Oceans Melting Greenland (OMG) (February, 2017)

Monday, February 27, 2017

Why pay attention to Greenland? Greenland’s ice sheets are melting and contributing to global sea level rise. There is enough ice on Greenland that global sea level can rise by 6 meters if it were all to melt. Satellites can tell us how much ice mass loss is occurring, thanks to GRACE, but that does not provide a full picture of the processes that are causing the melt. To better understand this question the Oceans Melting Greenland (OMG) project will survey Greenland’s glaciers and ice sheets and study how the oceans and ice interact with each other and the physical parameters that effect melt that cannot be captured by satellites. The data and information gathered will help enhance ocean and ice sheet models and make sea level predictions more accurate.

OMG is a 5-year mission that started in 2015. Most satellites cannot see through ice so OMG will be using shipboard (Figure 1), airborne (Figure 2) and in situ measurements to tell us what the satellites cannot. These data will help our understanding of what fundamentally affects the melt rate of Greenland’s ice by providing information on how the ocean interacts with the ice sheets, including circulation in the fjords, movement of ice sheets and the physical properties of the ocean water (Figure 3). In 2015 and 2016 bathymetric surveys were taken around Greenland’s coast to map the seafloor depth on the continental shelf and within the long narrow fjords that bring ocean water to ice sheets.  To measure the depth of the ocean on the continental shelf OMG used airborne gravimetry, AirGrav for short, a highly sensitive instrument that “weighs” the amount of water beneath the aircraft.  Ocean depths in fjords were measured using  Singlebeam and Multibeam Echo Sounders (MBES), highly accurate instruments that can create3D maps of the seafloor (Figures 4 and 5). These sonar surveys revealed valuable information about exactly where the ice and ocean come into direct contact. A Conductivity-Temperature-Depth (CTD) instrument is also deployed from the sonar ship allowing scientists to better understand how ocean properties change through space (Figure 6). The CTD measures the salinity (via conductivity), temperature, and depth of the water column, all of which is used to determine the circulation patterns that can lead to ice melt.

In 2016 OMG flew its first airborne radar campaign using the Glacier and Ice Surface Topography Interferometer (GLISTIN-A). GLISTIN-A is an airborne radar instrument that will measure the elevation and retreat of coastal glaciers (Figures 2 and 7). Each year the radar provides a high spatial resolution map of ice surface elevation, which will be used to measure how the ice sheets and glaciers are shrinking and retreating (Figure 8).

While CTDs are taken from the ship and provide accurate measurements of the water column they are limited to where the ship can navigate and how much ship time is available. To supplement the ship CTD measurements, similar measurements are made by dropping ocean probes from a plane. The Airborne eXpendable Conductivity Temperature and Depth (AXCTD) is a special type of CTD that can be dropped from a plane. After reaching the ocean, the AXCTD communicates the ocean temperature and salinity from the surface to the seafloor via radio waves to the airplane (Figure 9).

Both the AXCTD and GLISTIN-A radar airborne components will be repeated each year of the OMG mission.

The data collected as part of OMG will help us understand the interactions between the ice and the ocean, the overall rate of melt and identify areas are of greatest concern for high melt rate. The preliminary data can currently be found on the OMG web portal . As the data are further processed they will be distributed at PO.DAAC for long term archival so the data are available after the OMG mission ends. OMG results will be used to improve ice models so that they better reflect the physics occurring and provide a more accurate assessment of the ice melt in Greenland and sea level change.


PO.DAAC Science Team in collaboration with Josh Willis (OMG PI) and Ian Fenty (OMG)




Figure 1.Ship getting supplies in a Greenland harbor. (photo credit: https://omg.jpl.nasa.gov/images/2015-survey-p2/OMG-Photo-20150921055300-00.png)



Figure 2.Photo of NASA's Gulfstream-III aircraft on the tarmac at Armstrong Flight Research Center with the Glacier and Ice Surface Topography Interferometer GLISTIN-A radar instrument installed, taken on March 15, 2016. Image courtesy of NASA/Ken Ulbrich. (photo credit: https://omg.jpl.nasa.gov/images/2016-glistin/OMG-Photo-20160315220018-00.jpg)

https://omg.jpl.nasa.gov/images/graphics/OMG-GLISTIN-AXCTD.gif  https://omg.jpl.nasa.gov/images/graphics/OMG-Ship-AIRGrav.gif

Figure 3. The top animation depicts an aircraft with the Glacier and Ice Surface Topography Interferometer (GLISTIN-A) instrument observing changes in the thickness and retreat of the glacier front as well as an aircraft deploying Airborne eXpendable Conductivity, Temperature and Depth (AXCTD) probes to measure ocean temperature and salinity on the shelf. The bottom animation depicts a ship collecting measurements of the depth and shape of the sea floor, as well as an aircraft measuring free-air gravity with the AIRGrav instrument, which also provides information about the depth of the ocean. (photo credit: https://omg.jpl.nasa.gov/portal/gallery/graphics)



Figure 4. Cruise track for the first phase of the MBES survey and CTD stations. (photo credit: https://omg.jpl.nasa.gov/images/graphics/OMG_Phase1_Cruise.jpg)



Figure 5.Bathymetry measured by MBES. On the right is a map showing the location of the bathymetry survey above.



Figure 6. “Ocean temperatures on the inner shelf and two fjords at the end of a major cross-shelf trough. Yellow symbols indicate CTD locations in (a). Atlantic Water likely approaches the inner fjord along the southern edge of the deep trough. (b) Some Atlantic Water flows into Upernavik Isfjord toward the glaciers while some continues north toward Cornell Glacier in Ryders Ford. Atlantic Waters are warmest near the entrance of Upernavik Isfjord (3.8°C at 350 m) and cooler at the entrance of Ryders Fjord (2°C at 350 m). Vigorous mixing from plumes of ascending fresh subglacial discharge likely enhances glacier melting at Upernavik North Glacier and warms the outflowing mid-depth waters. In contrast, no evidence of similar mixing and middepth warming is found at the shallower Cornell Glacier” (Fenty et al. 2016).


Figure 7.“The sampling strategy for the yearly Airborne eXpendable conductivity, temperature, and depth (AXCTD) and Glacier and Ice Surface Topography Interferometer (GLISTIN-A) campaigns. Over most of the continental shelf, AXCTD probe spacing (represented by yellow diamonds) is approximately 50 km, adequate to resolve the largescale spatial variations of Atlantic Water. AXCTD campaigns will take place in late September and early October when seasonal sea ice cover is near its annual minimum. GLISTIN-A swaths, shown as red 10 km bands are located across or near the faces of most of Greenland’s marine terminating glaciers. Green swaths in the northwest and north sector are lines that were missed in the March 2015 survey due to instrument problems” (Fenty et al. 2016).



Figure 8.“Surface elevation observed by the GLISTIN-A radar at Jakobshavn Isbræ. GLISTIN-A elevations are shown over Google Earth satellite imagery. The mean elevation in this region is estimated to be 609 ± 10 m above the sea level in the fjord. Three along-track elevation profiles in the inset show that the glacier rises about 1,000 m over a distance of roughly 35 km from the front” (Fenty et al. 2016).



Figure 9.A test flight deploying an AXCTD. The parachute can be seen after the AXCTD is deployed. Click on image to see the video. (video credit: https://io.jsc.nasa.gov/photos/11879/video/jsc2016m000524.mp4)