoscale eddies are ubiquitous in the ocean, and typically exhibit different properties to their surroundings (Fig 2), allowing them to transport properties such as heat, salt and carbon around the ocean. For example, Agulhas eddies carry water with properties associated with the Indian ocean far into the South Atlantic. In the Southern Ocean, eddies account for the majority of oceanic poleward heat transport across the Antarctic Circumpolar Current. The water properties of eddies are also important in supplying nutrients to coastal zones and the surface ocean where plankton blooms may result. More than half of the kinetic energy of the ocean circulation is contained in the mesoscale eddy field, with the remainder largely contained in the large-scale circulation.

Where Do Mesoscale Eddies Come From?

The largest scale eddies emerge from instabilities of strongly horizontally sheared motions, particularly in boundary currents such as the Gulf Stream. These eddies often take the form of well defined rings extending to great depth. At slightly smaller scales, on the order of tens of kilometers, eddies are generated by the slumping of horizontal density gradients in a process known as baroclinic instability. Both of these formation processes lead to hot spots of eddy energy in the vicinity of western boundary currents and the Antarctic Circumpolar Current.

Fig. 1: False color image of ocean water color, from NASA’s Aqua MODIS satellite. Image courtesy of NASA-GSFC. The circular blue in the middle left is approximately 100km in diameter.

Fig. 2: Sea surface temperature from a fine-resolution (1/10° ⪝ 12 km grid spacing) from GFDL’s CM2.6 model. The view is of the South Atlantic and shows Agulhas eddies originating near the Cape of Good Hope (bottom right of image).

GFDL Research

The Role of Eddies in Climate

The ocean transports heat from the tropics to the poles, helping to maintain the extra-tropical climate. How the ocean transports heat varies by region, but in many regions, including the Southern Ocean, the mesoscale eddy heat transport is the dominant mechanism. In order to understand the role of eddies in climate, GFDL has developed global climate models that include ocean mesoscale eddies. These models are computationally expensive because the grid scales required to admit realistic eddies (

10km) are very small relative to the globe. Thus far, the finest resolution ocean used in a climate model is the 1/10 degree ocean component of the CM2.6, in which case the eddies are vigorous and largely reflect the level of energy suggested from satellite sea surface height measures.

In addition to their role in moving heat poleward, mesoscale eddies effect a vertical transfer of heat in the ocean, largely moving heat upwards to partially compensate for the downward heat transport by time mean fields. This vertical heat transport makes mesoscale eddies somewhat of a “gatekeeper” for ocean heat uptake, in which heat that otherwise could enter the deep ocean through mean circulation processes is in fact returned to the surface by transient mesoscale eddies. An accurate representation/parameterization of this vertical heat transport process remains an ongoing research aim at GFDL.

Do Eddies Affect Sea-level Change?

Steric sea-level change arises from changes in ocean density, which arise from changes in temperature and salinity. Questions about sea level change are largely split into global and regional questions. Global sea level rise occurs when the ocean warms, whereas regionally sea level changes due to the movement of heat and salt locally impacting on steric sea level patterns. For example, the ocean’s heat budget is sensitive to a balance of processes in regions where the ocean cools, many of which are known as deep-water formation sites. Such cooling at the ocean surface often results in the release of heat from the ocean interior, and in turn modifying sea level regionally.

In these “open-ocean deep-convection” sites, eddies play an important role in controlling the depth of convection by fluxing heat from the interior into the convection site. Additionally, possible changes in the Gulf Stream associated with a reduced Atlantic overturning circulation may give rise to local sea level rise along the US Atlantic coast, with details of such changes requiring models of resolutions sufficient to represent the ocean mesocale. Finally, the interactions between the ocean and land ice shelves occurs on very fine scale at high latitudes, with such interactions potentially impacting the stability of ice sheets and so represent perhaps the most important unresolved question in sea level studies. Each of these issues are being actively pursued as part of GFDL’s studies of sea level.

Representing Unresolved Mesoscale Eddies in Ocean Models

Most ocean climate models have a horizontal grid-spacing that is too coarse (typically 1° ⪝ 110 km) to explicitly represent any but the very largest eddies. Even in newer climate models the grid-spacing (typically ¼° ⪝ 30 km) is said to be only “eddy permitting”. Only in our finest resolution models (grid-spacing of order 1/10° ⪝ 12 km) do we consider the largest baroclinic eddies to be resolved, and even then, only in some parts of the ocean. In all these models, the important effects of the unresolved eddies must be parameterized – represented in terms of large-scale gradients of density or velocity.

Related Links

Featured Results

Visualizations

Publications

  • Griffies, Stephen M.,Michael Winton, Whit G Anderson, Rusty Benson, Thomas L Delworth, C O Dufour, John P Dunne, P Goddard, A K Morrison, Andrew T Wittenberg, J Yin, and Rong Zhang, February 2015: Impacts on ocean heat from transient mesoscale eddies in a hierarchy of climate models. Journal of Climate, 28(3), doi:10.1175/JCLI-D-14-00353.1.
  • Jansen, Malte, and Isaac M Held, August 2014: Parameterizing subgrid-scale eddy effects using energetically consistent backscatter. Ocean Modelling, 80, doi:10.1016/j.ocemod.2014.06.002.
  • Hallberg, Robert W., December 2013: Using a Resolution Function to Regulate Parameterizations of Oceanic Mesoscale Eddy Effects. Ocean Modelling, 72, doi:10.1016/j.ocemod.2013.08.007.
  • Delworth, Thomas L., Anthony Rosati, Whit G Anderson, Alistair Adcroft, Ventakramani Balaji, usty Benson, Keith W Dixon, Stephen M Griffies, Hyun-Chul Lee, Ronald C Pacanowski, Gabriel A Vecchi, Andrew T Wittenberg, Fanrong Zeng, and Rong Zhang, April 2012: Simulated climate and climate change in the GFDL CM2.5 high-resolution coupled climate model. Journal of Climate, 25(8), doi:10.1175/JCLI-D-11-00316.1.
  • Griffies, Stephen M., and R J Greatbatch, July 2012: Physical processes that impact the evolution of global mean sea level in ocean climate models. Ocean Modelling, 51, doi:10.1016/j.ocemod.2012.04.003.
  • Farneti, Riccardo, Thomas L Delworth, Anthony Rosati, Stephen M Griffies, and Fanrong Zeng, July 2010: The role of mesoscale eddies in the rectification of the Southern Ocean response to climate change. Journal of Physical Oceanography, 40(7), doi:10.1175/2010JPO4353.1.
  • Marshall, D, and Alistair Adcroft, April 2010: Parameterization of ocean eddies: Potential vorticity mixing, energetics and Arnold’s first stability theorem. Ocean Modelling, 32(3-4), doi:10.1016/j.ocemod.2010.02.001.
  • Hallberg, Robert W., and Anand Gnanadesikan, 2006: The role of eddies in determining the structure and response of the wind-driven Southern Hemisphere overturning: Results from the modeling eddies in the Southern Ocean (MESO) project. Journal of Physical Oceanography, 36(12), 2232-2252.

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