| 04 Dec 2025
On the fate of the Irminger Current water and its impact on the convection region in the Irminger Sea – A Lagrangian model study
Abstract. The Irminger Sea is one of the few places in the North Atlantic where dense water masses are formed through deep convection. In addition to atmospheric forcing, wintertime convection in the Irminger Sea interior can be impacted by the extent of restratification in the preceding years. In the Irminger Sea, the cold central basin is contrasted to the Irminger Current (IC) which carries warm and saline waters of subtropical origin. In this study, we investigate the potential impact of the IC on restratification of the Irminger Sea’s deep convection area (DCA), using a high-resolution regional model combined with Lagrangian particle tracking. We release particles over the upper 1500 meters of the IC in the eastern Irminger Sea and track them forward-in-time.
Of those particles, 38 % follow the boundary current circulation and 61 % enter the interior Irminger Sea. One percent leaves the Irminger Sea through Denmark Strait and across the ridge to the Iceland Basin. Of those entering the interior, about one half reaches the DCA, steered by mesoscale variability. On their way to the DCA, the IC waters cool and freshen but on average remain lighter than waters in the DCA and therefore have the potential to restratify the DCA. This westward spread of light IC waters constrains the extent of the DCA to the western Irminger Sea by enhancing the stratification in the eastern part of the basin.
This is an interesting paper on an important topic: boundary-interior exchange in the subpolar North Atlantic and its impact on restratification after deep convection. The authors analyze simulated particle trajectories as they spread from the relatively warm Irminger Current that flows northeastward west of the Reykjanes Ridge, quantifying the fractions that (1) follow the cyclonic boundary current around the Irminger Sea, (2) spread into the interior Irminger Sea, and (3) spread specifically into the deep convection area of the interior Irminger Sea. They go ont to investigate the relative buoyancy impact of waters from different depths and positions within the two-core Irminger Current.
The particle trajectories were calculated using a high-resolution (~2km; 216 vertical levels) configuration of the MIT/GCM covering the Irminger Sea, the Iceland Basin, and most of the Nordic Seas. This configuration has been around for a while and used in numerous studies. The authors provide convincing analyses that the model reasonably represents the primary circulation and hydrographic features of the area of interest (Irminger Sea). The particles are initiated in the Irminger Current west of the Reykjanes Ridge at depths from 0-1500m and tracked for up to 12 months (the simulation is only 12 Months long, covering the time period 9/1/2008 to 8/31/2009). The particles were initiated every three days for 9.5 months along a section crossing the Irminger Current at about 59N where moorings have been maintained since 2014 as part of the Overturning in the Subpolar North Atlantic Program (OSNAP). Total number of trajectories was 180,050, of varying lengths from 2.5-12 months.
I have two major comments about this work, and a few other comments.
First, I'm having difficulty interpreting the results since the simulated trajectories are of varying length but grouped all together in the analyses. The authors are up front about the varying trajectory lengths, but I'm concerned this leads to mis-leading results in some cases. For example, the fraction of particles that remains in the boundary current includes shorter trajectories that may have not had time to reach the interior. This statement, "For the BC particles we find a high particle density close to the top of the Reykjanes Ridge, mostly following the eastern IC core northward (Fig. 4d)" is technically correct, but the high concentration near the Reykjanes Ridge includes (I assume) many short trajectories that don't travel very far from their initiation position. So what does it mean that there is this high concentration near the Ridge? Similarly, consider the particles grouped as "DCA" (Deep Convection Area) and "non-DCA". According to the definitions, the former includes particles that enter the interior but never reach the DCA (estimated as 32%), whereas the latter includes only particles that make it to the DCA (estimated as 29%). It is reasonable I think to assume that it takes longer for particles to reach the DCA, and they have to pass through the interior non-DCA region to get there. So the shorter trajectories may not be long enough to reach the DCA and therefore end up in the non-DCA group. If all the particles were 12 months long (the maximum), a much higher fraction may reach the DCA. In my opinion, the fractional results only make sense if all the trajectories are the same length.
I spent a little bit of time trying to think about how the authors could address this problem. They could just use the first 2.5 months of all the trajectories. This won't be very satisfying though, since the travel times to the most interesting destinations (e.g., the DCA and Cape Farewell) are on average longer than that. Another possibility would be to recycle the model output so trajectories of the same length could be calculated. I think this is the better option, unless the authors can come up with another idea.
My second major comment is related but regards the interesting analysis of the buoyancy contributions to the DCA from the various regions within the Irminger Current (section 4 of the paper). The authors find that both the eastern and western Irminger Current cores contribute similar density anomalies to the DCA, but that Since 75% of the particles that reach the DCA are from the western Irminger Current core, it is concluded that the western core has more influence on restratification in the DCA. Again, I'm concerned about the variable length of the simulated trajectories. The eastern Irminger Current core is farther away from the DCA, so it will probably take longer for particles to get there. This may bias the contribution from the eastern core too low.
Other comments are listed below, some of which relate to the major comments above.
Line 16: somewhere in the abstract it should be mentioned that the simulation is for just one year
Line 26: Suggest to add a clarification here, something like “at least during the OSNAP observing period (2014-present)”
Line 56: Remove comma after “both”
Line 62: These areas were identified as regions of deep convection long before 2021. I suggest to re-phrase this sentence, perhaps adding more historical citations, to reflect that these areas have been known for some time as deep convection regions.
Line 73: should some of the papers by Pickart et all. Be included?
Line 75: Suggest to delete word “phase” at the end of the sentence.
Line 132: “much higher spatial resolution” compared to what? I think you mean compared to the observations—suggest to say that explicitly. The way it reads now, it sounds like you are saying this simulation has much higher spatial resolution than some other simulation.
Line 154: Should these Fried citations be labeled 2024a and 2024b?
Line 182: Suggest to add “particle” before the word “release”.
Line 199: Did you consider to recycle the velocity fields used to integrate the trajectories so as to get trajectories that are all the same length? See major comments.
Line 225: Need discussion on how these maps may be affected by the differing lengths of the trajectories. See major comments.
Line 233: There may be a bias here related to the fact that some trajectories are much shorter than others. It could be that all of the particles that cross the RR would spread into the interior Iceland Basin if they were all 1 year in duration.
Line 251: What is the meaning of 6% when some of the trajectories are not long enough to reach Cape Farewell?
Line 289: Add figure number before panel letter for clarity.
Line 353: I think “of” should be “or”.
Line 355: I found the text from this point to the end of the paragraph somewhat confusing. Some statements appear to be contradictory. I suggest to rewrite for clarity.
Line 389: Add comma after “convection area”
Line 390: I found this sentence somewhat confusing. If the particles lose heat and salt, it is not obvious whether they become more buoyant or less. That will depend on the relative impact of cooling and freshening. So I don’t think it’s obvious that these particles will add to the stratification. Maybe I’m missing something, but some clarification might be needed here. Maybe the authors mean “contribute to changes in stratification” rather than increase stratification, which is what I interpreted “add to the stratification” to mean.
Line 436: This statement (including the 99% value) could be mis-leading since some of the trajectories are very short and therefore don’t have enough time to leave the basin. Suggest to rephrase this sentence.
Line 487: Add “the” before “basin”.