Iceberg Drift
Installing an iceberg drift station on B15A during January, 2001.
Since March, 2000, our group has been involved in tracking the behavior of a large iceberg (B15) that calved from the east side of the Ross Ice Shelf. At the time of calving, this iceberg was the largest iceberg ever witnessed, and contained over 2000 cubic kilometers of ice (enough to supply the U.S. domestic, industrial and agricultural fresh water needs for about 10 years).
Read about our research here (PDF).
Get our data here (link to NSIDC data server/archive).
Project Overview
During the 8-year period of research, activities were divided between field work (in Antarctica) , data analysis/dissemination (in Chicago and elsewhere in the U.S.), model development and simulation, satellite image collection and archiving (done primarily by the AMRC at the University of Wisconsin), and education and public outreach (in Chicago, Evanston, and elsewhere in the U.S. and Antarctica).
Field work. Four field seasons were undertaken as the main focus of the research program. These seasons were closely aligned with two additional field seasons that were part of an initial SGER project (also funded by NSF in year 2000), giving a total of 6 separate field seasons devoted to research on "Earth's largest icebergs" (B15 and its "family" in the Ross Sea of Antarctica). Each of the 4 field seasons involved intensive field operations (usually using a combination of helicopter and fixed-wing, Twin Otter, support) in mid October to late November, making use of the sea-ice conditions (necessary to eliminate or reduce over-open-water flight) surrounding Ross Island and the icebergs, which were located through most of the project's logistical phases immediately north of Ross Island.
Notable highlights of the field work are listed as follows:
- Iceberg drift station deployment. During the 4-season effort (extended to 6-seasons, because 2 prior field seasons were supported by a related SGER project), 8 automatic weather stations (AWS) equipped with specialized sensors useful in understanding iceberg drift, melting and break-up (e.g., global positioning system (GPS) receivers, thermistor strings to measure firn temperature, solar flux pyranometers, etc.) were deployed on a total of 5 icebergs and 1 "nascent iceberg" located on the Ross Ice Shelf. Four of the 8 stations were deployed during the 4 seasons supported by this research grant (the other 4 were deployed during the SGER project, however, all 4 of these stations were revisited and were largely rebuilt during the research grant reported on here). Icebergs on which these stations were installed are: B15A, B15J, B15K, C16, C25 (originally Drygalski Ice Tongue) and Nascent Iceberg (still part of the Ross Ice Shelf). As of July, 2008, only 4 of the stations are still functioning. Stations on B15K, B15A and C25 (originally Drygalski Ice Tongue) have presumably fallen into the water.
Figure 1 - Location image (MODIS dated 9 November, 2004) of AWS/GPS stations deployed on icebergs during the course of the project. Stations on B15J and C16 were originally deployed during the SGER project that occurred immediately prior to this project. (One station on B15J is not shown because it was a small GPS-only station that functioned for a short period of time.) An additional station located near the iceberg calving front of the Ross Ice Shelf (at a site referred to as "Nascent Iceberg" in anticipation of future calving) is not shown, but is located approximately 300 km to the east of Cape Crozier.
Figure 2 - AWS/GPS iceberg station being installed on iceberg C16 during December, 2002.
- Seismometer deployment and recovery. During the 4 years of field effort associated with the research grant reported here, seismometers were deployed in various schemes. In October, 2003, 4 seismometer stations (with 5 seismometers) were deployed on iceberg C16 using PASSCAL instrumentation and with the help of Tim Parker, a PASSCAL technician. These seismometers were operated until mid January, 2004. At that time, a single seismometer was redeployed on C16 and a seismometer was installed at a test site near the airstrip at Willie Field on the McMurdo Ice Shelf. The 2004/2005 field season involved the installation of seismometers on icebergs B15A and B15K, the additional operation of the seismometer on iceberg C16 and the installation of a seismometer on Nascent Iceberg (Ross Ice Shelf). The 2005/2006 field season was notable in that iceberg B15A had drifted out of the Ross Sea. In a "daring" rescue mission, the seismometer on B15A was recovered using twin otter support and personnel from the FSTP field safety team in McMurdo Station. This seismometer recovery from B15A located off Cape Adare allowed analysis of seismic data obtained during the iceberg's break up off Cape Adare and formed the subject of considerable news media attention (including a piece on NPR's "All things considered"). In addition to the recovery of the seismometer from B15A during early November of 2005, the stations on C16 and Nascent Iceberg were maintained. A seismometer deployment on the thick landfast sea ice of McMurdo Sound was also made during the 2005/2006 season. The 2006/2007 field season marked the time when all seismometers were to be recovered from the field and the field effort to be wound down. Unfortunately, two seismometers, one on C16 and one on B15K, escaped the Ross Sea region, and their seismometers (including data) were lost.
Figure 3 - Location map of seismometer stations deployed on iceberg C16 during the 2003/4 field campaign. (a) MODIS image dated 26 December 2003. (b) Location of stations on C16 relative to the C16/B15A collision zone and to seismic stations on Ross Island run by the Mt. Erebus Volcano Observatory. Seismometers were also operated on icebergs B15A and B15K, and on Nascent Iceberg on the Ross Ice Shelf.
Figure 4 - During a "daring" twin-otter mission from McMurdo Station, the seismometer on B15A was rescued (including all data recovery) several days after B15A passed Cape Adare and broke up (as shown schematically on the map above). Also shown on the map above are other possible flight scenarios being considered as a means of recovering the seismometer and its data from the iceberg if the mission to the iceberg off Cape Adare had failed.
- Automatic camera deployment. During the 2003/4, 2004/5 and 2005/6 field seasons, automated camera systems (using Iridium telephone data connections) were deployed by Ronald Ross at various locations on icebergs C16, B15K and Nascent Iceberg. These cameras produced images of iceberg collision processes (e.g., the edge of iceberg C16 where iceberg B15A was in contact) and ice-shelf rifting processes (e.g., the rift near Nascent Iceberg on the Ross Ice Shelf expected to be a detachment rift for a new iceberg in the future). In 2006/2007 these cameras were given to support a project involving icebergs in the Weddell Sea (run by Ted Scambos of the NSIDC). Ronald Ross also provided time and engineering effort to develop an automatic camera that currently is in use near Cape Royds (to keep an "eye" on the penguin rookery and to monitor snow drift on Shackleton's hut).
Figure 5 - An automated "web cam" with Iridium data transfer capability was deployed on the edge of an iceberg-detachment rift on the Ross Ice Shelf near the Nascent Iceberg AWS/GPS station. The intention of this deployment was to observe the calving process visually; however, the iceberg did not calve during the period of the camera's deployment (nor has it calved at the time of this final report).
- GPS ice-shelf study. During the 2005/2006 field season, three GPS receivers were deployed on the Ross Ice Shelf near the Nascent Iceberg site to investigate two aspects of the ice shelf's motion: (1) horizontal fluctuation of the ice-shelf velocity field associated with daily tides and with stick-slip motions of Whillans Ice Stream, and (2) to determine if the ice shelf has accelerated since 1977, the time when ice-flow measurements were last made near the front of the ice Shelf. The GPS stations were established at "Nascent North", "Nascent South", and R13. The Nascent North and South stations were located on either side of the large rift immediately south of the AWS site deployed in 2004. The original Ross Ice Shelf Geophysical and Geophysical Survey station at R13 was reestablished about 50 km from Nascent Iceberg to re-determine the ice shelf velocity at a location where it was measured in 1977/8. The results of between 14 and 21 days of continuous GPS operation showed that the ice shelf was indeed responding to the ocean tide, with fluctuations in horizontal motion that were about equal in amplitude to the time-mean motion normally measured in ice-shelf flow studies. The analysis of the data from R13 revealed that there has been no significant change in the ice shelf velocity near the middle of its ice front.
Figure 6 - Location map of long-term GPS deployment made during November, 2005, to investigate tidal forcing of horizontal ice-shelf flow at the Nascent Iceberg site and at the RIGGS station called R13 (where previous flow velocity measurements were made during 1978).
- Iceberg and ice-shelf firn temperature studies. During the 2004-2006 field seasons, efforts were undertaken to deploy thermistor strings below the snow surface on iceberg C16 and at Nascent Iceberg on the Ross Ice Shelf. Data from these sensor strings are relayed via the ARGOS data transmission satellite system to monitor how typical Antarctic firn layers respond to rapid warming (e.g., as icebergs drift into warmer climates).
- Blowing snow observations. During the 2006 field season, efforts were undertaken to test sensors capable of observing fluxes of blowing snow on the Ross Ice Shelf. Sensors were deployed on the McMurdo Ice Shelf in anticipation of future field efforts designed to measure the contribution of blowing snow across the seaward ice front as a net mass balance term in the mass budget of the Antarctic Ice Sheet. This work continues under separate projects funded at the Lamont Doherty Earth Observatory.
- Engineering development and infrastructure. Throughout the 4 seasons of the project, a field test site was maintained on the McMurdo Ice Shelf near the Willie Field airstrip to test sensors, power systems and data acquisition systems. The purpose of this work was to improve on future field design of instrumentation.
Data Analysis/Dissemination. Various routine efforts to analyze and disseminate data were undertaken through the middle of 2006, when sufficient data was complied to allow the main research goals of the project to be worked on in earnest. From 2006 to the summer of 2008, about16 research papers (the count depends on how one interprets subjectively the relationship between the content of the paper and the goals of the research program) were prepared for submission to refereed journals (2 are currently under review, one was rejected and will be revised for resubmission), three PhD dissertations were defended, and two MS theses were prepared. The majority of these research papers were presented at various meetings, including those of the International Glaciological Society and the American Geophysical Union.
A major component of the data analysis and dissemination effort involved the recruitment of and collaboration with scientists from other institutions who work on similar projects. The two most notable scientists who participated in data analysis and dissemination were Jeremy Bassis (currently a post-doctoral researcher at the University of Chicago) and Richard Aster (currently chairman of the Earth and Environmental Science Program and the Geophysical Program Coordinator at New Mexico Tech). Both of these scientists' activities were funded by separate, unrelated projects, however both scientists contributed in a substantial manner to the overall success of the project reported here.
Meetings and invited seminars were regularly attended by project participants for both the dissemination of project research results and for general education and outreach. A notable example of this activity was the invited lecture by the project leader, Doug MacAyeal, at the 2008 IRIS workshop on the origins of iceberg seismicity.
In addition to the preparation of research papers for submission to technical and scientific journals, results from the research were deposited in the NSF Antarctic Glaciological Data Center (AGDC). This data center contains not simply a description of the data, but the data itself in a format easily accessible to users unfamiliar with the project. Although some seismic data resides at the AGDC, the majority of the data is in preparation for the IRIS Global Data Network, the site where data obtained using PASSCAL supported instrumentation is to be archived by agreement with the NSF.
Model Development and Simulation. Five basic areas of model development were addressed during the period of the project and are summarized as follows:
- Iceberg drift inverse model. A model of iceberg drift was created using adjoint trajectory methods (inverse methods) to determine the forces that govern iceberg calving and break up. During the course of the project's data gathering phase, two major icebergs were calved in response to collisions between fixed shelf ice and drifting icebergs. In the first case, iceberg C19 was calved in response to the Ross Ice Shelf being struck by iceberg B15A. In the second case, iceberg C25 was calved off the end of the Drygalski Ice Tongue in response to the ice tongue being struck by iceberg C16. These events were studied using iceberg drift modeling techniques to estimate the forces involved in the calving events.
- Iceberg drift in response to tide. One graduate student PhD project (ongoing) involves the simulation of iceberg drift in response to tidal forcing. This effort will culminate in the PhD dissertation of Mr. Young-Jin Kim.
- Iceberg drift in response to the Inverse Barometer Effect (IBE). One graduate student successfully defended his PhD (Ian Turnbull) on an analysis of how barometric anomalies (persistent low pressure cells) near Ross Island (particularly in the area immediately north of Mt. Erebus) may cause iceberg drift to "stagnate", and how icebergs can be "trapped" by pressure anomalies such as witnessed in the vicinity of Ross Island where B15A and B15J remained trapped (despite being adrift the entire time) for several years.
- Iceberg firn evolution. Olga Sergienko produced a model that predicts the temperature and melting within a firn layer on the surface of an iceberg. This work was conducted after she defended her PhD thesis (on another, but related topic).
- Ice Shelf response to ocean tide and tidal variations in ice stream inflow. Kelly Brunt created the first numerical model of an ice shelf's response to ocean tide. This model also investigated the sensitivity of ice-shelf flow to the tidal modulation of ice-stream input. The work formed the basis of her PhD thesis, which was successfully defended in Spring of 2008.
Satellite Image Collection and Archiving. Throughout the project, the icebergs of the Ross Sea were closely monitored using advanced meteorological satellite image analysis by the AMRC at the University of Wisconsin. This work, conducted primarily by Shelley Knuth and Jonathan Thom, culminated in a web-site product that provides a near daily satellite image analysis of the icebergs (see: http://amrc.ssec.wisc.edu/iceberg.php).
Education and Public Outreach. During the course of the research project, 3 graduate students received their PhD after writing dissertations that focused on data collected during the project (Olga Sergienko, 2005; Kelly Brunt, 2008 and Ian Turnbull, 2008). One PhD student continues to work on project data and may receive his degree sometime in the future (Young-Jin Kim, currently on leave of absence). One Master of Science degree was awarded based on analysis of project data (Marianne Okal, 2005).
A major component of the education of graduate students associated with this project involved non-traditional class experience and summer school experience in locations other than Chicago. In 2004, a 2 week long short course in glacial geology of New Zealand was offered to Olga Sergienko, Marianne Okal and Kelly Brunt. This course was repeated for Emily O'Donnell and Mac Cathles in 2005. All such courses were made possible by the fact that transit to New Zealand was accommodated by the field-work schedule. In addition to these non traditional classes, Olga Sergienko, Kelly Brunt and Mac Cathles attended glaciological summer schools in Sweden, Italy and in Woods Hole over the period of the project.
In addition to the supervision of graduate students, the PI's of the project were engaged in university teaching. During the period of the project, Doug MacAyeal received the University of Chicago "Provost's Teaching Award" for a course on Earth's dynamic environment and its relation to human evolution and settlement. This course made use of materials and views that could not have been available to the teacher were it not for his participation in Antarctic field work.
Throughout the award period, various contacts with the media and news services led to various reports on the project that were of interest to the general public. A piece on the break-up of iceberg B15A aired in October of 2006 on NPR's "All things considered" (see, http://www.npr.org/templates/story/story.php?storyId=6204027). The PI was featured also in Werner Herzog's film "Encounters at the End of the World", as well as in various television documentaries, including "Modern Marvels" (regarding polar icebreakers). The graduate students involved in the project also made various presentations to public school children in the Chicago region (and elsewhere) from time to time.
Figure 7 - One of the PI's participated in the filming of Werner Herzog's film "Encounters at the End of the World", and was interviewed on camera discussing aspects of human interest in icebergs.
Findings
A summary of major findings (see subheadings below) resulting from data collection, analysis and other research performed during the project is provided in short form here. Readers are directed to the publication list for further details.
A. Iceberg Drift and Break-up. Among the original objectives of the project was to develop a complete description of the drift and break-up of large tabular icebergs that calve from the ice shelves surrounding Antarctica. This objective was only partially met, however, heretofore unrecognized processes governing iceberg drift and break-up were discovered as a result of the study. These processes and features are listed as follows:
Figure 1 - Map summarizing iceberg drift from January 2001 to July 2008 for 5 icebergs equipped with AWS/GPS iceberg drift stations. Iceberg B15A (red line) lost its drift station in 2007, but is still adrift South of New Zealand. Icebergs C16 and B15J are still adrift along the coast of East Antarctica, and retain their AWS/GPS stations in excellent working order.
- Iceberg drift is primarily controlled by tides. The dominant kinetic energy of the various icebergs studied is associated with ocean tide interaction. Large tabular icebergs tend to "surf" the large-scale ocean tide (i.e., are influenced by the sea-surface tilt associated with the tide, much like a surfboard is influenced by a small sea swell approaching a beach). Iceberg tidal motion is the largest player in determining the actual forces of collisions that lead to iceberg-on-seabed or iceberg-on-ice-shelf induced break-up.
Figure 2 - Drift velocity of iceberg B15A observed by an iceberg AWS/GPS station. Both velocity components are strongly dominated by the diurnal tidal signal of the Ross Sea.
- Iceberg drift can "key" into coastal geometry and become "locked". One of the unanticipated mysteries of the evolution of B15A and B15J was the fact that they remained in the vicinity of Ross Island, without being grounded, for several years (from 2001 to 2004 for B15A). This apparent inability to drift clear of Ross Island is due to a "key" effect where the end of the iceberg "locks" into a constrained region (a "keyhole") created by the geometry of Ross Island, the Ross Ice Shelf, grounded iceberg C16 and various islands north of Ross Island.
Figure 3 - (left panel) Boundaries of accessibility (family of dark lines) to the AWS on iceberg B15 caused by constraints of coastal boundaries and shape of the southern end of B15A. (right panel) Actual drift record (dark line) of the AWS on iceberg B15A (measured by GPS associated with the AWS station) is indeed "locked" into a small region defined by the boundaries of accessibility (white lines).
- Iceberg drift forces are sufficient to cause calving from ice tongues and ice shelves. During a collision between iceberg C16 and the Drygalski Ice Tongue, AWS/GPS stations were active on both and could record the results of the collision (the calving of the tip of the ice tongue to create iceberg C25, which contained the original station deployed on the ice tongue). The forces (on the order of 2 x 1010 N) computed from the GPS records provide constraints on the iceberg calving process.
Figure 4 -Net force magnitude acting on icebergs C16 and C25 during a collision between C16 and the Drygalski Ice Tongue (C25). The moment of collision is defined as the point in time where the force on C25 (lower panel) rises from zero. These forces were estimated using GPS drift data collected on the icebergs.
- Iceberg drift is influenced by the Inverse Barometer Effect (IBE). The inverse barometer effect, or IBE, as described, e.g., by Wunsch and Stammer (1997), is a relation between sea-surface elevation change, h, and change in the local atmospheric pressure, ∆P, given by: h = ∆P/(rg), where r is the density of sea water, and g is the acceleration of gravity. This relation describes the upward deflection of the sea surface below low atmospheric pressure, and downward depression of the sea surface below high atmospheric pressure. Amplitude of the effect is very nearly 1 cm of sea-surface change for every 1 mbar (1 hPa) of surface-pressure change. In a study of atmospheric conditions in the Ross Island area, Monaghan and others (2005) found that yearly mean atmospheric pressure in Windless Bight, on the south side of Ross Island, is approximately 2 hPa higher than at Lewis Bay, on the other side of Ross Island. This difference suggests that, on average, the sea surface is approximately 2 cm higher in Lewis Bay than in Windless Bight. We shall argue below that icebergs are dynamically attracted to areas of low pressure (despite the high sea surface), and this may account for the lingering presence of icebergs B15A, B15J, B15K and C16 near Lewis Bay through the 2001-2005 period. The effect of sea-surface topography induced by atmospheric pressure loading on icebergs is counter-intuitive. As shall be explained below, icebergs are pushed up the sea-surface gradient by the atmospheric pressure difference. A way to quickly appreciate this counter-intuitive idea, is to make note of the fact that seawater density appears as a factor in the denominator of equation that defines the IBE (see above). The gradient in sea-surface elevation induced by a given pressure distribution is increased as seawater density is reduced. Thus, if the ocean were made of a fluid with a density equal to that of ice, i.e., 9/10ths that of seawater, the surface slopes induced by the IBE would be greater. Icebergs thus experience a sea-surface tilt that is too shallow to counterbalance the force of atmospheric pressure acting on the sides of the iceberg. This force tends to push the iceberg up the sea-surface gradient toward the area of low pressure.
Figure 5 -(Top panel) Barometric reading from the AWS on C16 for the time period from 1 March, 2002 to 1 October, 2003. Smoothly varying lines represent the 3-month running mean atmospheric pressure for C16 (thick, gray) and B15A (thin, black) used to evaluate pressure difference between the two stations. (lower panel) Trajectory of B15A's AWS site during the same time period (left), scatter plot of 3-month running mean pressure difference evaluated every 4 hours between C16 and B15A on a displaced trajectory (middle), points along the trajectory where C16's pressure was less than B15A's by 0.3 hPa (open circles, one hPa is equal to one millibar) and where C15's pressure was greater than B15A's by 1.0 hPa (crosses). The separation between crosses and open circles suggests that B15A tends to move more deeply into Lewis Bay when pressure within Lewis Bay (as represented by the barometer reading on C16) is less than that to the north (as represented by the barometer reading on B15A).
- Iceberg drift is influenced by the Taylor-Proudman effect. As described by Proudman (1916) and Taylor (1923), the horizontal velocity of hydrostatic, inviscid and barotropic (constant density) fluids is non-divergent and independent of vertical elevation in the water column in circumstances where the Rossby number is small. This is a simplified statement of the Taylor-Proudman theorem (see Pedlosky, 1997, section 2.7). Under this circumstance, the water column below a large tabular iceberg is expected to be horizontally non-divergent, and thus is expected to resist stretching or shrinking in the vertical dimension that would accompany changes in depth associated with iceberg movement across depth contours. As described in Pedlosky (1997, section 2.7), experiments in rotating tanks that demonstrate the Taylor-Proudman effect show that rigid bodies imbedded within the fluid, such as a sphere or cylinder, when towed horizontally, carry with them phantom bodies of fluid that are above and below the body. These "phantom" bodies of fluid that move with the towed rigid body are referred to as Taylor columns. Assuming that frictional effects at the base of a typical tabular iceberg are relatively small, one can speculate that a large tabular iceberg and the water beneath it comprise a Taylor column, and are thus constrained to move as a single vertically rigid entity. If this speculation is true, then the horizontal non-divergence of a Taylor column's flow suggests that tabular icebergs will have difficulty crossing contours of seabed depth, where vertical stretching or shrinking would demand horizontal convergence or divergence of the underlying water column, respectively. The AWS/GPS station drift trajectories of B15A and B15J both confirm the suspicion that the icebergs drift in a manner to minimize the change in water column thickness beneath them. Perhaps the most striking example of this effect is the movement of B15A through a sea-bed fracture zone (a mid-ocean ridge "gap" between two transform faults) as it drifted away from the Antarctic coastal zone (see figure below).
Figure 6 - Drift trajectory of B15A (observed by the iceberg's AWS/GPS station) during its exit from the coastal region of Antarctica superimposed on GEBCO ocean bathymetry. Notice the long drift leg parallel to the bathymetric channel associated with a sea floor transform fault. The fault is eventually crossed at the apex of the transform fault (mid-ocean ridge segment). This close connection between the iceberg drift trajectory and the bathymetry suggests that the Taylor-Proudman effect is applicable to large tabular icebergs.
Figure 7 - Example of possible Taylor-Proudman effects (Pedlosky, 1979) influencing the drift of B15J from January, 2006 to May, 2007. The iceberg's trajectory completes an orbit in the region between Franklin Island and seabed shoals to the north of Ross Island. This orbital pattern may reflect the tendency for the iceberg and underlying water column to move as a rigid package that can be trapped within basins defined by relatively deep bathymetry such as located between Franklin Island and Ross Island.
- Iceberg break-up may be stimulated by sea-swell effects. The break-up of B15A off the coast of Cape Adare occurred at a time when other aspects of its drift were unremarkable. The one remarkable environmental condition extant at the time of break-up was the arrival of sea swell (in the low frequency band, i.e., below 10 seconds period) generated by a distant storm occurring 7 days earlier in the Gulf of Alaska. The role of sea swell in iceberg break-up is the subject of some controversy, however, the results of research performed during the course of this project lend support to the idea that icebergs and ice shelves are subject to relentless mechanical motions associated with sea swell arriving from storm systems far from Antarctica.
Figure 8 - B15A immediately following break up off Cape Adare on 27 October, 2005 (Envisat radar image from 30 October, 2005, courtesy of European Space Agency).
Figure 9 - Analysis of sea swell in transit from the Gulf of Alaska to Antarctica during the time immediately prior to the break-up of B15A off Cape Adare (occurring at the time the swell arrived in the vicinity of B15A). a, GOES West image of storm on 21 October 2005 at 0000 UTC. b, Model wave-height estimate for 21 October 2005 at 1200 UTC (c.i. 0.6 m). c, Location map. d, Wave-height records of NOAA buoys 46001 and 51004. Blue bar represents time of lowest barometer reading at NOAA buoy 46001. e, Idealized and observed energy-density maxima on frequency vs. time plot. Red, yellow and blue circles on 21 October 2005 represent energy-density maxima at source, and grey ellipses represent energy-density maxima after deformation by dispersion. Predicted and observed microseism (for Pitcairn and Scott Base, labeled SBA in panel c) appear at frequencies above 0.08 Hz. Color patches represent observations abstracted from spectrograms at Pitcairn Island and various Antarctic sites (yellow=Pitcairn Island, light blue=B15A, red=Nascent Iceberg, dark blue=Scott Base) and are where the energy density associated with these observations exceeded 85% of the local mean. The patches have been trimmed (arbitrarily, removing vertical extents) to eliminate signal not associated with swell (e.g., earthquakes).
Figure 10 - Wind speed (middle panel in top to bottom order) and drift velocity (bottom panel) were unremarkable during the break up of B15A off Cape Adare on 27 October, 2005. Drift trajectory (inset, bottom panel) reveals that B15A was partially aground on a shallow shoal off Cape Adare at the time of break-up. B15A had been aground before without breaking up, thus some additional feature of the environment was at play to elicit the break-up.
Figure 11 - (upper panel, e) Amplitude of vertical motion on B15A measured by seismometer at the AWS/GPS site during the period when B15A broke up. (lower panel, f) Spectrogram of vertical motion observed by the seismometer (units of color are dB of spectral power density of vertical velocity, yellow/red color is high spectral density). The arrival of swell generated in the Gulf of Alaska (see figure 9 above) corresponded with the break-up of the iceberg.
B. Iceberg and Ice-Shelf Seismology. Another main objective of the project was to determine the source of harmonic tremor emanated by icebergs (IHT) and to determine the extent to which icebergs and ice shelves (near their calving front) are influenced by other seismic signals such as earthquakes, tsunamis and ocean swell. This objective was formulated in response to observations of IHT in the equatorial Pacific by the PI, Emile Okal, during the time B15 was calving and breaking into B15A and B15B. A summary of t seismological findings is provided below:
- Iceberg Harmonic Tremor (IHT). Iceberg harmonic tremor (IHT) emanating from tabular icebergs in the Southern Ocean and along calving margins of the Antarctic Ice Sheet is a complex, evolving signal at frequencies above approximately 0.5 Hz. IHT has been observed as T-phases on islands in the equatorial Pacific, as hydro-acoustic signals in the Indian Ocean, and by local and regional Antarctic seismic networks. To identify the IHT source mechanism and to understand its relevance to iceberg calving, evolution, and break-up, we deployed seismometers on a giant (25 km by 50 km) tabular iceberg called C16 in the Ross Sea, Antarctica, during a uniquely accessible period (austral summer, 2003/4) when it was aground against the northern shore of Ross Island. During the deployment period, C16 was in sporadic contact with another giant tabular iceberg, B15A, that was moving under the influence of local ocean currents. This study reveals that the C16-associated IHT was a manifestation of extended episodes of discrete, repeating stick-slip icequakes (typically thousands of individual sub-events per hour) produced when the cliff-like edges of the tabular icebergs underwent glancing, strike-slip collisions. IHT signal that we observed is thus not a phenomenon associated with iceberg elastic or fluid resonance modes, but is instead the consequence of long sequences of very regularly-spaced and similar pulses of seismic radiation from these constituent stick-slip sub-events. IHT represents a newly identified glaciogenic source of seismicity that can be used to improve our understanding of iceberg dynamics and possibly of ice-shelf disintegration processes.
Figure 12 -Two representative chevron-patterned iceberg harmonic tremor (IHT) episodes recorded at iceberg C16 seismic station B (see location map provided in "Activities" section of this final report). (a) and (b) Source-to-receiver oriented (183°E of N azimuth) longitudinal component of ground displacement for events recorded on 27 December 2003 and 19 January 2004, panels (a) and (b), respectively. (c) and (d) Spectrograms (low-pass filtered and resampled to 20 Hz using the MATLAB decimate command) of the seismograms shown in (a) and (b). IHT episodes display distinctive harmonic spectrogram character indicating a time-variable fundamental frequency with integer-multiple harmonics. Color bar indicates the log10 of power spectral density as a function of time (horizontal axis) and frequency (vertical axis) in the spectrograms expressed in units of dB of m2 s2 per Hz. Spectrograms were computed using the MATLAB spectrogram command with a 600-sample Hamming window with 500-sample overlap. For clearer visual presentation, the spectrograms are smoothed by a 3 pixel by 3 pixel averaging filter (using the MATLAB filter2 command). Aseismic "eyes" are seen in each pattern adjacent to where the fundamental harmonic reaches minimum frequency. (e) and (f) Longitudinal component of ground displacement during the aseismic eye periods highlighted in (a) and (b), respectively. Waveform polarity reverses following the aseismic eye (panel e), and amplitude of the initial pulse is enhanced during tremor restart following the eye (panels e and f). Timing of impulsive seismoacoustic phases generated at the start and end of these aseismic eyes establishes the source location. Waveform polarity reversal across the eyes, correlation between changes in the fundamental frequency and the GPS-derived velocity of B15A, and polarity of the P and S seismic radiation patterns establish the source mechanism of IHT as repeating stick-slip events at the C16/B15A collision zone.
- Iceberg and Ice-Shelf Response to Sea Swell and distantly sourced Tsunamis. In addition to the study of IHT (above), the seismic data collected on icebergs C16 and B15A, and Nascent Iceberg (on the Ross Ice Shelf) revealed the extent to which large floating ice masses respond to ocean swell and to earthquake generated tsunamis. Two tsunamis were recorded by the seismometers in late December of 2004. One was associated with the Sumatra Earthquake (Magnitude 9.3 on 26 December, 2004), the other was associated with a smaller, less publicized earthquake occurring near the Macquarie Islands (23 December, 2004). Equally striking is the fact that sea swell generated by storms in the Pacific, Indian and Southern oceans excites the strongest and most persistent motions of the icebergs and ice shelves in the frequency band below about 0.1 Hz. A seismometer operating on the floating Ross Ice Shelf near its seaward ice front (Nascent Iceberg) for 340 days (out of 730 days) during the 2004, 2005 and 2006 Antarctic field seasons recorded the arrival of 93 distantly sourced ocean swell events displaying frequency dispersion characteristic of surface gravity waves propagating on deep water. Comparison of swell event dispersion with the NOAA Wave Watch III (NWW3) ocean wave model analysis reveals that 86 of these events were linked to specific storms located in the Pacific, Southern and Indian oceans. Nearly all major storms in the NWW3 analysis of the Pacific ocean, in both northern and southern hemispheres, were linked to signals observed at the Nascent site during the period of seismometer operation. Swell-induced motion of the Ross Ice Shelf is found to increase by several orders of magnitude over the time period that sea ice surrounding Antarctica decreases from its maximum extent (in early October) to its minimum extent (in late February). The amplitude of vertical vibration of the ice shelf in the frequency band between 0.025 Hz and 0.14 Hz varies between tens of micrometers to millimeters as sea ice decays to its minimum seasonal extent. This suggests that climate influence on sea-ice extent may indirectly modulate swell energy incident on the calving margins of the Antarctic Ice Sheet. The largest swell signals observed on the Ross Ice Shelf come from storms in the tropical Pacific and Gulf of Alaska. The remoteness of these events emphasizes how the iceberg calving margin of Antarctica is connected to environmental conditions well beyond Antarctica.
Figure 13 - A 2-year spectrogram of vertical displacement constructed from the seismometer record at Nascent Iceberg. Color indicates the log10 of signal density as a function of time (horizontal axis) and frequency (vertical axis) expressed in units of dB of cm2 s2 Hz-1 ; red is higher density, blue is lower density, range between red and blue is 3 dB. Two major features are displayed by the spectrogram. First, the intensity of signal in the 0.025 Hz to 0.15 Hz range increases dramatically as the sea ice conditions in the Ross Sea and beyond become less concentrated and extensive [Gloerson et al., 1992]. Maximum signal intensity occurs in the late February to early March time periods, when sea ice concentration and extent is minimum. Minimum intensity occurs in the late October time periods, when sea ice concentration and extent is maximum. The second major feature displayed in the spectrogram is the sequence of ocean swell arrival events signifying teleconnection with swell-producing storms distant from Antarctica. The high-intensity (red/orange color) diagonal swaths seen in the record (tilting from lower left toward upper right), particularly from early January to end of March during both years of deployment (2005 and 2006) are sea-swell arrival events characterized by frequency-dispersed wave trains. Ninety three such swell events are identified in the seismometer record, 83 were identified with specific swell producing storms through comparison with NOAA Wave Watch III wave model analysis.
Figure 14 - Foci of swell origin for observed sea-swell-arrival events at Nascent Iceberg (denoted by gold star in the Ross Sea, Antarctica) determined from comparison of seismic data and the NOAA Wave Watch III (NWW3) analysis (main map, Robinson projection; small map, gnomonic projection). Swell origin foci are generally within the northern and southern hemisphere extratropical storm tracks, however two foci are associated with tropical typhoons. Triangles denote foci associated with the November to May period of 2004-2005. Circles denote foci associated with the October to May period of 2004-2005. Two dark dashed lines emanating from Nascent Iceberg (gold star) are great circles denoting limits unobstructed view where storms in the Pacific and Southern oceans have direct, unobstructed great circle paths to the seismometer observation site. Foci located within this realm are colored blue or red (depending on time period of observation denoted by symbol). The western limit is determined by the great circle connecting Nascent to Cape Adare, Antarctica. The eastern limit is determined by the great circle connecting Edward VII Peninsula to Nascent Iceberg. Foci located in parts of the Pacific, Indian and Southern oceans which have obstructed great circle paths to the seismometer site are indicated in gray. Diffraction may account for the observation of swell generated by sources that are shadowed by land masses, however we do not rigorously test this suggestion.
Figure 15 - Analysis of wave dispersion associated with trans-oceanic propagation. (a) Significant wave height from NWW3 for an 18 December, 2005, storm in the Gulf of Alaska (foci indicated by star, zone of all possible swell source foci compatible with spectrogram derived source-to-receiver distance indicated by two white lines). (b) - (f) Distribution of peak period of swell predicted by NWW3 analysis for 18 - 26 December, 2005, during which swell from the source indicated in panel (a) (denoted I in the figure) and swell from a previous storm which followed a similar path (denoted II) propagated south toward the Ross Sea (bottom center of maps). Red colors denote the advancing band of long-period swell (with frequency in the 0.05 Hz to 0.1 Hz band observed by the Nascent Iceberg seismometer). Deepest red color (16 - 20 s period) advances fastest southward across the Pacific, because in the deep-water limit, long waves have faster group velocity than short waves. Panels (d) and (f) show projected peak period fronts (dashed white lines) arriving at Nascent. (g) Spectrogram of seismometer signal at Nascent Iceberg showing arrival of swell from the 18 February storm (see panel a). Gray bands denote the times (labels appear on the bands in panel h) associated with the NWW3 analysis pictured in panels b - f. The onset of swell energy at 0.04 Hz at the time of panel f (26 December, 2005) agrees with the arrival of peak periods > 16 s implied by the NWW3 analysis. (h) Peak frequency (solid line) at a point of the NWW3 analysis just north of the Ross Sea indicated by a star in panel (b). The peak frequency at this point at times indicated by gray bands labeled b through f are in general agreement (i.e., within 0.02 Hz) with the frequency of maximum signal strength seen in the spectrogram above.
- Glaciogenic Tsunamis in the Antarctic Coastal Environment. In addition to the above phenomena, the seismic data revealed that impulsive creation of ocean surface-waves occurs frequently (e.g., 200 events per year in the Ross Sea) in the ice-shelf and iceberg covered environment of coastal Antarctica. The 368 events recorded by our field deployment suggest that these impulsive events are generated by glaciological mechanisms, such as (a) small-scale calving and edge wasting of icebergs and ice shelf ice fronts, (b) edge-on-edge closing and opening associated with iceberg collisions, and (c) possibly the impulsive opening of void space associated with ice-shelf rifting and basal crevasse formation. The observations described here provide a background of glaciogenic ocean-wave phenomena relevant to the Ross Sea and suggest that this phenomena may in the future be exploited (using more purposefully designed observation schemes) to understand iceberg calving and ice-shelf disintegration processes.
Figure 16 - Mechanisms by which icebergs and ice shelves can generate impulsive surface waves in the ocean (glaciogenic tsunamis).
Figure 17 - A catalogue of micro-tsunami events recorded in the Ross Sea by seismometers on icebergs C16 and B15A and on the Ross Ice Shelf at Nascent Iceberg. For each example a section seismogram of vertical channel (LHZ) seismometer output (units of counts, proportional to ground velocity) is displayed above a signal spectrogram of the event (dB of log10 of counts2 per Hz, color bars at bottom). Typical spectrogram patterns indicate linear frequency dispersion patterns (i.e., df/dt is approximately constant, where f is signal frequency and t is arrival time) consistent with surface-gravity wave dispersion associated with deep-water limits and null elastic-flexure effects. The slopes of the linear dispersion patterns are used to estimate distances each event's wave train traveled through open, ice-free water. The event in the upper right panel displays a close-up of the seismogram, because the waveform is particularly impulsive and well-distinguished from noise.
Figure 18 - Maps of micro-tsunami source loci associated with events recorded on iceberg C16 (station A, left panel), McMurdo Ice Shelf (middle panel) and Nascent Iceberg (right panel). Source loci are small circles mapped around the various receiver sites (seismometer sites) with radii determined from the dispersion characteristics of the wave arrival. While insufficient to unambiguously locate event sources (due to the stations not operating at the same time or not recording the same events simultaneously), the geographic small circles in each of the three panels are consistent with sources in the collision zones between the icebergs north of Ross Island, with sources around the edges of the icebergs, and with sources along the calving front of the Ross Ice Shelf or in the interior edges of ice-shelf rifts.
C. Ice-Shelf Tidal Motion Measurement and Modeling. Three stations near the calving front of the Ross Ice Shelf, Antarctica, recorded GPS data through a full spring-to-neap tidal cycle in November 2005. The data reveal a diurnal cycle of horizontal motion which varies both along with, and transverse to, the long-term average velocity direction, similar to tidal signals observed in other ice shelves and ice streams. Based on its periodicity, it is hypothesized that the signal is related to the tides, which are strongly diurnal in the Ross Sea. To assess the influence of the tide on the ice-shelf motion, two hypotheses are developed and a finite element model is created, based on creep flow, to test these hypotheses and to determine the mechanism, or combination of mechanisms, generating the velocity variations at the front of the Ross Ice Shelf. The first hypothesis addresses the direct response of the ice shelf to tidal forcing, such as forces due to sea-surface slopes or forces due to sub-ice-shelf currents and associated basal drag. The second hypothesis involves the indirect response of the ice-shelf flow to the tidal response of the Siple Coast ice streams, which feed the ice shelf. The diurnal, horizontal velocity variations previously observed in the ice streams have been described as either sinusoidal (similar to the smooth signal recorded in the GPS record at the front of the Ross Ice Shelf), or as an abrupt, tidally triggered, stick-slip motion, where most of the forward flow happens in short durations (on the order of 10 to 30 minutes) to velocities on the order of 10,000 m per year. The quasi-static creep-flow model developed to investigate the two hypotheses provides a relative sense of the significance of the direct and indirect forcing, but fails to simultaneously predict both the magnitude and the smooth, periodic nature of the signal observed at the front of the ice shelf.
Figure 19 - Maps of Ross Ice Shelf horizontal flow variation (relative to annual mean flow) at different points during the tidal cycle (colorbar indicates m per year velocity anomaly). Red dot on line-graphs indicate state of vertical displacement associated with the tide observed at Nascent Iceberg.
D. Analysis of Ice-Shelf Rift Imagery (Automated Web Cam). During the 2004-2005 Austral summer, an automated camera system was installed on the north rim of a large iceberg detachment rift on the Ross Ice Shelf to monitor its behavior. This study examines the accumulation of snow within the rift using simple photographic measurements. Results suggest that the snow accumulation rate inside the rift is about 10 times the rate on the snow surface of the ice shelf measured by an automatic weather station (AWS) located about a kilometer downwind of the rift, and confirms the suggestion that rifts entrap large volumes of blowing snow. A simple, analytical model of a rift-wall-hugging snow apron seen in the camera images suggests that this feature originates from ballistic in-fall of saltating snow that passes across the upstream edge of the rift. This model, and a simple ad hoc functional relationship between snow-volume flux and horizontal saltation velocity, allows estimation of the mass flux associated with snow saltation on the ice-shelf surface upwind of the rift. Based on the observed AWS wind speed, the flux suggested by this analysis appears to exceed the relationship between saltating snow flux and wind speed derived in wind-tunnel experiments. This excess may suggest, if not otherwise explained by observational shortcomings, that snow saltation on the Ross Ice Shelf begins, and has greater mass flux, at lower wind speeds than implied by the experiments.
Figure 20 - (upper left) Automated camera system in operation on the north edge of a rift on the Ross Ice Shelf. (upper right) Aerial view of the camera system. (lower panel) Schematic diagram of the rift-cross section directly in view (camera facing south) of the camera. Four processes are observed by the camera, from left to right in the illustration: burial of large firn blocks by snow accumulation, vertical sinking of the snowscape associated with hydrostatic equilibrium in the sea-water that supports the frozen rift floor, rift widening associated with a break in the frozen rift floor just beyond a mound that hides the break from the camera view, and finally growth of a snow apron that hugs the rift wall. A snow cornice are depicted on the south, camera facing wall of the rift, although this feature is very unstable and tends to collapse every 30 days or so. Distances of change are associated with each of the 4 processes, and are the values that best represent the measurements made on images obtained over the 136-day period of camera operation.
Figure 21 - (a) East view, (b) south view, and (c) west view produced during times of excellent visibility. The east view (a) is used to measure rift width (using the fact that the rift cliff was measured to be 30±1 m above the snow surface of the rift floor) and widening rate. (Panels d-g are not reproduced to a uniform scale.) (d) Snow cornice infall occurs roughly once every month and produces a debris field of moderately sized snow blocks at the foot of the south, upwind rift wall (top image, before; bottom image, after). (e) Snow cornice development and infall is the most identifiable example of snow influx to the rift floor because it is an episodic process associated with large visual changes to the rift lip morphology (top image, before; bottom image, after). (f) Objects in the foreground (approximately 65 m from the camera) appear to be buried by snow accumulation around them (top image, before; bottom image, after). The rate at which such objects are buried, based on image comparison, is assumed to reflect the general snow accumulation rate within the central part of the rift floor. (g) Comparison of images at the beginning (left) and end (right) of the camera deployment suggest that rubble in the foreground is being buried by high snow accumulation rates, that the mound of rubble in the foreground is sinking, perhaps by rotating downward toward the opposing rift wall, that the opposing rift wall is receding from the camera and the mound of rubble (with a break in the rift floor just beyond the rubble), and that a snow apron is growing against the opposing rift wall.
E. Iceberg Firn Temperature Analysis. Analysis of the firn temperature/depth profile on iceberg C16 is an ongoing process (and will continue as long as data is transmitted from C16). Initial results involve the application of inverse methods to the analysis of temperature data as a means of determining basic thermal parameters needed in future analysis. Two inverse methods are proposed as a means of estimating the thermal diffusivity of snow and firn from continuous measurements of their temperature. The first method is applicable to shallow depths where temperature experiences diurnal variations, and is based on the fact that phase and amplitude of these diurnal variations are functions of the thermal diffusivity. The second method is applicable to the deeper part of the firn layer, and is based on a simple least-squares estimation technique. The methods applied here differ from various methods used for borehole paleothermometry in that observations are continuous in time and performance constraints on model/data misfit can be applied over a finite temporal period. Both methods are tested on temperature records from thermistor strings operating in the upper 2.5 meters of firn on iceberg C16 (Ross Sea, Antarctica) from 2004 to 2007. Results of the analysis show promise in identifying melting events and the movement and refreezing of melt water within the snow/firn layer.
Figure 22 - Temperature evolution during the first 18 months of the operation of a thermistor string deployed in the top 16 m of the firn layer at Nascent Iceberg. Iceberg C16 is equipped with a similar firn thermistor string.
Figure 23 - Temperature evolution on iceberg C16. Snow/firn temperature (°C) observed at 7 cm depth (a), diurnal temperature variations at 7 and 15 cm depths (b), and a spectrogram (color bar shows the log10 of the power spectrum as a function of period and time) at 7 cm depth (c). The phase of diurnal variation in the 15-cm temperature record (green line, panel b) is shifted by about 3 hours relatively to the 7-cm record (blue line, panel b). Warm color (deeper red) in the spectrogram indicates that the signal varies strongly at the associated period range through the given time period. Periods of strong diurnal variation in the 7-cm temperature record are indicated by ellipses.
Summary
The project features a wide range of interdisciplinary findings that cover a range of results, from glaciological to seismological aspects of iceberg drift and behavior. While several of the major objectives of the research project have been met (e.g., observation of large, tabular iceberg drift; source mechanisms for the generation of iceberg tremor), many of the ancillary objectives (e.g., understanding how icebergs melt when they drift into warmer parts of the southern ocean) remain to be achieved. A welcome aspect of the project's results is that many of the findings were unanticipated, or resulted from aspects of research that developed in response to the nature of the field effort as the project evolved (e.g., study of blowing snow near iceberg calving rifts). Further information on the project's results can be found in the publications cited elsewhere in this final report.
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