Has anyone got any idea where I can read up the reasons for Arctic's Inverse stratification ?
Or can someone explain it here ?
Will try but am sure others can explain it better than I. The simple explanation is that with a sufficient salinity difference and a temperature gradient that is not too great, cooler fresher water is less dense than warmer saltier water. The arctic surface water is made fresher by precipitation, glacier runoff, iceberg and ice shelf melt water, river inflow (particularly over the shallow coastal shelf on the Siberian side) and also cycles of sea ice freeze and melt.
When seawater freezes it undergoes brine rejection as part of the crystallization process. How this can be viewed is water molecules at the freezing point impact the undersurface of other water molecules that are frozen or freezing and some percentage (it is at the scale where quantum mechanics kicks in so the behaviour is statistical) surrender kinetic energy, stick and enter the solid state thus releasing latent heat. Salt molecules are rejected in the process as not fitting into the crystalline structure. However, as I understand it some slight amount of salt becomes trapped and encapsulated in freezing water particles and incorporated into the ice. The rejected salt molecules create localized pockets of dense, very salty brine which rapidly sinks away into the depths. The sea ice as a consequence is substantially freshened versus the water from which it formed.
When the sea ice subsequently melts the next melt season the resultant melt water creates a freshened lens of relatively cool less dense and salty water that floats on the warmer saltier currents below, much of which is North Atlantic surface currents that dive under the less dense Arctic Surface Water as they enter the Arctic proper, particularly once reaching the Nansen Basin which underlies the CAB on the Atlantic side where greater depth lends itself to more complex stratification. A similar process occurs on the Pacific side to a lesser degree due to the narrowness of the Bering Strait.
As we are still undergoing a draw down of sea ice volume, freeze season extent is diminishing slower that melt season extent, and there is considerable ongoing glacier melt inflow, there is still plenty of freshened melt water each season to somewhat maintain the stratification. The danger is that we will start to run out of non-Greenland glaciers at some point, the shallow Siberian side which is only 2% of Arctic water volume subsequently loses much of its freshening river and glacier inflow and these peripheral seas then see further extent and thickness reductions which lessens the brine rejection mechanism and a vicious feedback occurs where a saltier Arctic results in less sea ice formation, since saltier water freezes slower and at a lower temperature, which leads to a further increase in salinity and even less ice. Looking at the pattern this year is worrying.
It gets worse, as I understand it the Arctic surface water in the CAB, specifically over the Amundsen Basin is only slightly less saline that the warmer Atlantic waters below which means it has always been on the razors edge of breaking down. In the past this did not matter as the central arctic was proof against melting out. But it is vulnerable, if it ever has a BOE it will start to mix with Atlantic surface water once no more fresh melt water is being added and that Atlantic water is much harder to refreeze. That being said, we may see freshening of the CAB in the near term as more CAB melt starts to occur each year, delaying things a bit. However models that suggest we may get a BOE every now and then in the future may be painting a too rosy picture. BOEs likely create an environment that favors more BOEs, at some point it is probably a runaway phenomenon.