source of image – NOAA
In the March 2012 issue of Physics today, there is a very informative article titled
by Carpenter Jeff R.and Mary-Louise Timmermans, 2012. Temperature steps in salty seas Physics Today. March 2012. Volume 65. Issue 3. pp. 66.
The article reports on findings with respect to the Arctic Ocean, and its vertical distribution of temperatures. It also presents information which indicates that the modeling of changes in this vertical distribution is quite a challenge.
The article includes the text [highlight added]
“Bodies of water tend to settle into a state in which the fluid density increases with depth. That tendency, called density stratification, is often a dominating influence on the physics of lakes and oceans. The phenomenon, however, is more complicated when the water contains dissolved salts. Along with heat, salts act to change the density of water—the higher the salinity, or concentration of dissolved salts, the denser the water. If salinity increases with depth, then a water body can maintain density stratification even as its temperature increases with depth. Likewise, density stratification is possible for the reverse situation in which temperature and salinity decrease with depth. But because salt and heat diffuse at different rates, those density-stratified states can become unstable.”
“…….the Arctic Ocean…..is similar to a lake in that it has a limited connection to the bordering Pacific and Atlantic oceans. Relatively warm and salty Atlantic waters enter the Arctic through narrow channels close to Greenland. Being slightly denser than the surface waters of the Arctic, the Atlantic water descends to a depth of a few hundred meters as it circulates. The surface waters of the Arctic are extremely cold and fresh due to water flowing into the ocean from rivers in the surrounding continents and contact with the cold Arctic air, among other things. That cold, fresh surface water overlies the warm, salty Atlantic inflow; thus the conditions needed for double diffusion are in place.”
Earlier in the article, the authors describe this “double diffusion” as
” [D]ouble diffusion…….requires at least two components that affect water density (usually temperature and salinity) and …. the components must have different molecular diffusion speeds. Double diffusion occurs over vast areas of the world’s oceans….the outcome is a staircase structure of the water column,….. The steps exist because vertical fluid motions are constrained by the stable density gradient. But neither how the staircases form nor what determines the thickness of the layers is entirely clear, and both constitute active areas of research.”
“The strong salinity stratification of ….. the Arctic Ocean limits mixing and effectively isolates the deep waters from the surface…..”
“The Arctic contains enough heat in the deep ocean to entirely melt the sea-ice pack. However, across much of the central Arctic Ocean, the staircase structure indicates that upward transport of deep heat is minimal. Density stratification acts as a cap on the transport of heat from the deep Arctic, and oceanographers are carefully watching the staircase for indications of changing heat transport.”
“Instruments tethered year-round to drifting sea-ice floes have enabled scientists to obtain a detailed picture of the double-diffusive Arctic staircase, even in the most remote and hostile regions of the ocean. The results of those intensive measurements show that each individual step of the staircase extends across almost the entire ocean basin. That means mixed layers on the scale of 1 m in the vertical have horizontal extents on the order of 1000 km, an aspect ratio of 106! A sheet of paper the size of a football field would have a similar aspect ratio.”“In addition to discovering the immense horizontal scales of the staircases, scientists have made advances in resolving the tiniest scales of variability. So-called microstructure profilers fall freely through the water column collecting measurements that can resolve turbulent fluctuations at a scale of just a couple of millimeters. Measurements …..in the Arctic Ocean have contributed to a growing body of evidence suggesting that the interfaces separating mixed layers are nonturbulent; transport across them is by molecular diffusion. Evidently, the individual staircase steps that stretch across vast regions of the Arctic Ocean are ultimately linked by the molecular collisions responsible for diffusion.”