Water staircases in seas

New Delhi,SHUBASHREE DESIKAN:Internal waves, which move vertically through the ocean, can sometimes pass through “water staircases,” which are steplike variations of density of water, in such as manner as to churn up the underlying warm, salty water, thereby increasing the temperature of the top, cooler layers, according to a paper published in Physical Review Fluids. This suggests a possible mechanism by which the upper layers of the Arctic Ocean warm up, causing the ice to melt.

Unlike the surface waves we usually observe, internal waves in teh ocean can stretch over scales of 10-100 kilometres.The photograph is used for illustrative purposes only. Photo: M. Karunakaran

Water staircases are steplike variations of density of water due to steplike changes in temperature and salinity. Though internal waves exist where the density gradually increases with depth, they cannot propagate where the density is uniform, for instance, within the steps of the staircase. While the passage of waves through a single step was studied earlier, the interaction between waves and multiple steps was not really understood well and is considered for the first time in this paper.

Ocean warming
The Arctic Ocean has inflows coming from the Pacific Ocean and Atlantic Ocean. In this, the top layers consist of cooler and less saline water and below that is a layer of water coming from the Atlantic Ocean which is more saline and warmer, too. The effect of salinity wins over that of temperature and so, though the water below is warmer, it is heavier than the cooler, less saline layer on top..

“Warm, but salty water — ultimately originating from the Atlantic Ocean resides near the bottom of the Arctic Ocean. If turbulence could somehow mix this water with that above, then, eventually, the surface could warm more rapidly, and this would increase the rate of sea-ice melt. One mechanism for mixing is the result of breaking internal waves,” says Bruce Sutherland, University of Alberta, Canada, the author of the paper, in an email interview.

The internal movement of water within seas from cold, dense regions to warmer, lighter ones constitutes the largest observable waves. These can stretch to wavelengths of over a hundred metres and travel from top to bottom. Like normal water waves, these propagate due to a variation of density of the water and not when there is constant density. In a staircase-like formation, though the density is constant within the step, there is a jump in density from one step to another. Hence, the wave’s energy can be transmitted from one interface to another.

Though not predominant, thermohaline staircases do occur in many places. “As well as the Arctic, they are persistent in the Caribbean Sea. This is due to surface heating and evaporation resulting in warm salty water overlying colder fresher water. They do not exist where there are strong currents, as in the Antarctic, which have enough energy to stir up any staircases that might form,” says Dr Sutherland.

Selective filtering
So the scenario is that when an internal wave strikes a density staircase, a part of its energy may be transmitted through the staircase. In other words, density staircases in the ocean can act to reflect short wavelength internal waves and transmit longer wavelength waves. This is analogous to the selective transparency of glass windows on modern buildings that have multilayered coatings designed to reflect red light (long wavelength light) and allow green-blue (shorter wavelength) light through

On reaching the ocean floor, the long-wavelength waves which have been transmitted cause turbulence and mix up the water. The warm waters then rise to the top and warm the top layers.

“Strong storms passing over the Arctic typically generate waves on the order of 10-100 km horizontal scale. As sea-ice retreats and storms passing over the increasingly non-ice-covered ocean generate more and more waves, it seems that still a substantial fraction of this energy can transmit to depth, and this may consequently cause mixing at depth that could bring those deep warm (but salty) waters closer to the surface. The filtering that my theory predicts suggests that the energy transmitted to depth might be moderately smaller than one would expect in the absence of density staircases. But, for the most energetic waves — those transmitted by storms — the decrease is not substantial,” says Dr Sutherland.