Tag Archives: sea level rise

Petermann Glacier Tidal Heaving

Some glaciers float on the ocean around Antarctica and Greenland. Petermann Gletscher in North Greenland is one of these. It spawned massive Manhattan-sized ice islands in 2010 and 2012. Could tides influence when and where such break-ups occur? After all, the tides under the floating glacier move the ice up and down. But how does a 50 km long, 15 km wide, and 300 m thick floating glacier pivots about its “hinge?” Does it do so like a rigid plate of steel or does it bend and buckle like jelly? I do not know, because nobody has measured the tidal motions of Petermann’s floating ice. So, one of many projects this summer will be to measure tides on Petermann with fancy GPS systems.

Shape of the floating part of Petermann Gletscher (right panel) drom laser altimeters along two tracks flown along the glacier in 2014 (left panel).

Shape of the floating portion of Petermann Gletscher from laser altimeters (right panel) along two tracks flown along the glacier in May of 2014 (left panel).

Martin Jakobsson of Stockholm University posed these questions, sort of, when he asked us American oceanographers, if we had any fancy GPS units to work with one he plans to put high on a cliff overlooking Petermann Fjord. He needs exact positions to map the bottom of the ocean. The cliff-GPS station is fixed while he moves about in a small boat that also has a GPS. Taking the difference of the raw travel times received by the cliff-GPS and the boat-GPS, he can reduce GPS position errors from several meters to several centimeters. People call this differential GPS and he wondered if we oceanographers had any use of it to perhaps give him the tidal corrections he also needs as the measures bottom depths from a boat. Well, this was not initially part of our plan and we did not get funded to study the glacier or the tides under it, but his question got me thinking while Alan Mix of Oregon State University did some organizing. One always squeezes extra science into such great opportunities. Discoveries lurk everywhere to inquiring minds.

Small survey boat loaded onto I/B Oden in Landskrona, Sweden, June 2015.

Small survey boat loaded onto I/B Oden in Landskrona, Sweden, June 2015.

Alan managed to find not one, not two, but three fancy GPS units from an organization that I had never heart of. It is called UNAVCO:

UNAVCO, a non-profit university-governed consortium, facilitates geoscience research and education using geodesy. We challenge ourselves to transform human understanding of the changing Earth by enabling the integration of innovative technologies, open geodetic observations, and research, from pole to pole.

“Geodetic observations” are measurements of locations on the earth’s surface. In the old days surveyors walked about with sextant, clocks, tripods, and optical devices to fix a location and reference it to another. Nowadays satellites and lasers do this faster, but I digress. Suffice it to say, UNAVCO is giving us 3 fancy GPS system to carry with us to Petermann Gletscher to make measurements of tides on the ice. So we can pick 3 locations on the ice where we leave these GPS for the 3-4 weeks next month. I have never done this before, so there will be lots of new learning.

Navigation during early Arctic exploration. Photo taken during a visit of the Peary MacMillan Arctic Museeum at Bowdoin University in Brunswick, Maine.

Navigation during early Arctic exploration. Photo taken during a visit of the Peary MacMillan Arctic Museeum at Bowdoin University in Brunswick, Maine.

I have worked with tides since plunging my head into tidal mud-flats of north-west Germany where I grew up and camping on the shores of the Conwy Estuary in North-Wales where I collected data for my MS thesis. Below I show a 4 week record from three locations in Nares Strait where the tidal elevations range from more than 4 meters at the southern entrance to less than 2 meters in Hall Basin next to Petermann Fjord. The data are from bottom pressure sensors that were deployed for 3-9 years, but I here only want to show the spring-neap cycle. So we already have some idea on how the tides in the ocean next to Petermann Glacier behave.

Sea level fluctuations in meters for 28 days at Discovery Harbor or Fort Conger, Canada near 81.7 N latitude (top), Alexandra Fjord, Canada near 78.9 N latitude (middle), and Foulke Fjord, Greenland near 78.3 N latitude (bottom).

Sea level fluctuations in meters for 28 days at Discovery Harbor or Fort Conger, Canada near 81.7 N latitude (top), Alexandra Fjord, Canada near 78.9 N latiude (middle), and Foulke Fjord, Greenland near 78.3 N latitude (bottom).

Models of tides in Nares Straits do really well if, and only if, the bottom topography is known. And this is where Martin’s mapping of the ocean floor in Petermann Fjord and our tidal observations on the floating glacier come together: We both need good bottom topography, we both use fancy GPS, and we both need to know tides to get accurate bottom depths and we need to know bottom depths to predict tides.

A Short Summary of Nares Strait Physics

The Arctic Ocean is a puddle of water covered by ice that melts, moves, and freezes. Grand and majestic rivers of Siberia and America discharge into the puddle and make it fresher than Atlantic Ocean waters. The fate of the Arctic freshwater helps decide if Europe and the US become warmer or colder, experience more or less storms, droughts, or floods, and if global sea level will rise or fall. In a nutshell: the fate of Arctic freshwater determines climate.

Arctic Ocean with Nares Strait study area (red box) with tide gauge locations as blue symbols and section of moored array as red symbol. Contours are bottom topography that emphasize ocean basins and continental shelf areas.

Arctic Ocean with Nares Strait study area (red box) with tide gauge locations as blue symbols and section of moored array as red symbol. Contours are bottom topography that emphasize ocean basins and continental shelf areas.

Nares Strait connects the Arctic and Atlantic Oceans to the west of Greenland. It is narrower than Fram Strait, but it transports as much fresh ocean water as does its wider sister facing Europe. Few people know this, including climate scientists who often model it with a bathymetry that is 10,000 years out of date from a time when Nares Strait did not yet exist. This is why the US National Science Foundation funded a group of oceanographers to use icebreakers, sensors, computers, and innovative engineering to collect and analyze data on the ice, the water, and the atmosphere.

Acoustic Doppler Current Profiler mooring deployment in Nares Strait from aboard the CCGS Henry Larsen in 2009.

Acoustic Doppler Current Profiler mooring deployment in Nares Strait from aboard the CCGS Henry Larsen in 2009.

Within days of the start of the grant I had to appear before the US Congress to answer questions on Petermann Glacier that discharges into Nares Strait. In 2010 a large 4-times Manhattan-sized ice islands broke off and people wanted to know if global warming was to blame. I was asked how ocean temperatures and currents relate to this and other events and what may happen next. My few data points were the only existing data for this remote region, but I had not yet had the time to analyze and publish much. Two years later another large 2-Manhattan sized ice island formed from the same glacier, but this time we were better prepared and people world-wide went directly to our data, thoughts, and stories when this blog was sourced in news papers in France, Germany, and China. Al Jezeraa, BBC, and PBS reported on it, too, giving me chance to connect via TV, radio, and pod-casting to a larger public.

Petermann Gletscher in 2003, 2010, and 2012 from MODIS Terra in rotated co-ordinate system with repeat NASA aircraft overflight tracks flown in 2002, 2003, 2007, and 2010. Thick black line across the glacier near y = -20 km is the grounding line location from Rignot and Steffen (2008).

Petermann Gletscher in 2003, 2010, and 2012 from MODIS Terra in rotated co-ordinate system with repeat NASA aircraft overflight tracks flown in 2002, 2003, 2007, and 2010. Thick black line across the glacier near y = -20 km is the grounding line location from Rignot and Steffen (2008).

While it was exciting and fun to share Nares Strait and Petermann Gletscher physics with a global audience, it is not what we had planned to do. Our goal was to put real numbers to how much water, ice, and freshwater was moving from the Arctic to the Atlantic via Nares Strait. So the next 3 years we labored through our extensive records to first describe and then to understand what was happening in Nares Strait. We found that ocean currents move water always to the south no matter if ice covers Nares Strait or not, no matter if the ice is moving or not, no matter which way the wind is blowing. The physical cause for this southward flow is that the sea level is always a few inches higher in the Arctic Ocean than it is in Baffin Bay and the Atlantic Ocean to the south.

Linear regression of volume flux  through Nares Strait from current meters with along-strait sea level difference from tide gauges (unpublished).

Linear regression of volume flux through Nares Strait from current meters with along-strait sea level difference from tide gauges. (unpublished).

We know, because we measured this with tide gauges that we placed in protected coastal bays. We recovered 3 sensors; most rewarding was the recovery of one sensor that we had failed to reach in 2005, 2006, 2007, and 2009, but in 2012 we finally got the instrument and 9-years of very good data. Batteries and computers inside were still running and recording. I have never seen as clean and as long a time series.

Results from a 2003-12 tide record shows as power spectra with named tidal constituents at diurnal (~24 hours) and semi-diurnal (~12 hours) periods. The red line is a modeled red noise spectra (unpublished).

Results from a 2003-12 tide record shown as a power spectra with named tidal constituents at diurnal (~24 hours) and semi-diurnal (~12 hours) periods. Data are shown as the relative amplitudes of oscillations at frequencies in cycles per day or cpd. The red line is a modeled red noise spectra (unpublished).

From satellite data that we analyzed as part of this grant, we know when the ice moves and when it stops moving. The freeze-up of Nares Strait comes in one of three forms: 1. Ice stops moving in winter, because an ice barrier (ice arch or ice bridge) forms in the south that blocks all southward motion of ice; 2. only new and young ice moves southward, because an ice barrier forms in the north that blocks all entry of Arctic ice into Nares Strait; and 3. Arctic ice moves freely through Nares Strait, because no ice barriers are present. Our 2003-12 study period covers years for each of these different ice regimes. And each of these regimes leads to very different ocean (and ice) flux as a result of very different ocean physics.

Data alone cannot make definite statements on what will happen next with our climate, but we know much new physics. The physics suggest certain balances of forces and energy for which we have mathematical equations, but these equations must be solved on computers that can only approximate the true physics and mathematics. These computer models are our only way to make predictions ito the future. The data we here collected and our analyses provide useful checks on existing models and will guide improved models.

June-10, 2012 MODIS-Terra image showing location of moored array that was deployed in Aug. 2009 to be recovered in Aug. 2012.

June-10, 2012 MODIS-Terra image showing location of moored array that was deployed in Aug. 2009.

Johnson, H., Münchow, A., Falkner, K., & Melling, H. (2011). Ocean circulation and properties in Petermann Fjord, Greenland Journal of Geophysical Research, 116 (C1) DOI: 10.1029/2010JC006519

Münchow, A., Falkner, K., Melling, H., Rabe, B., & Johnson, H. (2011). Ocean Warming of Nares Strait Bottom Waters off Northwest Greenland, 2003–2009 Oceanography, 24 (3), 114-123 DOI: 10.5670/oceanog.2011.62

Münchow, A., Padman, L., & Fricker, H. (2014). Interannual changes of the floating ice shelf of Petermann Gletscher, North Greenland, from 2000 to 2012 Journal of Glaciology, 60 (221), 489-499 DOI: 10.3189/2014JoG13J135

Münchow, A., Falkner, K., & Melling, H. (2014). Baffin Island and West Greenland Current Systems in northern Baffin Bay Progress in Oceanography DOI: 10.1016/j.pocean.2014.04.001

Rabe, B., Johnson, H., Münchow, A., & Melling, H. (2012). Geostrophic ocean currents and freshwater fluxes across the Canadian polar shelf via Nares Strait Journal of Marine Research, 70 (4), 603-640 DOI: 10.1357/002224012805262725

Ruins of Fort Conger in the High Arctic

Retreating from Fort Conger, the U.S. Army lost 68% of its men to death by starvation and drowning. They were delivered to the northern shores of Ellesmere Island within sight of northern Greenland by the SS Proteus on August 12, 1881 and were left with ample food and fuel to survive and explore comfortably for a years or so. Continue reading

Canyon below Ice at Petermann Gletscher

The Grand Canyon of Arizona stands tall in the mind as the Colorado River carved itself into 6000 feet of rock. A similar canyon has been discovered in northern Greenland near Petermann Gletscher. The canyon without a name is buried under 6000 feet of ice, but its size and scale Continue reading

Storm Surges, Global Warming, and Delaware Beaches

ADDENDUM (Nov.-7, 2012): Time lapse video from Delaware Sea Grant.

Rising seas and flood waters cause most of the damage during storms such as Sandy did last week. Tides, waves, and storms all contribute. We can debate how global warming impacts any of the above, but the arguments are involved. So lets assume, that neither tides, waves, nor storms are impacted by global warming, but that the globally averaged rise in sea level over the last 50 or 100 years is. This global warming induced sea level rise is about half a foot in 50 years (3 mm/year), but why would we care about global averages, when we live in Delaware? Furthermore, why worry about the whimpy surges we get ever 2-3 weeks. We don’t, we worry most about the most extreme events like Sandy and want to know how often they occur. Below I show a Sandy-like event to occur about once every 10 years. Furthermore, over time Delaware’s most extreme storm surges are rising twice as fast as global averages do. So, how much does the global warming impact our local flooding in Delaware?

Market Street on the beach in Lewes is in one of the lowest lying areas of town and takes its good old-time draining. This photograph looks northeastward toward the beach, just west of the intersection with Massachusetts Avenue. [Credit: Cape Gazette]

More than I initially thought: the largest storm surge that has hit Delaware was the Ash Wednesday storm on March 6, 1962 which added 5.8 feet to the regular tides and waves. I wrote about this yesterday using public NOAA data. This same storm today would add 6.8 feet to the regular tides and waves. For comparison, Sandy’s storm surge added 5.3 feet. So Sandy was a weak storm by comparison. If it had hit in 1962, it would have added only 4.3 feet. The difference of 1 foot in 50 years is due to steadily rising sea levels:

Largest storm surge at Lewes, Delaware each year from 1957 to present. The red line is a linear fit to the data. The slope indicates that the largest storm surge increase by almost 3 inches every 10 years.

On average each year has a larger largest surge than the year before. While this steady increase by 2.8 +/- 1.7 inches each 10 years is statistically significant (95% confidence), picking the extreme each year is perhaps not the best statistic as extremes do not happen often. Please note that a 95% confidence means that there is a 5% chance that the true increase is either smaller than 0.9 inches/decade or larger than 4.5 inches/decade.

What about the mean or average surge each year? From hourly data, I pick the middle surge, that is, half the surges each year are larger and half are smaller:

This increase of 1.4 +/- 0.2 inches per decade (95% confidence) is more in line of the global average. The uncertainty in this trend is smaller than that of the trend for the extreme, because the median sea level varies little from year to year, while the extreme value varies more from year to year. So, from these results we can conclude, that while the mean or median sea level at Lewes increases by perhaps 1.5 inches in 10 years, the extremes increase twice as fast. So, storm surges like Sandy will become more common than they are today mostly because of global warming.

Over the last 50 years we had at least 5 such events in 1962, 1968, 1996, 1998, and 2012. So, on average we have a Sandy-type storm surge greater than 5 feet every 10 years. This contradicts a Wilmington News Journal article today which quotes John Ramsey to describe “… Sandy as a 1-in-200-years storm, unlikely to be repeated anytime soon. That could give coastal communities time enough to deal with the real threats and realities of sea level rise and climate change.” There is no such time, as it is mis-leading to describe Sandy as a 1-in-200-year event when it has happened about every 10 years during the last 55 years. Instead of a 0.5% percent chance of a Sandy-like event to hit Lewes each year, I would raise this chance to be larger than 10%.