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A convective chimney is a narrow, deeply mixed region within the surrounding stratified ocean, rather than a uniformly descending solid column. Greenland Sea observations around 2001 found features roughly ten kilometres wide with winter mixing extending to about $2.5$ km. Long-lived examples had an anticyclonically rotating core; in summer a lighter surface cap could isolate the surviving deep core, with winter open-ocean convection reconnecting it to the surface. Wadhams's 2005 observations describe this structure. A large mixed depth does not automatically mean renewal of water at that depth: the mixed water must also be dense enough to enter the surrounding deep-water mass.

Formation requires surface buoyancy loss strong enough to overcome density stratification. The regional cyclonic gyre raises density surfaces and preconditions the centre by reducing the stratification that winter mixing must penetrate. Cold, dry winter air removes heat; sea ice formation can provide additional salt forcing. Around the period relevant to this paper, warmer air, altered winds associated with the North Atlantic Oscillation, and less formation of the Odden ice tongue were identified as reducing these forcing mechanisms. Fresh surface water also resists deep mixing. Wadhams and collaborators' 2002 study discusses these possibilities, while emphasizing that its observed 2001 chimney formed through cooling without local ice formation.

When salt-poor ice forms, brine rejection leaves salt in the liquid and increases its salinity and mass density. Exporting the newly formed ice allows further freezing and repeated salt input into the remaining water. Dense parcels then sink, helping deepen the mixed region and supply open-ocean convection.

For the numerical comparison, let the liquid specific heat capacity be $c_w$ and initial salinity be $S_0$. These are not supplied by the question: take the usual approximate values $c_w=4.0\mathrm{kJ}\mathrm{k}{\mathrm{g}}^{-1}{\mathrm{K}}^{-1}$ and $S_0=35$ psu for a numerical estimate. The ice heat capacity cannot replace $c_w$ during liquid cooling. Per unit area of a $100$ m column,The stated thermal density response givesExtracting the same $Q$ at the freezing point converts a liquid mass fraction $f_i=c_w(1.8)/L_f\simeq 0.02143$ into salt-free ice. Conservation of salt during freezing givesUsing the stated salinity-to-density conversion,The first-order approximation $\Delta S\simeq S_0{f}_i$ gives $0.75$ psu and a factor $4.2$. Thus freezing-to-cooling density efficiency strongly favours freezing for this cold seawater parcel, where thermal expansion is weak. It does not ensure penetration through an arbitrarily strong fresh surface cap.