Ask a Question

If you have any questions (about the Arctic Ocean, life at sea, careers in science, etc.) for me or any of the scientists and crew during the cruise, you can email at my school email address, and I will post your question(s) and the corresponding answer(s) below. Please include "Arctic Andy Q&A" in your email subject heading.

If you are a parent or teacher, encourage your children or students to submit questions. It would be great engage K-12 students, so they can learn directly from cruise participants while we are at sea.

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Thanks for following! I look forward to your questions!

Q and A:

#1: Based on photos of the CTD display from Shifty People, it looks like temperature change with depth is very chaotic in the upper layers of the water column — there are a few thermoclines, temperature increases to a maximum in the middle, and then gradually declines. Does this sound correct?

Please explain how the temperature changes with depth and why.

—Dave, in Okinawa, Japan

My response is based on a profile of temperature vs. depth from station 14, which I describe below.

The red profile is of temperature (x-axis) vs. depth (y-axis), which were both measured using the CTD sensor package on the rosette (overview in About the Cruise). The top panel extends from the ocean surface to a depth of 250 m, while the bottom panel continues from 250 m to 2250 m near the seafloor. Also included is a map showing the location of station 14. Note: Profile featured in Shifty People was from station 15, but station 14 sampled a slightly clearer temperature profile of the features in question.

Looking at the profile, the first thing you might notice is that nearly all of the water is subzero (<0°C), with exception of the warmer Atlantic Water, named after its origin, traced by the >0°C signal found at depths of about 250-1000 m (bottom panel). The reason the subzero Arctic water isn’t frozen is because it contains salt, which lowers the freezing point of seawater to approximately -1.8°C. If temperature drops below -1.8°C (it can and frequently does at the surface), sea ice forms, causing salt to be rejected as ice crystals are formed, brining the water below it.

Now, if we look at the top panel where there are large gradients and sharp peaks in temperature, I’ll attempt to explain why the temperature structure looks so chaotic there.

From reading About the Arctic Ocean, we know that the Polar Mixed Layer (PML) is a layer that extends from the surface to depths of 50 m in the Arctic Ocean. In oceanography, a “mixed layer” refers to the upper layer of the water column that is mixed near the surface due to wind and wave activity, resulting in a layer with homogeneous properties. If we look at the upper 50 m in the top panel, we surprisingly don’t see a homogeneous mixed layer — why is that?

My guess on why the PML is not homogeneous is because of the melting (or formation) of sea ice, which the PML is strongly influenced by (About the Arctic Ocean). Since sea ice is fresh due to salt rejection (see above), its melting point, like its freezing point, would be 0°C. Because of this, I suspect that the spikes in the temperature profile in the upper 50 m are due to sea ice melting. As this newly melted water entrains salt from neighboring seawater, it becomes denser and sinks, resulting in strong temperature gradients (a.k.a. thermoclines) in the upper 50 m where small, relatively warm layers penetrate the PML.

Layer formation, based on differences in temperature and salinity, is demonstrated in this short video.

Below the PML at about 60 m is a local temperature maximum (>0°C), which may result from the sea ice melting process described above. However, I suspect that this temperature spike is a signature of summer Bering Strait Water (sBSW). Underneath the sBSW is a strong temperature gradient (or thermocline) as the water temperature decreases to a minimum of about -1.4°C, referred to as winter Bering Strait Water (wBSW). Since the Bering Strait is only about 50 m deep, Pacific water flowing northward through the strait is strongly influenced by seasonal variations in temperature, resulting in it either being warmer (in summer) or colder (in winter). This summer and winter BSW continues on its northward journey, eventually moving off the Chukchi Shelf to create layers like those seen in the upper panel of the temperature profile.

The layers described above that compose the upper 200 m of the Canadian Basin are generally referred to as Polar Surface Water (PSW).

Underneath the PSW is the warm Atlantic Water, previously introduced above. Continuing downward is the Upper Polar Deep Water (UPDW), which is the deepest layer that spans the entire Arctic Ocean, eventually leaving through the Fram Strait (see map in About the Arctic Ocean). Below 2000 m at this station is the Canadian Basin Deep Water (CBDW), which is isolated from the Eurasian Basin by the Lomonosov Ridge. Near the bottom of the CBDW, temperature faintly increases, caused by geothermal heat input from below.

That’s it for my explanation of why temperature changes with depth, from top to bottom! Please, let me know if you have further questions about this Q&A, or if you have any other questions about the Arctic Ocean, life at sea or the science being conducted on the U.S. GEOTRACES Arctic Expedition!