Open ocean convection
This process has a crucial role in the formation of both bottom and intermediate water and in the large-scale thermohaline circulation, which largely determines global climate.
[2] Convection exists under certain conditions which are promoted by strong atmospheric forcing due to thermal or haline surface fluxes.
This may be observed in oceans adjacent to boundaries with either dry and cold winds above or ice, inducing large latent heat and moisture fluxes.
In sub-polar regions, the upper mixed layer starts deepening during late autumn until early spring, when the convection is at the deepest level before the phenomenon is weakened.
[2] The weak density stratification of the Labrador Sea is observed each wintertime, in depths between 1000 and 2000 m, making it one of the most extreme ocean convection sites in the world.
In winter, when the NAO is in positive phase above this region, the cyclonic activity is greater over the North Atlantic with an enhanced circulation of cold and dry air.
During this positive phase of NAO, the oceanic heat loss from the Labrador Sea is higher, contributing to a deeper convection.
[3] According to Holdsworth et al. (2015), during the negative phase of NAO which is associated with an absence of high frequency forcing, the average maximum mixed layer depth decreases more than 20%.
[5] In the northwestern Mediterranean Sea, deep convection occurs in winter, when the water undergoes the necessary preconditioning with air-sea fluxes inducing buoyancy losses at the surface.
[7] Additionally, according to Van Westen and Dijkstra, (2020), the formation of Maude Rise polynya which was observed in 2016 is associated with the subsurface convection.
In particular, the Maud Rise region undergoes preconditioning due to the accumulation of subsurface heat and salt, leading to a convection and favoring a polynya formation.
Preconditioning is referred to a period during which a cyclonic gyre-scale circulation and buoyancy forcing are combined to predispose a convective site to locally overturn.
A site is preconditioned when a laterally extended deep region of relatively weak vertical density stratification exists there, and it is capped by a locally shallow thermocline.
Cooling events lead to the second phase, deep convection, in which a part of the fluid column may overturn in numerous plumes that distribute the dense surface water in the vertical axis.
In this case, the convection cools and mixes a patch of water, creating a dense homogeneous cylinder, like a chimney, which ultimately collapses and adjusts under planetary rotation and gravity.
Thermobaricity is the effect in which sinking cold saline water is formed under freezing conditions, resulting in downward acceleration.
Additionally, many numerical and tank modeling experiments examine the role of rotation in the processes of the convection and in the morphology of the plumes.
[2] The chimneys of deep convection remain open for one to three months, during winter, in a quasi-stable state whereas they can collapse within a few weeks.
[2] Formation of the convection chimneys is preconditioned by two processes: strong heat fluxes from the sea-surface and cyclonic circulation.
During the initial stage of the intensive deepening of the chimney, when the baroclinic instability effects are assumed to be unimportant, the depth can be found as a function of time using the buoyancy forcing.
The left-hand side represents the time evolution of the total buoyancy anomaly accumulated in the time-depended chimney volume
If the buoyancy loss is maintained for a sufficient time period, then the sea-surface cooling weakens and the restratification phase starts.
Associated with the tilting isopycnal surfaces a thermal wind is set up generating the rim current around the edge of the convection regime.
, until the growing baroclinic instability begins to carry convected fluid outward while water from the exterior flows into the chimney.
At this moment, the rim current around the cooling region becomes baroclinically unstable and the buoyancy laterally is transferred by the instability eddies.
The final timescale is independent of the rate of rotation, increases with the radius of the cooling region r and decreases with the surface buoyancy flux Bo.
Therefore, the chimney of homogenized cold water erodes into several small conical structures, named cones, which propagate outward.
[15] Deep convective activity in the Labrador Sea has decreased and become shallower since the beginning of the 20th century due to low-frequency variability of the North Atlantic oscillation.
[16] Similarly, in the Greenland Sea, shallower deep mixed layers have been observed over the last 30 years due to the fall of wintertime atmospheric forcing.
The freshening of the surface waters due to enhanced meltwater from the Greenland Ice Sheet, have less density, making it more difficult for oceanic convection to occur.
