[Editor’s Note: Undergraduate Allison Einolf of Macalester College in Minnesota summarizes her work at the University of Delaware that was supervised by Andreas Muenchow as part of an NSF-funded summer internship.]
I’m about to fly to Thule, Greenland for a research expedition into the Nares Strait. We had planed to survey Petermann Fjord, but our proposed cruise track is facing an obstacle twice the size of Manhattan.
We’re heading up north to pick up instruments that have recorded current velocities, salinity, temperature, and ice thickness in Nares Strait since 2009. I’ve been working all summer on data retrieved on a similar cruise three years ago, focusing on what effects the ice arches have on currents north of the ice arches.
The ice arch existed in the Nares Strait the spring of 2008, but not in 2009, as can be seen in the images above. The images also show the locations of the instruments in the strait. I focused on three months, April through June of each year. During each year, both the currents and the prevailing winds in the area flow southward out of the strait towards the North Atlantic. The images above show the average current velocities from the first 100m at each instrument, but the current also changes with depth as can be seen in the graphs below. Each station is even-numbered 4-12 from Ellesmere Island to Greenland.
What I found is that the currents were stronger in 2009 without an ice arch, than in 2008 with an ice arch. Bottom currents are almost zero.
Why would the currents be stronger in 2009 compared to 2008? Our hypothesis was that with the ice arch present, the ice north of it can’t move much, but when the ice arch is not present, then the more mobile ice can transfer momentum from the wind to the ocean to drive currents, especially near the surface.
To test this hypothesis, I correlated ocean currents with wind data predicted from the regional circulation model of Roger Samelson from Oregon State University. The correlation coefficient r2 estimates the fraction of the ocean current’s variance that the wind can explain. Another interpretation is how closely a plot of wind versus current data resembles a straight line, with 1 being a straight line indicating perfect correlation and 0 being a random scatter indicating no correlation.
Looking at the plots, I find only weak correlations, since the highest it gets is around 0.2. We expected a stronger relationship between the wind at the surface during 2009 than in 2008. Even though the correlation values are very low, we do see a little of that. In 2008, the instrument that was closest to Ellesmere Island was the only one to have a higher correlation with wind at the surface, while in 2009, the other four stations have a higher correlation with the wind, even if it’s not a lot.
So, what does this mean? Does the wind from farther north affect the currents at that site after a few days? What else is creating the currents in Nares Strait? How did that change between 2008 and 2009? Why are the currents stronger in 2009 than in 2008? How is this all connected to the movements of the ice and how the ice is held back in years with an ice arch?
I do not have the answers yet, but I do know, that the local wind in Nares Strait doesn’t seem to be the major factor influencing currents, even though both the wind and the currents mainly flow out of the channel, south towards the North Atlantic. Perhaps currents are driven by the way the sea level in the north of Nares Strait differs from the sea level to the south (external pressure gradient). Or currents are driven by the way that temperature and salinity conspire to form internal pressure gradients. Nevertheless, the prevailing winds and currents will both be driving the ice island from Petermann Fjord south once it leaves the fjord. With an ice island moving towards our research area near Hans Island, our upcoming cruise will be facing a few icy challenges.
Very nice write up Allison!
A few thoughts – that you’ve undoubtedly already rejected.
Is there a possibility that atmospheric pressure differentials between Lincoln Sea and Baffin Bay could have apart to play in the observed differences in current. Pressure Seiches occur in long, narrow bodies of water as high atmospheric pressure forces water towards the low pressure end. Your not going to have a Seiche because Nares is open at both ends, but current velocity would still be affected.
I was also wondering whether, with the new data you’ll be retrieving, you will be using wind speed and direction as measured from Han Island. I know that canyons can experience very strong winds while just above the canyon walls things are calm. I’m sure you’ve taken this into consideration, but the Han Island data might prove more accurate.
A question I have that has nothing to do with your research is in regard to the gyre in Hall Basin. Last year a large chunk of MYI circled counter clockwise 3 times, damaging the fast ice in Peterman Fjord on two of the passes. It finally broke up on Joe island and I quit following what was left. The MYI seemed to rotate more rapidly than thinner bits also circling Hall, so I assumed that the gyre action was stronger at depth.
The questions are:
1) Is the gyre seasonal event, was this a unique/unusual occurrence or is this a regular year round feature.
2) If I’m correct in assuming that the gyre is stronger below the surface, how deep and how strong does it get?
I know you’re getting ready for your trip. If you’ve got better things to do than answer this I understand.
First of all, I would like to thank you for your comments and questions, and I would like to answer as many of your questions as possible, but my knowledge of the subject is very basic, so I may not be able to give the best answers. I am an undergraduate physics major, and all I know comes from a few papers I’ve read this summer.
As per the pressure gradient between Lincoln Sea and Baffin Bay, they may be an influence on the observed current, but that has already been accounted for in the regional atmospheric model of Roger Samelson at Oregon State University ( Samelson and Barbour, 2008), which was the wind model that I used to look for correlation with our current data. So any influence that the pressure gradient had on the currents might be a portion of the overall effects of wind that we found correlated to the currents, although it would be worth it to consider the pressure gradient’s direct influence on currents.
[Editor: Atmospheric pressure differences along Nares Strait drive winds. Pressure gradients in the ocean are a major component of the force balance, explaing perhaps 70% of the ocean current variance at times scales longer than 20 days, e.g., Muenchow and Melling (2008).]
Regarding Hans Island and the atmospheric data that is being recorded there, we will be interested in looking at that data and comparing it with the data we have from Samelson’s model and the relationship that the recorded data has with the current data. The canyon effect that you mention has been considered, and the model estimates low-level winds and surface wind stress, which might possibly be closer to the winds directly affecting the currents than the Hans Island measurements, which are taken above sea level. I don’t actually know much about the specifics of the Hans Island data, but more information can be found here.
[Editor: It would be possible to redo the 2009 ocean-wind correlations with Hans Island wind data, I suspect the results will be the same.]
Concerning a gyre in Hall Basin, my knowledge of that area is much more scattered, but Johnson, et al., 2011 published findings about a cyclonic gyre at the mouth of the Petermann Fjord that came from satellite imagery of mobile ice and calculations of geostrophic flow. Since the area has been difficult to measure during times when there is landfast ice, it is very difficult to know whether this is a seasonal phenomenon and how deep the gyre goes. I don’t know if there has been other research on the topic, since I’m new to the field myself, but I’m glad you brought it up because it gives me a lot to think about as I prepare for the research cruise. I hope I helped answer your questions and spur further inquiry and interest.
[Editor: The counter-clockwise velocity shear of southward ocean currents in Nares Strait is present both at the surface and below the surface even when the winds are in the opposite, northward direction. This velocity difference across the channel forms a gyre in both Hall and Kane Basins. Velocity differences of 0.2 m/s over 20-40 km are not uncommon. They emerge most clearly after the much larger tidal currents are removed.]
Thanks for the reply!
I hope you’re successful in retrieving the data (and that PII2012 stays out of the way). I won’t take up your time with more questions – at least until you’ve returned.
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