Reblog: The currents and winds of Nares Strait

Reblog of original Icy Seas post:

[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.

Nares Strait MODIS satellite imagery of the study area and ice arch April 21, 2008. Red dots are instrument locations. Arrows show current velocities.

Nares Strait MODIS satellite imagery of the study area and ice arch April 22, 2009. Red dots are instrument locations. Arrows show current velocities. Note the lack of the southern ice arch, but the presence of one north of the study area.

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.

Along-channel current velocities averaged over three months (April-June) for 2008 and 2009. Data from five different instruments are shown. The locations can be seen in the above map, and labels are in ascending order 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.

Filtered time series of current data from KS08 at 41m depth from April to June of 2008 and 2009, as well as wind data from Roger Samelson’s regional circulation model at 80.38 degrees N. The filter removes noise and tides.

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.

A plot of the r-squared values of correlation between wind and current data as they vary with depth. Data from five instruments are shown.The locations of the instruments can be seen in the above map, and labels are in ascending order from Ellesmere Island to Greenland.

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.


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