the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 May 2023
Linking Gulf Stream Air-Sea Interactions to the exceptional blocking episode in February 2019: A Lagrangian Perspective
Abstract. The development of atmospheric blocks over the North Atlantic European region can lead to extreme weather events like heatwaves or cold air outbreaks. Despite their potential severe impact on surface weather, the correct prediction of blocking lifecycles remains a key challenge in current numerical weather prediction (NWP) models. Increasing evidence suggests that latent heat release in cyclones, the advection of cold air (cold air outbreaks, CAOs) from the Arctic over the North Atlantic, and associated air-sea interactions over the Gulf Stream are key processes responsible for the onset, maintenance, and persistence of such flow regimes. In order to establish how air mass transformations over the ocean, and in particular over the Gulf Stream, affect the large-scale flow, we focus on an episode between 20 and 27 of February 2019, when a quasi-stationary upper-level ridge established over western Europe accompanied by an intensified storm track in the Northwestern North Atlantic. During that time a record-breaking warm spell occurred over Western Europe bringing temperatures above 20 °C to the United Kingdom, the Netherlands, and Northern France. The event was preceded and accompanied by the development of several, rapidly intensifying cyclones originating in the Gulf Stream region and traversing the North Atlantic. To explore the mechanistic linkage between the formation of this block and air-sea interactions over the Gulf Stream, we adopt a Lagrangian perspective, using backward and forward kinematic trajectories. This allows us to study the pathways and transformations of air masses forming the upper-level potential vorticity anomaly and interacting with the ocean front. We establish that more than one-fifth of these air masses interact with the Gulf Stream in the lower troposphere, experiencing intense heating and moistening over the region, due to the frequent occurrence of CAOs behind the cold front of the cyclones. Trajectories moistened within the cold sector of one cyclone, later ascent into the upper troposphere with the ascending air stream of a consecutive cyclone, fueled by the strong surface fluxes. These findings highlight the importance of CAOs in the Gulf Stream region with their intense coupling between the ocean and atmosphere for blocking development, and provide a mechanistic pathway linking air-sea interactions in the lower troposphere and the upper-level flow.
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RC1: 'Comment on egusphere-2023-905', Anonymous Referee #1, 07 Jul 2023
General Comments
First, I would like to recognise that a lot of work has gone into this article. This is a large analysis and has required a lot of thought by the authors and there are many interesting lines of evidence presented. Furthermore, I think the methods the authors use are the correct ones for understanding these very important processes. Nevertheless, I feel the paper is confused and lacks a clear focus – do you want to show that CAO air is modified, incorporated into an extratropical cyclone, and then transferred to upper levels to help develop a blocking event? If so, then they need to show this happening much more clearly. The paper shows each of these steps individually to some extent, but there is no coherent trail of evidence that shows the whole process happening. Furthermore, it seems the authors are so focussed on the “handover” phenomenon that they do not acknowledge that a minority of air parcels (~23% maximum) are subject to this process. They also do not use the trajectory method to show this happening in “real time” around LE2. It would be great to see the trajectories plotted running through a CAO, being modified over the Gulf stream, ascending in the WCB of LE2 and finally ending up in the NPVA, but the authors do not do this.
I am also worried that the way the analysis is presented overstates the role of CAO-GS “handover” mechanism in the overall makeup and development of the NPVA regions at upper levels. From my viewpoint, most of the trajectories (~77%) do not pass near the GS and are from a different origin (Gulf of Mexico for example) and therefore the influence of the implied low-level modification is relatively small. That does not mean that they are not important (I’m very willing to believe that they are) but the paper focuses so much on the ~23% of parcels that may have originated over the GS that they ignore much of the makeup of the NPVA. Again, I can easily believe that the ~20% of highly modified parcels that end up in the NPVA region can “tip the balance” between short-term ridging and persistent block development but the arguments are not presented in this way. The paper consistently focuses on the GS modification of CAOs as being the primary mechanism at work here whereas it is not the main mechanism.
Another concern is some of the lack of checking of details around figure contents, labelling, and captioning. It is consistent with some of the lack of detail presented in the text itself, which may be due to there being too many hypotheses, arguments, and datasets in the paper. Removing some of the unnecessary content would likely help clarify and improve this work significantly. Again, I would like to commend the authors for having done a tremendous amount of work here, but I would argue a third of it is not relevant. I recommend the authors review what they are trying to say in this paper and re-organise it to focus on the process they are interested in. The authors also need to give a much fairer/clearer reflection of the role of nonGS contributions in the overall makeup of the blocking event to give a better context to the role of the GS-induced “handover” mechanism.
I recommend the authors carefully consider the comments below. Removing unnecessary analysis, adding some finer detailed analysis (e.g., showing trajectories starting in a CAO, being modified, ascending in LE2 and ending up in the NPVA) and providing a much stronger acknowledgement of the relative importance of the GS trajectories to the non-GS trajectories would make this a fascinating and highly important paper. As it stands, the paper does not really show the processes described. Instead, the paper infers a lot of the processes, and provides too much emphasis on the mechanism that accounts for only 23% of the phenomenon under investigation.
Specific Comments
I would suggest removing all the GS trajectory analysis and the DI, DH and CAO1 trajectory analyses for NPVA. You are interested in the trajectories that end up in the NPVA so just stick with those trajectories. Also, it is important to make sure you quantify those NPVA GS trajectories in context of the nonGS trajectories as that tells you what proportion of the GS trajectories influence the NPVA. This is something that the paper severely lacks i.e., the proportional contribution of NPVA GS trajectories, which is the minority of trajectories. Furthermore, I find there are also so many acronyms that it is hard to keep track of what they all are. I also think the CAO2 trajectories are the most important, followed by the WCB trajectories (and the nonGS trajectories, which are in the majority). Both CAO2 and WCB are strongly tied to diabatic processes anyway, which negates the requirement to show the DH trajectories. I would then remove the whole GS section at the end (Section 3.5). If you wanted to do the PV trajectories, then you should do it on the NPVA trajectories as I believe they are the most important ones. This will help focus the paper much more and remove a lot of unnecessary text.
L60: Is it worth including the idealised study of Boutle et al. (2011) here too as they show the timescale for moisture adjustments in the boundary layer to be approximately 2.3 days for highly idealised situations and it fits in with the small timescale required for the “preconditioning” you mention.
L62-63: “However, the pathway of CAO influence on the upper-level flow is unclear” – not strictly true. These cold air outbreaks can be associated with a strong upper-level jet stream at their boundary with warmer sub-tropical air. This situation is seen frequently over the winter. The orientation of the jet stream in that situation (e.g., south-west to north-east) could potentially induce ridge building without any need for “preconditioning”. I would simply suggest removing this sentence as it is misleading and the content of the paragraph before this point is good on its own. Conversely, if you want to keep it, then you need to relate it back to the rest of the paragraph beforehand e.g. “the pathway from CAO, through diabatic modification of that cold air, subsequent transport through the WCB of a cyclone and then impacting the upper level flow is unclear” (or something similar to that).
L115-116: I’m confused by the last sentence. I think you probably need to add a figure showing how you have “visually” done this. Either as a response or as a supplementary figure. Also, the way it is written makes it seem like you’re looking at the “distribution of cold sectors” not “the distribution of the SSHF and PV within the cold sectors”, which is what I think you mean. Just needs a re-word and a small example as evidence.
L155-157: This is not clear as to what you’ve done to initiate these trajectories. “…equidistant grid of ∆x=100 km and ∆y=25 hPa vertically between 500 and 150 hPa within both NPVAs…” By Equidistant do you mean 100 km between each trajectory starting point or that there is a 100 x 100 km square area you start them within (I’m thinking it is the former)? Are the crosses in Fig 1. representative of the starting points of the back trajectories for the upper level NPVAs? If so, then you should state this in the text in section 2.2.1 and the Fig 1. caption to make it much clearer. If the above is true, then I think the black crosses in Figs 1e and 1g are the wrong way round (i.e., the shape of the NPVAs in Fig 1e match the distribution of crosses in 1g and vice versa). Please check this.
Fig 1e: The minor NPVA is difficult to see. Could you change it to a clearer colour to contrast with the existing blue shading? Possibly a magenta or pink colour would show better.
L285: “On average, more than 23% of the NPVA trajectories interact with the ABL over the Gulf Stream region…” so, most air parcels have no interaction with the Gulf Stream region? Therefore, the main cause of the NPVA development could be argued to come from other sources. Doesn’t that mean the mechanism you propose is not the most important one?
L295-324: The central idea behind this paper is that CAO air is modified around the Gulf Stream, ascends in WCBs within major extratropical cyclones and then impacts the upper-level circulation that led to the intense blocking event. If that is true, then why are there a larger fraction of CAO trajectories that end up in the NPVA (NPVA GS CAO) than travel through the WCB (NPVA GC WCB)? Surely, the percentage of CAO parcels should be less than or (at most) equal to the number of WCB trajectories, otherwise how does all the excess modified CAO air get into the upper troposphere? If the percentages are calculated as a “percentage of the fraction of NPVA GS” trajectories, then this needs to be clearly stated as the current plot is misleading by making it seem like CAO modification in more important than it actually is.
L321-323: “Moreover, there appears to be a connection between the increases in the fraction of WCB trajectories and the growing number of trajectories defined as GS CAO NPVA (blue and teal in Fig.3b; Tab.1).” This is very vague and I’m not sure I agree with the statement. Where exactly are they “connected”? Suggest removing or expanding.
L325-331: I disagree with this; the DI trajectories are in the minority for a lot of this. Also, in general, they only account for <20% of trajectories in the NPVA i.e., lots of air parcels from other sources. Furthermore, their presence in the NPVA may be a result of them descending from the upper troposphere to the mid/lower-troposphere and then ascending back above 500 hPa without ever interacting with the surface. Moreover, the ABL will be very shallow where the DIs are active (quite likely below 800 hPa). I think you’re inferring a lot here without showing any evidence. I don’t think this paragraph should be included unless you can show DI parcels that have specifically interacted with the ABL and ended up in the NPVA.
Figure 5a - the caption describes “Contributions of accumulated moisture uptakes to moisture present…” first – are these words referring to the “fractions explained” axis? If so, then that axis should be defined on the left with the “number of uptakes/day” the right (as you discuss fractions explained first). Also, the “fractions explained” label should be “accumulated moisture fraction of total moisture present (%)” to reflect the caption. Furthermore, I think you need to define what a “moisture uptake” is as the term is arbitrary. If it were quantified as an actual mass of water, then we could see how important the process is. If this moisture uptake were accumulated across all NPVA GS and NPVA nonGS trajectories, then we could see which has a bigger impact on the total NPVA moisture.
Figure 5b – I think this figure is misleading as it does not indicate the absolute values. If I look at the bars, both the NPVA GS and NPVA non-GS fractions add up to 100% i.e., they are relative to themselves. If NPVA nonGS accounts for 77% of all NPVA trajectories and NPVA GS accounts for 23% then the impact of the NPVA GS trajectories is significantly diminished. The plot currently implies that the NPVA nonGS and NPVA GS are equally important, which cannot be true if there is a 77/23 split. These fractions need to be presented as a percentage of all NPVA trajectories to gauge relative importance.
L370 and Fig 6c: Suggest changing “are related” to “are likely related” as you don’t show what level in the atmosphere these uptakes occur. If you could plot the pressure height at which the maximum uptake of each trajectory occurs (and where that is) then you could say “are related”. It is not inconceivable that a layer of moist air at e.g., 850 hPa was advected into the domain you’re looking at from a lower latitude before the last point in the back trajectory. The moisture would therefore “look” like it came from the Gulf Stream but could have originated elsewhere in the sub-tropics many days before. The inclusion of the word “likely” negates this by acknowledging that you’re not 100% certain.
L368-378 more generally: Just because the CAO index is co-incident with the moisture uptake field does not mean that the two are related. Your argument would be much better if you plotted the NPVA CAO2 GS trajectories’ moisture uptake frequency and contributions so that it could be compared to both the NPVA GS and NPVA nonGS to see the relative importance of the CAO modification process.
Fig 7/L379-394: The CS analysis doesn’t add much to this, and I think this figure would be much improved by removing all lines but the CAO2 line. That way you could clearly see the impact of LE1 and LE2 for creating the conditions for large moisture uptakes. I also don’t think lines 386-394 are necessary and can be removed.
L393-394: “It is worth noting that the episodes of extreme SLHF are significantly larger in magnitude for the NPVA GS trajectories than for the NPVA nonGS trajectories (Figure 7b).” – I think it is important to note here that large heat fluxes are the result of a large difference in characteristics between two different media. The heat fluxes are high because there is cold, dry air lying over a relatively warm, moist ocean. If the fluxes remain high, then the air must still be cold and dry. Low heat fluxes imply that the imbalance is much smaller (i.e., the air is relatively warmer and moister, which reflects the underlying surface conditions). So, while the SLFH may be more extreme in the NPVA GS (and possibly NPVA COA2 GS) trajectories, the overall moisture (and heat) content of the NPVA nonGS trajectories might be higher (and more important) in absolute terms – especially if there are a lot more NPVA nonGS trajectories than NPVA GS trajectories. I think you must employ some sort of weighting here to account for both the overall number of trajectories in each group and their total moisture content. A more modified air parcel may contain less moisture than an unmodified air parcel.
L395-396: “Our results are in agreement with other studies (e.g. Papritz and Grams, 2018; Aemisegger and Papritz, 2018; Hawcroft et al., 2012), indicating that CAOs play an important role in the water cycle of cyclones” I think you could actually show how important the role is by quantifying the contribution of CAO2 moisture to the total moisture in the NPVA if you follow through with the suggestions I’ve made above. That would be a neat and significant result (even if the value seemed relatively small).
L413: “and intensifies in the region of strong CAO left behind by LE1 (Fig.3b). The already moistened air is then fed into the ascending airstream of the LE2 cyclone” – I disagree with this. The plot shows the cyclone intensifying in the air behind the CAO region. Assuming a westward tilt of the system, it should be developing because of conditions upstream at upper levels. This statement appears to be suggestive of the CAO region causing the intensification, which is not actually shown. You also do not show that the “already moistened air” (assuming you mean modified CAO air) has been fed into the ascending air stream of LE2. You would need to show the specific trajectories that follow this path for me to be convinced this statement is true.
L416-429: Again, I think there is mainly inference here rather than proof. First, the statement on the NPVA GS “Trajectories begin their ascent into the upper troposphere on average 3.5 days after reaching maximum SLHF…” when combined with the statement in the previous paragraph stating “The ascent occurs approximately 54 hours after the maximum SLHF values” is indicative of the NPVA nonGS trajectories ascending well ahead of the NPVA GS trajectories, which is unsurprising because the high SLHF values imply that the air is still relatively cold and dry and needs more modification before it can be lifted. Also, when are Figs 8a and 8c representative of? There’s no information on when the maximum SLHF was occurring relative to the development of LE2. From Fig 3b, it is clear LE2 was very well developed by 2000Z on 21-02-2019, so working back from 2100Z on 24-02-2019, 2.6 days to ascend and 3.5 days to accumulate moisture means that these parcels were (on average) in a CAO approximately 6 days before and suggests they were associated with the modified CAO before the passage of LE1 (as can be seen in Fig 3a). So, it is unlikely that your proposed “hand over” process is working on the timescales you suggest. Again, if you were to plot the trajectories specifically associated with LE2 and they were clearly from the modified CAO following LE1 then I would believe your argument. As it stands, your own results suggest your proposed mechanism does not work on the timescales you propose.
L436-437: “NPVA nonGS trajectories tend to have a higher number of uptakes compared to NPVA GS trajectories, accounting for 75% of all NPVA trajectories.” I think this is a really important point, i.e., that most of the moisture uptake is not from the Gulf Stream region. Therefore, most of the air parcels reaching the NPVA region are not arriving there because of the “hand over” process.
L474: “Gulf Stream can be attributed to the influence of CAOs” – how have you shown this for these trajectories? Can you show that these trajectories/parcels were part of a CAO and then modified? As it stands you have only shown that these GS NPVA trajectories change their properties with time and have not shown that they were part of a CAO. You need to show the evidence before you can make this statement.
L507-508: “as evident from the temporal changes of sensible heat flux in the two types of trajectories. Indicating that processes occurring during CAOs may be responsible for the decrease of PV in the atmospheric boundary layer” – again, this is inferred but not actually shown. You need to show that these specific trajectories came from a CAO for this statement to be valid.
L509-521: I’m not sure what the purpose of this paragraph is. You discuss a “cold sector” but there isn’t really a cyclone nearby. I could believe a trailing cold front lies in the vicinity of the box in Fig 11a, but it is very remote from the parent cyclone. All the sentences on stratiform clouds are being used to infer the presence of a cold sector – why not just plot the vertical temperature field and show there is a thermal contrast? I think the paragraph is trying to explain the reason behind the negative PV, but there is a lot of inference here without showing the actual processes (e.g., evaporative cooling) happening. Finally, there is an inference that the cloud is “stratiform” in nature, but that is not shown conclusively either.
L530-531: “positioned ahead of the cold front”, what cold front? It is not shown on the figures. It is important to include where this feature is in the plots (or where you think it is at the very least).
L532-535: “Considering the handover mechanism’s predominance in our case study and the findings presented in Figure 10, it is reasonable to expect that their PV will increase within the next few hours, and they will be carried upwards into the upper troposphere by the ascending airstream of cyclone LE1 (Fig. 2).” – Again, you make this statement and then do not show the process that is central to it. Why not show the trajectories for this whole set of steps (cold air, modification, lifting in cyclone LE1 – or should this be LE2?) to make it completely clear that what you say is true. I haven’t seen any conclusive evidence to support this statement.
L547-562: I disagree with this paragraph given the timings I have noted above i.e., the timescales for your processes are indicative of air originating from the cold air outbreak that preceded LE1 as it takes ~6 days to modify the air enough to rise in the LE2 cyclone. The lack of a trajectory analysis showing these specific processes means that the mechanisms you describe have not been fully proven to occur. I accept they may be occurring (and it would be fascinating if they are) but you really have not provided enough evidence to support that.
L573-577: This whole paragraph is central to the point I am making. Why do the parcels originating outside the North Atlantic matter less than those that originate over the North Atlantic? Given, by your estimate, they constitute 77% of the make up of the NPVA region then they are surely the most important. The last sentence, “remote sources of moisture do not appear to be relevant for the air masses ascending into the block within the extratropical cyclones that formed in the North Atlantic in February 2019.” That is a true statement, but these air masses are VERY relevant for the block overall as they constitute the majority of the NPVA region from your trajectory analysis. The sentence seems to discount non-extratropical cyclone processed air as irrelevant, which it clearly is not.
Once the above has been considered, I recommend you return to the last paragraph of the introduction and clearly state your aims as a list of bullet points while being careful to make sure they tie in with the final outcomes of the paper. Stating the aims much more clearly would help this paper considerably.
Technical Corrections
L18: Change “ascent” to “ascend”
L109: I think “minima” should be “minimum” in the context you’ve used.
L119: Change “Block” to “block”. I can understand why you use upper case for “European Block” as a named thing, but in this case, it isn’t necessary unless you say “European Block” instead.
L150: You need to define what “GS” is here (I’m assuming Gulf Stream, but this is the first instance of the acronym being used and should be stated).
L199: Change “in, line” to “, in line”.
L232: Change “Europe has” to “Europe had”.
L236: Change “warm temperatures” to either “high temperatures” or “warm conditions”. Temperature can be high or low not warm or cold (analogous to that, you wouldn’t get wet and dry rainfall).
L262: Change “reinforces” to “reinforced”.
L266-267: “…from the west…” might read better than “…from western direction…”.
L268: Should this be Fig. 3c not 1c?
Fig 4 caption: You need to define what (a) and (b) are in the caption. Also, LE1 is labelled around 20th February in (a) whereas it is labelled between 18th and 19th February in (b). This should be corrected and consistent in both figures.
L283-295: Is it worth mentioning here that there are fewer trajectories initiated at lower levels than upper levels? To first order, you might ask “why do the percentage of NPVA GS trajectories not match the GS NPVA percentage?” given they should be starting / finishing in the same place if the tracers are to be believed (and therefore the same number). Just some clarification on why NPVA GS does not equal GS NPVA would be useful (1-2 sentences maximum). Again, this is coming from a non-expert for trajectories so the clarification may be useful to others.
L286: Should it be Fig 4b not 3b?
L302: Are those percent values for the CAO>0 or CAO>2K setups? You should clarify which. [In fact, what you say on L304-305 regarding this should be stated at L302]
L304: Do you mean Fig 4 instead of Fig 3 at the start of the line?
L307: after “atmosphere” include “(i.e., diabatic heating)” just to link better to the next sentences where you stop saying CAO and instead describe diabatic heating.
L309: Fig 4 not Fig 3?
L310: You can just say “exceeds 30%” as there’s no need to give a range when quoting and exceedance.
L311: Suggest changing “contrast to a larger fraction obtained by Yamamoto et al. (2021) ∼51,8%” to “contrast to the 51.8% obtained by Yamamoto et al. (2021) …”
L315: I don’t think “therefore after the application of the ascent criterion” is required. Please delete.
L320: Should “revolution” be “resolution”?
L322 and L324: Again – should these be Fig 4 not Fig 3?
L325: Change “undergoes” to “undergo”.
L339: Again, Fig 4 not Fig 3?
L409: Should this be Fig 8a not 6a?
L410: Should this be Fig. 8a not 6b?
L435: Should be Tab.2 not Tab/2.
L466: Change “have passed” to “has passed”
L503: “(Fig. ??a, b)” – please correct this.
References
Boutle, I.A., Belcher, S.E. and Plant, R.S. (2011), Moisture transport in midlatitude cyclones. Q.J.R. Meteorol. Soc., 137: 360-373. https://doi.org/10.1002/qj.783
Citation: https://doi.org/10.5194/egusphere-2023-905-RC1 -
RC2: 'Comment on egusphere-2023-905', Anonymous Referee #2, 21 Jul 2023
Review of ‘Linking Gulf Stream air-sea interactions to the exceptional blocking episode in February 2019: A Lagrangian perspective’ by Marta Wenta, Christian M. Grams, Lukas Papritz and Marc Federer
General comments
The paper constitutes a very complete piece of work tackling one important outstanding problem in dynamical meteorology, namely the connection between atmospheric blocking and surface processes and, in particular, the connection between the Gulf Stream, cold-air outbreaks, extratropical cyclone and blocking in the North Atlantic region, determinant of the European weather. The work relies on a variety of methods of analysis including cyclone tracking, the identification of negative PV anomalies and cold-air outbreaks, and Lagrangian trajectories. This work is fully in the scope of Weather and Climate Dynamics.
I give below a list of specific and technical comments that I believe can improve the paper by making it more understandable. Once these comments are considered I will be able to fully recommend the article for publication in this journal.
Specific comments
L25-26: ‘cold surges’ I can see that some of the papers referred to here do talk about cold surges or long-lived cold surges, but are these the same as ‘cold spells’. In my opinion ‘cold spell’ is more appropriate as it does not imply the motion of cold air, but the effect of the persistence of large-scale conditions over a given region for a sufficiently long period of time.
L56-57: ‘the warm sector of the consecutive cyclone’ Is there a more specific definition of ‘the consecutive cyclone’? Is this any consecutive cyclone or are the authors thinking here of a specific flow configuration? From reading the paper I would say that it’s better to talk about ‘consecutive cyclones’ in plural and that it’s not always obvious which cyclone could be named the consecutive one.
L67-71: I was not quite sure what was the argument around the wintertime poleward displacement of the jet stream and eddy heat fluxes. Is it the intensity or the position of the eddy heat flux that has the most influence on the position of the jet stream?
L147-148: In the list of traced variables along trajectories there are five that are single-level. However the trajectories are located in three dimensions. How is the assignment of the single-level variables to trajectories made? Is there a criterion on the vertical distance from the parcel to the surface (single-level) or is the assignment made based on the horizontal position of the parcel only?
L194-196: The CAO index is based on 850-hPa potential temperature. Is there any condition on the level of the parcel as the potential temperature vertical gradient will be very different for two parcels with the same potential temperature difference but at very different pressure (altitude).
L263-264: ‘The advection of cold air behind the cold front of LE2 resulted in another strong surface evaporation event…’ How is the causality being assessed? Or is it that the CAO index is somehow being used as an indicator of strong surface evaporation?
L323: I’m not convinced about the connection between increases in the fraction of WCB trajectories and increases in the number of GS CAO NPVA trajectories. I can see that there is coincidence to a certain degree but I would not say that this is systematic. Or perhaps I’m looking at this in the wrong way. Further explanation would be welcome. A similar comment is valid for L338-339.
L383: In what cases are the cold-sector uptakes not associated with a CAO? Does this tend to occur at higher latitudes where SST is climatologically coder?
L387: I’m not sure I’m looking at this in the right way. From Fig. 7 the proportion of CAOs peak between 14 and 15 Feb and the SHLF is minimum on 13 Feb but LE0 intensifies rapidly only until 16 Feb. Perhaps it would help to mark these events directly in the figure using vertical lines.
L391-293: If I’m following the argument, the idea here is that the CS of the cyclone should be completely covered by a CAO region. Since most uptakes take place in CAO regions in comparison with the cyclone’s CS regions, it follows that the uptakes must take place in CAOs that are not associated with that particular cyclone and therefore they must be associated with a preceding cyclone. If this interpretation is correct, then you might want to add ‘preceding’ in L392 “...occur in the CAO induced by the ‘preceding’ cyclone rather than…”. Even if my interpretation of your argument is correct, it might be worth elaborating in your explanation. How are cold sectors assigned to a given cyclone? Is this done using a neighbourhood of a given size (e.g. a circular region of a given radius) around the cyclone centre? How big is this area compared to the CAO regions and does the area identified as the cold sector depend on the definition of the cold sector? Are the cold sectors truly a subset of CAO regions?
L403: This comment is related to the comment to L147-148. The SLFH will be experienced only by trajectories intersecting or around the first model level. How is SLFH assigned to trajectories at higher altitudes?
L459: ‘do not interact’ Could the interaction be at upper levels by e.g. crossing above the maximum heating associated with extratropical cyclones, potentially leading to a reduction of potential vorticity?
L519: The argument is plausible but given that the lower trajectories are those that exhibit PV reduction I would have thought that this is more related to the strong surface heating rather than evaporative cooling. There is no direct demonstration of evaporative cooling being responsible for negative PV in this case.
Technical comments
L15: Even after reading the paper I am not quite sure what is meant by ‘one-fifth of these air masses’. How are the air masses being counted or what metric is being used to make such a statement?
L48: Change ‘pass through the region’ to ‘pass through a region’.
List of references and references in the text: Vanniére et al. (2017) and Vannière et al. (2017) are two different papers but the tilde over the first author’s name can’t be the symbol to distinguish them. I believe Benoît’s surname is Vannière.
L107-108: ‘eight neighbouring grid points’ I imagine this number of grid points is resolution dependent, is it?
L110: Is it worth applying the latitude related factor to the rapidly intensifying cyclone criterion given that the cyclones in the case study do intensify at a variety of latitudes?
L130 and L134: As I understand it, the regime mask is based on the weather regime PV pattern. However, the mask is described in L130 but the WR PV pattern is not defined until L133-134. I’d recommend switching the sentence order so that the text becomes clearer.
L134: What is the ‘cycle’ referred to in this line?
L134-135: I’d suggest indicating when both NPVAs formed in the first sentence simply because otherwise the reader is left wondering why you give this information about the minor NPVA but not about the main one. Of course, this comes in the next sentence, but I think re-ordering (re-writing) these two sentences would make the text easier to read.
L163-164: I don’t quite follow the additional criteria to avoid double-counting here and in section 2.2.3.
L169: Are there any special considerations to make when dealing with BL trajectories in terms of the parametrisation turbulence and other boundary layer processes?
L215: How is the start of the ascent defined? Are there any pressure fluctuations along trajectories that need to be smoothed out?
L245: I’m not sure I understand this sentence: “The close succession of cyclones over the North Atlantic [...] disrupted the normal progression of weather systems”. I would have attributed the disruption to atmospheric blocking rather than to the succession of cyclones.
L258: Should it be Fig. 1c rather than Fig. 1e?
L260: I’d recommend using the cyclone nomenclature already introduced instead of the cyclones’ start dates (e.g., LE1 instead of the one from 18 February).
L262: Change ‘reinforces’ for ‘reinforced’.
L264: By cyclone’s ascending air stream, do you mean the warm conveyor belt?
L264: Add ‘the’ between ‘from’ and ‘western’.
L268: Should it be Fig. 3c rather than Fig. 1c?
L268: It might be clearer to refer to the cyclones as LE0-3.
L304 and others: In general, what is included in the article is worth noting. I’d recommend avoiding saying ‘It is worth noting that…’ to simply say ‘Note that…’ However, I recognise this is a matter of personal preference.
L301-305: Rewrite these sentences so that you first mention the separation into two subsets to then quote the percentages, noting that they are based on the strong CAO subsets.
L308-309: It’s not completely surprising that most trajectories are diabatically heated by 2 K or more given that by construction the trajectories are required to ascend.
L310: Delete ‘the’ from ‘the upper-level blocks’.
L319: I understand that a reference to Madonna et al. (2014) is required here, but as it stands it’s not clear why. I’d say ‘... of 600 hPa within 48 h (green in Fig. 4), following Madonna et al. (2004).’
L320: I think it should be ‘evolution’ rather than ‘revolution’.
L404: Change ‘ascend’ for ‘ascent’.
L432: Should it be Fig. 6d rather than 8d?
L435: Change ‘Tab/2’ for ‘Tab. 2’.
L452: Change ‘Fig.8, d’ for ‘Fig. 8d’.
L476: There is no Fig. 8f.
L486: There is only one sub-section (3.5.1) in Section 3.5. Is the header needed?
L495: Change ‘level’ for ‘time’.
L503: Should it say ‘Fig. 10a,b’?
L507: Change ‘...trajectories. Indicating…’ for ‘...trajectories, indicating…’
L515: Why 36N? I would have said 40N.
L523: Change ‘exceed’ for ‘go below’.
L539: Delete ‘GS’ and add ‘that’ between ‘trajectories’ and ‘started’. ‘GS’ would be redundant if you say ‘trajectories that started from the Gulf Stream’.
L590: The piece that is still missing is the decay of the block. Is it that for some reason CAOs and cyclones stop occurring and the block decays? Or is it that their WCB outflows need to comply with certain features to be able to sustain the block?
L599: Should it say ‘... suggested by Methven (2015)’?
Figure 1. I’ve always found that crosses marking parcel locations obscure quite a lot of the field underneath. Is there any other way of representing this? What about dots or perhaps you could present not so many of them?
Figure 2: The stars are very difficult to see. Perhaps this could be improved by outlining them with a black line. The caption should say what they indicate.
Figure 4: The caption should explicitly say what are the (a) and (b) panels. Is the grey background necessary? The colours for DI and CAO(>0K) are too similar to each other and therefore difficult to distinguish.
Figure 6. Caption: (left column) includes (a). It’s perhaps better to say (c,e). Change (%3000 km^2) at the end of the caption to (%/3000 km^2)
Figure 7a: What are the intersections between categories, i.e. what categories are a subset of what categories and which categories should add to 100%? Add ‘interval between the 90th and 10th percentiles…’ The colours used for CS and rest are very similar to each other and therefore difficult to distinguish. Could you use different colours or different line thicknesses?
Figure 8: Caption: Delete ‘whem’. I might have missed it but I think panel (b) was not used.
Figure 10: The order in which the variables are listed in the caption does not correspond to the panels.
Citation: https://doi.org/10.5194/egusphere-2023-905-RC2 -
AC1: 'Comment on egusphere-2023-905', Marta Wenta, 21 Aug 2023
On behalf of my co-authors, I would like to address the Reviewers’ comments to the article titled ‘Linking Gulf Stream Air-Sea Interactions to the exceptional blocking episode in February 2019: A Lagrangian Perspective’. First, we would like to thank the reviewers for their time and effort in reviewing our manuscript. The feedback has been invaluable and we believe that it will greatly improve the content and presentation of the paper.
Following the feedback from the reviewers, we willimplement significant modifications to our manuscript, focusing more on our core objectives. Previous research, most notably by Cheung et al. (2023) and Joyce T. et al. (2019), has established that the Gulf Stream has an influence on large-scale dynamics. However, the precise mechanism by which signals from air-sea interactions translate into large-scale circulation remains partially understood, as underscored by studies such as those by Michel et al. (2023), Kwon et al. (2010), and Czaja et al. (2019). Particularly interesting is the fact that the majority of NPVA GS trajectories undergo a significant latent heat release prior to reaching the block. Steinfeld et al. 2019 and 2020 identified that these diabatically heated air masses play an important role in initiating and sustaining the block, even if they constitute just 30-40% of all trajectories released from the block. We recognize that the bulk of perspectives and analysis used in our initial manuscript might not have adequately addressed this, and the previous point. Thereby we obscured our initial intention to provide more dynamical insight in the processes establishing an impact of air-sea interaction on the upper-level large-scale flow.
To conclude, our objective is to identify the potential mechanisms by which the Gulf Stream and associated air-sea interactions in the western North Atlantic influence upper-level atmospheric patterns. We are not claiming that these trajectories are the sole drivers of block formation, but evidence from the aforementioned studies suggests they might play a certain role in block development. In the revised version of the manuscript, we will try to better convey this message.
In line with the reviewers' feedback, we recognize the need to refine and streamline our paper's focus. To achieve this, our primary attention will be directed toward the NPVA trajectories, with the goal of offering a clearer insight into their interplay with the Gulf Stream. We will remove much of the discussion of other trajectory subsets, which distracted from our main message. Besides that, we want to further emphasize the roles that cyclones and CAOs play in the lifecycle of NPVA GS trajectories. To support this, our approach will focus on a more in-depth synoptic analysis, ensuring a thorough understanding of these events and their implications.
To summarize, as we prepare for the paper's resubmission, we'll be integrating the aforementioned changes and reshaping the paper to align with the reviewers' suggestions. We trust that this updated version, which we plan to resubmit promptly, offers a clearer perspective and addresses all concerns highlighted in the reviews.
Bibliography:
Cheung, HN., Omrani, NE., Ogawa, F. et al. Pacific oceanic front amplifies the impact of Atlantic oceanic front on North Atlantic blocking. npj Clim Atmos Sci 6, 61 (2023). doi:10.1038/s41612-023-00370-x
Czaja, A., C. Frankignoul, S. Minobe, and B. Vannière, 2019: Simulating the Midlatitude Atmospheric Circulation: What Might We Gain From High-Resolution Modeling of Air-Sea Interactions? Curr Clim Change Rep, doi:10.1007/s40641-019-00148-5
Joyce, T. M., Kwon, Y.-O., Seo, H., & Ummenhofer, C. C. (2019). Meridional Gulf Stream shifts can influence wintertime variability in the North Atlantic storm track and Greenland blocking. Geophysical Research Letters, 46, 1702–1708. doi:10.1029/2018GL081087
Kwon, Y.-O., M. A. Alexander, N. A. Bond, C. Frankignoul, H. Nakamura, B. Qiu, and L. A. Thompson, 2010: Role of the Gulf Stream and Kuroshio–Oyashio Systems in Large-Scale Atmosphere–Ocean Interaction: A Review. J. Climate, 23, 3249–3281, doi:10.1175/2010JCLI3343.1.
Michel, S.L.L., von der Heydt, A.S., van Westen, R.M. et al. Increased wintertime European atmospheric blocking frequencies in General Circulation Models with an eddy-permitting ocean. npj Clim Atmos Sci 6, 50 (2023). doi:10.1038/s41612-023-00372-9
Steinfeld, D., Pfahl, S. The role of latent heating in atmospheric blocking dynamics: a global climatology. Clim Dyn 53, 6159–6180 (2019). doi:10.1007/s00382-019-04919-6
Steinfeld, D., Boettcher, M., Forbes, R., and Pfahl, S.: The sensitivity of atmospheric blocking to upstream latent heating – numerical experiments, Weather Clim. Dynam., 1, 405–426, doi: 10.5194/wcd-1-405-2020, 2020.
Citation: https://doi.org/10.5194/egusphere-2023-905-AC1
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-905', Anonymous Referee #1, 07 Jul 2023
General Comments
First, I would like to recognise that a lot of work has gone into this article. This is a large analysis and has required a lot of thought by the authors and there are many interesting lines of evidence presented. Furthermore, I think the methods the authors use are the correct ones for understanding these very important processes. Nevertheless, I feel the paper is confused and lacks a clear focus – do you want to show that CAO air is modified, incorporated into an extratropical cyclone, and then transferred to upper levels to help develop a blocking event? If so, then they need to show this happening much more clearly. The paper shows each of these steps individually to some extent, but there is no coherent trail of evidence that shows the whole process happening. Furthermore, it seems the authors are so focussed on the “handover” phenomenon that they do not acknowledge that a minority of air parcels (~23% maximum) are subject to this process. They also do not use the trajectory method to show this happening in “real time” around LE2. It would be great to see the trajectories plotted running through a CAO, being modified over the Gulf stream, ascending in the WCB of LE2 and finally ending up in the NPVA, but the authors do not do this.
I am also worried that the way the analysis is presented overstates the role of CAO-GS “handover” mechanism in the overall makeup and development of the NPVA regions at upper levels. From my viewpoint, most of the trajectories (~77%) do not pass near the GS and are from a different origin (Gulf of Mexico for example) and therefore the influence of the implied low-level modification is relatively small. That does not mean that they are not important (I’m very willing to believe that they are) but the paper focuses so much on the ~23% of parcels that may have originated over the GS that they ignore much of the makeup of the NPVA. Again, I can easily believe that the ~20% of highly modified parcels that end up in the NPVA region can “tip the balance” between short-term ridging and persistent block development but the arguments are not presented in this way. The paper consistently focuses on the GS modification of CAOs as being the primary mechanism at work here whereas it is not the main mechanism.
Another concern is some of the lack of checking of details around figure contents, labelling, and captioning. It is consistent with some of the lack of detail presented in the text itself, which may be due to there being too many hypotheses, arguments, and datasets in the paper. Removing some of the unnecessary content would likely help clarify and improve this work significantly. Again, I would like to commend the authors for having done a tremendous amount of work here, but I would argue a third of it is not relevant. I recommend the authors review what they are trying to say in this paper and re-organise it to focus on the process they are interested in. The authors also need to give a much fairer/clearer reflection of the role of nonGS contributions in the overall makeup of the blocking event to give a better context to the role of the GS-induced “handover” mechanism.
I recommend the authors carefully consider the comments below. Removing unnecessary analysis, adding some finer detailed analysis (e.g., showing trajectories starting in a CAO, being modified, ascending in LE2 and ending up in the NPVA) and providing a much stronger acknowledgement of the relative importance of the GS trajectories to the non-GS trajectories would make this a fascinating and highly important paper. As it stands, the paper does not really show the processes described. Instead, the paper infers a lot of the processes, and provides too much emphasis on the mechanism that accounts for only 23% of the phenomenon under investigation.
Specific Comments
I would suggest removing all the GS trajectory analysis and the DI, DH and CAO1 trajectory analyses for NPVA. You are interested in the trajectories that end up in the NPVA so just stick with those trajectories. Also, it is important to make sure you quantify those NPVA GS trajectories in context of the nonGS trajectories as that tells you what proportion of the GS trajectories influence the NPVA. This is something that the paper severely lacks i.e., the proportional contribution of NPVA GS trajectories, which is the minority of trajectories. Furthermore, I find there are also so many acronyms that it is hard to keep track of what they all are. I also think the CAO2 trajectories are the most important, followed by the WCB trajectories (and the nonGS trajectories, which are in the majority). Both CAO2 and WCB are strongly tied to diabatic processes anyway, which negates the requirement to show the DH trajectories. I would then remove the whole GS section at the end (Section 3.5). If you wanted to do the PV trajectories, then you should do it on the NPVA trajectories as I believe they are the most important ones. This will help focus the paper much more and remove a lot of unnecessary text.
L60: Is it worth including the idealised study of Boutle et al. (2011) here too as they show the timescale for moisture adjustments in the boundary layer to be approximately 2.3 days for highly idealised situations and it fits in with the small timescale required for the “preconditioning” you mention.
L62-63: “However, the pathway of CAO influence on the upper-level flow is unclear” – not strictly true. These cold air outbreaks can be associated with a strong upper-level jet stream at their boundary with warmer sub-tropical air. This situation is seen frequently over the winter. The orientation of the jet stream in that situation (e.g., south-west to north-east) could potentially induce ridge building without any need for “preconditioning”. I would simply suggest removing this sentence as it is misleading and the content of the paragraph before this point is good on its own. Conversely, if you want to keep it, then you need to relate it back to the rest of the paragraph beforehand e.g. “the pathway from CAO, through diabatic modification of that cold air, subsequent transport through the WCB of a cyclone and then impacting the upper level flow is unclear” (or something similar to that).
L115-116: I’m confused by the last sentence. I think you probably need to add a figure showing how you have “visually” done this. Either as a response or as a supplementary figure. Also, the way it is written makes it seem like you’re looking at the “distribution of cold sectors” not “the distribution of the SSHF and PV within the cold sectors”, which is what I think you mean. Just needs a re-word and a small example as evidence.
L155-157: This is not clear as to what you’ve done to initiate these trajectories. “…equidistant grid of ∆x=100 km and ∆y=25 hPa vertically between 500 and 150 hPa within both NPVAs…” By Equidistant do you mean 100 km between each trajectory starting point or that there is a 100 x 100 km square area you start them within (I’m thinking it is the former)? Are the crosses in Fig 1. representative of the starting points of the back trajectories for the upper level NPVAs? If so, then you should state this in the text in section 2.2.1 and the Fig 1. caption to make it much clearer. If the above is true, then I think the black crosses in Figs 1e and 1g are the wrong way round (i.e., the shape of the NPVAs in Fig 1e match the distribution of crosses in 1g and vice versa). Please check this.
Fig 1e: The minor NPVA is difficult to see. Could you change it to a clearer colour to contrast with the existing blue shading? Possibly a magenta or pink colour would show better.
L285: “On average, more than 23% of the NPVA trajectories interact with the ABL over the Gulf Stream region…” so, most air parcels have no interaction with the Gulf Stream region? Therefore, the main cause of the NPVA development could be argued to come from other sources. Doesn’t that mean the mechanism you propose is not the most important one?
L295-324: The central idea behind this paper is that CAO air is modified around the Gulf Stream, ascends in WCBs within major extratropical cyclones and then impacts the upper-level circulation that led to the intense blocking event. If that is true, then why are there a larger fraction of CAO trajectories that end up in the NPVA (NPVA GS CAO) than travel through the WCB (NPVA GC WCB)? Surely, the percentage of CAO parcels should be less than or (at most) equal to the number of WCB trajectories, otherwise how does all the excess modified CAO air get into the upper troposphere? If the percentages are calculated as a “percentage of the fraction of NPVA GS” trajectories, then this needs to be clearly stated as the current plot is misleading by making it seem like CAO modification in more important than it actually is.
L321-323: “Moreover, there appears to be a connection between the increases in the fraction of WCB trajectories and the growing number of trajectories defined as GS CAO NPVA (blue and teal in Fig.3b; Tab.1).” This is very vague and I’m not sure I agree with the statement. Where exactly are they “connected”? Suggest removing or expanding.
L325-331: I disagree with this; the DI trajectories are in the minority for a lot of this. Also, in general, they only account for <20% of trajectories in the NPVA i.e., lots of air parcels from other sources. Furthermore, their presence in the NPVA may be a result of them descending from the upper troposphere to the mid/lower-troposphere and then ascending back above 500 hPa without ever interacting with the surface. Moreover, the ABL will be very shallow where the DIs are active (quite likely below 800 hPa). I think you’re inferring a lot here without showing any evidence. I don’t think this paragraph should be included unless you can show DI parcels that have specifically interacted with the ABL and ended up in the NPVA.
Figure 5a - the caption describes “Contributions of accumulated moisture uptakes to moisture present…” first – are these words referring to the “fractions explained” axis? If so, then that axis should be defined on the left with the “number of uptakes/day” the right (as you discuss fractions explained first). Also, the “fractions explained” label should be “accumulated moisture fraction of total moisture present (%)” to reflect the caption. Furthermore, I think you need to define what a “moisture uptake” is as the term is arbitrary. If it were quantified as an actual mass of water, then we could see how important the process is. If this moisture uptake were accumulated across all NPVA GS and NPVA nonGS trajectories, then we could see which has a bigger impact on the total NPVA moisture.
Figure 5b – I think this figure is misleading as it does not indicate the absolute values. If I look at the bars, both the NPVA GS and NPVA non-GS fractions add up to 100% i.e., they are relative to themselves. If NPVA nonGS accounts for 77% of all NPVA trajectories and NPVA GS accounts for 23% then the impact of the NPVA GS trajectories is significantly diminished. The plot currently implies that the NPVA nonGS and NPVA GS are equally important, which cannot be true if there is a 77/23 split. These fractions need to be presented as a percentage of all NPVA trajectories to gauge relative importance.
L370 and Fig 6c: Suggest changing “are related” to “are likely related” as you don’t show what level in the atmosphere these uptakes occur. If you could plot the pressure height at which the maximum uptake of each trajectory occurs (and where that is) then you could say “are related”. It is not inconceivable that a layer of moist air at e.g., 850 hPa was advected into the domain you’re looking at from a lower latitude before the last point in the back trajectory. The moisture would therefore “look” like it came from the Gulf Stream but could have originated elsewhere in the sub-tropics many days before. The inclusion of the word “likely” negates this by acknowledging that you’re not 100% certain.
L368-378 more generally: Just because the CAO index is co-incident with the moisture uptake field does not mean that the two are related. Your argument would be much better if you plotted the NPVA CAO2 GS trajectories’ moisture uptake frequency and contributions so that it could be compared to both the NPVA GS and NPVA nonGS to see the relative importance of the CAO modification process.
Fig 7/L379-394: The CS analysis doesn’t add much to this, and I think this figure would be much improved by removing all lines but the CAO2 line. That way you could clearly see the impact of LE1 and LE2 for creating the conditions for large moisture uptakes. I also don’t think lines 386-394 are necessary and can be removed.
L393-394: “It is worth noting that the episodes of extreme SLHF are significantly larger in magnitude for the NPVA GS trajectories than for the NPVA nonGS trajectories (Figure 7b).” – I think it is important to note here that large heat fluxes are the result of a large difference in characteristics between two different media. The heat fluxes are high because there is cold, dry air lying over a relatively warm, moist ocean. If the fluxes remain high, then the air must still be cold and dry. Low heat fluxes imply that the imbalance is much smaller (i.e., the air is relatively warmer and moister, which reflects the underlying surface conditions). So, while the SLFH may be more extreme in the NPVA GS (and possibly NPVA COA2 GS) trajectories, the overall moisture (and heat) content of the NPVA nonGS trajectories might be higher (and more important) in absolute terms – especially if there are a lot more NPVA nonGS trajectories than NPVA GS trajectories. I think you must employ some sort of weighting here to account for both the overall number of trajectories in each group and their total moisture content. A more modified air parcel may contain less moisture than an unmodified air parcel.
L395-396: “Our results are in agreement with other studies (e.g. Papritz and Grams, 2018; Aemisegger and Papritz, 2018; Hawcroft et al., 2012), indicating that CAOs play an important role in the water cycle of cyclones” I think you could actually show how important the role is by quantifying the contribution of CAO2 moisture to the total moisture in the NPVA if you follow through with the suggestions I’ve made above. That would be a neat and significant result (even if the value seemed relatively small).
L413: “and intensifies in the region of strong CAO left behind by LE1 (Fig.3b). The already moistened air is then fed into the ascending airstream of the LE2 cyclone” – I disagree with this. The plot shows the cyclone intensifying in the air behind the CAO region. Assuming a westward tilt of the system, it should be developing because of conditions upstream at upper levels. This statement appears to be suggestive of the CAO region causing the intensification, which is not actually shown. You also do not show that the “already moistened air” (assuming you mean modified CAO air) has been fed into the ascending air stream of LE2. You would need to show the specific trajectories that follow this path for me to be convinced this statement is true.
L416-429: Again, I think there is mainly inference here rather than proof. First, the statement on the NPVA GS “Trajectories begin their ascent into the upper troposphere on average 3.5 days after reaching maximum SLHF…” when combined with the statement in the previous paragraph stating “The ascent occurs approximately 54 hours after the maximum SLHF values” is indicative of the NPVA nonGS trajectories ascending well ahead of the NPVA GS trajectories, which is unsurprising because the high SLHF values imply that the air is still relatively cold and dry and needs more modification before it can be lifted. Also, when are Figs 8a and 8c representative of? There’s no information on when the maximum SLHF was occurring relative to the development of LE2. From Fig 3b, it is clear LE2 was very well developed by 2000Z on 21-02-2019, so working back from 2100Z on 24-02-2019, 2.6 days to ascend and 3.5 days to accumulate moisture means that these parcels were (on average) in a CAO approximately 6 days before and suggests they were associated with the modified CAO before the passage of LE1 (as can be seen in Fig 3a). So, it is unlikely that your proposed “hand over” process is working on the timescales you suggest. Again, if you were to plot the trajectories specifically associated with LE2 and they were clearly from the modified CAO following LE1 then I would believe your argument. As it stands, your own results suggest your proposed mechanism does not work on the timescales you propose.
L436-437: “NPVA nonGS trajectories tend to have a higher number of uptakes compared to NPVA GS trajectories, accounting for 75% of all NPVA trajectories.” I think this is a really important point, i.e., that most of the moisture uptake is not from the Gulf Stream region. Therefore, most of the air parcels reaching the NPVA region are not arriving there because of the “hand over” process.
L474: “Gulf Stream can be attributed to the influence of CAOs” – how have you shown this for these trajectories? Can you show that these trajectories/parcels were part of a CAO and then modified? As it stands you have only shown that these GS NPVA trajectories change their properties with time and have not shown that they were part of a CAO. You need to show the evidence before you can make this statement.
L507-508: “as evident from the temporal changes of sensible heat flux in the two types of trajectories. Indicating that processes occurring during CAOs may be responsible for the decrease of PV in the atmospheric boundary layer” – again, this is inferred but not actually shown. You need to show that these specific trajectories came from a CAO for this statement to be valid.
L509-521: I’m not sure what the purpose of this paragraph is. You discuss a “cold sector” but there isn’t really a cyclone nearby. I could believe a trailing cold front lies in the vicinity of the box in Fig 11a, but it is very remote from the parent cyclone. All the sentences on stratiform clouds are being used to infer the presence of a cold sector – why not just plot the vertical temperature field and show there is a thermal contrast? I think the paragraph is trying to explain the reason behind the negative PV, but there is a lot of inference here without showing the actual processes (e.g., evaporative cooling) happening. Finally, there is an inference that the cloud is “stratiform” in nature, but that is not shown conclusively either.
L530-531: “positioned ahead of the cold front”, what cold front? It is not shown on the figures. It is important to include where this feature is in the plots (or where you think it is at the very least).
L532-535: “Considering the handover mechanism’s predominance in our case study and the findings presented in Figure 10, it is reasonable to expect that their PV will increase within the next few hours, and they will be carried upwards into the upper troposphere by the ascending airstream of cyclone LE1 (Fig. 2).” – Again, you make this statement and then do not show the process that is central to it. Why not show the trajectories for this whole set of steps (cold air, modification, lifting in cyclone LE1 – or should this be LE2?) to make it completely clear that what you say is true. I haven’t seen any conclusive evidence to support this statement.
L547-562: I disagree with this paragraph given the timings I have noted above i.e., the timescales for your processes are indicative of air originating from the cold air outbreak that preceded LE1 as it takes ~6 days to modify the air enough to rise in the LE2 cyclone. The lack of a trajectory analysis showing these specific processes means that the mechanisms you describe have not been fully proven to occur. I accept they may be occurring (and it would be fascinating if they are) but you really have not provided enough evidence to support that.
L573-577: This whole paragraph is central to the point I am making. Why do the parcels originating outside the North Atlantic matter less than those that originate over the North Atlantic? Given, by your estimate, they constitute 77% of the make up of the NPVA region then they are surely the most important. The last sentence, “remote sources of moisture do not appear to be relevant for the air masses ascending into the block within the extratropical cyclones that formed in the North Atlantic in February 2019.” That is a true statement, but these air masses are VERY relevant for the block overall as they constitute the majority of the NPVA region from your trajectory analysis. The sentence seems to discount non-extratropical cyclone processed air as irrelevant, which it clearly is not.
Once the above has been considered, I recommend you return to the last paragraph of the introduction and clearly state your aims as a list of bullet points while being careful to make sure they tie in with the final outcomes of the paper. Stating the aims much more clearly would help this paper considerably.
Technical Corrections
L18: Change “ascent” to “ascend”
L109: I think “minima” should be “minimum” in the context you’ve used.
L119: Change “Block” to “block”. I can understand why you use upper case for “European Block” as a named thing, but in this case, it isn’t necessary unless you say “European Block” instead.
L150: You need to define what “GS” is here (I’m assuming Gulf Stream, but this is the first instance of the acronym being used and should be stated).
L199: Change “in, line” to “, in line”.
L232: Change “Europe has” to “Europe had”.
L236: Change “warm temperatures” to either “high temperatures” or “warm conditions”. Temperature can be high or low not warm or cold (analogous to that, you wouldn’t get wet and dry rainfall).
L262: Change “reinforces” to “reinforced”.
L266-267: “…from the west…” might read better than “…from western direction…”.
L268: Should this be Fig. 3c not 1c?
Fig 4 caption: You need to define what (a) and (b) are in the caption. Also, LE1 is labelled around 20th February in (a) whereas it is labelled between 18th and 19th February in (b). This should be corrected and consistent in both figures.
L283-295: Is it worth mentioning here that there are fewer trajectories initiated at lower levels than upper levels? To first order, you might ask “why do the percentage of NPVA GS trajectories not match the GS NPVA percentage?” given they should be starting / finishing in the same place if the tracers are to be believed (and therefore the same number). Just some clarification on why NPVA GS does not equal GS NPVA would be useful (1-2 sentences maximum). Again, this is coming from a non-expert for trajectories so the clarification may be useful to others.
L286: Should it be Fig 4b not 3b?
L302: Are those percent values for the CAO>0 or CAO>2K setups? You should clarify which. [In fact, what you say on L304-305 regarding this should be stated at L302]
L304: Do you mean Fig 4 instead of Fig 3 at the start of the line?
L307: after “atmosphere” include “(i.e., diabatic heating)” just to link better to the next sentences where you stop saying CAO and instead describe diabatic heating.
L309: Fig 4 not Fig 3?
L310: You can just say “exceeds 30%” as there’s no need to give a range when quoting and exceedance.
L311: Suggest changing “contrast to a larger fraction obtained by Yamamoto et al. (2021) ∼51,8%” to “contrast to the 51.8% obtained by Yamamoto et al. (2021) …”
L315: I don’t think “therefore after the application of the ascent criterion” is required. Please delete.
L320: Should “revolution” be “resolution”?
L322 and L324: Again – should these be Fig 4 not Fig 3?
L325: Change “undergoes” to “undergo”.
L339: Again, Fig 4 not Fig 3?
L409: Should this be Fig 8a not 6a?
L410: Should this be Fig. 8a not 6b?
L435: Should be Tab.2 not Tab/2.
L466: Change “have passed” to “has passed”
L503: “(Fig. ??a, b)” – please correct this.
References
Boutle, I.A., Belcher, S.E. and Plant, R.S. (2011), Moisture transport in midlatitude cyclones. Q.J.R. Meteorol. Soc., 137: 360-373. https://doi.org/10.1002/qj.783
Citation: https://doi.org/10.5194/egusphere-2023-905-RC1 -
RC2: 'Comment on egusphere-2023-905', Anonymous Referee #2, 21 Jul 2023
Review of ‘Linking Gulf Stream air-sea interactions to the exceptional blocking episode in February 2019: A Lagrangian perspective’ by Marta Wenta, Christian M. Grams, Lukas Papritz and Marc Federer
General comments
The paper constitutes a very complete piece of work tackling one important outstanding problem in dynamical meteorology, namely the connection between atmospheric blocking and surface processes and, in particular, the connection between the Gulf Stream, cold-air outbreaks, extratropical cyclone and blocking in the North Atlantic region, determinant of the European weather. The work relies on a variety of methods of analysis including cyclone tracking, the identification of negative PV anomalies and cold-air outbreaks, and Lagrangian trajectories. This work is fully in the scope of Weather and Climate Dynamics.
I give below a list of specific and technical comments that I believe can improve the paper by making it more understandable. Once these comments are considered I will be able to fully recommend the article for publication in this journal.
Specific comments
L25-26: ‘cold surges’ I can see that some of the papers referred to here do talk about cold surges or long-lived cold surges, but are these the same as ‘cold spells’. In my opinion ‘cold spell’ is more appropriate as it does not imply the motion of cold air, but the effect of the persistence of large-scale conditions over a given region for a sufficiently long period of time.
L56-57: ‘the warm sector of the consecutive cyclone’ Is there a more specific definition of ‘the consecutive cyclone’? Is this any consecutive cyclone or are the authors thinking here of a specific flow configuration? From reading the paper I would say that it’s better to talk about ‘consecutive cyclones’ in plural and that it’s not always obvious which cyclone could be named the consecutive one.
L67-71: I was not quite sure what was the argument around the wintertime poleward displacement of the jet stream and eddy heat fluxes. Is it the intensity or the position of the eddy heat flux that has the most influence on the position of the jet stream?
L147-148: In the list of traced variables along trajectories there are five that are single-level. However the trajectories are located in three dimensions. How is the assignment of the single-level variables to trajectories made? Is there a criterion on the vertical distance from the parcel to the surface (single-level) or is the assignment made based on the horizontal position of the parcel only?
L194-196: The CAO index is based on 850-hPa potential temperature. Is there any condition on the level of the parcel as the potential temperature vertical gradient will be very different for two parcels with the same potential temperature difference but at very different pressure (altitude).
L263-264: ‘The advection of cold air behind the cold front of LE2 resulted in another strong surface evaporation event…’ How is the causality being assessed? Or is it that the CAO index is somehow being used as an indicator of strong surface evaporation?
L323: I’m not convinced about the connection between increases in the fraction of WCB trajectories and increases in the number of GS CAO NPVA trajectories. I can see that there is coincidence to a certain degree but I would not say that this is systematic. Or perhaps I’m looking at this in the wrong way. Further explanation would be welcome. A similar comment is valid for L338-339.
L383: In what cases are the cold-sector uptakes not associated with a CAO? Does this tend to occur at higher latitudes where SST is climatologically coder?
L387: I’m not sure I’m looking at this in the right way. From Fig. 7 the proportion of CAOs peak between 14 and 15 Feb and the SHLF is minimum on 13 Feb but LE0 intensifies rapidly only until 16 Feb. Perhaps it would help to mark these events directly in the figure using vertical lines.
L391-293: If I’m following the argument, the idea here is that the CS of the cyclone should be completely covered by a CAO region. Since most uptakes take place in CAO regions in comparison with the cyclone’s CS regions, it follows that the uptakes must take place in CAOs that are not associated with that particular cyclone and therefore they must be associated with a preceding cyclone. If this interpretation is correct, then you might want to add ‘preceding’ in L392 “...occur in the CAO induced by the ‘preceding’ cyclone rather than…”. Even if my interpretation of your argument is correct, it might be worth elaborating in your explanation. How are cold sectors assigned to a given cyclone? Is this done using a neighbourhood of a given size (e.g. a circular region of a given radius) around the cyclone centre? How big is this area compared to the CAO regions and does the area identified as the cold sector depend on the definition of the cold sector? Are the cold sectors truly a subset of CAO regions?
L403: This comment is related to the comment to L147-148. The SLFH will be experienced only by trajectories intersecting or around the first model level. How is SLFH assigned to trajectories at higher altitudes?
L459: ‘do not interact’ Could the interaction be at upper levels by e.g. crossing above the maximum heating associated with extratropical cyclones, potentially leading to a reduction of potential vorticity?
L519: The argument is plausible but given that the lower trajectories are those that exhibit PV reduction I would have thought that this is more related to the strong surface heating rather than evaporative cooling. There is no direct demonstration of evaporative cooling being responsible for negative PV in this case.
Technical comments
L15: Even after reading the paper I am not quite sure what is meant by ‘one-fifth of these air masses’. How are the air masses being counted or what metric is being used to make such a statement?
L48: Change ‘pass through the region’ to ‘pass through a region’.
List of references and references in the text: Vanniére et al. (2017) and Vannière et al. (2017) are two different papers but the tilde over the first author’s name can’t be the symbol to distinguish them. I believe Benoît’s surname is Vannière.
L107-108: ‘eight neighbouring grid points’ I imagine this number of grid points is resolution dependent, is it?
L110: Is it worth applying the latitude related factor to the rapidly intensifying cyclone criterion given that the cyclones in the case study do intensify at a variety of latitudes?
L130 and L134: As I understand it, the regime mask is based on the weather regime PV pattern. However, the mask is described in L130 but the WR PV pattern is not defined until L133-134. I’d recommend switching the sentence order so that the text becomes clearer.
L134: What is the ‘cycle’ referred to in this line?
L134-135: I’d suggest indicating when both NPVAs formed in the first sentence simply because otherwise the reader is left wondering why you give this information about the minor NPVA but not about the main one. Of course, this comes in the next sentence, but I think re-ordering (re-writing) these two sentences would make the text easier to read.
L163-164: I don’t quite follow the additional criteria to avoid double-counting here and in section 2.2.3.
L169: Are there any special considerations to make when dealing with BL trajectories in terms of the parametrisation turbulence and other boundary layer processes?
L215: How is the start of the ascent defined? Are there any pressure fluctuations along trajectories that need to be smoothed out?
L245: I’m not sure I understand this sentence: “The close succession of cyclones over the North Atlantic [...] disrupted the normal progression of weather systems”. I would have attributed the disruption to atmospheric blocking rather than to the succession of cyclones.
L258: Should it be Fig. 1c rather than Fig. 1e?
L260: I’d recommend using the cyclone nomenclature already introduced instead of the cyclones’ start dates (e.g., LE1 instead of the one from 18 February).
L262: Change ‘reinforces’ for ‘reinforced’.
L264: By cyclone’s ascending air stream, do you mean the warm conveyor belt?
L264: Add ‘the’ between ‘from’ and ‘western’.
L268: Should it be Fig. 3c rather than Fig. 1c?
L268: It might be clearer to refer to the cyclones as LE0-3.
L304 and others: In general, what is included in the article is worth noting. I’d recommend avoiding saying ‘It is worth noting that…’ to simply say ‘Note that…’ However, I recognise this is a matter of personal preference.
L301-305: Rewrite these sentences so that you first mention the separation into two subsets to then quote the percentages, noting that they are based on the strong CAO subsets.
L308-309: It’s not completely surprising that most trajectories are diabatically heated by 2 K or more given that by construction the trajectories are required to ascend.
L310: Delete ‘the’ from ‘the upper-level blocks’.
L319: I understand that a reference to Madonna et al. (2014) is required here, but as it stands it’s not clear why. I’d say ‘... of 600 hPa within 48 h (green in Fig. 4), following Madonna et al. (2004).’
L320: I think it should be ‘evolution’ rather than ‘revolution’.
L404: Change ‘ascend’ for ‘ascent’.
L432: Should it be Fig. 6d rather than 8d?
L435: Change ‘Tab/2’ for ‘Tab. 2’.
L452: Change ‘Fig.8, d’ for ‘Fig. 8d’.
L476: There is no Fig. 8f.
L486: There is only one sub-section (3.5.1) in Section 3.5. Is the header needed?
L495: Change ‘level’ for ‘time’.
L503: Should it say ‘Fig. 10a,b’?
L507: Change ‘...trajectories. Indicating…’ for ‘...trajectories, indicating…’
L515: Why 36N? I would have said 40N.
L523: Change ‘exceed’ for ‘go below’.
L539: Delete ‘GS’ and add ‘that’ between ‘trajectories’ and ‘started’. ‘GS’ would be redundant if you say ‘trajectories that started from the Gulf Stream’.
L590: The piece that is still missing is the decay of the block. Is it that for some reason CAOs and cyclones stop occurring and the block decays? Or is it that their WCB outflows need to comply with certain features to be able to sustain the block?
L599: Should it say ‘... suggested by Methven (2015)’?
Figure 1. I’ve always found that crosses marking parcel locations obscure quite a lot of the field underneath. Is there any other way of representing this? What about dots or perhaps you could present not so many of them?
Figure 2: The stars are very difficult to see. Perhaps this could be improved by outlining them with a black line. The caption should say what they indicate.
Figure 4: The caption should explicitly say what are the (a) and (b) panels. Is the grey background necessary? The colours for DI and CAO(>0K) are too similar to each other and therefore difficult to distinguish.
Figure 6. Caption: (left column) includes (a). It’s perhaps better to say (c,e). Change (%3000 km^2) at the end of the caption to (%/3000 km^2)
Figure 7a: What are the intersections between categories, i.e. what categories are a subset of what categories and which categories should add to 100%? Add ‘interval between the 90th and 10th percentiles…’ The colours used for CS and rest are very similar to each other and therefore difficult to distinguish. Could you use different colours or different line thicknesses?
Figure 8: Caption: Delete ‘whem’. I might have missed it but I think panel (b) was not used.
Figure 10: The order in which the variables are listed in the caption does not correspond to the panels.
Citation: https://doi.org/10.5194/egusphere-2023-905-RC2 -
AC1: 'Comment on egusphere-2023-905', Marta Wenta, 21 Aug 2023
On behalf of my co-authors, I would like to address the Reviewers’ comments to the article titled ‘Linking Gulf Stream Air-Sea Interactions to the exceptional blocking episode in February 2019: A Lagrangian Perspective’. First, we would like to thank the reviewers for their time and effort in reviewing our manuscript. The feedback has been invaluable and we believe that it will greatly improve the content and presentation of the paper.
Following the feedback from the reviewers, we willimplement significant modifications to our manuscript, focusing more on our core objectives. Previous research, most notably by Cheung et al. (2023) and Joyce T. et al. (2019), has established that the Gulf Stream has an influence on large-scale dynamics. However, the precise mechanism by which signals from air-sea interactions translate into large-scale circulation remains partially understood, as underscored by studies such as those by Michel et al. (2023), Kwon et al. (2010), and Czaja et al. (2019). Particularly interesting is the fact that the majority of NPVA GS trajectories undergo a significant latent heat release prior to reaching the block. Steinfeld et al. 2019 and 2020 identified that these diabatically heated air masses play an important role in initiating and sustaining the block, even if they constitute just 30-40% of all trajectories released from the block. We recognize that the bulk of perspectives and analysis used in our initial manuscript might not have adequately addressed this, and the previous point. Thereby we obscured our initial intention to provide more dynamical insight in the processes establishing an impact of air-sea interaction on the upper-level large-scale flow.
To conclude, our objective is to identify the potential mechanisms by which the Gulf Stream and associated air-sea interactions in the western North Atlantic influence upper-level atmospheric patterns. We are not claiming that these trajectories are the sole drivers of block formation, but evidence from the aforementioned studies suggests they might play a certain role in block development. In the revised version of the manuscript, we will try to better convey this message.
In line with the reviewers' feedback, we recognize the need to refine and streamline our paper's focus. To achieve this, our primary attention will be directed toward the NPVA trajectories, with the goal of offering a clearer insight into their interplay with the Gulf Stream. We will remove much of the discussion of other trajectory subsets, which distracted from our main message. Besides that, we want to further emphasize the roles that cyclones and CAOs play in the lifecycle of NPVA GS trajectories. To support this, our approach will focus on a more in-depth synoptic analysis, ensuring a thorough understanding of these events and their implications.
To summarize, as we prepare for the paper's resubmission, we'll be integrating the aforementioned changes and reshaping the paper to align with the reviewers' suggestions. We trust that this updated version, which we plan to resubmit promptly, offers a clearer perspective and addresses all concerns highlighted in the reviews.
Bibliography:
Cheung, HN., Omrani, NE., Ogawa, F. et al. Pacific oceanic front amplifies the impact of Atlantic oceanic front on North Atlantic blocking. npj Clim Atmos Sci 6, 61 (2023). doi:10.1038/s41612-023-00370-x
Czaja, A., C. Frankignoul, S. Minobe, and B. Vannière, 2019: Simulating the Midlatitude Atmospheric Circulation: What Might We Gain From High-Resolution Modeling of Air-Sea Interactions? Curr Clim Change Rep, doi:10.1007/s40641-019-00148-5
Joyce, T. M., Kwon, Y.-O., Seo, H., & Ummenhofer, C. C. (2019). Meridional Gulf Stream shifts can influence wintertime variability in the North Atlantic storm track and Greenland blocking. Geophysical Research Letters, 46, 1702–1708. doi:10.1029/2018GL081087
Kwon, Y.-O., M. A. Alexander, N. A. Bond, C. Frankignoul, H. Nakamura, B. Qiu, and L. A. Thompson, 2010: Role of the Gulf Stream and Kuroshio–Oyashio Systems in Large-Scale Atmosphere–Ocean Interaction: A Review. J. Climate, 23, 3249–3281, doi:10.1175/2010JCLI3343.1.
Michel, S.L.L., von der Heydt, A.S., van Westen, R.M. et al. Increased wintertime European atmospheric blocking frequencies in General Circulation Models with an eddy-permitting ocean. npj Clim Atmos Sci 6, 50 (2023). doi:10.1038/s41612-023-00372-9
Steinfeld, D., Pfahl, S. The role of latent heating in atmospheric blocking dynamics: a global climatology. Clim Dyn 53, 6159–6180 (2019). doi:10.1007/s00382-019-04919-6
Steinfeld, D., Boettcher, M., Forbes, R., and Pfahl, S.: The sensitivity of atmospheric blocking to upstream latent heating – numerical experiments, Weather Clim. Dynam., 1, 405–426, doi: 10.5194/wcd-1-405-2020, 2020.
Citation: https://doi.org/10.5194/egusphere-2023-905-AC1
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Christian M. Grams
Lukas Papritz
Marc Federer
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