As kindly suggested I apply to those of you who 'get it' here: Why should there be any connection between ocean depth and surface ice. The assumption that deep water in the CAB should somehow protect the ice keeps popping up, but why should that be the case?
I don't really 'get it' being ocean dynamics complicated even for oceanographers but I am going to give my simplified two cents.
- There is a mixed layer, top 50 m. This layer is called mixed because, thanks to the action on the surface by winds, waves, etc., the mechanical input is able to mix differences in temperature and salinity by induced turbulence. It is strong enough to keep stratified layers from forming.
Curiously the mixed layer has an increased salinity in most oceans except in the Arctic Ocean, which mixed layer is much fresher than at higher depths. The mixed layer ends by an abrupt salinity increase called halocline.
What makes the water of Arctic ocean mixed layer "fresh" and cold relatively? I think it's rivers, and the fact that the halocline comes before the increase of temperature, deeper under the halocline. So thanks to this strong stratification layer, the heat from beneath won't reach the mixed layer, staying relatively cold, and relatively fresh. I think there's a physical explanation for the halocline coming at lower depth than the temperature increase from deep waters, but I don't remember it. If it was not the case, there would not be Arctic ice, probably.
- Currents are entering the Arctic from Bering Strait bring saltier warmer water from the Pacific (not as salty as Atlantic water).
- As these currents push their way along the shallow shelves of Chukchi, ESS, and Alaskan Coast, they encounter ice and surrounding water with much fresher water. There is some mixing between these currents and the marginal zones with broken ice, thanks to the weather and to the current eddies themselves. Since the depths are relatively small around 50 m, the currents are constrained and there is time for direct interaction with the ice and surrounding water.
- These currents reach the continental shelves and (although slightly diluted due to the mixing they have encountered in the long distance from the Pacific) they sink because their density is higher than surrounding Arctic Ocean water and suddenly the 50 m bottom is gone. They surely tended to sink in the shelf as well, but again, since the depth is relatively small, the currents are constrained, are affected by mixing, and transport heat to the Arctic ice. But once they fall to 200, 300m, below the halocline, this heat will not affect the Arctic ice (at least immediately).
- What happens with the Atlantic Currents? The depths at Barents sea are actually more varied, I think it is deeper in general (and I wonder if that is what causes that some years the ice in the Barents side persists for longer). Also the Atlantic currents are extremely massive compared to Bering inflow. But anyway, as soon as they reach the depths beyond Svalbard and FJL, these currents sink if they didn't sink before.
What happens at depth is beyond my understanding.
PS. Wikipedia page of the Arctic brings some interesting facts...